_Evaluation of High-early Strength PCC Mixtures Used in Full Depth Repairs

Evaluation of high-early strength PCC mixtures usedin full depth repairs

Neeraj Buch a,*, Thomas J. Van Dam b, Karl Peterson b, Larry Sutter c

a Department of Civil and Environmental Engineering, 3556 Engineering Building, Michigan State University, E. Lansing, MI 48824, USAb Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA

c School of Technology, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA

Received 15 March 2006; received in revised form 31 July 2006; accepted 25 September 2006Available online 21 November 2006

Abstract

The research described herein was conducted as a part of NCHRP Project 18-04B, Early Opening-to-Trac Portland Cement Concretefor Pavement Rehabilitation [Van Dam TJ. Durability of Early-Opening-to-Trac portland cement concrete for pavement rehabilita-tion. Final Report, NCHRP Web Document 76, Transportation Research Board, National Research Council, Washington, DC; 2005.].In the study fourteen dierent high-early strength portland cement concrete (PCC) full-depth repair mixtures (two replicates or batcheswere prepared for each mixture for a total of 28 batches) were designed for a 68 h opening to trac window. The fresh and hardenedconcrete properties, freeze-thaw durability and microstructural properties of the concrete were assessed as part of the laboratory testingprogram. As a result of this assessment it was determined that high-early strength PCC mixtures of adequate strength could be produced,but that interactions between the various constituents could result in durability problems in some mixtures, particularly those made withType III cement and high-range water reducers. It is recommended that durability-related testing be conducted on such mixtures toensure longevity of the repair.! 2006 Elsevier Ltd. All rights reserved.

Keywords: Microstructure; Freeze-thaw durability; Fast track; High-early strength

1. Introduction

The research described herein was conducted as a partof NCHRP Project 18-04B, Early Opening-to-Trac Port-land Cement Concrete for Pavement Rehabilitation [1]. Withincreasing trac volume in urban areas, delays duringpavement rehabilitation are becoming less tolerated. Tominimize delays, state highway agencies (SHAs) useearly-opening-to-trac (EOT) rehabilitation strategiesthat allow work to be completed at night or during periodsof low trac. Generally, portland cement concrete (PCC)used in these applications is expected to gain sucientstrength to carry trac within 624 h after placement.But concerns have been raised that such high-early strengthmaterials lack durability, resulting in the need for addi-

tional lane closure to replace failing repairs. Rigorousrequirements for mix design and strength developmenthave usually been stipulated for EOT concrete applica-tions, often with limited consideration given to materialsand construction aspects that influence long-term perfor-mance and durability.

The main dierence between high-early strength andnormal repair concrete mixtures is that strength gain mustoccur much more rapidly in the former, thus more cement,less water, a cocktail of admixtures, and aids to retain heatare commonly used. Most SHAs allow the use of eitherType I or Type III portland cement in their high-earlystrength PCC mixtures. When specified, the minimumcement contents varies from state to state, ranging from440 to 550 kg/m3 (740925 lb/yd3) for Type I cement and390490 kg/m3 (660825 lb/yd3) for Type III cement. Inaddition, the use of accelerators is almost universally spec-ified for mixtures containing Type I cement, with the most

0950-0618/$ - see front matter ! 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.conbuildmat.2006.09.006

* Corresponding author. Tel.: +1 517 432 0012; fax: +1 517 432 1827.E-mail address: [email protected] (N. Buch).

www.elsevier.com/locate/conbuildmat

Available online at www.sciencedirect.com

Construction and Building Materials 22 (2008) 162174

Constructionand Building

MATERIALS

Jonathas Iohanathan Felipe de Oliveira

common accelerator being calcium chloride. Some mix-tures specified with Type III cement do not require theuse of an accelerator, relying solely on the early strengthgain characteristics of the cement. The water-to-cement(w/c) ratio for the high-early strength PCC mixtures varieswidely, with maximum allowable values ranging from 0.33to 0.49.

Based on a review of several SHAs construction speci-fications it was found that the acceptability of a high-earlystrength PCC mixture is commonly linked exclusively tostrength requirements. For example, the time to openingcriteria used by SHAs varied from as early as 4 h (Kansasand Ohio) to as late as 12 h (Maryland and Minnesota). Insome cases, only a time to opening criteria was specified,but in others, strength was used as the sole criteria foropening. A few SHAs used both a strength and time toopening criteria. Florida for example specifies that thecompressive strength must exceed 21 MPa (3000 psi) in24 h while allowing a 6 h time to opening. In the case ofNew York, the repair was opened to trac once the surfacetemperature of the repair reached 65 "C (150 "F).

Research conducted under the Strategic HighwayResearch Project (SHRP) found that an opening criteriausing a third-point modulus of rupture of 2.1 MPa(300 psi) or a compressive strength of 13.8 MPa(2000 psi) was reasonable [2]. Recommendations made bythe American Concrete Pavement Association (ACPA)are not based on strength, but instead on opening time[3] and in the NHIs PCC Pavement Evaluation and Reha-bilitation guidelines, both minimum strength requirementsand time to opening requirements are recognized as beingacceptable [4]. The flexural strength criteria dier slightlyfrom that presented by SHRP, with a requirement of1.7 MPa (250 psi) for third-point loading and 2.1 MPa(300 psi) for center-point loading.

In this paper the authors report on an extensive labora-tory study in which 14 dierent high-early strength PCCmixtures (two replicates or batches were made for eachmixture for a total of 28 batches) designed for 68 h open-

ing were produced and tested. The testing included assess-ment of the properties of the fresh and hardened concrete,freeze-thaw durability, and the microstructural character-ization of the concrete.

2. Experimental program

2.1. Experimental matrix

To evaluate the properties of the high-early strengthPCC mixtures a factorial experimental design approachwas adopted, allowing for two or more levels to be setfor each based on knowledge of concrete technology as itpertains to high-early strength PCC mixtures, concentrat-ing on variables of greatest interest. The material combina-tions are presented in Table 1, where an alpha-numericdesignation (A-1 through N-14) was assigned to eachmixture.

The seven independent variables considered in the studyhave either 2 or 3 levels. In total, 288 material combina-tions are possible, of which 14 material/curing combina-tions were evaluated. Two batches were made for eachmixture. Thus, a total of 28 batches of PCC mixtures weremade and evaluated. The ranges for the seven independentvariables were carefully selected after considering the infor-mation contained in the NCHRP 18-04A Final Report [5],numerous published articles, specifications used by variousSHAs in the construction of high-early strength PCC repairmaterials. The first six variables (cement type, cement fac-tor, w/c ratio, coarse aggregate type, accelerator type, andwater reducer type) are justified for inclusion in this studyas each is a common variable in the mix design/proportion-ing process, as is evidenced by their inclusion in most SHAspecifications. Further, each is suspected to have a directeect on the microstructure of portland cement concreteand must be considered when studying the durability ofEOT PCC materials.

The justification for the inclusion of the final variable,curing, was thought to potentially have a significant impact

Table 1Summary of high-early strength PCC mixture designs used in laboratory study

Combination 1 2 3 4 5 6 7

Cement type Cement factor w/c ratio Coarse aggregate Accelerator Water reducer Curing

A-1 Type I 425 kg/m3 (716 lb/yd3) 0.4 Carbonate Non-chloride No 23 "C (73 "F)B-2 Type I 525 kg/m3 (885 lb/yd3) 0.4 Carbonate Non-chloride No 23 "C (73 "F)C-3 Type I 525 kg/m3 (885 lb/yd3) 0.4 Carbonate Non-chloride No Heated blanketD-4 Type I 525 kg/m3 (885 lb/yd3) 0.4 Siliceous Non-chloride No Heated blanketE-5 Type I 425 kg/m3 (716 lb/yd3) 0.4 Siliceous Non-chloride No 23 "C (73 "F)F-6 Type I 525 kg/m3 (885 lb/yd3) 0.4 Carbonate Non-chloride No 23 "C (73 "F)G-7 Type I 525 kg/m3 (885 lb/yd3) 0.4 Carbonate Non-chloride Type F 23 "C (73 "F)H-8 Type I 525 kg/m3 (885 lb/yd3) 0.4 Carbonate No Type E 23 "C (73 "F)I-9 Type I 425 kg/m3 (716 lb/yd3) 0.4 Carbonate Calcium chloride No 23 "C (73 "F)J-10 Type I 525 kg/m3 (885 lb/yd3) 0.4 Carbonate Calcium chloride No 23 "C (73 "F)K-11 Type III 425 kg/m3 (716 lb/yd3) 0.4 Carbonate Non-chloride Type F 23 "C (73 "F)L-12 Type III 525 kg/m3 (885 lb/yd3) 0.4 Carbonate Non-chloride Type F 23 "C (73 "F)M-13 Type III 525 kg/m3 (885 lb/yd3) 0.4 Carbonate Non-chloride Type F Heated blanketN-14 Type III 425 kg/m3 (716 lb/yd3) 0.4 Carbonate Calcium chloride Type F 23 "C (73 "F)

N. Buch et al. / Construction and Building Materials 22 (2008) 162174 163

















on the durability of EOT PCC repairs. It has been welldemonstrated in field and laboratory studies that manyof these PCC mixtures obtain relatively high temperatures(in excess of 65 "C [150 "F]) in the first few hours afterplacement due to the high heat of hydration that occurs.This could lead to thermally induced stress and/or possiblealteration of the primary hydration products, negativelyaecting durability.

2.2. Specimen preparation

Laboratory specimens were prepared in accordance withAASHTO T 126 at the Michigan Department of Transpor-tations (MDOT) Construction and Technology Labora-tory in Lansing, Michigan. The fresh concrete was mixedin a 0.4 m3 (14 ft3) capacity Lancaster mixer, ensuring thatall specimens needed for a given mixture were produced ina single batch, thereby reducing experimental variability.As noted, two batches were made of each material combi-nation. Furthermore, the order in which batches were pre-pared was randomized to avoid systematic error. Table 2lists the number of specimens that were fabricated fromeach batch and how they were tested, including cylinders,beams, prisms, and rings.

All coarse aggregates were sieved into standard size frac-tions and then combined to the desired grading meetingASTM C33 specifications. The aggregate volume fractionhad to change for the various mixtures as cement factorand w/c ratio varied, but the proportion of coarse aggre-gate to fine aggregate was held constant. The aggregateswere brought to SSD condition prior to mixing to ensureaccuracy in the resulting w/c ratio. Molds were strippedfrom the specimens 6 h after casting.

2.3. Tests conducted on laboratory prepared specimens

Once the specimens were molded, various tests wereconducted to assess the characteristics of interest. The sametesting procedures were followed for all mixtures. Asshown in Table 2, the material characterization tests wereclassified as follows:

! Properties of fresh concrete.! Freeze-thaw durability.! Microstructural characterization.

2.4. Properties of fresh concrete

Testing of fresh concrete was conducted in accordancewith the relevant test methods, and the results used to ver-ify workability, air content, and establish maturity trends.This testing was necessary to ensure that the mixtures pro-duced and tested could be constructed, and thus have prac-tical application. The properties of fresh concrete measuredfor each mixture include the slump (AASHTO T 119), aircontent (AASHTO T 152), unit weight (AASHTO T121), and maturity (ASTM C 1074). The measured air con-tents were also compared to the air-void system parametersobtained from the hardened concrete as determined byASTM C 457, as it is known that a specified air contentof fresh concrete does not necessarily equate to adequateair-void system parameters to protect against damageincurred due to freezing and thawing. The establishedmaturity trends are also valuable in that they provide abasis for early opening criteria, as well as an understandingof the heat of hydration characteristics for the variousmixtures.

2.5. Freeze-thaw durability

Two freeze-thaw durability tests were conducted onspecimens that were cured for 28-days. AASHTO T 161Procedure A was used to assess the durability of concretesubjected to cyclic freezing and thawing. Specimens weretested to 300 cycles in strict accordance to the temperaturecycling regime specified in the test method and the dilationwas measured.

Of almost equal value is that the microstructure of thetest specimens was characterized after testing. The degreeof microcracking in the AASHTO T 161 specimens wasassessed. This permitted evaluation of whether significantalterations attributable to the environmental conditioning

Table 2Test details associated with the laboratory evaluation including number of specimens required for each batch

Test attribute Test name/Equipment Measured property No. of specimens Specification

Properties of freshconcrete

Slump Concrete workability One per batch AASHTO T 119-93Air content Total air content of fresh concrete One per batch AASHTO T 152-93Maturity and time of setting Time of curing and temperature of

strength gain/time and depth2 ASTM C 1074/

AASHTO T131-93Freeze-thaw

durabilityResistance of concrete to freezingand thawing

Loss of concrete stiness due to freeze-thaw damage

2 AASHTO T 161-93

Microstructuralcharacterization

Compressive strength Concrete strength 8 AASHTO T 22-92Flexural strength Concrete flexural strength 4 AASHTO T 97-86Stereo optical microscopy Air-void system parameters 2 ASTM C 457 and C

856Petrographic optical microscopywith UV dye impregnation

Microstructure analysis, reactionproducts, and evidence of deterioration

Same specimens asstereo microscopy

ASTM C 856

Scanning electron microscopy(SEM)

Microstructure analysis, elementalanalysis

Same specimens asstereo microscopy

ASTM C 856

164 N. Buch et al. / Construction and Building Materials 22 (2008) 162174

occurred. This is important since identification of the fail-ure mechanism is necessary to address any deficienciesand to verify that the distress observed was indeed due tofreezing and thawing.

2.6. Microstructural characterization

Five separate test methods were conducted under themicrostructural characterization test attribute. The firsttwo tests are compressive strength (AASHTO T 22) andflexural strength (AASHTO T 97). Compressive strengthtesting was conducted for the mixtures at 6 h, 8 h, 24 h,and 28 days. Flexural strength testing was conducted at6 h and 8 h.

Microscopic evaluation was conducted to gain anunderstanding of how the microstructure varied from mix-ture to mixture and to determine how the microstructuralfeatures impacted material performance. In this way,improved knowledge on how to produce high-earlystrength PCC mixtures having a durable microstructurewas obtained. Further, the results of the microstructuralcharacterization were correlated, to the degree possible,with the freeze-thaw durability testing in an attempt to linkthe results of simple laboratory tests to the more advancedtesting.

Specifically, ASTM C 457 was used to determine the air-void system parameters from polished slabs observed usingthe stereo optical microscope. The air-void system param-eters include the air content, spacing factor, specific sur-face, and paste to air ratio. Petrographic evaluation inaccordance with ASTM C 856 was also conducted on thinsections. Due to the nature of petrographic analysis, a sub-jective rating system was developed to rate the degree ofpaste inhomogeniety (on a scale of 1 to 3, with 1 beinghomogenous and 3 being highly inhomogenous) and degreeof microcracking (on a scale of 1 to 3, with 1 being free ofmicrocracks and 3 indicating a high degree of microcrack-ing). As described, beams that had been subjected toAASHTO T 161 were sectioned and the degree of micro-cracking measured (reported as l/mm2) using a high-reso-lution flatbed scanner.

3. Laboratory test results and discussion

3.1. Properties of fresh concrete

Table 3 summarizes the descriptive statistics associatedwith fresh concrete tests. The target slump for the PCCmixtures was 150 mm. With the exception of mixture G-7, which has slump values in excess of 250 mm (10 in.),all other mixture resulted in acceptable workability.Mixture G-7 was a high cement content (525 kg/m3), highw/c ratio (0.40), calcium chloride accelerated mixture witha Type F HRWR, making it dicult to create the desiredmixture with a lower slump. The fresh concrete unit weightranged from 2.15 to 2.30 Mg/m3. The target air content forvarious mixtures ranged from 4.5% to 7.5%.

The distribution of the laboratory measured air contentof fresh concrete for the various mixtures is shown inFig. 1. For the most part, the measured air contents werewithin the desired range with a few notable exceptions.One mixture that was a bit problematic, having air contentslightly higher than desired was E-5, which was a lowcement content (425 kg/m3), high w/c ratio (0.40) mixturemade with a non-chloride accelerator. Of greater concernto durability was that three of the eight mixtures made withType III cement had low measured fresh concrete air con-tent. All mixtures containing Type III cement were madeusing a Type F HRWR since the fineness of the cementmade it impossible to adequately mix these mixtures with-out it. The high and low values for air observed in the mix-tures contributed to overall higher variability as reflected inthe coecient of variability for this parameter (Table 3).

The maturity results followed expected trends. However,it should be noted that some of the mixtures (C-3, D-4, andM-13) were cured at higher ambient temperatures (65 "C),contributing to a noticeable increase in maturity at a givenage. Within a category, there are little noticeable dierencesdue to dierent cement content, w/c ratio, accelerator type,or type of cement.

3.2. Freeze-thaw durability

The resistance of concrete to damage due to freezing andthawing (AASHTO T161) test results indicate that the dila-tion values varied greatly from mixture to mixture. In gen-eral, approximately 20% of the specimens exceeded thethreshold of 0.02 mm/mm dilation. Most of these speci-mens were made with Type III cement, which would ini-tially suggest that the cement type might play a role. Butsince all of the Type III cement mixtures contained theType F HRWR as well, this conclusion cannot be drawndirectly. Furthermore, mixture N-14 made with Type IIIcement did not show exceptionally high dilations. This par-ticular mixture contained the calcium chloride acceleratorinstead of the non-chloride accelerator. The distributionof average dilation for the fourteen mixture designs is illus-trated in Fig. 2. The negative dilations indicate that someof the specimens actually experienced small levels ofshrinkage. This is normal, indicating that no damage hasoccurred and that the durability factor in these cases wouldbe 100.

The argument that the Type F HRWR may be contrib-uting to high dilation values is supported by the results

Table 3Descriptive statistics of the fresh concrete properties

Mixture property Average Standarddeviation

Coecient ofvariation (%)

Slump (mm) 115.20 66.12 57.40Air content (%) 5.66 1.02 18.10Maturity 8 h 385.8 68.22 17.70

24 h 1160.4 262.54 22.6Unit weight

(Mg/m3)2.186 0.05 2.27


from mixture G-7-A, which is the only mixture made withType I cement and the Type F HRWR and the non-chlo-ride accelerator. The dosage of the Type F admixturewas higher for G-7-A than G-7-B (G-7-B was made at alater date, and the admixture dosage was reduced to tryto correct the high slump), whereas the dosage of air entrai-ner was increased (both G-7-A and G-7-B had measuredair contents of 5.2%). Similar results were observed forone of the Type III mixtures where the first replicate (K-11-A) was made with a higher HRWR dosage and lowerair entrainer dosage than the second replicate (K-11-B).This suggests that an interaction between the variousadmixtures may have negatively aected the air-void sys-tem in these high cement content, low w/c ratio mixtures.

3.3. Microstructural characterization

The microstructural characterization of the mixturesincluded compressive and flexural strength testing, stereo

optical microscopy, petrographic microscopy, electronmicroscopy, and scanning with a flatbed scanner to assesscrack length in specimens that were subjected to freeze-thaw testing.

Fig. 3a and b summarizes the strength variability withina batch of concrete mixture design. The descriptive statis-tics (average and standard deviation) were computed basedon four replicates. A summary of the strength data fromthe various PCC mixtures is presented in Figs. 4a throughc. The initial design criteria included a minimum compres-sive strength of 13.8 MPa (2000 psi) and a flexural strengthof 2.1 MPa (300 psi) within the 68 h opening time criteria.Of the 28 batches 19 did not achieve the desired 6-h com-pressive strength, with three of the mixtures that achievedthe desired early strength undergoing high temperaturecuring. This demonstrates the importance of increased tem-perature in achieving early strength. Interestingly, the fourbatches cured at laboratory ambient temperatures thatmade the minimum compressive strength criteria were from

-0.02

-0.01

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0.07

A-1

-A

B-2

-A

C-3

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D-4

-A

E-5-

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F-6-

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G-7

-A

H-8

-A

I-9-A

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K-1

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Mixture ID

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dila

tion

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m/m

m

Replicate 1 Replicate 2 Dilation threshold

Fig. 2. Average dilation for the high-early strength mixtures.

0

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A-1 B-2 C-3 D-4 E-5 F-6 G-7 H-8 I-9 J-10 K-11 L-12 M-13 N-14Mixture ID

Avg.

% a

ir co

nten

t

Fig. 1. Air content for the high-early strength mixtures (dashed lines indicated desired target range).


two mixtures made with Type I cement, one of which waslow cement content, high w/c ratio and the other highcement content, low w/c ratio. Within 8 h, only five batchesfrom three mixtures (all made with Type I cement) did notmeet the minimum compressive strength requirement. Ofthe mixtures that failed to meet the minimum compressivestrength criteria within 8 h, one (G-7) used a Type Eadmixture whereas the other (H-8) used a Type F admix-ture. The fifth batch (J-10-B) had unexpectedly lowstrengths.

The flexural strength results were slightly better, with 12of the 28 batches passing the 6-h criteria of 2.1 MPa(300 psi). These were the same mixtures that passed thecompressive strength criteria, with the addition of bothbatches from mixture K-11 and one from L-12. These lattertwo mixtures were made with Type III cement. Only fourbatches did not meet the flexural strength criteria at 8 h,three of which were from mixtures G-7 and H-8, and onewas J-10-B, which is the same batch that had low compres-sive strengths.

These results indicate that meeting high early strengthcriteria is not assured at 6 h, although the heat generatedduring hydration assists in this process. Further, admix-tures can have a significant eect on early strength gain,with the type of water reducer and accelerator playing arole. Ultimately, only a few mixtures had diculty inachieving the desired strength within 8 h, and these wereprimarily mixtures made with Type I cement and a waterreducer.

Fig. 4c shows the relationship between early age com-pressive and flexural strengths for all mixtures. It is recom-mended that if the concrete compressive strength is to beused during construction to estimate flexural strength, therelationship should be established for the actual job mix.

Stereo optical microscopy was used in accordance withASTM C 457 to collect air-void system parameters thatrelate to the ability of the concrete to resist damage fromcyclic freezing and thawing in a saturated state. The desiredair content was 6 1.5%. Based on ASTM C 457, theresults of which are included in Table 4, the average

Fig. 3. Concrete mixture strength variability (sample size = 4) (a) Concrete mixture variability Compressive strength (b) Concrete mixture variability Flexural strength.


measured air content of the hardened concrete was 5.64%.In Fig. 5, the dashed horizontal lines indicate the desiredrange for air content. In general, most mixtures had aircontents that fell within the desired range. The most obvi-ous deviation was in three of the four mixtures made withType III cement (L-12, M-13, and N-14), which had aircontents significantly below the desired value and mixturesD-4, E-5 and K-11 exhibit air contents greater than 7.5%.However, the lower air voids are typically more criticalto the concrete freeze-thaw durability, whereas higher aircontent can impact strength.

In general, the total air content of a mixture is notthought to be nearly as important in protecting the pasteagainst freeze-thaw damage as is the size and spacing ofthe entrained air bubbles. The most common parameterused in North America to assess the spacing of air bubblesis the spacing factor, which is the average distance from apoint in the paste to the surface of the nearest air bubble.As shown in Table 4, the average spacing factor is0.1591 mm (0.006263 in.). The average value is below theASTM C 457 criterion set for protecting the paste againstdamage (0.200 mm [0.008 in.]). Further, it can be seen inFig. 6 that all of the spacing factors exceeding the limitingcriterion (as indicated by the dashed horizontal line) are inmixtures made with Type III cement and the Type FHRWR, even though one batch made with Type III cementand Type F HRWR had acceptable spacing factors (K-11-B). These findings further illustrate that the combination ofconstituents, the fine cement, and the admixtures used

made it dicult to create the desired air-void system. Inter-estingly, most of these mixtures had acceptable fresh aircontents (although not all), illustrating the need to betterassess the air-void system in fresh concrete for such mix-tures. Fig. 7a illustrates the relationship between air bubblespacing factor and dilation and Fig. 7b illustrates the rela-tionship between air bubble spacing and the correlatedASTM C 666 durability factor (DF) based on the followingrelationship developed and used by the Michigan Depart-ment of Transportation:

DF e0:01630:2772%dilation2

0:02016

After stereo optical microscope evaluation, thin sectionswere made to assess the microstructural characteristics ofthe concrete. This evaluation was conducted on specimensthat were kept at laboratory ambient conditions for longerthan 56 days. Those made strictly for petrographic analysiswere initially moist cured at 23 "C for seven days. Using thepetrographic microscope, the paste homogeneity andmicrocracking was assessed on a scale of 13 for eachbatch. High levels of inhomogeneity indicate that the den-sity of the hydrated cement paste varies due to diculties inuniformly dispersing the cement grains and/or in obtaininguniform hydration. The results of this analysis are shown inTable 4. The mixtures rated with the highest degree ofinhomogeneity were those made with Type III cement,although some batches made with Type III cement hadgood paste uniformity (e.g. N-14-A). In comparing

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xu

rals

tren

gth,

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.

8-hr tes ts 6-hr tes ts Linear (6-hr tes ts ) Linear (8-hr tes ts )

a b

c

Fig. 4. Measured compressive and flexural strengths for the various high-early strength mixtures. (a) Average compressive strength data for the high-earlystrength mixtures (dashed line indicates the opening criteria). (b) Average flexural strength data for the high-early strength mixtures (dashed line indicatesthe opening criteria). (c) Early age comparison between compressive and flexural strength results.


mixtures made with Type I cement, the eect of the Type FHRWR is observed in that mixture G-7 had greater pasteuniformity than B-2. Similar benefits were not noted withthe Type E water reducer, where moderate inhomogeneitywas observed both in the 68 h mixture (H-8).

A small degree of microcracking is a common feature inconcrete, and thus is not in and of itself of great conse-quence. A rating of 1 indicated an absence of micro-cracking. Moderate microcracking (rating of 2),although not desirable, is thought not to indicate a prob-lem with the mixture and would not be assumed to have agreat impact on the durability of the concrete. But thesevere microcracking (rating of 3) noted in all of thesemixtures is out of the ordinary, and may be an indicatorof potential problems in the future for these high earlystrength materials.

These same thin sections were evaluated using theESEM in backscatter electron mode. As was seen usingthe stereo optical microscope, very dierent air-void sys-tems were observed between the mixtures prepared usinga Type I cement and those with a Type III cement, withair bubbles being far more abundant and smaller in theType I cement mixtures. This observation is illustrated inFig. 8. It is also observed that the hydrated cement pastewas more uniform in the Type III cement mixtures, primar-ily a result of the finer cement, which left fewer unhydratedcement grains. The SEM images also revealed that thehydrated cement paste in mixtures made with the non-chlo-ride accelerator was distinctly more uniform than that inthe mixtures made with calcium chloride.

The final microstructural observation was the estimationof crack length per unit area of concrete subjected tofreeze-thaw testing (AASHTO T 161) as summarized inTable 4. It is observed that significant microcracking waspresent in the high-early strength concrete mixtures. Thisis not surprising as significant microcracking was originallyobserved in the mixtures using petrographic means.

Table 4Summary of microstructural assessment of high-early strength mixtures

Mixture Stereoopticalmicroscopeobservations

Petrographicmicroscoperatings

Crackingin freeze-thawbeams(lm/mm2)

Percentair

Spacingfactor(mm)

Pastehomogeneity

Microcracking

A-1-A 5.04 0.1519 2 3 15.670A-1-B 4.68 0.1252 2 3 0.546B-2-A 5.33 0.1427 2 3 33.713B-2-B 5.55 0.1383 2 3 21.368C-3-A 6.48 0.1225 2 3 52.232C-3-B 6.12 0.1206 2 3 42.735D-4-A 9.94 0.1393 2 3 12.821D-4-B 5.91 0.1447 1 3 15.195E-5-A 7.42 0.1360 2 3 12.583E-5-B 9.44 0.1091 2 3 4.036F-6-A 4.47 0.1774 1 3 33.476F-6-B 5.26 0.1584 1 3 17.094G-7-A 6.34 0.1570 1 3 30.627G-7-B 6.48 0.1401 1 3 29.677H-8-A 6.20 0.1103 2 3 23.742H-8-B 4.03 0.1179 2 3 32.051I-9-A 4.54 0.1075 1 3 8.072I-9-B 4.47 0.1121 2 3 8.072J-10-A 5.55 0.1178 1 3 30.627J-10-B 5.91 0.1171 1 3 28.965K-11-A 6.20 0.2320 2 3 17.094K-11-B 9.65 0.1364 2 3 3.561L-12-A 3.67 0.2867 2 3 26.353L-12-B 4.39 0.1997 3 3 41.311M-13-A 4.25 0.2398 3 3 42.735M-13-B 3.89 0.2298 3 3 53.656N-14-A 3.60 0.2585 1 3 0.000N-14-B 3.17 0.2252 2 3 16.144

Average 5.642 0.15908 1.8 3 23.3627St. Dev 1.7624 0.051278 0.63 0 15.00881COV 31.2% 32.2% 35.3% 0 64.2%

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

A-1 B-2 C-3 D-4 E-5 F-6 G-7 H-8 I-9 J-10 K-11 L-12 M-13 N-14

Mixture ID

AST

M C4

57 m

easu

red

air,

%

Fig. 5. Air content measured using ASTM C 457 (dashed lines indicated desired target range).


Microcracking correlated in an imprecise way to the mea-sured dilation values, but when cracking is plotted againstcement factor for all mixtures as in Fig. 9, it is easilyobserved that more cracking is measured in higher cementfactor mixtures. This observation likely reflects that there ismore paste present in such mixtures thus more cracking,but it may also suggest high cement content mixtures aremore susceptible to microcracking.

4. Statistical analysis

To study the eect of each individual factor (single fac-tor eects) on a test result, all other mixture factors must bekept constant. Any resulting dierence in the test result canthen be attributed to the variation of the factor understudy. For example, to study the eect of an increase ordecrease in a mixtures cement factor, test results for mix-

tures A and B can be compared. Both used Type I cement,have a 0.40 w/c ratio, contain carbonate coarse aggregate,utilized a non-chloride accelerator with no water reducerand were cured at 23 "C. The only dierence between thetwo mixtures is that they use a dierent cement factor, withmixture A containing 425 kg/m3 of cement whereas mix-ture B used 525 kg/m3 of cement. Table 5 lists the possiblemixture pairs that can be used to study the eect of eachmixture factor. For each test conducted, the mean, vari-ance and 95% confidence intervals for each factor was cal-culated. To determine if a mixture variable aects a certaintest result, p-values are calculated. A p-value of less than0.05 indicates that the mixture variable has a significantimpact on that test result.

Varying the cement type (mixtures G and L) in the mix-ture from Type I to Type III cement had a significantimpact on the (i) air void volume (p = 0.0232), (ii) concrete

0.0000

0.0500

0.1000

0.1500

0.2000

0.2500

0.3000

A-1 B-2 C-3 D-4 E-5 F-6 G-7 H-8 I-9 J-10 K-11 L-12 M-13 N-14Mixture ID

Spac

ing

fact

or, m

m.

Spacing factor criterion = 0.2000 mm

Fig. 6. Spacing factor measured using ASTM C 457.

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500Spacing factor,mm

Dila

tion

, m

m/m

m

exceed spacing factor threshold

exceed dilation threshold

G -7A

N-14B

N-14A

L -12A

L -12 B

M-13B

K -11 AM-13A

Fig. 7a. Relationship between spacing factor and dilation due to freezing and thawing.


Fig. 8. Dierence in air-void system from Type I (Mixture E-19) cement on left and Type III (Mixture M-27) on the right. Magnification 50.

0

10

20

30

40

50

60

70

80

90

100

0.0000 0.0500 0.1000 0.1500 0.2000 0.2500 0.3000 0.3500Spacing Factor (mm)

Dur

abili

ty F

act

or

ex ceed s ac in factor th resh old

D ura b ility F actor th res h old

M-13AK -11A

M-13B

L -12BG -7A

L -12A

N-14A

Fig. 7b. Relationship between spacing factor and calculated durability factor due to freezing and thawing.

0

10

20

30

40

50

60

400 425 450 475 500 525 550 575 600Cement factor, kg/m3

Cra

ck le

ngt

h, m

icro

ns/m

m2

Fig. 9. Cement factor versus crack length per unit area for all mixtures.


workability as determined by the slump test (p = 0.0343)and (iii) 68 h mechanical properties (p = 0.01). Air voidvolume was found to be higher in mix G (type I) than inmix L (type III).

By changing the cement factor from 425 to 525 kg/m3

(mixtures A versus B), significant impacts occurred in the(i) concrete workability as determined by the slump test(p = 0.0136); and (ii) mechanical properties (p = 0.005).

Similarly, comparing the eect of cement factor of 425versus 525 kg/m3 in calcium chloride accelerated mixtures(mixtures I versus J: Type I cement, calcium-chloride accel-erator and no water reducer) resulted in significantlyimpacting the (i) air void volume (p = 0.0141); (ii) concreteworkability (p = 0.0250) and (iii) concrete mechanicalproperties (p = 0.0310).

Varying the w/c ratio (mixture B versus F) from 0.36to 0.40 caused significant eect on the (i) mechanicalproperties of the hardened concrete (p = 0.02); (ii) work-ability of the fresh concrete (p = 0.0246); and (iii) freeze-thaw dilation (p = 0.0249). The majority of these mechan-ical tests are either strength (22, 24, 25, 26 and 27) ordensity (30, 31, 33 and 34) tests. The other test results thatare significant to a change in w/c ratio are coecient ofthermal expansion (1), freeze-thaw dilation (19) and slump(20). The coecient of thermal expansion, freeze-thawdilation, significant strengths and bulk densities of mixtureF (w/c = 0.36) were higher than those of mix B (w/c =0.40).

The eects of coarse aggregate (mixtures A versus E; Cversus D) were significant on the (i) density of the hardenedconcrete specimen (p = 0.0298) regardless of cement factorand curing regime; (ii) air-void spacing factor (p = 0.0190)if the cement of 525 kg/m3 is used and cured at a tempera-ture of 65 "C. In this case, the air-void spacing factor islower while the slump is higher when carbonate aggregate(71-47) is used (Mix C) in comparison to siliceous aggre-gate (95-10) is used (Mix D). On the other hand, the coarseaggregate does not have an eect upon these tests if thecement factor is 425 kg/m3 and the curing temperature is23 "C (Mixes A and E).

The use of dierent accelerators with a cement factor of425 kg/m3 of Type I cement with no water reducer (mix-tures A and I) significantly (p < 0.05) impacted concreteworkability and hardened mechanical properties. Howeverthe same concrete properties were not significantly aectedwhen using the two dierent accelerators if 525 kg/m3 of aType I cement is used with no water reducer or if 525 kg/m3 of Type III cement is used with a Type F high-rangewater reducer.

Accelerator type is an important factor for the thermaland strength properties of concrete if the mixtures aremade using 425 kg/m3 of a Type I cement with type Fwater reducer (mixtures K and N) and it is not importantfor the other mixture combinations. Mix K (non-chlorideaccelerator) resulted in a higher coecient of thermalexpansion than mix N (calcium chloride accelerator).Mix N on the other hand tested to have a higher 24 h T

able5

Mixture

pairsusedin

68hopeningtime

Factor

Mixes

used

Cem

enttype

Cem

entfactor(kg/m3)

w/c

Ratio

Coarse

aggregate

Accelerator

Water

reducer

Curingtemperature

Cem

enttype

GandL

I/III

525

0.40

Carbonate

Non-Chl

F23

"CCem

entfactor

AandB

I425/525

0.40

Carbonate

Non-Chl

No

23"C

IandJ

ICaC

lNo

KandL

III

Non-Chl

Fw/c

Ratio

BandF

I525

0.40/0.36

Carbonate

Non-Chl

No

23"C

Coarse

aggregate

AandE

I425

0.40

Carb/Silic

Non-Chl

No

23"C

CandD

525

65"C

Accelerator

AandI

I425

0.40

Carbonate

Non-Chl/CaC

lNo

23"C

BandJ

I525

No

KandN

III

425

FWater

reducer

BandG

I525

0.40

Carbonate

Non-Chl

No/F

23"C

Curingtemperature

BandC

I525

0.40

Carbonate

Non-Chl

No

23"C/65"C

LandM

III

F


compressive strength and eective diusion coecient thandid mix K.

The eect of water reducers was investigated by compar-ing mixtures B and G. The statistical analysis of the dataindicated that the admixture significantly (p < 0.05)impacted the air-void system and the mechanical propertiesof the hardened concrete. Mix B, which used no waterreducing admixture, resulted in better air-void system char-acteristics and significant increases in strength than did mixG which utilized a Type F water reducing admixture.

The eects of curing on the various fresh and hardenedconcrete properties were investigated by comparing mix-tures B and C and mixtures L and M. The dierent curingtemperatures significantly (p < 0.05) impacted the paste-to-air ratio and air-void spacing factor, freeze-thaw beamdilation, and the 28-day compressive strength when TypeI cement with no water reducer was used. The curing tem-perature eect was significant for the 6 and 8 h compressivestrength tests when Type III cement was used with a TypeF water reducer.

Table 6 summarizes the level of significance for eachmixture variable on the properties of the fresh and hard-ened concrete mixtures.

! An Important mix factor is significant to that test nomatter what other combinations of mix variables areused.

! A factor that is important under Certain Conditionsis important to that test for some of the mixtures butnot in every case where that factor was varied.

! A factor that is Not Important is not significant tothat particular test in any of the tested mixtures.

5. Conclusions

The results presented in this paper are part of a compre-hensive laboratory and field study sponsored by theNCHRP that was undertaken to evaluate the fresh, hard-ened, volumetric and microstructural properties of high-early strength concrete used in concrete pavement rehabil-itation. Based on the results of the experimental programand statistical analysis the following conclusions can bedrawn:

1. For the specific cements used, the cement type had anotable impact on various measures of early concretestrength, with Type III cements producing higher

strengths at a given age than Type I cements. Thisincreased rate of hydration was also evidenced in thematurity readings.

2. As expected, the coarse aggregate type had a very signif-icant impact on measures of density, with denser aggre-gate resulting in denser concrete. Mixtures made withthe gravel coarse aggregate had the highest coecientof thermal expansion, whereas mixtures made with thecarbonate aggregate had the least scaling. Aggregatetype also aected some of the strength properties ofthe mixtures, most notably compressive strength.

3. In many instances, the use of the finer Type III cement,in conjunction with the Type F HRWR, negatively influ-enced the air-void system parameters, most acutelywhen calcium chloride accelerator was used. Thisresulted in generally poorer freeze-thaw performance.It is noted that only single sources of Type III cementand Type F HRWR were used, and thus no generalizedconclusions regarding the suitability of Type III cementand/or Type F HRWR can be drawn.

4. All the high-early strength mixtures had significantlyhigh levels of microcracking as assessed viewing thin sec-tions under a petrographic microscope. High cementcontent mixtures had relatively greater amounts ofmicrocracking, likely due to the increased paste content.

5. The use of the Type F HRWR in the mixtures reducedthe air content and negatively impacted a number ofthe air-void system parameters, resulting in poorerfreeze-thaw performance. It also negatively impactedthe early strength characteristics of the concrete whenType I cement was used. Mixtures containing Type EWR exhibited poor paste homogeneity and very lowearly strengths.

6. Due to the unique characteristics of these materials andthe increased potential for interactions between the var-ious mixture constituents, it is recommended that trans-portation agencies apply a thorough testing regimeduring the mixture design process and employ a QC/QA program during construction to ensure the produc-tion of durable high-early strength mixtures.

Acknowledgements

The authors acknowledge the financial support givenby the National Cooperative Highway Research Programand the assistance provided by Dr. Amir Hanna, theNCHRP Senior Program Ocer overseeing this work

Table 6Factor significance for concrete properties

Concrete property Important Factors important under certain conditions Not important

Workability (slump) Cement type, w/c ratio, water reducer Cement factor, coarse aggregate, accelerator Remaining factorsPhysical (density) w/c ratio, coarse aggregate Cement factor Remaining factorsAir-void system Cement type, water reducer Coarse aggregate, cement factor, accelerator, curing regime Remaining factorsFreeze-thaw dilation w/c ratio Curing regime Remaining factorsStrength Water reducer, cement type, w/c ratio Coarse aggregate type, cement factor, curing regime Accelerator






described. Furthermore, the authors acknowledge MDOTfor providing invaluable assistance with the concrete mix-ture design development, specimen fabrication and test-ing.

References

[1] Van Dam TJ. Durability of Early-Opening-to-Trac portlandcement concrete for pavement rehabilitation. Final Report, NCHRPWeb Document 76, Transportation Research Board, NationalResearch Council, Washington, DC; 2005.

[2] Whiting D, Nagi M, Okamoto P, Yu T, Peshkin D, Smith K, et al.Optimization of highway concrete technology. SHRP-C-373, Strategic

Highway Research Program, National Research Council, Washington,DC; 1994.

[3] American Concrete Pavement Association (ACPA). Concrete pavingtechnology guidelines for full-depth repair. TB002-02P, Skokie,Illinois; 1995.

[4] Hoerner TE, Smith KD, Yu HT, Peshkin DG, Wade MJ. PCCPavement Evaluation and Rehabilitation, Reference Manual, NHICourse Number 131062, Federal Highway Administration, USDepartment of Transportation; October 2001. 410pp.

[5] Zollinger DG. Assessment of durability performance of Early-Opening-to-Trac portland cement concrete. Final Report, NCHRPProject 18-04A, National Cooperative Highway Research Program,Transportation Research Board, National Research Council, Wash-ington, DC; 1998.


Evaluation of high-early strength PCC mixtures used in full depth repairsIntroductionExperimental programExperimental matrixSpecimen preparationTests conducted on laboratory prepared specimensProperties of fresh concreteFreeze-thaw durabilityMicrostructural characterization

Laboratory test results and discussionProperties of fresh concreteFreeze-thaw durabilityMicrostructural characterization

Statistical analysisConclusionsAcknowledgementsReferences

_Evaluation of High-early Strength PCC Mixtures Used in Full Depth Repairs

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