Post-tensioning buys longer spans and thinner floors at the price of specialist work and permanent restrictions on drilling. Here is the trade in plain terms.
A conventional reinforced concrete slab resists load through steel bars that act once the concrete cracks slightly. A post-tensioned slab contains high strength strands that are stressed after the concrete hardens, putting the slab into compression and allowing a thinner section over a longer span. For an owner the decision touches floor to floor height, column spacing, programme and, permanently, what you may and may not drill into afterwards.
A conventional reinforced slab at a glance
Reinforcing bars are fixed and the concrete is cast around them. Capacity comes from the steel working in tension once the concrete cracks under load. The slab is thicker for a given span, uses more concrete and reinforcement per square metre, and needs no specialist stressing operation. Every competent contractor can build one, and any competent engineer can later assess or modify it.
A post-tensioned slab at a glance
Ducts or sheathed strands are laid to a designed profile, concrete is cast, and once it reaches a specified strength the strands are stressed with a jack and anchored. The compression delays cracking, reduces deflection and allows a thinner slab across longer spans with fewer columns. It is a specialist operation with its own testing, records and permanent safety implications.
Slab thickness and floor height
A conventional reinforced slab: Thicker for a given span, which raises the overall building height or reduces the clear height in each storey.
A post-tensioned slab: Thinner for the same span, which can save height across many floors or free up space for services.
Column grid
A conventional reinforced slab: Favours closer columns, which constrains car park layouts and open plan spaces.
A post-tensioned slab: Permits wider spacing, producing fewer columns and more usable or lettable arrangements.
Material quantities
A conventional reinforced slab: More concrete and conventional reinforcement per square metre.
A post-tensioned slab: Less concrete and less ordinary reinforcement, offset by the cost of strands, anchorages and the stressing operation.
Skills and supervision needed
A conventional reinforced slab: Standard trades, with inspection focused on bar position, cover and cover concrete quality.
A post-tensioned slab: A specialist subcontractor, calibrated jacks, elongation records and strict control of the stressing sequence.
Programme
A conventional reinforced slab: Simple cycles, but thicker slabs take longer to reach striking strength and use more formwork support.
A post-tensioned slab: Faster floor cycles are possible because stressing allows earlier striking, if the specialist is available when needed.
Future alterations
A conventional reinforced slab: Openings and fixings can be formed later with structural advice and conventional scanning.
A post-tensioned slab: Strands are under permanent tension, and cutting one is dangerous and structurally serious, so drilling and coring need scanning and engineering approval every time.
Records you must keep
A conventional reinforced slab: Drawings, bar bending schedules, inspection records and cube results.
A post-tensioned slab: All of that plus tendon layout drawings, stressing records with elongation measurements, grouting records and anchorage details.
When a conventional reinforced slab wins
- Modest spans, typical villa and small building column grids.
- A contractor pool without post-tensioning experience and no specialist available.
- Buildings where owners will later cut openings, fix heavy equipment or alter layouts.
- Projects where simplicity of future maintenance outweighs material efficiency.
When a post-tensioned slab wins
- Long spans and column free areas, such as car parks, showrooms and open plan floors.
- Buildings where reducing structural depth per floor allows an extra storey within a height limit.
- Projects where saving concrete and reinforcement across many repeated floors is material.
- Floors where deflection control matters for long spans supporting brittle finishes.
How to decide
Choose post-tensioning when the geometry demands it and the repetition pays for it: long spans, a need for column free space, a height restriction that a thinner floor helps you live within, or enough repeated floors that the material saving outweighs the specialist cost. Stay conventional for a villa or a small building with normal spans, for a project where no experienced post-tensioning subcontractor is realistically available, and for any building whose owner expects to cut, core and modify the slabs over its life. If the answer is genuinely balanced, ask the structural engineer to price both options at concept stage, because the decision becomes hard to reverse once the layout has been drawn around it.
For a post-tensioned option, the evidence an owner should insist on is the complete stressing record set: tendon layout drawings marked as built, the jack calibration certificates, measured elongations compared against the calculated values, the sequence in which tendons were stressed, and the grouting records for bonded systems. Elongation is the practical proof that the force in the strand is what the design assumed. Keep the tendon drawings somewhere the facilities team will find them, because the first person to drill a hole in that slab needs them, and that person may be working for you years from now.
Domira builds and supervises projects across Saudi Arabia, and this is the level of detail its site teams work at. Send the drawings or the site, and Domira will tell you what the specification has to say before anyone prices it.
Frequently Asked Questions
Can you drill into a post-tensioned slab?
Only after scanning to locate the tendons and with engineering approval for the position and depth. Cutting a stressed tendon releases a large force, is dangerous to the operative and removes capacity from the slab. Many buildings restrict drilling into post-tensioned floors to defined zones. Keep the tendon layout drawings available to anyone who will ever fix something to the ceiling.
Is a post-tensioned slab cheaper than a conventional slab?
It uses less concrete and less ordinary reinforcement, but it adds strands, anchorages, a specialist subcontractor and the stressing operation. Whether it is cheaper depends on span, repetition and local availability of specialists. On long spans with many repeated floors it frequently wins, while on short spans in a small building it rarely does. Price both from the same structural engineer.
What is tendon elongation and why is it recorded?
When a strand is stressed it stretches by a predictable amount for the applied force. Measuring the actual elongation and comparing it with the calculated value confirms that the force in the tendon is correct and that there is no blockage, friction problem or slipped anchorage. Elongation outside the permitted tolerance triggers investigation before the slab is accepted.
Do post-tensioned slabs crack?
They are designed to crack less, because the compression introduced by the tendons delays the onset of cracking and reduces deflection. They can still crack from shrinkage, restraint from stiff walls or columns, thermal movement or overload. Cracking in a post-tensioned floor should always be reported to a structural engineer rather than filled, because its causes differ from conventional slab cracking.
Have a Property Question?
Domira can help you navigate property, financing and investment decisions in Saudi Arabia. Speak with our team for an initial consultation.
