Prestressing: Pretensioning, Post-tensioning, Differences, Types, Processes, Advantages & Disadvantages
Prestressing in concrete is the process of the application of stress in concrete. The stress could be added before casting the concrete, or after the concrete casting.
When stress is added before concrete casting, it is known as pre-tensioning whereas, if stress is added after casting concrete, it is called post-tensioning.
We have basically two types of pre-stressing viz, pre-tensioning and post-tensioning. The differences between the two are as stated above.
For pretensioning, stress is applied to concrete through the use of strands of considerable tensile strength before concrete is place.
For post-tensioning, sheathings are embedded in concrete prior to casting, and strands are passed through the sheathings. Concrete is then placed, cured for 28 days and allowed to attain maximum (usually its 28 days) strength. The strands are then tensioned using power jacks or other suitable apparatus while the tensioning force is measured via a calibrated guage.
The tension stress stretches the strands and enable them to compress the concrete hence, imperfections in concrete such as cracks could be closed by tensioning.
While concrete is good in compression, it has considerable low tensile strength, hence the utilization of reinforcing bars in concrete.
Reinforced concrete too, can only stretch for limited spans. The longer we increase the span, the more the likelihood of deflections and vibrations.
With prestressing, we can execute constructions over longer spans than is attainable with PCC and RCC.
Some differences, advantages and disadvantages between the two prestressing methods include:
- Pretensioning done in factories, is more suitable for precast construction than post tensioning. Whereas, post tensioning can be done both in factories and on site.
- Post tensioning is suitable for any length of member. This prestressing method is suitable for very long span bridges, as there is no limit to the length of suitability for post tensioning.
- Because pre-tensioning does not involve the use of sheathing, cost of sheathing is eliminated hence, it is cheaper than post tensioning.
- Pre-tension is more popular and is a more preferred method because the structural members are are mostly smaller, precast and easily transferrable
- Since durability of post tensioning depends on the type of anchors used for holding the strands. Hence, pre-tensioning is more durable.
- While moulds are used to produce pre-tensioned members, they are not a necessity in post tensioned members which can be cast insitu. Cables, strands or tendons which are stretched or tensioned with jacks are normally used.
- Development of pre-tensioning is due to steel and concrete bonding, whereas post-tensioning development is anchored on bearing and its efficacy is subject to the anchors.
- Pre tensioning has high degree of strength loss (more than 17%) while loss of strength in post tensioning is not more than 15%.
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