Sharring experiences in urban infrastructure delivery.

Pavers In Crosswalks-Water Issues

By: Tom Davis Category: Sidewalk Features

Concrete unit pavers in crosswalks make a distinctive pathway and adds a classy touch to the intersection. By using different colors and placement  patterns  they can be a memorable place-making amenity. This post and several others that will follow will address placement configuration and issues that drive up maintenance cost.Cross-section of paver crosswalk installation in downtown Houston Texas.

Concrete unit pavers are typically provided for walking, non-traffic areas. There are thicker pavers for vehicular traffic duty. But do not let the “traffic duty” rating fool you as they are no better than the thinner version if the foundation below it is not built right and stays in a fully supportive configuration.

A typical installation in downtown Houston is shown in the cross-section in this  first image.  Note that the pavers are placed on a one inch thick bed of sand and cradled in a suppressed area of the reinforced concrete street. The face of the curb-and-gutter at each end of the walkway forms the wall wall of the cradle on the lower outside edges. Of course the walkway matches the slope of the street. Keep in mind when reading the paragraphs  below  the condition of the sand layer sloping down to the gutter as the water from rains in the sand layer must follow that path.Standing water over uneven, broken pavers.

The most destructive issue that paver crosswalks face–IMHO–is saturation of the supporting sand layer with the repetitive loading of passing traffic. The water serves as a lubricant that allows the sand to move thereby causing uneven support to the pavers. As one end of a paver  settles the gap between the pavers opposite the settling end opens and the sand between them washes out. Remember, the pavers are separated and locked in place by a thin sand layer but when the one end settles typically the adjacent paver also settles, the top edges are then forced together and spalling occurs.

The photo here  shows the worst case where  the sand can be seen washed out from the seperating layer and the one-inch support layer.

What is the solution? First there is no solution to prevent maintenance  of pavers in all locations. You can do at least two things to reduce the frequency and the degree of difficulty of the maintenance assuming the installation was designed and  installed correctly.

1.   In the crosswalks in downtown Houston we installed a drain at the gutter ends to drain the supporting sand layer. The drain is piped to the adjacent inlet if there is one. I believe that some of the randomness in the location of the problem crosswalks is indication that some of the drains are working where settlement has not been as great. In those walks that show signs of spalling and cracking the solution is to take them out, check the drain and reinstall.Note sand washing to surface and spalding of corners.

2.  The manufacturer’s warning to re-sand regularly–to replace the sand between the pavers– should be followed as that always allows the water quickly into the 1″ layer below and  then the system begins to unravel as described above.

Failures are also created by the loading and unloading of passing vehicles. That is obvious but it is not such a simple issue and I will share my observations in another post.


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