Structural Engineer Pocket Book

176 Structural Engineer's Pocket Book
Summary of material properties
Density
17 to 24 kN/m 3 depending on the density of the chosen aggregate.
Compressive strength Design strengths have a good range. F cu ˆ 7 to 60 N/mm 2 .
Tensile strength
strength.
Poor at about 8 to 15% of F cu . Reinforcement provides flexural
Modulus of elasticity This varies with the mix design strength, reinforcement content
and age. Typical short-term (28 days) values are: 24 to 32 kN/mm 2 . Long-term values are
about 30 to 50% of the short-term values.
Linear coefficient of thermal expansion
8to12 10 6 C
Shrinkage As water is lost in the chemical hydration reaction with the cement, the
concrete section will shrink. The amount of shrinkage depends on the water content,
aggregate properties and section geometry. Normally, a long-term shrinkage strain of
0.03% can be assumed, of which 90% occurs in the first year.
Creep Irreversible plastic flow will occur under sustained compressive loads. The amount
depends on the temperature, relative humidity, applied stress, loading period, strength
of concrete, allowed curing time and size of element. It can be assumed that about 40%
and 80% of the final creep occurs in one month and 30 months respectively. The final
(30 year) creep value is estimated from sf/E, where s is the applied stress, E is the modulus
of elasticity of the concrete at the age of loading and f is the creep factor which varies
between about 1.0 and 3.2 for UK concrete loaded at 28 days.