Summary Report on Dowel Bar Retrofit for Rigid Pavements Stg2_4.8_DBR... · 2009-04-28 ·...

DISTRIBUTION Stage 2, April 28, 2009

UCPRC-SR-2008-03

December 2008Summary Report: UCPRC-SR-2008-03

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Authors:John T. Harvey, Erwin Kohler, Nicholas Santero, Yi Bian,

Mauricio Mancio, Cruz Carlos, Jr.

Partnered Pavement Research Program (PPRC) Contract Strategic Plan Element 4.8: Dowel Bar Retrofit for Rigid Pavements

PREPARED FOR: California Department of Transportation

Division of Research and Innovation

Office of Roadway Research

PREPARED BY:University of California

Pavement Research Center

UC Davis, UC Berkeley


UCPRC-SR-2008-03 ii

DOCUMENT RETRIEVAL PAGE Summary Report: UCPRC-SR-2008-03

Title: Summary Report on Dowel Bar Retrofit for Rigid Pavements

Authors: John T. Harvey, Erwin Kohler, Nick Santero, Yi Bian, Maurcio Mancio, and Cruz Carlos, Jr.

Prepared for: Division of Research and Innovation Office of Roadway Research Caltrans

FHWA No.: Work Submitted April 28, 2009

Date:December 2008

Strategic Plan Element No.: 4.8

Status: Draft Version No.:Stage 2

Abstract: This report summarizes investigations undertaken by the University of California Pavement Research Center between 2000 and 2008 to assess Caltrans strategies for the dowel bar retrofit (DBR)of rigid pavements, specifically jointed plain concrete pavement. The overall objectives and questions to be answered by the study are reviewed and the investigations undertaken to answer those questions, namely desktop studies and laboratory and full-scale experiments are discussed. Reports and recommendations from each study are listed, as are some details about how the recommendations have been implemented.

Keywords Dowel bar retrofit, rehabilitation, concrete pavement, joints, faulting, grinding, life cycle cost analysis, corrosion

Proposals for implementation All recommendations from the DBR investigations are summarized in this report.

Related documents • Research Report: Construction and Test Results from Dowel Bar Retrofit HVS Test Sections 553FD, 554FD, and

555FD: US 101, Ukiah, Mendocino County, by J. Harvey, A. Ali, D. Hung, J. Uhlmeyer, L. Popescu, D. Bush, K. Grote, J. Lea, and C. Scheffy. (February 2003) UCPRC-RR-2003-03.

• Research Report: Construction and Test Results on Dowel Bar Retrofit HVS Test Sections 556FD, 557FD, 558FD, and 559FD: State Route 14, Los Angeles County at Palmdale, by Yi Bian, John Harvey, and Abdikarim Ali. (March 2006) UCPRC-RR-2006-02.

• Research Report: Laboratory Evaluation of Corrosion Resistance of Steel Dowels in Concrete Pavement, by Mauricio Mancio, Cruz Carlos Jr., Jieying Zhang, John T. Harvey, and Paulo J. M. Monteiro. (January 2007) UCPRC-RR-2005-10.

• Research Report: Performance of Dowel Bar Retrofit, by E. Kohler, J. Harvey, B. Steven, and N. Santero. (December 2007) UCPRC-RR-2007-10.

• Technical Memorandum: Survey Results of Dowel Bar Retrofit Projects in California, by Erwin Kohler and Nick Santero. (December 2007) UCPRC-TM-2007-07.

• Research Report: Life-Cycle Cost Analysis of Dowel Bar Retrofit, by N. Santero, E. Kohler, and J. Harvey. (December 2007) UCPRC-RR-2007-11.

• Research Report: Fiber-Reinforced Polymer (FRP) Dowel Bar Laboratory Tests Results, by Yi Bian, John T. Harvey, and Erwin Kohler. (September 2008) UCPRC-RR-2007-01.

• Research Report: Finite Element Analysis of Dowel Bar Retrofit Alternatives, by Y. Bian, G. Jie, and J. Harvey. (March 2008) UCPRC-RR-2008-06.

Signatures John T. Harvey First Author

Erwin Kohler Technical Review

David Spinner Editor

John T. Harvey Principal Investigator

T. J. Holland Caltrans Contract Manager


UCPRC-SR-2008-03 iii

DISCLAIMER The contents of this report reflect the views of the authors who are responsible for the facts and

accuracy of the data presented herein. The contents do not necessarily reflect the official views

or policies of the State of California or of the Federal Highway Administration. This report does

not constitute a standard, specification, or regulation.

ACKNOWLEDGMENTS The University of California Pavement Research Center would like to acknowledge the support

and advice provided by:

• The Washington State Department of Transportation for assistance with inspection of

field test sections and provision of a failed DBR joint cut from an interstate highway and

cores from various locations;

• The Caltrans Corrosion Laboratory for assistance with preparation of thin slices, chloride

content analysis, and peer review of results;

• The Caltrans Maintenance and Construction staff in District 1 and District 7 for help with

field test section construction and operations; and

• The Caltrans Headquarters Materials, Engineering and Testing Services (METS) staff for

assistance with coordination of field work and guidance.

This summary report was prepared under the technical direction of William Farnbach and Tom

Pyle for the Caltrans Pavement Standards Team (PST), under the chairmanship of Phil

Stolarski, Tom Hoover, and Peter Vacura. The technical representative for the Division of

Research and Innovation was T. Joseph Holland. The contract manager was Michael Samadian

and later Joe Holland, assisted by Alfredo Rodriguez. The authors would like to thank all those

involved for support and advice on this project.


UCPRC-SR-2008-03 iv

PROJECT OBJECTIVES

The work presented in this report was carried out by the University of California Pavement

Research Center (UCPRC) through the Partnered Pavement Research Center (PPRC)

agreement as PPRC Strategic Plan Element 4.8: “Dowel Bar Retrofit (DBR) for Rigid

Pavements.” Tasks for this project focus on the four objectives listed below that were agreed

upon with Caltrans in 2001. This report summarizes the work completed for all the objectives.

1. Field accelerated pavement testing with the Heavy Vehicle Simulator (HVS). The HVS

testing was performed to quickly collect full-scale data to answer questions regarding

loading under slow, heavy traffic, although with heavier loads than normally occur under

real traffic. This HVS testing compares performance of retrofitted joints and cracks with

those not retrofitted. Associated testing included use of the Falling Weight Deflectometer

(FWD) to measure load transfer efficiency (LTE) and other pavement properties. Several

generic types of dowels are included in the field test sections.

2. Field live-traffic testing. Field live-traffic testing was conducted to collect field data on a

long-term basis under real loads. This testing enables calibration of HVS and analysis

results, and includes instrumented sections in California and a compilation of

performance data from existing DBR projects throughout California and the United

States: This allows calibration of HVS and analysis results to field project results.

3. Laboratory testing of materials. This was undertaken to permit evaluation of additional

variables that cannot be included in HVS testing, such as corrosion of the dowels and

dowel types not included in the field test sections. Laboratory testing is also used to

characterize materials used in the HVS test sections.

4. Modeling and Analysis.

a. Finite element analysis of doweled concrete pavement joints. This analysis was

used to provide performance prediction of other options without testing, and to

extrapolate HVS results. It was also used to evaluate the effects of some

construction quality issues on expected performance.

b. Compilation of performance data from existing DBR projects throughout

California and the United States. This allows for calibration of HVS and analysis

results to field project results.

c. Life-cycle cost analysis is performed to determine the cost effectiveness of DBR

compared to the alternative of grinding without DBR.

This document completes the delivery of the work of PPRC SPE 4.8.


UCPRC-SR-2008-03 v

TABLE OF CONTENTS Project Objectives...................................................................................................................... iv List of Figures ............................................................................................................................vi List of Tables..............................................................................................................................vi 1 Introduction .....................................................................................................................1

1.1 Background......................................................................................................................1 1.2 Project Overview, Study Objectives, and Outcomes........................................................3 1.3 Timeline ...........................................................................................................................6

2 Field Accelerated Pavement Testing.............................................................................9 2.1 Literature Survey: Models for Faulting and Ride Quality................................................10 2.2 Ukiah Heavy Vehicle Simulator Test Sections...............................................................11 2.3 Palmdale Heavy Vehicle Simulator Test Sections .........................................................14 2.4 Reports ..........................................................................................................................22

3 Field Observations of Performance and Measurement of Field Chloride Contents.........................................................................................................................23

3.1 Chloride Content in In-Service Pavements in Washington State ...................................23 3.2 Review of Field Performance Across California and Several Other States ...................24 3.3 Reports ..........................................................................................................................28

4 Laboratory Studies .......................................................................................................29 4.1 Laboratory Corrosion Testing of Metallic Dowels...........................................................29 4.2 Laboratory Testing of Fiber-Reinforced Polymer (FRP) Dowels....................................34 The overall conclusions from laboratory testing of FRP dowels were: ....................................39 4.3 Reports ..........................................................................................................................40

5 Modeling ........................................................................................................................41 5.1 Finite Element Analysis and Performance Estimates ....................................................41 5.2 Life Cycle Cost Analysis and Comparison with Alternatives ..........................................45 5.3 Reports ..........................................................................................................................51

6 Summary of Recommendations ..................................................................................53


UCPRC-SR-2008-03 vi

LIST OF FIGURES

Figure 1: Faulting on a rigid pavement, photo taken looking upstream (courtesy of

L. Khazanovich). .....................................................................................................................2 Figure 2: Timeline for the dowel bar retrofit study........................................................................6 Figure 3: Microphoto of holiday in epoxy-coating. ......................................................................33 Figure 4: Flexural testing fixture..................................................................................................35 Figure 5: Flexural testing fixture with failing dowel. ....................................................................35 Figure 6: Shear fixture. ...............................................................................................................36 Figure 7: Shear fixture with failing dowel. ..................................................................................37

LIST OF TABLES

Table 1: Objectives and Outcomes..............................................................................................7 Table 2: Timetable of Testing on Palmdale Sections.................................................................15 Table 3: Summary of Palmdate Dowel Bar Retrofit Test Sections ............................................16 Table 4: Typical Properties of GFRP and Steel Bars (Trejo et al., 2000) ...................................35 Table 5: Properties of the Two Types of FRP Dowel Bars Used in This Study ..........................36 Table 6: Summary of Implementation of Recommendations by Caltrans...................................57


UCPRC-SR-2008-03 vii

CONVERSION FACTORS SI* (MODERN METRIC) CONVERSION FACTORS

APPROXIMATE CONVERSIONS TO SI UNITS

APPROXIMATE CONVERSIONS FROM SI UNITS

Symbol Convert From Multiply By Convert To Symbol

LENGTH

mm millimeters 0.039 inches in

M meters 3.28 feet ft

Km kilometers mile mile

AREA

Mm2 square millimeters 0.0016 square inches in2

M2 square meters 10.764 square feet ft2

VOLUME

M3 Cubic meters 35.314 cubic feet ft3

MASS

Kg kilograms 2.202 pounds lb

TEMPERATURE (exact degrees)

C Celsius 1.8C+32 Fahrenheit F

FORCE and PRESSURE or STRESS

N newtons 0.225 poundforce lbf

kPa kilopascals 0.145 poundforce/square inch lbf/in2

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM

E380. (Revised March 2003)


UCPRC-SR-2008-03 viii


UCPRC-SR-2008-03 1

1 INTRODUCTION

1.1 Background

Caltrans operates a state highway network of more than 49,000 lane miles (78,000 lane-

kilometers). In 2000, when this study was being considered, 32 percent of the network consisted

of rigid pavement (portland cement concrete), mostly on truck routes in urban areas with heavy

traffic volumes. At that time, 48 percent of the rehabilitation projects and 41 percent of the lane-

miles requiring immediate attention were rigid pavements that were mostly constructed between

1959 and 1974, and designed for 20-year lives based on traffic volumes and loads estimated at

that time. As of 20071, rigid pavements still made up 32 percent of the network lane-miles, and

31 percent of the distressed lane-miles.

Faulting is a distress of rigid pavements in which there is a difference in elevation between slabs

at the transverse joints (Figure 1) that affects the pavement’s ride quality. This elevation

difference makes the edge of the slab upstream of a transverse joint (or crack) higher than the

edge downstream of the joint, causing a pavement profile that repeatedly steps down at the

joints. Elevation differences of even a few millimeters cause noticeable increases in the

roughness of the pavement, as measured by the International Roughness Index (IRI). Faulting

is primarily caused by high deflections at the joint and lack of load transfer efficiency (LTE)

across it, which results in the two sides of the joint acting independently as truck axles pass

over it. The independent action causes local damage to the base beneath the slab at the joint

and migration of base material from under the downstream slab to under the upstream slab; and

in some cases, tilting of the slab through embedment of the upstream edge into the base and

uplift of the other end of the slab. The presence of water at the joint accelerates the faulting

mechanism.

LTE is provided through the base layer, through aggregate interlock between the slab faces at

the joint, and through use of dowels as load-transfer devices that transmit load across the joint.

With the exception of a few test sections, jointed concrete pavements in California built before

1999 do not have dowels at their transverse joints. The main reason for not including them—in

addition to the material and labor costs associated with including dowels—was their reputation

1 California Department of Transportation—2007 State of the Pavement Report. 2008. Sacramento, CA: Caltrans Maintenance Program. Pavement Management Information Branch.


UCPRC-SR-2008-03 2

for difficult construction, which originated during the 1950s and 1960s when many of the state’s

rigid pavements were constructed. The prime issues during that period were with the dowels’

alignment and location, as improper positioning could lead to early pavement failure. Further,

typical practice at the time included use of small-diameter dowels, on the order of 19 to 25 mm

(3/4 to 1 in.), which caused high bearing stresses in the surrounding concrete. Even when these

dowels were placed correctly, they loosened and were not particularly effective in reducing

faulting2.

In addition, many transverse cracks act as joints that are susceptible to faulting because

Caltrans rigid pavements, except for a few experimental sections, are all plain-jointed concrete,

which means they lack the continuous steel reinforcement that would hold together the two

sides of the transverse cracks. To control faulting development, Caltrans has historically relied

on improving the non-erodability of base materials and on aggregate interlock.

Figure 1: Faulting on a rigid pavement, photo taken looking upstream (courtesy of L. Khazanovich).

Nearly all of the rigid pavements in California currently needing rehabilitation were built with

cement-treated bases (CTBs). Caltrans has found that in many cases these bases have

deteriorated through cracking and erosion, and that they have been ineffective in preventing

rapid development of faulting. A 1979 study performed by McLeod and Monismith indicated that

faulting generally occurred within 1 million to 4 million equivalent single-axle loads (ESALs) after 2 Hveem, F. N. 1949. A Report of an Investigation to Determine Causes for Displacement and Faulting at the Joints in Portland Cement Concrete Pavements. California Division of Highways, Materials and Research Department (M&R), Sacramento, California. (May 17).


UCPRC-SR-2008-03 3

construction3. With today’s traffic rates, this represents less than five years of traffic for many of

Caltrans’ concrete freeways.

Caltrans became interested in Dowel Bar

Retrofit (DBR) as a possible solution to

faulting problems in the late 1990s, after

positive reports of successful construction

and economical performance from other state

highway agencies, such as the Washington

State Department of Transportation (WSDOT).

DBR consists of the installation of dowel bars

in concrete pavements originally built without

them. Installation includes sawing slots for the dowels across transverse joints, inserting the

dowels, and grouting them in place. In most cases this is followed by diamond grinding to

remove faulting and to smooth the grout surface where the dowels were installed. The joint load

transfer provided by the dowels improves the structural response of the pavement and

significantly slows the development of new faulting under truck traffic.

Partly on the basis of good results obtained from several early experimental sections in

California, and successful early performance (up to 10 years at the time) documented by

WSDOT in the late 1990s, Caltrans proceeded with implementation of DBR. However, in 2001

Caltrans began expressing concerns over dowel failures resulting from misaligned dowels and

the loss of backfill material from DBR slots in several of its projects. Other concerns included the

impact on the life of DBR of corrosion resistance of different dowel types, in part based on

experience with dowel bar corrosion in Washington and Minnesota.

1.2 Project Overview, Study Objectives, and Outcomes

The work summarized in this report was performed as part of a project originally proposed in

2000 by the Caltrans Headquarters Division of Design. Other Caltrans divisions participating in

oversight of the project included Headquarters Materials Engineering and Testing Services

3 Macleod, D. R., and Monismith, C. L. 1979. Performance of Portland Cement Concrete Pavement. Department of Civil Engineering, Institute of Transportation Studies, University of California, Berkeley. (February).


UCPRC-SR-2008-03 4

(METS): Office of Rigid Pavement Materials and Structural Concrete, as well as Caltrans

Districts 1 and 7.

This research was intended to provide Caltrans with information needed for to help make

decisions regarding selection of DBR and its design and construction in order (1) to help

determine where DBR may be a cost-effective strategy for rehabilitating rigid pavement; (2) to

help obtain best performance where DBR is selected as the preferred rehabilitation strategy;

and (3) to help obtain best performance for new dowels with respect to corrosion. This work was

completed as part of Partnered Pavement Research Program (PPRC) Strategic Plan Element

4.8, “Dowel Bar Retrofit of Rigid Pavements.”

The objectives of the study and the outcomes are shown in Table 1. The table also identifies the

sections of this report where the results are summarized.

To answer the study questions, the study including the following investigations:

Field Accelerated Pavement Testing with the Heavy Vehicle Simulator (HVS). A review of the

literature was performed in 2001 that included identification of models for predicting faulting

performance, and the relation of ride quality to faulting. Six test sections were constructed on

US 101 near Ukiah. These six sections consisted of two sets of replicate sections: one trafficked

by the HVS and one opened to live traffic. Each set of replicate sections included one control

section with no DBR, one DBR section including two transverse joints, and one DBR section

including a transverse joint and a transverse crack. The HVS sections were monitored in terms

of LTE and joint deflections. Four DBR test sections were also constructed on the shoulder of

SR 14 near Palmdale for HVS testing. These sections included transverse joints with four

epoxy-coated steel dowels per wheelpath, three epoxy-coated steel dowels per wheelpath, four

fiber-reinforced polymer (FRP) dowels per wheelpath, and four grout-filled hollow stainless steel

dowels per wheelpath. All of these sections were trafficked with the HVS until the slab or the

dowels failed. They were monitored in terms of LTE and joint deflections. There were not live-

traffic sections at Palmdale.

Field Live-Traffic Testing and Other Field Performance Reviews. From 2001 to 2007, live-traffic

sections on US 101 at Ukiah were monitored in terms of LTE, joint deflections, and fault height.

A review was also made of field performance on construction projects built on the state highway


UCPRC-SR-2008-03 5

network that were previously reviewed by a Caltrans consultant, and additional test sections

placed by Caltrans. Other state highway agencies were contacted regarding field performance

of DBR projects. To further investigate dowel corrosion, chloride content analyses were

performed on cores taken around dowels at various locations in Washington State with different

levels of salting practice to compare the concrete chloride contents in the laboratory beams with

those in the field. One of the few DBR joints to fail in Washington was extracted from an

interstate highway and brought to California for forensic sampling.

Laboratory Testing of Materials. Laboratory corrosion testing was performed on seven kinds of

steel dowel: bare carbon steel, stainless-steel clad, grout-filled hollow stainless steel, dual-

phase steel, carbon steel–coated with bendable epoxy (green color code; Designation

ASTM A775), and carbon steel–coated with nonbendable epoxies (purple and gray color codes;

Designation ASTM A934). These dowels were subjected to half-cell potential measurements

following ASTM C876, and measurements using the linear polarization resistance (LPR)

technique following ASTM G59. These tests were performed on dowels cast in concrete beams

with joints and subjected to chloride conditioning. Laboratory testing was also performed on

FRP dowels to measure flexural stiffness, flexural strength, flexural fatigue, shear strength, and

shear fatigue performance. FRP dowels were tested without conditioning, and after conditioning

under alkaline solution, water, and ultraviolet light.

Concrete and grout strengths were tested for HVS test sections.

Modeling. Finite element analysis of slabs with DBR using steel and FRP dowels was performed

to estimate the stresses at the interface between the dowel and the backfill grout and the

stresses at the interface of the slot between the grout and the concrete slab. Parametric

analyses were performed to calculate the sensitivity of the compressive bearing stresses at the

grout-dowel interface to dowel stiffness, wheel load, and subgrade stiffness. Parametric

analyses were also performed to estimate stresses at the grout-dowel and grout-slab interfaces

for different axle loads and configurations. Grout-slab shear stresses were compared with shear

strengths for this interface measured by Caltrans in the laboratory.

Performance estimates from accelerated pavement testing and the review of field performance

of sections throughout California (including experimental sections at Colfax, where DBR was

placed on Interstate 80 including experimental dowel types, and mainline construction projects


UCPRC-SR-2008-03 6

in southern California) and the United States were used to perform life-cycle cost analysis

(LCCA) comparing DBR with alternative strategies.

1.3 Timeline

• An overview of the project timeline is shown in Figure 2.

HVS Testing US101 Ukiah

HVS Testing SR14 Palmdale

Lab test corrosion

Lab test FRP dow els

Monitor live traffic US101

Finite Element Analysis

US Performance Evaluation

Life Cycle Cost Analysis

'99 '00 '01 '02 '03 '04 '05 '06 '07 '08 '091999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Figure 2: Timeline for the dowel bar retrofit study.


UCPRC-SR-2008-03

Table 1: Objectives and Outcomes

Study Objectives Outcome Section* Examine literature and use existing faulting and ride quality models to perform sensitivity analyses to identify key variables expected to affect DBR performance. Evaluate the structural adequacy of DBR options (number of dowels per joint, dowel type, placement on transverse cracks, presence of longitudinal cracks), in terms of Load Transfer Efficiency (LTE) by accelerated pavement testing of DBR test sections with the Heavy Vehicle Simulator, and compare with joints with no DBR.

Identification of variables causing increases in roughness for concrete pavement. Estimation of years to reach state IRI trigger values for concrete pavement without dowels. Recommendations for FWD testing and deflection analysis for concrete pavement. Comparison of load transfer efficiency (LTE) and joint deflections for DBR compared with

control under HVS loading and different temperatures. Construction guidelines for DBR from WSDOT. Comparison of LTE and joint deflections for DBR under HVS loading: 3 and 4 dowels per

joint; steel, FRP, and grout-filled hollow stainless steel dowels and different temperatures.

2.1 2.1 2.2 & 2.3 2.2 2.2 2.3

Evaluate the performance of DBR on in-service pavements under live traffic including monitored test section, Caltrans construction projects, and construction projects in other states. Determine chloride contents in concrete slabs near dowels.

Summary of DBR field performance on nearly all sections in California and across the U.S. Measurement of chloride contents in concrete for mountain passes and other locations

with different levels of de-icing compound use, from WSDOT slabs.

3.2 3.1

Determine relative corrosion resistance of different metallic dowel types through laboratory testing, and compare resistance with expected chloride exposure Determine engineering properties and durability of fiber-reinforced polymer (FRP) dowels through laboratory testing including alkali, water, and UV conditioning.

Comparison of corrosion resistance of seven kinds of steel dowel, including different coatings and different types of steel from laboratory accelerated testing.

Comparison of chloride contents in laboratory tests and WSDOT slabs; preliminary recommendation for dowel type in different areas.

Evaluation of performance-related properties of FRP dowels under mechanical loading. Evaluation of durability of FRP dowels when exposed to alkaline solutions, water, and

ultra-violet conditioning; recommendation for testing for acceptance.

4.1 4.1 4.2 4.2


UCPRC-SR-2008-03 8

Study Objectives Outcome Section* Evaluate stresses in DBR/slab systems affecting performance using Finite Element Analysis and use to estimate performance under different conditions Use life-cycle cost analysis to evaluate cost-effectiveness of DBR compared to alternatives

Determination of stresses for DBR in different dowel types and surrounding grout for different load scenarios.

Estimation of faulting performance using FHWA models and calculated bearing stresses in grout for different load scenarios.

Estimation of fatigue life of FRP dowels for expected stresses. Estimation of risk of exceeding grout/slab shear strength for different grout types tested

by Caltrans. Comparison of life-cycle cost for DBR, grinding alone and asphalt overlay. Evaluation of sensitivity of life-cycle costs to different variables. Recommendations for projects where DBR is most likely to be the most cost-effective

solution.

5.1 5.1 5.1 5.1 5.2 5.2 5.2

* Section in this report in which the objective is discussed.


UCPRC-SR-2008-03

2 FIELD ACCELERATED PAVEMENT TESTING

Accelerated pavement testing was carried out using the Heavy

Vehicle Simulator (HVS) on DBR and control test sections

constructed in the truck lane of US 101 near Ukiah and on the

shoulder of SR 14 near Palmdale. The purpose of the HVS

testing was:

• To evaluate the effect that DBR of transverse joints and

cracks has on their load transfer efficiency (LTE), then

compare those results with the LTE of joints and cracks

without DBR,

• To evaluate the effects on LTE of different numbers of dowels in the wheelpath, and

• To evaluate the effects on LTE of different dowel materials.

The HVS test sections at Ukiah included a 35-year-old existing pavement in a wet environment,

while those in Palmdale included a four-year old pavement built to 1960s standards in a dry

environment.

Extensive deflection testing was performed on all HVS test sections using the Falling Weight

Deflectometer (FWD) and different load levels, day and night, and at different times of the year

for the following stages of pavement condition:

• Before DBR

• After DBR but before HVS testing

• After HVS testing

The deflection data identified the baseline LTE for the original pavement, the improvement in

LTE caused by DBR for each of the different designs and dowel types used, and any reduction

in LTE caused by HVS trafficking. The effects of pavement temperature on LTE were measured

for each of these factors.

HVS tests followed these three failure criteria:

• Fatigue cracking of the concrete slab,

• Major damage to the DBR joints, or

• Loss of LTE of the trafficked joint or crack.


UCPRC-SR-2008-03 10

As an initial step, the literature available in 2001 was reviewed for past observations of faulting

performance, and to identify existing performance models. Models were sought to identify

critical variables associated with faulting of pavements without dowels, and to relate faulting to

ride quality. Similar models were used with HVS data and modeling data to estimate DBR

performance in later investigations. Models were also identified in the literature that related ride

quality to faulting in terms of the old Present Serviceability Rating (PSR) roughness

measurement parameter used in the 1993 AASHTO Design Guide and the International

Roughness Index (IRI) used in the Caltrans PMS. The models relating IRI to faulting and

faulting to load and environment variables are from a study sponsored by the Federal Highway

Administration in the early 1990s commonly referred to as “RIPPER.”

2.1 Literature Survey: Models for Faulting and Ride Quality

The primary findings identified in the 2001 literature survey and sensitivity analyses performed

using the faulting and ride quality models were:

• Caltrans rigid pavements built between 1950 and 1997 do not have dowels at the

transverse joints. Nationally calibrated models and previous performance observations in

California indicate that these pavements develop faulting sufficient to significantly affect

ride quality after about 5 million Equivalent Single Axle Loads (ESALs).

• Faulting has a very strong effect on ride quality, which is the primary factor in road users’

perception of pavement condition. Faulting also has a much greater impact on the

International Roughness Index (IRI) value than do transverse cracking, full-depth repairs,

or spalling of joints. Models indicate that faults as small as 1.7 mm to 3 mm reduce ride

quality sufficiently to result in a high prioritization for maintenance or rehabilitation in the

Caltrans Pavement Management System (IRI greater than 3.36 m/km [213 inches/mile]).

• The RIPPER model for faulting indicates that pavements in climate regions with more

days per year on which the air temperature exceeded 32°C (90°F) have the slowest

development of faulting compared to those with fewer days with those high temperatures.

This is due to slab expansion which closes the joints, resulting in greater aggregate

interlock and greater load transfer efficiency at those temperatures.

• The RIPPER model for faulting underpredicted the measured faulting prior to DBR

construction for the 35-year old pavement without dowels at the Ukiah test site, which

included a sensitivity analysis for past ESALs.


UCPRC-SR-2008-03 11

2.2 Ukiah Heavy Vehicle Simulator Test Sections

The existing pavement at Ukiah, built in 1967,

consisted of concrete slabs approximately

200 mm thick (0.67 ft) on cement-treated base

approximately 90 mm thick (3.5 in.). The joints

were cut at a skewed angle of 9.5 degrees (1:6

ratio), typical of Caltrans construction at that

time. The pavement had been subjected to an

estimated 4 million ESALs and had fault heights

between 2 mm and 8 mm (measured on adjacent joints without DBR in 2006). The HVS test

sections at Ukiah included:

• Section 553FD, which had two transverse cracks that were dowel bar retrofitted, with

433,974 total HVS repetitions applied.

• Section 554FD, which had two sawn joints and a longitudinal crack and was dowel bar

retrofitted across the sawn joints, with 472,578 total HVS repetitions applied.

• Section 555FD which was a control section and was not retrofitted with dowel bars.

Testing was performed across two sawn joints and uncracked slabs, with 169,679 total

HVS repetitions applied.

Trafficking on the HVS sections at Ukiah consisted of 40 and 90 kN (9,000 and 22,250 lb) loads

on a dual truck wheel. Channelized (no wander), bidirectional loading was conducted on the

wheelpath over the center of the dowel bar group for all tests. The test sections were in a deep

cut with water draining into the pavement from the cut slopes. The pavement was tested from

February to May 2001. Heavy rain fell on the sections during the testing of the first two sections,

while there were drier conditions during testing of the third and final sections. None of the

sections failed due to cracking or failure of the dowels.

Findings from FWD deflection tests on the Ukiah test sections and the 19 adjacent existing

slabs within the closure built without dowels were:

• For pavement that was built without dowels:

o With regard to load transfer efficiency of joints and cracks without dowels: Measured

LTE varies widely at temperatures below 35°C although generally a temperature

increase below this threshold level will increase load transfer efficiency. At


UCPRC-SR-2008-03 12

temperatures above 35°C, LTE is nearly always above 90 percent even for joints that

had LTE as low as 25 percent at lower temperatures. This effect is due to slab

expansion, which results from increasing temperature, closing the joints between

slabs and thus increasing aggregate interlock and LTE.

o With regard to backcalculated concrete and subgrade moduli (subgrade here refers

to all layers combined beneath the concrete slabs) and subgrade k-value: Concrete

modulus decreases and subgrade modulus and k-value increase with increasing

temperature. The relations between backcalculated moduli and k-value and

temperature are approximately linear. These effects are due to slab curling, with the

center of the slab tending to lift off of the underlying layers with increasing

temperatures, and with the corners of the slab tending to lift off of the underlying

layers with decreasing temperatures.

o With regard to deflections measured at the corners and centerline transverse joints:

Deflections at both locations decreased with increasing temperature. Corner

deflections were more sensitive to temperature than were centerline transverse

deflections. There is a great deal of variance for both locations, particularly at lower

temperatures. However, for both locations the relation between deflections and

temperature is approximately linear. This effect is due to slab curling and slab

expansion with increasing temperature. The slabs curl downward at the corners and

joints with increasing temperature, which increases contact with the underlying layers

and therefore decreases deflections. Slab expansion with increasing temperatures

closes the joints and increases aggregate interlock, which results in load sharing

between slabs and therefore decreases deflections.

o With regard to maximum deflection differences measured at the corners and

centerline transverse joints: The trends are the same as those for deflections.

Maximum deflection differences approach zero at temperatures above 30°C to 35°C.

o These results indicate that the energy applied to the underlying layers through

deflections and the shearing deflections applied to the underlying layers through

maximum deflection differences (also measured by LTE, although LTE is normalized

with respect to deflection magnitude) are greatly decreased at high temperatures.

The majority of faulting damage is therefore concluded to primarily occur during the

night and during the winter in California.

o LTE values, backcalculated moduli and k-values were found to be relatively

insensitive to load magnitude.


UCPRC-SR-2008-03 13

• Load transfer efficiency, deflections, and maximum deflection differences measured at

the corners and centerline transverse joints on the DBR sections follow the same trends

with regard to temperature and load effects as the measurements for the non-doweled

sections. However, LTE on the DBR sections almost never went below 80 percent, and

deflections and deflection differences were much smaller on the dowel bar–retrofitted

sections than on the non-doweled sections even at low temperatures. Backcalculated

moduli and k-values on the doweled sections followed the same trends as those for non-

doweled sections, and the effects were very similar because these parameters were

taken from the center of the slab, which is relatively unaffected by load transfer at the

joints.

Findings from the Ukiah HVS tests:

• Following construction of the Ukiah test

sections WSDOT prepared a detailed step-

by-step inspection guideline for the

construction process for its own use and for

Caltrans. A senior WSDOT engineer (Jeff

Uhlmeyer) was on site providing quality

inspection during construction, and he

prepared the guidelines following the

construction.

• The non-doweled test section (555FD) had low LTE and high deflections at the joint prior

to HVS testing. LTE did not drop significantly after HVS loading; however, deflections

increased significantly. These results indicated significant degradation of the aggregate

interlock at the joint. Significant pumping was observed on 555FD during wet weather.

• There was no measured loss of LTE on the dowel bar–retrofitted sections (553FD and

554FD). However, there was a measured increase in deflections at the joint under HVS

loading, indicating degradation of aggregate interlock or the dowel-concrete contact

surface (or both), or degradation of the underlying materials beneath the joint. No

pumping was observed on 553FD or 554FD. Wet weather continued during testing of

554FD, but stopped soon after the beginning of testing on 553FD in early April.

• Maximum deflection differences across the joint increased a great deal on Section

555FD under HVS loading, indicating degradation of aggregate interlock or dowel-

concrete contact surface (or both), or degradation of the underlying materials beneath


UCPRC-SR-2008-03 14

the joint. Maximum deflection differences increased somewhat on Section 554FD under

HVS loading and did not appear to increase at all on Section 553FD.

• It was thought that the reason for the lack of loss of LTE on the DBR sections might

have been insufficient load repetitions applied. It was not clear at the time why

deflections increased under loading (indicating damage to the aggregate interlock and

dowel-concrete contact surface), but LTE did not decrease. Similar performance was

later observed on the Palmdale HVS DBR sections.

• Extrapolation of the results from Sections 553FD and 554FD was found to be difficult.

However, based on extrapolation of the deflection results under HVS loading, it was

estimated that the DBR sections can withstand approximately 1.3 to 2 million repetitions

of the 90 kN load before joint deflections become as large as those of the non-doweled

control section (555FD). Using the Caltrans 4.2 load equivalence exponent, this number

corresponds to 40 million to 60 million equivalent 80 kN (18,000 lb) single axle loads.

This extrapolated estimate should be treated with caution. Later results from the

Palmdale sections also indicated little or no loss of LTE on DBR sections under many

more heavy load repetitions than were placed at Palmdale. The changes in maximum

deflection difference across the retrofitted joints and crack never approached the level of

those on the non-doweled joints, indicating that the design life extrapolations may have

been somewhat conservative.

2.3 Palmdale Heavy Vehicle Simulator Test Sections

In Palmdale, existing pavement research test

sections were used for the DBR test sections.

Originally built in 1998 to evaluate long-life rigid

pavement designs using deflection and HVS

testing, the DBR test sections were configured in a

way that used parts of the center slabs untrafficked

by the HVS in the earlier experiment. However,

most of the joints had undergone traffic of millions

of heavy loads as part of the earlier HVS testing.

The DBR test sections were in the part of the Palmdale test pavements that did not include

dowels in their original construction. The pavement consisted of 3.7-m wide (12 ft) concrete

slabs averaging 200 mm thick, on top of 100 mm of cement-treated base (CTB) designed to


UCPRC-SR-2008-03 15

meet pre-1964 specifications, on 150 mm of aggregate subbase. The joints were cut

perpendicular to the untied shoulders. As part of the previous research project, the concrete mix

design included a blend of Type I/II cement and fast-setting hydraulic cement concrete (FSHCC)

designed to provide high early strength.

The timetable in Table 2 includes the original construction, previous HVS testing, DBR

construction, HVS testing on the DBR sections, FWD testing, and coring.

Table 2: Timetable of Testing on Palmdale Sections

Event Time Construction of original

doweled and non-doweled pavements

June 1998

FWD Test No. 1 June 19, 1998

FWD Test No. 2 June 23, 1998

FWD Test No. 3 August 1998

FWD Test No. 4 September 1998

FWD Test No. 5 January 1999

FWD Test No. 6 March 1999 HVS tests on original HVS

sections June 1999–January 2001

FWD Test No. 7 February 2001 DBR construction on untested

non-doweled joints and cracks

June 28–30, 2001

HVS Test 558FD August 2001–March 2002

HVS Test 556FD March–August 2002

FWD Test No. 8 April 2002

HVS Test 557FD August–October 2002

FWD Test No. 9 October 2002

HVS Test 559FD October 2002–March 2003

FWD Test No. 10 April 2003

FWD Test No. 11 June 2003

Coring of DBR section June 2003

FWD Test No. 12 February 2004


UCPRC-SR-2008-03 16

Four sections were dowel bar retrofitted for HVS test sections as summarized below in Table 3.

Trafficking of the four HVS test sections was programmed to provide similar temperature and

rainfall to enable the following comparisons:

• Four epoxy-coated steel dowels per wheelpath (556FD) versus three dowels per

wheelpath (557FD), and

• Epoxy-coated steel dowels (559FD, Joint 32) versus grout-filled hollow stainless steel

dowels (559FD, Joint 33) versus fiber-reinforced polymer (FRP) dowels (558FD), all with

four dowels per wheelpath.

Trafficking consisted of 60 kN (13,500 lb) and 90 kN (20,250 lb) dual-wheel truck-tire loading

and 150 kN (33,750 lb) aircraft, single-wheel loading. The final load used was 150 kN

(33,750 lb) on an aircraft wheel, with tire pressure maintained at 1450 kPa (210 psi).

Channelized (no wander), bidirectional loading was conducted on the wheelpath for all tests

over the center of the dowel bar group.

None of the sections failed due to cracking or failure of the dowels.

FWD testing was performed before DBR construction across all of the doweled and non-

doweled sections of the original Palmdale test sections. The same FWD testing was repeated

periodically across the same locations after DBR construction but before HVS testing, and after

HVS testing.

Table 3: Summary of Palmdate Dowel Bar Retrofit Test Sections

HVS Test

Section

Joint or Crack

Number Type of Dowels Number of

Dowels Number of HVS Load

RepetitionsJoint 41 Epoxy-coated steel Crack 2 Fiber-reinforced polymer 558FD Joint 42 Fiber-reinforced polymer

Four per wheelpath

2,208,578

Joint 38 Epoxy-coated steel 556FD Joint 39 Epoxy-coated steel Four per

wheelpath 2,208,546

Joint 35 Epoxy-coated steel 557FD Joint 36 Epoxy-coated steel Three per wheelpath 1,121,600

Joint 32 Epoxy-coated steel

Crack 1 Grout-filled, hollow stainless steel in

one wheelpath; epoxy-coated steel in other wheelpath

559FD

Joint 33 Hollow stainless steel

Four per wheelpath 2,001,497


UCPRC-SR-2008-03 17

Findings from construction of the Palmdale test sections:

• From construction of the test sections:

o The backfill grout material strength exceeded the Caltrans specifications requiring

3.5 MPa flexural strength at one day and 35 MPa compressive strength at one day.

The specification of 21 MPa at three hours was not checked due to delays in testing;

however, the material had a compressive strength of 37 MPa at eight hours, which

indicates high early strength for the mix. The twenty-eight-day strengths for the

backfill grout were high: 6.4 MPa flexural and 63 MPa compressive under standard

curing conditions.

o Cores taken from the DBR slots after HVS testing was completed showed that all of

the dowels cored were above the mid-depth of the slab and at least some were very

near the top of the slab. These conditions indicate the test sections were not

“perfect” and may resemble field construction in terms of dowel placement variability.

The results indicate that placement of the dowels at mid-depth can be difficult even

when productivity is not the major issue.

o Most of the dowel slots appeared to be in good condition after construction.

o Observations after construction include the following:

• Foam backer board slightly out of place on several joints.

• Apparent segregation (lack of fine materials) in the grout in some slots.

• Cracking on one set of slots that were placed across a set of unconnected,

parallel transverse cracks. This may indicate what will occur if there is no clear

joint or crack on which to place the foam backer board.

• Findings from FWD deflection testing after previous HVS testing and before DBR

o The average LTE in the Palmdale North Tangent test sections—with originally

installed dowels in the joints, some with a widened lane and others with a tied

shoulder—was 91 percent, with almost no difference between daytime and nighttime

measurements. The average LTE on the non-doweled sections was 33 percent in

the day at an average surface temperature of 10°C, and 25 percent at night at an

average surface temperature of 3°C. The influence of temperature is expected,

because higher temperatures result in slab expansion, which closes the joint and

increases aggregate interlock at the joint.


UCPRC-SR-2008-03 18

o Temperature gradient has a significant effect on backcalculated stiffnesses, with

higher slab and supporting layer stiffnesses and supporting layer k-values found

when testing was performed at night. This is attributed to negative nighttime

temperature gradients (cooler on top) in the concrete slabs, causing curl that places

the slab center in closer contact with the supporting layers.

o Both of these findings agree with findings from the Ukiah DBR HVS sections.

• Findings from FWD deflection testing after DBR and

before HVS testing

o The joints with originally installed dowels, many of

which had been previously trafficked to slab-

cracking failure, had high LTE, which was nearly

insensitive to temperature. Average LTE was 96

percent at a slab surface temperature of 34°C; 94

percent at 13°C; 93 percent at 23°C; and 94 percent

at 9°C. The standard deviation across the many

joints in each section was always less than 2.5

percent.

o At higher temperatures the LTE of the non-doweled and DBR joints was nearly the

same as that of the originally installed dowel joints. The average LTE was 97 percent

at 34°C, with no difference between non-doweled and DBR joints. At 23°C, the

average non-doweled LTE was 85 percent while the average DBR LTE was

82 percent.

o The non-doweled joints had lower LTE than the DBR joints under cooler nighttime

temperatures. The average LTE for the non-doweled joints was 73 percent at 13°C;

and 57 percent at 9°C. At 9°C, individual non-doweled joint LTE ranged between

27 and 93 percent, indicating the inconsistency of relying on aggregate interlock for

LTE. The greater values are likely joints that were not trafficked during the original

HVS testing that did not have their aggregate interlock damaged. This large range of

LTE at low temperatures for the non-doweled joints also indicates that thermal

contraction under low temperatures is not uniform across all joints.

o The DBR significantly improved the LTE of the previously non-doweled joints, and

reduced the sensitivity of the LTE to temperatures. The average LTE of the DBR

joints was 83 percent at 13°C; and 79 percent at 9°C.


UCPRC-SR-2008-03 19

o Three dowels per wheelpath typically had somewhat lower nighttime LTE than that of

four dowels per wheelpath, regardless of the dowel type used for four dowels per

wheelpath, although three dowels per wheelpath was generally much better than for

non-doweled joints.

o Backcalculated slab stiffnesses and the stiffnesses and k-values for the supporting

layers were similar to those obtained before DBR, and generally showed the

expected temperature sensitivity. As with all of the FWD results, the backcalculated

stiffnesses and k-values appeared to have no load sensitivity (linear behavior).

• Findings from HVS Test Results

o The HVS test sections had these failure criteria: fatigue cracking of the concrete

slab, major damage to the DBR joints, or loss of LTE of the loaded joint or crack. All

of the HVS test sections failed only by fatigue cracking of the concrete slab. Neither

of the other two types of failure occurred on any of the test sections. HVS trafficking

was stopped on each test section as shown in Table 3.

o Fatigue life of the slab with three dowels per wheelpath was substantially shorter

than the other test sections that had four dowels per wheelpath. Fatigue life was

similar for all three strategies with four dowels per wheelpath. Longer fatigue life,

higher LTE, and lower deflections indicated better performance under these test

conditions by four dowels per wheelpath than three dowels per wheelpath.

o Two years after construction and HVS testing was completed on all sections,

transverse cracking was seen in the grout on some slots. Slight separation was seen

between the grout and the slab on some slots but the grout did not come out of any

slots and the cracks remained tightly interlocked.

o The following observations about joint deflections and LTE are with respect to

measurements made under HVS loading using Joint Deflection Measurement

Devices (JDMDs) mounted on the DBR joints during HVS testing. Larger joint vertical

deflections and lower LTE correlate with increased rate of faulting and roughness

development.

• As expected, all the sections showed an increase in initial joint maximum vertical

deflection with increase in load (60 to 90 to 150 kN).

• All the sections showed a sensitivity of joint maximum vertical deflection to

temperature, with deflections decreasing with increased temperature. This is


UCPRC-SR-2008-03 20

expected and is caused by slab expansion closing the joint and increasing

aggregate interlock at the joint.

• All of the DBR joints except one showed an increase in joint maximum deflection

after HVS trafficking—measured using the same 60 kN measurement load—that

could not be attributed to temperature changes. The increases came under the

90 and 150 kN loading. The only joint not showing an increase was in Section

558FD which had FRP dowels. The other joint with FRP dowels in the same

section behaved the same as all the other joints.

• Joints with four epoxy-coated steel dowels per wheelpath had much smaller joint

vertical deflections than did the alternatives (four FRP dowels, four hollow

stainless steel dowels, and three epoxy-coated steel dowels per wheelpath). The

alternatives had deflections similar to each other. At comparable temperatures,

joints with three epoxy-coated steel dowels had the largest deflections.

• All of the sections showed a slight increase in initial LTE with increase in load (60

to 90 to 150 kN). Under HVS loading, and using the moving wheel definition of

LTE described in the report, the LTE is nearly always greater than 90 percent on

the DBR joints, regardless of temperature and load.

• All but one of the joints showed a sensitivity of LTE to temperature, with LTE

increasing with increased temperature. This is expected and is caused by slab

expansion closing the joint, and increasing aggregate interlock at the joint. The

temperature susceptibility was very low compared to non-doweled joints. The

DBR joints with hollow stainless steel dowels showed no sensitivity to

temperature.

• All of the DBR joints showed little or no decrease in LTE after HVS trafficking, as

measured under the measuring load of 60 kN.

• Joints with three epoxy-coated steel dowels per wheelpath had lower LTE than

did all of the joints with four dowels per wheelpath, regardless of the type of

dowel.

• The following findings regarding joint deflections and LTE are with respect to

measurements made using the FWD after HVS testing, from October 2002 through

February 2004.

o The HVS trafficking caused almost no change in LTE for all of the DBR joints. Each

of the DBR strategies tested by the HVS provided LTE that was not substantially

damaged by heavy traffic loading.


UCPRC-SR-2008-03 21

o Backcalculated stiffnesses from slab center deflection measurements showed the

same day-to-night differences observed in the results from before DBR construction,

as expected.

• Findings from Comparison of DBR Joints with Originally Installed Dowel Joints at

Palmdale from FWD Measurements

o When the temperature is above 33°C all joints, with or without dowels, have

consistently high LTE, likely mostly carried by aggregate interlock. Effects of dowels

(original or DBR) are not apparent under these conditions.

o At lower temperatures, the DBR joints had somewhat less LTE than joints with

originally installed dowels, although LTE was still always greater than 80 percent and

was usually 85 to 90 percent with four dowels per wheelpath. There was also greater

variability in the LTE between different DBR joints than for joints with originally

installed dowels.

• Findings regarding Reduction of Built-in Slab Curling from DBR

o The Equivalent Built-In Temperature Difference (EBITD) backcalculated on several

slabs before and after DBR showed that the DBR may have reduced the EBITD by

about 2°C based on FWD deflections. EBITD is the permanent warping in the slab

caused primarily by higher shrinkage at the surface than at the bottom. High EBITD

increases tensile stresses in the slab that cause longitudinal and corner cracking.

Further investigation regarding the effect of DBR on EBITD is recommended, and

should be based on FWD deflection measurements before and after DBR on future

DBR projects.


UCPRC-SR-2008-03 22

o If a beneficial reduction in EBITD does it occur, it would likely only occur when the

DBR backfill grout sets during late afternoon and early evening, when the

temperature gradient in the slab is most positive (hotter on top) and the slab is the

closest to being flat. Any future investigation of changes in EBITD caused by DBR

should check to see if EBITD is dependent on the slab temperature gradient at the

time that the backfill grout sets.

2.4 Reports

The following reports were prepared for this phase of the study:

HARVEY, J. T., Ali, A., Hung, D., Uhlmeyer, J., Popescu, L., Bush, D., Grote, K. Lea, J., and

Scheffy, C. 2003. Construction and Test Results from Dowel Bar Retrofit HVS Test Sections

553FD, 554FD and 555FD: US 101, Ukiah, Mendocino County. Report prepared for the

California Department of Transportation. Pavement Research Center, Institute of Transportation

Studies, University of California. UCPRC-RR-2003-03.

Y. BIAN, J.T. Harvey, and A. Ali. March 2006. Construction and Test Results on Dowel Bar

Retrofit HVS Test Sections 556FD, 557FD, 558FD, and 559FD: State Route 14, Los Angeles

County at Palmdale. Research report prepared for the California Department of Transportation

(Caltrans) by the University of California Pavement Research Center, UC Davis and Berkeley.

UCPRC-RR-2006-02.


UCPRC-SR-2008-03 23

3 FIELD OBSERVATIONS OF PERFORMANCE AND MEASUREMENT OF FIELD CHLORIDE CONTENTS

3.1 Chloride Content in In-Service Pavements in Washington State

Field observations were made to assess the risk of dowel corrosion in DBR projects. The results

are also applicable to dowels placed in new concrete slabs.

Chloride analyses were performed on concrete cores taken from near the transverse joints of

field slabs at six locations in Washington State to evaluate field chloride contents for situations

with different de-icing chemical applications that span the range similar to California. The ages

of the slabs were approximately 20 to 45 years, and they were in different locations in the state,

from mountain passes with heavy de-icing applications, to relatively frost-free coastal regions.

In addition, three 40-year old slabs with two failed DBR joints between them were extracted by

WSDOT from Interstate 90 (I-90) after 13 years of service following DBR and shipped to the

University of California Pavement Research Center (UCPRC) laboratory at the UC Berkeley

Richmond Field Station. Cores were also taken from that slab at various locations around the

dowels.

The samples taken from different levels in the cores

taken from the six locations and the failed slab were

tested by the Caltrans chemistry laboratory or

Construction Testing Laboratory in Illinois. The

results were compared with chloride contents of

laboratory concrete specimens with embedded

dowels that were subjected to accelerated chloride

conditioning (the results of which are described in

Chapter 4 of this report) so that the relative chloride exposure of the field and laboratory could

be compared.

The following findings are drawn from the extracted WSDOT field slabs and the cores taken

from slabs at various locations in Washington State.


UCPRC-SR-2008-03 24

• Chloride concentrations close to pavement joints are significantly higher than in other

areas of the pavement. At the joint, easier access and accumulation of chlorides leads to

higher, localized concentrations.

• When a joint is present, the chloride ions do not diffuse through the concrete (or grout)

from the top; instead, they migrate through the open joint along the dowel.

• Corrosion was observed under the epoxy coating of the dowels extracted from the

WSDOT I-90 slab sent to Richmond. In the extracted slabs from which cores were taken

at the joints, a considerable amount of corrosion product was verified by means of visual

inspection. The corrosion occurred underneath the epoxy coating on the central region

of the dowel beneath the joint. The corrosion is likely to have contributed to the loss of

load transfer efficiency (LTE) of the joint because of the low-strength corrosion products

at the interface between the concrete and the dowel. Lack of centering of one dowel

over the transverse joint is also likely to have contributed to the low LTE.

3.2 Review of Field Performance Across California and Several Other States

A survey was undertaken to evaluate the

performance of DBR in the field in California

and other states. The study consisted of

reviewing available performance information

from the literature and in interviews with

DOT staff in states that have used DBR. It

also consisted of reviewing the performance

of experimental sections in California,

including an update of condition information

on the Ukiah and Colfax sites. Data collected

in 2004 by Caltrans and Applied Research Associates (ARA) consultants was reviewed, and

some additional analysis of performance was completed. In 2006 and 2007 an additional field

survey of DBR joint condition was performed in many of the sections previously surveyed by

Caltrans and ARA in 2004.


UCPRC-SR-2008-03 25

The study found:

• The review of the most recent performance information showed that nearly 2,000 lane-

miles of DBR have been carried out by various state departments of transportation

throughout the United States. This number may be an underestimate as it comes from

industry and not from official state records. The states with the most DBR lane-miles are

Washington, Oklahoma, North Dakota, and Kansas. Other states, such as Iowa and

Florida, have not used DBR in a long time because of the poor performance observed

on early projects in which the technique was used.

• The overall performance of DBR throughout the United States cannot be clearly

classified as good or bad. Varied examples were found: for instance, some states (such

as Kansas) have observed excellent behavior; other states (such as Wisconsin) have

imposed moratoriums, and still other states (such as Florida) stopped using DBR after a

few projects. A review of the cases in most states that use DBR shows that there have

been projects with both failures and excellent performance. It is important to note that

even for those DBR projects reported as having serious problems, the agencies perceive

that load transfer capacity has been restored and that the dowel bars continue to

perform.

• The reported problems with DBR can be classified in the following broad categories:

o Cracking and debonding of the backfill material. o Damage occurring in the original concrete pavement around the DBR slots. o Spalling of backfill material.

• By far the most common type of distress is cracking and debonding of the backfill

material, which may or may not lead to loose material on the surface. This distress has

been observed in all the states that have used DBR. Various construction details were

commented on by the different states to reduce the risk of this type of distress. The

relative importance of maintenance problems associated with this type of distress also

varied across the states.


UCPRC-SR-2008-03 26

• The performance of the experimental sites in California, namely Ukiah, Palmdale, and

Colfax, has been very good. LTE levels are much higher than before the retrofit, and the

backfill material presented no problems, except for very isolated cases of backfill

transverse hairline cracks in some of the Palmdale sections. The variety of conditions

encountered in the experimental sections lead to the conclusion that it is possible to

achieve good DBR performance in a wide range of situations:

o In wet, dry, and freezing climates;

o With high and low traffic levels (in terms of volume and load);

o With three or four dowel bars per wheelpath;

o At retrofitted joints as well as at retrofitted transverse cracks; and

o With various types of dowel bars (epoxy-coated steel, hollow stainless, and fiber-

reinforced polymer).

• The Ukiah and Palmdale sections had weeks to cure before trafficking, which likely

improved the performance of the DBR grout. The curing time before the Colfax section

was opened to traffic is not known.

• The monitoring of the instrumentation and follow-up deflection testing by the UCPRC on

the Ukiah sections loaded under live-traffic conditions from 2001 and evaluated in 2007,

confirmed findings from the loading with the Heavy Vehicle Simulator (HVS). Information

from this test site, combined with results of evaluation of the Colfax test site, allowed the

following conclusions to be drawn:

o Retrofitting joints and cracks with dowel bars stops or at least significantly delays

faulting development. Assessment of faulting six years (Ukiah) or nine years (Colfax)

after DBR revealed zero faulting on the DBR joints. Nonretrofitted joints adjacent to

test sections exhibited 2 mm to 9 mm of faulting. It must be noted that nonretrofitted


UCPRC-SR-2008-03 27

joints were actually located between retrofitted joints and were therefore subjected

over the years to the exact same environmental and traffic loading as the DBR joints.

o Since faulting is eliminated, the rate of roughness progression is greatly reduced on

retrofitted sections. Although the difference in International Roughness Index (IRI)

could not be observed in the Ukiah sections because dowels were installed in only

two joints per section, measurements at the Colfax section, where groups of 30 joints

were retrofitted, show this to be the case.

o The presence of dowels means an improvement in LTE, with values that can range

from less than 30 percent to more than 80 percent. In addition, DBR reduces but

does not eliminate the susceptibility of LTE to temperature changes. This means that

although retrofitted joints still present lower LTE at low temperatures, these LTE

values are still higher than those for nonretrofitted joints.

o Data from the Ukiah section, where the subgrade conditions were near saturation for

extended periods, suggest that the greater level of LTE achieved after DBR slowly

deteriorates. In general terms, at this site the retrofit increased the LTE level from 40

to 80 percent, but over 4.5 years the LTE had been steadily reduced to

approximately 60 percent. Unfortunately, LTE at Ukiah was measured between

wheelpaths. Data from the Colfax section indicates that retrofit with steel dowels

elevated LTE from 20 to 85 percent, and it remained the same for at least 3.5 years.

For the joints retrofitted at Colfax with fiber-reinforced polymer (FRP) dowels, the

LTE went from 20 to 70 percent and stayed at that level. More recent data is not

available.

• The Caltrans experience with DBR has not been

especially positive. The performance of DBR in the

evaluated projects is mostly good, but there are

cases of unacceptable behavior. In the locations

where the dowel bars have not become loose

because of deterioration in the backfill material or

extreme misalignment of the dowels, the load

transfer efficiency has generally remained good. In the locations where the backfill has

deteriorated (cracking of the backfill leading to eventual disintegration of the backfill

which then comes out of the pavement), the problems generally began appearing shortly

after the retrofit. Reanalysis of the Caltrans-ARA investigation of 2004 showed that

about 4 percent of the evaluated slots were rated as in the worst category on a 0-to-4


UCPRC-SR-2008-03 28

scale used (4 being the worst condition), and another 4 percent were rated as 3. These

results mean that approximately 8 percent of the slots of these projects presented

extensive cracking or had been repaired. The other 92 percent of slots were in different

levels of good condition, with 56 percent considered to be in perfect condition, without

even hairline cracks. Smoothness, LTE, and dowel alignment tests all indicated that

DBR was performing well. When Caltrans conducted additional field and comprehensive

laboratory investigations, it found that projects on Interstate Route 8 and Interstate

Route 405 had problems due to poor workmanship, where the DBR construction work

was not carried out according to specifications.

• The poor performance of some DBR projects should not be attributed to inadequacy of

the technique, but to poor execution due to lack of experience. That many projects that

have performed well should be taken as an indication that well-executed DBR projects

will last and will extend the life of jointed concrete pavements.

3.3 Reports


M. MANCIO, Carlos, C., Jr., Zhang, J., Harvey, J. T., and Monteiro, P. J. M. January 2007.

Laboratory Evaluation of Corrosion Resistance of Steel Dowels in Concrete Pavement.

Research Report prepared for the California Department of Transportation (Caltrans), Division

of Research and Innovation by the University of California Pavement Research Center, UC

Davis and Berkeley. UCPRC-RR-2005-10. (Chloride contents of Washington State DOT slabs

and slab extracted from Interstate 90 is discussed in Section 3.4.1 of this report.)

KOHLER, E., and Santero, N. December 2007. Survey Results of Dowel Bar Retrofit Projects in

California. Technical Memorandum prepared for the California Department of Transportation

(Caltrans), Division of Research and Innovation by the University of California Pavement

Research Center, UC Davis and Berkeley. UCPRC-TM-2007-07.

KOHLER, E., Harvey, J., Steven, B., and Santero, N. December 2007. Performance of Dowel

Bar Retrofit. Report prepared for the California Department of Transportation (Caltrans),

Division of Research and Innovation by the University of California Pavement Research Center,

UC Davis and Berkeley. UCPRC-RR-2007-10.


UCPRC-SR-2008-03 29

4 LABORATORY STUDIES

4.1 Laboratory Corrosion Testing of Metallic Dowels

A number of problems can arise that can

compromise the performance of the pavement and

lead to premature failure if corrosion of the dowels

occurs, whether on DBR projects or when dowels

are used in new pavement. These problems include

(a) the loss of dowel cross-section, which reduces load transfer capability, and (b) the

accumulation of corrosion products, which restricts the free expansion and contraction of the

slabs. A laboratory study was performed to evaluate the risk of corrosion for different kinds of

metallic dowels, with results applicable to both DBR and new pavements.

Dowel bar corrosion has been investigated in the field and laboratory in the past, which has led

to the widespread use of epoxy-coated steel dowels in concrete pavements in place of bare

carbon steel. Deformed steel reinforcement in sound concrete is protected from corrosion by a

passive film formed due to the high pH (12.5–13.5) of concrete pore solutions. This thin

protective film slows the corrosion reaction rate to very low levels. However, if the passive layer

is broken or dissolves, then the metal reverts to active behavior and rapid corrosion can occur.

The process of steel corrosion in reinforced concrete structures is subdivided into an initiation

stage and a propagation stage.

For dowels in concrete pavements, the initiation stage is very short because of easy access to

the dowels by aggressive agents through the joints; therefore the corrosion performance of the

system depends largely on the properties of the steel dowel being used. Caltrans and other

agencies seal the joints of concrete pavements in order to minimize the ingress of water and

fine debris into the joints. The effectiveness of this joint-sealing practice in preventing

aggressive agents from accessing dowels is unknown, and sealing the joint from the surface

does not completely protect the dowel from exposure to water and debris because these can

still reach it from the sides of the joint or from beneath it. Aggressive agents can also access the

dowels by penetrating the concrete.


UCPRC-SR-2008-03 30

Seven kinds of steel dowels were evaluated in this study:

• Bare carbon steel

• Stainless steel-clad

• Grout-filled hollow stainless steel

• Microcomposite steel

• Carbon steel coated with flexible epoxy (green color code, Designation ASTM A775),

and

• Carbon steel coated with nonflexible epoxies (two types: purple and gray color codes,

Designation ASTM A934).

The stainless-clad bars have a core of carbon steel covered by an outer layer (approximately

5 mm thick) of stainless steel. The ends of the stainless-clad dowels do not have stainless steel

cladding, but they do have a protective paint coat. Epoxy-coated bars were also epoxy-coated

at the ends. The stainless hollow dowels consisted of a hollow stainless steel cylinder with a

wall thickness of approximately 5 mm, filled with a cementitious grout. Microcomposite steel

refers to microstructurally designed steels with a dislocated lath structure (laths of martensite

alternating with thin films of austenite) in which the formation of carbides is avoided.

Corrosion testing was performed in the laboratory by casting dowels (38 x 457 mm [1.5 ×

18 in.]) into flexural beam–sized specimens (150 x 150 x 559 mm [6 × 6 × 22 in.]) with a

transverse joint directly over the middle of the dowel.

Two test methods were used in the study to assess corrosion:

• Half-cell potential measurements, indicative of the probability of corrosion activity of the

reinforcing steel located beneath the half-cell (ASTM C876).

• Linear Polarization Resistance (LPR), a well-established method for determining

corrosion rate by using electrolytic test cells.

A major concern with the LPR technique is uncertainty about the area of the steel bar that is

affected by the current from the counter electrode. In this study, the counter electrode was

placed over a fabricated joint filled with NaCl solution and located above the dowel. Since

virtually all the current will flow through the salt solution—which represents a path of very low

resistance compared to concrete—it has been assumed that the area polarized corresponds to

that part of the dowel exposed to the NaCl solution inside the joint. Epoxy-coated bars, however,


UCPRC-SR-2008-03 31

could not be evaluated quantitatively using the LPR technique. It was found that the corrosion

that occurred in the epoxy-coated dowels was localized and concentrated in small holidays

(pinholes, voids, defects, etc.) and defects on the coating, and therefore the assumption about

the exposed area does not hold for these bars.

In order to facilitate the identification of

corroded areas in the epoxy-coated

dowels and the evaluation of the role of

defects in the development of localized

corrosion, the epoxy-coated dowels

were checked for holidays. This was

achieved using a low-voltage holiday

detector tester before casting the

dowels in concrete beams. This

mapping of coating defects was used to

check against locations of corrosion,

identified during the visual inspections of corroded dowels after conditioning. Every epoxy-

coated bar examined had one or more defects on the coating, especially along the edges at the

ends. These dowels were shipped from the manufacturer directly to the laboratory and were

subjected to careful handling in the laboratory.

Chloride analyses were performed on concrete cores extracted from the laboratory beams for

comparison with the chloride contents of the WSDOT slabs to evaluate the relative

aggressiveness of the laboratory testing.

The corrosion testing was performed in three phases:

• Phase I: Four types of dowels were cast in concrete beams with joints. The four types of

dowels investigated were: carbon steel; stainless steel clad; stainless hollow; and carbon

steel coated with flexible epoxy. The specimens were subjected to a corrosive

environment (weekly wet and dry cycling with 3% NaCl solution ponded on top of the

beams, permitting access of the corrosive solution through the simulated joint) at two

temperatures: cold (4.4ºC) and hot (40 to 43ºC). No mechanical loading was placed on

the beams. Half-cell potential was monitored for six months, and visual inspection of the


UCPRC-SR-2008-03 32

corroded dowels was made at the end of testing. Three replicates for each dowel type

were tested.

• Phase II: All seven types of dowels were tested. A more permeable concrete was used

than in Phase I. Corrosion was accelerated by exposing the samples to cycles of a 3.5%

NaCl solution at room temperature for a period of 18 months. Half-cell potential tests,

LPR curves, visual inspections, chloride-

content analyses, and scanning electron

microscopic (SEM) investigations were

carried out. Four replicates of each type of

steel dowel were tested in this phase.

• Phase III: Comparison of the chloride

contents of the laboratory specimens and

the WSDOT field specimens.

Findings from the corrosion study were:

• Phase I findings:

o Carbon steel dowels present the shortest corrosion initiation period—when chlorides

have direct access to the bar through the joint, the initiation stage can be

disregarded and the corrosion propagation phase begins immediately. Epoxy-coated

dowels exhibited a considerably lengthened initiation period, while the stainless

hollow and stainless clad dowels provided the highest resistance to the onset of

corrosion.

o From visual inspections after six months of cyclic ponding, it was observed that the

carbon steel dowels exhibited uniform corrosion along the bar. Epoxy-coated dowels

had localized corrosion at defects—mostly at the ends of the bars where the coating

was most vulnerable to damage. No visible corrosion was observed on either the

stainless steel hollow bars or stainless clad bars.

• Phase II findings:

o Corrosion is not uniform in the epoxy-coated specimens, but is instead concentrated

at localized defective areas (e.g., pinholes, voids, etc.). Given that epoxy is an

electrical insulator, polarization only happens at very small locations (defective

areas) that cannot be accounted for in the calculation of the polarization resistance

(Rp) term. Therefore, the epoxy-coated dowels cannot be quantitatively evaluated

with the other dowels and must be evaluated qualitatively.


UCPRC-SR-2008-03 33

Figure 3: Microphoto of holiday in epoxy-coating.

o The carbon steel dowels exhibited the

lowest values of Rp and therefore have

the smallest resistance to charge transfer

across the interface. Carbon steel dowels

are therefore expected to have the fastest

rate of corrosion propagation among the

types included in this study.

o Microcomposite steel dowels exhibited polarization resistance approximately 35

times larger than carbon steel dowels, while stainless clad and stainless hollow bars

had about 73 times greater polarization resistance. This observation indicates that

the microcomposite steel dowels exhibit much greater resistance to corrosion

propagation than carbon steel dowels, but not as much as the stainless clad and

hollow bars.

o Based on corrosion current density results, it was verified that the carbon steel

dowels exhibited very rapid corrosion while microcomposite steel exhibited a

moderate level and stainless steel–clad and stainless steel hollow proceeded at low

rates of corrosion.

o Visual inspections of the corroded dowels revealed heavy and mostly uniform

corrosion along the carbon steel dowels, light corrosion in the microcomposite steel

dowels, and no visible corrosion in the stainless steel–clad and stainless steel hollow

bars. For the epoxy-coated dowels, the visual inspections generally revealed that

visible corrosion was not widespread, but did occur at a few localized defective

areas, generally at holidays and at the edges of the bar ends. No significant

difference was observed on the performance of nonflexible and flexible epoxy-coated

dowels.

o In general, the microscopic investigation by scanning electron microscopic (SEM)

matches well the results anticipated by the electrical measurements and visual

inspections. However, the analysis focused mostly on the corroded areas of each

sample, and revealed corroded areas that were not visible to the naked eye.


UCPRC-SR-2008-03 34

o Statistical analyses of the results show that in all cases, the type of steel dowel has a

statistically significant effect on the quantitative parameters studied (i.e., half-cell

potential, polarization resistance, and corrosion current density).

• Phase III Chloride Comparison findings:

o In the field cores, it was verified that the chloride threshold for carbon steel was

exceeded in five out of six projects.

o In the laboratory samples, with open joints located above the dowels, the chloride

concentrations were more constant along the depth profile, as compared to the field

conditions in which the chlorides had to diffuse through the concrete or migrate

through a narrower joint.

o The use of a 3.5% NaCl solution for laboratory experiments may lead to higher

chloride concentrations than those found in the field specimens, greatly accelerating

the corrosion process compared to the field. As a result of this aggressive

environment, corrosion could be observed in nearly all samples in only 18 months of

exposure.

o Laboratory results can be used to comparatively evaluate the corrosion resistance of

different materials exposed to the same aggressive environment. However, the

chloride concentration analyses indicate that the actual field conditions and local

environment should be taken into account when choosing appropriate material for a

given project.

4.2 Laboratory Testing of Fiber-Reinforced Polymer (FRP) Dowels

FRP dowels are a potential alternative to metallic dowels. Their primary advantage compared to

metallic dowels is that they are not subject to corrosion by chlorides. Their performance as used

for DBR was evaluated with the HVS at the Palmdale test sections (Chapter 2 of this report) and

in a full-scale pilot project on I-80 near Colfax (Chapter 3 of this report).

FRP is a composite material made of a polymer (plastic) matrix (such as polyester, epoxy, vinyl

ester) and a reinforcing fiber (such as glass, carbon, aramid, or other reinforcing material).

Parallel fibers are embedded in matrix materials to form a fibrous composite. The matrix serves

(1) to bind the fiber together, (2) to transfer loads to the fibers, and (3) to protect them against

environmental attack. The properties of FRP are influenced by the properties of their constituent

materials, their distribution, and the interaction among them.


UCPRC-SR-2008-03 35

Figure 4: Flexural testing fixture.

Currently, three types of fiber are commonly used

in FRP: glass, carbon, and aramid fiber, and the

corresponding FRP materials are named Glass

FRP (GFRP), Carbon FRP (CFRP), and Aramid

FRP (AFRP), respectively. Popular matrix

materials include epoxy and polyester.

GFRP, using electrical grade glass fibers, is the typical material used for FRP dowels because

of their low cost and generally good performance-related properties. The disadvantages of glass

fibers are their poor abrasion resistance and poor adhesion to polymer matrix in the presence of

moisture. Typical properties compared with those of steel are shown in Table 4.

Table 4: Typical Properties of GFRP and Steel Bars (Trejo et al., 2000)

Tensile Strength Tensile Stiffness Direct Shear Strength Bar

Type GPa ksi GPa Ksi MPa ksi

Specific Gravity

GFRP 500–

1,200 70–175 41–55

6,000–

8,000 150 22 1.5–2.0

Steel 483–690 70–100 200 29,000 520 75 7.9

A laboratory study was performed to answer the following two questions regarding FRP dowels:

• Are the mechanical properties of FRP dowels adequate to perform acceptably

(compared to steel dowels) in transferring traffic load across concrete pavement joints?

• Are the mechanical characteristics of FRP negatively affected by environmental factors

other than chloride corrosion?

Figure 5: Flexural testing fixture with failing dowel.


UCPRC-SR-2008-03 36

In this study, two types of glass FRP (GFRP) dowel bars from different manufacturers were

tested (and are referred to as A bars and B bars) and both included E-glass fibers. Polyester

and epoxy matrices were used for the A bar and B bar matrix materials, respectively. Standard

size bars were used in the laboratory testing. In both bar types, fibers were embedded in

parallel in the matrix materials. The dowels were 1.5 in. (38 mm) in cross-section diameter and

18 in. (457 mm) in length. The properties of the two types of bars, which were provided by the

manufacturers, are shown in Table 5.

Table 5: Properties of the Two Types of FRP Dowel Bars Used in This Study

Type A Type B

Glass fiber content 70 percent Min. 65 percent, typical 72 to

73 percent

Glass type E-type glass E-type glass

Matrix type Polyester resin Epoxy

The following mechanical properties were evaluated:

• Flexural strength and flexural fatigue (ASTM D6272–00, D44760-03);

• Flexural stiffness under different temperatures and loading frequencies; and

• Shear strength and shear fatigue (ASTM D4475–02).

Figure 6: Shear fixture.

The flexural and shear tests were conducted

using control as well as conditioned FRP

dowel bars. The conditioning process

consisted of exposing the specimens to:

• Alkaline solution,

• Water, and

• Ultraviolet radiation.

FRP dowel bars, especially glass FRP dowel bars, can be highly sensitive to alkaline attack.

The alkali solution can attack the glass fibers, eventually dissolving the glass. Since concrete

pore solution is highly alkaline, with a pH level of about 12.5 to 13.5, and the dowel bars are


UCPRC-SR-2008-03 37

embedded in concrete slabs, it is important to understand whether the matrix adequately

protects the fibers from such an attack.

The water absorbed by polymers can soften the polymer structure and lead to FRP stiffness

loss. Moisture can also weaken the fiber–matrix interfacial bond, and at the chemical level the

presence of moisture provides a polar group which may replace the fiber–matrix bonds, thereby

adversely affecting the interface adhesion.

Although dowel bars will not be exposed to UV rays once they are embedded in the concrete

pavement, they can be exposed to sunlight during transport, storage, and handling. UV radiation

has wavelengths between 290 nm and 400 nm, and most FRP materials have bond dissociation

energy in this wavelength range. Under UV radiation, the bond dissociation between fiber and

matrix will be initiated and chemical changes may be induced in the matrix material in the

presence of oxygen, which could result in stress concentrations and lead to the initiation of

rupture at a stress level much lower than the failure stress for an original FRP material.

Replicate FRP dowel bars were subjected to alkaline solution and water conditioning for three

months and UV radiation for two months

(only one type of conditioning for any one

dowel). They were then tested for flexural

stiffness, strength and fatigue, and shear

strength and fatigue. Unconditioned dowels

were tested as controls for comparison with

the conditioned dowels.

Figure 7: Shear fixture with failing dowel.

Findings from the laboratory flexural testing of FRP dowels were:

• Type B dowels (with epoxy matrix) were found to be more than twice as strong as typical

steel dowels, while Type A dowels (with polyester matrix) were approximately 30 percent

stronger than steel dowels. The elastic modulus of fiber-reinforced polymer (FRP)

dowels is approximately 20 percent that of steel dowels. The elastic moduli of both types

of FRP bars are similar, although the modulus of the Type B bars was slightly higher

(15 percent) than that of the Type A bars.


UCPRC-SR-2008-03 38

• The stiffness of both types of FRP dowel is not influenced by applied loading frequency.

However, the dowel stiffness is affected by testing temperature: Dowels exhibit 10 to

15 percent lower stiffness at 40°C than at 20°C and 5°C.

• The alkali solution–conditioned dowels showed considerably lower stiffness compared to

the control dowels; while the stiffnesses of water- and UV-conditioned Type A bars were

slightly less than the stiffness of the control dowels. For Type A bars, the dowel bar

strength is 5.6% lower after being conditioned in water and 8.5% lower after alkali

solution conditioning, compared with control bars.

• Higher variability in flexural strength was found on the conditioned Type B bars

compared to the Type A bars, especially on those that were submerged in alkali solution

(other conditioning consisted of water submersion and UV exposure). Many of the alkali-

conditioned Type B bars had minor cracks and highly variable strength after the

conditioning. The tested strength values ranged from 468 to 714 MPa. If more bars were

tested, it is very possible that even lower strength values could have been found. Thus it

is believed that Type B bars would be unreliable if they were subjected to the highly

alkaline solutions found in concrete pore water.

• The fatigue life of both types of FRP bars is similar when they have not been conditioned

with water, alkali solution, or ultraviolet light, and with more than a million cycles of

fatigue life at a flexural stress-to-strength ratio of 0.5. Very good correlation exists

between fatigue life and stress ratio. Conditioning of the specimens had a minor effect

on flexural fatigue life, considering the stress was reduced to match the reduced strength.

• During fatigue tests, all dowel bars tend to fail when bar stiffness drops to half of the

original stiffness of the control (unconditioned) bars. In other words, a 50 percent

reduction in stiffness, due either to repetitive loading or environmental damage, can

cause dowel failure.

Findings from the laboratory shear testing of FRP dowels were:

• The shear strength of Type B bars (epoxy matrix) is higher than that of Type A bars

(polyester matrix). The strength ratio between the two types of bars is very similar in

shear as it is in flexure, with Type A bars having 55 percent of the strength of the Type B

bars.

• Conditioning had a negligible effect on the shear strength of the bars, except for the

alkali-conditioned Type B bars.


UCPRC-SR-2008-03 39

• Type A bars showed better fatigue performance than the Type B bars at high stress

ratios. At a low stress ratio (0.3 to 0.4), both types of FRP will survive repetitive loading

in excess of 10 million cycles.

The overall conclusions from laboratory testing of FRP dowels were:

• It is known from the literature survey that although the stiffness and strength of

composite FRP materials depend mainly on the properties of the fibers, the matrix binds

the fibers and protects them from environmental attack. Consequently, when the matrix

remains intact, FRP materials with high fiber strength/stiffness exhibit high

strength/stiffness. However when matrix material is damaged or is penetrated by

aggressive agents from the environment, an FRP composite’s strength and stiffness can

decrease significantly.

• From the test results presented in this report, it appears that the polyester resin matrix

that was used for the Type A bars successfully protected the E-glass fibers during the

bars’ conditioning with water, an alkali solution, and ultraviolet radiation. However, the

epoxy resin that was used for the Type B bars was found to be susceptible to alkali

attack; the mechanical properties of the alkali solution–conditioned Type B bars were

significantly damaged compared to the unconditioned Type B bars. E-glass fibers are

highly prone to alkali attack, so a matrix material must be able protect them against an

alkali solution to guarantee the durability of the GFRP material.

The laboratory testing of FRP dowel bars answers the following questions:

1. Are the mechanical properties of FRP adequate to perform acceptably (compared to

steel dowels) in transferring traffic load across concrete pavement joints? The answer is:

Yes, measured strength, stiffness, and fatigue life indicate that FRP dowel bars can

perform well.

2. Are the mechanical characteristics of FRP negatively affected by environmental

conditions? The answer is: Yes, but potential risks can be identified, and susceptible

dowels can be eliminated if they are tested for alkali attack and water damage before

acceptance.


UCPRC-SR-2008-03 40

4.3 Reports


MANCIO, M., Carlos, C. Jr., Zhang, J., Harvey, J. T., and Monteiro, P. J. M. January 2007.

Laboratory Evaluation of Corrosion Resistance of Steel Dowels in Concrete Pavement.

Research Report prepared for the California Department of Transportation (Caltrans), Division

of Research and Innovation by the University of California Pavement Research Center, UC

Davis and Berkeley. UCPRC-RR-2005-10.

BIAN, Y., Harvey, J., and Kohler, E. September 2008. Fiber-reinforced Polymer (FRP) Dowel

Bar Laboratory Tests Results. Report prepared for the California Department of Transportation

(Caltrans), Division of Research and Innovation by the University of California Pavement

Research Center, UC Davis and Berkeley. UCPRC-RR-2007-01.


UCPRC-SR-2008-03 41

5 MODELING

5.1 Finite Element Analysis and Performance Estimates

A finite element analysis model of concrete slabs with

dowel bar retrofit (DBR) was developed and used to

calculate critical stresses in the grout around the dowels

and at the interface between the grout and the concrete slab. The 3D finite element model of

DBR slabs was developed using ABAQUS, a commercial finite element analysis package.

A typical DBR geometry was included in the model. The grout-to-dowel interface was modeled

using a concrete-smeared cracking model with displacement-dependent tension stiffening. The

ABAQUS model was calibrated by comparing the stress distributions and deflection values it

produced with those produced by EverFE, a three-dimensional finite element program

developed for the Washington State Department of Transportation.

The model was used to find the following:

• Compressive bearing stress in the DBR grout around the dowel as a function of dowel bar

stiffness, subgrade k-value, and wheel-load magnitude for a single wheel. Compressive

bearing stress is important because it has been found to correlate with loosening of the

dowel bar, which leads to loss of LTE in the joint and to faulting.

• Tensile and shear stress at the interface between the DBR grout and the concrete slab at

the edges of the dowel bar slot cut into the slab as a function of axle load and axle type.

These results were calculated assuming FRP and steel dowels. The calculated stresses

were compared to laboratory measurements of the shear strength of the grout-to-slab

(grout/slab) interface performed by the Caltrans METS staff at the Transportation Laboratory.

These results are important because Caltrans has had failures of DBR associated with

failures of the bonding between the grout placed in the DBR slot and the concrete slab.

• Tensile and shear stress in FRP dowels as a function of axle load and axle type. These

results were then used to estimate dowel bar fatigue life based on laboratory strength and

fatigue test results. These results are important in estimating whether FRP dowels

themselves will fail under heavy traffic loading.

• Bearing stress in the grout at the interface between the grout and the dowel bar for FRP and

steel dowel bars as a function of axle load and axle type. These results were used to


UCPRC-SR-2008-03 42

estimate faulting performance for FRP and

steel dowels using an empirical-mechanistic

regression equation developed from field

performance observations by Yu, et al (the

previously mentioned RIPPER model).

Another RIPPER model was used to estimate

DBR design lives using Caltrans and FHWA

International Roughness Index (IRI) criteria.

Findings from the finite element modeling were:

• The parametric analyses performed to evaluate compressive bearing stress in the DBR

grout around the dowel for a single-wheel load and as a function of dowel bar stiffness,

subgrade k-value, and wheel-load magnitude showed that the critical bearing stress in

the grout at the grout/dowel interface was reduced when dowel stiffness, higher

subgrade k-value (stiffness), and lower wheel load were increased.

• The results for a single-wheel load indicate that FRP dowels have shorter DBR lives

than steel dowels because their lower stiffness (30 to 36 GPa versus 200 GPa for steel)

results in greater bearing stress on the grout surrounding the dowels, which causes

dowel loosening under traffic. However, results for axle loads as opposed to single-

wheel loads showed smaller differences in bearing stress for different dowel stiffnesses.

• The parametric analysis of grout/concrete and grout/dowel interfacial stresses and

deflections for FRP dowels with different axle configurations, as opposed to single-wheel

loads, showed that maximum compressive bearing stresses on the grout around the

dowels were 4.70 and 3.96 MPa (682 and 575 psi) for the single- and tandem-axle

California legal load limits, respectively. The axle-load cases included steering axles,

dual-single axles, and dual-tandem axles, including California legal axle loads on the

dual-single and dual-tandem axles. Maximum tensile stresses on the grout around the

dowels, occurring in a localized area at the mouth of the dowel hole at the joint, were

7.97 and 6.82 MPa (1,142 and 990 psi) for the single- and tandem-axle California legal

load limits, respectively. These calculated tensile stresses are greater than the expected

tensile strengths of grout or concrete.

• The load on an axle was found to produce lower bearing stresses in the grout around the

dowels than the half-axle load on one wheel. This finding indicates that Heavy Vehicle

Simulator (HVS) tests, which use a single wheel, can be expected to cause greater


UCPRC-SR-2008-03 43

bearing stresses than an axle load with the same load on each wheel. This result is likely

due to the eccentricity of the one-wheel load on the slab as opposed to an axle load.

• Maximum tensile stress values calculated at the interface between the grout and the

concrete slab ranged between 0.76 and 0.89 MPa (110 and 129 psi) for the range of

axle load cases analyzed, with the greatest maximum tensile stress calculated for the

California legal single-axle load. Maximum shear stress values calculated at the

grout/slab interface ranged between 0.15 and 0.27 MPa (22 and 39 psi) for the same

cases, with the greatest maximum shear stress calculated for the California legal

tandem-axle load.

• Review of the results of bond tests that the METS Office of Concrete Pavement

performed on the bond strength between DBR grout and concrete slabs taken from

cores indicated that they were unavoidably biased, and nonconservatively, because

many of the bonds failed during coring and could not be tested. The METS bond shear

strength data, including consideration of its variability, was compared with data for

grout/concrete shear stresses calculated with the model. The comparison indicated that

for the projects with magnesium-phosphate grout, approximately 5 percent of the

interfaces between the dowel bar grout and the concrete slab can be expected to fail by

shearing whenever a single or tandem axle with the California legal load limit passes

over them. The probability of bond failure is expected to be much lower for the DBR slots

with modified high-alumina grout.

• A similar analysis for steel dowels was performed. The calculated maximum

compressive bearing stresses on the grout around the dowels were 4.53 and 3.72 MPa

(657 and 540 psi) for the single- and tandem-axle California legal load limits,

respectively. Maximum tensile stresses on the grout around the dowels, occurring in a

localized area at the mouth of the dowel hole at the joint, were 8.72 and 7.36 MPa

(1,266 and 1,068 psi) for the single- and tandem-axle California legal load limits,

respectively. These calculated tensile stresses are greater than the expected tensile

strengths of grout or concrete, as was the case for the FRP dowels.

• Maximum tensile stress values calculated at the interface between the grout and the

concrete slab ranged between 0.88 and 0.99 MPa (128 and 144 psi) for the range of

axle-load cases analyzed, with the same greatest maximum tensile stress calculated for

the legal California single- and tandem-axle loads. Maximum shear stress values

calculated at the grout/slab interface ranged between 0.24 and 0.42 MPa (35 and 61 psi)


UCPRC-SR-2008-03 44

for the same cases, with the greatest maximum shear stress calculated for the legal

California single-axle load.

• Finite element analyses performed by Applied Research Associates (ARA) for Caltrans

were compared with the steel-dowel results calculated as part of this study. The ARA

model was for a single-axle legal load and somewhat different slab configuration and a

more critical load location at the edge of the slab. The ARA results showed much greater

calculated shear stresses at the grout/slab interface than those found in this study, and

slightly smaller tensile stresses at the grout/dowel interface.

• The METS grout/concrete bond-strength data, again considering its variability, was

compared with the data on shear stresses at the grout/concrete interface calculated as

part of this study. The ratios of shear bond strength to interface shear stress indicated

that for the projects with magnesium-phosphate grout, approximately 5 to 10 percent of

the interfaces between the dowel bar grout and the concrete slab can be expected to fail

by shearing whenever a single or tandem axle with the California legal load limit passes

over them. The probability of bond failure is expected to be much lower for the DBR slots

with modified high-alumina grout.

• The shear stresses calculated by ARA for Caltrans for the case of a California legal

single-axle load placed at the edge of a slab with skewed joints are much greater than

the mean grout/slab shear strength measured by METS and indicate that the shear

strength would be exceeded almost every time that loading situation occurred.

• It was also noted that the METS shear strength values were taken from cores that had

completed most of their strength gain. Lower strengths would be expected prior to

completion of the strength-gain process for the grouts, when the DBR slots are opened

to traffic before strength gain is substantially completed.

• Comparison of calculated shear and tensile stresses in FRP dowels under the various

axle-load configurations included in the study indicated that FRP dowels can withstand

more than 1 billion load cycles without failing from fatigue. On the basis of traffic

volumes on heavy-duty California highways, the FRP dowel bar fatigue life—that is, the

life of the dowel itself with respect to cracking of the dowel—is approximately 78 years.

• Expected faulting and roughness design life [equivalent single-axle loads (ESALs) to IRI

failure values] for FRP dowels were calculated using grout/dowel compressive bearing

stresses from the finite element model analysis for a legal-limit dual-single-axle load and

mechanistic-empirical prediction models from an FHWA study commonly referred to as

RIPPER. The predicted design lives of DBR with FRP dowels ranged between 46 and


UCPRC-SR-2008-03 45

30 years for 1 and 5 million ESALs per year in the design lane, respectively, and the

current Caltrans IRI criteria of 3.16 m/km (224 in./mi). For the FHWA IRI criteria of

2.4 m/km (170 in./mi), the predicted design lives are between 31 and 21 years for

1 million and 5 million ESALs per year in the design lane, respectively. Similar

calculations were made for steel dowels and showed predicted design lives of DBR

between 46 and 30 years for 1 million and 5 million ESALs per year in the design lane,

respectively, for the current Caltrans IRI criteria, and 31 and 21 years for the current

FHWA IRI criteria. It was noted that the RIPPER models were calibrated using only steel

dowels, and they were also calibrated only for originally installed dowels, not DBR. The

lower strengths of DBR grout would be expected to reduce the design lives, so these

values should be considered as upper bounds.

5.2 Life Cycle Cost Analysis and Comparison with Alternatives

In order to determine how much of the

current Caltrans state highway network

the DBR studies results apply to a review

was undertaken of the Caltrans historical

Pavement Condition survey (PCS)

database. Dowel bar retrofit (DBR) can be

applied only to pavements that meet the

following three basic conditions: (1) the

surface layer is portland cement concrete (PCC), (2) the PCC is non-doweled jointed plain

concrete pavement (JPCP), and (3) the PCC has a minimal amount of cracking.

A life-cycle cost analysis (LCCA) was conducted to compare the cost-effectiveness of DBR with

other maintenance and rehabilitation techniques for concrete pavements. Agency and user

costs were estimated under various conditions, using data from Caltrans, industry, and

academic sources. User costs, in terms of additional delay caused by construction of DBR and

alternatives and subsequent maintenance and rehabilitation, were estimated using the Federal

Highway Administration (FHWA) software package called RealCost and the Caltrans Life-Cycle

Cost Analysis Procedures Manual, which was developed by Caltrans and the UCPRC.


UCPRC-SR-2008-03 46

DBR eliminates or at least considerably

reduces the progression of step faulting. For

this LCCA, DBR was also assumed to reduce

slab cracking by means of reduced deflections

at the joints, which in turn decreases concrete

stresses. Calibrated mechanistic-empirical or

empirical models are not available for

predicting this assumed extension of cracking

life. Instead, an analysis approach was used that makes the extension of fatigue life a variable.

Factors considered in the analysis included:

- Initial remaining life: This takes into account the structural condition of the pavement that is

a candidate for DBR. The analysis considered 10, 20, and 30 years of expected fatigue life

remaining.

- Grinding life: This captures scenarios for grinding life in the absence of DBR, which

determines the interval between grindings if the fatigue life is longer than the grinding life.

The analysis considered 10, 12, 15, 17, and 20 years. These values are based on results of

a report by Caltrans and Applied Research Associates (ARA) reviewing Caltrans PMS data.

- Analysis period: This study used a 40-year analysis period. It fits the planning horizon for the

activities considered and meets the recommendations of the FHWA.

- User cost variables: These include traffic growth, closure details (time of day/week, number

of lanes affected), and traffic distribution (rural vs. urban, percentage of trucks). For this

analysis, all closures were considered to be on weeknights from 10:00 p.m. to 6:00 a.m. and

to affect only one lane of traffic. The chosen annual growth rate was 1.5%.

- DBR performance: To account for the uncertain maintenance cost of DBR (due to failed

backfill material), analyses in this study were run using a failure rate of 0%, 3%, and 6% per

year. Results were also produced for the cases of plus/minus 10% from the expected DBR

initial cost ($120,000/ln-mi).

- Discount rate: A discount rate of 4% for LCCA was used, as is typically done by Caltrans. It

should be noted that this value has a significant impact on the results.

The comparison was based on a five-mile rural stretch of highway with an initial annual average

daily traffic (AADT) load of 38,500 vehicles, 24 percent of which were trucks. The section is


UCPRC-SR-2008-03 47

based loosely on a DBR site on Route 99 in Kern County. The final results of the LCCA were

relatively unaffected by the details of the case study.

The methodology consisted of comparing maintenance and rehabilitation scenarios, with and

without DBR. The activities considered included grinding, three levels of CPR (concrete

pavement rehabilitation, which means slab replacements, grinding, and joint sealing), hot-mix

asphalt overlays of two different thicknesses, and repairs to the asphalt overlays. These

activities were used to create several realistic maintenance schedules. The cash flow in each

case was computed. The results were obtained in terms of the fatigue life extension needed for

DBR to become cost-effective.

Findings from this investigation are listed below.

With regard to the extent of lane-miles that could potentially benefit from DBR:

• According to the PCS, the total lane-miles of PCC that have less than 10 percent first-

stage cracking and 5 percent third-stage cracking is roughly 10,200 lane-miles, out of

12,800 total lane-miles of PCC pavement across the state. The result of the above

analysis is that there are approximately 8,200 lane-miles of pavement that are possible

candidates for DBR.

• A further refinement to this value would be to stratify it by traffic level because

pavements with higher levels of truck traffic would benefit more from a doweled structure.

This, however, is not easily accomplished using the available databases, and therefore

stratifying was only done in terms of truck and nontruck lanes. According to the PCS,

37 percent of the PCC lanes are truck lanes. Therefore, roughly 3,000 lane-miles of PCC

are in truck lanes and currently do not contain dowels. Because this represents roughly

25 percent of the total PCC pavement in the state, DBR could have a significant impact

as a maintenance strategy, provided those lane-miles have sufficient remaining fatigue

life to make DBR potentially economically competitive. These sections are likely to be

good candidates for DBR, if they have not had diamond grinding within the past 8 to 12

years.

• Caltrans has used diamond grinding through its HM-1 or HA-22 programs on

approximately 3,400 lane-miles in the last 10 years (according to the State of the

Pavement reports), with roughly 1,750 lane miles in the past five years. The final

summation indicates that about 1,000 lane-miles of truck lanes may be possible


UCPRC-SR-2008-03 48

candidates for DBR if they have substantial fatigue life remaining (as discussed below),

and about 4,000 total lane-miles that might benefit from DBR, if it is the most cost-

effective approach to maintain smoothness.

With regard to the question that this investigation tried to answer regarding whether DBR is an

economically competitive option for rehabilitation of concrete pavements, a single, simple

answer could not be found that applied to the entire network. However, some important

conclusions were drawn from the LCCA regarding the cost-effectiveness of DBR for different

kinds of segments in the network, as follows:

Conditions that Make DBR Economically Effective

• The benefits of DBR will not be immediately realized since both DBR and grinding

alone immediately produce a smoother pavement. The avoidance and delay of future

maintenance and repair (M&R) activities from DBR only occurs after significant time

has passed. But the LCCA results show that DBR can be a cost-effective solution

under certain conditions.

- DBR may be the most cost-effective treatment on rigid pavements with relatively

long remaining fatigue lives (~30 years) since the investment in DBR is recaptured

through avoiding future grindings and extending the initial fatigue life.

- DBR will most likely not be the most cost-effective treatment on pavement sections

with short remaining fatigue lives (~10 years) since much of the capital investment in

DBR will probably not be fully recovered.

- DBR will definitely not be the most cost-effective treatment when a PCC pavement

needs to undergo a major rehabilitation in the near future since the DBR benefits will

not have sufficient time to be realized.

Expected Fatigue Life Extension from DBR for It to Be Cost-Effective

• Under the typical current California case analyzed (initial cost of $120,000 per lane-

mile, 3 percent failed DBR slots per year, 10-year grinding life, and 20 years

remaining fatigue life), DBR is the best solution if it provides at least a 44 percent

increase in fatigue life, which means that DBR must extend the cracking life of the

pavement by nine years. The 44 percent is for a typical case, but sensitivity analysis

showed that, depending on the assumptions made, this value can range from

20 percent to 96 percent. Some state agencies, like those of Kansas, Washington,


UCPRC-SR-2008-03 49

Oklahoma, and South Dakota, seem to believe that such fatigue life extensions are

possible, and have conducted extensive rehabilitation with DBR.

Consideration of Reduced Thickness from Grinding and Other Variables

• If DBR is limited to pavements with 30 years of remaining fatigue life, which is to say

pavements with almost no cracking (could be less than 5 percent slabs with first-

stage cracking), then an increase in fatigue life from DBR between 20 percent and

60 percent (depending on life of the grinding-only treatment and the initial cost)

would be sufficient to make it cost-effective. If the slab thickness reduction from

grinding and associated fatigue life reduction is taken into account, then the results

show that using a 40-year planning horizon and a 4 percent discount rate, DBR must

increase the fatigue life of the pavement between 9 percent and 42 percent to be a

cost-effective solution, depending on grinding life and effect of thickness reduction.

Reasonableness of Fatigue Life Extension from DBR

• A simple, quick analysis indicated that DBR could potentially increase the fatigue

cracking life of PCC slabs. A reasonable upper bound might be 50 percent, primarily

based on judgment and the fact that for many cases, such as the single-axle case

analyzed, DBR provides no fatigue cracking benefit, while for others, such as the two

axles-on-the-slab case analyzed, DBR can reduce tensile stresses and therefore

increase fatigue cracking life. Determining the actual extent of a potential fatigue

cracking life increase would require a detailed study as the increase depends on

pavement structure, subgrade, climate, load locations, axle types, and other factors.

Agency versus User Costs

• Although this analysis considered both user costs and agency costs, the latter drove

the results. The user costs, while substantially higher for the DBR cases, accounted

for only a very small percentage of the total costs. In this analysis, agency costs

were generally two orders of magnitude larger than the accompanying user costs for

a given activity. For user costs to make a significant impact in the results, long traffic

queues would have to develop as a result of the construction. Queues such as these

can be produced by using high traffic growth rates and/or peak-time construction

closures, both of which are contrary to the assumptions used in this analysis.

However, it is realistic to assume that Caltrans will take measures to mitigate the


UCPRC-SR-2008-03 50

inconvenience posed to users through the Traffic Management Plan (TMP) when

performing the retrofit. This includes using nighttime and off-peak closure schedules

whenever possible.

Sensitivity to Construction and Maintenance Costs

• The only initial construction cost that had a notable effect on the results was that of

the DBR. A 10 percent change in the initial cost of DBR resulted in a 5 percent to 10

percent change in the fatigue life extension needed to be life-cycle cost competitive.

Economies of scale (which arise when DBR is implemented as a statewide rigid

pavement capital maintenance activity) or other economic efficiency gains can

reduce DBR construction costs.

Consideration of Life and Frequency of Grinding Alone Alternative

• The initial cost of grinding alone and the

longevity of grinding without DBR are

much less important to the results than

the number of grinds that need to be

performed over the remaining fatigue life

of the JPCP. When the base case

requires multiple grindings, the

equivalent DBR case can avoid these

future activities, thus saving the costs

associated with them. The resulting loss

of fatigue life is exacerbated when multiple grindings are necessary over the service

life of the pavement. Therefore, when faulting rather than cracking is the anticipated

future trigger distress after the initial activity (DBR or grinding alone), DBR becomes

a more feasible alternative because of its ability to improve load transfer efficiency

and therefore avoid future grindings.

Effect of Annual Maintenance Costs

• The annual maintenance costs also play a key role in determining the cost-

effectiveness of DBR. Although California has encountered varied maintenance

demand on its finished DBR projects because of early failures of DBR slots, DBR

has performed much better on a nationwide scale. It is not unreasonable to expect


UCPRC-SR-2008-03 51

that maintenance demand can approach zero failed slots per year if experienced

contractors are selected and if underperforming backfill materials are ruled out.

Closer inspection during the construction process would also help to ensure a high

quality initial product. The use of warranty contracts for DBR projects might also be a

useful solution.

5.3 Reports


1. BIAN, Y., Jie, G., and Harvey, J. March 2008. Finite Element Analysis of Dowel Bar Retrofit

Alternatives. Report prepared for the California Department of Transportation (Caltrans),

Division of Research and Innovation by the University of California Pavement Research

Center, UC Davis and Berkeley. UCPRC-RR-2008-06.

2 SANTERO, N., Kohler, E., and Harvey, J. Life-Cycle Cost Analysis of Dowel Bar Retrofit.

Report prepared for the California Department of Transportation (Caltrans), Division of

Research and Innovation by the University of California Pavement Research Center, UC

Davis and Berkeley. UCPRC-RR-2007-11.


UCPRC-SR-2008-03 52


UCPRC-SR-2008-03 53

6 SUMMARY OF RECOMMENDATIONS

Based on the research carried out in the studies

detailed in this report, a number of

recommendations were made that responded to

questions raised by Caltrans between 2000 and

2007 regarding dowel bar retrofit (DBR).

Observations are presented grouped by the

four identified objectives (Table 1), plus

additional related recommendations derived from observations of construction practices.

Details on Caltrans’ current implementation status for each recommendation are provided in

Table 2.

Objective 1. Evaluate the structural adequacy of DBR options in terms of Load Transfer Efficiency (LTE) by accelerated pavement testing of DBR test sections, and compare with joints with no DBR.

The recommendations from this objective are:

1. In order to characterize joint behavior, backcalculated moduli, and k-value adequately,

Falling Weight Deflectometer (FWD) testing should be performed at two temperatures,

preferably at least 10°C different, with at least one temperature less than 67° F (20° C).

2. Deflections and deflection differences increase in approximately a linear relation with

increasing load magnitude, therefore multiple loads are probably not necessary to

characterize joint behavior using the FWD. The load used for characterization should be

near the legal load limit, and overload testing is probably not necessary.

3. Using the conclusions developed in this report, a protocol should be developed and

implemented for evaluation of rigid pavements using the FWD. This protocol should

include:

a. The recommendations for testing described above, and

b. A process for interpreting the deflection data and developing estimates of

concrete and subgrade modulus, k-value, load transfer efficiency (LTE), and

other useful information for rigid pavement analysis.


UCPRC-SR-2008-03 54

4. Four steel dowels per wheelpath should be used for heavier traffic applications of DBR

because four dowels provides greater LTE than three steel dowels per wheelpath.

5. Use FRP dowels with four dowels per wheelpath in locations that would otherwise call

for three dowels per wheelpath because this design has similar LTE to three steel

dowels per wheelpath because of the lower stiffness of FRP. The effect on long-term

performance is uncertain. Caution with regard to FRP dowels should be noted from the

recommendations under Objective 3, below.

Objective 2. Evaluate the performance of DBR on in-service pavements under live traffic, including monitored test sections, Caltrans construction projects, and construction projects in other states. Determine chloride contents in concrete slabs near dowels.

Results from this objective are incorporated in recommendations under other objectives.

Objective 3. Determine relative corrosion resistance of different metallic dowel types through laboratory testing, and compare resistance with expected chloride exposure. Determine engineering properties and durability of fiber-reinforced polymer dowels through laboratory testing.

For metallic dowels the recommendations from this objective are:

1. The presence of corrosion at the bar ends and along the bar from ponding water on dowels

cast in concrete in the laboratory indicates that chlorides can pass all the way to the bar

ends from the joint along the horizontal interface between the dowel and the concrete, or

through the concrete. For this reason it is recommended that uncoated carbon steel dowels

continue to not be used.

2. Epoxy dowels present some risk of corrosion, primarily localized at holidays and the ends of

the bars. Based on this finding, it is recommended that:

a. Quality control checks to control holidays should be implemented.

b. Bar ends should be painted or coated with epoxy, and care must be taken with

epoxy-coated dowels during shipping, storage, and installation. Corrosion will be

exacerbated if the bar ends are not coated (observed on various Caltrans

construction sites) or if the coated ends are damaged during storage, transport,

and installation.


UCPRC-SR-2008-03 55

3. It is recommended that the use of FRP, stainless steel–clad, hollow stainless steel, or

microcomposite steel dowels be considered for locations with high risk of high chloride

exposure (such as on mountain passes and marine environments), where exposure to

corrosive solutions is anticipated. The selection of a specific corrosion-resistant dowel

should be based on further field investigations and cost differences.

4. It is recommended that a field study be performed at several mountain pass locations to

measure the chloride content of snowmelt after sand/salt application for comparison with the

chloride content of the solution used in the laboratory testing in Phases I and II and the core

results from Phase III. Some coring should be performed to check chloride contents for year-

round exposure comparison. The results of this study should be used to further refine the

risk assessment in these critical locations.

For FRP dowels the recommendations from this objective are:

5. If FRP dowel bars are used in pavement, it is recommended that representative dowel bars

be conditioned by alkali solution and that water be tested before acceptance for use on the

project. An outline of a testing method for reference has been provided in the appendix of

UCPRC-RR-2007-1 (Fiber-reinforced Polymer (FRP) Dowel Bar Laboratory Test Results).

Objective 4. Evaluate stresses in DBR/slab systems effecting performance using Finite Element Analysis, and use to estimate performance under different conditions. Use Life-cycle Cost Analysis to evaluate cost-effectiveness of DBR compared to alternatives.

The following recommendations are from the LCCA results. In general, DBR is best suited for

JPCP sections that have a substantial fatigue life remaining and that are susceptible to faulting.

Because external inputs are highly sensitive, it is impossible to claim that DBR should (or should

not) be used under all circumstances.

The decision to use DBR should be made on a project-by-project basis using the tables

generated in this study. However, the results are interpreted, greatly simplified, as showing that:

1. DBR should not be performed on pavements containing more than 5 percent cracked

slabs (third-stage cracking, either with slabs presenting interconnected cracks or having

previously been replaced). The 5 percent cracking limit is subjective, but it was selected

to reflect shorter fatigue life remaining in the candidate pavement. Even if slabs have


UCPRC-SR-2008-03 56

been replaced and the current cracking level has therefore been reduced below 5

percent, the fact that slabs had been replaced should be interpreted as an indicator that

the fatigue life of the original remaining slabs is about to be exhausted.

2. Another greatly simplified interpretation is that DBR should generally be performed on

pavements exhibiting fewer than 2 percent cracked slabs. This will prevent faulting of

transverse cracks that may be expected to appear relatively soon after the DBR.

3. There is a grey area between the 2 percent and 5 percent where greater reliance on

engineering judgment is required as part of project review. The primary factor to

consider is the rate of faulting development. If there is sufficient fatigue life, then DBR is

more likely to be life-cycle cost-effective where the rate of faulting development is high.

Factors increasing the rate of faulting development are:

a. Erodability of the base. DBR will be more cost-effective if the base is susceptible

to erosion, which would lead to a faster rate of faulting development if grinding is

performed without DBR.

b. Poor aggregate interlock, Smaller aggregates and lower strength aggregates will

tend to lose load transfer efficiency faster, leading to faster faulting development.

Loss of aggregate interlock also occurs where slabs have had greater shrinkage

or have higher coefficients of thermal expansion, leading to opening of the joint.

c. Traffic forecasting. DBR will be more cost-effective in situations with heavier

traffic.

Each of these factors contributes to load transfer efficiency, which, combined with historical

measurements of faulting development (current fault height divided by cumulative truck

traffic or age), will give an indication of fault development rate.

Minor suggestion. This suggestion applies to construction practices, and falls outside the

original objectives.

Construction inspection on DBR projects should follow the WSDOT guidelines. Inspectors

trained to use the guidelines should be on-site during all of the construction process.


UCPRC-SR-2008-03 57

Table 6: Summary of Implementation of Recommendations by Caltrans

Details on Implementation by Caltrans Objective 1: Evaluate the structural adequacy of DBR options in terms of Load Transfer Efficiency (LTE) by accelerated pavement testing of DBR test sections, and compare with joints with no DBR. 1.1 FWD testing should be performed at two temperatures, preferably at least 10°C different, with

at least one temperature less than 67°F (20°C). Not formally implemented to date, many studies are conducted at night for traffic closure reasons. Temperature limit recommended less than 70°F (21°C) in MTAG Ch. 6.

1.2 Do not use multiple loads for FWD testing; use one load near the legal load limit. Not formally implemented to date.

1.3 Develop a protocol for interpreting deflection data to estimate concrete and subgrade moduli and subgrade k-value, load transfer efficiency, and other useful parameters. Implemented in CalBack software.

1.4 Use four dowels per wheelpath for DBR on heavy traffic applications. Not formally implemented in DBR guidelines to date. MTAG Ch. 6 states that three per wheelpath are adequate.

1.5 Consider use of FRP dowels in locations with high risk of corrosion. Use FRP dowels with four dowels per wheelpath in locations that would otherwise call for three dowels per wheelpath. Not formally implemented in DBR guidelines to date.

Objective 2: Evaluate the performance of DBR on in-service pavements under live traffic, including monitored test sections, Caltrans construction projects, and construction projects in other states. Determine chloride contents in concrete slabs near dowels. Results from this objective are incorporated in recommendations under other objectives. Objective 3: Determine relative corrosion resistance of different metallic dowel types through laboratory testing, and compare resistance with expected chloride exposure. Determine engineering properties and durability of Fiber-reinforced Polymer (FRP) dowels through laboratory testing. 3.1 Do not use uncoated carbon steel dowels. Caltrans does not use uncoated steel dowels. 3.2 Perform holiday checks on epoxy-coated dowels, paint, or coat bar ends. Not yet implemented

to date. MTAG Ch. 6 calls for inspection for damage or abrasion of epoxy coating 3.3 Consider use of FRP, stainless steel clad, hollow stainless steel, or microcomposite steel

dowels be considered for locations with high risk of high chloride exposure. Not formally implemented in DBR guidelines to date.

3.4 Perform field study to assess chloride exposure of dowels on mountain passes for comparison with results of this study. Study not yet performed to date.

3.5 Test FRP dowels by conditioning with alkali solution and water prior to acceptance for a project. Caltrans is not currently using FRP dowels. Not formally implemented in DBR guidelines to date.

Objective 4: Use Finite Element Analysis to evaluate stresses in DBR/slab systems affecting performance and use results to estimate performance under different conditions. Use Life-cycle Cost Analysis to evaluate cost-effectiveness of DBR compared to alternatives. 4.1 Do not use DBR on pavements containing more than 5% cracked slabs (third-stage cracking).

Not formally implemented in DBR guidelines to date. MTAG Ch. 6 recommends 10% limit on cracked or otherwise structurally damaged slabs.

4.2 Where possible, only use DBR on pavements that exhibit less than 2% cracked slabs. See above.

4.3 For pavements with between 2% and 5% slabs cracked, consider project-specific factors when deciding whether to use DBR. See above.

Additional recommendations from findings

Rec

omm

enda

tions

Follow WSDOT guidelines during construction inspection; have inspector on site during entire construction process. Similar guidelines included in MTAG, although some details are covered in WSDOT guidelines not included in MTAG.

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