Concrete is a composite material composed mainly of water, aggregate, and cement. Usually, there are
additives and reinforcements included to achieve the desired physical properties of the finished material. When these ingredients are mixed together, they form a fluid mass that is easily molded into shape. Over time, the cement forms a hard matrix that binds the rest of the ingredients together into a durable stone-like material with many uses.
Fine and coarse aggregates make up the bulk of a concrete mixture. Sand, natural gravel, and crushed stone
are used mainly for this purpose.
Concrete production is the process of mixing together the various ingredients—water, aggregate, cement, and any additives—to produce concrete. Concrete production is time-sensitive. Once the ingredients are mixed, workers must put the concrete in place before it hardens. In modern usage, most concrete production takes place in a large type of industrial facility called a concrete plant, or often a batch plant. general usage, concrete plants come in two main types, ready mix plants, and central mix plants. A ready mix plant mixes all the ingredients except water, while a central mix plant mixes all the ingredients including water. A central mix plant offers more accurate control of the concrete quality through better measurements of the amount of water added, but must be placed closer to the work site where the concrete will be used since hydration begins at the plant.
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Workability of Concrete
It is the amount of useful work done internally to produce full compaction. Workability is the ability of a fresh
(plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the
concrete’s quality. Workability depends on water content, aggregate (shape and size distribution), cementitious
content, and age (level of hydration) and workability can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures increases concrete workability.
Factor affecting workability
The factors helping concrete to have more lubricating effect to reduce internal friction for helping easy compaction are given below:
Water content:- cement requires a water/cement ratio of about 0.25 for hydration and a water/cement ratio of 0.15 for filling the voids in the gel pores. In other words, a water/cement ratio of about 0.38 would be required to hydrate all the particles of cement and also to occupy the space in the gel pores Water content in a given volume of concrete, will have significant influences on the workability. The higher the water content per cubic meter of
concrete, the higher will be the fluidity of concrete, which is one of the important factors affecting workability. More water can be added, provided a correspondingly higher quantity of cement is also added to keep the water/cement ratio constant so that the strength remains the same.
Abrams’ law (also called Abrams’ water-cement ratio law) law states the strength of a concrete mix is inversely proportional to the mass ratio of water to cement. As the water content in releases, the strength of concrete decreases.
where x is water cement ratio and A,B are constant
Mix proportions: Aggregate/ cement ratio is an important factor influencing workability. The higher the aggregate/cement ratio, the leaner is the concrete. In lean concrete, less quantity of paste is available for providing lubrication, per unit surface area of aggregate and hence the mobility of aggregate is restrained. On
On the other hand, in the case of rich concrete with a lower aggregate/cement ratio, more paste is available to make the mix cohesive and fatty to give better workability.
- Higher the ⁄ ratio, leaner is the mix and the leaner mix is less workable.
- The lesser the ⁄ ratio, rich the mix, and the more workable.
- Flaky particles reduce workability because of higher surface area
As per IS 456 clause 184.108.40.206, average 28 days compressive strength of at least three 150mm cubes prepared
with water proposed to be used shall not be less than 90% of the average strength of three similar concrete
cubes prepared with distilled water.
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Size of aggregates:- The bigger the size of the aggregate, the less the surface area and hence less amount of water is required for wetting the surface, and less matrix or paste is required for lubricating the surface to reduce internal friction. For a given quantity of water and paste, a bigger size of aggregates will give higher workability. The above of course will be true within certain limits.
Surface texture: The influence of surface texture on workability is again due to the fact that the total surface area of rough-textured aggregate is more than the surface area of a smooth rounded aggregate of the same volume. From the earlier discussions, it can be inferred that rough textured aggregate will show poor workability and smooth or glassy textured aggregate will give better workability. A reduction of inter-particle frictional resistance offered by smooth aggregates also contributes to higher workability.
The shape of aggregates: The shape of the aggregate influences the workability in good measure. Angular,
elongated, or flaky aggregate makes the concrete very harsh when compared to rounded aggregates or cubical-shaped aggregates. Contribution to better workability to rounded aggregate will come from the fact that for the
given volume or weight it will have less surface area and fewer voids than the angular or flaky aggregate. Not only that,
being round in shape, the frictional resistance is also greatly reduced. This explains the reason why river sand and
gravel provide greater workability to concrete than crushed sand and aggregate.
Grading of aggregates: This is one of the factors which will have maximum influence on workability. A well-graded aggregate is the one that has the least amount of voids in a given volume. Other factors being constant, when the total voids are less, the excess paste is available to give a better lubricating effect. With an excess amount of paste, the mixture becomes cohesive and fatty which prevents the segregation of particles. Aggregate particles will slide past each other with the least amount of compacting effort. The better the grading, the less is void content and the higher the workability. The water-cement ratio needed for cement to complete its hydration process ranges from 0.22 to 0.25. The existence of additional water in the mixture is needed for ease of concrete placing and finishing (workability of concrete). Reducing the water content in a mixture may result in a stiffer mixture, which reduces the workability and increases potential placement problems. A concrete is said to be workable if it can be easily mixed, easily placed, and easily finished. Workable concrete should not show any segregation or bleeding.
Segregation – segregation is said to occur when coarse aggregates depart out from the finer aggregate. i.e. get the concentration of coarse aggregates at one place and finer aggregates at another. Segregation is caused when concrete is dropped from a considerable height or changes in direction while placing concrete. The danger of segregation can be reduced by the use of air entrainment. Segregation can be reduced by air entrainment in concrete.
Bleeding – Bleeding of concrete is said to occur when excess water comes up at the surface of the concrete. Bleeding is a function of (a) air velocity, (b) temperature, and (c) humidity. If the rate of bleeding is roughly equal to the rate of evaporation, then bleeding will not cause any problem. If the rate of bleeding is less than the rate of evaporation, then the surface becomes dry, because of which cracks appear on it. Bleeding can be reduced by using fine cement/use pozzolans. Bleeding is lower with finer cement, cement having high C3A content, or when calcium chloride is added. Rich mixes are less prone to bleeding than lean ones. Air entrainment effectively reduces bleeding.
Efflorescence –White substance coming at top of concrete surface. The presence of chloride in water is responsible for effloresces. So, seawater should not be used for RCC. But if for the entire life, the structure will be submerged in the sea then seawater can be used.
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