Sonic, electromagnetic and impulse radar investigation of stone masonry bridges

Abstract

Masonry arch bridge assessment is overviewed strategically. It is proposed that where a bridge fails the load-carrying capacity assessment by a small margin, and it looks to be in good condition, that further investigation be undertaken using NDT techniques. The NDT techniques of radar, sonic and conductivity methods are reviewed and the results from sonic and conductivity NDT surveys are discussed. It is concluded that these techniques are effective and that tomographic plots aid the interpretation of data. It is shown on a full scale twin-arch masonry bridge that both sonics and conductivity could identify the masonry wall thickness. However with respect to sonics, great attention must be paid to recording the data. Meticulous work is also necessary during the preparation and analysis of the sonic data, but good results can be achieved when the correct grid density is set in relation to the goal of the test. The conductivity technique has demonstrated the ability to be a rapid, low cost, non-contacting technique from which tomographic cross-sections, inhomogeneity identification, moisture movement detection over time and layering within the masonry can be achieved. It should be noted that the conductivity method has given detailed information up to a certain depth into the structure, but no data could be collected at deeper locations. Nonetheless its extreme low cost, particularly in terms of time in the post data-collection phase, makes it convenient for repetitive surveys.