Interpreting Low Tensile Bond Results on a Pier Repair
On a bridge pier repair, the specification requires ASTM C1583 tensile bond tests at one per 50 m², with a minimum individual result of 1.0 MPa; failure in the substrate is acceptable. The repair area is 300 m², and 10 tests are performed. Results are 1.2, 1.1, 1.4, 0.9, 0.4, 1.3, 0.5, 1.1, 1.0, and 1.2 MPa. The three low results (0.4, 0.5, 0.9) failed at the repair/substrate interface; the others failed in the substrate above 1.0 MPa. The contractor wants to average all results, which gives about 1.01 MPa, and accept the work. The low tests are near a construction joint and were run 3 days after placement. The specification requires testing at 7 days.
1. Evaluate the contractor's proposal to accept by averaging. What is the correct disposition?
Read the worked answer
Reject averaging. The specification sets a minimum individual result, so one result below 1.0 MPa fails that location even if the mean is above 1.0. The 0.4 and 0.5 MPa failures at the interface are clear nonconformances; 0.9 MPa also fails the individual minimum. In addition, the tests were run at 3 days when the specification requires 7 days, so the results may not be valid for acceptance. The correct disposition is to treat the tested locations as failed, investigate the cause, and retest at the specified age after corrective work. Do not use the substrate failures above 1.0 MPa to offset the low interface failures.
2. What are the likely causes of the low interface failures, and how would you investigate them?
Read the worked answer
Low interface failures near a construction joint suggest substrate preparation or moisture problems: laitance, dust, form oil, or a cold joint; substrate not saturated-surface-dry or standing water; repair mortar placed too dry; insufficient compaction; or curing started too late. The 3-day test age also matters because cementitious bond strength is still developing. Investigate by reviewing placement records and photographs, cutting cores at the failed locations, and inspecting the interface for voids, dust, or a weak layer. Check substrate moisture and profile, and run additional tests away from the joint at 7 days. If cores show a continuous weak interface, remove and replace the affected area.
3. What corrective action and documentation would you require before final acceptance?
Read the worked answer
Remove the failed patches back to sound substrate, re-prepare, and reapply using the approved method; extend the repair beyond the low-test area until sound material is reached. Retest at 7 days with the specified frequency and individual minimum. Document the nonconformance, test locations, photographs, core results, corrective method, and retest results. Update the inspection and test plan and the as-built records. Final acceptance should be based on passing individual tests plus visual soundness, not on an average. If the owner accepts any deviation, it must be through a formal engineered acceptance with a service-life rationale.