chain drag/ tap-hammer tests

Excel Building Surveying Ltd have recently been commissioned to complete chain drag and tap-hammer testing to various sites including London, Southampton, York and Bournemouth. The testing was requested in order to determine the presence of defects within various concrete slabs and balconies. The presence of defects is audibly distinguishable by dragging a chain about a surface and listening for the change in pitch. Further focussed chain dragging is used to determine the extent of defect (typically de-lamination). The defective area is then marked up on site ready for measurement and cost estimates of repair. The defective areas usually comprise a minor part of the whole and therefore help to minimise costs. Chain drag testing around a number of carpark decks in York quickly identified areas warranting further investigation. Core sampling through a defective sounding slab revealed 2no layers of concrete with very poor bond between each layer. Further defective sounding areas were found to contain delaminating concrete due to corroding reinforcement exacerbated by the use of road salts. Chain drag testing to a new-build factory slab in Southampton resulted in a number of localised defects being quickly established. Subsequent repairs formed approximately 5% of the surface meaning work could still proceed around the repairs and costs were limited by the extent of defects. Tap-hammer testing is a long-established methodology of quickly identifying defects in concrete, plaster ceilings and the like. The surface is repeatedly tapped using a light hammer and changes in pitch are readily discernible. The equipment varies but the theory is similar to that of the chain drag. Defective areas are quickly identifiable meaning further investigations can be focused resulting in lower test costs. Recent hammer testing to a number of balconies in Bournemouth suggested that damage was limited to visibly obvious defects. Further testing (half-cells, dust sample analysis and compressive strength testing of cores) showed the balconies to be in fair condition meaning the urgency of future maintenance requirements could be reduced.