Construction Materials, Management and Estimating
Questions split into material specifications - cement chemistry, concrete, bricks, steel - and project management - CPM/PERT, crashing and estimating. This domain rewards precise definitions and recall of IS code provisions.
Core rules
- Cement: tricalcium aluminate (C₃A) causes flash set and high heat of hydration; C₃S gives early strength; C₂S gives long-term strength; gypsum retards C₃A.
- Workability is measured by slump for concrete, flow for mortar and compaction factor; slump is not a measure of strength.
- IS 456 curing: at least 7 days for OPC and 10 days for blended cements.
- First class bricks are table moulded, well burnt, give a ringing sound and absorb less than 20% water.
- CPM: total float = LS − ES; free float = total float − (LS of successor − ES of current); the critical path has zero float.
- PERT expected time = (a + 4m + b)/6; variance = ((b − a)/6)².
- Crash cost slope = (crash cost − normal cost)/(normal duration − crash duration); crash the cheapest critical activity first.
Worked example
An activity normally takes 10 days at ₹20,000; crashed it takes 6 days at ₹28,000. Cost slope = (28,000 − 20,000)/(10 − 6) = ₹2,000 per day. Shortening the project by 3 days through this activity costs ₹6,000, provided it remains critical and no parallel path becomes critical first.
Traps
Confusing total float with free float; crashing a non-critical activity and expecting the duration to fall; adding contingency to every rate-analysis item; and mistaking the Vicat standard consistency test (33 mm plunger) for a strength test.