Humboldt H-2987 Concrete Rebound Hammer - Manual (Page 4)

rebound numbers that are lower than they should be. Using the Rebound Ham-
mer on new concrete may damage it if the strength is less than 1,000 PSI.
If you are testing concrete with a lot of surface carbonation, the rebound read-
ings will be significantly higher than normal. The thicker the layer of carbonated
concrete, the higher the rebound numbers.
Holding the Rebound Hammer against Walls, Ceilings, and Floors
The Rebound Hammer will work against any concrete surface as long as you
keep it perpendicular to that surface. The Rebound Hammer can be used
against a ceiling (upward), a floor (downward), or a wall (forward). You will need
to consider gravity when estimating the compressive strength of concrete. The
rebound number found when pressing up against a ceiling will differ from the
number that appears if you are pressing down against a floor. Please review the
strike angles A, B, and C and the correlation curves in “Reading your Results”
below.
Specialty Concrete
The Rebound Hammer is designed to estimate the compressive strength of stan-
dard concrete. The correlation diagram for standard concrete is printed on the
side of your Concrete Rebound Hammer. It shows the correlation curves deter-
mined when the compressive strength of standard concrete, as measured with
your Rebound Hammer, is directly compared to the compressive strength from
laboratory tests of concrete cylinders or cubes.
This diagram is not designed for use with:
• Light weight concrete
• Concrete using artificial aggregates
• High performance concrete
In order to gain an accurate rebound number for these specialty materials, you
will need to create your own correlation curves diagram intended for this use.
See the standards ACI 228.1R-2010 and ASTM C-805-08.
Reading your Results
Make at least ten readings from a concrete surface and discard the highest and
lowest rebound numbers. Average the remaining eight numbers. If desired, take
a few test readings before you complete your series of ten regular tests. Use the
average rebound number to estimate the strength of the concrete. Compare
your average rebound number to the chart shown on your Concrete Rebound
Hammer. Consider the diagram shown here: