Technical Report: Design Elements of Porous Asphalt

According to “WAPA Porous Asphalt Pavements Design Guide” (2015), it is determined that the design of a porous asphalt mix entails selecting an appropriate aggregate grading and blending it with a target binder content. The stability of its aggregate gradation is dependent on increasing the coarse aggregate matrix's stone-to-stone contact. The coarse aggregates are held together by the asphalt mortar, which is made up of asphalt, fine aggregates, and mineral filler. As a result, asphalt binder, aggregates, and mineral filler are critical components to consider.  

The design of porous asphalt pavements (Figure 3), unlike conventional pavements. The Federal Highway Administration (2012) provides a technical brief on the porosity and permeability of porous asphalt pavements, with recommendations for testing and evaluation of these properties. The pavements are typically built over an uncompacted subgrade to maximize infiltration through the soil. Above the uncompacted subgrade is a base course also known as a 'stone reservoir'. It is typically several inches thick, consisting of uniformly graded, clean crushed stone/gravel with 40% voids serving as a structural layer and storing water for a short period as it infiltrates into the soil below. Next, is a layer of geotextile fabric which typically lies between the porous asphalt layer and the base course layer. It is a permeable fabric which acts as a filter to prevent sediment and debris from clogging the pores of the porous asphalt layer while allowing water to pass through. It is an important element in the design of porous asphalt pavement as it helps to maintain the pavement's permeability and overall performance.

The last layer, which is the top layer consists of one or more layers of porous asphalt mixes with interconnected voids that allow water to flow through the pavement into the stone reservoir. These porous asphalt layers are usually 2-4 inches thick. It consists of asphalt binder, aggregates, sands, and recycled materials. By limiting the amount of fines, the porous mixture allows for more air voids. (Typically between 16% and 22% is recommended.) 

According to Dylla and Hansen (2015), it stated that porous pavements are usually not designed to store the precipitation in the car park. Overflow should therefore be incorporated into the design to prevent stored stormwater from reaching the surface layers. In this design, it is integrated by connecting a discharge drain pipe in the base course layer. The base course layer is connected to a discharge drain pipe. The discharge drain pipe is then connected to the public drain/water collection point which will eventually be drained away respectively.

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