Application of semi-rigid composite permeable pavements in road network

Use of Permeable Paving Systems (PPS) in road transport networks has been gaining more attention due to benefits such as minimising surface run-off, decreasing the risk of flash flooding and reducing pollutants deposited in waterways. Simultaneously the increasing stockpiles of waste tyre and its non-biodegradable nature leading to negative environmental impacts and associated hazards have posed increased attention to seek innovative sustainable solutions for reducing tyre stockpiles. Crumb rubber has enhanced properties that allow it to be a suitable substitution for rock aggregate in PPS. By using mixtures of tyre and rock aggregates and engineered level of binders the flexibility of PPS can be optimized based on the application to accommodate differential settlements thus reducing degradation and cracking commonly observed in conventional permeable pavement systems. This can ultimately lead to an increase in serviceability and minimising costly maintenance works and more importantly expand the range of applications permeable pavements can be used in. This study reports on the mechanical behaviour of a range of semi-rigid PPS products incorporating tyre crumbs for surface paving with high drainage capability for mitigation of storm-water run- off.

The transient stiffness-deformation behaviour of a range of permeable pavement systems with varying tyre crumb content and binder type under variable loadings was investigated. In the first step, different mixing ratios of crushed rocks (rigid aggregate) and tyre crumbs (soft aggregate) were tested in the laboratory to find the optimum mixtures fit for different applications. Constraint modulus and shear wave velocity tests were carried out to establish the mixtures which their performance changes from rigid-type to soft-type with varying pressure level. Results of this step provide an insight into the formation of force chains in the mixtures ultimately providing the most formidable mix ratio resulting in transitional soft to rigid behaviour.

In the next step different binder types (polyurethane based) and percentages were added to selected blends to investigate the performance of polymer bonded tyre-rock aggregates under static and dynamic loads. Test results suggest that the quality of binder only shows its impact for tyre contents higher than 30%. In blends with higher tyre contents the force chains mostly pass through tyre aggregates and hence the deformation of tyre aggregates puts more strain on binder film causing the lower grade binders to fail. Same low grade binders show satisfactory results when tested with mixtures up to 30% tyre content.

Preliminary test results suggest that the optimum mixture of tyre and rock aggregate with the right binder type and ratio can produce a permeable pavement system suitable for car parks and other lightly trafficked areas including pedestrian footpaths.