Materials Used in Reinforced Cement Concrete (R.C.C.)Work
Before the introduction of ordinary portland cement, the lime was used as a cementing or binding material. Most of the cement concrete work in building constructions are done with ordinary portland cement.
But other special varieties of cement such as rapid hardening cement, high alumina cement, and colored cement are used under certain circumstances. The cement should comply with all the standard requirements.
These are the inert or chemically inactive materials that form the bulk part of cement concrete. These are bonded together by means of a cement. The aggregates are classified into two categories i.e., fine and coarse:
The materials which are passing through the BIS test sieve no. 480 is termed as a fine aggregate. Generally, the natural river sand is used as a fine aggregate. But at some places, where natural river sand is not available economically, the finely crushed stone (stone dust) may be used as a fine aggregate.
The material which is retained on the BIS test sieve no. 480 is designated as a coarse aggregate. Generally, the broken stone is used as a coarse aggregate. The nature of work decides the maximum size and shape of the coarse aggregate. For thin slabs and walls, the maximum size of coarse aggregate should be limited to one-third the thickness of the concrete section to be cast.
The aggregates to be used for reinforced cement concrete work should be hard, durable, and clean. The aggregates should be completely free from any kind of oils, lumps of clay, organic and vegetable matter, fine dust, etc. The presence of all such debris prevents adhesion of aggregates with the concrete and hence reduces the strength of concrete.
Steel reinforcing bars shall be in the form of round bars of mild steel or deformed steel of standard specifications and shall be free from corrosion, loose rust scales, oil, grease, paint, etc.
The steel bar shall be round and diameters of bars may vary from 5 mm to 40 mm and capable of being bent (doubled over) without fracture. Bars shall be hooked and bent accurately and placed in position as per design and drawing and bonded together tight with 20 S.W.G. annealed steel wire of standard specification at their point of intersection.
Steel bars shall be bent by applying gradual and even motion, steel bars of 40 mm diameter and greater may be bent by heating it to dull red and allowed to cool slowly without immersing in water or quenching.
Joints in the steel bar should be avoided as far as possible when joints have to be made an overlap of 40 times diameter bars shall be given with the proper hooks at the ends and joints should be staggered.
Bigger diameter bars should be joined by welding and tested before placing them in position.
While concreting steels bars shall be given side and bottom covers of concrete by placing precast cover blocks underneath of 1:2 cement mortar 2.3 cm X 2.5 cm in sections and thickness of the specified cover, 4 cm to 5 cm for beam and 1 cm to 2 cm for a slab.
During laying and compacting of concrete the reinforcing bars should not move from their positions and bars of the laid portions should not be disturbed.
This is the least expensive ingredient but the most important ingredient of concrete. The water, which is being used for making concrete, should be clean and free from harmful impurities and debris such as oil, alkali, acid, etc.
In general, the water which is fit for drinking purposes should be used for making concrete.
Centering and Shuttering
Centering and shuttering shall be made with timber or steel plate close and tight to prevent any leakage, with necessary props, bracings, and wedges, sufficiently strong and stable and should not yield on laying concrete and placed in such a way that they can be slackened and removed gradually without disturbing the concrete.
No plastering should be made on the concrete surface before proper curing.
A coat of oil should be applied over the shuttering or paper should be spread to have a smooth and finished surface and to prevent adherence of concrete and this will make removal of the formwork easy.
For slab and beam small camber should be given in centering, 1 cm per 2.5 m with a maximum of 4 cm. Centering and shuttering should not be stripped before 14 days in general ( 4 days for R.C.C. columns, 10 days for roof slab, and 14 days for beams).
The centering and shuttering shall be removed carefully so that no part is disturbed or damaged.
The proportion of Cement Concrete
Cement concrete shall be of 1:2:4 proportion by volume for slabs, beams, and lintels, and 1:1.5:3 proportion for columns unless otherwise specified.
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