Concrete Technology
Concrete technology tests the relationship between the mix, the water-cement ratio, workability and the strength you finally get. Expect questions on nominal mixes, aggregate limits, slump, cube testing and curing.
Composition and grades
Concrete is cement, fine aggregate, coarse aggregate, water and optionally admixtures. Nominal site mixes: 1:3:6 for blinding and mass concrete (roughly C15), 1:2:4 for general reinforced work (about C20/25), 1:1.5:3 for stronger work (C25/30 to C30/37). Water-cement ratio is normally 0.45–0.55; strength falls as water rises - every extra litre of water added per 50 kg bag costs strength and increases shrinkage and permeability.
Aggregates and workability
Maximum aggregate size 20 mm, not more than one quarter of the section thickness and not more than three quarters of the clear bar spacing. Workability is measured by the slump test, typically 25–75 mm for site concrete; a collapsed slump means a wet, segregated mix.
Strength and curing
Characteristic strength is measured on 150 mm cubes at 28 days; 7-day results are usually about 65–70% of the 28-day value. Moist-cure for at least 7 days - ponding, wet sacks, polythene. Hot dry weather makes curing more, not less, important.
Worked example
Cement content of 1 m³ of 1:2:4 concrete. Dry-volume factor 1.54; total parts = 1 + 2 + 4 = 7. Cement volume = 1.54 ÷ 7 = 0.220 m³. At a cement density of 1440 kg/m³: 0.220 × 1440 = 317 kg, which is 317 ÷ 50 = 6.3 bags of 50 kg per cubic metre. Sand = 0.220 × 2 = 0.44 m³ and coarse aggregate = 0.220 × 4 = 0.88 m³. At w/c 0.50 the water is 0.50 × 317 = 158 litres. Reasoning: volumes must be converted to dry volume first because the aggregates bulk and the voids between particles are filled by finer material.
Traps that cost marks
Quoting a mix as a grade (1:2:4 is a mix, C20/25 is a grade); adding water on site to make placing easier; ignoring damp sand, which bulks and over-sands the mix; judging strength by colour or by how fast it sets; testing cubes at 7 days against a 28-day target; forgetting curing altogether.