Structural Concrete - Hassoun

16 Chapter 2 Properties of Reinforced Concrete
2.1.2 Properties and Proportions of Concrete Constituents
Concrete is a mixture of cement, aggregate, and water. An increase in the cement content in the
mix and the use of well-graded aggregate increase the strength of concrete. Special admixtures are
usually added to the mix to produce the desired quality and strength of concrete.
2.1.3 Method of Mixing and Curing
The use of mechanical concrete mixers and the proper time of mixing both have favorable effects on
strength of concrete. Also, the use of vibrators produces dense concrete with a minimum percentage
of voids. A void ratio of 5% may reduce the concrete strength by about 30%.
The curing conditions exercise an important influence on the strength of concrete. Both moisture
and temperature have a direct effect on the hydration of cement. The longer the period of moist
storage, the greater the strength. If the curing temperature is higher than the initial temperature of
casting, the resulting 28-day strength of concrete is reached earlier than 28 days.
2.1.4 Age of Concrete
The strength of concrete increases appreciably with age, and hydration of cement continues for
months. In practice, the strength of concrete is determined from cylinders or cubes tested at the
age of 7 and 28 days. As a practical assumption, concrete at 28 days is 1.5 times as strong as at
7 days: The range varies between 1.3 and 1.7. The British Code of Practice [2] accepts concrete
if the strength at 7 days is not less than two-thirds of the required 28-day strength. For a normal
portland cement, the increase of strength with time, relative to 28-day strength, may be assumed as
follows:
Age 7 days 14 days 28 days 3 months 6 months 1 year 2 years 5 years
Strength ratio 0.67 0.86 1.0 1.17 1.23 1.27 1.31 1.35
2.1.5 Loading Conditions
The compressive strength of concrete is estimated by testing a cylinder or cube to failure in a few
minutes. Under sustained loads for years, the compressive strength of concrete is reduced by about
30%. Under 1 day sustained loading, concrete may lose about 10% of its compressive strength.
Sustained loads and creep effect as well as dynamic and impact effect, if they occur on the structure,
should be considered in the design of reinforced concrete members.
2.1.6 Shape and Dimensions of Tested Specimen
The common sizes of concrete specimens used to predict the compressive strength are either 6 ×
12-in. (150 × 300-mm) or 4 × 8-in. (100 × 200-mm) cylinders or 6-in. (150-mm) cubes. When a
given concrete is tested in compression by means of cylinders of like shape but of different sizes,
the larger specimens give lower strength indexes. Table 2.1 [4] gives the relative strength for various
sizes of cylinders as a percentage of the strength of the standard cylinder; the heights of all cylinders
are twice the diameters.
Sometimes concrete cylinders of nonstandard shape are tested. The greater the ratio of specimen
height to diameter, the lower the strength indicated by the compression test. To compute the
equivalent strength of the standard shape, the results must be multiplied by a correction factor.
Approximate values of the correction factor are given in Table 2.2, extracted from ASTM C 42/C
42 M. The relative strengths of a cylinder and a cube for different compressive strengths are shown
in Table 2.3.