Structural Concrete - Hassoun

2.12 Creep 25
3. Type, Amount, and Gradation of Aggregate. The smaller the size of aggregate particles, the
greater the shrinkage. The greater the aggregate content, the smaller the shrinkage [7].
4. Ambient Conditions, Moisture, and Temperature. Concrete specimens subjected to moist conditions
undergo an expansion of 200 to 300 × 10 −6 , but if they are left to dry in air, they shrink.
High temperature speeds the evaporation of water and, consequently, increases shrinkage.
5. Admixtures. Admixtures that increase the water requirement of concrete increase the shrinkage
value.
6. Size and Shape of Specimen. As shrinkage takes place in a reinforced concrete member, tension
stresses develop in the concrete, and equal compressive stresses develop in the steel.
These stresses are added to those developed by the loading action. Therefore, cracks may
develop in concrete when a high percentage of steel is used. Proper distribution of reinforcement,
by producing better distribution of tensile stresses in concrete, can reduce differential
internal stresses.
The values of final shrinkage for ordinary concrete vary between 200 and 700 × 10 −6 .For
normal-weight concrete, a value of 300 × 10 −6 may be used. The British Code [8] gives a value of
500 × 10 −6 , which represents an unrestrained shrinkage of 1.5 mm in a 3-m length of thin, plain
concrete sections. If the member is restrained, a tensile stress of about 10 N/mm 2 (1400 psi) arises.
If concrete is kept moist for a certain period after setting, shrinkage is reduced; therefore, it is
important to cure the concrete for a period of no fewer than 7 days.
Exposure of concrete to wind increases the shrinkage rate on the upwind side. Shrinkage
causes an increase in the deflection of structural members, which in turn increases with time.
Symmetrical reinforcement in the concrete section may prevent curvature and deflection due to
shrinkage.
Generally, concrete shrinks at a high rate during the initial period of hardening, but at later
stages the rate diminishes gradually. It can be said that 15 to 30% of the shrinkage value occurs in
2 weeks, 40 to 80% occurs in 1 month, and 70 to 85% occurs in 1 year.
2.11.2 Expansion Due to Rise in Temperature
Concrete expands with increasing temperature and contracts with decreasing temperature. The
coefficient of thermal expansion of concrete varies between 4 and 7 × 10 −6 per degree Fahrenheit.
An average value of 5.5 × 10 −6 per degree Fahrenheit (12 × 10 −6 per degree Celsius) can be
used for ordinary concrete. The British Standard Institution [8] suggests a value of 10 −5 per degree
Celsius. This value represents a change of length of 10 mm in a 30-m member subjected to a change
in temperature of 33 ∘ C. If the member is restrained and unreinforced, a stress of about 7 N/mm 2
(1000 psi) may develop.
In long reinforced concrete structures, expansion joints must be provided at lengths of 100 to
200 ft (30 to 60 m). The width of the expansion joint is about 1 in. (25 mm). Concrete is not a good
conductor of heat, whereas steel is a good one. The ability of concrete to carry load is not much
affected by temperature.
2.12 CREEP
Concrete is an elastoplastic material, and beginning with small stresses, plastic strains develop
in addition to elastic ones. Under sustained load, plastic deformation continues to develop over a
period that may last for years. Such deformation increases at a high rate during the first 4 months
after application of the load. This slow plastic deformation under constant stress is called creep.