Structural Concrete - Hassoun

2.2 Compressive Strength 17
Table 2.1
Effect of Size of Compression Specimen on Strength of
Concrete
Size of Cylinder
in. mm Relative Compressive Strength
2 × 4 50× 100 1.09
3 × 6 75× 150 1.06
6 × 12 150 × 300 1.00
8 × 16 200 × 400 0.96
12 × 24 300 × 600 0.91
18 × 36 450 × 900 0.86
24 × 48 600 × 1200 0.84
36 × 72 900 × 1800 0.82
Table 2.2
Strength Correction Factor for Cylinders of Different Height–Diameter Ratios
Ratio 2.0 1.75 1.50 1.25 1.10 1.00 0.75 0.50
Strength correction factor 1.00 0.98 0.96 0.93 0.90 0.87 0.70 0.50
Strength relative to standard cylinder 1.00 1.02 1.04 1.06 1.11 1.18 1.43 2.00
Table 2.3 Relative Strength of Cylinder versus Cube [6]
Compressive strength (psi) 1000 2200 2900 3500 3800 4900 5300 5900 6400 7300
(N/mm 2 ) 7.0 15.5 20.0 24.5 27.0 24.5 37.0 41.5 45.0 51.5
Strength ratio of cylinder to cube 0.77 0.76 0.81 0.87 0.91 0.93 0.94 0.95 0.96 0.96
2.2 COMPRESSIVE STRENGTH
In designing structural members, it is assumed that the concrete resists compressive stresses and
not tensile stresses; therefore, compressive strength is the criterion of quality concrete. The other
concrete stresses can be taken as a percentage of the compressive strength, which can be easily
and accurately determined from tests. Specimens used to determine compressive strength may be
cylindrical, cubical, or prismatic.
Test specimens in the form of a 6-in. (150-mm) or 8-in. (200-mm) cube are used in Great
Britain, Germany, and other parts of Europe.
Prism specimens are used in France, Russia, and other countries and are usually 70 × 70 ×
350 mm or 100 × 100 × 500 mm. They are cast with their longer sides horizontal and are tested,
like cubes, in a position normal to the position of cast.
Before testing, the specimens are moist cured and then tested at the age of 28 days by gradually
applying a static load until rupture occurs. The rupture of the concrete specimen may be caused
by the applied tensile stress (failure in cohesion), the applied shearing stress (sliding failure), the
compressive stress (crushing failure), or combinations of these stresses.
The failure of the concrete specimen can be in one of three modes [5], as shown in Fig. 2.1.
First, under axial compression, the specimen may fail in shear, as in Fig. 2.1a. Resistance to failure
is due to both cohesion and internal friction.