1、Standard Method of Test for Measuring Early-Age Compression Strength and Projecting Later-Age Strength AASHTO Designation: T 276-17 Technical Section: 3c, Hardened Concrete Release: Group 1 (April 2017) ASTM Designation: C918-13 American Association of State Highway and Transportation Officials 444
2、North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-3c T 276-1 AASHTO Standard Method of Test for Measuring Early-Age Compression Strength and Projecting Later-Age Strength AASHTO Designation: T 276-17 Technical Section: 3c, Hardened Concrete Release: Group 1 (April 2017) ASTM Designation
3、: C918-13 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 specimens are stored under standard curing conditions and the measured temperature history is used to compute a maturity index that is related to strength ga
4、in. 1.2. This test method also covers a procedure for using the results of early-age compressive-strength tests to project the potential strength of concrete at later ages. 1.3. The values stated in SI units or inch-pound units are to be regarded separately as standard. The values stated in each sys
5、tem may not be exact equivalents; therefore; each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard. 1.4. The text of this standard references notes and footnotes that provide explanatory material. These notes and fo
6、otnotes (excluding those 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 a
7、nd health practices and determine 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
8、 Concrete T 22, Compressive 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: C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method 2017 by the American
9、 Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3c T 276-2 AASHTO C1231/C1231M, Standard Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Concrete Cylinders C1768/C1768M, Standard Pr
10、actice for Accelerated Curing of Concrete Cylinders 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, maturity index, and temperaturetime factor. 3.2. Definition of Terms Specific to
11、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 relationship between compressive strength and the logarithm of the maturity index.
12、 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: ( )log logMmS Sb M m=+ (1) where: SM= predicted strength at maturity index, M; Sm= measu
13、red 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 developed by performing compressive strength tests at various ages, computing the correspo
14、nding 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 prediction equation. 3.2.3. projected strength, nthe potential strength estimated b
15、y 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 23 or in accordance with R 39. The temperature of a representative specimen is
16、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 procedure is presented for acquiring a series of compressive strength values and the c
17、orresponding 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. 2017 by the American Association of State Highway and Transportation Officials.All rights re
18、served. 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 on its measured strength at an age as early as 24 h. The early-age test results provide
19、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 curing depends upon the materials comprising the concrete. In this test method, it
20、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 standard curing conditions. The user of this method shall verify that the test data used
21、 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 line, the principle of this test method is applicable provided an approximate equatio
22、n 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 intervals developed in accordance with the appendix are helpful in evaluating pre
23、dictions. 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. APPARATUS 6.1. Equipment and Small ToolsFor fabricating specimens and measuring the charac
24、teristics 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 M 205M/M 205. 6.3. Temperature Recorder: 6.3.1. A device is required to monitor and record the temperature of a test specimen as a function of
25、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 for the first 48 h and 1 h or less thereafter. The temperature recording device shall be accurate
26、 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. Note 2Commercial maturity instruments use specific values of the datum temperature to evaluate th
27、e 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 properties of the fresh concrete in accordance with R 39, R 60, or T 23. 2017 by the American Association
28、 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 the specimens in accordance with R 39 or T 23, whichever is applicable. Record the time when mol
29、ding 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 24 h. Maintain a record of the concrete temperature during the entire curing period. 8.3. Cappi
30、ng and TestingFor specimens cured in accordance with T 23 or R 39, remove the specimens from the molds as soon as practicable after 24 h. For specimens subjected to accelerated curing, remove molds at elapsed times prescribed in ASTM C1768/C1768M. Cap the specimen in accordance with T 231 or ASTM C1
31、231/C1231M. 8.3.1. The capping materials, if used, shall develop, at the age of 30 min, a 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
32、22 at the age of 24 h or later. Record the strength and the age at the time of the test. 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 tim
33、e of the test by using the manual procedure described in the section titled Maturity Functions 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
34、 used to project the strength of the concrete at some later age, determine the projected 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
35、 accordance with R 39. Use the procedure in Section 8 to obtain compressive strength values 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
36、 28 days, the data shall include tests at the desired later age (see Section 5.2). Strength 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 accor
37、dance with T 23. 9.2. The constant b for use in the prediction equation (see Equation 1) 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 th
38、e average cylinder strength versus the logarithm of the maturity index. Compute the best-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) 2017 by the American Association of State Highway and Transpo
39、rtation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3c T 276-5 AASHTO where: 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 c
40、orrect equation has been determined. 9.2.2. Manual plottingPrepare a sheet of semilog graph 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 matu
41、rity index. Determine the best-fitting straight line by drawing a line that visually minimizes 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-ax
42、is (see Figure X1.1). This slope is the value of b for use in the prediction equation (see 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
43、as the temperaturetime factor in degree-hours, three cycles are generally appropriate. If 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
44、 it is desired to check the accuracy of the first estimate of the value of b, fabricate companion 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 re-estimated by use of the equation: (
45、)( )log logmb SS M m=(3) where: = indicating that the values are to be added; S = measured 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.
46、1. As stated in Section 12, the variability of early-age compressive strength obtained by this 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
47、. Use of the results from this test method to predict specification compliance of strength at later ages must be applied with caution because strength requirements in existing specifications and codes are not based on early-age testing. 10.2. Develop a one-sided confidence interval for the projected
48、 strength for use in the acceptance decision. The confidence interval is based on the measured 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
49、 specification requirements if the following condition is satisfied: 2017 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 SM (SL+ K) (4) where: SM= projected strength at designated age; and SL= specified lower limit, specifically, the specified strength at the designated age. 0.95, 1dnSKdtn= + (5) where: d = average difference between the measured and projected strength. ( )11nnMiiiSS ddnn= =(6) where: S = measured st
