03-08-2012, 03:54 PM
Prestressed Concrete Structures
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Stress-strain Curves for Concrete
Curve under uniaxial compression
The stress versus strain behaviour of concrete under uniaxial compression is initially
linear (stress is proportional to strain) and elastic (strain is recovered at unloading). With
the generation of micro-cracks, the behaviour becomes nonlinear and inelastic. After the
specimen reaches the peak stress, the resisting stress decreases with increase in strain.
IS:1343 - 1980 recommends a parabolic characteristic stress-strain curve, proposed by
Hognestad, for concrete under uniaxial compression (Figure 3 in the Code).
Creep of Concrete
Creep of concrete is defined as the increase in deformation with time under constant
load. Due to the creep of concrete, the prestress in the tendon is reduced with time.
Hence, the study of creep is important in prestressed concrete to calculate the loss in
prestress.
The creep occurs due to two causes.
1. Rearrangement of hydrated cement paste (especially the layered products)
2. Expulsion of water from voids under load
If a concrete specimen is subjected to slow compressive loading, the stress versus
strain curve is elongated along the strain axis as compared to the curve for fast loading.
This can be explained in terms of creep. If the load is sustained at a level, the increase
in strain due to creep will lead to a shift from the fast loading curve to the slow loading
curve (Figure 1-6.5).
Shrinkage of Concrete
Shrinkage of concrete is defined as the contraction due to loss of moisture. The study of
shrinkage is also important in prestressed concrete to calculate the loss in prestress.
The shrinkage occurs due to two causes.
1. Loss of water from voids
2. Reduction of volume during carbonation
The following figure shows the variation of shrinkage strain with time. Here, t0 is the time
at commencement of drying. The shrinkage strain increases at a decreasing rate with
time. The ultimate shrinkage strain (εsh) is estimated to calculate the loss in prestress.