Concrete Strength Development

7/29/2019 Concrete Strength Development

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MATU RITY OF CONCRETE-STRENGTH DEVEL OPME NTby

HECTOR GARCIA-MONZON, B.S.C.E.A THESIS

INCIVIL ENGINEERING

Submitted to the Graduate Facultyof Texas Tech University inPartial Fulfillment ofthe Requirements forthe Degree of

MASTER OF SCIENCEIN

CIVIL ENGINEERINGApproved

Chairperson of the Committee

Accepted

Dean of the Graduate SchoolMay, 2003


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ACKNOWLEDGEMENTS

I wou ld like to thank all of those who helped m e make this possible. I wo uld liketo especially thank my parents, Armando and Cristina Garcia, for supporting me in mydecision to continue my education. W ithout them and their upbringings, I would not bethe person I am today. I wou ld also like to give thanks to Dr. Phelan and Dr. Senadheerafor giving me the opportunity to work on this project and for their support and guidancethroughout the journey.

Along the way, I have also had the privilege of working with some great peop lewh o have also becom e some great friends. This close group of friends is know n as theBridge crew and includes the following: Tom Gardner, Jacob Bice, Carlos M orales,Cesar Calderon, Daniel Rasband, Brittany Boen, Nestor M endoza, Gene Bow les, andothers who have come and gone throughout the project. I would like to thank the B ridgeCrew for all their help and support, and for all the fun times and memories we had duringthis research. I wo uld like to give a special thanks to Carlos for encou raging m e when Iwas slacking and making me laugh over the little things (wing mo del).

I would also like to thank the guys in the Civil Engineering shop; Frank, Mario,Pau l, and Dou g, for all their help. It is through them that I learned how to direct theconstruction of the field sites by observing their practices on the first field site.

Also, I would like to thank some very good fiiends for believing I couldaccomplish anything I set out to do: Felipe, Chris, Alvin, Nick, Ethan, Julie, and K im.

Thank you, everybody so much.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS i iLIST OF TABL ES viLIST OF FIGURES viiiCHAPTER1. INTROD UCTION 1

1.1 Significance of Research 31.2 Current Strength Determ ination 61.3 Us e of the Ma turity Metho d 71.4 Past Implem entation of Maturity Method 81.5 Research App roach 91.6 Thes is Form at 11

2. HYDRA TION, CURING, AND MATURITY 132.1 Com position and Hyd ration of Concrete 132.2 Con crete Curing 182.3 Curing Temperature 202.4 Maturity 22

2.4.1 Nu rse-Saul M aturity Function 242.4.2 Nurse-Saul Equivalent Age Function 262.4.3 Arrhenius Equivalent Age Function 27

2.5 Maturity Function Comparison 292.6 Strength Gain Mod els 31

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2.6.1 Freiesleben Hansen (FH) Strength Gain Mo del 322.6.2 Natural Log (LN) Strength Gain Mod el 33

2.7 Summ ary 353. TEST SETUP AND APPROACH 37

3.1 Bridge Deck Field Setup 393.1.1 Field Test Site (Construction Stage I) - Footing s,Abutment Walls 403.1.2 Field T est Site (Construction Stage II) - BearingStrip, Panel Placement 423.1.3 Field Test Site (Construction Stage III) - CIPDeck Preparation 453.1.4 Field Test Site (Construction Stage IV) - Instrumentation 473.1.5 Field Test Site (Deck Pour) 493.1.6 Field Test Site (Curing) 51

3.2 Field Cast Cylinder Setup 533.3 Lab Cast Cylinder Setup 5 83.4 Bridge Deck Sampling and Testing 623.5 Field Cylinder Testing 663.6 LMA T Cylinder Testing 693.7 Expe cted Results for Test Setup 70

4. MATU RITY PREDICTIONS AND CONVENTIONALSTRENGTH ANALY SIS 72

4.1 Bridge Deck Tem perature Histories 724.2 FMA T Cylinder Temperature Histories 76

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4.3 FMA T Cylinder and Strength Development 804.3.1 Arrhenius Equivalent Age Strength-Maturity Curves 804.3.2 Nurse-Sa ul Equivalent Ag e Strength-Maturity Curves 84

4.4 Equivalent Age Comparison of FMA T Cylinders 874.5 Com parison of Bridge Deck In-Situ Strength to Predicted Strength 904.6 Gen eration of LM AT Strength-Maturity Curves 1024.7 In-Situ Strength Com pared to LM AT Predicted Strength 1084.8 hi-Situ Strength Com pared to CO NV T Cylinder Strength 116

5. DISCUSSION AND CONCL USIONS 1215.1 Strength Determ ination Based on the Maturity Metho d 1225.2 CO NV T Strength Determination Findings 1255.3 Future Research 1265.4 Con clusion 127

REFERENCES 129APPENDIXA. BRIDGE DECK IN-SITU STRENGTH DATA 132B. FMA T CYLINDER STRENGTH DATA 153C. LMAT CYLINDER STRENGTH DATA 169

D. CONVT CYLINDER STRENGTH DATA 185E. ARR EQUIVALENT AGES FOR BRIDGE DECK, FMAT, ANDLMAT CYLINDERS 198F. N-S EQUIVALENT AGES FOR BRIDGE DECK, FMAT, ANDLMAT CYLINDERS 203


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LIST OF TABLES

1.1 No men clature 51.2 Cylinde r Typ es 51.3 M iscellaneous Ab breviations 61.4 Cylinde r Typ es and Quan tity 111.5 Te sting Perform ed 112.1 Ordin ary Portland Cem ent Chem ical Com poimds 142.2 Characteristics and General Use of Portland Cem ent Typ es 142.3 Typical Chem ical Com position Percentages of Portland Cement 153.1 Mix Designs 383.2 Bridge Deck Descriptions 403.3 Schedule for Bridge Deck Sampling and Testing 643.4 Con crete Core Correction Factors 653.5 FM AT Cylinder Testing Schedule 683.6 CON VT Strength Cylinder Testing Schedule 693.7 Testing of LM AT Cylinders 704.1 Coefficients for FH M odel (FMA T Cylinders) 824.2 Coefficients for LN Mo del (FMA T Cylinders) 854.3 Equivalent Ag e Values Based on Arrhenius and N urse-Saul(FMA T Cylinders) 904.4 Equivalent A ge Values Based on Arrhenius and Nurse-Saul(Bridge Decks) 92

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4.5 FTW In-situ Strength Com pared to FM AT Predicted Strength 954.6 SAT In-situ Strength Com pared to FM AT Predicted Strength 964.7 HO U In-situ Strength Com pared to FM AT Predicted Strength 974.8 Coefficients for FH Mo del (LMA T Cylinders) 1034.9 Coefficients for LN Strength Gain Mo del (LM AT Cylinders) 1034.10 FTW In-Situ Com pared to LM AT Predicted Strength - FH Mod el 1104.11 SAT In-Situ Com pared to LMA T Predicted Strength - FH Mo del 1114.12 ELP In-Situ Comp ared to LMA T Predicted Strength - FH Model 1124.13 FTW hi-Situ Compared to LMA T Predicted Strength - LN Model 1134.14 SAT In-Situ Compared to LMA T Predicted Strength - LN Model 1144.15 ELP In-Situ Com pared to LM AT Predicted Strength - LN Mo del 1154.16 In-Situ Strength Com pared to CON VT Cylinder 120

V l l


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LIST OF FIGURES

1.1 Strength Com parisons 22.1 Rate of Heat Evolution during the Hyd ration of C3S 162.2 Sau l's Maturity Rule 232.3 Com parison of Maturity Functions 302.4 Strength-Maturity Curve Based on FH Model 332.5 Strength-Maturity Curve Based on LN Model 353.1 Abutm ent Wall 423.2 Bridge Deck Double Bay Cross-section 433.3 Placed Concrete Beams on Abutment Walls 443.4 Placing Pre-cast Panels on Beams 443.5 Placed Pre-cast Panels on Bridge Deck (Double Bay) 453.6 Reb ar Mat in One Section of the Bridge Deck (Double Bay) 463.7 Com pletion of Phase 3 (Single Bay) 473.8 Shielded Type "T " Thermocouples 493.9 Placem ent of Concrete 5 03.10 Freshly Poured Deck with Finish and Curing Compound 523.11 Finished Bridge Deck with Curing Media 533.12 Preparing Field Cast Cylinders 543.13 Dem olded Field Cast Cylinders 5 63.14 Lime Bath and Cylinders with Embedded Thermocouples 57

V l l l


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3.15 Slump Test 603.16 Lab Cylinders with Embedded Thermocouples in Lime Bath 603.17 Calorime ter Drum s and Data Logger 623.18 Coring Machine 633.19 Four-Inch Conc rete Core Being Retracted 634.1 FTW Bridge Deck Temperature History 754.2 SAT Bridge Deck Temperature History 754.3 HO U Bridge Deck Temperature History 764.4 FTW FMA T Cylinder Temperature History 784.5 SAT FMA T Cylinder Temperature History 794.6 HO U FM AT Cylinder Temperature History 794.7 FTW FM AT Cylinders - FH Model 824.8 SAT FM AT Cylinders - FH Model 834.9 HO U FM AT Cylinders - FH Model 834.10 FTW FMA T Cylinders - LN Model 854.11 SAT FMA T Cylinders - LN Mod el 864.12 HO U FM AT Cylinders - LN Model 864.13 FTW FMA T Equivalent Age Comparison 884.14 SAT FMA T Equivalent Age Comparison 894.15 HO U FM AT Equivalent Age Comparison 894.16 FTW Bridge Deck Equivalent Age Comparison 934.17 SAT Bridge Deck Equivalent Age Comparison 93

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4.18 HO U Bridge Deck Equivalent Age Comparison 944.19 FTW hi-Situ Strength Com pared to FM AT Predicted Strength 954.20 SAT In-Situ Strength Com pared to FMA T Predicted Strength 964.21 HO U In-Situ Strength Com pared to FM AT Predicted Strength 974.22 FTW LMA T Cylinders - FH Model 1054.23 SAT LM AT Cylinders - FH Model 1064.24 ELP LM AT Cylinders - FH Model 1064.25 FTW LMA T Cylinders - LN Model 1074.26 SAT LMA T Cylinders - LN Model 1074.27 ELP LM AT Cylinders - LN Model 1084.28 FTW In-Situ Compared to FMAT and LMAT Predicted Strength(FH Mo del) 1104.29 SA T In-Situ Com pared to FMAT and LM AT Predicted Strength(FH Mo del) 1114.30 EL P In-Situ Com pared to LMA T Predicted Strength (FH M odel) 1124.31 FTW Li-Situ Compared to FMAT and LMAT Predicted Strength(LN Mo del) 1134.32 SAT In-Situ Compared to FMAT and LMAT Predicted Strength(LN Mo del) 1144.33 EL P In-Situ Com pared to LM AT Predicted Strength (LN Mo del) 1154.34 FTW hi-Situ Strength Com pared to CO NV T Cylinder Strength 1174.35 SAT In-Situ Strength Com pared to CON VT Cylinder Strength 1184.36 HO U In-Situ Strength Com pared to CON VT Cylinder Strength 1184.37 EL P In-Situ Strength Com pared to CON VT Cylinder Strength 119


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CHAPTER 1INTRODUCTION

There continues to be increasing pressurefi-omowners, contractors, and thepub lic to open bridge d ecks sooner to full traffic loads. As a result, a set of criteria orguidelines is needed to determine when concrete bridge decks can safely be opened.Today, current practice allows many bridge decks and concrete pavements to be openedto traffic once a desired com pressive strength is achievedfi-oma representative field-casttest cylinder. Thou gh gen erally imtrue, man y believe that this strength value serves as ame asure of the durability of the placed concrete. The maturity method is investigated inthis thesis to determine (a) how well field cast concrete strengths can be predicted fi-omgenerated strength-maturity cu rves, and (b) what effects different wet mat curingdurations have on strength development and overall strength.

The ob jective of this thesis is to compare the compressive strengths of in-situfour-inch diameter cores takenfi-ombridge decks with predicted strengths based on (a)strength-maturity curves developedfi-omsix-inch diameter field-cast cylinders, and (b)strength-m aturity curves developed from six-inch diameter lab-cast cylinders. Inaddition, a comparison is made between in-situ strength and the conventional cylinderstrength metho d which is currently used to determine the cast-in-place strength. Resu ltsfrom these tests lead to an evaluation to establish which method (i.e., maturity method orconven tional metho d) is more appropriate to assess in-situ comp ressive sfrength. Thecomparisons just described are illustrated in Figure 1.1.


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hi-SituConcreteStrengthofBridgeDeckSlab

compared to ->

compared to ->

compared to ->

Predicted sfrength from strength-maturitycurves developed from field-cast cylindersPredicted sfrength from strength-maturitycurves developed from lab-cast cylindersSfrength assessment from conventionalfield-cast cylinders

Figure 1.1 Sfrength Comparisons

The com parisons made in this thesis as shown in Figure 1.1 support the use of thema turity meth od as a mea ns to assess the compressive sfrength of concrete, provided thatthere is little to no variation between concrete mixes used to pour con crete decks anddevelop sfrength-maturity curves. W ith proper curing of bridge deck slabs, it has beenfound that 4 days of curing is adequate to attain compressive sfrengths equivalent to 10days o f curing, but it is still not know n if 4 days of curing is adequate for concretedurability. Also , it was discovered that there exists some issues concerning qualityconfrol during concrete placement such as adding excessive water to a concrete mix , thuschang ing its sfrength gain characteristics. All of these findings will be discussed in moredetail within this thesis.


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1.1 Significance of ResearchFollow ing specifications d eveloped in 1 993, the minim um requirem ent for wet

mat cu ring in Tex as is eight days for Type I and III cement and ten days for T ype II, I/II,or mix designs with fly ash, (TxDO T, 1993). These specifications also require aminim um of 14 and 21 days after termination of wet mat curing before pavem ents areopen to V A ton construction fraffic and full traffic load, respectively. As a result, bridgedecks were open to full fraffic anywhere from 29 to 31 days from the completion of theconcrete pour. Special provisions to Item 420 - Concrete Structures, which w ere vmtte nin 1995 and adopted in 1998, allow opening the bridge deck to construction and fullfraffic much sooner than the 14 and 21 days required in the 1993 specifications.How ever, these special provisions do not reduce the wet mat curing period (TxDO T,1998).

According to the special provisions, once the concrete is cured the minimumnum ber of day s, the pavement is allowed to dry for one day to prepare it for siufacefreatment. An ad ditional day typically is needed to apply any required surface freatmen t.The bridg e deck is then op ened to full fraffic only if the required comp ressive designsfrength has been g ained. Th us, bridge decks can essentially be open to traffic in only 10to 12 days from the initial pour, depen ding on the type of cement and if all the abovecriteria have be en met. As a result, the current special provisions represent a significantreduction in the number o f days required before bridge decks can be open to traffic.

Althou gh, the num ber of days before full traffic load is allowed on bridg e deck shas de creased from 1993 to 1998 in Texas, there is still a desire to reduce the required


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curing time . The con cern today is the effects shorter wet mat curing durations will haveon the overall performance and durability of the concrete. By investigating differentcuring du rations (e.g., 0, 2 ,4 , 8, 10, and 14 days ), either (a) future updates to currentspecifications can be mad e based on results from this project or (b) the results of thisresearch m ay reveal that current practice is satisfactory and no change in thespecifications is necessary.

Since there is a need to open new and/or reconstructed bridge decks as soon aspos sible, a minimu m appropriate curing duration must be specified without sacrificingthe overall design life of the bridge deck. To accomplish this, a major test program w asundertaken by TxDO T and Texas Tech University w hich included both field and labtesting.

Field testing included five full-scale bridge decks that were exposed to differentcuring du rations. The curing durations studied in this research project were for 0, 2, 4, 8,10, and 14 days of wet mat curing (0-day curing refers to no moist curing). The com pletefield test setup and field testing plan is described in Sections 3.1, 3.2, 3.4, and 3.5.

Lab testing included the five mix designs used in field testing as well as two othermix designs used in the state. In the lab, sfrength-maturity curves were developed onlyfor curing du rations of 0, 4, and 10 days with an additional set of lab-cast cylinders thatwere cu red for 7 days. This thesis focuses on compressive sfrength results for the 0-, 4-,and 10-day wet mat cure durations. As will be discussed later in this thesis, these datesare representative o f the range of findings. The lab testing plan is discussed in more d etailin Sections 3.3 and 3.6.


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The entire set of results for the different curing durations m entioned above will beincluded in the final report subm itted to TxDO T wh ich will include sfrengthdevelopment, deterioration, and damage resistance of the concrete, in order to establishappropriate wet mat curing times.

Since this project is very extensive and the data points are num erous, anomenclature and abbreviations have been adopted to present the data in an easier form.This allow s tables and graphs to be less cluttered and better vmderstood. Thenom enclature and abbreviations used in this thesis are presented in Tables 1.1, 1.2, and1.3.

LBBELPFTWSA THOUPHRATL

Table 1.1 NomenclatureDescription

Lubbock D istrict MixEl Paso District MixFort Worth District MixSan Antonio District MixHouston District MixPharr District MixAtlanta District Mix

FMATLM AT

CONVT

Table 1.2 Cylinder TypesDescription

maturity based on field-cured 6" diameter cylindersmaturity based on lab-cured 6" diameter cylindersconventional 6" diameter field-cured cylinders


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Table 1.3 Miscellaneous Abbreviationsin-situN-SARRLNFH

Descriptioncast-in-place concrete from bridge deck (usually 4" diameter cores)Nurse-Saul - Method for calculating maturityArrhenius - Method for calculating maturitynatural log model - Sfrength gain model to predict strengthFreiesleben Hansen model - Strength gain model to predict sfrength

1.2 Current Sfrength DeterminationDetermining a representative sfrength of in-situ co ncrete can be a tedious process.

Currently numerous concrete cylinders and flexural beams are cast on site and tested as ameans to verify whether compressive and flexural requirements for the in-situ concretehave been met. Even then, due to a number of factors includ ing size effects, differencesin temperatures during curing, concrete placement practices, and even the curing processitself, compressive and flexural sfrengths from cast-on-site specimens are not trulyrepresentative of the actual strength of the in-situ concrete. Further, Popovics (199 8)states that cast-on-site cy linders are not representative of in-situ conc rete due to thestandardized m anner in wh ich cylinders are prepared, since such a standard does not existfor cast-in-place concrete. Usually, cylinders, whether cured in the field or lab, yieldhigher sfrengths than co res taken from the in-situ concrete. As a result, in-situ sfrengthmay be lower than believed when assessing sfrength based on conventional cylinders.

Also, a problem arises as to when to test the site poured specim ens. Usuallyconfractors make an educated guess or rely on specifications set by the engineer wh endetermining the proper time that the in-situ co ncrete has achieved a desired sfrength.


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They th en test the site cast specimens to verify if indeed the desired comp ressive strengthhas been achieved . This process can require several trials, as the contractor will notalway s guess correctiy. On the other hand, comp ressive strengths may have beenachieved even before the first set of conventional cylinders is tested without theconfractor k now ing it. By having a better indicator as to when in-situ com pressivesfrengths have first been achieved, confractors are able to continue with co nstruction andpossibly complete projects ahead of schedule.

Even tho ugh comp ressive sfrength is used as an indicator to allow load ing ofconcrete bridge deck s, it does not, howev er, generally indicate the durability of theconcrete. For an accurate durability determination, additional tests are required. Thesetests include rapid chloride permeability test (RCPT), ponding, scaling, freeze-thaw,abrasion, etc. which will be included in the final report submitted to TxDOT.

1.3 U se of the Maturity Metho dOne possible solution to eliminate the "guess work" as to when in-situ concrete

has achieved adequate sfrength g ain is to utilize the maturity concept. The m aturitymethod, which is based on the concept of maturity, has been shown to be a reliable meansof determining in-situ concrete strength within 10% of developed strength-maturitycurves based on research conducted at the University of Texas by Constantino andCarrasquillo (1 998). Als o, with the use of the maturity method, fewer cylinders have tobe prepared in the field. T hese cylinders are tested when in-situ concrete has attained acertain ma turity index or maturity equivalent age that is representative of a desired


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sfrength. This is to further verify that in-situ concrete has attained a comp ressive strengthas predicted by developed strength-maturity curves. Although some industryprofessionals feel the use of sfrength-maturity curves will eliminate the need for all fieldcast cylinders, it remains goo d practice to have a redundant m ethod for final sfrengthdetermination . Thou gh m aturity is used to assist in assessing the concrete compressivesfrength at any given ag e, it could possibly be used to estimate con crete durability ifadequate testing and data are available to define a correlation between maturity anddurability.

1.4 Past Implemen tation of Maturity M ethodThe ma turity concept has been successfully used in many areas of the Un ited

States including in Dallas on the No rth Central Expressway. This 5-year project wasfinished nine mo nths ahead of schedule and had a total cost savings of $60 million. Thematurity method helped accelerate this project by completely eliminating the traditionalprocess of "preparin g, curing, and breaking concrete sam ples" to verify in-situ sfrength(Texas Quality Initiative, 1999). frnplementation of the maturity method allowed for in-situ sfrengths of the concrete to govern the construction progress as oppo sed to therequired 7-day wait before beams could be tested. Concrete usually reached designstrengths in as few as 4 days, thus allowing constioiction to proceed (Texas QualityInitiative, 1999). No t only did the use of the maturity concept help save money and time,but it also helped improve the efficiency of work crews and equipm ent by speeding u pstage construction. As a result, congestion from detoured fraffic was short lived.


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1.5 Research ApproachThe m aturity method has been show n to be successful in predicting sfrength over

the past few de cades. Add itional site applications must be comp leted, how ever, beforesufficient confidence is gained for widespread use of the method. The approach taken forthis research project was to study seven different b ridge deck concrete mix designs usedin seven different dish icts in the State of Texas. The seven mix designs tested are for thefollowing dishicts: Lubbock, El Paso, Fort Worth, San Antonio, Houston, Pharr, andAtlanta.

Of these seven mix designs, five of them (Lubbock, El Paso, Fort Worth, SanAn tonio , and Houston ) were used to construct full scale bridge decks in different climaticregions in Texas. These bridge decks were divided into six sections each. Each sectionreceived a different curing duration (e.g., 0, 2, 4, 8, 10, or 14 days). The 0-day curesection of the bridge deck slab received no moist curing, only a curing compoundapplication. Concrete curing temperatures were monitored for each section for 28 days todetermine a maturity equivalent age for each section.

Along with the bridge deck, 36 field-cured maturity (FMA T) cylinders wereprepared. Curing durations of 0, 4, and 10 days were used for the FMA T cylinders aswill be discussed in Section 3.2. The 0-day cure FM AT cylinder received no m oistcuring after demolding, whereas the 4- and 10-day cure FMAT cylinders were immersedin a lime bath for 4 and 10 days respectively. FM AT cylinders were used to developsfrength-maturity curves. This was done by monitoring concrete curing temperatureswith embedded thermocouples. Maturity equivalent ages were calculated using the


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recorded con crete curing temperatures. W ith average comp ressive strengths from threeFM AT cylinders for each test date, (i.e., 1, 3, 7, 14, and 28 days as described in Section3.5), and their corresponding maturity equivalent ages, a strength-maturity relationshipwas developed for each curing duration.

Also, 48 conventional field-cured (CONVT) cylinders were prepared at the fieldsite. The se cylinders are similar to those used in common construction practice todetermine the in-situ sfrength of a concrete structure. CON VT cylinders were cured fordurations o f 0, 2, 4, 8, 10, and 14 days. These were also cured by imm ersing thespecimens in a lime bath for the specified am ount of days. CON VT cylinders were testedon days 7 , 14, and 28 as described in Section 3.5.

As w ell as constructing these field sites and preparing field cast specimens, sevenmix designs w ere tested in the lab. A total of 42 lab-cured maturity (LMA T) cy linderswere prepa red. Curing durations of 0, 4, 7, and 10 days were studied as will be discussedin Section 3.3. Concrete curing temperatures were monitored and maturity equivalentages were calculated. Ma turity equivalent ages and compressive data from LMA Tcylinders (at 1, 3, 7, 14, and 28 days) were used to develop sfrength-maturity curvesbased on lab curing conditions.

Table 1.4 and 1.5 illusfrate the different 6-inch diameter cylinders used in thisproject as well as testing phases performed for each mix design. As can be seen fromTable 1.5, mixes for Fort Worth and San Anton io have a complete set of tests. FMATcylinders w ere not performed for the Lubbock and El Paso mixes because o f difficultiesexperienced in setting up the temperature data acquisition portion of this test. Pharr and

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Atlanta mixes did not have a bridge deck, FMA T, or CONV T strength testing phasebecau se they w ere strictly tested in the lab. As a result of having different testing ph asesfor the concrete mix designs, only the highlighted portions of the testing phases arediscussed in this thesis. The Lu bbock mix design w as omitted from this thesis due toinadequate specimen sampling which yielded little data.

CylinderTypeFMATLMATCONVT

QTY.364248

Table 1.4 C ylinder Types and Qu antityDescriptionfield-cured cylinders used to develop sfrength-maturity curveslab-cured cylinders used to develop strength-maturity curvesfield-cured cylinders used to assess in-situ sfrength conventionally

DistrictLBBELPFTWSA THOUPHRATL* Testim

Table 1.5BridgeDeckXXXXX

; Schedulec

Testing PerformecFMAT

XXX

for July

LMATXXXX*XX2003

CONVTXXXXX

Note: Highlighted Testing Discussed in Thesis

1.6 Thesis FormatThe format for this thesis is as follows. Chapter 2 serves as a backgroun d an d

provides an overview of (a) the chemical composition and concrete hydration process, (b)the curing process, (c) the maturity concept of concrete, and (d) sfrength gain prediction

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mo dels. Chapter 3 discusses field-site and lab setups and testing procedu res performedby Texas T ech University researchers. Chapter 4 provides an interpretation of the datagained from this research. Chapter 5 discusses findings, conclusions, and suggestedfuture research. The append ices to this thesis are as followed:

Appendix A: Bridge Deck In-Situ Sfrength DataAppendix B: FMAT Cylinder Sfrength DataAppendix C: LMAT Cylinder Sfrength DataAppendix D: CONV T Cylinder Sfrength D ataAppendix E : ARR Equivalent Age for Bridge Deck, FMA T, and LMA T cylindersAppendix F: N-S Equivalent Age for Bridge Decks, FMAT , and LM AT cylinders.

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CHAPTER 2HYDRATION, CURING, AND M ATURITY

To fully appreciate the concept of concrete maturity, knowledge of the chemicalcom position , hydration process, and curing of concrete is needed. These variables aredepend ent on each other and each affects strength gain. A brief backgrou nd on thehydration of concrete including a discussion of the numerous factors w hich affect theprocess is provided. In addition, the history of the maturity concept and the two commonmatu rity functions in use, the Nu rse-Saul and the Arrhenius functions, are discussed. Acom parison of the determined m aturity values from both functions is provided. Als o,sfrength gain mo dels for predicting concrete strength at any given time after placem entare presented and discussed.

2.1 Composition and Hydration of ConcreteThe hydration of concrete is an exothermic chemical process. Heat generated

from this process is primarily du e to the interaction between w ater and tricalcium silicatewithin the cement (M indess et al., 2002 ). Althoug h other chemicals are present inPortland cement, they generally have minor roles in the production of heat during thehydration pro cess. The four major constituents of portland cement are listed in Table 2. 1.Also, note that the shorthand notation presented is not the complete composition of allconstituen ts in a given mix. How ever the terms listed in Table 2.1, and their respectivepercentages, generally represent the characters of a given mix.

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Table 2.1 O rdinary Portland C ement Chemical C ompounds(Mindess et al., 2002)Chemical Name

Tricalcium silicateDicalcium silicateTricalcium aluminateTetracalcium aluminoferrite

Shorthand NotationC3SC2SC3A

C4AF

W ith variations in amounts of these chemical compou nds, different types ofPortland cemen t exist. AST M C I 50-99 lists five distinct types of portland cem ents,Types 1, II, III, rv , and V. Depen ding on the physical and chemical requirements forspecific construction projects, (e.g., high early strength, high sulfate resistance, etc.), thetype of Portland cem ent used is selected. The five portland cement types along with theircharacteristics, and general use are shown in Table 2.2.

Table 2.2 Characteristics and General Use of Portland Cement Types (PCA, 1992)Cement Type

1IIIIIIVV

CharacteristicNormal cement (general purpose)Moderate sulfate resistanceHigh early strengthLow heat of hydrationHigh sulfate resistance

General UsePavements, floors, bridgesDrainage structuresCold weather placementMassive concrete structuresExposure to high sulfate environments

Every chemical compound serves a purpose in the composition of portlandceme nt. If rapid hardening is required, higher concentrations of C3S is present in thecemen t. To decrease large amou nts of heat of hydration during the hydration proces s,C2S is increased. For resistance against sulfates attack, C3A content is lowered in the

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ceme nt. C4AF is largely responsible for lowering clinkering tempe rature during theprodu ction of portland cement and also contributes to the cem ent's color. Typicalchemical composition percentages of the four major components of portland cement areshown in Table 2.3.

Table 2.3 Typical Chemical Composition Percentages of Portland Cement(PCA, 1995)

ConstituentC3SC2SC3A

C4AF

Portland Cement Type1

5519107

II5124611

III5619107

IV2849412

V384349

As m entioned earlier in this section, C3S is the primary source of heat evolutionand is of major interest during the hydration process. Also , heat generated from thiscom poun d is easily measured, as opposed to C2S, which is also a major con stituent inPortland cements. The hydration process of C3S, which can be shown with a calorimetriccurve such as that shown in Figure 2.1, goes through the following five d istinctive stages:

I. Initial Hyd rolysisII. Induction periodIII. Accelerationrv. DecelerationV . Steady State.

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Stage V

Figure 2.1 Rate of H eat Evolution du ring the Hydration of C3S(Mindess et al., 2002)

During initial hydro lysis (Stage I), heat is generated rapidly from the release ofcalcium and hydro xide ions from the cement particles. This process lasts approximately15 min utes from the time water first comes into contact with cement. After this shorthydrolysis stage, the induction, or dormant, period (Stage II) begins and lasts anywherefrom 2 to 4 hours. Com pletion of the induction period marks the initial set of concrete.At this stage , portland cement concrete (PCC) is in a plastic state, as a num ber of ions inthe paste are needed before crystallization can occur. Once a certain concenfration ofions is achieved, crystal nuclei form and hydration prod ucts start to form in the concrete.This renewed chemical reaction produces heat in the concrete (Mindess et al., 2002).

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As the tem perature rises in the concrete, the hydration rate of the concrete isaccelerated and the concrete gains sfrength rapidly (PC A, 1997). This is referred to asthe acceleration stage (Stage III) and lasts for about 4 to 8 hours. Hyd ration productsconsisting of a hydrate layer formed around the surface of C3S particles are produced at ahigh rate. Du ring this stage, final set of the concrete is achieved and early hardeningbegin s. It is at the end of this stage that the maxim um rate of heat generation is achieved.

Follow ing the acceleration stage is the deceleration stage (Stage IV). Thedeceleration stage is much like the acceleration stage with the exception that thehydration p rocess decelerates throughout this stage. The deceleration results from theconstant grow th of the hydrate layer around the C3S particles. The hydrate layer beginsto obstruct the path of free pore water in the concrete from reaching un hydrated C3Sparticles. W ith ever increasing pore water path obstruction from grow ing hydrate layers,the hydration process approaches the steady state stage (Stage V), where hydrationcontinues to o ccur, but at a very slow and steady rate. This stage starts about 18 to 36hours from initial contact betwee n water and cement. Once this stage has been reached,hydration approaches 100% completion asymptotically (Mindess et al., 2002).According to Powers (1947), cement particles passing a 200-mesh sieve will in timebecom e fiiUy hydrated if adequate moisture is provided, either from mix w ater or sometype of curing. He also states that the hydration rate is negligibly low for coarse particles(i.e., those retained by a 200-mesh sieve), due to a hydrate layer surrounding them . Thegrowing hydrate layer around the coarse particles prevents moisture from reaching theinterior of the coarse cement particles and thus complete hydration ceases.

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2.2 Concrete CuringFor Portland cem ents, typically the highest rate of heat generation occurs during

the first 24 ho urs, (i.e.. Stages I through IV ), after initial contact between water andcement, and a large amount of heat due to hydration evolves within the first three days(PC A, 1997). As is well know n, the reaction rate for any chemical process is sensitive totemp erature. Pow ers (1947) also suggests that concrete should be kept as "saturated asposs ible until the original water-filled space has been filled with hydration prod ucts."Thus the temperature of the concrete and the availability of moisture are critical duringthe first four hydration stages of concrete in order to ensure proper hyd ration(Constantino and Carrasquillo, 1998).

Keeping the concrete moist or saturated prevents evaporation of mix water and isusually performed by water curing such as sprinkling water, ponding, or using moistcoverings and sealed curing methods which keep the concrete saturated. Sealed curinginvolves the use of waterproof paper, plastic sheeting, or curing compounds to seal in mixwater and prevent pore water evaporation from the pores within the concrete (Mindess etal., 2002).

The use of lawn sprinklers is one method of sprinkling water on concrete and

keep ing it saturated. Sprinkling tends to consume large amou nts of water, however, andis not economical when water supplies at the site are limited.

Curing concrete by way of ponding requires maintaining a layer of water on theconcre te. Pondin g typically reduces the need for large amounts of water if the barriers

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can be mad e watertight. Althoug h, if pond ing is performed with the use of earth or sanddikes, large quantities of water are needed along with constant supervision.

Exa mp les of saturated coverings include cotton mats and burlap. These coveringsare: (1) applied on top of the concrete, (2) saturated w ith water, and (3) generally keptcontinually m oist for the duration of the curing period. Periodic supervision is necessaryto ensure drjang of the saturated coverings does not occur.

If water curing is desired, it should be applied as soon as possible to prevent thesealing of capillary pores due to drying. How ever, it cannot be applied too early due toexisting bleed w ater, which increases the water cem ent ratio of the cement paste of thetop surface. W ith premature w ater curing, additional moisture is added to the top surface,further increasing the water cem ent ratio and possibly causing scaling of the concretesurface. By beginn ing the wet mat curing process before the end of the induction period(Stage 11), the concrete is prevented from drying out at the edges.

One metho d of sealed curing involves plastic sheeting or waterproof paper. Thismed ia should be applied to the surface of the concrete as soon as possible. How ever, itcannot be placed too soon o r marring of the surface will occur. Plastic sheeting trapsevaporated m oisture within the sheeting. The moisture condenses on the sheeting andredistributes itself over the surface of the concrete. This condensed mo isture helpscontinue the hydration process.

The use of curing compounds or membranes is another method of sealed curing.Curing com pounds h ave become a very popular form of curing as they can be appliedmuch sooner than sheets or mats. Although curing compounds have found w ide

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accepta nce, they provide the least amoimt of curing protection. Als o, they do notnecessa rily prevent comp lete evaporation of mix water. Instead, they only retard thisproc ess, as it is typically imp ossible to com pletely seal the entire concrete deck surfacewith the comp ound. As a result of an impermeab le layer forming on the concrete surface,mix water generally is the primary means for hydration, with some help possible frommoist curing if applied later. The use of curing compo unds is best after there has beensome initial time of moist curing (e.g., sprinkling water, ponding, or saturated covering).However, currently in Texas and in most other states, a curing compound typically isappUed before moist curing (Phelan, 2001).

2.3 Curing TemperatureAcco rding to K ehl and C arrasquillo (1998), factors that affect the curing

temperature of concrete are the following:1. Types and quantities of cementitious materials2. Size and shape of the concrete section3. Insulating effectiveness of the formwork4. Concrete placing temperature5. Ambient conditions

6. Curing condition s.The am ount and type of cement in a concrete mix w ill have a great effect on the

temp erature produced . The use of pozzolan ic material also affects the heat of hydration.The replacement of cement with fly ash usually lowers the total temperature rise of the

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concrete as well as delays the time it takes to reach the maximum temperature (Kehl andCarrasq uillo, 1998). For this reason, it is recomm ended that the curing period beextended for mix designs that have pozzolanic material, which TxDOT required twoadditional curing days for such concrete mixes (TxDOT, 1993). According to ACI 308,cemen ts with pozzolanic material should be cured until 70% of the specified comp ressivesfrength is attained.

In addition to the type of cement used , the size of the concrete structural m emberis important. Internal heat of hydration dissipates quickly in thin sections due to the largesurface area to volum e ratio. In other words , heat generated by thin sections has a shorterdistance to fravel before it is released into its surroundings. Research conducted b y Kehland Carrasquillo show that intemal temperature profiles of thin unprotected sectionsclosely follow am bient temperature profiles (1998 ). W hen pouring thin concretesections, proper cvuing protection can be critical to achieving an acceptable amount ofhyd ration. Du e to the thin section scenario, ambient temperatures are an important factorin determining the intem al temperature. Since there is generally an absence oftemperature extremes throughout a thin section, as opposed to a massive section, thematurity method for determining sfrength development can be performed with relativeease (i.e., relatively few thermocouple sensors are required) because large temperaturegradients do not exist.

Heat is dissipated in massive concrete section pours at a very slow rate, resultingin a temperature profile resem bling adiabatic con ditions in the interior of the concrete.Different heat loss rates between m assive structures and standard specimens such as

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cylinders or beams can pose problems when using the maturity method to predict in-situsfrength (Constantino and Carrasquillo, 1998). The same correlation difficulties alsooccur for cylinder and flexure beam tests for massive section sfrength prediction.

Other factors influencing concrete curing temperature are formwork properties,concrete temperature at placement, ambient temperature profile, and curing conditions.Dep ending on the insulation qu ality of the formw ork, h eat can either be maintained in theconcrete wh ile curing or lost to the environment. The temperature of the concrete duringmixin g will determine the time at which heat generation begin s. If the temperature of theconcrete m ix is approxim ately 70 F , then the mix is well on its way to hydrating asopposed to a mix , which has a lower temperature (e.g., 40 F). A concrete mix beginningwith a low tem perature w ill struggle to produce sufficient h eat, and completion of thehydration process is at risk. Favorable ambient temperatures can contribute heat from theenvironm ent and help the hydration process. In addition, curing conditions can alsoaffect concrete curing temp eratures. Providing proper curing conditions can assureadequate m oisture, sufficient insulation from heat loss, and protection from evaporation,wh ich will help ensure continued hy dration and proper sfrength gain.

2.4 MaturityAn approach to determine strength development of concrete based on the maturity

me thod started as early as 1950 (Malho tra, 1991). Nu rse (1949) believed that acombination of temperature history and curing time could summarize the effects ofdifferent steam curing cycles. Thu s, Nu rse first developed the concept of "m aturity" for

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use in steam curing conditions, but without considering the use of a datum temperature.Nurse also did not take into account actual concrete temperatures, but used chambercuring temperatures instead. After reviewing research performed b y Nu rse, Saul (1951)linked "maturity" as an indicator of strength gain, dependant on the product of concretetemp erature and time. Saul then proposed the "maturity rule " as the following:"Concrete of the same mix at the same maturity (reckoned in temperature-time) hasapproximately the same sfrength whatever combination of temperature and time go toma ke up that maturity." Figure 2.2 demonsfrates this statement.

Time TimeM 1 = M 2 = M MMaturity, TTF

Figure 2.2 Sau l's Mattirity Rule (Kehl et. al., 1998)

With the development of maturity functions, temperature and time can beconverted into a maturity index (i.e., temp erature-time factor or equivalent ag e), which inturn ind icates the sfrength developmen t of concrete through a sfrength-maturity

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relationsh ip. Currently, two main maturity functions are in use. The functions are theNurse-Saul Maturity function and the Arrhenius Equivalent Age function, usedrespectively to calculate a temperature-time factor and equivalent age. Both functionshave adv antages and disadvantages as will be discussed. Although there are otherfunctions used to determine con crete maturity, they generally have not demonstrated thereliability and/or ease of use of the two functions m entioned above.

2.4.1 Nurse-Saul Maturity FunctionAs mentioned earlier. Nurse suggested that the product of concrete temperature

history and time could be used to determine sfrength. Later, Saul determined that acom bination o f temperature and time alone was not satisfactory. He recognized that adatum temp erature, which is the lowest temperature at which concrete gains sfrength, w asnecessary to accurately determine the sfrength gain of concrete. W ith the comb inedfindings of Nurse and Sau l, a function dependent on time and temperature history withrespect to a datum tem perature was developed (Carino, 1984). Equation 2-1 is theresulting Nurse-Sau l M aturity function expressed as a temperature-time factor.

M^=Y,(T-TJM [2-1]0

wheret - time after pourA t = elapsed time between temperature readingsMs - concrete maturity at age / expressed as a "temperature-time factor"

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T = average temperature between A tTo - datum tem perature, usually taken as 14 F (-10 C).From the equation, the two major parameters are the temperature and the elapsed

time after placemen t of concrete. Ideally, mo nitoring of concrete temperatures shouldstart when w ater first comes into contact with cem ent to capture the short and rapidtemp erature rise associated with Stage I of concrete hydration. Since this stage occursrapidly (usually 15 min utes), the maturity index for this time period is negligible. Also ,cvirrently it is impossible to mon itor and record concrete temperatures wh en w ater firstcome s into contact at the mixing plant. Therefore, temperature monitoring starts soonafter concrete placement.

It should also be noted that the function is linear and that the tem perature-timefactor is merely the area underneath the temp erature-time curve as shown in Figure 2.2.The datum temperature. To, is the temp erature at which concrete will continue to gainsfrength as long as the concrete reaches final set, (i.e., the end of Stage 111), before theconcrete temperature drops below 32 F (0 C) and mix water begins to freeze. Once theconcrete temp erature falls below the datum tem perature, concrete ceases to gain sfrength.

One draw back to the Nurse-Saul maturity function as shown in Equation 2-1 isthat extreme fluctuations in curing temperatures must be kept to a minimum, as the

equation does not account for a hydration rate of change due to increasing or decreasingcuring tempe ratures. Ano ther drawback is that the units of temperature-time, for example150 F-hours are not convenient to grasp. Further, due to the unique expression of thismatu rity index (i.e., temperature-time factor), the Nu rse-Saul maturity function is

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difficult to comp are to other maturity fiinctions such as the Arrhenius Eq uivalent Ag efunction, w here matu rity is expressed as a fimction of time.

Nev ertheless, the Nurse-Sau l maturity method is widely used. Researchconducted by Constantino and Carrasquillo at the University of Texas have found errorswith the Nurse-Sau l function to be typically less than 10% when co mpared to in-situsfrengths of the more scientifically correct Arrhenius m ethod (1998 ). Also the sim plicityof the Nu rse-Saul function is easily programm able into a computer (i.e., a spreadsheet)for maturity index calculation, if more adv anced software is not available.

2.4.2 Nurse-Saul Equivalent Age FunctionTo com pare the maturity value expressed by the Nurse-Saul M aturity function

with the Arrhen ius Equ ivalent Age fimction. Equation 2-1 is modified and a referencetempe rature is infroduced. The addition of a reference temperature allows the equation tobe ex pressed in imits of equivalent age as opposed to a temperature-time factor.Equ ivalent ag e is defined as "the curing age at a constant standard reference temperaturethat results in the same relative sfrength gain as under the actual temperature history "(Carino, 1984). The Nurse-Saul E quivalent Age function can be written as shown inEquation 2-2.

t ,(T-TJAtt =-5 [2-2]en swhere

tens = Nurse-Saul equivalent age at the reference temperature

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Tr - reference tem perature, usually taken as 68 F (20 C).Other terms in Equation 2-2 have been defined previously in Equation 2-1.

2.4.3 A rrhenius Equivalent Age FunctionSince Equations 2-1 and 2-2 do not consider the rate of chemical reaction, a more

scientifically correct model is necessary to determine maturity based on changes in therate of curing temperatu re. The Arrhenius Equivalent A ge function, based on theArrhenius equation, does consider the rate of hydration due to fluctuations in curingtemp eratures. It is a know n fact that temperature chan ges influence the rate of thechemical reactions in the concrete during the hydration process (Constantino andCarrasqu illo, 199 8). The base Arrhenius equation, Equation 2-3 , accounts for theincreasing probability of molecule collisions with increasing temperature. In turn, thehigh co llision rate results in a higher kinetic energy, which affects the activation energy,E. Activation energy, which is dependent on the temperature of the concrete, is theenergy required to ensure chemical reaction. Higher temperatures result in ease ofchem ical reaction, wh ereas low temperatures will imped e the reaction rate. The variable"C " in the Arrhenius b ase equation is termed the frequency factor and is the frequency ofproperly oriented collision between reacting molecules.

Ek^ = Ce "^ [2-3]

whereT~ given absolute temperature (Kelvin)kr - rate constant at a given absolute temperature, T

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C = frequency factorR - universal gas constant (8.314 J/(K*mol))E = activation energy

E = 33500 J/mol when r > 293 K (68 F, 20 C)E = 33500 + 1470* (293-r) when T< 293 K (68 F, 20 C).

A maturity index can be determined from the rate constant given in Equation 2-3by computing a ratio between the rate constant at a given temperature (kr), and a rateconstant at a reference temperature (kr). An equivalent age can thus be determined fromthe ratio {kj/K ) and applying intervals of time. Thu s, the Arrhenius Equivalent Ag etakes the form shown in Equation 2-4.

e0tea=Y,

where

1 _L_1R\ T T A t [2-4]

tea = Arrhen ius equivalent age at the reference tem peratureTr = standard absolute reference temperature (293 K)T- average absolute temperature between A t (Kelvin)E and R defined in Equation 2-3A t defined in Equation 2- 1.

The standard reference tem perature, kr, used in this equation is 293 K elvin, wh ichis equivalent to 68 F (20 C ). Equation 2-4 takes into account the rate of hydration b yincludin g activation energy. As shovra in the definition of Equation 2-3, activation

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energy rem ains constant at 3.35 kJ/mol for temperatures above 293 K. For temperaturesbelow this point, more kinetic energy is necessary to ensure chemical reactions. Thus,chemical reactions above 293 K are carried out with relative ease as opposed to thosebelow 293 K. Althou gh, the Arrhenius Equivalent Age function describes the chemicalreaction more correctly, the complexity of its use without a computer program is difficult,as opposed to the Nurse-Sau l Equivalent Age fimction. Com parison of Equation 2-2 and2-4 are made in the following section.

2.5 Maturity Function ComparisonW ith both of the maturity fimctions expressed in units of time, a comparison

betw een the fimctions is possible with respect to an age conversion factor. The ageconversion factor, O L, "converts a curing interval A t to the equivalent curing interval at thestandard reference tem perature" (Malhofra, 1991, 104). The Nurse-Saul fimction.Equation 2-2 can thus be w ritten as shown in Equation 2-5.

t =ya A t [2-5]ens / I m "-"^


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[2-7]where

a =eI 1

y^r T [2-8]With the given age conversion factors. Equation 2-6 and 2-8 can be compared to

determine what effect temperature during A t has on m aturity values calculated fromEqu ations 2-2 and 2-4. This plotted comparison is shown in Figure 2.3 .

4.50

Arrhenius14 23 32 41 50 59 68 77 86 95

Concrete Curing Temperature ("F)Figure 2.3 Comparison of Maturity Functions

104 113 122

To understand Figure 2.3, consider a curing temperature of approximately 95 Fover an interval of one hour, the Nurse-S aul function would indicate a maturityequivalent age of about 1.5 ho urs, whereas the Arrheniu s function wou ld indicate about 2hou rs. As noted before, the Nurse-Sau l function is linear in nature. It also has lower age

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conversion factors when compared to the Arrhenius fimction once the curing temperatureexceed s 68 F (20 C). In confrast, the Nurse-Sau l function has slightly higher ageconversion factors when curing temperatures range from approximately 23 F to 59 F(-5 C to 15 C). Both functions have a conversion factor of 1 when the curingtemp erature is 68 F (20 C) as is expected since this is the reference temperature. Aslong as curing tem peratures do not vary significantly from the reference temperature,equivalent ag es should be similar whether using the Arrhenius or Nurse-Saul function.As will be discussed in Section 4.4, constant curing temperatures of 68 F (20 C) arenever achiev able in the field due to diurnal temperature fluctuations. For most sites inTex as, the actual concrete temp erature exceeds the reference tempe rature, (i.e., 68 F).For this reason , equivalent ages based on the Arrhenius function are higher than those forthe Nurse-Saul function.

2.6 Sfrength Gain M odelsOnce m aturity values (i.e., equivalent ages), are know n, they must be related to a

sfrength gain mo del. This sfrength gain model in tum can be used to predict concretesfrength at any time. Strength gain models for each maturity fimction are discussed in thefollowing sections.

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2.6.1 Freiesleben Hansen (FH) Sfrength Gain ModelFreiesleben Hansen and Penderson (1985) suggested that the strength gain model

used for the Arrhenius equation should be very similar to the relationship between heat ofhyd ration and maturity as shown in Equation 2-4. The Freiesleben Hansen (FH ) strengthgain mod el is shown in Equation 2-9.

S(U = S ,e''-^ [2-9]where

S(tea) - compressive sfrength as a fimction of equivalent tim e (tea)Sinf- final value of sfrength ST- sfrength time constanttea defined previously in Equation 2-4.Sinf is the asymp totic value of concrete strength associated with an infinite

equivalent age. The strength time constant, r, is the maximum equivalent age value ofthe differential of comp ressive sfrength (5) with respect to the log of tea as shown inEquation 2-10.

dSr = max [2-10]d\og{tJAll variables previously defined in Equations 2-9 and 2-4.The sfrength curvature factor, a, is the rate of sfrength developm ent. W ith the aid

of Quadrel, a comp uter software developed b y Digital S ite Systems, Inc. (DSS), thevalues oiSirif, T, and flrare easily determined. These values are determined by inp utting

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cylinder compressive sfrength data with its corresponding maturity value into theQuadrel softwire. This data is then analyzed by using a nonlinear regression routine tofind a curve o f best fit as shown in Figure 2.4.

Compressive Strength Data^ F H best fit100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0

Maturity asArrhenius Equivalent Age (hrs)Figure 2.4 Sfrength-Maturity Curve Based on FH M odel

2.6.2 Nahiral Log (LN) Sfrength Gain ModelSfrength gain using the Nurse-Saul function is modeled with a natural log curve.

Plowman (1956) observed that strength data plotted with respect to the natural logarithmof the ma turity value resemb led a sfraight line . He therefore based sfrength gain on anatural log curve. Plo wm an's equation. Equation 2.11 shows the natural log sfrengthcurve w ith som e modifications to the original equation to express sfrength as a functionof the Nurse-Saul equivalent age.

S(t^) = a,\n(teJ +


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whereS(tes) - compressive sfrength as a function of equivalent age (4,)ai - natural log best-fit cu rve constant02 = natural log best-fit curve constanttens defined in Equation 2-2.Altho ugh , a best-fit curve for the natural log strength gain model is easier to

calculate, som e limitations exist. First, the natural log strength gain model equation,Equation 2-11, predicts continual increasing sfrength w ith increasing maturity. Second,due to the linear characteristic of the Nurse-Saul eq uation. Equation 2-2 is not valid atvery early maturity values (Malhofra, 1991). Sfrength-maturity curves developed fromEquation 2-11 are similar to Figure 2.4. An example of a strength-maturity curve basedon the Natural L og (LN ) sfrength gain model is shown in Figure 2.5. Sfrength-maturitycurves based on Equa tion 2-2 are discussed in more detail in Section 4.3.2.

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aEoo 0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0Maturity as Nurse-Saul Equivalent Age (hrs)F ^ e 2.5 Sfrength-Maturity Curve based on LN Model

2.7 Summary

Determining in-situ strength and behavior of concrete can be accomplished moreeasily with a backgro und of the maturity method, as provided in this chapter. Byknow ing the m any factors which affect sfrength gain due to (1) the chemical com positionof cement, (2) the hydration process , and (3) curing metho ds, better interpretations of testresults are poss ible. As w ill be discussed in Chapter 4, bridge deck sections that receiveno m oist curing (i.e., 0-day cure), but only a curing com pound, ex hibit significantiydifferent sfrength g ains as opposed to sections with some duration of wet mat curing.

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Monitoring and recording temperature profiles of the bridge deck and field castcylinders helps determine maturity values by applying either the Nurse-Saul or ArrheniusEqu ation, Equa tions 2-2 or 2-4, respectively. Strength-maturity curves can then bedeveloped from the field-cured cylinders and compared to in-situ strengths from thecorrespo nding m aturity value of the bridge deck. As will be discussed in Chapter 4,predicted sfrengths from b oth m odels (Equations 2-9 and 2-11) give satisfactory resultswith the exception of the 0-day cure sfrength-maturity cu res. Also , a sfrength comparisonis made betw een fraditional cylinder breaks and the in-situ strength.

The app roach and testing procedure to accom plish sfrength determination ispresented in Chapter 3. Althoug h, the overall objective of this TxDO T project is to assesthe effect of different wet m at curing durations on the durability on in-situ concrete, thefocus of Chapter 3 will be on comp ressive strength.

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CHAPTER 3TEST SETUP AND APPROACH

As a combined effort between TxDOT and Texas Tech University, researchers atTexas Tech University have begun a process that potentially will lead to the developmentof new guidelines as to when b ridge deck s can be open to (a) constiaiction traffic and (b)full fraffic without sacrificing concrete durability. To achieve this objective, various mixdesigns have been subjected to different curing durations to observe what effect, if any,this will have on the durability of concrete. Wet m at curing durations tested are for 2, 4,8, 10, and 14 days from the time of pour. Also, no "moist curing " is considered and willhereafter be referred to as 0-day cure. As mentioned in Section 1.1, the focus of thisthesis is on the 0-, 4-, and 10-day cure field site pou rs.

To achieve this objective, seven concrete mix designs used in different districtswith in Texa s are being tested in lab facilities located at Texas Tech University. Of theseven mix designs , five were actually tested in the field in five different TxD OT districts.At each site, a relatively small-scale, though full-size, bridge deck was constructed. Inaddition, numerous concrete specimens (e.g., cylinders, beams, freeze-thaw) were castalongside the bridge deck. By having bridge decks throughout the state, as opposed tosimp ly in a laboratory, realistic environmental effects are inherently considered w henassessing the quality of a particular concrete mix . Mix designs used in this research arelisted in Table 3.1.

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.1COUQ

u1

Dsrcs

u

k2

hJ

naca 3000I 2000" 1000

0

8000o 7000; | ' 6 o o o 4

bOc 5000C / 5u> 40003000I 2000

^ 10000

0-Day Cure Data 0- D ay Cure Fit 4-Day Cure Data 4- D ay Cure Fit 10-Day Cure Data 10-Day Cure Fit0 100 200 300 400 500 6 00 700 800 900 1000 1100 1200 1300

Maturity as Arrhenius Equivalent Age (hours)

Figure 4 .8 SAT FMAT Cyl inders - FH Model

i^x ^ ^ ^

- I

ri- \ 1 - 1 i 1

0-Day Cure Data 0 -D a y Cure Fit 4-Day Cure Data 4-Day Cure Fit 10-Day Cure Data 10-Day Cure Fit1 1 1 1 1 I

100 2 00 300 400 500 600 700 800 900 1000 1100 1200 1300Maturity as Arrhenius Equivalent Age (hrs)

Figure 4 .9 HOU FMAT Cyl inders - FH Model

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4.3.2 Nurse-Saul Eouivalent Age Strength-Mattiritv CurvesIn an attempt to verify which mattirity function best serves to predict con crete

compressive sfrengths, strength-mattirity curves were also developed for the Nurse-SaulEq uivalen t Age function. The same temperattire history data was used to determine amattirity value based on Eq uation 2.10 w ith a datum temperattire of 14 F (-10 C ) and areference temperature of 68 F (20 C).

S(t,^) = a,ln(t^^) + a, [2-11]

Coefficients for the Natural L og Sfrength Gain Mo del (LN mo del) are listed inTable 4.2. Figures 4.10 through 4.12 show the developed sfrength-maturity curves basedon the Nurse-S aul E quivalent A ge fimction w ith a natural log best-fit curve.

As discussed in Section 2.5, maturity values using the Nurse-Saul Equivalent Agefunction are lower than those for the Arrhenius E quivalent Age function for curingtemperatures above 68 F (-10 C). Since curing temperatures were usually above 68F,it should be exp ected that equivalent ages du e to the Arrhenius function will be higherthan those for the Nurse-Saul function. Maturity values using the Arrhenius and Nurse-Saul fimctions for the FM AT cylinders are discussed and compared in more detail inSection 4.4.

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Table 4.2 Coefficients for LN Model (FMAT Cylinders)Dist r ic t

FTW

SAT

HOU

Curing Durat ion041004100410

a i663.1839.3918.6864.71191.31166.01056.81233.71139.0

32-597.7

-1261.3-1645.4464.8-928.9-873.4

-1282.4-1589.7-1272.3

9000.-- 8000CO^ 7000|j 6000I 5000> 4000

S 2000^ 1000 0-Day Cure Data 4-Day Cure Data 10-Day Cure Data 0-Day Cure Fit 4-Day Cure Fit 10-Day Cure Fit

100 200 300 400 500 600 700Maturity as Nurse-Saul Equivalent Age (hrs)

800 900

Figure 4.10 FTW FMAT Cylinders - LN Model

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8000^ 7000^ 6000 4 - *00

5000^ 4000.> 3000g- 20006 1000

0

* r T ^ ^ ^ ^ ^

1

^ 1

0-Day Cure Data 0-Day Cure Fit 4-Day Cure Data 4-Day Cure Fit1 1 1

10-Day Cure Data 10-Day Cure Fit1 1 1

0 100 200 300 400 500 600 700 800Maturity as Nurse-Saul Equivalent Age (hours)

900 1000

Figure 4.11 SAT FMAT Cylinders - LN Model

8000si

.^.c4-e>u

- 4 - *C/5>Su

o

7000600050004000300020001000

^l _ ^ ^ : : - = = S =

^^^\\^^^''^

*

* P ^ ^ ^

1

0-Day Cure Data 0-Day Cure Fit> , 4-Day Cure Data 4-Day Cure Fit

1 1

10-Day Cure Data 10-Day Cure Fit1 1

100 200 300 400 500 600 700 800Maturity as Nurse-Saul E quivalent Age (hrs)

900 1000

Figure 4.12 HOU FMAT Cylinders - LN Model

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4.4 Equivalent Ape Comparison of FMAT CylindersEqu ivalent a ges for different concrete mixes and curing durations are compared

for both the Arrhenius and Nurse-Saul fimctions in Table 4.3 and illustrated in Figures4.13, 4.14 , and 4.15. From Table 4.3 and Figures 4.1 3, 4.14, and 4.15, it is shown thatthe Arrh enius function yields a higher equivalent age as opposed to the Nurse-Saulfunction. This is accounted for by having curing temperatures above 68 F (20 C). Forexample, when comparing the equivalent ages for the Fort Worth distiict mix with a 4-day curing duration and Day 7 testing, equivalent ages for the Arrhenius and Nurse-Saulfunctions are 226 and 202 hou rs, respectively. Referring back to the temperature historyfor the FTW FMAT cylinders in Figure 4.4, curing temperatures typically were above 68F , with the exception of Days 8 through 13. Therefore, equivalent ages for theArrhen ius function were higher, even w ith curing temperatures below 68 F for a 5-dayperiod. Of course, had the curing temperatures been below 68 F for the entire curingduration, the Nurse-Sau l function would give a higher maturity equivalent age.

Altho ugh both of these fimctions are being investigated, it is not the intent of thisresearch to determ ine which function best determines the "true" equivalent age of theconcrete but to m ake an attempt to determine w hich fimction is more suitable to predictthe sfrength o f concrete.

The m aturity values listed in Table 4.3 are the equivalent ages used to develop thesfrength-maturity curves for the FMA T cylinders. As mentioned in Chapter 2, thematu rity value and com pressive sfrength for the 0-day cure cylinders were used as thefirst data point for Day 1 testing for the 0-, 4-, and 10-day cure cylinders. Since all

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cylinders were capped and kept in plastic molds the first 24 hours, the 0-day cure cylinderessentially experienced the same amount of hydration as the 4 and 10-day cure cylmders.It is for this reason that maturity values for the 4- and 10-day cure FMAT cylinders arethe same for Day 1. Similarly, the 10-day cure cylinders experience the same curingcond itions as the 4-day cure cylinders up until Day 4. Thus, the maturity value andcom pressive sfrengtii for the 4-day cure cylinders on Day 3 are used for the developm entof the 10-day cure maturity curve and 10-day cure cylinders on Days 1 and 3.

(U1

1300120011001000

90080 070 060050 0400300200100

0

D 0-Day Arrhenius El 0-Day Nurse-SaulD 4-Day Arrhenius 0 4-Day Nurse-SaulD 10-Day Arrhenius B 10-Day Nurse-Saul

r M ~ l T f l g l ^ M M J ;

g

D ay l Day 3 Da y? Day 14Test Age (Calendar Days)

Day 28

Figure 4.13 FTW FMA T Equivalent Age C omparison

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Day 1 Day 3 Day 7 Day 14Test Age (Calendar Days) Day 28

Figure 4.14 SA T FMA T Equivalent Age Comparison

Ag(h

uve

1300120011001000900800700600500400300200100

D 0-Day Arrhenius 0 0-Day Nurse-Saul 4-Day Arrhenius B 4-Day Nurse-SaulD 10-Day Arrhenius 0 10-Day Nurse-Saul

I g?r-i-:-r-MI

Day 1 Day 3 Day 7 Day 14Test Age (Calendar Days)

Day 28

Figure 4.15 HOU FM AT E quivalent Age C omparison

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Table 4.3 Equivalent Ag e Values Based on Arrhenius and Nurse-Saul(FMAT Cylinders)District

FT W

SAT

HOU

EquivalentAge FunctionArrhenius

Nurse-Saul

Arrhenius

Nurse-Saul

Arrhenius

Nurse-Saul

CuringDuration041004100410041004100410

Equivalent Age for Testinq Davs (hrs)1383838323232565656414141333333292929

31029696888686139137137107107107899090828383

725122622321220220130829428524023623422 5221219199198198

1437734934835133934060957855647 446 545 947 346 446 841 441 141 5

2873470470069968668 512411197116993799391599898 699085 0846850

4.5 Co mparison of Bridge Deck In-Situ Strength to Predicted StrengthW ith the develop ment of strength-maturity curves, in-situ concrete sfrength can

be compared to predicted sfrengths from both the Arrhenius and Nurse-Saul generatedcurves . First, the in-situ equivalent age of the bridge deck is determined. By utilizing thetempe rature history o f each concrete deck section, two equivalent ages for each sectionwere determined using both the Arrhenius and Nurse-Saul fimctions. Thus, two maturityequivalent ages were comp uted for each section of the bridge deck. W hen d eterminingequivalen t ag es, it is assumed that an entire bridge deck section (i.e., 0, 2, 4, 8, 10, or 14-

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day cure section) experienced approximately the same curing temperature and thus hasthe same maturity value throughout that specific concrete slab section.

As shown in Table 3.3, three concrete cores from each curing section of thebridge deck w ere sam pled and tested on Days 4, 7, 10, 14, and 28 with the exception oftiie 10-day cure section. Concrete cores were sampled and tested only on Days 10, 14,and 28 for the 10-day cure section because cu ring cotton m ats and plastic sheetingcovered this section during earlier testing days. Since no samples were taken for Days 4and 7 for the 10-day cure slab, no com pressive data was available for these days.Therefore, m aturity values for the bridge deck on specified sampling and testing days, asshow n in T able 3 .3, were d etermined to predict concrete sfrength of the deck based on thedeveloped F M AT sfrength-maturity cu rves. Com puted equivalent ages for the bridgedeck usin g the A rrhenius and Nurse-Saul fimctions are listed in Table 4.4 and shown inFigures 4.16, 4.17, and 4.18.

Ag ain, it is observed that higher maturity values existed for the Arrheniusfimction as opposed to the Nu rse-Saul function. Also , maturity values for the 0-, 4-, and10-day cure sections of the bridge deck do not differ significantiy from each other on anygiven testing day. This can be attributed to the similar curing temperature experienced byeach section of the bridge deck as show n in Figures 4.1 through 4.2 since equivalent ageis solely based on temperature and time and not the amount of moisture availability.

Since concrete temp erature data for the 0- and 4-day curing portions of theHouston bridge deck were unavailable as mentioned in Section 4.1, maturity values for

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tile 10-day cure section were used for the 0- and 4-day curing sections of the Houstonbridge deck as shown in Table 4.4.

Table 4.4 Equivalent Age Values Based on Arrhenius and Nurse-Saul(Bridge Decks)District

FTW

SAT

HOU

EquivalentAge FunctionArrhenius

Nurse-Saul

Arrhenius

Nurse-Saul

Arrhenius

Nurse-Saul

CuringDuration041004100410041004100410

Equivalent Age for Testinq Davs (hrs)4

19820420416616816821021721 7157161161144144144141141141

728729 229 224524624 633834634625926326 3272272272231231231

1034233032931431530746 747 748536 1366370368368368318318318

1441 942 940 838939038365166068 051251752652052052044 544 5445

2877178876474 2745737127712831306100410081019114111411141947947947

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"3

1300120011001000900800700600500400300200100

D 0-Day Arrhenius E3 0-Day Nurse-Saul 4-Day Arrhenius 13 4-Day Nurse-SaulD 10-Day Arrhenius B 10-Day Nurse-Saul

Day 4 Day 7 Day 10 Day 14Test Age (Calendar Days)

Day 28

Figure 4.16 FTW Bridge Deck Equivalent Age C omparison1300120011001000

M_^ui)


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3orW

1300120011001000

90 080 070 060 0500400300200100

0

10-Day Arrhenius B 10-Day Nurse-Saul

mDay 4 Day 7 Day 10 Day 14

Test Age (Calendar Days)Day 28

Figure 4.18 H OU Bridge Deck Equivalent Age Comparison

Once m aturity values w ere determined for the bridge decks, sfrength predictionswere mad e based on developed sfrength-maturity curves from the FM AT cylinders. TheFreiesleben Han sen and natural log sfrength gain models with their respectivecoefficients as shown in T ables 4.1 and 4.2 were used to determine predicted concretebridge d eck sfrengths. Sfrengtii gain mod els, coefficients, and in-situ equivalent ageswere entered in a spreadsheet to compare the accuracy of both the Arrhenius and Nurse-Saul Equivalent A ge functions. Com parison of in-situ to predicted sfrengths for theFTW , SAT , and HO U bridge decks based on sfrength-maturity cmves from FM ATcylinders are shown in Tables 4.5 through 4.7 and shown in Figures 4.19 through 4.21.

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Day 4 Day? Day 10 Day 14Test Age (Calendar Days)

Day 28

Figure 4.19 FTW In-Situ Sfrength Compared to FMAT Predicted Sfrength

Table 4.5 FTW In-Sitti Sfrength Compared to FMAT Predicted Sfrength

0-daycure

4-daycure

10-daycure

In-Situ Strength (psi)ARR Predicted Strength (psi)

% ErrorN-S Predicted Strength (psi)

% ErrorIn-Situ Strength (psi)

ARR Predicted Strength (psi)% Error

N-S Predicted Strength (psi)% Error

In-Situ S trength (psi)ARR Predicted Strength (psi)


% Error

Test Ag4

21912899

322791

2731223256

43037

3_-

727173144

163054

1234703563

33360

3_-

e (Calendar Days)10

25173253

293218

2836033659

23566

135403767

63614

2

1427963373

213359

2038243851

13747

239593949

13817

4

2832103700

153788

1844774229

64290

442624401

34419

4

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80007000

) 6000JStog 5000^ 4000 - HI 3000^ 2000

1000 -

Jl [kn

10-Day In-Situ D 0-Day Arrhenius 0 0-Day Nurse-Saul14-Day In-Situ D 4-Day Arrhenius 13 4-Day Nurse-Saul10-Day In-Situ D 10-Day Arrhenius 0 10-Day Nurse-SaulI l a w ^ i w a ^ iia aia i w i iDay 4 Da y? Day 10 Day 14

Test Age (Calendar Days) Day 28

Figure 4.20 SAT fri-Sitti Strength C ompared to FM AT P redicted Sfrength

Table 4.6 SAT In-Sitti Strength Compared to FMAT Predicted Strength

0-daycure

4-daycure

10-daycure



% ErrorIn-Situ Strength (psi)


N-S Predicted Strength (psi)% ErrorIn-Situ Strength (psi)



Test Afl42917

500171

483966

48155340

115122

6-

731675411

715271

6654045927

105710

6-

e (Calendar Days)103365

565868

555765

55726290

136102

1052906148

166022

14

1434295886

725860

7158826624

136514

1155256504

186432

16

2839896272

576442

6266167205

97310

1164647122

107202

11

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80007000

. 6000Ien

5000^ 40002Io 3000 2000

1000 0-Day In-Situ4-Day In-Situ10-Day In-Situm m

0-Day Arrhenius E3 0-Day Nurse-SaulD 4-Day Arrhenius 0 4-Day Nurse-SaulD 10-Day Arrhenius S 10-Day Nurse-SaulW M M W M M m M M ! l jlDay 4 D ay ? Day 10 Day 14

Test Age (Calendar Days) Day 28

Figure 4.21 HOU In-Sitti Sfrength Compared to FMAT Predicted Strength

Table 4.7 H OU fri-Sitti Sfrength Com pared to FMA T Predicted Strength

0-daycure

4-daycure

10-daycure

In-Situ S trength (psi)ARR Predicted Strength (psi)


% ErrorIn-Situ S trength (psi)





% Error

Test Ag4

72063924

463949

4555214720

154517

18_

774894506

404471

4058725483

75127

13

e (Calendar Days)10

82374749

424807

4262235788

75519

1162755472

135291

16

1480274998

385161

3668546092

115932

1463755759

105672

11

2881655463

335961

2768556629

36866

169376279

106534

6

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As shovm in Figures 4.19 through 4.21, predicted strengths from developedsfrengtii-maturity curves for tiie 0-day cure sections deviate drastically from the in-sittasfrengths o f the bridge deck. This is a result of not providing any type of moist curing tothe surface of the deck after the initial pour and only applying a curing com pound .Apparently, the curing compound did not provide adequate protection against moistureloss for the 0-day cure portion of the deck slab during the first 24 hours of hydration,wh ich is a critical period d uring hardening and sfrength gain for concrete. On the otherhand , the 0-day cure cylinders were capped after preparation for the initial 24 hours afterthe pour. By preven ting m oisture loss for the 0-day cure cylinders during the first 24hours , adequate co ncrete hydration occurred from the concrete mix water and resulted inhighe r sfrength gains for these cylinders, thus higher strength data was used to developsfrength-maturity curves. As a result of higher sfrength data for the 0-day cure FMA Tcylind ers, sfrength-maturity curves developed u sing either the FH or LN sfrength gainmo dels predicted hig her sfrengths for the bridge deck.

From Figures 4.19 and 4.20 and referring to Tables 4.5 and 4.6, predictedsfrengths for the 0-day cure section, whether using the Arrhenius (FH m odel) or Nurse-Saul (LN m odel) fimctions, were higher than actual m -situ sfrengths for the section.Predicted sfrengths for the Fort Worth and San Antonio bridge decks deviated anywherefrom 12 to 72 % of in-situ 4-inch diameter core sfrengths.

In confrast, predicted sfrengths for the 0-day cure Hou ston bridge deck w ereactually low er than tiiose of the acttial in-sihi bridge deck sfrength w ith no curing. Thedeviation for the 0-day cure Houston bridge deck ranged anywhere from 27 to 46% of

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acttial in-sitti sfrength. Al so, it is seen in Figure 4.21 that 0-day cure sections of thebridg e deck had higher com pressive strengths than the 4- and 10-day cure sections. Thisobservation was not expected as it is normally believed that concrete with some curingwo uld hav e higher compressive sfrengths than concrete with no curing. Reasons for thisdiscrepan cy co uld be due to any, or a combination, of the following: (a) high relativehum idity at the H ouston site resulting in ongoing curing for the 0-day cure section and(b) lack of quality confrol of the delivered concrete. Most of the Houston bridge deck(i.e., 4-, 10-, and 14-day cure sections), FMAT and CONVT cylinders were poured with aconcrete m ix h aving a slump of 6 inches, which d eviated from the 3 V^-inch designslum p. The remain ing sections (0- and 2-day cure sections) were poured from a concretemix h aving a slum p of 4 inches - though the second mix delivered from the plant shouldhave been identical to the first.

W hen c omp aring the sfrength predictions for the 4- and 10-day cure sections, itwas observed that deviations from the in-situ sfrength were much less when compared tothe 0-day c ure section. For the 4- and 10-day cure sections, the predicted sfrength errorsranged anyw here from 0.3 to 17.7% and 1.0 to 16.4% with the use of the Arrhenius andNu rse-Saul fimctions, respectively. Even with lower percentage errors for the 4- and 10-day cure sections, it was evident that error percentages w ere still somewhat large whencom pared to errors for the Fort Worth and San Antonio concrete mixes. Usually apercentag e error of 10% or less would seem to be more desirable, even though currentlythere is no known rule of thumb or guideline to determine an acceptable percentage error.

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For in stance, the sfrength predictions for the 4- and 10-day cure section of theFort W ortii bridge d eck were very similar to the in-situ concrete sfrength. The 4- and 10-day cu re sfrengtii prediction s on ly varied 1 to 6% from in-sitti strength using either theArrhenius or Nurse-Saul functions as shown in Table 4.5. With percentage errors at orbelow 6% for the 4- and 10-day cure sections of the Fort Worth bridge deck, one isinclined to have co nfidence in using the maturity method as a means to predict in-placesfrength. No te that this agreement is mu ch closer than was found using ttaditionalcylinder breaks as discussed in Section 4.8.

Predicted sfrength percentage errors for the San Antonio and Ho uston bridgedecks we re slightly larger than those for the Fort W orth bridge deck. The percentagedifferences betw een the predicted strengths and the 4- and 10-day cure San Anton io in-situ sfrengths ranged from 6 to 18% using either the Arrhenius or Nurse-Saul functionswith their respective sfrength g ain mo dels. For Houston, the differences ranged from 1 to18%.

Altho ugh, there are some percentage errors above 10% for the San Antonio andHouston bridge deck, most of the percentages fall below the 15%. Of the predictedsfrengths, on ly 5 of the 32 strengths had percentage errors greater that 15 % . In fact, 27 ofthe 32 predictions had an error of 15% or less. Again, since there is no guideline todetermine an acceptable error, engineering judgment should be used to determine if theseerrors w ould be acceptable.

As observed, percen tage errors varied for each of the test sites and could be aresult of scatter of the 4-inch diameter in-situ core or FM AT cylinder sfrength data. The

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compressive strength data for in-situ cores and FMAT cylinders is shown in Appendix Aand B respectiv ely. Referring to Tables 4.5 through 4.7, one should notice that some ofthe in-situ sfrengths for earlier test dates have higher compressive strengths than for latertest dates . For instance on the 0-day cure section for the Fort Worth D eck (Table 4.5),com pressive sfrengths of 2717 and 2517 psi were attained on Day 7 and 10, respectively.Of cou rse, it is highly unlikely that concrete compressive sfrength actually went down. Inthis ca se, it is theorized that the variability of con crete sfrengths is a result of poorconcrete placement practices such as lack of consolidation when vibrating concrete,variability of current testing practices, and/or the non-homogeneous nature of concrete.

Also, concrete m ixes used for the bridge deck did not necessarily reflect the actualmix design as stated in the specifications. For example, the Houston concrete mixspecified a 3 '/2-inch design slum p, but the trucks delivered a concrete mix w ith slumps of6 and 4 inches. On num erous o ccasions, exfra water was added to the concrete mix, as iscommon practice when placing concrete in the field. Addmg exfra water to the mix wasusually the decision of the contractor in order to increase the workab ility of the con crete.

Altho ugh it is obviou s that there was a lack of quality confrol d min g co ncreteplacem ent, it w as not the intent of this project to monitor quality confrol. The objectiveof the overall project is to determine appropriate wet m at curing durations to en suredurability of concrete with current concrete placemen t practices. This meant havingconfractors p our con crete in the same m anner as would be done on any other constiuctionproject. As a result of the variability of field practices, Texas Tech researchers believedit would b e beneficial to perform lab tests on these same mixe s.

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4.6 Generation of LMAT Strength-Maturity CurvesAn other portion of testing involved the generation of sfrength-maturity curves

from LM AT cy linders. These developed curves were then used to see how well strengthscould b e predicted for tiie bridge deck. Three of the concrete mixes used in the field sitehave been tested. These include concrete mixes for the Fort Worth, San An tonio, and ElPaso districts.

Th e process for developing lab maturity curves was performed in the sameman ner as for the FMA T cylinders. Curing concrete temperatures for the cylinders weremo nitored by thermo couples and recorded by dataloggers. Temp erature histories for theLMAT cylinders were then utilized to determine maturity equivalent ages using both theArrhe nius and N urse-Saul Equivalent A ge fiinctions for days when cylinder testing wasperformed. Cylinder breaks w ere performed on Days 1, 3, 7, 14, and 28 as recommend edby A STM CI 07 4-9 3. The scheduled testing and number of cylinder breaks are shown inTable 3.7.

Onc e all compressive data had been collected and linked w ith its correspondingequivalen t age, strength-maturity cm-ves were generated. Maturity curves for the LMA Tcylinders w ere developed with the FH sfrength gain model using Quadrel software andthe LN strength gain m odel using a spreadsheet with their respective equivalent ages asdiscussed in Section 4.3.1 and 4.3.2. Coefficients for both of the sfrength gain models aretabulated in Tables 4.8 and 4.9.

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Table 4.8 Coefficients for FH Model (LMAT Cylinders)Dist r ic t

FT W

SAT

ELP

Curing Durat ion041004100410

S|nf6814.3310212.62

L7702.385719.427950.589882.334129.258300.767732.54

a.4394.4098.6048.5936.7046.742.528.71

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Concrete Strength Development

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