ASTM C805/C805M-18

Standard Test Method for Rebound Number of Hardened Concrete

SIGNIFICANCE AND USE
5.1 This test method is applicable to assess the in-place uniformity of concrete, to delineate variations in concrete quality throughout a structure, and to estimate in-place strength if a correlation is developed in accordance with 5.4.  
5.2 For a given concrete mixture, the rebound number is affected by factors such as moisture content of the test surface, the type of form material or type of finishing used in construction of the surface to be tested, vertical distance from the bottom of a concrete placement, and the depth of carbonation. These factors need to be considered in interpreting rebound numbers.  
5.3 Different instruments of the same nominal design may give rebound numbers differing from 1 to 3 units. Therefore, tests should be made with the same instrument in order to compare results. If more than one instrument is to be used, perform comparative tests on a range of typical concrete surfaces so as to determine the magnitude of the differences to be expected in the readings of different instruments.  
5.4 Relationships between rebound number and concrete strength that are provided by instrument manufacturers shall be used only to provide indications of relative concrete strength at different locations in a structure. To use this test method to estimate strength, it is necessary to establish a relationship between strength and rebound number for a given concrete and given apparatus (see Note 1). Establish the relationship by correlating rebound numbers measured on the structure with the measured strengths of cores taken from corresponding locations (see Note 2). At least two replicate cores shall be taken from at least six locations with different rebound numbers. Select test locations so that a wide range of rebound numbers in the structure is obtained. Obtain, prepare, and test cores in accordance with Test Method C42/C42M. If the rebound number is affected by the orientation of the instrument during testing, the strength relationship is applicabl...
SCOPE
1.1 This test method covers the determination of a rebound number of hardened concrete using a spring-driven steel hammer.  
1.2 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.  
1.3 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.  
1.4 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: 14-Dec-2018
• ICS: 91.100.30 - Concrete and concrete products
• Technical Committee: C09 - Concrete and Concrete Aggregates
• Drafting Committee: C09.64 - Nondestructive and In-Place Testing

Relations

• Replaces: ASTM C805/C805M-13a - Standard Test Method for Rebound Number of Hardened Concrete
• Effective Date: 15-Dec-2018
• 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 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 E18-18 - Standard Test Methods for Rockwell Hardness of Metallic Materials
• Effective Date: 01-Jul-2018
• Refers: ASTM C125-18a - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jul-2018
• Refers: ASTM C42/C42M-18 - Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete
• Effective Date: 15-Mar-2018
• Refers: ASTM C125-18 - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jan-2018
• Refers: ASTM E18-17 - Standard Test Methods for Rockwell Hardness of Metallic Materials
• Effective Date: 01-Jul-2017
• Refers: ASTM C125-16 - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 15-Dec-2016
• Refers: ASTM C42/C42M-16 - Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete
• Effective Date: 01-Oct-2016
• Refers: ASTM C125-15b - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 15-Dec-2015
• Refers: ASTM C125-15a - Standard Terminology Relating to Concrete and Concrete Aggregates
• Effective Date: 01-Jul-2015

Overview

ASTM C805/C805M-18: Standard Test Method for Rebound Number of Hardened Concrete is an internationally recognized procedure developed by ASTM International to determine the rebound number of hardened concrete using a spring-driven steel hammer, commonly known as a rebound hammer or Schmidt hammer. This non-destructive test provides critical information about the uniformity, quality variations, and relative strength of in-place concrete in structures. It plays an essential role in quality control, structural evaluation, and maintenance of concrete buildings and infrastructure.

Key Topics

• Test Principle: The method involves using a spring-driven steel hammer to impact a plunger placed against the concrete surface. The rebound distance (or speed) of the hammer after impact is measured and reported as the dimensionless rebound number.

• Assessment Parameters: The rebound number is influenced by:

  • Surface moisture content
  • Type of formwork material or construction finish
  • Vertical position from the base of the concrete placement
  • Depth of surface carbonation

• Comparison of Results: For reliable data, comparative tests should utilize the same instrument, or differences between equipment should be quantified with side-by-side testing on comparable surfaces.

• Correlating Strength: Manufacturer-supplied relationships between rebound number and concrete strength only indicate relative strength. For strength estimation, users must establish correlations with compressive strengths determined in accordance with ASTM C42/C42M by extracting and testing cores from the structure.

• Limitations: The test method is not suitable for acceptance or rejection of concrete but is valuable for evaluating in-place consistency and identifying potential problem areas.

Applications

ASTM C805/C805M-18 is widely used in the construction, inspection, and maintenance sectors for various practical purposes:

• Quality Control: Verify uniformity and detect variations in concrete quality during or after construction without causing structural damage.
• Structural Assessment: Assess existing concrete structures for degradation or non-uniformity as part of routine maintenance programs or before major renovations.
• Relative Strength Estimation: Estimate relative compressive strength where direct sampling is impractical, after developing the required strength correlation.
• Historic Structures and Repair: Evaluate the integrity of older concrete in heritage structures while minimizing invasive tests.
• Diagnostic and Investigative Purposes: Identify areas of potential concern that warrant further investigation, such as low-strength zones, poor workmanship, or exposure-related deterioration.

Related Standards

ASTM C805/C805M-18 references and is complemented by several important standards and guidelines:

• ASTM C42/C42M - Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete. Essential for establishing rebound number-strength correlations.
• ASTM C125 - Terminology Relating to Concrete and Concrete Aggregates. Provides standard definitions for terms used.
• ASTM E18 - Test Methods for Rockwell Hardness of Metallic Materials. Used for verifying the hardness of the rebound hammer's anvil.
• BS EN 13791 - Assessment of In-Situ Compressive Strength in Structures and Pre-Cast Concrete Components. An international complementary standard for in-situ strength evaluation.
• ACI 228.1R - In-Place Methods to Estimate Concrete Strength. Offers further guidance on correlation procedures.

Practical Value

By implementing ASTM C805/C805M-18, professionals gain a rapid, cost-effective, and non-destructive means of assessing the quality and uniformity of concrete structures. The test is especially useful when combined with core sampling to develop reliable strength correlations, supporting evidence-based decisions in construction quality assurance, structural diagnostics, and preventive maintenance planning.

Keywords: rebound number, hardened concrete, nondestructive testing, ASTM C805, rebound hammer, concrete strength, concrete evaluation, quality control, construction standards

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: C805/C805M − 18
Standard Test Method for
Rebound Number of Hardened Concrete
This standard is issued under the fixed designation C805/C805M; 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* 2.2 Other Standard:
BS EN 13791 Assessment of In-Situ Compressive Strength
1.1 This test method covers the determination of a rebound
in Structures and Pre-Cast Concrete Components
number of hardened concrete using a spring-driven steel
hammer.
3. Terminology
1.2 The values stated in either SI units or inch-pound units
3.1 Definitions:
are to be regarded separately as standard. The values stated in
3.1.1 For definitions of terms used in this test method, refer
each system may not be exact equivalents; therefore, each
to Terminology C125.
system shall be used independently of the other. Combining
4. Summary of Test Method
values from the two systems may result in non-conformance
with the standard.
4.1 A steel hammer impacts, with a predetermined amount
of energy, a metal plunger in contact with a concrete surface.
1.3 This standard does not purport to address all of the
Either the distance that the hammer rebounds is measured or
safety concerns, if any, associated with its use. It is the
the hammer speeds before and after impact are measured. The
responsibility of the user of this standard to establish appro-
test result is reported as a dimensionless rebound number.
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
5. Significance and Use
1.4 This international standard was developed in accor-
5.1 This test method is applicable to assess the in-place
dance with internationally recognized principles on standard-
uniformity of concrete, to delineate variations in concrete
ization established in the Decision on Principles for the
qualitythroughoutastructure,andtoestimatein-placestrength
Development of International Standards, Guides and Recom-
if a correlation is developed in accordance with 5.4.
mendations issued by the World Trade Organization Technical
Barriers to Trade (TBT) Committee.
5.2 For a given concrete mixture, the rebound number is
affected by factors such as moisture content of the test surface,
2. Referenced Documents the type of form material or type of finishing used in construc-
2 tion of the surface to be tested, vertical distance from the
2.1 ASTM Standards:
bottom of a concrete placement, and the depth of carbonation.
C42/C42M Test Method for Obtaining and Testing Drilled
These factors need to be considered in interpreting rebound
Cores and Sawed Beams of Concrete
numbers.
C125 Terminology Relating to Concrete and Concrete Ag-
5.3 Different instruments of the same nominal design may
gregates
give rebound numbers differing from 1 to 3 units. Therefore,
C670 Practice for Preparing Precision and Bias Statements
tests should be made with the same instrument in order to
for Test Methods for Construction Materials
compare results. If more than one instrument is to be used,
E18 Test Methods for Rockwell Hardness of Metallic Ma-
terials perform comparative tests on a range of typical concrete
surfaces so as to determine the magnitude of the differences to
be expected in the readings of different instruments.
5.4 Relationships between rebound number and concrete
This test method is under the jurisdiction of ASTM Committee C09 on
Concrete and ConcreteAggregates and is the direct responsibility of Subcommittee
strengththatareprovidedbyinstrumentmanufacturersshallbe
C09.64 on Nondestructive and In-Place Testing.
used only to provide indications of relative concrete strength at
Current edition approved Dec. 15, 2018. Published February 2019. Originally
different locations in a structure. To use this test method to
approved in 1975. Last previous edition approved in 2013 as C805/C805M – 13a.
DOI: 10.1520/C0805_C0805M-18. estimate strength, it is necessary to establish a relationship
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 Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036, http://www.ansi.org.
*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
C805/C805M − 18
between strength and rebound number for a given concrete and 6.2 Abrasive Stone, consisting of medium-grain texture
given apparatus (see Note 1). Establish the relationship by silicon carbide or equivalent material.
correlating rebound numbers measured on the structure with
6.3 Verification Anvil, used to check the operation of the
the measured strengths of cores taken from corresponding
reboundhammer.Aninstrumentguideisprovidedtocenterthe
locations (see Note 2). At least two replicate cores shall be
rebound hammer over the impact area and keep the instrument
taken from at least six locations with different rebound
perpendicular to the anvil surface. The anvil shall be con-
numbers. Select test locations so that a wide range of rebound
structed so that it will result in a rebound number of at least 75
numbers in the structure is obtained. Obtain, prepare, and test
for a properly operating instrument (see Note 6). The manu-
cores in accordance with Test Method C42/C42M.Ifthe
facturer of the rebound hammer shall stipulate the type of
rebound number is affected by the orientation of the instrument
during testing, the strength relationship is applicable for the verification anvil to be used and shall provide the acceptable
same orientation as used to obtain the correlation date (see range of rebound numbers for a properly operating instrument.
Note 3). Locations where strengths are to be estimated using The anvil manufacturer shall indicate how the anvil is to be
thedevelopedcorrelationshallhavesimilarsurfacetextureand supported for verification tests of the instrument, and shall
shall have been exposed to similar conditions as the locations
provide instructions for visual inspection of the anvil surface
where correlation cores were taken. The functionality of the
for surface wear.
rebound hammer shall have been verified in accordance with
NOTE6—Asuitableanvilhasincludedanapproximately150mm[6in.]
6.4 before making the correlation measurements.
diameter by 150 mm [6 in.] tall steel cylinder with an impact area
NOTE 1—SeeACI 228.1R or BS EN 13791 for additional information
hardened to an HRC hardness value of 64 to 68 as measured by Test
on developing the relationship and on using the relationship to estimate
Methods E18.
in-place strength.
NOTE 2—The use of molded test specimens to develop a correlation
6.4 Verification—Rebound hammers shall be serviced and
may not provide a reliable relationship because the surface texture and
verifiedannuallyandwheneverthereisreasontoquestiontheir
depth of carbonation of molded specimens are not usually representative
of the in-place concrete.
proper operation. Verify the functional operation of a rebound
NOTE 3—The use of correction factors to account for instrument
hammer using the verification anvil described in 6.3. During
orientation may reduce the reliability of strength estimates if the correla-
verification, support the anvil as instructed by the anvil
tion is developed for a different orientation than used for testing.
manufacturer.
5.5 This test method is not suitable as the basis for accep-
tance or rejection of concrete.
NOTE 7—Typically, a properly operating rebound hammer and a
properly designed anvil should result in a rebound number of about 80.
6. Apparatus
The anvil needs to be supported as stated by the anvil manufacturer to
obtain reliable rebound numbers. Verification on the anvil does not
6.1 Rebound Hammer, consisting of a spring-loaded steel
guarantee that the hammer will yield repeatable rebound numbers at other
hammer that, when released, strikes a metal plunger in contact
points on the scale. At the user’s option, the rebound hammer can be
with the concrete surface. The spring-loaded hammer must
verified at lower rebound numbers by using blocks of polished stone
travel with a consistent and reproducible speed. The rebound
having uniform hardness. Some users compare several hammers on
number is based on the rebound distance of the hammer after
concrete or stone surfaces encompassing the usual range of rebound
it impacts the plunger, or it is based on the ratio of the hammer numbers encountered in the field.
speed after impact to the speed before impact. Rebound
numbers based on these two measurement principles are not
7. Test Area and Interferences
comparable.
7.1 Selection of Test Surface—Concrete members to be
NOTE4—Severaltypesandsizesofreboundhammersarecommercially
tested shall be at least 100 mm [4 in.] thick and fixed within a
...

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: C805/C805M − 13a C805/C805M − 18
Standard Test Method for
Rebound Number of Hardened Concrete
This standard is issued under the fixed designation C805/C805M; 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 the determination of a rebound number of hardened concrete using a spring-driven steel hammer.
1.2 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.
1.3 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.
1.4 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:
C42/C42M Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete
C125 Terminology Relating to Concrete and Concrete Aggregates
C670 Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials
E18 Test Methods for Rockwell Hardness of Metallic Materials
2.2 Other Standard:
BS EN 13791 Assessment of In-Situ Compressive Strength in Structures and Pre-Cast Concrete Components
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this test method, refer to Terminology C125.
4. Summary of Test Method
4.1 A steel hammer impacts, with a predetermined amount of energy, a metal plunger in contact with a concrete surface. Either
the distance that the hammer rebounds is measured or the hammer speeds before and after impact are measured. The test result
is reported as a dimensionless rebound number.
5. Significance and Use
5.1 This test method is applicable to assess the in-place uniformity of concrete, to delineate variations in concrete quality
throughout a structure, and to estimate in-place strength if a correlation is developed in accordance with 5.4.
5.2 For a given concrete mixture, the rebound number is affected by factors such as moisture content of the test surface, the type
of form material or type of finishing used in construction of the surface to be tested, vertical distance from the bottom of a concrete
placement, and the depth of carbonation. These factors need to be considered in interpreting rebound numbers.
This test method is under the jurisdiction of ASTM Committee C09 on Concrete and Concrete Aggregates and is the direct responsibility of Subcommittee C09.64 on
Nondestructive and In-Place Testing.
Current edition approved Dec. 15, 2013Dec. 15, 2018. Published January 2014February 2019. Originally approved in 1975. Last previous edition approved in 2013 as
C805 – 13.C805/C805M – 13a. DOI: 10.1520/C0805_C0805M-13a.10.1520/C0805_C0805M-18.
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.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
*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
C805/C805M − 18
5.3 Different instruments of the same nominal design may give rebound numbers differing from 1 to 3 units. Therefore, tests
should be made with the same instrument in order to compare results. If more than one instrument is to be used, perform
comparative tests on a range of typical concrete surfaces so as to determine the magnitude of the differences to be expected in the
readings of different instruments.
5.4 Relationships between rebound number and concrete strength that are provided by instrument manufacturers shall be used
only to provide indications of relative concrete strength at different locations in a structure. To use this test method to estimate
strength, it is necessary to establish a relationship between strength and rebound number for a given concrete and given apparatus
(see Note 1). Establish the relationship by correlating rebound numbers measured on the structure with the measured strengths of
cores taken from corresponding locations (see Note 2). At least two replicate cores shall be taken from at least six locations with
different rebound numbers. Select test locations so that a wide range of rebound numbers in the structure is obtained. Obtain,
prepare, and test cores in accordance with Test Method C42/C42M. If the rebound number ifis affected by the orientation of the
instrument during testing, the strength relationship is applicable for the same orientation as used to obtain the correlation date (see
Note 3). Locations where strengths are to be estimated using the developed correlation shall have similar surface texture and shall
have been exposed to similar conditions as the locations where correlation cores were taken. The functionality of the rebound
hammer shall have been verified in accordance with 6.46.4 before making the correlation measurements.
NOTE 1—See ACI 228.1R or BS EN 13791 for additional information on developing the relationship and on using the relationship to estimate in-place
strength.
NOTE 2—The use of molded test specimens to develop a correlation may not provide a reliable relationship because the surface texture and depth of
carbonation of molded specimens are not usually representative of the in-place concrete.
NOTE 3—The use of correction factors to account for instrument orientation may reduce the reliability of strength estimates if the correlation is
developed for a different orientation than used for testing.
5.5 This test method is not suitable as the basis for acceptance or rejection of concrete.
6. Apparatus
6.1 Rebound Hammer, consisting of a spring-loaded steel hammer that, when released, strikes a metal plunger in contact with
the concrete surface. The spring-loaded hammer must travel with a consistent and reproducible speed. The rebound number is
based on the rebound distance of the hammer after it impacts the plunger, or it is based on the ratio of the hammer speed after
impact to the speed before impact. Rebound numbers based on these two measurement principles are not comparable.
NOTE 4—Several types and sizes of rebound hammers are commercially available to accommodate testing of various sizes and types of concrete
construction.
6.1.1 A means shall be provided to display the rebound number after each test.
NOTE 5—Methods of displaying rebound numbernumbers include mechanical sliders and electronic displays. Instruments are available that will store
the rebound numbers, which can then be transferred to a computer for analysis.
6.1.2 The manufacturer shall supply rebound number correction factors for instruments that require such a factor to account for
the orientation of the instrument during a test. The correction factor is permitted to be applied automatically by the instrument. The
manufacturer shall keep a record of test data used as the basis for applicable correction factors.
6.2 Abrasive Stone, consisting of medium-grain texture silicon carbide or equivalent material.
6.3 Verification Anvil, used to check the operation of the rebound hammer. An instrument guide is provided to center the rebound
hammer over the impact area and keep the instrument perpendicular to the anvil surface. The anvil shall be constructed so that it
will result in a rebound number of at least 75 for a properly operating instrument (see Note 6). The manufacturer of the rebound
hammer shall stipulate the type of verification anvil to be used and shall provide the acceptable range of rebound numbers for a
properly operating instrument. The anvil manufacturer shall indicate how the anvil is to be supported for verification tests of the
instrument, and shall provide instructions for visual inspection of the anvil surface for surface wear.
NOTE 6—A suitable anvil has included an approximately 150 mm [6 in.] diameter by 150 mm [6 in.] tall steel cylinder with an impact area hardened
to an HRC hardness value of 64 to 68 as measured by Test Methods E18.
6.4 Verification—Rebound hammers shall be serviced and verified annually and whenever there is reason to question their
proper operation. Verify the functional operation of a rebound hammer using the verification anvil described in 6.3. During
verification, support the anvil as instructed by the anvil manufacturer.
NOTE 7—Typically, a properly operating rebound hammer and a properly designed anvil should result in a rebound number of about 80. The anvil needs
to be supported as stated by the anvil manufacturer to obtain reliable rebound numbers. Verification
...