F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

2.5(a)
Strength of concrete
Strength of concrete 25
The compressive strength of concrete is the most common measure for
judging the quality of concrete. In the UK, the characteristic strength of
concrete (see Section 1.4) is based on the 28-day cube strength; that is, the
crushing strength of standard 150 mm cubes at an age of 28 days after
mixing; 100 mm cubes may be used if the nominal maximum size of the
aggregate does not exceed 25 mm. Procedures for making and testing the
cubes are given in BS 1881:Parts 108 and 116. In the USA, 6 x 12 in (150 x
300 mm nominal) cylinder specimens are tested, in accordance with ASTM
standard C39, to determine the cylinder strength. The range of cube
strengths for reinforced and prestressed concrete work is usually from 25 to
60 N/mm..:.
The cylinder strength is usually only about 70-90% of the cube strength.
The difference is due to the frictional forces which develop between the
platen plates of the testing machine and the contact faces of the test
specimen. These end forces produce a multiaxial stress state which
increases the compressive strength of the concrete. The multiaxial stress
effects are significant throughout the cube; in the cylinder, however, the
height/width ratio is sufficiently large for the mid-height region to be
reasonably free from these effects. For practical purposes the cylinder
strength may be taken as the uniaxial compressive strength of concrete.
Nowadays, tests are rarely carried out to measure directly the tensile
strength of concrete, mainly because of the difficulty of applying a truly
concentric pull. A method for measuring the indirect tensile strength,
sometimes referred to as the splitting tensile strength, is described in
BS 1881 and ASTM standard C496. The test, which was developed
independently in Brazil and in Japan, consists essentially in loading a
standard concrete cylinder (300 x 150 mm diameter in the UK; 12 x 6 in
diameter in the USA) across a diameter until failure occurs, by splitting
across a vertical plane (Fig. 2.5-1(a) ). If the concrete cylinder behaves as
a linearly elastic body, the distribution of the horizontal stresses along the
vertical diameter would be as shown in Fig. 2.5-1(b), with a uniform
tensile stress of 2 Find/ over most of the diametrical plane. Of course, the
concrete cylinder does not behave exactly as a linearly elastic body, but
both BS 1881 and ASTM C496 state that the splitting tensile strength It
may be taken as
It = ;~ (N/mm 2 ) (2.5-1)
where F = maximum applied force (N);
d = cylinder diameter (mm);
I = cylinder length (mm).
For practical concrete mixes such as those used in reinforced and
prestressed concrete construction, the splitting tensile strength generally
varies from about 118 to 1112 of the cube strength.
The determination of the flexural strength of concrete, as described in
BS 1881 and ASTM standard C78, consists essentially of testing a plain