Concrete Technology and Reinforced Concrete Design
Concrete technology and reinforced concrete design covers concrete ingredients, workability, mix design, curing, and limit-state design of RCC members to IS 456:2000. Expect questions on grades, water-cement ratio, cover, singly reinforced sections and shear.
Core rules
- M20 means characteristic cube strength of 20 N/mm² at 28 days; target mean strength = fck + 1.65σ.
- IS 456 exposure limits: maximum w/c 0.55, 0.50, 0.45, 0.40 and 0.35 for mild, moderate, severe, very severe and extreme exposure; nominal cover for slabs is 20, 30, 45, 50 and 75 mm respectively.
- Limit state of collapse: Mu = 0.36 fck b xu(d − 0.42xu); xu/d is limited to 0.48 for Fe 415 and 0.46 for Fe 500.
- Minimum tension steel in slabs is 0.12% of gross area for Fe 415/500 and 0.15% for Fe 250.
- Workability tests: slump, compaction factor and Vee-Bee; curing sustains hydration and reduces shrinkage.
Worked example
A singly reinforced beam has b = 230 mm, d = 450 mm, fck = 20 N/mm², fy = 415 N/mm² and M = 120 kNm. Limiting moment = 0.138 fck b d² = 0.138 × 20 × 230 × 450² = 128.5 kNm, so the section is singly reinforced. Then M/(fck b d²) = 120 × 10⁶/(20 × 230 × 450²) = 0.1288. Ast = 0.5 fck/fy [1 − √(1 − 4.6 × 0.1288)] bd = 0.0241 × 0.3616 × 103,500 ≈ 902 mm².
Traps
Mixing cube and cylinder strengths; using nominal mixes where design mix is required; taking clear cover as effective cover; applying the Fe 250 xu/d limit to Fe 415.