Determining Broken Wires in Unbonded Seven-Wire Strands Using Penetration Tests

Abstract

Unbonded seven-wire prestressing strands provide the primary reinforcement in post-tensioned concrete slabs. Strand corrosion can result in broken wires, which require expensive inspections and strand replacements. During these inspections, strands that are suspected of being corroded are exposed and inspected visually and a screwdriver penetration test is performed. Penetration of the screwdriver between any two adjacent wires is an indication that one of the wires may be broken or that the strand is unstressed or de-stressed. In this study, penetration tests were conducted on corroded strands with broken wires. The purpose of the study was twofold: to determine the distance from an inspection location where a wire break may be detected using penetration tests and to determine how corrosion products between the wires of a strand affect the accuracy of the penetration tests. During testing, corroded strands were stressed to typical service-load levels and the load and boundary conditions of an unbonded, post-tensioned concrete slab were simulated. Two outer wires were cut in succession, and penetration tests were performed along the strand. It was observed that penetration can be achieved between a broken and an unbroken wire within a region where the strain in the broken wire is less than about 200*e.