Mullins Effect and Crack Growth in Natural Rubber Vulcanizates during Heat Aging and Cyclic Loading

The effect of thermal aging and cyclic loading on mechanical properties and development of cracks in natural rubber vulcanizates was studied. After aging at 70oC and 110oC vulcanizates were subjected to cyclic loading. At a certain number of loading cycles, the samples were conducted in a tension test. At the aging condition of 70oC, the static tensile properties of material stay almost unchanged even after 88 aged hours and 8000 loading cycles. On the contrary, the dynamic fatigue resistance of vulcanizates decreases with increasing aging time. These results are attributed to the post-curing and the development of microcracks that might be caused by Mullins effect: in the case of static loading, the strain-induced crystallization may prevent cracks growth, but in the case of cyclic loading the strain-induced crystallization does not occur, so cracks develop without hindrance. However, at 110oC both static properties and dynamic fatigue resistance of material reduced dramatically because at high temperature the heat degradation exceeds both post-curing and strain-induced crystallization. Crack formation and propagation were examined by a digital optical microscope in the progress of cyclic loading. Results showed that natural rubber vulcanizate filled with carbon black has the best crack growth resistance (CGR) while the addition of modified and unmodified silica reduces CGR of materials. Moreover, the vulcanizate with unmodified silica has the lowest CGR.

Experimental Performance of Polyester-Fiber-Based Soil Geogrids against Reflective Cracks

Road pavements may be subjected to deterioration problems caused by excessive traffic loads, thousands of load repetitions, environmental and climatic conditions, freeze-thaw effects, and insufficient design of mixtures. The formation of reflective cracks is the most common structural problem that occurs on a road’s superstructure over time. Special applications requiring expensive geosynthetics, such as glass-fiber-based geogrids (GG) or carbon-fiber-based geogrids, are generally used to solve this problem. Therefore, in this study, the utility of polyester-fiber-based soil geogrid (PG) is investigated as an alternative solution in relation to experimental performance analysis. Three-point bending tests are realized to custom-engineered hot mix asphalt plate specimens, with and without geogrids, under full-capacity static loading, cyclic loading, and dynamic fatigue loading conditions. Although the same bending strength levels are seen for plate specimens, including GG and PG reinforcements with a 100 kN/m tensile strength, PG-containing specimens show a 42% greater performance in the case of cyclic loading. Moreover, PG-reinforced plates have greater elastic behavior than GG plates, up to 49.4% within a dynamic fatigue test at the end of 15,000 cycles. As a result, it is shown that reflective cracks can be more effectively delayed by the use of soil PGs, rather than GGs, in relation to overall performance and service life.