Using ground penetrating radars to detect internal defects in concrete foundation slabs

Introduction. The co-authors have analyzed a monolithic reinforced concrete piled raft foundation of a multi-storied residential building under construction. The mission of this research effort is to obtain trustworthy information about the internal structure of a foundation slab, to detect and survey internal defects, if any. This research project is to capture potential problems that may accompany the inspection of a foundation, to generate awareness about ground penetrating radar surveys and their methods that can help to optimize operating processes in the process of inspection. Materials and methods. The co-authors have employed a method of ground penetrating radar surveying, performed using a regular mesh of orthogonal projections over an easily accessible surface area of a raft foundation and several antennas producing sounding signals with centre frequencies varying within the range of 1,500 and 2,000 MHz. Results. A number of internal defects has been detected in the structure of a foundation slab, including several horizontal cold joints, cavity pockets and honeycombs. Their presence was later confirmed by control drilling and core material sampling. The information thus obtained was later generalized and entered into surface maps of cold joints, that depicted both the relief and the layout of detected defects in space. The analysis of core material chips has proven that reflecting boundaries are the same as those of the core material destruction; it has also demonstrated the presence of air pockets and the proofs of poor quality concrete mix compaction. Conclusions. The resolution of the ground penetrating radar method is sufficient to identify features of reinforced concrete slabs significant for their structure; it enables researchers to obtain trustworthy information about the internal structure of a foundation slab and make conclusions about the presence or absence of internal defects inside it, including cold joints, cavity pockets or honeycombs.