Prediction Of Lifetime Service Performance Of Polypropylene Geotextiles

The use of polypropylene (PP) geotextiles has expanded to such an extent that nowadays they are almost certain to be include in any construction work. But geotextiles may be expected to survive a number of environmentally related challenges with regards to durability. As a result, extensive researches have been done on the ageing process of PP material to evaluate its long-term service performances. The objective of this study is to develop a long-term lifetime prediction model for PP geotextiles used in the channel project at Yangtze River.The long-term performances of PP geotextiles under three outdoor environments were evaluated. During one year of outdoor weathering tests under atmospheric conditions in Shanghai, the geotextiles were sampled and the tensile strength of the samples measured. In addition, the environmental factors, such as UV irradiation, ambient temperature and rainfall, were recorded to study the influence of these factors on the ageing of the samples. It shows that under the weathering conditions outdoors in Shanghai, UV irradiation plays an important role on the ageing of PP geotextiles. A linear relationship can be built between the reduction of tensile strength of samples and the cumulated UV irradiation. Other environmental factors, such as ambient temperature and rainfall, have no significant effects on the linear relationship. At the same period of the outdoor tests under atmospheric conditions, PP geotextiles were buried under a layer of local soil with different thicknesses to evaluate the long-term performance of the samples under the condition. The tensile strength of samples show that theenvironmental factors inside the soil such as temperature, humidity, content of oxygen, microorganisms and chemicals, etc will give no noticeable effects on the ageing of the geotextiles. The long-term performance of PP geotextiles 2-meter under water was also examined in water area of Hengsha Dock, the site of the project. The tensile strength of samples shows that water, microorganisms and chemicals under water have no notable influence on the ageing of the geotextiles. The results of the three weathering tests mentioned show that the reduction of tensile strength is mainly due to the photo-oxidation of PP geotextiles, therefore, further study should focus on the photo-oxidation process of PP materials.Both accelerated weathering tests (with fluorescent lamps) and outdoor weathering tests were then conducted to study the relationship between the macro-property (tensile strength) and micro-structure of PP filaments, which were used to weave the geotextiles. Tensile strength, intrinsic viscosity (viscosity average molecular weight), melting point and degree of crystallization of the filaments under both weathering conditions were then measured. Results show that, under both weathering conditions, the reduction of melting point and the degree of crystallization of PP filaments are not consistent with the reduction of tensile strength of the corresponding samples. However, a linear correlation can be developed between the reduction of tensile strength and the reduction of intrinsic viscosity of the filaments samples under both weathering conditions. It shows that, during photo-oxidation of PP filaments, the reduction of tensile strength can be characterized by the reduction of intrinsic viscosity of the corresponding samples. It means that a relationship can be built between the macro-property and the micro-structure of PP filaments during photo-oxidation.Influence of the intensity of UV irradiation on the photo-oxidation of PP filaments is then investigated. Three intensities of UV irradiationwere provided in accelerated weathering tests. One year of outdoor weathering tests were also conducted. The intrinsic viscosity of the filaments samples under different weathering conditions was then measured. In addition, FTIR spectrometry was used to analyze the corresponding samples. The FTIR results show clearly that the samples under accelerated weathering tests and outdoor weathering tests degrade with the same degradation mechanism of hydroperoxides, when the intensity of UV irradiation in the accelerated weathering tests is lower than 162.58 W/m2. But when the intensity of UV irradiation in the accelerated weathering test is 325.25 W/m2, the degradation mechanism of hydroperoxide will be different with that in the outdoor weathering tests. The study shows the possibility, by choosing a suitable UV irradiation intensity in accelerated tests, that the same kinetics of main oxidation route during photo-oxidation of the filament samples could be followed under both accelerated and outdoor weathering tests. It is actually the basic requirement if the weathering performance of PP filament outdoors is to be evaluated by means of accelerated tests.On the condition that the same kinetics of main oxidation route is followed, the weathering performance of PP filaments under atmospheric conditions is predicted by means of the "energy equivalent" principle. Based on the smallest residuals of variances of the testing data, a best-fit curve is obtained to show the relationship between the tensile strength of PP filaments under outdoor weathering tests and the accelerated ones with a low intensity of UV irradiation. The corresponding equation of the curve can be used to estimate the residual tensile strength of the PP filaments after certain period of outdoor service.The photo-oxidation of PP filaments under different loads is investigated finally. Results show that the greater the mechanical stress on the PP samples is, the more quickly the tensile strength of the samplereduces. Safe stress of the PP filaments in the present study is 10% of the tensile strength. It means that the influence of stress on the ageing of PP geotextiles can be ignored if the maximum load of the PP geotextiles is lower than the safe stress.It is aware that the present study focuses on the PP geotextiles used in the channel project at Yangtze River and the weathering conditions are the atmospheric conditions in Shanghai. Different PP materials under different weathering conditions may give different weathering performances and different lifetime prediction equations. However, the long-term service performance of PP geotextiles can still be evaluated by the present method if the local weather records can be obtained.