Ultrasonic tomography of grouted duct post-tensioned reinforced concrete bridge beams

Some concern exists over the safety and durability of the 600 post-tensione d bridges in the UK, and the much larger number worldwide. The objective of the work reported herein was to identify voiding in the metallic tendon du cts in these bridges. Voiding can give rise to two sets of problems: (a) po ssible ingress of chlorides, which would cause corrosion; and (b) a lack of redistribution of stress within the beam. It was against this background t hat it was important to first of all identify the extent of voiding in post -tensioned bridges. The new technique of ultrasonic tomography was used for the trials reported in this paper. Two test beams were examined: a 10 m long beam at the Trans port Research Laboratory (TRL), Crowthorne, UK and a short test beam constr ucted at Stanger Science and Environment, Elstree, UK. The ducts in the TRL beam were 40 mm in diameter. This is smaller than would normally be encoun tered in a post-tensioned bridge beam. A more usual duct diameter would be 100-110 mm with a cover of around 125 mm. The second test beam at Stanger S cience and Environment, Elstree contained 100-mm diameter ducts. The time-of-flight tomography data obtained demonstrated that it is a poten tially highly successful method of investigating post-tensioned concrete be ams. The method is somewhat time consuming and so should be used in conjunc tion with a simpler testing method, e.g. sonic impact-echo, which identifie s areas of interest. The smaller the ducts to be investigated, the smaller the required distances between testing stations. This therefore significant ly increases the testing time. (C) 2001 Published by Elsevier Science Ltd.