Determination of the Effective Bond Strength for Near-Surface Mounted Titanium-Alloy Bars in Reinforced Concrete

Abstract

The repair and strengthening of existing structures have become active research areas in civil engineering with the objective of developing efficient means to extend the service life of bridges to avoid the high cost of replacing transportation infrastructure. Strengthening reinforced concrete members using near-surface mounted (NSM) titanium-alloy bars (TiABs) has recently emerged as a feasible option to provide a simple and economical solution over other alternatives such as carbon fiber-reinforced polymers. However, experimental investigations are still needed to establish and verify design procedures and to enable structural reliability studies. The AASHTO Guide for Design and Construction of Near-Surface Mounted Titanium-alloy Bars for Strengthening Concrete Structures provides an assumed average bond strength of 1 ksi for TiABs that have hooked anchorage. The AASHTO Guide does not provide any information on any other type of anchorage or TiAB mounting method. This project investigated: (i) the effective bond strength for bonded TiAB with hooked anchorage (hooked-bonded) to achieve yielding and the assumed 1 ksi of bond strength, (ii) the effective bond strength for bonded TiAB with no hooked anchorage at the bar ends (straight-bonded) to achieve yielding, and (iii) the flexural behavior of TiAB that are unbonded along the length of the bar with hooked anchorage (hooked-unbonded). Fifteen test specimens, including one control, with varying TiAB anchorage types and bond lengths were tested. Average bond strengths of 1.09 ksi, 0.82 ksi, and 0.55 ksi yielded a hooked-bonded #4 TiAB in a NSM flexural bending application. The specimen that exhibited 1.09 ksi of average bond strength achieved the expected strength from the AASHTO Guide’s nominal moment equation adapted for the use of NSM TiAB. However, improved ductility was observed with longer bonded lengths that represent a lower average bond strength such as 0.82 ksi. The AASHTO Guide average bond strength recommendation of 1.0 ksi for hooked-bonded anchorage method appears to be a reasonable effective bond strength designation based on the testing conducted in this research, but large-scale testing is recommended to gain greater confidence for use in design documents. Average bond strengths of 0.55 ksi, 0.41 ksi, and 0.34 ksi yielded a straight-bonded #4 TiABs in a NSM flexural bending application. The specimen that exhibited the 0.55 ksi of average bond strength achieved the expected strength from the AASHTO Guide’s nominal moment equation adapted for the use of NSM TiAB for hooked-bonded anchorage. However, more ductile behavior was observed with longer bonded lengths that represent a lower average bond strength such as 0.41 ksi. 0.5 ksi for straight-bonded anchorage methods appears to be a reasonable effective bond strength designation based on the testing conducted in this research, but large-scale testing is recommended to gain greater confidence for use in design documents. Specimens mounted with hooked-unbonded TiAB achieved yielding and similar capacities as the hooked-bonded and straight-bonded strengthening methods with larger crack widths and a lower stiffness. The observed low stiffness and large crack widths supports other researchers’ recommendations to use hooked-unbonded TiAB for temporary operations.