Materials and Specifications
Materials behaviour and specification
You are tested on choosing materials by performance and writing a specification a contractor can price and a clerk of works can enforce. Cement: ordinary Portland 42.5 for general work, sulphate-resisting for marine and sulphate-bearing soils, blended or pozzolana cements for lower heat and better durability. Aggregates must be graded, clean and free of clay; the water-cement ratio governs strength and durability more than cement content. Blocks: 225 mm and 150 mm sandcrete, properly cured and tested for compressive strength. Timber: seasoned to limit movement, free of large knots and fungal attack, treated against termites. Steel: specified by grade and cover.
Writing specifications
Use the three-part structure of general, products and execution, and prefer performance requirements where the contractor carries design responsibility. Do not specify by brand alone; cite a standard (NIS, BS, EN, ASTM) or an approved equal. Name the tests: slump for workability, cube crushing at 7 and 28 days, sieve analysis, and block strength tests.
Worked example
For a wall in 450 x 225 mm blocks with 10 mm joints, the modular face is 0.46 x 0.235 = 0.1081 m², so 1 / 0.1081 = 9.25 blocks per m². A 20 m² wall needs 185 blocks; add 5% for breakage and cutting, giving 195 blocks, ordered in whole pallets.
Traps
Specifying a mix by ratio and assuming the resulting strength; substituting an equivalent material without written approval; ignoring curing; accepting blocks that were never cured or tested.