Basic Concepts: Drainage

The typical pavement is a roadway – a long, relatively narrow strip of pavement which drains to structures along the curbs or out towards ditches. Water is not likely to remain on the pavement surface for long periods. Even water that gets into the pavement layer through cracks will flow to the pavement edges.

Parking lots are configured differently. They usually have a large pavement surface typically shaped into large “bowls” sloped to storm water structures separated by several hundred feet. This construction has high and low points. High points will dry quicker than lower areas. Increased pavement weathering and structural failures can be expected on the lower areas because they stay wetter longer.  Additionally, water will infiltrate the pavement structure through pavement cracks or landscaping saturating and weaken the pavement structure. Vehicle loading could breakdown the pavement layer. Note on the top photograph the rougher surface near the drain at the lower left corner. The photo below shows a pile of sand next to the drainage grate.

The sand comes out of the pavement surface during the weathering process. Weathering is the natural aging at the top of the pavement layer. Sunlight and moisture combine to oxidize the asphalt cement. The oxidation causes the asphalt cement to become brittle and to break away from the pavement surface. Particles of asphalt cement and sand are carried by storm water to drainage structures. In addition to hardening and reducing the pavement thickness, the oxidation creates tensile stress in the pavement surface. To relive the tension, the pavement surface will crack. Cracking will happen more frequently within the drainage paths, so will excess storm water entering the pavement structure, more pavement failures can be expected along stormwater drainage paths and near drainage structures.  A simple example is the daily washing at a restaurant. The water flows to the nearby stormwater structure, slow damaging the pavement along the way.

Zimmer Consultants Photographs

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