Concrete Technology and Material Properties

Reinforced concrete design starts from the materials. This lesson covers the concrete properties that GATE CE asks about, using the values of IS 456 and related Indian standards.

Cement

• Bogue compounds: C₃S (early strength), C₂S (later strength), C₃A (fast setting, heat; sulphate attack), C₄AF.
• Types: OPC (33, 43, 53 grades), PPC, PSC, rapid hardening, low-heat, sulphate-resisting.
• Tests: fineness, normal consistency (Vicat), initial setting time (≥ 30 min for OPC) and final setting time (≤ 600 min), soundness (Le Chatelier, autoclave), compressive strength of mortar cubes.

Fresh concrete

• Abrams' law: strength falls as the water–cement ratio rises. Workability rises with water, so admixtures (plasticisers) are used to get both.
• Workability tests: slump (most common), compaction factor, Vee-Bee (stiff mixes), flow table.
• Problems: segregation and bleeding.

Strength

• Characteristic strength f_ck: the strength below which not more than 5% of results fall. Target mean strength for mix design: f_ck + 1.65 s (s = standard deviation).
• Grades: M20 means f_ck = 20 MPa (150 mm cubes at 28 days).
• Flexural strength (modulus of rupture): f_cr = 0.7 √f_ck.
• Modulus of elasticity: E_c = 5000 √f_ck (MPa).
• Steel: E_s = 2 × 10⁵ MPa, so the modular ratio m = E_s/E_c (working stress method uses m = 280/(3σ_cbc)).

Long-term behaviour

• Creep: strain growing under sustained load; the creep coefficient is larger for concrete loaded at an early age.
• Shrinkage: volume loss as concrete dries, independent of load.
• Durability: IS 456 sets a minimum cement content, a maximum w/c ratio and a minimum grade for each exposure condition, and nominal cover (for example 20 mm for mild, 30 mm moderate, 45 mm severe exposure).

Worked example

M25 concrete has f_ck = 25 MPa, and site control has s = 4 MPa.

• Target mean strength = 25 + 1.65 × 4 = 31.6 MPa.
• E_c = 5000 √25 = 25,000 MPa; f_cr = 0.7 × 5 = 3.5 MPa.

Practice