Most fluid-applied polyurethane membrane systems are composed of two layers such as a main polyurethane layer and a top coating layer. It was sometimes found out in actual buildings that a top coating layer is ruptured by movement of cracks in substrate. This kind of defect hasn't been reported till now. The purpose of the paper is to make clear the reason why this kind defect occurred and to propose effective ways to improve durability of it. Three works such as a dynamic outdoor exposure test of polyurethane membrane systems, measurement of elongation of top coating layers of them and a fatigue test of dumbbell specimens of them were carried out in the study. Firstly test specimens of various thick polyurethane membrane systems applied on substrate were exposed outdoors for five years. The artificial cracks prepared at the center of the specimens were repeatedly moved once a week during test and top coating layers of the specimens were observed. After a few months later, fine cracks appeared in the coating layers of some specimens and then propagated into the polyurethane layers underneath them.
To understand why cracking started in a top coating layer, it is needed the information on how a top coating layer is extended by joint movement of a substrate. Then, the specimens on which target scales for measuring elongation previously printed on the surface top coating layers were extended to joint width of 2.0mm in incremental steps, and measured elongation rate of the surface coating layers. The highest elongation was observed in the top coating layer just on the artificial crack of the substrate, and it increased according as thickness of a beneath polyurethane layer decreased.
Secondly, repeated extension tests of dumbbell specimens of membrane systems were carried out at various elongation rates to know the fatigue properties of them. Any rupture didn't appeared in the specimens subjected to the elongation rate of 10％. However top coating layers ruptured in the specimens over 25% in elongation, which is almost equivalent to the elongation rate measured in the membrane system of 1mm thick at crack width of 0.5mm in the previous tests.
Based on the above tests results, the three ways to avoid cracking in a top coating layer of a fluid-applied polyurethane membrane system were proposed, such as making a membrane system with sufficient thickness of polyurethane layer, improving the ability of fatigue resistance of a top coating layer, and laying an air-permeable sheet underneath of a polyurethane layer.
Finally, the conclusion of the study was summarized follows,
(1) It was found out through dynamic outdoor exposure tests that cracking first appeared in a top coating layer and propagated into underneath polyurethane layer.
(2) This is because that fatigue resistance of a top coating layer is lower than that of a polyurethane layer.
(3) Three ways to improve fatigue resistance of a polyurethane membrane system were proposed.
