PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 283

Head of
testing machine

Steel ball

1 in. min. 1 in. min.

L
3

Specimen Load-applying

and
support blocks

d

Steel rod Steel ball

Bed of
testing machine

L
3

L
3

L
3

Span length, L

FIGURE 7.33 Apparatus for flexure test of concrete by third-point
loading method (ASTM C78). Copyright ASTM. reprinted with permission.

Note that third-point loading ensures a constant bending moment without
any shear force applied in the middle third of the specimen. Thus, Equation 7.5
is valid as long as fracture occurs in the middle third of the specimen. If frac-
ture occurs slightly outside the middle third, the results can still be used
with some corrections. Otherwise the results are discarded.
For normal-weight concrete, the flexure strength can be approximated as

f œ R = 1 0.62 to 0.83 2 2 (7.6a)

f œ

R = 1 7.5 to 10 2 2 (7.6b)

Equation 7.6a is used for SI units, where both R and are in MPa, where-
as Equation 7.6b is used for U.S. customary units, where both R and

f¿ c

f¿ are

in psi.

■

7.5.4 Rebound Hammer Test

The rebound hammer test, also known as the Schmidt hammer test, is a non-
destructive test performed on hardened concrete to determine the hardness
of the surface (Figure 7.34). The hardness of the surface can be correlated, to
some extent, with the concrete strength. The rebound hammer is commonly
used to get an indication of the concrete strength. The device is about 0.3 m
(1 ft) long and encloses a mass and a spring. The spring-loaded mass is released
to hit the surface of the concrete. The mass rebounds, and the amount of re-
bound is read on a scale attached to the device. The larger the rebound, the
harder is the concrete surface and, therefore, the greater is the strength. The
device usually comes with graphs prepared by the manufacturer to relate