Building Materials and Construction Basics
What this domain tests
This section checks whether you can identify a material, quote its standard specification and place it correctly in the construction sequence. Expect direct questions on brick classes, cement grades and setting times, sand silt and bulking, concrete grades, and the order of substructure and superstructure items.
Core rules and concepts
- Common burnt clay bricks (IS 1077): deep red colour, metallic ringing sound, water absorption not more than 20% for lower classes (15% for higher classes), frog upward in masonry.
- OPC 33/43/53 grade (IS 269, IS 8112): initial setting time not less than 30 minutes, final setting not more than 600 minutes, soundness checked by Le Chatelier apparatus.
- Sand for concrete: silt and clay content under 5%; bulking of sand must be allowed for when batching by volume.
- Concrete M20 means characteristic compressive strength of 20 N/mm² at 28 days; a water-cement ratio near 0.5 gives workable, durable concrete; curing 7 days minimum for OPC, 10-14 days in hot dry weather.
- Sequence: excavation, PCC, footing, plinth beam, DPC, superstructure, lintel, roof slab, finishing.
- DPC is laid continuous at plinth level, 40-50 mm thick cement mortar 1:2 with waterproofing compound, and must not be broken by door thresholds without a step-down detail.
Worked example
Cement needed for 1 m³ of 1:6 cement mortar. Wet volume = 1 m³; add 30-33% for voids, so dry volume = 1.33 m³. Cement = 1.33 ÷ 7 = 0.19 m³. One bag of cement = 0.035 m³, so 0.19 ÷ 0.035 = 5.4 bags, i.e. about 274 kg. Sand = 1.33 × 6 ÷ 7 = 1.14 m³.
Traps that cost marks
Swapping initial and final setting times; treating 1 m³ of mortar as needing 1 m³ of dry sand; ignoring bulking of sand; naming curing days without saying which cement type.