How many readings are required at a single test location?

A minimum of 9 valid rebound readings should be taken within a designated test area to compute a reliable median or mean.

What is the minimum spacing allowed between two test impact points?

Impact points must be spaced at least 20 mm apart from each other and at least 20 mm away from any edge or structural joint.

Can the rebound hammer be used to directly predict compressive strength?

No, it is primarily an index of surface hardness and concrete uniformity. Direct strength prediction has a wide error margin (±25%) unless correlated with core tests.

How does surface wetness affect the rebound number?

A wet concrete surface typically yields a lower rebound number than a dry surface, underestimating the strength by up to 20%. The surface condition should ideally be dry.

IS 516 Part 5/Sec 2

: 2021

Methods of Tests for Strength of Concrete - Part 5: Non-Destructive Testing of Concrete - Section 2: Rebound Hammer

CurrentFrequently UsedTesting MethodStructural Engineering · Concrete

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This standard specifies the method for assessing the in-situ compressive strength, uniformity, and surface hardness of hardened concrete using a Rebound Hammer. It is a non-destructive testing (NDT) code frequently used by structural and quality control engineers to quickly evaluate existing concrete structures.

Specifies the method for determining the rebound number of concrete using a rebound hammer.

Quick Reference Values

minimum test area sizeApproximately 300 mm x 300 mm

minimum distance between impact points20 mm

minimum distance from edge20 mm

minimum thickness of member100 mm (without special backing)

number of readings per areaMinimum 9 readings

accuracy of strength prediction uncalibrated± 25%

accuracy with core correlation± 15% to 20%

Practical Notes

Rebound hammer primarily tests the surface layer (approx 30 mm depth). Do not rely on it as the sole indicator of core structural strength.

Surface carbonation hardens the concrete surface, artificially inflating the rebound number. Always grind away the carbonated layer before testing for accurate strength estimation.

The instrument must be held perfectly perpendicular to the test surface. If testing at an angle (upwards or downwards), correction factors must be applied based on manufacturer curves.

For reliable compressive strength evaluation, establish a project-specific correlation curve between rebound numbers and actual core test strengths.