ASTM C918/C918M-20

Standard Test Method for Measuring Early-Age Compressive Strength and Projecting Later-Age Strength

SIGNIFICANCE AND USE
5.1 This test method provides a procedure to estimate the potential strength of a particular test specimen based upon its measured strength at an age as early as 24 h.4 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 test specimens and strength achieved at some later age under standard curing 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 standard curing conditions. The user of this test 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 line, the principle of this test method is applicable provided an appropriate equation is used to represent the non-linear 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 1: Confidence intervals developed in accordance with 10.2 are helpful in evaluating projected strengths.  
5.4 This test method is not intended for estimating the in-place strength of concrete. Practice C1074 provides procedures for using the measured in-place maturity index to estimate in-place strength.
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 or accelerated 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 strength of concrete at later ages.  
1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.  
1.4 The text of this standard references notes and footnotes which provide explanatory material. These notes and footnotes (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, health, and environmental practices and determine the applicability of regulatory limitations prior to use. (Warning—Fresh hydraulic cementitious mixtures are caustic and may cause chemical burns to skin and tissue upon prolonged exposure.)2  
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 31-Mar-2020
• ICS: 91.100.30 - Concrete and concrete products
• Technical Committee: C09 - Concrete and Concrete Aggregates
• Drafting Committee: C09.61 - Testing for Strength

Relations

• Replaces: ASTM C918/C918M-13 - Standard Test Method for Measuring Early-Age Compressive Strength and Projecting Later-Age Strength
• Effective Date: 01-Apr-2020
• Refers: ASTM C31/C31M-24a - Standard Practice for Making and Curing Concrete Test Specimens in the Field
• Effective Date: 01-Apr-2024
• Refers: ASTM C670-24a - Standard Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials
• Effective Date: 01-Feb-2024
• Refers: ASTM C670-24 - Standard Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials
• Effective Date: 01-Jan-2024
• Refers: ASTM C31/C31M-24 - Standard Practice for Making and Curing Concrete Test Specimens in the Field
• Effective Date: 01-Jan-2024
• Refers: ASTM C39/C39M-23 - Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens
• Effective Date: 01-Dec-2023
• Refers: ASTM C617/C617M-23 - Standard Practice for Capping Cylindrical Concrete Specimens
• Effective Date: 01-Oct-2023
• Refers: ASTM C1231/C1231M-23 - Standard Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Cylindrical Concrete Specimens
• Effective Date: 01-Oct-2023
• Refers: ASTM C125-19a - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 15-Dec-2019
• Refers: ASTM C125-19 - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jan-2019
• Refers: ASTM C125-18b - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Oct-2018
• Refers: ASTM C125-18a - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jul-2018
• Refers: ASTM C125-18 - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jan-2018
• Refers: ASTM C31/C31M-18 - Standard Practice for Making and Curing Concrete Test Specimens in the Field
• Effective Date: 01-Jan-2018
• Refers: ASTM C1074-17 - Standard Practice for Estimating Concrete Strength by the Maturity Method
• Effective Date: 15-Dec-2017

Overview

ASTM C918/C918M-20 specifies the standard test method for measuring the early-age compressive strength of concrete and projecting its potential later-age strength. This method provides a proven procedure to estimate concrete strength based on early-age test results, which can be as soon as 24 hours after casting. By establishing a relationship between the early-age strength and the maturity index, this standard assists in monitoring the consistency of concrete production and supports effective process control.

This test method is applicable to cylindrical concrete specimens prepared and cured under standard or accelerated conditions, with precise temperature monitoring to compute the maturity index. Projections are specific to mixtures utilizing identical materials and proportions as those used to develop the initial strength-maturity relationship.

Key Topics

• Early-Age Strength Measurement: Methods to prepare, cure, and test concrete specimens for compressive strength at early ages (minimum 24 hours), using established ASTM practices for sampling and cylinder preparation.
• Maturity Index Calculation: Utilization of temperature histories, as recorded by embedded sensors, to compute the maturity index which reflects the cumulative effect of time and temperature on strength gain.
• Prediction Equation: Development and application of a prediction equation that relates compressive strength to the logarithm of the maturity index, allowing the projection of future strengths based on early-age data.
• Process Quality Control: Early results provide fast feedback on production variability, enabling timely process adjustments.
• Limitations: The projections are valid only for concrete mixes with the same constituents and proportions as used in the initial calibration. Estimation of in-place strength is not within this standard's scope; for that, see ASTM C1074.

Applications

ASTM C918/C918M-20 is widely used by:

• Concrete producers and contractors: To assess the consistency and early performance of concrete batches, allowing adjustments and quality assurance during production.
• Laboratories: For establishing baseline strength-maturity relationships and projecting later-age strengths efficiently.
• Construction teams: For process control in projects where rapid feedback on concrete strength is important, such as fast-track construction or during accelerated curing processes.
• Quality control/assurance processes: To detect variability in materials, curing, or mixing operations early in the production chain.
• Research and development: For evaluating the impact of new cementitious mixtures, admixtures, or curing methods on early and projected strengths.

It is important to note that the method is not intended for assessing the actual in-place strength of concrete structures. For estimating in-place strength using field maturity indices, ASTM C1074 should be referenced.

Related Standards

ASTM C918/C918M-20 references and is complemented by several other key ASTM standards, including:

• ASTM C31/C31M – Practice for Making and Curing Concrete Test Specimens in the Field
• ASTM C39/C39M – Test Method for Compressive Strength of Cylindrical Concrete Specimens
• ASTM C1074 – Practice for Estimating Concrete Strength by the Maturity Method
• ASTM C192/C192M – Practice for Making and Curing Concrete Test Specimens in the Laboratory
• ASTM C470/C470M – Specification for Molds for Forming Concrete Test Cylinders Vertically
• ASTM C617/C617M – Practice for Capping Cylindrical Concrete Specimens
• ASTM C1231/C1231M – Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Cylindrical Concrete Specimens
• ASTM C1768/C1768M – Practice for Accelerated Curing of Concrete Cylinders

Practical Value

ASTM C918/C918M-20 delivers tangible benefits for quality assurance in concrete construction. By enabling early estimation of compressive strength and providing data for process control, it helps avoid costly delays and ensures that concrete meets required performance characteristics. This test method supports consistent quality, timely adjustments, and efficient use of materials in both laboratory and field applications, making it an essential tool in concrete technology and construction quality management.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: C918/C918M − 20
Standard Test Method for
Measuring Early-Age Compressive Strength and Projecting
Later-Age Strength
This standard is issued under the fixed designation C918/C918M; the number immediately following the designation indicates the year
of 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* 2. Referenced Documents
1.1 This test method covers a procedure for making and 2.1 ASTM Standards:
curing concrete specimens and for testing them at an early age. C31/C31M Practice for Making and Curing Concrete Test
The specimens are stored under standard or accelerated curing Specimens in the Field
conditions and the measured temperature history is used to C39/C39M Test Method for Compressive Strength of Cylin-
compute a maturity index that is related to strength gain. drical Concrete Specimens
C125 Terminology Relating to Concrete and Concrete Ag-
1.2 This test method also covers a procedure for using the
gregates
results of early-age compressive-strength tests to project the
C192/C192M Practice for Making and Curing Concrete Test
potential strength of concrete at later ages.
Specimens in the Laboratory
1.3 The values stated in either SI units or inch-pound units
C470/C470M Specification for Molds for Forming Concrete
are to be regarded separately as standard. The values stated in
Test Cylinders Vertically
each system may not be exact equivalents; therefore, each
C617/C617M Practice for Capping Cylindrical Concrete
system shall be used independently of the other. Combining
Specimens
values from the two systems may result in non-conformance
C670 Practice for Preparing Precision and Bias Statements
with the standard.
for Test Methods for Construction Materials
1.4 The text of this standard references notes and footnotes C1074 Practice for Estimating Concrete Strength by the
Maturity Method
which provide explanatory material. These notes and footnotes
(excluding those in tables and figures) shall not be considered C1231/C1231M Practice for Use of Unbonded Caps in
Determination of Compressive Strength of Hardened Cy-
as requirements of the standard.
lindrical Concrete Specimens
1.5 This standard does not purport to address all of the
C1768/C1768M Practice forAccelerated Curing of Concrete
safety concerns, if any, associated with its use. It is the
Cylinders
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
3. Terminology
mine the applicability of regulatory limitations prior to use.
3.1 Definitions:
(Warning—Fresh hydraulic cementitious mixtures are caustic
3.1.1 For definitions of terms used in this test method, refer
and may cause chemical burns to skin and tissue upon
to Terminology C125.
prolonged exposure.)
3.2 Definitions of Terms Specific to This Standard:
1.6 This international standard was developed in accor-
3.2.1 potential strength, n—the strength of a test specimen
dance with internationally recognized principles on standard-
that would be obtained at a specified age under standard curing
ization established in the Decision on Principles for the
conditions.
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
3.2.2 prediction equation, n—the equation representing the
Barriers to Trade (TBT) Committee.
straight-line relationship between compressive strength and the
logarithm of the maturity index.
3.2.2.1 Discussion—The prediction equation is used to proj-
This test method is under the jurisdiction of ASTM Committee C09 on
ect the strength of a test specimen based upon its measured
Concrete and ConcreteAggregates and is the direct responsibility of Subcommittee
C09.61 on Testing for Strength.
Current edition approved April 1, 2020. Published May 2020. Originally
approved in 1980. Last previous edition approved in 2013 as C918 – 13. DOI: For referenced ASTM standards, visit the ASTM website, www.astm.org, or
10.1520/C0918_C0918M-20. contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Section on Safety Precautions, Manual of Aggregate and Concrete Testing, Standards volume information, refer to the standard’s Document Summary page on
Annual Book of ASTM Standards, Vol. 04.0.2. the ASTM website.
*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
C918/C918M − 20
early-age strength. The general form of the prediction equation maturity index cannot be approximated by a straight line, the
used in this test method is: principle of this test method is applicable provided an appro-
priate equation is used to represent the non-linear relationship.
S 5 S 1b log M 2 log m (1)
~ !
M m
5.3 Strength projections are limited to concretes using the
where:
same materials and proportions as the concrete used to estab-
S = projected strength at maturity index M,
M
lish the prediction equation.
S = measured compressive strength at maturity index m,
m
b = slope of the line, NOTE 1—Confidence intervals developed in accordance with 10.2 are
helpful in evaluating projected strengths.
M = maturity index under standard curing conditions, and
m = maturity index of the specimen tested at early age.
5.4 This test method is not intended for estimating the
The prediction equation is developed by performing in-place strength of concrete. Practice C1074 provides proce-
compressive strength tests at various ages, computing the dures for using the measured in-place maturity index to
corresponding maturity indices at the test ages, and plotting estimate in-place strength.
thecompressivestrengthasafunctionofthelogarithmofthe
maturity index. A best-fit line is drawn through the data and 6. Apparatus
the slope of this line is used in the prediction equation.
6.1 Equipment and Small Tools, for fabricating specimens
3.2.3 projected strength, n—the potential strength estimated
and measuring the characteristics of fresh concrete, shall
by using the measured early-age strength and the previously
conformtotheapplicablerequirementsofPracticesC31/C31M
established prediction equation.
or C192/C192M.
6.2 Molds shall conform to the requirements for cylinder
4. Summary of Test Method
molds in Specification C470/C470M.
4.1 Cylindrical test specimens are prepared and cured in
6.3 Temperature Recorder:
accordance with the appropriate sections of Practice C31/
6.3.1 A device is required to monitor and record the
C31M, in accordance with Practice C192/C192M, or in accor-
temperature of a test specimen as a function of time. Accept-
dance with Practice C1768/C1768M. The temperature of a
abledevicesincludethermocouplesorthermistorsconnectedto
representative specimen is monitored during the curing period.
continuous chart recorders or digital data-loggers. For digital
Specimens are tested for compressive strength at an early age
instruments, the recording time interval shall be ⁄2 h or less for
beyond 24 h, and the concrete temperature history is used to
the first 48 h and1hor less thereafter. The temperature
compute the maturity index at the time of test.
recording device shall be accurate to within 1 °C [62 °F]
4.2 A procedure is presented for acquiring a series of
6.3.2 Alternative devices include commercial maturity in-
compressive strength values and the corresponding maturity
struments that automatically compute and display the
indices at different ages. These data are used to develop a
temperature-time factor or the equivalent age as described in
prediction equation, that is, used subsequently to project the
Practice C1074.
strengths at later ages based upon measured early-age
NOTE 2—Commercial maturity instruments use specific values of the
strengths.
datum temperature to evaluate the temperature-time factor or of the
Q-value to evaluate equivalent age. Refer to the Appendix of Practice
5. Significance and Use
C1074 for additional explanation and recommendations.
5.1 This test method provides a procedure to estimate the
6.4 Accelerated curing apparatus shall conform to Prac-
potential strength of a particular test specimen based upon its
tice C1768/C1768M.
measuredstrengthatanageasearlyas24h. Theearly-agetest
results provide information on the variability of the concrete
7. Sampling
production process for use in process control.
7.1 Sample and measure the properties of the fresh concrete
5.2 The relationship between early-age strength of test
in accordance with Practices C31/C31M or C192/C192M.
specimens and strength achieved at some later age under
standard curing depends upon the materials comprising the
8. Procedure for Early-Age and Projected Strengths
concrete. In this test method, it is assumed that there is a linear
relationship between strength and the logarithm of the maturity
8.1 Mold and cure the specimens in accordance with the
index. Experience has shown that this is an acceptable approxi- standard curing procedure in Practice C31/C31M, in accor-
mation for test ages between 24 h and 28 days under standard
dance with Practice C192/C192M, or in accordance with one
curing conditions. The user of this test method shall verify that of the accelerated curing methods in Practice C1768/C1768M,
the test data used to develop the prediction equation are
whichever is applicable. Record the time when molding of the
represented correctly by the linear relationship. If the underly- specimens is completed.
ing relationship between strength and the logarithm of the
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
For additional information, seeSignificanceofTestsandPropertiesofConcrete
record of the concrete temperature during the entire curing
and Concrete-Making Materials, ASTM STP 169C, Chapter 15, “Prediction of
Potential Concrete Strength at Later Ages,” 1994. period.
C918/C918M − 20
8.3 Capping and Testing—For specimens cured in accor-
m = maturity index.
dance with Practice C31/C31M or Practice C192/C192M,
Plot the best-fit straight line on the same graph as the data to
remove the specimens from the molds as soon as practicable
verify that the correct equation has been determined.
after 24 h. For specimens subjected to accelerated curing,
9.2.2 Manual Plotting—Prepare a sheet of semi-log graph
remove molds at the elapsed times prescribed in Practice
paperwiththey-axisrepresentingcompressivestrengthandthe
C1768/C1768M. Cap the specimens in accordance with Prac-
logarithmic scale (x-axis) representing the maturity index (see
tice C617/C617M or Practice C1231/C1231M.
Note 3). Plot the strength values from 9.1 versus the corre-
8.3.1 The capping materials, if used, shall develop, at the
sponding maturity index. Determine the best-fitting straight
age of 30 min, a strength equal to or greater than the strength
line by drawing a line that visually minimizes the distances
of the cylinders to be tested.
between the points and the line. The slope of the line is the
8.3.2 Do not test specimens sooner than 30 min after
vertical distance, in units of stress, between the intersection of
capping.
the line with the beginning and the end of one cycle on the
8.4 Determine the cylinder compressive strength in accor-
x-axis (see Fig. X1.1).This slope is the value ofb for use in the
dance with Test Method C39/C39M at an age of 24 h or later.
prediction equation (see Eq 1).
Record the strength and the age at the time of the test. The age
NOTE 3—The scale for the y-axis and the number of cycles in the
of the cylinder is measured to the nearest 15 min from the time
semi-log graph paper should be chosen so that the data fill up as much of
of molding. Strength at each test age shall be the average
the paper as possible. When the maturity index is expressed as the
strength of at least two cylinders.
temperature-time factor in degree-hours, three cycles are generally appro-
priate. If the maturity index is expressed as the equivalent age in hours,
8.5 Determinethematurityindexatthetimeoftestbyusing
two cycles are appropriate.
the manual procedure described in the section titled Maturity
9.3 Use the constant, b, and Eq 1 to determine the projected
Functions in Practice C1074 or by using a maturity instrument.
strength based on early-age test results.
Record the maturity index, m, of the early-age test specimens.
NOTE 4—If it is desired to check the accuracy of the first estimate of the
8.6 When the data representing the compressive strength
value of b, fabricate companion specimens to those for testing at an early
and the maturity index,m, are to be used to project the strength
age, cure them in accordance with the standard curing procedure in
of the concrete at some later age, determine the projected
Practice C31/C31M, record their temperature histories and test them at 28
strengthbyusingthepredictionequationdeterminedinSection days. The value of b is re-estimated by use of the following equation:
9.
~S 2 S !
( m
b 5 (3)
log M 2 log m
9. Procedure for Developing Prediction Equation ~ !
(
where:
9.1 Develop a prediction equation for each concrete to be
used on the job. Prepare specimens in accordance with Practice S = measured compressive strength at M,
M = maturity index
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: C918/C918M − 13 C918/C918M − 20
Standard Test Method for
Measuring Early-Age Compressive Strength and Projecting
Later-Age Strength
This standard is issued under the fixed designation C918/C918M; the number immediately following the designation indicates the year
of 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 and curing concrete specimens and for testing them at an early age. The
specimens are stored under standard or accelerated 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
strength of concrete at later ages.
1.3 The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each
system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the
two systems may result in non-conformance with the standard.
1.4 The text of this standard references notes and footnotes which provide explanatory material. These notes and footnotes
(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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use. (Warning—Fresh hydraulic cementitious mixtures are caustic and may cause
chemical burns to skin and tissue upon prolonged exposure.)
1.6 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
C31/C31M Practice for Making and Curing Concrete Test Specimens in the Field
C39/C39M Test Method for Compressive Strength of Cylindrical Concrete Specimens
C125 Terminology Relating to Concrete and Concrete Aggregates
C192/C192M Practice for Making and Curing Concrete Test Specimens in the Laboratory
C470/C470M Specification for Molds for Forming Concrete Test Cylinders Vertically
C617/C617M Practice for Capping Cylindrical Concrete Specimens
C670 Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials
C1074 Practice for Estimating Concrete Strength by the Maturity Method
C1231/C1231M Practice for Use of Unbonded Caps in Determination of Compressive Strength of Hardened Cylindrical
Concrete Specimens
C1768/C1768M Practice for Accelerated Curing of Concrete Cylinders
3. Terminology
3.1 Definitions:
This test method is under the jurisdiction of ASTM Committee C09 on Concrete and Concrete Aggregates and is the direct responsibility of Subcommittee C09.61 on
Testing for Strength.
Current edition approved Dec. 1, 2013April 1, 2020. Published January 2014May 2020. Originally approved in 1980. Last previous edition approved in 20072013 as
C918 – 07.C918 – 13. DOI: 10.1520/C0918_C0918M-13.10.1520/C0918_C0918M-20.
Section on Safety Precautions, Manual of Aggregate and Concrete Testing, Annual Book of ASTM Standards, Vol. 04.0.2.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*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
C918/C918M − 20
3.1.1 Refer to Practice For C1074 for the definitions of the following terms:terms used in datum temperature,this test method,
equivalent age,refer to maturity,Terminology C125maturity 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 specimen that would be obtained at a specified age under standard curing
conditions.
3.2.2 prediction equation, n—the equation representing the straight-line relationship between compressive strength and the
logarithm of the maturity index.
3.2.2.1 Discussion—
The 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:
S 5 S 1b log M 2 log m (1)
~ !
M m
where:
S = projected strength at maturity index M,
M
S = measured compressive strength at maturity index m,
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 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 prediction equation.
3.2.3 projected strength, n—the 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 sections of Practice C31/C31M, in
accordance with Practice C192/C192M, or in accordance with Practice C1768/C1768M. 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 procedure 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.
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
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 test specimens and strength achieved at some later age under standard curing
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 standard curing conditions. The user of this test 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 line, the principle of this test method is applicable
provided an appropriate equation is used to represent the non-linear 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 1—Confidence intervals developed in accordance with 10.2 are helpful in evaluating projected strengths.
5.4 This test method is not intended for estimating the in-place strength of concrete. Practice C1074 provides procedures for
using the measured in-place maturity index to estimate in-place strength.
For additional information, see Significance of Tests and Properties of Concrete and Concrete-Making Materials, ASTM STP 169C, Chapter 15, “Prediction of Potential
Concrete Strength at Later Ages,” 1994.
C918/C918M − 20
6. Apparatus
6.1 Equipment and Small Tools, for fabricating specimens and measuring the characteristics of fresh concrete, shall conform to
the applicable requirements of Practices C31/C31M or C192/C192M.
6.2 Molds shall conform to the requirements for cylinder molds in Specification C470/C470M.
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 time. Acceptable devices
include thermocouples or thermistors connected to continuous chart recorders or digital data-loggers. For digital instruments, the
recording time interval shall be ⁄2 h or less for the first 48 h and 1 h or less thereafter. The temperature recording device shall be
accurate to within 1 °C [62 °F]
6.3.2 Alternative devices include commercial maturity instruments that automatically compute and display the temperature-time
factor or the equivalent age as described in Practice C1074.
NOTE 2—Commercial maturity instruments use specific values of the datum temperature to evaluate the temperature-time factor or of the Q-value to
evaluate equivalent age. Refer to the Appendix of Practice C1074 for additional explanation and recommendations.
6.4 Accelerated curing apparatus shall conform to Practice C1768/C1768M.
7. Sampling
7.1 Sample and measure the properties of the fresh concrete in accordance with Practices C31/C31M or C192/C192M.
8. Procedure for Early-Age and Projected Strengths
8.1 Mold and cure the specimens in accordance with the standard curing procedure in Practice C31/C31M, in accordance with
Practice C192/C192M, or in accordance with one of the accelerated curing methods in Practice C1768/C1768M, 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 24 h. Maintain a record of the concrete temperature during the entire curing period.
8.3 Capping and Testing—For specimens cured in accordance with Practice C31/C31M or Practice C192/C192M, remove the
specimens from the molds as soon as practicable after 24 h. For specimens subjected to accelerated curing, remove molds at the
elapsed times prescribed in Practice C1768/C1768M. Cap the specimens in accordance with Practice C617/C617M or Practice
C1231/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
cylinders 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 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 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 test by using the manual procedure described in the section titled Maturity
Functions 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 strength and the maturity index, m, are to be 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 accordance with Practice
C192/C192M. Use the procedure in Section 8 to obtain compressive strength values and the corresponding maturity indices at the
times of testing. These data shall include tests at ages of 24 h, 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 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 9.1, and provided the specimens are cured in
accordance with the section on standard curing of Practice C31/C31M.
9.2 The constant b for use in the prediction equation (see Eq 1) is established using one of two alternative methods: (1) by
regression analysis, or (2) by manual plotting.
9.2.1 Regression Analysis—Convert 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-fit straight line to the points using an appropriate calculator
or computer program. The straight line has the following equation:
S 5 a1b log m (2)
m
C918/C918M − 20
where:
S = compressive strength at m,
m
a = intercept of line,
b = slope of line, and
m = maturity index.
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 Plotting—Prepare a sheet of semi-log 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 9.1 versus t
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