NDT TESTS ON CONCRETE

Rebound Tests
The rebound hammer is a surface hardness tester for which an empirical
correlation has been established between strength and rebound number.
The only known instrument to make use of the rebound principle for
concrete testing is the Schmidt hammer, which weighs about 4 lb (1.8 kg)
and is suitable for both laboratory and field work. It consists of a springcontrolled
hammer mass that slides on a plunger within a tubular housing.
The hammer is forced against the surface of the concrete by the spring and
the distance of rebound is measured on a scale. The test surface can be
horizontal, vertical or at any angle but the instrument must be calibrated in
this position.
Calibration can be done with cylinders (6 by 12 in., 15 by 30 cm) of the same
cement and aggregate as will be used on the job. The cylinders are capped
and firmly held in a compression machine. Several readings are taken, well
distributed and reproducible, the average representing the rebound
number for the cylinder. This procedure is repeated with several cylinders,
after which compressive strengths are obtained.
Limitations and Advantages
The Schmidt hammer provides an inexpensive, simple and quick method of
obtaining an indication of concrete strength, but accuracy of ±15 to ±20 per
cent is possible only for specimens cast cured and tested under conditions
for which calibration curves have been established. The results are affected
by factors such as smoothness of surface, size and shape of specimen,
moisture condition of the concrete, type of cement and coarse aggregate,
and extent of carbonation of surface.
Pull-Out Tests
A pull-out test measures, with a special ram, the force required to pull from
the concrete a specially shaped steel rod whose enlarged end has been cast
into the concrete to a depth of 3 in. (7.6 cm). The concrete is simultaneously
in tension and in shear, but the force required to pull the concrete out can be
related to its compressive strength. The pull-out technique can thus
measure quantitatively the in-situ strength of concrete when proper
correlations have been made. It has been found, over a wide range of
strengths, that pull-out strengths have a coefficient of variation comparable
to that of compressive strength2.
Limitations and Advantages
Although pullout tests do not measure the interior strength of mass
concrete, they do give information on the maturity and development of
strength of a representative part of it. Such tests have the advantage of
measuring quantitatively the strength of concrete in place. Their main
disadvantage is that they have to be planned in advance and pull-out
assemblies set into the formwork before the concrete is placed. The pullout,
of course, creates some minor damage. The test can be nondestructive,
however, if a minimum pull-out force is applied that stops short
of failure but makes certain that a minimum strength has been reached.
This is information of distinct value in determining when forms can be
removed safely.