Construction Materials and Their Properties
This domain tests whether you can name a material, state the property that governs its use, and quote the acceptance limit a laboratory test checks. Items appear as property matching, standard-test limits, and site situations asking why a material was chosen.
Core properties and limits
- Strength: the load carried before failure. For burnt clay bricks it is the class number in N/mm² (IS 1077 classes run 3.5 to 35).
- Water absorption: bricks should not absorb more than 20% of their dry mass, tightened to 15% for classes above 12.5.
- Workability: how easily mortar or concrete is placed and compacted; slump and compaction factor measure it.
- Cement grades 33, 43 and 53 (IS 269, IS 8112, IS 12269) are 28-day strengths of 1:3 mortar cubes, not concrete strengths.
- Setting time of OPC: initial not less than 30 minutes, final not more than 600 minutes.
- Aggregate grading and fineness modulus control voids and water demand; sand should carry no more than about 3% silt by volume.
- Steel grades Fe 415, Fe 500 and Fe 550D (IS 1786) are fixed by yield strength, with elongation and bend tests.
- Timber fails through moisture movement: shrinkage warps boards, and knots or shakes cut permissible stress.
Worked example
A brick weighs 2400 g oven-dry and 2760 g after 24 hours immersion. Absorption = (2760 − 2400) ÷ 2400 × 100 = 360 ÷ 2400 × 100 = 15%. Against the 20% limit for class 12.5 it passes comfortably; against the 15% limit for class 17.5 and above it sits exactly on the limit, so a second sample is required before acceptance.
Traps that cost marks
Dividing by the saturated mass instead of the dry mass; quoting cement grade as a concrete grade; swapping initial and final setting times; assuming a higher brick class allows more absorption when the limit is actually stricter; ignoring bulking of sand when batching by volume.