Prestressed concrete extends the range of reinforced concrete by introducing compressive stress into the member before it carries any load. This pre-compression offsets the tensile stress that loading produces, allowing the concrete to carry loads over longer spans, with shallower sections, and with fewer cracks than ordinary reinforced concrete. Understanding how prestress works and how prestressed members are detailed is increasingly important as longer-span structures become common in commercial and infrastructure projects.
The Basic Principle
Plain concrete is strong in compression but weak in tension — its tensile strength is roughly one-tenth of its compressive strength. When a beam bends under load, the bottom fibre goes into tension and cracks. Reinforcement carries the tension across the crack, but the crack still forms. In a prestressed beam, high-strength steel tendons are tensioned before (pre-tensioning) or after (post-tensioning) the concrete is cast. The tensioned tendon pulls the ends of the beam together, putting the entire cross-section into compression. When the beam bends under load, the applied tensile stress must first overcome this pre-compression before any net tension — and cracking — can develop.
Pre-tensioning vs Post-tensioning
In pre-tensioned members, the tendons are tensioned against external abutments before concrete is cast around them. When the concrete reaches sufficient strength, the tendons are released and the prestress is transferred to the concrete by bond. Pre-tensioning is done in a factory — precast concrete beams, hollow-core slabs, and railway sleepers are all typically pre-tensioned. The process is well-controlled and produces consistent quality.
In post-tensioned members, ducts are cast into the concrete along the tendon profile. After the concrete gains strength, the tendons are threaded through the ducts, tensioned against anchorage plates at each end, and then grouted (bonded post-tensioning) or left ungrouted in a corrosion-inhibiting grease (unbonded post-tensioning). Post-tensioning is done on site and is suited to in-situ construction — flat slab buildings, bridges, and transfer beams are commonly post-tensioned.
Tendon Profiles
The profile of the tendon through the member — its shape from end to end — is designed to produce a prestress force that counteracts the bending moment from applied loads. In a simply supported beam, the maximum bending moment is at mid-span, so the tendon is draped to its lowest point at mid-span (putting the most compression where the greatest tension would otherwise develop) and rises toward the supports. In a continuous beam or flat slab, the profile reverses over interior supports to counteract the hogging moment there.
Reading Prestressed Concrete Drawings
Prestressed concrete drawings include a tendon layout plan (showing the position of tendons in plan), a tendon profile drawing (showing the vertical profile of each tendon through the member depth), anchorage details (the hardware at each end of the tendon where it bears against the concrete), and a stressing schedule (the sequence in which tendons are stressed, to what force, and what elongation is expected). The stressing schedule is critical — stressing tendons in the wrong sequence or to the wrong force can crack the concrete or damage the anchorages.
Applications
Post-tensioned flat slabs are now the default structural system for multi-storey car parks, office buildings, and residential towers in many markets — they achieve longer spans with shallower slabs than conventional reinforced concrete, reducing overall building height and material use. Prestressed precast beams and hollow-core slabs dominate the industrial building and multi-storey car park market. In infrastructure, post-tensioned concrete box girder bridges span hundreds of metres using segmental construction techniques.
Conclusion
Prestressed concrete is not a specialised niche — it is the structural system behind many of the most common building types encountered in practice. Understanding the difference between pre-tensioning and post-tensioning, how the tendon profile is designed, and what the stressing schedule documents gives anyone working with structural drawings a much more complete picture of how these structures actually work.