Concrete Technology
The exam asks about concrete grades and nominal mixes, water-cement ratio, workability tests, cube testing and curing - usually as one small calculation and several definition-level items.
Grades and mixes
- M20 means characteristic cube compressive strength of 20 N/mm² at 28 days. Nominal mixes: 1:3:6 for M10, 1:2:4 for M15, 1:1.5:3 for M20, 1:1:2 for M25.
- Water-cement ratio controls both strength and workability: a lower ratio gives higher strength but harsher concrete; typical values are 0.4 to 0.6.
Workability and fresh concrete
- Slump cone is 300 mm high; a slump of about 25–75 mm suits normal RCC work. Slump measures workability only, never strength.
- Compaction factor test and Vee-Bee consistometer are the alternatives. Excess water causes segregation and bleeding.
Testing and curing
- Test 150 mm cubes, three per sample, cured 28 days; 7-day strength is roughly 65–70% of the 28-day value.
- Cure by ponding, wet gunny bags or spraying, for at least 7 days with ordinary Portland cement and longer with blended cements.
Worked example
A 150 mm cube fails at 450 kN. Area = 150 × 150 = 22,500 mm². Stress = 450,000 N ÷ 22,500 mm² = 20 N/mm². The sample meets M20; had it failed at 380 kN the result would be 16.9 N/mm² and would fail.
Marks-losing traps
- Using the cube's face perimeter or the mould volume instead of the 22,500 mm² area.
- Quoting slump as a strength measure.
- Forgetting that volume batching is affected by bulking of sand, so weight batching is preferred for good concrete.