Prestressed Concrete Explained Simply

Concrete is strong in compression but weak in tension, and that single fact shapes almost everything about how we use it. Prestressed concrete is an elegant answer to that weakness: instead of waiting for the structure to be loaded and then cracking in tension, we deliberately squeeze the concrete in advance so that the everyday loads have to overcome that built-in compression before any tension appears.

The result is a material that can span further, carry more and stay thinner than ordinary reinforced concrete. This article explains how the prestressing force is applied, the difference between pre-tensioning and post-tensioning, where each is used, and the considerations that matter when prestressed concrete is specified, particularly on the kind of slabs and transfer structures common in Singapore.

Why prestressing exists

In a normal reinforced concrete beam, the underside goes into tension when the beam bends under load. Concrete cracks at relatively low tension, so steel reinforcement is placed there to carry it once the concrete has cracked. This works well, but it accepts cracking as part of normal behaviour, and the section has to be fairly deep to keep deflections and crack widths in check. Our overview of reinforced concrete sets out that conventional approach.

Prestressing changes the starting point. By introducing a permanent compressive force, usually low down where tension would otherwise develop, the engineer pre-loads the section so that applied loads first cancel out that compression before they can put the concrete into tension. Cracking is delayed or avoided, the section behaves more stiffly, and far longer spans become practical at a shallower depth.

How the prestress is applied

The compressive force comes from high-strength steel tendons, strands or bars that are stretched and then anchored against the concrete. When the stretched steel tries to return to its original length, it drags the concrete into compression. There are two principal ways to do this, and the difference is mainly about timing.

Pre-tensioning

In pre-tensioning the strands are stretched first, before the concrete is cast. They are tensioned between strong anchorage points in a casting bed, then the concrete is poured around them and allowed to harden. Once it has gained enough strength, the strands are released. As they shorten, the bond between steel and concrete transfers the force, putting the member into compression.

Pre-tensioning is a factory process, so it suits precast elements made repeatedly under controlled conditions, such as hollow-core floor planks, beams and bridge girders. These arrive on site ready to install, which speeds up construction.

Post-tensioning

In post-tensioning the concrete is cast first, with hollow ducts left in place along the path the tendons will follow. After the concrete has hardened, the tendons are threaded through, stretched with hydraulic jacks against the hardened concrete, and locked off at anchorages at the ends. The ducts may then be filled with grout to protect the steel and bond it to the structure, or the tendons may be left unbonded inside a protective sheath.

Post-tensioning is carried out on site and is well suited to in-situ floor slabs and transfer beams. It is widely used for prestressed concrete flat slabs in commercial and residential buildings, where it allows thin slabs to span long distances between columns with few or no downstand beams.

The idea of tendon profile

One of the most powerful features of post-tensioning is that the tendon can be draped, following a curved profile that dips low at midspan and rises near the supports. Because the tendon is curved, its tension produces an upward force along the span that directly opposes the downward load of the floor.

This is sometimes described as load balancing: the prestress is tuned so that it counteracts a chosen portion of the permanent load, leaving the concrete carrying much less net bending. The slab is effectively held up partly by the tendons rather than by its own bending stiffness alone, which is why prestressed members can be so slender.

Where prestressed concrete is used

Prestressing earns its keep wherever spans are long, depth is precious or cracking must be controlled tightly. Common applications include:

Long-span floor slabs in offices, car parks and apartments, where post-tensioning reduces slab depth and the number of columns.
Transfer structures that carry columns from above across a clear space below, such as over a lobby or car park.
Bridges and viaducts, where precast pre-tensioned girders or post-tensioned segments span between piers.
Water-retaining structures, where keeping the concrete in compression helps prevent the cracks that would otherwise leak.
Precast floor planks and beams, delivered ready to install for faster construction.

Reducing slab depth has a knock-on benefit across a tall building: shaving height from each floor can save significant overall height or allow an extra storey within the same envelope. That whole-building thinking is part of what a structural engineer does when weighing a structural system.

Considerations and trade-offs

Prestressing is powerful but it demands precision and discipline. Several factors need careful attention.

Anchorage zones. The whole prestress force funnels into the concrete at the anchorages, creating intense local stresses that need dedicated reinforcement.
Prestress losses. The initial force does not stay constant; it reduces over time as concrete shrinks and creeps and as the steel relaxes, and the design must allow for this.
Drilling and coring. A post-tensioned slab contains live, highly stressed tendons. Cutting or coring into one without locating the tendons first can be dangerous and damaging, so the tendon layout must be known before any penetration.
Corrosion protection. Because the strands are highly stressed, protecting them from corrosion through grouting or sheathing is essential to long-term durability.
Specialist work. Stressing operations require trained operatives and calibrated equipment, and the sequence of casting and stressing has to be planned carefully.

These points are why alterations to prestressed structures are so sensitive. Forming a new opening in a post-tensioned slab is not a routine cut; it needs the tendons traced and the structure re-checked by a Professional Engineer, which ties directly into how A&A works in Singapore are regulated.

Prestressed concrete and durability

Done well, prestressing improves long-term performance because keeping the concrete in compression suppresses the cracking that lets water and chlorides reach the steel. Done poorly, the consequences concentrate at the highly stressed tendons and anchorages, so quality of grouting, anchorage detailing and construction control all matter more than in ordinary reinforced concrete.

If you are assessing an existing prestressed structure, perhaps before a renovation or a change of use, a structural engineer’s report is the right starting point to establish its condition and how it was built before any work is contemplated.

Frequently asked questions

What is the difference between prestressed and reinforced concrete?

Reinforced concrete relies on steel to carry tension after the concrete cracks, whereas prestressed concrete applies a permanent compressive force in advance so the concrete largely avoids cracking under everyday loads.

What is the difference between pre-tensioning and post-tensioning?

Pre-tensioning stretches the strands before the concrete is cast and is typically a factory precast process, while post-tensioning stretches tendons after the concrete has hardened and is usually carried out in situ on site.

Can I drill into a post-tensioned slab?

Not without first locating the tendons, because they are under very high tension and cutting one can be dangerous and structurally damaging, so any penetration should be planned with the tendon layout known and checked by an engineer.

Why are prestressed slabs thinner than normal slabs?

The prestress counteracts much of the bending and deflection caused by the floor load, so the slab does not have to rely on depth alone for stiffness and strength, allowing longer spans at a shallower thickness.

Is prestressed concrete common in Singapore?

Yes, post-tensioned flat slabs and transfer structures are widely used in commercial and residential buildings here because they save depth and reduce the number of columns and beams.

Specifying or assessing a prestressed structure and want it reviewed by an engineer? Talk to our consultancy or get in touch.