Overstressing does not appear to be beneficial for stabilized wires which as a result of heat treatment, have 0.1 percent proof stress in excess of 85 percent of tensile strength since such wires suffer very little permanent deformation when overstressed.

The loss of prestress in post tensioned members in following is loss of slip due to anchorages and friction, in post tensioned beams which are bounded, before grouting action prestress in steel is affected by bending action of member.

In addition to loss of prestress in post and pre tensioning members, re may be losses of prestress due to sudden changes in temperature, especially in steam curing of pretensioned units before bonding of tendon to concrete, prestress in tendon will be influenced by bending of member.

The value of creep coefficient ?f? depends upon various factors such as humidity duration of load applied, age of loading and effective section thickness and effective section thickness is defined for uniform sections as twice cross sectional area divided by exposed perimeter, it can be assumed about 4060 and 80 percent, respectively of final creep develops during first, six and 30 months under load when concrete is exposed to conditions of constant relative humidity.

The shrinkage of concrete in prestressed members is due to gradual loss of moisture which results in changes in volume, rich mixes exhibit relatively greater shrinkage than lean mixes since contraction of cement gel increases with cement content, shrinkage also depends upon degree of hardening of concrete at commencement of drying and shrinkage of concrete in prestressed members results in a shortening of tensioned wires and hence contributes to loss of stress.

The phenomena of relaxation of stress occur as a result of creep in steel and relaxation of stress in steel changes according to variation of percentage of creep, phenomenon of creep is influenced by chemical composition, micro structures, grain size and variables in manufacturing process, which results in changes in internal crystal structure.

The losses in prestress in pretensioned member are four types: Losses due to elastic deformation of concrete, Loss due to relaxation of stress in steel, Loss due to creep of concrete, Loss due to shrinkage of concrete, in pretensioned members, due to prestress present at ends of members tendon bends upwards due to bending action and tendon becomes shortened, this is not considered as loss in prestress.

Anchorage slip is distance moved by friction wedges (in post tensioned members) after releasing jacks at ends of member and before wires get fixed perfectly in wedges, loss during anchoring which occurs with wedge type grips is normally allowed for on site by over-extending tendon in prestressing operation by amount of draw in before anchoring.

Loss of stress due to wobble effect, which depends upon local deviations in alignment of cable and wobble effect is also known as wave effect, friction coefficient values for wave effect k are 0.15 per 100m for normal conditions, 1.5 per 100m for thin walled ducts where heavy vibrations are encountered and in or adverse conditions.

Loss of stress due to anchorage slip = (E_s?/l),
For a 30m long beam, loss of stress = (210×10³×5)/(30×1000) = 35n/mm²,
Loss of stress = 35/1000×100 = 3.5%.