Concrete Technology
Scope of the domain
Concrete technology covers ingredients, grades, workability, batching, mixing, placing, compaction, curing and testing. The exam tests nominal mixes, water-cement ratio and the meaning of cube strength.
Core rules and concepts
- Concrete = cement + fine aggregate + coarse aggregate + water, with admixtures if needed. Aggregates occupy most of the volume.
- Grade M20 means characteristic compressive strength of 20 N/mm² at 28 days, tested on 150 mm cubes.
- Nominal mixes by volume: M10 = 1:3:6, M15 = 1:2:4, M20 = 1:1.5:3, M25 = 1:1:2.
- Water-cement ratio controls strength and durability; a lower w/c ratio generally gives higher strength if the mix is workable.
- Workability is measured by slump, compaction factor or Vee-Bee test. Slump is not a strength test.
- Curing keeps concrete moist so cement hydration can continue; 28 days is the standard reference age.
Worked example
A mix uses 50 kg of cement and a water-cement ratio of 0.50. Water required = 0.50 × 50 = 25 kg. Since 1 kg of water is about 1 litre, the quantity is 25 litres. Reasoning: the ratio is by mass of water to mass of cement; do not add water to improve workability without adjusting the mix, because extra water lowers strength and durability.
Traps
Do not call M20 a 1:2:4 mix; that is M15. Do not say slump measures strength. Do not use 7-day strength as the grade value. Do not forget that cube strength is characteristic strength, not the average of any single cube.