Concrete is very strong in compression, however relatively weak when put in tension (ie. pulled apart), hence the use of steel reinforcement in concrete, to take the tension forces. At Holcim we can provide options to form a very efficient form of construction is where the reinforcement within the concrete is "tensioned" after the concrete is cast, thereby putting the concrete in compression.
The reinforcement lies within cables, which are grouted once the steel is tensioned. Adopting post tensioned concrete means that a designer can achieve greater span widths and thinner slabs for the same design load.
Post-tensioned concrete is typically specified having an early age strength requirement. Generally, a compressive strength of 22 MPA is required before stressing can be completed. This strength requirement is specified to match the builder's construction cycle time (normally at 3 or 4 days) - the faster the post-tensioning can occur, the faster the construction cycle.
Given the early age strength requirement, post-tensioned concrete is classified as a Special Class concrete by AS1379. There is currently debate on how best to assess post-tensioned concrete in terms of early age strength measurement and also how best to optimise mix designs for constructional applications.