Concrete Properties
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Introduction
Concrete has relatively high compressive strength, but significantly lower tensile strength, and as such is usually reinforced with materials that are strong in tension (often steel). The elasticity of concrete is relatively constant at low stress levels but starts decreasing at higher stress levels as matrix cracking develops. Concrete has a very low coefficient of thermal expansion, and as it matures concrete shrinks. All concrete structures will crack to some extent, due to shrinkage and tension. Concrete which is subjected to long-duration forces is prone to creep.
Concepts and Formulas
Typical properties of normal strength Portland cement concrete:
- Density : 2240 - 2400 kg/m3 (140 - 150 lb/ft3)
- Compressive strength : 20 - 40 MPa (3000 - 6000 psi = 432 - 864 ksf)
Compressive strength of concrete (% of 28-day strength) vs age (days)
- Flexural strength : 3 - 5 MPa (400 - 700 psi = 57.6 - 100.8 ksf)
- Tensile strength : 2 - 5 MPa (300 - 700 psi = 43.2 - 100.8 ksf)
- Modulus of elasticity : 14000 - 41000 MPa (2 x 106 - 6 x 106 psi = 288000 - 864000 ksf)
The American Concrete Institute (ACI) suggests the following equation for the modulus of elasticity:
where
wc = weight of concrete (pounds per cubic foot)
f'c = compressive strength of concrete at 28 days (psi)
AASHTO Load and Resistance Factor Design Manual, or "LRFD" suggests the following equation:
where
K1 = correction factor for aggregate source (taken as 1.0 unless determined otherwise)
wc = weight of concrete (pounds per cubic foot (pcf))
f'c = compressive strength of concrete at 28 days (psi)
A handy approximate equation:
- Permeability : 1 x 10-10 cm/sec (2.8346 x 10-7 ft/day)
- Coefficient of thermal expansion : 10-5 oC-1 (5.5 x 10-6 oF-1)
- Drying shrinkage : 4 - 8 x 10-4
- Drying shrinkage of reinforced concrete : 2 - 3 x 10-4
- Poisson's ratio : 0.20 - 0.21
- Shear strength : 6 - 17 MPa (870 - 2465 psi)
- Specific heat : 0.75 kJ/kg K (0.18 Btu/lbm oF (kcal/kg oC))
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