FACTORS AFFECTING STRENGTH OF CONCRETE

 In general, concrete consists of coarse and fine aggregate, cement, water, and—in many cases— different types of admixtures. The materials are mixed together until a cement paste is developed, filling most of the voids in the aggregates and producing a uniform dense concrete. The plastic concrete is then placed in a mold and left to set, harden, and develop adequate strength. The strength of concrete depends upon many factors and may vary within wide limits with the same production method. The main factors that affect the strength of concrete are described next. 

Water–Cement Ratio 

The water–cement ratio is one of the most important factors affecting the strength of concrete. For complete hydration of a given amount of cement, a water-cement ratio (by weight) equal to 0.25 is needed. A water–cement ratio of about 0.35 or higher is needed for the concrete to be reasonably workable without additives. This ratio corresponds to 4 gal of water per sack of cement (94 lb) (or 17.8 lb per 50 kg of cement). Based on this cement ratio, a concrete strength of about 6000 psi may be achieved. A water–cement ratio of 0.5 and 0.7 may produce a concrete strength of about 5000 and 3000 psi, respectively.

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. 

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. 

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:

 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.

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.