Invited Student Paper-A Procedure for Load Rating ...docs.trb.org/prp/17-01754.pdf · 1 Invited...

Cuaron, Jáuregui, and Weldon 1

Invited Student Paper-A Procedure for Load Rating Reinforced Concrete Slab Bridges 1

without Plans 2 3

Alain M. Cuaron, Corresponding Author 4 Graduate Research Assistant 5 New Mexico State University 6 Department of Civil Engineering 7 P.O. Box 30001, Las Cruces, NM 88003 8 Tel: 915-490-2957; Email: [email protected] 9

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David V. Jáuregui, PhD, PE 11 Professor and Department Head 12 New Mexico State University 13

Department of Civil Engineering 14 P.O. Box 30001, Las Cruces, NM 88003 15

Tel: 575-646-3801; Email: [email protected] 16 17

Brad D. Weldon, PhD 18 Associate Professor 19 New Mexico State University 20

Department of Civil Engineering 21 P.O. Box 30001, Las Cruces, NM 88003 22

Tel: 575-646-1167; Email: [email protected] 23 24 25

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41 Word count: 4989 + 10 x 250 words = 7489 42

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45 Submission Date: November 15, 201646


1 Graduate Student, Department of Civil Engineering, New Mexico State University, Las Cruces,

NM 88003 2 Department Head, Department of Civil Engineering, New Mexico State University, Las Cruces,

NM 88003 3 Associate Professor, Department of Civil Engineering, New Mexico State University, Las

Cruces, NM 88003

Invited Student Paper-A Procedure for Load Rating Reinforced Concrete Slab Bridges 1

without Plans 2 3 Alain M. Cuaron1, David V. Jáuregui2, and Brad D. Weldon3 4

5

Abstract 6 In New Mexico, bridges without plans are currently an issue as in other states in the U.S. Standard 7 techniques cannot be used for load rating due to the lack of design plans that contain necessary 8 information such as material properties and the amount and location of the steel reinforcement. In 9

this study, the state departments of transportation were surveyed regarding their load rating 10 policies and procedures for planless bridges. Many states reported a significant amount of bridges 11 without plans in particular reinforced concrete bridges and the rating approaches vary from state 12 to state. This project was conducted for the New Mexico Department of Transportation (NMDOT) 13

to develop an effective method to load rate reinforced concrete slab bridges without plans and 14 obtain more representative estimates of load-carrying capacity. Twenty-three reinforced concrete 15

slab bridges (simple and continuous span) were load rated and evaluated using simple analytical 16 and non-destructive testing techniques. The procedure includes field measurement of the bridge 17

dimensions and geometry, Windsor Probe testing of the concrete strength, scanning of the hidden 18 steel reinforcement, generation of as-built drawings, and modeling the bridge with a load rating 19 software. The procedure provided the NMDOT with as-built plans as well as more realistic load 20

rating results reinforced concrete slab bridges without design plans. Comparisons are made with 21 other state policies to illustrate the differences in the load ratings. 22

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Keywords: reinforced concrete slab bridge, no plans, load rating, steel detection, concrete testing. 27

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RESEARCH OBJECTIVE 1

The objective of this research was to develop a method to better determine load ratings for 2

reinforced concrete slab bridges without plans in New Mexico. The rating procedure was 3

developed in accordance with the AASHTO Manual (1) and instructions given by the New Mexico 4

Department of Transportation (NMDOT). The project required site visits to the selected bridges 5

to obtain basic geometry and dimensional measurements. In addition, the field work included 6

rebar scanning and Windsor Probe testing. Based on the data collected in the field and simple 7

analytical procedures to confirm the reinforcement layout, as-built drawings were generated and 8

used to develop the bridge models for load rating. A total of twenty-three slab bridges were 9

evaluated for the NMDOT in this study. 10

The study reported herein continues on previous work conducted at New Mexico State 11

University (NMSU) that focused on the evaluation of prestressed concrete bridges without plans 12

using proof testing (2). Though load testing is an effective way to determine load ratings of 13

planless bridges, it requires a significant amount of time and resources. The use of proof testing 14

on reinforced concrete slab bridges is more challenging compared to prestressed concrete bridges 15

due to the presence of pre-existing cracks and the different sources on non-linearity. However, 16

load testing of reinforced concrete slab bridges may be avoided due to the relatively short spans 17

and easily accessible components that allows the use of rebar scanning and concrete strength 18

testing to determine the hidden reinforcement and material properties needed to evaluate the bridge 19

capacity. 20

Other universities have also investigated the load rating of concrete bridges without plans 21

including the University of Delaware (3), Case Western Reserve University (4), and Colorado 22

State University (5). In general, load testing is the most common approach used to determine the 23

reinforcement layout as well as estimate the capacity of planless concrete bridges (3, 4). Very 24

little research has been done to evaluate planless concrete bridges without the use of load testing. 25

Researchers from Colorado State University developed an approach for estimating the load 26

carrying capacity of bridges with unknown material properties using structural analysis techniques 27

and conservative assumptions for the concrete compressive and steel yield strengths following the 28

AASHTO Manual (5). Though this simple approach provided more accurate load rating, the 29

estimated capacity for several bridges remained below the HS-20 design load. Ultimately, 30

recommendations were made to use experimental testing of the material properties along with load 31

testing to provide a better estimate of the material strengths and likely, larger load ratings. 32

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STATE SURVEYS 1 A questionnaire was developed and submitted to the fifty state Departments of Transportation 2 (DOTs) regarding their load rating procedures for bridges without plans and thirty-three 3 responded. The survey was filled out by the state’s bridge rating engineer or responsible member 4

of the bridge management team. The survey included the following questions: 5 6

How many planless bridges do you have in your inventory and what are the main load 7 rating challenges? 8

Does your agency have an established procedure for the load rating of planless bridges? 9

For concrete bridges, how are the steel reinforcement layout and the material strengths 10 estimated? Is non-destructive testing employed? 11

What computer software program does your agency use to perform load ratings? 12

Does your State conduct any form of load testing to aid in rating bridges, and if so, what 13 kind of tests are conducted and how many bridges are tested per year? 14

15 As shown in Figure 1, the majority of the state DOTs that responded reported more than 100 16 concrete bridges without plans in their inventory. Several of the states provided the number of 17 concrete bridges without plans in their state. The percentage of concrete bridges without plans for 18

these states are as follows starting with the largest: 34% (NH), 28% (UT), 23% (MT), 21% (ND), 19 20% (KY), 19% (NM), 16% (NV), 9% (OR), 8% (ID), 7% (TX), 6% (WI), 4% (DE), 3% (WY), 20

3% (TN), 2% (IL), 1% (CO), 1% (WA), and 0.5% (MN). In addition, 85% of the respondents 21 stated that bridges without plans are generally a concern in their state. Most states indicated that 22 the main challenge in load rating of bridges without plans is the unknown material properties and 23

reinforcement details. The most common procedure involves field work and the use of engineering 24 judgment by a professional engineer (PE). To estimate the strength and layout of the steel 25

reinforcement, approximately 79% follow the AASHTO Manual and about 33% conduct a 26

physical inspection and/or use non-destructive testing such as rebar scanning. To estimate the 27

concrete compressive strength, about 76% use the AASHTO Manual values while 10% use a 28 Schmidt Hammer or Windsor Probe. The load rating software program, used by 58% of the states 29

is the AASHTOWare BrR program and 24% use BRASS. About 52% conduct load tests on 30 bridges without plans; (diagnostic testing being the most common), ¾ of which test 1-5 bridges 31 per year. Furthermore, 65% contract with a consultant for load testing, 12% conduct in-house 32

testing, and 23% use a combination of consultant and in-house testing. The instrumentation 33 employed by approximately 71% of the state DOTs include strain and deflection gauges and a 34

dump truck is most commonly used to apply load (6). 35 36

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1 FIGURE 1 Concrete Bridges without Plans in the United States 2 3

Of the thirty-three states that responded, eight provided documentation and/or references 4 describing the steps involved in the load rating procedure which are summarized below. 5 6

Florida DOT Policy 7 For reinforced concrete bridges without plans, the Florida DOT will first obtain field 8

measurements of the bridge and then use standard bridge plans and/or codes corresponding to the 9 year built to approximate the reinforcing steel. The bridge is then analyzed and load rated. If the 10 reinforcement cannot be estimated and the bridge shows no distress, a load rating may be assigned 11

by a PE using engineering judgment (7). If the bridge does show some distress, then non-12 destructive testing (e.g., load testing) is performed to determine the bridge capacity (7). 13

14

Idaho DOT Policy 15 An initial search for design drawings is done for a reinforced concrete bridge without plans in 16

Idaho. If plans cannot be found, a PE assigns the rating factors based on HS20 truck loading and 17 the NBI condition ratings as shown in Table 1. For concrete bridges with an NBI condition rating 18

of 4 or less, the bridge is rated for legal loads and load posted (8). 19

20 TABLE 1 Inventory and Operating Ratings used in Idaho based on NBI Condition Rating 21 (8) 22 23

Lowest NBI

Condition Rating

Rating Factor Rating in Tons (IR)

Inventory Operating Inventory Operating

9 1.00 1.67 36.00 60.00

8 1.00 1.67 36.00 60.00

7 0.86 1.45 31.00 52.00

6 0.64 1.06 23.00 38.00

5 0.50 0.84 18.00 30.00

4 0.33 0.56 12.00 20.00

3 0.17 0.28 6.00 10.00 2 0.08 0.09 3.00 3.00 1 0.00 0.00 0.00 0.00

24 25 26


Maryland DOT Policy 1 Bridges without plans in Maryland are evaluated and load rated by a PE using the guidelines given 2 in the “Policy and Procedure Memorandum for Structural Load Ratings” which states the 3 following (9): 4

5

Inventory and operating factors for legal vehicles can be taken as 1.0 if the bridge has no 6 defects or damage and if the bridge has been carrying normal traffic. 7

If defects and/or structural damage are found, the Structural Inspection and Remedial 8 Engineering Division is informed and the bridge is assessed to determine the need for 9 posting, load testing, or repair. 10

11 The state will conduct load testing on occasion, specifically proof testing, of bridges without plans 12 when it is not possible to otherwise provide reliable load ratings. The load ratings are based on 13

the standard vehicle that produces the largest stress and vehicles that produce larger stresses are 14 prohibited from crossing the bridge (9). 15

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Michigan DOT Policy 17 Michigan utilizes the AASHTO Manual guided by engineering judgment to load rate reinforced 18 concrete bridges without plans. The state lists several items that should be considered by the PE 19

including: material properties and design vehicle based on year of construction; measurable 20 structural dimensions; condition of primary members; load path redundancy; changes since 21 original construction; and comparable structures of known design. An inventory and operating 22

load rating factor are determined by a PE based on the controlling vehicle. When more than one 23 vehicle type produces the same live load effects (moment and/or shear), the heavier vehicle is 24

recorded. Regarding load posting of bridges without plans, if the planless bridge has been carrying 25 normal traffic for a decent amount of time, no distress or any serious damage is evident, and the 26

bridge gets inspected regularly insuring it is performing satisfactorily, then the bridge does not 27 need to be load posted (10). 28

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Missouri DOT Policy 30 The procedure for load rating reinforced concrete bridges without plans in Missouri advocates the 31 use of load testing under direction by a PE. Bridges are generally load tested if an accurate loading 32

history of the bridge is not available. The yield stress of the reinforcing steel is assumed to be 33 33 ksi unless the material age is known (such as from a mill test). If the grade of steel is known, load 34 ratings are based on allowable stresses provided in a table (11). The concrete compressive strength 35 is taken as 2,363 psi or less with the inventory and operating allowable concrete compressive 36 stresses also provided in a state specific table. 37

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Oregon DOT Policy 1 An approximate load rating is performed to evaluate concrete bridges without plans in Oregon 2 based on comparisons of simple-span positive live load moments. If the bridge has successfully 3 carried the legal loads without any major problems for a long period of time, the capacity is 4

assumed to be equal to the legal load that produced the largest load effect. Under the assumption 5 that the legal loads procedure a smaller load effect compared to the HS20 load, the HS inventory 6 rating is considered proportional to the legal load effects. The inventory and operating HS ratings 7 are determined based on Equations 1 and 2 shown below. 8 9

𝐼𝑛𝑣𝑒𝑛𝑡𝑜𝑟𝑦 𝐻𝑆 = (20.0) ∗ (𝑀𝐿𝐸𝐺𝐴𝐿

𝑀𝐻𝑆20) ∗ (𝐶𝐹) (Eq. 1) 10

11

𝑂𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝐻𝑆 = (𝐼𝑛𝑣𝑒𝑛𝑡𝑜𝑟𝑦) ∗ (5

3) (Eq. 2) 12

13 where MHS20 is the maximum positive live load moment for 1 lane of HS-20 or HL-93 loading 14 (truck loading only); MLEGAL is the maximum positive live load moment for 1 lane of the Legal 15

Loading; and CF is the condition factor given in Table 2 (12). 16 17

TABLE 2 Condition Factor (12) 18 19

NBI Item 59, Superstructure

Rating Condition Factor CF

5 "Fair Conditon" or better 1.00

4 "Poor Conditon" 0.50

3 "Serious Condition" 0.25

2 "Critical Condition" 0.12

20 Texas DOT Policy 21 Texas follows the State Bridge Inspection Manual to evaluate and load rate reinforced concrete 22

bridges without plans. Initially, the state will perform a bridge inspection and if the bridge shows 23 no signs of structural distress, an HS-15 Inventory Rating and HS-20 Operating Rating are 24 assigned. Subsequently, the state will load post the bridge if it is over 4 years old and the NBI 25 condition rating is less than 5 for Item 58 (Deck) or less than 6 for Item 59, 60, or 62 26

(Superstructure, Substructure, or Channel). Figure 2 summarizes the state procedure for load 27 rating concrete bridges without plans. This procedure may be followed if the five considerations 28 given in Table 3 are met. If structural distress is evident, a structural analysis of the structure is 29

required to determine the load rating (13). Also, if the bridge was designed prior to 1950, the 30 amount of reinforcing steel is based on a comparative design rating (13). 31 32


1 2

Note: *Permit Trucks with gross or axle weights that exceed the state legal load limits will not be 3

allowed to use these bridges. 4 I.F.-Inspection Frequency. 5 Refer to AASHTO Manual for Bridge Evaluation, Chapter 6, Section B. 6

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FIGURE 2 Texas Procedure for Load Rating Concrete Bridges without Plans (13) 8

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TABLE 3 Considerations for Load Rating Concrete Bridges without Plans in Texas (13) 10 11

Considerations that must be met

1 It has been carrying unrestricted traffic for many years

2 There are no signs of significant distress

3

Ensure bridge exhibits proper span-to-depth ratios of the main members, which indicates that the

original design was by competent engineers. In general, this consideration means that for simple

span structures the span-to-depth ratio of main members should not exceed approximately 20.

Span-to-depth ratios exceeding this ratio may indicate that the designer did not properly consider

reasonable design truck loadings.

4 Construction details such as slab thickness and reinforcement cover over any exposed reinforcing

to specifications current at the time of the estimated construction date.

5 Appearance of the bridge shows that construction was done by a competent builder.

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Utah DOT Policy 1 Utah separates the load rating procedures of reinforced concrete bridges without plans into four 2 cases which depend on the available documentation. Case II applies when the available documents 3 specify the design live load. In this case, the rating factors are computed by dividing the maximum 4

factored simple span moment caused by the design vehicle shown on the design plans by the 5 moment caused by the rating vehicle of interest. Case III applies when the design load is unknown 6 and the Average Daily Truck Traffic (ADTT) is less than 10. The rating factors are calculated 7 similarly to Case II using the moment ratios, however, the design vehicle is selected based on the 8 year of construction. In Case IV, advanced calculations and diagnostic load testing is used to 9

determine a load rating for the bridge. Again, the rating factors are estimated based on the moment 10 ratios and the test vehicle is used in place of the design vehicle. The assumed design load is an H-11 15 vehicle for bridges built before 1961, an HS-15 vehicle for bridges built from 1961 to 1970, 12 and an HS-20 vehicle for bridges built after 1970. Case I is used when all other cases do not apply. 13

For this case, the rating factors are calculated by dividing the controlling moment, the maximum 14 factored simple span moment caused by legal load or permit loads, by the factored maximum 15

simple span moment caused by the vehicle. The rating factor determined from Cases I-III are 16 reduced by a condition factor. The condition factor is equal to 1.0 when the superstructure 17

condition rating is 5 or greater. Bridges with a superstructure condition rating of 4 will have a 18 condition factor ranging between 0.6-0.8 and for a rating of 3, the condition factor ranges between 19 0.3-0.5. For bridges with a superstructure condition rating of 2 or less, it is recommended that the 20

bridge be closed for repair or replacement (14). 21 22

LOAD RATING PROCEDURE 23 24

Field Measurements and Concrete Strength 25 Performing a thorough field inspection is key in the load rating of reinforced concrete bridges 26

without plans. Several measurements must be obtained during the inspection to generate the as-27 built drawings and load rate the bridge including the following: span length(s); width of bridge; 28 bearing lengths; slab thickness; steel reinforcement details; bridge skew; curb and railing 29

dimensions; utilities (such as pipelines); pier dimensions; and wearing surface thickness. 30 The measured concrete strength for slab bridges is estimated using a Windsor Probe. Three 31

probes are shot into the concrete and the exposure lengths are measured using a micrometer. A 32 rebar scanner is used to locate the steel and mark the concrete surface so probes may be driven 33

without hitting the reinforcing bars. All three probes must embed into the concrete with an exposed 34 probe measurement of ± 0.33 inches between probes (15). Each probe must be at least 7 inches 35 from any other probe and at least 4 inches from the edge of a concrete surface (15). From the 36 probe testing, aggregates are collected to determine the hardness (i.e., Mohs number) from a 37 Mineral Scratch Test (15). Using the Mohs numbers along with the probe measurements, a 38

measured concrete compressive strength is determined (15). 39 Of the twenty-three bridges evaluated in this project, the Windsor Probe test failed (i.e., 40

required number of probes did not embed) at twelve bridges and therefore, the measured concrete 41 strength could not be estimated in the field. At the remaining eleven bridges, the measured 42 concrete strengths were successfully measured with the Windsor Probe and were significantly 43 higher than the assumed design values ranging from 4,970 psi to 8,230 psi. Most bridges showed 44 a measured concrete compressive strength of 6 ksi or 7 ksi. 45

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Steel Layout 1 2 Hilti Ferroscan 3 The mild steel details for reinforced concrete slabs are determined utilizing a Ferroscan non-4

destructive testing system which measures variations in magnetic resonance caused by the 5 presence of reinforcing steel. A Hilti PS 200 S scanner is used to scan the slab for hidden 6 reinforcement and a PSA 100 monitor is connected to the scanner to display the results (16). 7 Strategic locations are selected for scanning to estimate the rebar size, spacing, cover, and length. 8 Two scanning options are used to determine the reinforcement layout (blockscans and quickscans) 9

which are described in the following paragraphs. 10 11 Blockscans A blockscan consists of a series of 2 ft. X 2 ft. square grids of scanned areas. Using 12 the blockscan option, scans may be uploaded to the monitor as an image to display the 13

reinforcement. The scanner is moved horizontally and vertically across the 2 ft. X 2 ft. square 14 grids and the data from the blockscans are transferred to an Excel file to conduct the analysis and 15

determine the bar size, cover, and the spacing between bars (16). Figure 3 is an example blockscan. 16 17

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FIGURE 3 Bridge Blockscan 20 21 For simple supported slabs, four 6 ft. X 2 ft. blockscans (three consecutive 2 ft. X 2 ft. 22 blockscans) are generally taken on the bottom of the slab at the abutment and at midspan. Two of 23

the 6 ft. X 2 ft. blockscans are taken at the middle of the slab width while the other two blockscans 24 are taken near the slab edge. At locations where the quickscans (discussed in the next section) 25 identify a change in bar spacing between midspan and abutment, additional blockscans are taken 26

in that area to determine the change in the reinforcement layout. 27

For continuous reinforced concrete slab bridges, the same blockscan procedure described 28 above for simple supported slabs is used for the exterior spans. If the exterior spans are identical, 29 only one (the most accessible) needs to be scanned. For the interior span, four 6 ft. X 2 ft. 30 blockscans are taken at midspan on the slab soffit (two at the middle of the width and two near the 31 edge). The top of the continuous slab is measured with two 6 ft. X 2 ft. blockscans taken at each 32

pier location near the shoulder areas. 33 34

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Quickscans A quickscan is another option used to estimate the hidden reinforcement layout. For 1

this option, the scanner is moved in one direction across the slab perpendicular to the reinforcement 2 bars. The quickscan results provide the position and the depth of the reinforcing bars in the scanned 3 area (16). Figure 4 is an example quickscan. 4

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FIGURE 4 Bridge Quickscan 8 9 For simple supported bridges, quickscans are taken at the tenth points along the length on 10

the bottom of the slab (across the entire width). Each quickscan is reviewed to count the number 11 of bars. In cases where the quickscans show a different number of reinforcing bars, extra 12

quickscans are taken to determine where the bar spacing changes. Similarly, for continuous 13 reinforced concrete slabs, quickscans are taken at the tenth points on the bottom of each span and 14 on the top of the slab at and near the pier locations (i.e., directly above the pier, and at 25% and 15

50% of the span length of the adjacent spans). Extra quickscans are also taken to determine the 16 start and end location(s) and the locations where the bar spacing changes for the top reinforcing 17

bars. If the bridge has a wearing surface, quickscans are typically taken close to the curb where 18 there is no wearing surface or a thinner layer is present. For bridges with heavy traffic, quickscans 19

are taken from the edge of the slab to about 25% of the slab width or right before the traffic lane 20 for safety purposes. Figure 5 show the blockscan and quickscan locations for a simple and 21 continuous reinforced concrete slab bridge. 22


1 (a) (b) 2

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FIGURE 5 Scan Locations for (a) Simple and (b) Continuous Slab Bridges 4 5 Estimation of Bottom and Top Reinforcement 6

Blockscans taken on the bottom of a reinforced concrete slab are analyzed to determine bar size 7 and cover averages using Excel. The data for each blockscan consists of a series of bar size 8 measurements as well as the cover for each reinforcing bar in the scanned area. The bar size and 9

cover measurements are then averaged to estimate the bottom reinforcement layout. 10 The estimate of the top reinforcement for continuous slab bridges is done using the same 11

procedure for the bottom reinforcing bars. However, the concrete cover for the top reinforcement 12 is typically larger (as much as three inches or more) which leads to problems estimating the 13

reinforcement bar size in the blockscan. In such cases, the spacing and cover of the top 14 reinforcement are determined using the quickscans and the bar size is estimated using the historic 15

ratio of top to bottom area of steel per foot as described in the next paragraph. 16 The historic top to bottom steel area ratio was investigated through a review of the design 17 details for several continuous slab bridges with plans in the New Mexico bridge inventory. The 18

review included approximately 30 continuous slabs which showed that the top steel area in the 19 negative moment regions is about 10% greater than the bottom steel area in the positive moment 20 region. Thus, in cases where the top reinforcement could not be accurately detected by the rebar 21

scanner, the top steel area at the pier was assumed 10% larger than the bottom steel area at midspan. 22 Using the results from the quickscans taken on the top of the slab along with the steel area estimate, 23 the reinforcement layout (spacing, cover, and bar size) was established for the negative moment 24

region. 25 The yield strength for both bottom and top reinforcing steel bars is determined based on 26

the age of the bridge. Using the AASHTO Standard Specifications for Highway Bridges that 27 pertained to the year the bridge was constructed, the strength of steel is determined. 28

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Top and Bottom Steel Estimation 1

Apart from the bridge scans, the area of steel per linear foot was estimated using Equations 3 2 through 10 in accordance with the AASHTO Standard Specifications for Highway Bridges (17) 3 and based on the age of the bridge. With the steel area estimate along with the scan data and 4

historic top to bottom steel ratios, the reinforcing bar size, spacing and cover can be finalized. 5 6

𝐼 =50

𝐿+125 (Eq. 3) 7

8 𝐸 = (4 + 0.06 ∗ 𝑆) (Eq. 4) 9

10

𝐷𝐹 =𝑊

𝐸 (Eq. 5) 11

12 𝑀𝐷𝐿 = 𝑀𝐷𝐶 + 𝑀𝐷𝑊 (Eq. 6) 13

14

𝑀𝑢 = (ϒ𝐷 ∗ 𝑀𝐷𝐿) + (ϒ𝐿 ∗ 𝑀𝐿𝐿 ∗ (1 + 𝐼) ∗ 𝐷𝐹 ∗1

2) (Eq. 7) 15

16

𝑎 =𝐴𝑠∗𝐹𝑦

0.85∗𝑓′𝑐∗𝑏 (Eq. 8) 17

18

ɸ𝑀𝑛 = 𝑀𝑢 = ɸ ∗ 𝐴𝑠 ∗ 𝐹𝑦 ∗ (𝑑 −𝑎

2) (Eq. 9) 19

20

𝐴𝑠 =

ɸ∗𝐹𝑦∗𝑑 ±√(ɸ∗𝐹𝑦∗𝑑)2−4∗(ɸ∗𝐹𝑦2

1.7∗𝑓′𝑐∗𝑏

)∗𝑀𝑢

2∗ɸ∗𝐹𝑦2

1.7∗𝑓′𝑐∗𝑏

(Eq. 10) 21

22

In the equations above, I represents the impact factor which is based on L, the length of the 23

portion of the span that is loaded to produce the maximum stress in the member. E is the width of 24 the slab over which a wheel load is distributed, which is a function of the effective span length S. 25

The distribution factor DF, is a function of W (slab width of 1 ft) and E. 𝑀𝐷𝐶 is the moment 26

applied by the slab, curbs, railings, and utility loads and 𝑀𝐷𝑊 is the moment applied by the bridge 27

wearing surface. 𝑀𝐷𝐿 is the moment due to the total dead loads (sum of 𝑀𝐷𝐶 and 𝑀𝐷𝑊). 𝑀𝑢 is 28

the factored moment for the bridge due to the applied dead and live loads. ϒ𝐷 and ϒ𝐿 are the load 29

factors applied to the dead and live load moments, respectively. 𝑀𝐿𝐿 is the maximum live load 30 moment produced by the lane load (with concentrated load), tandem, or truck load (typically an 31 HS-20 truck). Fy is the yield strength of the reinforcing steel bars; f’c is the concrete compressive 32 strength; d is the effective depth of the tensile reinforcement; a is the depth of the compressive 33

block; and b is the width of the concrete section. The design moment ɸ𝑀𝑛, is set equal to the 34

factored moment, 𝑀𝑢 and solved for the area of steel, As using Equation 10, for the top and bottom 35 reinforcement. 36 37

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As-Built Drawings and Load Rating Model 1 Based on the field measurements as well as the estimated reinforcement layout, as-built drawings 2 are produced following the AutoCAD as-built template provided by the NMDOT. The drawings 3 provide a structure plan, profile, and typical section view of the bridge. Along with the different 4

bridge views, the plans include important notes and information such as personnel that performed 5 the field measurements, day the measurements were taken, name of individual that prepared the 6 bridge drawings, unusual features (e.g. pipelines and non-standard railing), wearing surface, and 7 material properties. The as-built drawings will also include the inventory and operating ratings 8 (discussed in the paragraph below). 9

Once the as-built drawings are completed, the bridge is modeled using the AASHTOWare 10 BrR software to determine the load ratings for the bridge. The BrR results include the inventory 11 and operating rating factors, the controlling locations, and the limit states. Inventory and operating 12 ratings are determined based on the rating vehicle (typically an HS-20 truck). In cases where the 13

operating rating is below 1.0, a legal load rating is performed under the following federal and state 14 legal loads: Type 3, Type 3-3, Type 3S2, NM 2, NM 3A, NM 3B, NM 4, NM 5A, and NM 5B. 15

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LOAD RATING RESULTS 17 The load rating results for the twenty-three bridges evaluated during this project are shown in 18 Table 4 and includes the inventory and operating rating factors using a nominal concrete 19 compressive strength of 3 ksi and the measured concrete strength from the Windsor Probe testing. 20

Table 5 provides a comparison of the load rating results with the results determined following the 21 Idaho and Oregon DOT procedures for reinforced concrete slab bridges without plans. 22

The increase in the load ratings using the measured concrete compressive strength from the 23 Windsor Probe varied. Some load ratings only increased by about 4% where others significantly 24 increased by approximately 47%. On average, the load ratings increased by about 16% using the 25

Windsor Probe concrete strength. 26

In general, the load rating results estimated following the Idaho and Oregon procedure 27 produced lower values compared to the ratings determined with the approach developed in this 28 project. In some cases, the Idaho and Oregon load ratings were as much as 69% and 62% smaller 29

than the NMSU load ratings, respectively. On average, the Idaho and Oregon results were about 30 35% and 30% smaller compared to the NMSU results, respectively. 31

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TABLE 4 Load Rating Results Based on Concrete Strength 1

2

Bridge

Number

Year

Built

Controlling

Span

Length

Span/Depth

Ratio

Location

(%)

f'c (psi) Rating Factor Rating Factor

Windsor

Probe

Results

f'c = 3 ksi f'c = Windsor Probe

Results

Inventory Operating Inventory Operating

1001 1939 25'-2" 16.78 60 NA 0.83 1.39 NA NA

1002 1939 25'-0" 17.14 60 NA 0.81 1.35 NA NA

1003 1938 20'-3½" 13.16 50 NA 0.90 1.49 NA NA

1005 1938 22'-10" 17.13 50 NA 0.51 0.85 NA NA

2371 1955 13'-0" 17.33 60 8050 0.66 1.10 0.71 (8%) 1.19 (8%)

2391 1955 15'-6" 18.60 40 NA 0.72 1.20 NA NA

2396 1930 14'-6" 27.62 40 NA 0.40 0.66 NA NA

7489 1964 19'-0" 19.00 60 7800 0.75 1.24 1.10 (47%) 1.83 (47%)

7490 1964 27'-5" 25.31 50 6467 0.30 0.51 0.39 (29%) 0.65 (28%)

7492 1966 28'-0" 22.40 60 6925 0.57 0.95 0.68 (19%) 1.13 (19%)

7825 1950 30'-9" 20.50 60 4967 1.56 2.61 1.69 (8%) 2.83 (8%)

8257 1977 15'-10¾" 12.72 60 7800 0.67 1.12 0.94 (40%) 1.58 (41%)

8279 1965 20'-4" 18.07 40 7263 1.47 2.46 1.60 (8%) 2.66 (8%)

8282 1976 33'-½" 22.03 40 NA 1.09 1.82 NA NA

8486 1984 20'-2" 16.13 50 7525 1.30 2.16 1.40 (8%) 2.34 (8%)

8662 1986 40'-0" 21.33 50 6133 0.78 1.30 0.86 (11%) 1.44 (11%)

8760 1985 38'-6" 27.18 50 5800 0.81 1.36 0.90 (11%) 1.51 (11%)

8799 1981 23'-1" 22.61 50 7250 0.42 0.70 0.46 (10%) 0.78 (10%)

8827 1991 38'-2½" 23.51 50 5908 0.94 1.57 0.98 (4%) 1.63 (4%)

9077 1930 10'-10" 14.44 70 7667 0.72 1.20 0.75 (5%) 1.26 (5%)

9122 1998 10'-10" 16.25 40 NA 0.42 0.71 NA NA

9203 1960 10'-10½" 14.50 60 7733 0.65 1.09 0.70 (6%) 1.16 (7%)

9446 1939 26'-4½" 17.58 100 NA 1.07 1.79 NA NA

3 TABLE 5 Load Rating Results Based On Idaho and Oregon DOT Procedure 4

5

Bridge Number

NMSU Procedure Idaho DOT Procedure Oregon DOT Procedure

Inventory Operating Inventory Operating Inventory Operating

1001 0.833 1.392 0.33 (60%) 0.56 (60%) 0.43 (48%) 0.72 (48%)

1002 0.808 1.350 0.50 (38%) 0.84 (38%) 0.87 (7%) 1.45 (7%)

1003 0.895 1.494 0.33 (63%) 0.56 (63%) 0.43 (52%) 0.71 (52%)

1005 0.506 0.845 0.50 (1%) 0.84 (1%) 0.88 (42%) 1.46 (42%)

2371 0.661 1.104 0.64 (3%) 1.06 (4%) 0.76 (13%) 1.26 (13%)

2391 0.718 1.199 0.50 (30%) 0.84 (30%) 0.80 (10%) 1.33 (10%)

2396 0.397 0.663 0.64 (38%) 1.06 (38%) 0.79 (50%) 1.32 (50%)

7489 0.745 1.243 0.64 (14%) 1.06 (15%) 0.85 (12%) 1.42 (12%)

7490 0.302 0.505 0.64 (53%) 1.06 (52%) 0.79 (62%) 1.32 (62%)

7492 0.566 0.945 0.50 (12%) 0.84 (11%) 0.81 (30%) 1.36 (30%)

7825 1.562 2.609 0.50 (68%) 0.84 (68%) 0.79 (49%) 1.32 (49%)

8257 0.672 1.121 0.50 (26%) 0.84 (25%) 0.81 (17%) 1.34 (17%)

8279 1.471 2.457 0.50 (66%) 0.84 (66%) 0.85 (42%) 1.42 (42%)

8282 1.092 1.824 0.50 (54%) 0.84 (54%) 0.79 (28%) 1.32 (28%)

8486 1.295 2.163 0.64 (51%) 1.06 (51%) 0.85 (34%) 1.42 (34%)

8662 0.773 1.291 0.64 (17%) 1.06 (18%) 0.77 (0.1%) 1.29 (0.1%)

8760 0.813 1.358 0.64 (21%) 1.06 (22%) 0.78 (4%) 1.30 (4%)

8799 0.421 0.703 0.64 (34%) 1.06 (34%) 0.88 (52%) 1.46 (52%)

8827 0.937 1.565 0.50 (47%) 0.84 (46%) 0.78 (17%) 1.30 (17%)

9077 0.717 1.197 0.64 (11%) 1.06 (11%) 0.71 (2%) 1.18 (2%)

9122 0.424 0.707 0.50 (15%) 0.84 (16%) 0.71 (40%) 1.18 (40%)

9203 0.654 1.092 0.64 (2%) 1.06 (3%) 0.71 (8%) 1.18 (7%)

9446 1.070 1.786 0.33 (69%) 0.56 (69%) 0.42 (61%) 0.70 (61%)

6 7


SUMMARY AND CONCLUSIONS 1 Many states are implementing field testing and analytical methods to determine the load carrying 2 capacity of bridges without plans. A simple procedure was developed for New Mexico to load 3 rate reinforced concrete slab bridges without plans and included obtaining field measurements, 4

estimating the material properties based on the age of the bridge and Windsor Probe testing, and 5 approximating the reinforcement layout utilizing a rebar scanner and structural analysis 6 techniques. 7

Overall, the rebar scanner was found to be effective for determining the bar size, spacing 8 and cover of bottom steel reinforcement. However, to determine the top reinforcement layout, 9

specifically in the negative moment regions, the scanner is not as effective due to the large concrete 10 cover in these areas. The AASHTO based method and the assumption that the top steel area at the 11 pier is 10% larger than the bottom steel area at midspan provide a good estimate of the top 12 reinforcement in cases where the steel cannot be measured with the scanner. The Windsor Probe 13

testing results showed significantly larger concrete compressive strengths compared to assumed 14 design values. Thus, the final load rating models submitted to the NMDOT are conservative due 15

to the use of 3 ksi nominal concrete compressive strength. 16 This project illustrated that load ratings assigned to a bridge without the use of 17

experimental and theoretical analysis may be overly conservative and may not realistically 18 represent a bridge’s load carrying capacity. The use of basic non-destructive testing equipment 19 along with the implementation of simple structural analysis techniques, proved to be an effective 20

method for estimating the load carrying capacity of planless concrete slab bridges. 21

22 REFERENCES 23 24 1. American Association of State Highway and Transportation Officials (AASHTO). Manual 25

for Bridge Evaluation, 2nd Edition, Washington, D.C, 2011. 26

2. Aguilar, C. Load Rating a Prestressed Concrete Double-Tee Beam without Plans by Field 27 Testing. In Transportation Research Record: Journal of the Transportation Research Board, 28 No. 1302, Transportation Research Board of the National Academies, Washington, D.C., 29

2015, pp. 90-99. 30 3. Huang, J., Shenton, H.W. Load Rating Concrete Bridge without Plans. Structures Congress, 31

American Society of Civil Engineers, University of Delaware, Newark, Delaware, 2010. 32 4. Eitel, A., Huckelbridge, A., Capaldi, N. Development of a Load Test for the Evaluation and 33

Rating of Short-Span Reinforced Concrete Slab Bridges. Federal Highway Administration-34 Ohio Department of Transportation, Case Western Reserve University, Cleveland, Ohio, 35 2002. 36

5. Taylor, Z., Amini, O., and van de Lindt, J.W. Approach for Establishing Approximate Load 37 Carrying Capacity for Bridges with Unknown Design Properties. Colorado State University. 38

Fort Collins, Colorado, 2011. 39 6. Cuaron, A. A Procedure for Load Rating Reinforced Concrete Slab Bridges without Plans. 40

MS Thesis in Civil Engineering, New Mexico State University, Las Cruces, New Mexico, 41 2016. 42

7. Potter, W., Florida Department of Transportation. Load Rating Bridges with No As-Built 43 Plans Questionnaire. New Mexico State University, Las Cruces, New Mexico, 2014. 44

8. Bridges with Unknown Structural Components. Idaho Manual for Bridge Evaluation, 45 Section 6.1.4. Idaho, 2013. 46


9. Maryland Department of Transportation. Load Rating Bridges with No As-Built Plans 1

Questionnaire. New Mexico State University, Las Cruces, New Mexico, 2014. 2 10. Chynoweth, M.J., Michigan Department of Transportation. Load Rating Bridges with no As-3

Built Plans Questionnaire. New Mexico State University, Las Cruces, New Mexico, 2014. 4

11. Missouri Department of Transportation. Live Load Distribution Factors and Load Testing. 5 Missouri, 1994. 6

12. Oregon Department of Transportation. ODOT LRFR Manual, Section 8. Oregon, 2013. 7 13. Texas Department of Transportation. Load Rating Bridges with No As-Built Plans 8

Questionnaire. New Mexico State University, Las Cruces, New Mexico, 2014. 9

14. Utah Department of Transportation. Method for Rating Factors. Utah, n.d. 10 15. American Society for Testing and Materials (ASTM). Standard Test Method for Penetration 11

Resistance of Hardened Concrete. West Conshohocken, Pennsylvania, 2010. 12 16. Hilti Corporation. Hilti PS250/200-S. Germany, 2012. 13

17. American Association of State Highway and Transportation Officials (AASHTO). Standard 14 Specifications for Highway Bridges, 17th Edition, Washington, D.C, 2002. 15

16

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