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Portland cement is a blend of finely pulverized clinker, produced by burning at high temperature materials containing lime, alumina, iron and silica in pre-determined proportion to give the desired end properties. Normally, gypsum or its derivatives are added during grinding stage to control setting.

As recommended by BIS, if cement is more than three months old then it should be tested for its strength before being taken into use.

The reactions which are responsible for hardening of cement are characterized by liberation of heat. This is referred to as Heat of Hydration. In mass concreting works such as dam etc. where dissipation of generated heat is not possible, the temperature of concrete increases. On subsequent cooling, cracks develop. Hence, it is essential to use a cement with low hydration for massive structures.

The cement gets the shade from the nature of raw material used, which may be different from factory to factory. Further, the colour of the finished concrete is controlled largely by the colour of aggregates and to a lesser extent by the colour of the cement. Apprehension against any cement on the basis of the colour is aesthetically and technically unjustified.

The following precautions are to be taken for storage of cement:

  • 1.Keep the cement bags in a dry and moisture free store.
  • 2.Bags should be stacked on wooden planks.
  • 3.Bags should not be stacked against the wall.
  • 4.Don't pile more than 15 bags high and arrange the bags in header and stretcher fashion.
  • 5.The principles of first-in first out (FIFO) should be followed in removing bags.
  • i.The colour of cement should be uniform. It should be grey colour with a light greenish shade.
  • ii.Open the bag & see that lumps should not be present in the bag. It will ensure that no setting has taken place.
  • iii.Cement should feel smooth when touched or rubbed in between fingers. If it is felt rough, it indicates adulteration with sand.
  • iv.If a small quantity of cement is thrown in a bucket of water, the particles should float for some time before it sink.

Concrete is a mixture of cement, sand, stone aggregates and water & Mortar is a mix of cement, sand and water, to be used for brick works/block works and plaster. A mortar is a mixture of sand, cement and water. It is essentially a concrete without coarse aggregates.

If a concrete mix is placed in and around a cage of steel rods, it is called Reinforced Cement Concrete (RCC).

The term 'curing' is used to include maintenance of a favourable environment for the continuation of chemical reactions, i.e. retention of moisture within, or supplying moisture to the concrete from an external and protection against extremes of temperature.

Ponding, continuous sprinkling, covering with wet cloth, cotton mats or similar materials, covering with specially prepared paper, polyethylene, sealing coat applied as a liquid commonly known as 'curing compound' which hardens to form a thin protective membrane, are some of the methods by which concrete is cured.

Steel reinforcements are used, generally, in the form of bars of circular cross section in concrete structure. They are like a skeleton in human body. Plain concrete steel or any other reinforcement is strong in compression but weak in tension. Steel is one of the best forms of reinforcements, to take care of those stresses and to strengthen concrete to bear all kinds of loads.

Lap length is the length overlap of bars tied to extend the reinforcement length. Lap length about 50 times the diameter of the bar is considered safe. Laps of neighbouring bar lengths should be staggered and should not be provided at one level/line.

Again the term "Anchorage length" signifies the additional length of steel of one structure required to be inserted in other at the junction. For example, main bars of beam in column at beam column junction, column bars in footing etc. The length requirement is similar to the lap length mentioned in previous question or as per the design instructions.

Reinforcement should be free from loose rust, oil paints, mud etc. it should be cut, bent and fixed properly. The reinforcement shall be placed and maintained in position by providing proper cover blocks, spacers, supporting bars, laps etc. Reinforcements shall be placed and tied such that concrete placement is possible without segregation & compaction possibly by an immersion vibrator.

Plaster protects structure from temperature variations; external attacks of sulphates, chlorides, etc. Plaster also provides smooth & aesthetic surface on RCC & Brickwork surface.

The following thumb rules are necessary for good concrete practices.

  • 1.Cement Use fresh cement of approved quality and preferably from the same source. Store it properly to prevent deterioration.
  • 2.Aggregates Use well graded aggregates, free from silt, organic matter and other undesirable impurities and preferably from the same source Batching
  • 3.Batching materials by weight is preferable. It batched on volume basis, use measurement boxes in units of 35 litres which is the capacity of one 50kg bag of cement. The cement should, in any case, be batched only by weight and preferably by whole bags.
  • 4.Correction for bulking of aggregates in necessary
  • 5.Quantity of mixing water: use minimum mixing water, consistent with the degree of workability required for easy placing and compaction of concrete.
  • 6.Mixing: Use a mixing machine. Avoid hand mixing. In the case of hand-mixing, mix cement and sand first and the add aggregates
  • 7.Transporting: Avoid drying out, segregation and setting. Do not use set concrete
  • 8.Placing: Place concrete in its final position before setting starts. Avoid segregation of material and disturbance of the formwork. Lay concrete in suitable layers without any break of continuity.
  • 9.Compaction: Use proper hand tools or vibrators, avoid over-vibration.
  • 10.Finishing: Finish after a little stiffening.
  • 11.Curing: Keep concrete moist as given IS-456. Concrete will PPC or PSC need more curing period. Avoid wetting and drying cycles.
  • 12.Formwork: Use formwork which is rigid and closely fitted, with sufficient strength to support the wet concrete.