The principles of prestressing, why it is used in structural design, and the inspector's role in verifying that prestressed concrete is installed correctly.
Principles of Prestressing and the Inspector's Role
Prestressed concrete works by introducing a compressive force into the concrete element before service loads are applied. When loads are applied, they must first overcome this pre-compression before the concrete sees any net tension. Since concrete is strong in compression but weak in tension, this precompression dramatically improves the element's ability to carry bending loads without cracking. The result is that prestressed elements can span farther, carry more load, and use less material than comparable conventionally reinforced concrete.
Prestressed concrete is produced by one of two methods. Pre-tensioning stresses the strands before the concrete is placed - the strands are pulled tight between abutments in a casting bed, concrete is placed around them, and when the concrete hardens sufficiently, the strands are released. The force transfers from the strand to the concrete through bond. Post-tensioning places tendons in ducts within the concrete element; after the concrete hardens, the tendons are stressed using hydraulic jacks and anchored against the member ends. Post-tensioning is performed in the field and is where the special inspector plays the most active role.
IBC Section 1705.3 – Special inspection of concrete, including prestressed elements; ACI 318 Chapter 25 – Prestressing strand and tendon requirements; ACI 318 Chapter 26 – Construction requirements for prestressed concrete; PTI DC10.5 – Specification for unbonded single strand tendons.
Prestressed concrete inspection requires more technical preparation than conventional reinforced concrete inspection because the stressing operation involves calculated elongation values, load-elongation relationships, and acceptance tolerances that vary by tendon type and design. Before a stressing operation, the inspector must have reviewed the post-tensioning shop drawings, the approved stressing procedure, and the elongation calculations prepared by the post-tensioning contractor or engineer.
Pre-construction prestressed concrete inspection review: (1) Obtain and review approved post-tensioning shop drawings showing tendon layout, profile, and anchorage details. (2) Review the stressing procedure and elongation calculation package. (3) Confirm the tendon specification and ASTM material standard. (4) Verify jack calibration certificates are current. (5) Confirm grouting specification and grout mix design for bonded tendons. (6) Identify mandatory inspection hold points - stressing, grouting, and encapsulation verification. (7) Understand the stressing sequence and any phased stressing requirements.
Prestressing Materials and Specifications
The most common prestressing tendon is seven-wire strand - six wires helically wound around a center wire. The strand's high tensile strength, typically 270,000 psi for low-relaxation strand, allows it to be stressed to the high levels required for effective prestressing. ASTM A416 governs the properties and testing requirements for prestressing strand. The inspector must verify that delivered strand is accompanied by mill certifications conforming to ASTM A416 and that the certifications reference the specific heat and coil numbers being used on the project.
Post-tensioned tendons may be unbonded or bonded. Unbonded tendons are encased in a plastic sheath pre-coated with corrosion-inhibiting grease; after stressing, they rely on the anchor hardware to transfer force to the concrete. Bonded tendons are placed in metal or plastic ducts; after stressing, the ducts are grouted with a cement grout that bonds the strand to the surrounding concrete. Each type has different inspection requirements. Unbonded tendon inspection focuses on sheath integrity, grease coverage, and anchor encapsulation. Bonded tendon inspection adds grout injection verification.
ASTM A416 – Standard specification for low-relaxation seven-wire strand for prestressed concrete; ASTM A882 – Standard specification for epoxy-coated seven-wire prestressing strand; PTI DC10.5 – Specification for unbonded single strand tendons; ACI 318 Section 20.3 – Prestressing strand properties.
The high tensile strength that makes prestressing strand effective also makes it notch-sensitive and susceptible to hydrogen embrittlement from certain corrosive environments. Strand must never be arc-welded, subjected to stray electrical currents, or exposed to chlorides without adequate protection. These are not theoretical concerns - failures have occurred when strand was mishandled during construction.