Effect of Temperature-Time History on Concrete Strength in Mass ...

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Email: alper.yikici@mail.wvu.edu. 13. 14. Dr. Roger H.L. Chen (Corresponding Author). 15. Professor. 16. Department of Civil and Environmental Engineering.
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Effect of Temperature-Time History on Concrete Strength in Mass Concrete Structures Tahsin Alper Yikici, M.Sc. Research Assistant Department of Civil and Environmental Engineering College of Engineering and Mineral Resources West Virginia University PO Box 6103 Morgantown, WV 26506-6103 Ph: 304-293-4013 Fax: 304-293-7109 Email: [email protected] Dr. Roger H.L. Chen (Corresponding Author) Professor Department of Civil and Environmental Engineering College of Engineering and Mineral Resources West Virginia University PO Box 6103 Morgantown, WV 26506-6103 Ph: 304-293-9925 Fax: 304-293-7109 Email: [email protected] Submission Date: November 15, 2012 Number of Words in Text: 3515 Number of Tables: 4 Number of Figures: 8 Total Equivalent Number of Words: 6515

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ABSTRACT Concrete maturity method is a popular non-destructive testing method to estimate inplace strength development of concrete structures. Many state highway agencies adopted procedures for using maturity method to obtain better quality control while monitoring in-place strength development in real time. In this study, maturity method was used to estimate in-place strength of large concrete placements. Four 6-foot cube blocks were constructed in four different locations and calibration curves for those concrete mixtures were established using 6x12 inch cylinder specimens collected from the construction site. Temperature sensors were embedded in specific locations throughout the depth of the cubes, and the equivalent age of the in-place concrete was calculated. 4-inch diameter core samples, with 6-foot in length, were taken from the cubes at four-day after construction and the core strengths were compared with the predicted strengths using maturity. In addition, activation energy values were determined in the laboratory and used for equivalent age calculations as recommended in ASTM C 1074. According to the test results, the concrete top surface strength prediction is always higher than the actual core strength. For three cube constructions, core results from mid-section were close to the predicted strengths and core results from the bottom section were higher than the predicted values. Results show that in-place concrete strength is being influenced by several factors other than temperature, including the location of the sample in the structure, lack of compaction quality, higher air content and in-situ water-cement ratio, so that establishing a reliable maturity and in-place strength relationship is rather difficult within given circumstances. The results of this study provide useful information to examine the accuracy of the maturity method used in the estimation of in-place concrete strength in large structures.

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INTRODUCTION The strength of properly batched, placed and vibrated concrete does not depend only on the curing time, but also on the temperature-time history. This concept is known in the concrete industry as maturity concept. According to the maturity concept, an empirical relationship can be established between temperature-time history and strength development of the concrete in order to predict strength of in-place concrete during the curing period (1). ASTM C 1074 recommends maturity method as “a technique for estimating concrete strength that is based on the assumption that samples of given concrete mixture attains equal strength if they attain equal values of maturity index”(2). The method assumes that the temperature-time history of concrete can be used to develop a strength-maturity curve that is specific to each mix design. By preparing these correlation curves, the strength development of in-place concrete can be estimated by monitoring the concrete temperatures in real time. Consequently, this information can be used to make decisions (e.g. time of formwork removal, or time of post-tensioning) that save time and reduce the construction cost (1). Furthermore, monitoring concrete temperatures, especially in mass concrete pours, can be used to prevent high internal concrete temperatures and large temperature gradients that are specified by several state agencies in order to reduce the possibility of thermal cracking. Many state transportation agencies have already instituted procedures or are still conducting research projects to implement the maturity method to predict in-place concrete strength. According to the West Virginia Department of Highway (WVDOH) survey results conducted in 2007, twenty-five out of thirty-six states used the maturity concept mainly as a substitute for early cylinder compressive strength to allow formwork to be removed or pavements to be opened to traffic (3). In 2008, Auburn University employed maturity method on several precast, prestressed girders and a bridge deck. They concluded that the method can be used accurately for estimating in-place concrete strength up to an equivalent age of seven days (4). University of Washington researchers reported in 2009 that the maturity method was used in three different Portland cement concrete pavement (PCCP) projects in order to open traffic faster. Only one out of the three trials was successfully conducted (3). Similarly, University of Maryland evaluated maturity method for use in pavements and they concluded that the procedure is very sensitive to the constituent materials and concrete mixtures. They recommended taking extreme pre-cautions in order to obtain maximum accuracy when using maturity method for field applications (5). Furthermore, the cross over effect (1) due to high temperature curing has been shown to limit the applicability of maturity method in predicting the behavior of concrete that has high early temperature, such as mass concrete construction. One of the objectives of this study is to investigate the applicability of maturity method to estimate the in-place concrete strength of large bridge sub-structure elements, such as piers, pier footers, pier caps or abutments, using WVDOH approved Class B concrete mixtures that are currently used in bridge projects. Class B concrete, as described in WVDOH Standard Specifications, has minimum 3,000 psi (20 MPa) 28-day design strength with optimum 4-inches slump and 7% target air. Class B concrete may be designed using supplementary cementitious materials such as fly-ash, ground granulated blast furnace slag (GGBFS) or micro-silica with 564 pound per cubic yard (330 kg/m3) target cement content and 0.49 maximum water-cementitious ratio. In addition, this study outlines the effect of the strength development from the temperature

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variations in concrete throughout the depth of 6-ft concrete cubes. This paper presents test results from four different 6-ft cube constructions and the predicted in-place concrete strength using a maturity function based on concrete equivalent age. RESEARCH METHODOLOGY Six-ft concrete cube blocks were constructed at different locations in West Virginia, using Class B concrete from local ready-mix plants. Temperature sensors were instrumented, fresh concrete properties were determined and 6x12 inch cylinders were taken for the maturity test. Core samples were taken from the hardened concrete cube block and the measured compressive strengths from the core samples were compared to the predicted strengths from equivalent age calculations. Activation energy values for the concrete mixtures were determined in the laboratory following ASTM C 1074 (2). SIX-FT CUBE CONSTRUCTION Six-ft concrete cubes were constructed at four different WVDOH districts (D1, D5, D6 and D9), located in Charleston (D1), Lewisburg (D9), Martinsburg (D5), and Wheeling (D6), pouring approximately nine cubic-yards of concrete in each cube provided by local ready-mix concrete plants. The concrete mix design for each casting is given in Table 1. The cube blocks were constructed two feet in the ground on a two inch layer of #57 limestone. Each cube was instrumented with temperature loggers attached on a rebar cage (Figure 1). A schematic of the sensor locations is given in Figure 2. Concrete was poured directly from the mixer truck without pumping and then was subjected to vibration in order to get sufficient compaction. Ordinary surface finish using wood-float rubbing was applied on the top surface. The concrete surface was maintained completely and continuously moist during the seven-day curing period. After the concrete placement the top of the block was covered with white polyethylene sheeting. If necessary, concrete blankets were used on top surface as well as around the formwork on the side surfaces. One of the purposes of the six-foot cube constructions was to investigate strength development of in-place concrete by monitoring the temperature distribution in concrete and investigate the applicability and the limitations of the maturity concept for large concrete placements throughout West Virginia.

(a) Instrumentation of the rebar cage

(b) Sensors attached to the rebar

FIGURE 1 Mounting of the temperature loggers.

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TABLE 1 One yd 3 Theoretical Mix Design Item Cement (TYPE I/II), lbs Fly-Ash (TYPE F). lbs GGBFS (Grade 100), lbs Water, lbs Coarse Aggregate (#57), lbs Fine Aggregate, lbs Target Air Content, % w/cm

D1 Class B Fly-Ash 470 75 245 1775 1255 7.0 0.45

D9

D5 Class B GGBFS 423 141 276 1815 1225 7.0 0.49

Class B 564 262 1723 1299 7.0 0.45

D6 Class B – 1 Extra 658 260 1750 1111 7.0 0.40

NOTE: 1 lb = 0.454 kilograms (kg)

6’-0”

6

13

1’-6”

1

6’-0”

2

14

7

3

9

11

10

1’-6”

3’-0”

4 12

8

5 1’-0”

1’-0” 2’-0”

3’-0”

SIDE VIEW

FIGURE 2 Schematic of the sensor locations.

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Experimental Work In order to establish the maturity-strength relationship of each mix, 6x12 inch cylinders were cast during the cube constructions. Additionally, two 6x12 inch cylinders were embedded with temperature loggers recording hourly temperature history. All cylinders were placed inside insulated boxes to reduce the effect from the ambient conditions overnight and then transported the next day to temperature controlled curing tanks at the district material laboratory (Figure 3). Compressive strength of the concrete was determined at 1, 3, 7, 14 and 28 days, testing at least two cylinders at each age. In addition to that, at 4-days, 4-inch-diameter by 6-foot long core samples were taken from the hardened concrete cube (Figure 5) and a total of six 4x8 inch cylinder specimens were extracted from the core along the 6-ft length (Figure 6). The specimens were prepared and tested immediately after coring to represent the in-place compressive strength of the concrete cube at different depth. A schematic drawing that shows the cut locations and specimen designations is presented in Figure 4.

(b) Maturity cylinders

(c) Cylinders on the field

(a) Concrete placement

FIGURE 3 D5-Cube construction and sampling. 1 foot (30.5 cm) 2”

8” 1C

4” 1R

Top of the core 8”

2C

4”

8”

4”

2R

3C

3R

8” 4C

4” 4R

36”

8” 5C

4”

8”

5R

6C

2”

36”

FIGURE 4 Core specimen cut locations and designations.

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(a) Coring machine

(b) 6-ft core

FIGURE 5 Six-ft cube coring.

2’0”

X

6’0”

X

28 days O X

4 days O

X: Temperature sensors O: Coring positions

56 days O X

X

X

28 days O

X 1’0”

2’0”

6’0”

TOP VIEW FIGURE 6 Schematic of the coring locations.

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TEST RESULTS AND DISCUSSION Determination of Activation Energy The activation energy of concrete mixtures was determined experimentally following ASTM C 1074-10 A1 procedure. It requires establishing the compressive strength versus age relationship of 2-inch mortar cubes cured at three different temperatures (2). The mortar was proportioned to have a fine-aggregate to cement- ratio equal to the coarseaggregate to cement ratio of the concrete mixture. Specimens were cured at three different temperatures: high (104°F), low (50°F), and laboratory temperature (73°F). Three cubes were tested at six different times in compression following the recommended test schedule by Tank R. C. (6), based on equal temperature-time factors for different curing temperatures. Upon the completion of the compressive strength tests, hyperbolic equation (1,2,7) was used to fit the set of data to determine the best fit regression parameters, such as the limiting strength, Su, the rate constant of strength gain, k, and the dormant period t0, for three different curing temperatures. After that, natural logarithm of the k-values versus reciprocal curing temperature in Kelvin was plotted. From that, the negative slope of the line is obtained which equals to the value of the activation energy divided by the universal gas constant (R), also known as Q. This calculation is based on the Arrhenius function that is being used to explain the temperature dependence of the rate constant, k (1,7). Apparent activation energy values for the Class B concrete containing fly ash and GGBFS were determined according to ASTM C 1074 procedures. The calculation of this value requires the determination of several parameters using the linear hyperbolic equation:

S  Su

k (t  t 0 ) 1  k (t  t 0 )

where: S= average strength of the cubes at age t t= test age in hours Su=limiting strength t0= age when strength development assumed to begin k= rate constant. It was found that the hyperbolic strength-age function can properly model the strength development with the lowest goodness of fit R-square value 0.93 for each set of experiment. The apparent activation energy values were calculated approximately 45,900 J/mol and 44,750 J/mol for Class B Fly-Ash and Class B GGBFS concrete mixtures, respectively.

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Maturity Calculations Equivalent age approach was used to establish maturity-strength relationship. The actual age of the concrete was converted to its equivalent age at a specified temperature. Equivalent age can be calculated according to the following “Arrhenius Equation”:

where: te = equivalent age Q = activation energy divided by the gas constant (R), Ta = average temperature of the concrete during time interval, Ts = specified (reference) temperature (typically 23°C) Δt = the time interval A calibration curve was prepared from strengths of the laboratory cured specimens using the recorded temperature-time history of the cylinders. The calibration curve can be used to estimate the in-place concrete strength if temperature history of the structure is known (1,2,8). The calibration curve that represents the strength gain of the concrete was modeled using the linear hyperbolic model suggested by ASTM C 1074-10 (2). The age when strength development assumed to begin (t0) was set equal to the final setting time of the concrete. Figure 8 shows the compressive strength versus equivalent age relationships based on the cylinder compressive strength results. The equivalent age of the D1 and D5 concrete were calculated using the measured activation energy values of 45,900 J/mol and 44,750 J/mol, respectively. The equivalent age of the D9 and D6 concrete were calculated using an assumed activation energy value of 41,800 J/mol based on the model proposed by Han S.H. (9).

FIGURE 7 Concrete mix design calibration curve, strength vs equivalent age.

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Core Strength The compressive strength results from the concrete cores obtained at 4-day from each 6-ft cube are listed in Table 2. The test results show that there is a significant strength difference along the depth, between the top (1C) position and the bottom (6C) position. 1C position appears to be the weakest and 5C and 6C positions are the strongest. This appears to coincide with observation by other researchers that cores usually have lower strength near the top surface and the strength increases with depth (10). The core test results clearly indicate the variations from the conditions occurred during concrete placement. During D9 cube construction concrete was delivered in two separate trucks and the air content measured on the field was 7.8% and 9.5%, respectively. The unexpected difference in air content may be the reason that shows a large variation in strength between the cores 3C and 4C positions. During D6 cube construction the slump of the fresh concrete was only 1.75 inches and vibration was very difficult. Hence, honeycombing was observed at the mid-height section from the concrete surfaces. The effect of the segregation and honeycombing on the core strengths was detected between core samples 3C and 4C. Furthermore, there is a possibility of strength reduction due to drilling operations. The coefficient of variation of strength estimation using 4-in diameter cores was presented 4 to 5.5%. (11) However, it is really difficult to separate out the errors due to on-site quality control issues, such as concrete placement, compaction, air-content, actual water to cement ratio, etc. (10). TABLE 2 Concrete Compressive Strength from the 6-ft Core at 4-day 1C

2C

3C

4C

5C

6C

Depth from the surface, inch D1 Cube

2”-10" 3,160

14"-22" 4,670

26"-34" 4,830

38"-46" 4,690

50"-58" 4,850

62"-70" 4,930

D5 Cube

3,880

4,790

4,790

4,870

4,790

5,300

D9 Cube D6 Cube

2,420 4,460

2,660 5,710

* 4,100

3,620 3,310

3,670 5,250

4,010 5,250

Note: 1 inch = 2.54 cm, * the core was broken at 3C position

In-Place Concrete Strength Prediction Even though the maturity method is more reliable in predicting the relative strength than the absolute strength (1), in this study, it was assumed that the 28-day (equivalent-age) cylinder strength and the in-place concrete strength are same, hence, the developed maturity-strength relationship is used to predict the concrete strength in the cube. In order to estimate the in-place concrete strength, temperature sensors were installed at critical locations in the 6-ft cubes. The locations were selected to be representative of the temperatures at the locations of coring due to symmetry. In-place concrete strengths were estimated using strength-equivalent age calibration curves (Figure 7) for each concrete mixture. The equivalent ages of the concrete at three locations were calculated based on the temperature-time history of three specific locations in the 6-ft cubes, corresponding to sensor #6 (top section), #7 (mid-section) and #8 (bottom section) and

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given in Table 3. These sensors #6, #7 and #8 were located at 2”, 36” and 70” from the cube top surface, respectively. Figure 8 shows a typical temperature-time history of those three locations up to 14 days (from the D5 Cube). In addition, the predicted strengths are compared to the 4-day core strength results; “1C” representing the top position, “3C” and “4C” representing the center position, and 6C representing the bottom position of the cubes. The results show that the top surface predicted strength (#6) is always higher than the actual core strength at all four cubes. For D1, D5 and D9 cubes, concrete strength at the mid-section (#7) were close to the predicted strength, however the core strength results are higher than the predicted values at the bottom section (#8). Essentially due to on-site quality issues mentioned earlier, it is noticed that in D6 case the core strengths are lower than the predicted strengths at each position. TABLE 3 In-place Concrete Strength Prediction Compared with the Core Strength Results

Core Strength, psi

Equivalent age, days

Predicted Strength, psi

Core Strength, psi

Equivalent age, days

Predicted Strength, psi

Core Strength, psi

D6

Predicted Strength, psi

14.1 3,920 3,160 #6 21.2 4,060 4,760 #7 13.9 3,920 4,930 #8 Note: 1 psi = 6.89 kPa

D9

Equivalent age, days

Core Strength, psi

D5 Predicted Strength, psi

Equivalent age, days

Sensor

D1

11.7 18.2 10.5

4,310 4,970 4,130

3,880 4,830 5,300

17.3 31.1 19.0

3,370 3,500 3,400

2,420 3,620 4,010

20.3 24.5 10.0

6,330 6,400 5,970

4,460 3,710 5,250

FIGURE 8 Measured concrete temperature-time history from D5 Cube.

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In addition to the 4-day core, there are 28-day and 56-day cores (Figure 6) extracted from the cubes, and the compressive test results from these core specimens are shown in Table 4. As expected, the long-term strength of the concrete from these core specimens can not be predicted by the linear hyperbolic strength-maturity model (1,4). Modification of the maturity method is needed for the prediction of long-term concrete strength development, especially considering high-early temperature effects, such as those seen in mass concrete. TABLE 4 Compressive Strength from the 6-ft Core at 28-day and 56 day

Depth from the surface, inches

1C 2"-10"

2C 14"-22"

3C 26"-34"

4C 38"-46"

5C 50"-58"

6C 62"-70"

28 Days (center)

4,750

5,640

5,600

4,950

6,460

6,540

29 Days (corner)

4,370

5,600

5,640

5,490

6,070

5,900

56 Days

4,690

6,130

5,920

5,820

6,370

6,410

28 Days (center)

4,460

6,080

5,820

5,570

5,630

6,960

28 Days (corner)

4,510

4,800

5,150

6,040

5,700

6,590

76 Days*

4,180

5,750

5,580

5,310

6,090

7,430

28 Days (center)

2,960

2,670

2,520

3,710

3,630

4,120

28 Days (corner)

3,150

2,630

2,510

3,790

3,740

4,210

56 Days

3,350

2,730

2,640

4,000

3,840

4,330

28 Days (center)

6,010

6,440

5,150

6,490

6,210

6,230

28 Days (corner)

5,730

6,160

5,450

5,980

6,090

6,400

56 Days

5,390

6,530

6,160

6,590

6,630

6,440

D1 CUBE

D5 CUBE

D9 CUBE

D6 CUBE

Note: 1 inch = 2.54 cm * Coring was delayed due to drilling equipment malfunction

SUMMARY AND CONCLUSIONS Many states, including West Virginia are interested in using ASTM C 1074 Maturity Method for the benefits of increasing quality control, accelerating construction time, or reducing number and cost of sampling and testing standard cylinders. On the other hand, accuracy, effectiveness, and reliability of the test method to estimate in-place concrete strength have been a concern, especially for the concrete under high temperature differential curing such as in the case of mass concrete. The purpose of this study is to investigate the applicability of the maturity method on large concrete pours using regular Class B Concrete in West Virginia. Four different concrete mix designs were investigated in four 6-ft cube constructions in different districts. Maturity-strength calibration curves for these mixes were established and concrete temperatures inside the cubes were monitored in order to calculate equivalent concrete age using the measured activation energy values. The in-place concrete strength was determined by testing core samples extracted from cubes and results were compared with the predicted values. Based on the test results the following conclusions can be made:

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1. Compressive strength-age development of concrete can be represented by testing the corresponding mortar mixture following ASTM C 1074-A1. The hyperbolic strength-age relationship can be used to model strength development at different temperatures. Activation energy values for concrete mixtures including supplementary cementitious materials were determined testing mortar cubes prepared in the laboratory. 2. Test results show that the in-place concrete core strengths of the 6-ft cube close to the concrete top surface were overly estimated using ASTM C1074 maturity method. Effect of variable temperature curing in large structures cannot be accurately predicted using the current maturity calculation with linear hyperbolic equation. Further study is needed to modify the maturity calculation for its application in mass concrete with high early-age temperature. 3. The error in estimating in-place concrete strength using equivalent age method is unpredictable partly because the concrete in-place strength is highly dependent on the quality control on-site. The variables include in-situ water-cementitious ratio, air content, vibration/consolidation, and finishing. ACKNOWLEDGEMENT The authors acknowledge the support provided by the FHWA and West Virginia Transportation Division of Highways for the project RP#257-Pre-liminary Analysis of Use of Mass Concrete in West Virginia. Special thanks are extended to our project monitors Michael A. Mance, Donald Williams and Ryan Arnold of WVDOH. The assistance received from the Materials Control and Soils Testing Division (WVDOH MC&ST), and WVDOH District 1, District 5, District 9 and District 6 Bridge and Materials divisions are especially acknowledged. REFERENCES 1. Carino N. J. “The Maturity Method,” Chapter 5 in Handbook on Nondestructive Testing of Concrete, 2nd Edition, Malhotra V. M. and Carino N. J., Eds., CRC Press, Boca Raton, Fl, 2004. 2. ASTM C 1074-10. Standard Practice for Estimating Concrete Strength by the Maturity Method. ASTM Standards, ASTM International, West Conshohocken, PA. 3. Muench S., Pierce L.M., Kinne C., Uhlmeyer J.S. and Anderson K. W. Use of Maturity Method In Accelerated PCCP Construction. WSDOT Research Report, WA-RD 698.1. Washington State Department of Transportation, 2009. 4. Wade, S.A., Barnes R.W., Schindler, A.K. and Nixon J.M. Evaluation of the Maturity Method to Estimate Concrete Strength in Field Applications. ALDOT Research Report, Highway Research Center and Department of Civil Engineering at Auburn University, 2008. 5. Hosten M. A., Johnson R. Implementation of the Concrete Maturity Meter for Maryland. State Highway Administration Research Report, Report No. MD-11SP708B4K. Morgan State University, 2011. 6. Tank, R. C. The Rate Constant Model for Strength Development of Concrete. Ph.D. Dissertation submitted at Polytechnic University of New York, June 1988, 209 pp.

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7. Brooks A. G., Schindler A. K. and Barnes R. W. Maturity Method Evaluated for Various Cementitious Materials. Journal of Materials in Civil Engineering. ASCE, December 2007, pp. 1017-1025 8. Poole, T.S. and Harrington P.J. An Evaluation of the Maturity Method (ASTM C 1074) for Use in Mass Concrete. Technical Report SL-96-16. U.S Army Corps of Engineers, Vicksburg, MS, 1996. 9. Han S.H., Kim J.K., Park Y.D. Prediction of Compressive Strength of Fly Ash Concrete by New Apparent Activation Energy Function. Cement and Concrete Research, Vol. 33, 2003, pp. 965–971. 10. Neville, A. M. Properties of Concrete. Pearson Education Ltd, England, 2002. 11. Bartlett F. M., Precision of in-place concrete strengths predicted using core strength correction factors obtained by weighted regression analysis, Structural Safety, Vol. 19, Issue 4, 1997, pp. 397-410.

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