Cement concrete is an important construction material. Its importance is increasing every day. It is a composite material consisting of a binding material, which is essentially cement, inert mineral Filler, known as aggregates and water. Thus cement concrete can be prepared by mixing above ingredients in a fixed proportion. Green concrete is a plastic mass, which can be moulded into any desired shape. This is its main advantage as a construction material.
Aggregate, coarse and fine combined occupy about 75% space within a given mass of concrete and the rest of 25% space is filled by water, cement and air voids.
Functions of Aggregate and Cement Paste:
The aggregate has following three main functions:
1. It provides a mass of particles, which resists the action of applied loads, abrasion, percolation of moisture and the action of the weather.
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2. It reduces the volume changes resulting from setting and hardening process and from moisture changes in the cement paste.
3. It provides relatively cheap filler for the cementing material.
The properties of concrete resulting from the use of a particular aggregate depend upon the following factors:
1. The mineral characteristics of the aggregate particles. It influences particularly strength, durability and elasticity of concrete.
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2. Surface characteristics of the particles. It has a marked influence on the workability of fresh concrete and bond in the hardened concrete.
3. Grading of aggregate. It has a marked influence on the workability, density and economy of concrete.
4. Amount of aggregate per unit-volume of concrete. It has a marked influence on the cost and volume changes due to drying.
Cement Paste for Concrete:
It is a binder material, which is formed by the hydration of cement. After adding water to the cement, it begins to hydrate and forms a gel. Cement needs 23% water of its weight for hydration and 15% for filling the space or voids of gel. Thus for complete hydration of cement 38% of water of weight of cement is required.
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If more water is added to cement, paste, it will evaporate forming capillary voids, resulting honey combed structure of the paste. This will reduce the strength and other properties of concrete. The gel occupies about 2.1 times the volume of dry cement and forms cement paste with fine aggregate. The strength of concrete depends upon the physical characteristics of this gel.
Functions of Cement Paste:
Followings are the main functions of the cement paste:
1. It provides lubrication to the coarse aggregate and helps in compaction of the green concrete.
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2. It fills the voids in between the inert-aggregates.
3. It provides a plastic mass, which can be moulded in any shape.
4. It provides strength and water tightness to the concrete in the hardened state.
The properties of hardened paste depend upon the following factors:
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(a) Characteristics of cement,
(b) Water-cement ratio, and
(c) Completeness of hydration of cement.
General Proportion of Ordinary Concrete:
The properties of green i.e., freshly prepared concrete and hardened concrete depend upon the relative proportions of the component ingredients. In fresh concrete the coarse aggregate is suspended in the cement paste. In this state the cement paste should not only coat the aggregate, but also should fill the voids between them.
The consistency of the mass is controlled by the fluidity of the paste and gradation and shape of aggregate particles. For almost all works, plastic consistency is desirable as too wet or too harsh a mixture will produce defective products.
In case of hardened concrete, strength and porosity is one function of the density of paste, which in turn is controlled by water-cement ratio in the mix. Thus there are practical limits to the proportions of cement, water and aggregate (coarse as well as fine aggregate) in normal concrete. The general proportions of practical mixes are shown in the lower diagram of Fig. 1.1.
All mixes are of the same consistency. In this figure, the ratio of cement to combined aggregate varies from 1:3 to 1:9. From the figure it will be seen that the volume of aggregate varies from 66% for 1:3 ratio to 76% for 1:9 cement aggregate ratio and the cement ratio varies from 16% to 7% making cement and aggregate as 82% to 83% of the volume of concrete. Thus in lean mixes the quantity of cement is even less than 50% than in the rich mixes.
The water content varies from 16% to 18% of the total volume of concrete. The amount of water for 1:3 mix is 18%, 16% for 1:6 mix and 17.3% for 1:9 mix. Thus it is clear from the above that water content is practically constant for all mixes while the cement content varies considerably. Hence in lean mixes greater water cement ratio is used. Thus for the same consistency an intimate relation exists between the richness of mix and water cement ratio.
The upper diagram of Fig. 1.1 shows the relative proportion of cement, combined water and free water in the total volume of paste of the mixes represented by the lower diagram. From the diagram it will be seen that per unit of paste, the amount of combined water practically remains the same for all mixes. Also to produce a workable mix about 70% more water is required to be added than is necessary for the hydration of cement.
Actually about 25% to 30% water is required for hydration of cement, which is added to a workable mix. Thus pastes having high water cement ratios contain more free or uncombined water than those having low water-cement-ratios. As free water occupies effective pore space, the paste in lean mixes has more porous structure than rich mixes. Actually the properties of the concrete are governed by the physical structure of the gel, (properties of fresh concrete).
Advantages of Concrete:
Concrete is considered superior than other construction materials due to the following reasons:
1. Green or freshly prepared concrete can be moulded in any desired shape easily.
2. Concrete can be mixed at places many kilometers away from the actual site of work and used without any loss of property of the good concrete.
3. Concrete of any desired compressive strength can be prepared.
4. Using steel along with concrete any type of structure can be built.
5. Almost all types of structures can be built with R.C.C.
6. As concrete has high compressive strength, it can be used economically.
7. Light weight and fire resistant concrete can also be prepared. Light weight concrete is useful for partition work.
8. Materials used in the production of concrete are easily available.
9. It can be pumped and placed even in very difficult locations.
10. It can be sprayed and can be used to rectify small cracks and repair work by gunitting or shot creating.
11. It provides quickness and ease in the construction of monolithic members of the structures.
12. It provides good architectural look to the structure.
13. It is quite durable and not liable to be attacked by white ants or termite.
14. The maintenance cost of structure made of concrete is less.
15. It is fire resistant and can also be used as sound proof by replacing the coarse aggregate either by light weight aggregate or using foam concrete.
Disadvantages of Concrete:
Concrete has following disadvantages:
1. Its tensile strength is very low of the order of 10% of the compressive strength. Thus it develops cracks. To avoid the development of cracks, steel is used as reinforcement.
2. Due to rise in temperature, concrete expands and due to fall in temperature it shrinks. Thus to avoid the development of deformation in concrete structures, expansion joints are provided at suitable locations and interval.
3. The fresh concrete shrinks very much on drying and hardened concrete expands on getting moisture, and again shrinks on drying.
4. Concrete is not fully impermeable material. Hence to store salty water, special treatment of the surface is essential, more specially at joints.
5. For structures which come in contact with alkaline and sulphate mixed water, special cement should be used as ordinary cement will disintegrate easily.
6. Due to the more weight of concrete, more reinforcement is needed, which increases the cost of the structure.
7. Due to sustained load, more creep develops in the concrete. Hence at the time of design, provision for creep should also be kept.