Progressive Investigation and Void Detection of Post-Tensioned Slab Bridge Tendons

摘要

Twin bridge structures provide a grade separated crossing for Manitoba Provincial Trunk Highway (PTH) 8 over PTH 101. The superstructure consists of a continuous 5-span cast-in-place post-tensioned concrete slab. Constructed in 1973, the structures required a detailed condition assessment, strength evaluation, and preliminary design for rehabilitation. Due to the critical importance of the post-tensioning tendons to contribute to the load carrying capacity of the structure, it was determined that a progressive investigation and void detection assessment of the tendons was necessary. Void detection testing of in-situ post-tensioning tendons was performed using the MALA Geoscience Imaging Radar Array (MIRA). Readings were obtained in the negative moment region where the tendons are nearest to the deck surface. Readings were obtained along one metre strips on both sides of all supporting structures. MIRA readings were verified by strip-removal of deck concrete and examination of tendons in four locations. The tendons were opened and examined for the presence of voids, grout was sampled for chloride content, and the strands were inspected for signs of distress. The detailed condition assessment was carried out in accordance with the Ministry of Transportation Ontario (MTO) Structure Rehabilitation Manual and consisted of visual inspection, concrete cover measurements, delamination survey, corrosion potential readings, and a coring program to determine concrete compressive strength, chloride content, air-void content, and to perform petrographic analysis. Strength evaluation (load rating) of the twin structures was carried out in accordance with the AASHTO Manual for Bridge Evaluation (2nd Edition, 2011) under the Strength I limit state. Trucks evaluated were the HS 25 lane, HS 30 lane, HSS 25 truck, HL-93 design truck, and TAC legal loads (39.5, 56.5, and 62.5 tonne configurations). The structure was designed for the HS 20-44 live load. The progressive investigation and void detection assessment of the tendons was an important part of the condition assessment and vital for the accurate strength evaluation of the twin structures.