g PWQ
2
2r X 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 1.8 kg and is suitable for both laboratory and field work. It consists of a
spring-controlled hammer mass that slides on 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 tests surface can be horizontal, vertical or at any angle but the
instrument must be calibrated in the position. A typical rebound hammer is shown in Fig 10.38.
v2 2e
X The Schmidt hammer provides an inexpensive, simple and quick
method of obtaining an indication of concrete strength, but accuracy of ± 15 to ± 20 % is
possible only for specimens cast cured and tested under condition 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.
w
2
X The basic principle of concrete maturity is that increase in strength is a function
of time and temperature, and that any standard of maturity in terms of strength must include
both temperature and period of curing. The maturity of the concrete at any instant can be
calculated by integration of temperature as a function of time if complete records of time-
temperature changes are maintained. It must be emphasized, however, that in measuring
maturity no property of the concrete itself is measured. If the concrete design and placing are
good, the test will indicate whether the concrete has been adequately cured; it will not indicate
the quality of the concrete.
2w
X Concrete absorbs X-rays and -rays passing through it and the degree
of absorption depends on the density of concrete. These rays, while passing through concrete,
are partly absorbed and partly scattered. The scattered radiation can be shielded from the
measuring device and the density of concrete determined by the degree of absorption of the
rays traversing a direct path of known length. Radium and radio-cobalt are used as source of
rays. Radium has the advantage that its activity can be regarded as constant since it takes 1000
to 2000 years for its activity to be reduced by half. However, radio-cobalt whose activity
reduces to half in just five years is preferred because it is quite cheap.
v 2 2e
X Radioactive methods of testing concrete can be used to detect the
location of reinforcement, measure density and perhaps establish whether honeycombing has
occurred in structural concrete units. Gamma radiography is increasingly accepted in developed
countries. The equipment is quite simple and running costs are small, although the initial price
can be high. Concrete up to 45 cm thick can be examined without any difficulty. The maximum
thickness of concrete which -rays can penetrate is 90 cm and in field practically it is 60 cm only.
-rays, produced by radio active materials, are highly penetrative and are therefore preferred.
X-rays involve very high voltages and make use of very expensive equipments restricting their
use in field.
2
2
2w
Any of the methods discussed above may be used to assess the quality of concrete depending
upon the following considerations.