Prestressed Concrete Explained: Pre-tensioning and Post-tensioning

Ordinary reinforced concrete is a partnership between two materials: concrete handles compression while steel reinforcement handles tension. The strategy works, but it accepts a fundamental limitation. Because steel and concrete are bonded together, both must strain together before the steel can develop meaningful tension. This means the concrete in the tension zone must crack before the reinforcement can carry significant load. Those cracks are typically harmless at service conditions, but they allow moisture and chlorides to reach the reinforcement, accelerating corrosion and limiting service life, particularly in aggressive environments. Prestressed concrete solves this by introducing a deliberate compressive force into the member before loads are applied, so that when service loads arrive they first reduce the pre-compression before any tension can develop. Done correctly, the concrete never cracks in tension at all.

Why Prestressing Works: The Basic Mechanism

Consider a simply supported beam. Under a uniformly distributed load, the bottom fibre is in tension and the top fibre is in compression. In a prestressed beam, a steel tendon positioned below the centroid is tensioned and anchored. The tendon pulls the bottom of the beam upward and puts the entire cross-section into compression, with the greatest compression at the bottom where the eccentricity is largest. When service load is applied, it adds tension at the bottom. If the pre-compression is sufficient, the net stress at the bottom fibre remains compressive or just reaches zero tension: no crack forms.

The stress distribution at any cross-section can be tracked with the equation: f = P/A ± P·e·y/I ± M·y/I, where P is the prestress force, A is the gross cross-section area, e is the eccentricity of the tendon from the centroid, I is the moment of inertia, y is the distance from the centroid to the fibre of interest, and M is the applied moment. Engineers must ensure that at transfer (when prestress is first applied) and at service (with full loads), stresses remain within ACI 318 or AASHTO LRFD allowable limits. ACI 318 Table 24.5.3 sets limits on concrete tensile stress at service; AASHTO LRFD Section 5.9 governs bridge girders.

Pre-tensioning: Fabrication Against Abutments

Pre-tensioning is performed in a precast plant before the concrete is cast. Long steel strands, typically low-relaxation 7-wire strands conforming to ASTM A416 Grade 270, are threaded through the formwork and anchored to massive fixed abutments at each end of the casting bed. The strands are stressed to approximately 75 percent of their ultimate tensile strength, typically around 1,395 MPa (202 ksi), using hydraulic jacks. Fresh concrete is then poured around the tensioned strands. After curing to a specified release strength, typically 28 MPa (4,000 psi) minimum, the strands are cut at each end of the individual members.

When the strands are cut, they attempt to shorten elastically, but because they are bonded to the surrounding concrete, they instead compress the concrete. The prestress is transferred into the concrete purely through bond between the strand and the concrete matrix. At the cut ends, there is a transfer length, typically about 50 to 60 strand diameters, over which the strand stress builds from zero to its full effective prestress. This transfer length must be accounted for in shear and flexural design near the beam ends.

Pre-tensioned members camber upward when the prestress is released, because the eccentric force causes the beam to bow. Initial camber can range from a few millimetres in short members to 75 mm or more in long bridge girders. Predicting long-term camber accurately is challenging because creep and shrinkage continue to increase camber over months and years after fabrication, and live load deflection partially counteracts it. Camber prediction errors are a common source of problems in precast bridge construction, particularly for composite deck slab systems where haunch thickness depends on assumed camber.

Post-tensioning: Stressing After Concrete Hardens

Post-tensioning stresses the tendons after the concrete member has been cast and has reached sufficient strength. Before casting, flexible plastic or steel ducts are positioned along the desired tendon profile inside the formwork. These ducts create channels within the hardened concrete through which the tendons will later be threaded. The tendon profile is typically parabolic, following the moment diagram shape, with the tendon at its lowest point at midspan where positive moment is maximum and rising toward the ends or over supports.

After the concrete reaches its specified strength, usually 28 MPa or higher for stressing, high-strength strands or bars are threaded through the ducts. One end of the tendon is typically fixed with a dead-end anchor, and the other end is engaged by a hydraulic jack seated against an anchor plate cast into the concrete. The jack extends the tendon to the target stress, and a wedge-type or nut anchor is then set to maintain the tension after the jack is removed. The tendon force is transferred to the concrete at the anchor locations, and from there distributed into the body of the member through the compression zone.

After stressing, grouted post-tensioning systems inject cementitious grout into the duct under pressure, encapsulating the tendon for corrosion protection and providing bond. Unbonded post-tensioning, commonly used in building slabs, leaves the tendon coated in grease and wrapped in a plastic sheathing. Unbonded tendons are faster to install and allow easier future repairs, but if a tendon is severed, its full length loses prestress. For critical applications such as bridges, bonded systems are generally required by AASHTO.

Prestress Losses

The initial jacking stress is never the effective prestress that acts over the life of the member. A series of loss mechanisms reduce the tendon force from the moment of stressing onward. Engineers must account for all of them to calculate the net effective prestress.

Elastic shortening occurs immediately when the prestress is transferred into the concrete. As the concrete compresses, the tendon shortens with it, losing a portion of its elongation. For pre-tensioned members, elastic shortening loss is typically 5 to 10 percent. Friction losses occur in post-tensioned members where the tendon presses against the duct wall as it bends along a curved profile. Friction loss is calculated using the Wobble and curvature coefficients from AASHTO LRFD Eq. 5.9.5.2.2a-1: the longer and more curved the tendon path, the greater the friction loss. Anchor set loss occurs when the jack is released and the wedge anchor seats, allowing the tendon to shorten by 6 to 10 mm.

Long-term losses continue over years. Concrete creep, the continued shortening of concrete under sustained compressive stress, reduces tendon elongation steadily. Concrete shrinkage causes the member to shorten independently of load. Steel relaxation, the gradual reduction in stress in a tendon held at constant elongation, reduces the force in the tendon over time. Low-relaxation strands, the current standard, experience relaxation losses of roughly 2.5 percent of the initial stress over 50 years. Total long-term losses, including creep, shrinkage, and relaxation, typically range from 15 to 25 percent of the initial jacking stress in typical bridge girders.

Kern Distance and Practical Limits on Tendon Eccentricity

The concept of the kern is central to prestressed concrete design. The kern is a region within the cross-section such that if the prestress resultant acts anywhere within the kern, no tensile stress develops in the concrete. For a rectangular section, the kern extends from d/6 above to d/6 below the centroid, where d is the section depth: the familiar "middle third" rule. For a flanged or I-shaped section, the kern limits are calculated from the section properties.

At transfer, with only the prestress and beam self-weight, the tendon eccentricity must not create unacceptable tension at the top of the beam. Under full service loads, the tendon eccentricity must be sufficient to overcome the applied moment and keep the bottom fibre in compression. This combination of requirements defines the feasible zone for the tendon centroid at each cross-section. In long bridge girders, the tendon may be positioned very close to the bottom fibre at midspan, sometimes within 75 mm of the concrete surface, to maximise eccentricity and thus prestress moment.

Partial prestress, permitted by modern codes, allows tensile stresses in the concrete at service loads up to defined limits (typically 0.5*sqrt(f'c) in MPa for Class T members per ACI 318), with some mild steel reinforcement added to control crack width. Full prestress, which maintains the entire section in compression under all service conditions, is required for bridge girders in aggressive environments and for liquid-containing structures.

Applications and Span Advantages

Prestressed concrete enables spans that would be uneconomical or impractical with ordinary reinforced concrete. Precast pre-tensioned bridge girders such as the AASHTO Type VI and bulb-T sections commonly span 36 to 55 metres. Post-tensioned spliced girders can achieve 70 metres. Segmental box girder bridges, assembled from short precast segments stressed together by post-tensioning tendons, have achieved spans exceeding 300 metres with the cantilever erection method.

In building construction, post-tensioned flat plate slabs allow column-free spans of 12 to 18 metres at slab thicknesses of only 200 to 250 mm, reducing floor-to-floor height and allowing more storeys in a given building height. Parking structures almost universally use post-tensioned construction because it eliminates most cracking and prevents chloride-laden melt water from reaching reinforcement, dramatically extending service life.