Plain Cement Concrete (PCC) is a construction material generally used as a binding materials and is composed of cement, (commonly Portland Cement) and other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate made of gravels or crushed rocks such as limestone or granite, plus a fine aggregate such as sand), water, and chemical admixtures.
In my earlier articles, I discussed types of specifications, purpose of specifications, RCC specifications, Specifications for Brick masonry in Cement Mortar, Coursed Rubble Masonry Specifications. In this article, we are going to discuss in detail various specifications that are to be given before the initiation of PCC work.
Specifications for Plain Cement Concrete (PCC)
Aggregate shall be of invert materials and should be clean, dense, hard, sound, durable, non-absorbent and capable of developing good bond with mortar.
Coarse aggregate shall be of hard broken stone of granite or similar stone, free from dust, dirt and other foreign matters. The stone ballast shall be of 20mm size and smaller. All the coarse material should be retained in a 5mm square mesh and should be well graded such that the voids do not exceed 42%.
Fine aggregate shall be of coarse sand consisting of hard, sharp and angular grains and shall pass through a screen of 5mm square mesh. Sand shall be of standard specifications, clean and free from dust, dirt and organic matter. Sea sand shall not be used.
Cement shall be fresh Portland cement of standard ISI specifications and shall have the required tensile and compressive stresses and fineness.
Water shall be clean and free from alkaline and acid matters and suitable for drinking purposes.
1:2:4 (cement : sand : stone ballast) by volume when specified. Minimum compressive strength of concrete of 1:2:4 proportion shall be 140 kg/cm2 in 7 days.
Mixing shall be done on masonry platform or sheet iron tray.
Stone ballast, sand and cement shall be put into cement concrete mixer to have the required proportions.
Regular slump test should be carried out to control the addition of water and to maintain the required consistency. A slump of 7.5cm to 10cm may be allowed for building work.
Formwork centering and shuttering shall be provided as required as per the standard specification before laying concrete to confine to support or to keep the concrete in position. The inner surface of shuttering shall be oiled to prevent concrete sticking to it.
Concrete shall be laid gently (not thrown) in layers not exceeding 15cm and compacted by pinning with rods and tamping with wooden tampers or with mechanical vibrating machine until a dense concrete is obtained.
After about two hours of laying of concrete, when the concrete has begun to harden, it shall be kept damp by covering with wet gunny bags or wet sand for 24 hours.