Properties of hardened concrete

Hardened concrete properties decide the performance of a structure in service. The main ones:

1. Strength

• Compressive strength: the basic property, tested on 150 mm cubes at 28 days (IS 516); characteristic strength $f_{ck}$ is the value below which not more than 5% of results fall (IS 456, Cl. 2.2). Grades M20 to M40 are common.
• Tensile strength: direct tension is difficult to test, so the splitting test or the flexure test is used. Split tensile strength is about $0.5\sqrt{f_{ck}}$ to $0.6\sqrt{f_{ck}}$, and IS 456 (Cl. 6.2.2) gives the flexural strength as $f_{cr} = 0.7\sqrt{f_{ck}}$.
• Shear and bond strength (design shear stress $\tau_c$ from IS 456 Table 19; bond stress $\tau_{bd}$ from Cl. 26.2.1.1).

2. Elasticity

The stress-strain curve is non-linear. The short-term modulus is $E_c = 5000\sqrt{f_{ck}}$ N/mm$^2$ (IS 456, Cl. 6.2.3.1). Poisson's ratio is 0.15 to 0.20.

3. Volume change and time-dependent behaviour

• Shrinkage: strain about 0.0003 (Cl. 6.2.4).
• Creep: creep coefficient 1.1 to 2.2 (Cl. 6.2.5.1).
• Thermal: coefficient of expansion about $10\times10^{-6}$ per °C.

4. Durability and permeability

Resistance to water penetration, chemicals, freeze-thaw, abrasion and carbonation; it depends on W/C, cement content and compaction (IS 456 Tables 4 and 5).

5. Other properties

• Density (unit weight): 2400 kg/m$^3$ for plain concrete and 2500 kg/m$^3$ for reinforced concrete (IS 456, Cl. 19.2.1).
• Fatigue resistance, impact resistance and fire resistance.

Strength increases with age, lower W/C, good compaction and good curing.