In-depth defect inspection of precast double walls using non-destructive techniques

Abstract

Precast double walls are frequently used for building applications and consist of a pair of precast reinforced concrete shells connected by a lattice girder creating an inner cavity. Once placed at its final position this cavity is filled with freshly cast and compacted concrete. Aside from benefits such as increased ease in installation, reduced cost and shorter construction time also some pitfalls are recognized. Due to the restrained space and the presence of the truss girder undesired defects (e.g. voids, honeycombing, etc.) might occur without being noticed, as the outer wall elements also act as formwork. This actually might reduce the load capacity and jeopardize the structural integrity of the concrete structure. Therefore, different non-destructive tests are being used in order to assess the filling degree of the cavity in the double walls and to localize defects, with varying success ratio (i.e. the detectability). Different techniques were applied on a test wall with intentionally embedded defects (polystyrene blocks) with varying diameter: rebound hammer, ultrasonic pulse velocity via direct transmission, ultrasonic shear-wave tomography, ground penetrating radar and active infrared thermography. The success and the limits of these techniques for in depth defect localization are being discussed in this paper. The effect of the selected detection method and the surface-on-depth ratio of the defect itself has an effect on its detectability.