AASHTO T 276-2015 Standard Method of Test for Measuring Early-Age Compression Strength and Projecting Later-Age Strength.pdf

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1、Standard Method of Test for Measuring Early-Age Compression Strength and Projecting Later-Age Strength AASHTO Designation: T 276-15 ASTM Designation: C918-07 American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-3c T 276

2、-1 AASHTO Standard Method of Test for Measuring Early-Age Compression Strength and Projecting Later-Age Strength AASHTO Designation: T 276-15 ASTM Designation: C918-07 1. SCOPE 1.1. This test method covers a procedure for making and curing concrete specimens and for testing them at an early age. The

3、 specimens are stored under standard curing conditions and the measured temperature history is used to compute a maturity index that is related to strength gain. 1.2. This test method also covers a procedure for using the results of early-age compressive-strength tests to project the potential stren

4、gth of concrete at later ages. 1.3. The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only. 1.4. The text of this standard references notes and footnotes that provide explanatory material. These notes and footnotes (excluding those

5、in tables and figures) shall not be considered as requirements of the standard. 1.5. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and d

6、etermine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: M 205M/M 205, Molds for Forming Concrete Test Cylinders Vertically R 39, Making and Curing Concrete Test Specimens in the Laboratory R 60, Sampling Freshly Mixed Concrete T 22, Compressi

7、ve Strength of Cylindrical Concrete Specimens T 23, Making and Curing Concrete Test Specimens in the Field T 231, Capping Cylindrical Concrete Specimens 2.2. ASTM Standards: C617/C617M, Standard Practice for Capping Cylindrical Concrete Specimens C670, Standard Practice for Preparing Precision and B

8、ias Statements for Test Methods for Construction Materials C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method C1231/C1231M, Standard Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Concrete Cylinders 2015 by the American Associatio

9、n of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3c T 276-2 AASHTO 3. TERMINOLOGY 3.1. Definitions: 3.1.1. Refer to ASTM C1074 for the definition of the following terms: datum temperature, equivalent age, maturity, maturity function

10、, maturity index, and temperaturetime factor. 3.2. Definition of Terms Specific to This Standard: 3.2.1. potential strengththe strength of a test specimen that would be obtained at a specified age under standard curing conditions. 3.2.2. prediction equationthe equation representing the straight-line

11、 relationship between compressive strength and the logarithm of the maturity index. 3.2.2.1. DiscussionThe prediction equation is used to project the strength of a test specimen based upon its measured early-age strength. The general form of the prediction equation used in this test method is: ( )lo

12、g logMmS Sb M m=+ (1) where: SM= predicted strength at maturity index, M; Sm= measured compressive strength at maturity index, m; b = slope of the line; M = maturity index under standard curing conditions; and m = maturity index of the specimen tested at early age. The prediction equation is develop

13、ed by performing compressive strength tests at various ages, computing the corresponding maturity indices at the test ages, and plotting the compressive strength as a function of the logarithm of the maturity index. A best-fit line is drawn through the data and the slope of this line is used in the

14、prediction equation. 3.2.3. projected strengththe potential strength estimated by using the measured early-age strength and the previously established prediction equation. 4. SUMMARY OF TEST METHOD 4.1. Cylindrical test specimens are prepared and cured in accordance with the appropriate section of T

15、 23 or in accordance with R 39. The temperature of a representative specimen is monitored during the curing period. Specimens are tested for compressive strength at an early age beyond 24 h, and the concrete temperature history is used to compute the maturity index at the time of test. 4.2. A proced

16、ure is presented for acquiring a series of compressive strength values and the corresponding maturity indices at different ages. These data are used to develop a prediction equation that is used subsequently to project the strengths at later ages based upon measured early-age strengths. 2015 by the

17、American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3c T 276-3 AASHTO 5. SIGNIFICANCE AND USE 5.1. This test method provides a procedure to estimate the potential strength of a particular test specimen based upon its

18、 measured strength at an age as early as 24 h. The early-age test results provide information on the variability of the concrete production process for use in process control. 5.2. The relationship between early-age strength of a test specimen and strength achieved at some later age under standard c

19、uring depends upon the materials comprising the concrete. In this test method, it is assumed that there is a linear relationship between strength and the logarithm of the maturity index. Experience has shown that this is an acceptable approximation for test ages between 24 h and 28 days under standa

20、rd curing conditions. The user of this method shall verify that the test data used to develop the prediction equation are represented correctly by the linear relationship. If the underlying relationship between strength and the logarithm of the maturity index cannot be approximated by a straight lin

21、e, the principle of this test method is applicable provided an approximate equation is used to represent the nonlinear relationship. 5.3. Strength projections are limited to concretes using the same materials and proportions as the concrete used to establish the prediction equation. Note 1Confidence

22、 intervals developed in accordance with the appendix are helpful in evaluating predictions. 5.4. This test method is not intended for estimating the in-place strength of concrete. ASTM C1074 provides procedures for using the measured in-place maturity index to estimate in-place strength. 6. APPARATU

23、S 6.1. Equipment and Small Toolsfor fabricating specimens and measuring the characteristics of fresh concrete; shall conform to the applicable requirements of R 39 or T 23. 6.2. Moldsshall conform to the requirements for cylinder molds in Specification M 205M/M 205. 6.3. Temperature Recorder: 6.3.1.

24、 A device is required to monitor and record the temperature of a test specimen as a function of time. Acceptable devices include thermocouples or thermistors connected to a continuous chart recorder or digital data-loggers. For digital instruments, the recording time interval shall be 1/2h or less f

25、or the first 48 h and 1 h or less thereafter. The temperature recording device shall be accurate to within 1C (2F). 6.3.2. Alternative devices include commercial maturity instruments that automatically compute and display the temperaturetime factor or the equivalent age as described in ASTM C1074. N

26、ote 2Commercial maturity instruments use specific values of the datum temperature to evaluate the temperaturetime factor or of the Q-value to evaluate equivalent age. Refer to the appendix of ASTM C1074 for additional explanation and recommendations. 7. SAMPLING 7.1. Sample and measure the propertie

27、s of the fresh concrete in accordance with R 39, R 60, or T 23. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3c T 276-4 AASHTO 8. PROCEDURE FOR EARLY-AGE AND PROJECTED STRENGTHS 8.1. Mold and cure

28、the specimens in accordance with R 39 or T 23, whichever is applicable. Record the time when molding of the specimens is completed. 8.2. Embed a temperature sensor into the center of one of the specimens of the sampled concrete. Activate the temperature recording device. Continue curing for at least

29、 24 h. Maintain a record of the concrete temperature during the entire curing period. 8.3. Capping and TestingRemove the specimens from the molds as soon as practical after 24 h. Cap the specimens in accordance with T 231. 8.3.1. The capping materials, if used, shall develop, at the age of 30 min, a

30、 strength equal to or greater than the strength of the cylinder to be tested. 8.3.2. Do not test specimens sooner than 30 min after capping. 8.4. Determine the cylinder compressive strength in accordance with T 22 at the age of 24 h or later. Record the strength and the age at the time of the test.

31、The age of the cylinder is measured to the nearest 15 min from the time of molding. Strength at each test age shall be the average strength of at least two cylinders. 8.5. Determine the maturity index at the time of the test by using the manual procedure described in the section titled Maturity Func

32、tions in ASTM C1074 or by using a maturity instrument. Record the maturity index, m, of the early-age test specimens. 8.6. When the data representing the compressive strength and the maturity index, m, are to be used to project the strength of the concrete at some later age, determine the projected

33、strength by using the prediction equation determined in Section 9. 9. PROCEDURE FOR DEVELOPING PREDICTION EQUATION 9.1. Develop a prediction equation for each concrete to be used on the job. Prepare specimens in accordance with R 39. Use the procedure in Section 8 to obtain compressive strength valu

34、es and the corresponding maturity indices at the time of testing. These data shall include tests at ages of 24 h and 3, 7, 14, and 28 days. If the age for which the projected strength is to be determined exceeds 28 days, the data shall include tests at the desired later age (see Section 5.2). Streng

35、th at each age shall be the average strength of at least two cylinders. 9.1.1. Field data are acceptable, provided they furnish all of the information in Section 9.1 and provided the specimens are cured in accordance with T 23. 9.2. The constant b for use in the prediction equation (see Equation 1)

36、is established using one of two alternative methods: (1) by regression analysis or (2) by manual plotting. 9.2.1. Regression AnalysisConvert the values of the maturity indices by taking their logarithms. Plot the average cylinder strength versus the logarithm of the maturity index. Compute the best-

37、fit straight line to the point using an appropriate calculator or computer program. The straight line has the following equation: Sm= a + b log m (2) where: 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable la

38、w.TS-3c T 276-5 AASHTO Sm= compressive strength at m; a = intercept of line; b = slope of line; and m = maturity index. 9.2.1.1. Plot the best-fit straight line on the same graph as the data to verify that the correct equation has been determined. 9.2.2. Manual plottingPrepare a sheet of semilog gra

39、ph paper with the y-axis representing compressive strength and the logarithmic scale (x-axis) representing the maturity index (see Note 3). Plot the strength values from Section 9.1 versus the corresponding maturity index. Determine the best-fitting straight line by drawing a line that visually mini

40、mizes the distance between the point and the line. The slope of the line is the vertical distance, in units of stress, between the intersection of the line with the beginning and the end of one cycle on the x-axis (see Figure X1.1) This slope is the value of b for use in the prediction equation (see

41、 Equation 1). Note 3The scale for the y-axis and the number of cycles in the semilog graph paper should be chosen so that the data fill up as much of the paper as possible. When the maturity index is expressed as the temperaturetime factor in degree-hours, three cycles are generally appropriate. If

42、the maturity index is expressed as the equivalent age in hours, two cycles are appropriate. 9.3. Use the constant, b, and Equation 1 to determine the projected strength based on early-age test results. Note 4If it is desired to check the accuracy of the first estimate of the value of b, fabricate co

43、mpanion specimens to those for testing at an early age, cure them in accordance with T 23, record their temperature histories, and test them at 28 days. The value of b is reestimated by use of the equation: ( )( )log logmb SS M m=(3) where: = indicating that the values are to be added; S = measured

44、compression strength at M; M = maturity index corresponding to test at 28 days; Sm= measured compressive strength at m; and m = maturity index corresponding to early-age test. 10. INTERPRETATION OF RESULTS 10.1. As stated in Section 12, the variability of early-age compressive strength obtained by t

45、his test method is the same or less than that obtained from traditional test methods. Thus, results are applicable for rapid assessment of variability for process control and signaling the need for adjustments. Use of the results from this test method to predict specification compliance of strength

46、at later ages must be applied with caution because strength requirements in existing specifications and codes are not based upon early-age testing. 10.2. Develop a one-sided confidence interval for the projected strength for use in the acceptance decision. The confidence interval is based on the mea

47、sured difference between projected and measured strengths at a designated age. Usually, such an interval is developed at a 95 percent confidence level, and the decision is to accept the concrete as conforming to specification requirements if the following condition is satisfied: SM (SL+ K) (4) 2015

48、by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3c T 276-6 AASHTO where: SM= projected strength at designated age; and SL= specified lower limit, specifically, the specified strength at the designated age.

49、 0.95, 1dnSKdtn= + (5) where: d = average difference between the measured and projected strength. ( )11nnMiiiSS ddnn= =(6) where: S = measured strength after standard curing up to designated age; di = the difference between the ithpair of strength values; n = number of paired (Smand S) values used in the analysis; t0.95,n1= value from the t-distribution at the 95 percent level for n 1 degrees of freedom; and Sd = standard deviation for the difference between the measured and projected strengths. Sd = 21()( 1)niiddn=(7) 1