Detailed Specifications for Cement Concrete provides a step by step description and specifications for various phases of concrete production and application such as materials, mix proportions, mixing of content, workability, formwork, laying and curing. This specification helps as a guide for how to carry out cement concrete works for various purposes.
Work Procedure for Cement Concrete Works
1. Materials for Cement Concrete
Cement concrete is composed of cement, aggregates and water. Aggregates are of two types, namely fine aggregates (sand) and coarse aggregates. The aggregate should be inert (non-reactive) with clean, dense, hard, sound, durable, and non-absorbent grains capable of developing a good bond with the cement mortar.
Cement should be fresh Portland cement or Pozzolana Portland cement (as per the requirement or specification) and should have the required tensile and compressive strength and fineness.
Fine aggregate or sand should be coarse with hard, sharp and angular grains and should pass through 5mm (3/16”) square sieves or mesh. This should be of standard quality, clean from dust, dirt and organic matters. Sea sand should not be used for concrete works. Fine aggregates can also be made up of crushed stones or manufactured sand if specified.
These should be hard broken stones of granite or similar stone, free from dust, and other foreign materials. The size of stone ballast should be 20mm (0.75 inches) and less and should be retained on 5mm (0.25 inch) square mesh. These should belong to the high-quality grades so that voids do not exceed 42%.
The size of coarse aggregates should be as specified depending on the thickness of concrete and nature of work. For example, the size of coarse aggregates for building works should be 20mm and for road work and mass concrete works, 40mm to 60mm.
Water should be potable and free from alkaline and acidic matters.
2. Proportioning of Cement Concrete
The proportions selected for cement concrete are as per the design and strength requirements. The proportion can be 1:2:4 (M15 concrete) or 1:1.5:3 for M20 concrete. The proportion of 1:2:4 concrete denotes the ratio of cement: sand: coarse aggregates by volume unless specified. Minimum compressive strength of concrete of 1:2:4 mix proportion should be 140 kg/sq.cm or 2000 lbs/sq.in on 7 days.
3. Measurement of Materials
Sand and coarse aggregates shall be measured by volume with boxes. Cement need not be measured by box, one bag of cement of 50kg weight should be considered as 1/30 cu.m or 1.2 cu.ft volume. Size of measured boxes may be 30 cm x 30 cm x 38 cm or 35 cm x 35 cm x 28 cm equivalent to the content of one bag of cement.
All materials should be dry and, in case, if damp sand is used, compensation should be made by adding sand to the extent required for bulking of sand.
4. Mixing of Cement Concrete
Mixing of concrete should be machine-based to get the best quality. For small works, hand mixing by batches can be allowed.
Coarse aggregates, sand and cement are put into the cement concrete mixer in the required proportion. For concrete of 1:2:4 mix proportion, first four boxes of coarse aggregates, then two boxes of sand and one bag of cement is put into the machine mixer. The mixer is then revolved to mix the materials in a dry state and then water is added gradually to the required quantity, i.e. 25 to 30 liters (5 to 6 gallons) per bag of cement to have the required water-cement ratio.
The mixing should be thorough and should have a plastic mix of uniform color. The time for mixing can be 1.5 to 2 minutes per rotation for thorough mixing. Mixed concrete should be unloaded on a masonry platform or the wheelbarrow for transportation and placement. The output of the concrete mixer is 15 to 20 mix per hour.
Hand mixing of concrete should be done on a masonry platform or sheet iron tray. For concrete of 1:2:4 mix proportion, first two boxes of sand and one bag of cement should be dry-mixed thoroughly. Then, a dry mix of cement and sand is placed over a stack of 4 boxes of stone aggregates and the whole mixture is dry-mixed, turning at least three times to have a uniform mix.
Water is then added slowly and gradually with a water-can while the contents are being mixed. Generally, 25 to 30 liters (5 to 6 gallons) of water is added for every bag of cement. The contents should be mixed to give a plastic mix of required workability and water-cement ratio. The contents should be mixed thoroughly turning at least three times to give a uniform concrete.
Read More: Hand Mixing of Concrete
5. Checking for Concrete Slump
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 10 cm (3 inches to 4 inches) may be allowed for building work and 4 cm to 3 cm (1.5 inch to 2 inches) may be allowed for road work.
6. Formwork for Concrete Works
Formwork (centering and shuttering) should be provided as required as per the standard specifications before laying of concrete to confine or to support or to keep the concrete in position. The inner surface of the formwork should be oiled with formwork oils to prevent concrete from sticking to it.
The base and formwork over which concrete has to be placed should be cleaned by sprinkling water before concrete is placed. Forms should not be removed before 14 days in general, side forms may, however, be removed after 3 days of concreting. Formwork should be removed slowly and carefully without disturbing and damaging the concrete.
7. Laying of Concrete
Concrete should be laid gently in layers not exceeding 15 cm or 6 inches and compacted by pinning with rods and tamping with wooden tampers or with mechanical concrete vibrating machines until a dense concrete mix is obtained.
For important works, mechanical vibrating machines should be used, for thick or mass concrete, immersion type vibrators, and for thin concrete, surface concrete vibrators should be used for compacting concrete. Over vibration should be avoided to prevent segregation of concrete. After removal of concrete formwork in due time, the concrete should be free from honeycombing, air holes or any other defect.
Concrete should be laid or placed continuously. If the laying of concrete is suspended for the rest of the day or for the following day, the end should be sloped at an angle of 30 degrees and made rough for further jointing. When the work is resumed, the previous sloped portion should be roughened, cleaned, watered and grout of neat cement should be applied, and the fresh concrete should be placed. For successive layers of concrete, the upper layer of concrete should be placed before the lower layer has set.
8. Curing of Concrete
After about two hours of laying when the concrete has begun to harden, it should be kept damp by covering with wet gunny bags or wet sand for 24 hours and then curing by flooding with water making mud walls of 7.5 cm or 3 inches high or by covering with wet sand or earth and kept damp continuously for 15 days.
If specified, curing may be done by covering concrete with a special type of waterproof paper for 15 days to prevent moisture escaping or evaporating.
Read More on Curing of Cement Concrete – Time and Duration