1、Designation: C 918/C 918M 07Standard Test Method forMeasuring Early-Age Compressive Strength and ProjectingLater-Age Strength1This standard is issued under the fixed designation C 918/C 918M; the number immediately following the designation indicates the yearof original adoption or, in the case of r
2、evision, the year of last revision. A number in parentheses indicates the year of last reapproval.A superscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This test method covers a procedure for making andcuring concrete specimens and for testing the
3、m at an early age.The specimens are stored under standard-curing conditions andthe measured temperature history is used to compute a maturityindex that is related to strength gain.1.2 This test method also covers a procedure for using theresults of early-age compressive-strength tests to project the
4、potential strength of concrete at later ages.1.3 The values stated in either SI units or inch-pound unitsare to be regarded separately as standard. The values stated ineach system may not be exact equivalents; therefore, eachsystem shall be used independently of the other. Combiningvalues from the t
5、wo systems may result in non-conformancewith the standard.1.4 The text of this standard references notes and footnoteswhich provide explanatory material. These notes and footnotes(excluding those in tables and figures) shall not be consideredas requirements of the standard.1.5 This standard does not
6、 purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards
7、:2C 31/C 31M Practice for Making and Curing Concrete TestSpecimens in the FieldC 39/C 39M Test Method for Compressive Strength of Cy-lindrical Concrete SpecimensC 192/C 192M Practice for Making and Curing ConcreteTest Specimens in the LaboratoryC 470/C 470M Specification for Molds for Forming Con-cr
8、ete Test Cylinders VerticallyC 617 Practice for Capping Cylindrical Concrete Speci-mensC 670 Practice for Preparing Precision and Bias Statementsfor Test Methods for Construction MaterialsC 1074 Practice for Estimating Concrete Strength by theMaturity MethodC 1231/C 1231M Practice for Use of Unbonde
9、d Caps inDetermination of Compressive Strength of Hardened Con-crete Cylinders3. Terminology3.1 Definitions:3.1.1 Refer to Practice C 1074 for the definitions of thefollowing terms: datum temperature, equivalent age, maturity,maturity function, maturity index, and temperaturetime factor.3.2 Definiti
10、ons of Terms Specific to This Standard:3.2.1 potential strength, nthe strength of a test specimenthat would be obtained at a specified age under standard curingconditions.3.2.2 prediction equation, nthe equation representing thestraight-line relationship between compressive strength and thelogarithm
11、 of the maturity index.3.2.2.1 DiscussionThe prediction equation is used toproject the strength of a test specimen based upon its measuredearly-age strength. The general form of the prediction equationused in this test method is:SM5 Sm1 b log M 2 log m! (1)where:SM= projected strength at maturity in
12、dex M,Sm= measured compressive strength at maturity index m,b = slope of the line,1This test method is under the jurisdiction of ASTM Committee C09 onConcrete and Concrete Aggregates and is the direct responsibility of SubcommitteeC09.61 on Testing for Strength.Current edition approved July 15, 2007
13、. Published August 2007. Originallyapproved in 1980. Last previous edition approved in 2002 as C 918 02.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standard
14、s Document Summary page onthe ASTM website.1*A Summary of Changes section appears at the end of this standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.M = maturity index under standard curing conditions, andm = maturity index o
15、f the specimen tested at early age.The prediction equation is developed by performing compressivestrength tests at various ages, computing the corresponding maturityindices at the test ages, and plotting the compressive strength as afunction of the logarithm of the maturity index. A best-fit line is
16、 drawnthrough the data and the slope of this line is used in the predictionequation.3.2.3 projected strength, nthe potential strength estimatedby using the measured early-age strength and the previouslyestablished prediction equation.4. Summary of Test Method4.1 Cylindrical test specimens are prepar
17、ed and cured inaccordance with the appropriate sections of Practice C 31/C 31M or in accordance with Practice C 192/C 192M. Thetemperature of a representative specimen is monitored duringthe curing period. Specimens are tested for compressivestrength at an early age beyond 24 h, and the concretetemp
18、erature history is used to compute the maturity index atthe time of test.4.2 A procedure is presented for acquiring a series ofcompressive strength values and the corresponding maturityindices at different ages. These data are used to develop aprediction equation, that is, used subsequently to proje
19、ct thestrengths at later ages based upon measured early-agestrengths.5. Significance and Use5.1 This test method provides a procedure to estimate thepotential strength of a particular test specimen based upon itsmeasured strength at an age as early as 24 h.3The early-age testresults provide informat
20、ion on the variability of the concreteproduction process for use in process control.5.2 The relationship between early-age strength of testspecimens and strength achieved at some later age understandard curing depends upon the materials comprising theconcrete. In this test method, it is assumed that
21、 there is a linearrelationship between strength and the logarithm of the maturityindex. Experience has shown that this is an acceptable approxi-mation for test ages between 24 h and 28 days under standardcuring conditions. The user of this test method shall verify thatthe test data used to develop t
22、he prediction equation arerepresented correctly by the linear relationship. If the underly-ing relationship between strength and the logarithm of thematurity index cannot be approximated by a straight line, theprinciple of this test method is applicable provided an appro-priate equation is used to r
23、epresent the non-linear relationship.5.3 Strength projections are limited to concretes using thesame materials and proportions as the concrete used to estab-lish the prediction equation.NOTE 1Confidence intervals developed in accordance with 10.2 arehelpful in evaluating projected strengths.5.4 This
24、 test method is not intended for estimating thein-place strength of concrete. Practice C 1074 provides proce-dures for using the measured in-place maturity index toestimate in-place strength.6. Apparatus6.1 Equipment and Small Tools, for fabricating specimensand measuring the characteristics of fres
25、h concrete, shallconform to the applicable requirements of Practices C 31/C 31M or C 192/C 192M.6.2 Molds shall conform to the requirements for cylindermolds in Specification C 470/C 470M.6.3 Temperature Recorder:6.3.1 A device is required to monitor and record thetemperature of a test specimen as a
26、 function of time. Accept-able devices include thermocouples or thermistors connected tocontinuous chart recorders or digital data-loggers. For digitalinstruments, the recording time interval shall be12 h or less forthe first 48 h and1horless thereafter. The temperaturerecording device shall be accu
27、rate to within 1 C 62 F6.3.2 Alternative devices include commercial maturity in-struments that automatically compute and display thetemperature-time factor or the equivalent age as described inPractice C 1074.NOTE 2Commercial maturity instruments use specific values of thedatum temperature to evalua
28、te the temperature-time factor or of theQ-value to evaluate equivalent age. Refer to the Appendix of PracticeC 1074 for additional explanation and recommendations.7. Sampling7.1 Sample and measure the properties of the fresh concretein accordance with Practices C 31/C 31M or C 192/C 192M.8. Procedur
29、e for Early-Age and Projected Strengths8.1 Mold and cure the specimens in accordance with thestandard curing procedure in Practice C 31/C 31M or inaccordance with Practice C 192/C 192M whichever is appli-cable. Record the time when molding of the specimens iscompleted.8.2 Embed a temperature sensor
30、into the center of one of thespecimens of the sampled concrete. Activate the temperaturerecording device. Continue curing for at least 24 h. Maintain arecord of the concrete temperature during the entire curingperiod.8.3 Capping and TestingRemove the specimens from themolds as soon as practical afte
31、r 24 h. Cap the specimens inaccordance with Practice C 617 or Practice C 1231/C 1231M.8.3.1 The capping materials, if used, shall develop, at theage of 30 min, a strength equal to or greater than the strengthof the cylinders to be tested.8.3.2 Do not test specimens sooner than 30 min aftercapping.8.
32、4 Determine the cylinder compressive strength in accor-dance with Test Method C 39/C 39M at an age of 24 h or later.Record the strength and the age at the time of the test. The ageof the cylinder is measured to the nearest 15 min from the timeof molding. Strength at each test age shall be the averag
33、estrength of at least two cylinders.3For additional information, see Significance of Tests and Properties of Concreteand Concrete-Making Materials, ASTM STP 169C, Chapter 15, “Prediction ofPotential Concrete Strength at Later Ages,” 1994.C 918/C 918M 0728.5 Determine the maturity index at the time o
34、f test by usingthe manual procedure described in the section titled MaturityFunctions in Practice C 1074 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 strengthand the maturity index, m, are to be used to
35、 project the strengthof the concrete at some later age, determine the projectedstrength by using the prediction equation determined in Section9.9. Procedure for Developing Prediction Equation9.1 Develop a prediction equation for each concrete to beused on the job. Prepare specimens in accordance wit
36、h PracticeC 192/C 192M. Use the procedure in Section 8 to obtaincompressive strength values and the corresponding maturityindices at the times of testing. These data shall include tests atages of 24 h, 3, 7, 14, and 28 days. If the age for which theprojected strength is to be determined exceeds 28 d
37、ays, the datashall include tests at the desired later age (see 5.2). Strength ateach age shall be the average strength of at least two cylinders.9.1.1 Field data are acceptable, provided they furnish all ofthe information in 9.1, and provided the specimens are cured inaccordance with the section on
38、standard curing of PracticeC 31/C 31M.9.2 The constant b for use in the prediction equation (see Eq1) is established using one of two alternative methods: (1)byregression analysis, or (2) by manual plotting.9.2.1 Regression AnalysisConvert the values of the ma-turity indices by taking their logarith
39、ms. Plot the averagecylinder strength versus the logarithm of the maturity index.Compute the best-fit straight line to the points using anappropriate calculator or computer program. The straight linehas the following equation:Sm5 a 1 b log m (2)where:Sm= compressive strength at m,a = intercept of li
40、ne,b = slope of line, andm = maturity index.Plot the best-fit straight line on the same graph as the data toverify that the correct equation has been determined.9.2.2 Manual PlottingPrepare a sheet of semi-log graphpaper with the y-axis representing compressive strength and thelogarithmic scale (x-a
41、xis) representing the maturity index (seeNote 3). Plot the strength values from 9.1 versus the corre-sponding maturity index. Determine the best-fitting straightline by drawing a line that visually minimizes the distancesbetween the points and the line. The slope of the line is thevertical distance,
42、 in units of stress, between the intersection ofthe line with the beginning and the end of one cycle on thex-axis (see Fig. X1.1). This slope is the value of b for use in theprediction equation (see Eq 1).NOTE 3The scale for the y-axis and the number of cycles in thesemi-log graph paper should be ch
43、osen so that the data fill up as much ofthe paper as possible. When the maturity index is expressed as thetemperature-time factor in degree-hours, three cycles are generally appro-priate. If the maturity index is expressed as the equivalent age in hours,two cycles are appropriate.9.3 Use the constan
44、t, b, and Eq 1 to determine the projectedstrength based on early-age test results.NOTE 4If it is desired to check the accuracy of the first estimate of thevalue of b, fabricate companion specimens to those for testing at an earlyage, cure them in accordance with the standard curing procedure inPract
45、ice C 31/C 31M, record their temperature histories and test them at28 days. The value of b is re-estimated by use of the following equation:b 5(S 2 Sm!(log M 2 log m!(3)where:S = measured compressive strength at M,M = maturity index corresponding to test at 28 days,Sm= measured compressive strength
46、at m, andm = maturity index corresponding to early-age test.10. Interpretation of Results10.1 As stated in Section 12, the variability of early-agecompressive strength obtained by this test method is the sameor less than that obtained from traditional test methods. Thusresults are applicable for rap
47、id assessment of variability forprocess control and signaling the need for adjustments. Use ofthe results from this test method to predict specificationcompliance of strengths at later ages must be applied withcaution because strength requirements in existing specifica-tions and codes are not based
48、upon early-age testing.10.2 Develop a one-sided confidence interval for the pro-jected strength for use in the acceptance decision. The confi-dence interval is based on the measured differences betweenprojected and measured strengths at a designated age. Usuallysuch an interval is developed at a 95
49、% confidence level, andthe decision is to accept the concrete as conforming tospecification requirements if the following condition is satis-fied:SM. SL1 K! (4)where:SM= projected strength at designated age,SL= specified lower limit, specifically, the specifiedstrength at the designated age,K 5 d1 t0.95, n21sd=n(5)d = average difference between the measured and projectedstrength.d5(i 5 1nSM2 S!in5(i 5 1ndin(6)S = measured strength after standard curing up todesignated age,di= the difference between the ith pair of strengthvalues,n = number of paired (SMand S)
