# ASTM C918C918M-13

Designation: C918/C918M − 13Standard Test Method forMeasuring Early-Age Compressive Strength and ProjectingLater-Age Strength1This standard is issued under the fixed designation C918/C918M; the number immediately following the designation indicates the yearof original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.A superscript epsilon (´) 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 them at an early age.The specimens are stored under standard or accelerated curingconditions and the measured temperature history is used tocompute a maturity index 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 thepotential 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 two 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 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. (Warning—Freshhydraulic cementitious mixtures are caustic and may causechemical burns to skin and tissue upon prolonged exposure.)22. Referenced Documents2.1 ASTM Standards:3C31/C31M Practice for Making and Curing Concrete TestSpecimens in the FieldC39/C39M Test Method for Compressive Strength of Cylin-drical Concrete SpecimensC192/C192M Practice for Making and Curing Concrete TestSpecimens in the LaboratoryC470/C470M Specification for Molds for Forming ConcreteTest Cylinders VerticallyC617/C617M Practice for Capping Cylindrical ConcreteSpecimensC670 Practice for Preparing Precision and Bias Statementsfor Test Methods for Construction MaterialsC1074 Practice for Estimating Concrete Strength by theMaturity MethodC1231/C1231M Practice for Use of Unbonded Caps inDetermination of Compressive Strength of Hardened Con-crete CylindersC1768/C1768M Practice forAccelerated Curing of ConcreteCylinders3. Terminology3.1 Definitions:3.1.1 Refer to Practice C1074 for the definitions of thefollowing terms: datum temperature, equivalent age, maturity,maturity function, maturity index, and temperature–time factor.3.2 Definitions of Terms Specific to This Standard:3.2.1 potential strength, n—the strength of a test specimenthat would be obtained at a specified age under standard curingconditions.3.2.2 prediction equation, n—the equation representing thestraight-line relationship between compressive strength and thelogarithm of the maturity index.3.2.2.1 Discussion—The prediction equation is used to proj-ect 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 Sm1b~log M 2 log m! (1)where:SM= projected strength at maturity index 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 Aggregatesand is the direct responsibility of SubcommitteeC09.61 on Testing for Strength.Current edition approved Dec. 1, 2013. Published January 2014. Originallyapproved in 1980. Last previous edition approved in 2007 as C918 – 07. DOI:10.1520/C0918_C0918M-13.2Section on Safety Precautions, Manual of Aggregate and Concrete Testing,Annual Book of ASTM Standards, Vol. 04.0.2.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.*A Summary of Changes section appears at the end of this standardCopyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1M = maturity index under standard curing conditions, andm = maturity index of the specimen tested at early age.The prediction equation is developed by performingcompressive strength tests at various ages, computing thecorresponding maturity indices at the test ages, and plottingthe compressive strength as a function of the logarithm of thematurity index. A best-fit line is drawn through the data andthe slope of this line is used in the prediction equation.3.2.3 projected strength, n—the potential strength estimatedby using the measured early-age strength and the previouslyestablished prediction equation.4. Summary of Test Method4.1 Cylindrical test specimens are prepared and cured inaccordance with the appropriate sections of Practice C31/C31M, in accordance with Practice C192/C192M, or in accor-dance with Practice C1768/C1768M. The temperature of arepresentative specimen is monitored during the curing period.Specimens are tested for compressive strength at an early agebeyond 24 h, and the concrete temperature history is used tocompute the maturity index at the 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 project 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.4The early-age testresults provide information 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 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 the 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 represent 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 1—Confidence intervals developed in accordance with 10.2 arehelpful in evaluating projected strengths.5.4 This test method is not intended for estimating thein-place strength of concrete. Practice C1074 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 fresh concrete, shallconform to the applicable requirements of Practices C31/C31Mor C192/C192M.6.2 Molds shall conform to the requirements for cylindermolds in Specification C470/C470M.6.3 Temperature Recorder:6.3.1 A device is required to monitor and record thetemperature of a test specimen as a 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 be1⁄2 h or less forthe first 48 h and1horless thereafter. The temperaturerecording device shall be accurate to within 1 °C [62 °F]6.3.2 Alternative devices include commercial maturity in-struments that automatically compute and display thetemperature-time factor or the equivalent age as described inPractice C1074.NOTE 2—Commercial maturity instruments use specific values of thedatum temperature to evaluate the temperature-time factor or of theQ-value to evaluate equivalent age. Refer to the Appendix of PracticeC1074 for additional explanation and recommendations.6.4 Accelerated curing apparatus shall conform to Prac-tice C1768/C1768M.7. Sampling7.1 Sample and measure the properties of the fresh concretein accordance with Practices C31/C31M or C192/C192M.8. Procedure for Early-Age and Projected Strengths8.1 Mold and cure the specimens in accordance with thestandard curing procedure in Practice C31/C31M, in accor-dance with Practice C192/C192M, or in accordance with oneof the accelerated curing methods in Practice C1768/C1768M,whichever is applicable. Record the time when molding of thespecimens is completed.8.2 Embed a temperature sensor 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 Testing—For specimens cured in accor-dance with Practice C31/C31M or Practice C192/C192M,remove the specimens from the molds as soon as practicableafter 24 h. For specimens subjected to accelerated curing,remove molds at the elapsed times prescribed in PracticeC1768/C1768M. Cap the specimens in accordance with Prac-tice C617/C617M or Practice C1231/C1231M.4For 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.C918/C918M − 1328.3.1 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.8.4 Determine the cylinder compressive strength in accor-dance with Test Method C39/C39M 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 averagestrength of at least two cylinders.8.5 Determine the maturity index at the time of test by usingthe manual procedure described in the section titled MaturityFunctions in Practice 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 strengthand the maturity index, m, are to be used to 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 with PracticeC192/C192M. 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 days, 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 standard curing of PracticeC31/C31M.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 Analysis—Convert the values of the matu-rity indices by taking their logarithms. 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 a1b log m (2)where:Sm= compressive strength at m,a = intercept of line,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 Plotting—Prepare a sheet of semi-log graphpaper with the y-axis representing compressive strength and thelogarithmic scale (x-axis) 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, 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 3—The scale for the y-axis and the number of cycles in thesemi-log graph paper should be chosen 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 constant, b, and Eq 1 to determine the projectedstrength based on early-age test results.NOTE 4—If 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 inPractice C31/C31M, record their temperature histories and test them at 28days. 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 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 rapid 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 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 bas