In practice, the existing test methods for welded seams of polymeric waterproofing membranes appear to be costly and of limited value. Several arguments seem to favor the application of contact-less and imaging, nondestructive test methods. However, it has to be proven first that these methods yield reliable data even under harsh conditions in tunneling. Thermography has indicated defects and in-homogeneities in waterproofing layers in other areas of applications (such as e.g., bridge constructions). The feasibility of using thermographic methods or other nondestructive test methods, eventually in combination shall be evaluated under laboratory and (if the second phase of the project will be approved after the first year) also under tunnel conditions.
The main aim of the project is to prepare a feasibility study on the use and applicability of thermography methods for quality control of mainly hand welded seams of polymeric waterproofing membranes for applications in, e.g., tunneling and other areas.
The use of thermographic methods to seals in general and specifically on geo-membranes in bridge and road construction has been investigated and delivered there in field trials evidence of defects or in-homogeneities. Preliminary tests in the laboratory to tunnel sealing systems of the AlpTransit construction project has also shown that larger adherent or non-adherent points of welds of plastic waterproofing membranes can in principle be distinguished with thermography. Before use in tunnel conditions but the process and the individual test parameters may have been optimized depending on the system and the detection limits are determined. Different combinations of materials, complex geometries and varying ground conditions and environmental factors are factors whose impact on the feasibility are still unknown.
The main output of the project is the final report.
The laboratory tests of the feasibility of using impulse or lock-In thermography for assessing the quality of hand-welded seams of polymeric waterproofing membranes for underground construction have shown that defects that had been simulated by inserting a foreign object and the width of the seams of regular welded waterproofing membranes could be determined and that adhering seams could be distinguished from seams that were loosely lying above each other or simply overlapping. The tests were performed on waterproofing membranes made from polyethylene and PVC which had different colours.
There was no conclusive relation between result of the measurement and either the colour or the material of the polymeric waterproofing membrane. Whether it is possible to detect so-called “kissing bonds”, i.e., locally disbonded, but continuously closely touching waterproofing membranes could not be clarified due to the lack of suitable test objects.
The tests further showed no conclusive evidence that differences observed in the bond quality of the welded seams that were, e.g., quantitatively determined by destructive tensionpeel or shear testing could be detected by lock-in thermography. Therefore, one important requirement for further studies on the effects under real construction conditions has not been satisfied. Within the current project, an approach for manufacturing welded seams with variable bonding quality has been developed and validated. It was further shown that welded seams of PVC waterproofing membranes that were manufactured following this procedure yielded a clearly higher scatter in the bond quality (coefficient of variation of 50%) than those of polyethylene waterproofing membranes (coefficient of variation 16%).
Adherend and non-adherend welded seams could be well distinguished by these methods. Hence, the width of the welded seams can be determined using lock-in thermography. The adhesion of the seams, however, could not be measured. Since a qualitative or semi-quantitative determination of the adhesion of the seam is essential for an assessment of the quality as well as of the expected service life time of the seams, thermography can not solve this problem to the required extent. Other test methods which could be used instead, often show certain disadvantages compared with thermography that limit their applicability. Examples are wave- or vibration-based test methods which require mechanical contact with the t