Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > Rebound Hammers
 Q:
Can rebound hammers reliably measure compressive
strength?
On a recent project, the contractor forgot to make
compressive strength cylinders on a job that required them. Now
the contractor is proposing to use a rebound hammer (Photo) on the
in-place concrete to estimate the compressive strength. Is this
acceptable?
A: Rebound hammers test the surface hardness of concrete,
which cannot be converted directly to compressive strength. ASTM
C 805-97, Standard Test Method for Rebound Number of Hardened Concrete,
states that "because of the inherent uncertainty of estimating
strength with a rebound number, the test is not intended as the
basis for acceptance or rejection of concrete."
There are many factors other than concrete strength
that influence rebound hammer test results, including surface smoothness
and finish, moisture content, coarse aggregate type, and the presence
of carbonation. Although rebound hammers can be used to estimate
concrete strength, the rebound numbers must be correlated with the
compressive strength of molded specimens or cores taken from the
structure. The procedure used to develop this relationship is described
in ACI 228.1R-03, In-Place Methods to Estimate Concrete Strength,
American Concrete Institute.
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