Concrete is an important construction material consisting of ingredients that are inert materials, viz., coarse and fine aggregates and binding materials, viz., cement or lime. Cement concrete is widely used as plain concrete, reinforced concrete, pre-stressed concrete and precast concrete members.
Properties of coarse and fine aggregates and the quality of water to make quality cement concrete is discussed in the following sections.
8.4.1 Fresh and Hardened Concrete
Cement concrete is prepared to a plastic consistency by adding water and this hardens on curing. Plastic concrete is called fresh concrete, which is a freshly mixed material that can be moulded into any shape. The concrete formed to the required shape and cured is said to be hardened concrete. The properties of the concrete, in both the stages, depend on the relative quality and quantity of cement, aggregates and water.
8.4.2 Aggregates
Aggregates are the major and important constituent of concrete. They form the body of the concrete as it occupies 70–80% of the volume of concrete. Although aggregates were originally considered to be inert material, they have recently been found to be chemically active to some extent. Cement is the only factory-made component, whereas aggregates (both coarse and fine) and water are naturally available materials.
Concrete can be considered as a two-phase material, viz., paste phase and aggregate phase. Paste phase is the role played by cement and water in concrete whereas aggregate phase is the role played by aggregates (both fine and coarse) in concrete. The requirement of paste phase has been dealt in previous chapter. Now the aggregates phase is discussed.
1. Classification
Aggregates may be classified as (i) normal weight aggregates, (ii) light-weight aggregates and (iii) heavy-weight aggregates.
Normal weight aggregates can be further classified into natural and artificial aggregates. They are also classified as coarse or fine aggregates depending on the size. Natural and artificial aggregates are as follows:
2. Coarse Aggregate
As stated above, coarse aggregates are produced by the disintegration and crushing of rock. Coarse aggregates are usually those particles which are retained on an IS 4.75 mm sieve.
Stones that are hard and durable such as granite, basalt, quartzite provide good coarse aggregate. Naturally available river gravels and shingle obtained from sea beaches after removing shale may be used.
Blast furnace slag is used for making light-weight concrete. Brick-bats are used for lime concrete or for temporary or cheap concrete work. For reinforced concrete only crushed rock aggregates of size 20 mm are generally considered as satisfactory.
The aggregates should be absolutely clean, free from organic matter and other impurities. The aggregate must be capable of resisting weather.
The grading of coarse aggregate is very important for getting good quality concrete. Good grading of coarse aggregate implies that the quantity of aggregate used should contain all standard fractions of aggregate in required proportions such that the sample contains minimum voids. Thus, a sample of well-graded aggregate (both coarse and fine) containing minimum voids requires a minimum paste phase. Minimum paste require less quantities of cement and water. This is further to mean increased economy, higher strength, lower shrinkage and greater durability.
Fineness modulus is obtained by adding the cumulative percentages of aggregates retained on each sieve (ranging from 80 mm to 150 micron) and dividing their sum by 100. Fineness modulus is an empirical factor, larger the figure, coarser is the material. For a good concrete, the value of fineness modulus of coarse aggregate should be between 6 and 8.5.
One of the methods of arriving at the practical grading is by the trial and error method. Different size fractions of aggregates can be mixed, and the one that gives the maximum weight or minimum void may be used.
3. Fine Aggregate
Fine aggregate is sand, which is usually obtained from rivers or lakes. Sometimes beach sand is also used. In places where sand is not available or a large quantity of sand is needed, crushed stone dust is used. The fineness modulus (F.M.) of sand should be around 2 to 3.2. The following limits may be used as guidance.
Water
Water is an important ingredient of concrete. As a general guidance, if the water is fit for drinking, it is fit for making concrete. However, some water containing small traces of sugar is not suitable for concrete. Another yardstick adopted is that if the pH is between 6 and 8 the water is considered to be suitable.
The best method is to use the water from the particular source and sample concrete is prepared for testing. The concrete is tested for 7 days and 28 days compressive strengths; if 90% result is obtained it can be considered to be suitable.
The following guidelines may also be adopted:
- Neutralizing 100 ml sample of water using phenoplhaline as an indicator should not require more than 5 ml of 0.02 N NaOH.
- Neutralizing 100 ml of sample of water using a mixed indicator should not require more than 25 ml of 0.02 N H2SO4.
BIS (IS: 456–2000) recommends the permissible limits for solids enlisted in Table 8.1.
Table 8.1 Materials and permissible solids
Source: IS: 456–2000.
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