determines the compressive strength of concrete

Concrete Mix

The concrete mix is an important aspect of RCC slab because it determines the compressive strength of concrete and also its durability in terms of weather resistance and water permeability. In engineering practice, the concrete mix is denoted by M15, M20, M25 (and more richer mixes as well). where the number denotes the strength of a cube in N/mm2 of concrete made using similar mix proportions. The mix proportions should are determined on the basis of the strength requirements of concrete and the grading and size of aggregates. However, ordinarily, the mix proportions are specified as 1:2:4 (M15), 1:1.5:3 (M20), 1:3:6 (M10)  - for cement:sand:aggregates. For instance, 1:2:4 is commonly specified as mix for casting RCC slabs in rural areas, where as less stronger mixes such as 1:4:8 are specified for Plain Cement Concrete applications such as base concrete for footings.

As per IS 456:2000 – Code of Practice for Plain and Reinforced Concrete, the following grades of concrete are recommended for various exposure conditions which would affect its durability –

Exposure condition	Minimum cement content – kg/m3	Maximum water cement ratio	Min grade of concrete
Mild	300	0.55	M20
Moderate	300	0.50	M25
Severe	320	0.45	M30
Very severe	340	0.45	M35
Extreme	360	0.40	M40

Reinforcement

The reinforcement steel for slabs is normally Tor steel (cold-twisted deformed bars) or TMT (Thermo Mechanically Treated) bars which have recently become more common on account of their better resistance to corrosion. The bars are available in standard diameters ranging from 6mm to 40mm. The actual specification for reinforcement is calculated on the basis of structural requirements. However, for most rural applications, where spans normally donot exceed 3.5 metres, 8 and 10 mm dia steel bars are used, the bigger dimater used as main reinforcemment along the span of the slab. As a general guide, minimum reinforcement in RCC slabs in either direction should not be less than 0.15% of the gross cross-sectional area of concrete and the maximum diameter of main reinforcement should be 1/8th-1/10th of the slab thickness. Slab being a crucial element of construction of the house, the advice of non professionals should not be followed by the owner.

·         Minimum lap length should be 50 times the diameter of bar- 50d

·         Development length should be at least sixty times the diameter of bar- 60d

·         Steel should be bent at 45 degree

·         Cover blocks should be used to maintain covers to steel bars. The following covers should be maintained -

·         Column- 40 mm

·         Beam- 25 mm

·         Slab- 20 mm

·         Footing- 50mm