Concrete Technology and RCC Design
Concrete Technology covers making and testing concrete; RCC Design applies the IS 456:2000 limit state method to beams and slabs. The exam rewards clear statements of grades, water-cement ratio, workability tests and the flexural formulae.
Concrete basics
M20 means a mix designed for a 28-day cube strength of 20 N/mm2, tested on 150 mm cubes after wet curing. Abrams law states that strength falls as the water-cement ratio rises. For mild exposure the IS 456 durability limits are a minimum cement content of 300 kg/m3 and a maximum w/c of 0.55. Workability is measured by slump for concrete, compaction factor for low-workability mixes and the Vee-Bee apparatus for very dry mixes.
Limit state design
Factored moment Mu = 0.36 fck b xu (d - 0.42 xu), with xu limited to xu,max, which is 0.48d for Fe415 and 0.46d for Fe500. For a singly reinforced balanced section Mu,lim = 0.138 fck b d squared with Fe415. Steel area follows from Mu = 0.87 fy Ast (d - 0.42 xu).
Worked example
A beam has b = 230 mm, d = 450 mm, M20 concrete and Fe415 steel. Mu,lim = 0.138 x 20 x 230 x 450 squared = 128.5 kNm. A factored moment of 100 kNm is below this, so the section is singly reinforced. Minimum steel of 0.85 bd/fy must still be provided even if the calculated area is smaller.
Traps
- Using characteristic loads directly without the partial safety factors 1.5 for dead and live load and 1.15 for steel.
- Writing fy instead of 0.87 fy in the moment equation.
- Quoting 7-day strength where 28-day strength is required.
- Forgetting nominal cover, which ranges from 20 to 40 mm depending on exposure, and development length.