Building Materials and Construction
This domain tests whether you can name a material, state the property or IS limit that governs its acceptance, and choose it correctly for a situation. Questions come as property-matching, IS-limit recall, and short descriptions of masonry and foundations.
Stones, bricks and timber
- Igneous rocks (granite, basalt) are strongest and used for heavy work; sedimentary (sandstone, limestone) are softer; metamorphic (marble, slate) split easily - slate for roofing, marble for flooring.
- A good burnt clay brick is uniform in colour, gives a clear ringing sound when struck, has sharp edges, and has no lime nodules (which cause "lime blowing" - pitting after wetting).
- Water absorption of common burnt clay bricks is limited to about 20% of dry weight, and first-class bricks are expected to give roughly 10 N/mm² crushing strength.
Cement and mortar
- OPC grades 33, 43 and 53 denote 28-day compressive strength in N/mm². Initial setting time is a minimum of 30 minutes; final setting time a maximum of 600 minutes.
- Soundness is tested with the Le Chatelier apparatus (unsoundness from free lime); fineness by sieving or Blaine; strength on 70.6 mm mortar cubes.
Construction rules
- English bond (alternate header and stretcher courses) is the strongest; Flemish bond alternates within a course and is chosen for appearance.
- A damp-proof course at plinth level (1:2:4 concrete or bitumen) stops rising damp.
Worked example
A brick weighs 3.00 kg dry and 3.60 kg after 24 hours of soaking. Absorption = (3.60 − 3.00) / 3.00 × 100 = 20%. This is exactly at the permissible limit, so the brick is acceptable but with no margin; a sample reading 25% must be rejected.
Marks-losing traps
- Confusing setting time with curing: 30 minutes is a minimum for initial set, not a maximum.
- Writing that lime and cement are interchangeable - fat lime slakes and hardens by carbonation, cement by hydration.
- The efflorescence test detects soluble salts, not strength.