Prestressed Concrete and Special Structural Systems
This domain tests the mechanics and detailing of prestressed members: how prestress is applied, how losses accumulate, how service stresses are verified, and where post-tensioned slabs, precast systems and shear walls differ from ordinary reinforced concrete.
Systems
Pretensioned members are stressed in a plant against a bed and bonded to the concrete at transfer; post-tensioned members are stressed after the concrete hardens, through ducts that may be grouted (bonded) or left unbonded. Grouting protects strands and provides bond; unbonded systems rely on anchorages and are common in slabs.
Losses
Immediate losses include elastic shortening (pretension), friction and wobble (post-tension) and anchorage set. Time-dependent losses are creep, shrinkage and relaxation of the steel. Total losses typically run 15 to 25 percent, so use effective prestress, not jacking force, in service checks.
Service checks
Compare concrete stresses to the ACI 318 limits for the applicable class: compression at transfer and at service, and tension limits for uncracked and transition members. Check cracking moment, decompression, camber and end-zone bursting reinforcement, and include secondary moments in continuous post-tensioned members.
Worked example
A strand with fpu = 1860 MPa is jacked to 0.75 fpu = 1395 MPa. With 15 percent total losses the effective stress is about 1186 MPa. For 140 mm2 of strand, effective prestress is 1186 times 140 = 166 kN per strand, which is the force used for service-load stress calculations.
Traps
Using jacking stress in service checks; forgetting elastic shortening in pretensioned members; leaving secondary moments out of continuous post-tensioned analysis; ignoring end-zone reinforcement and anchorage bearing stresses.