Concrete grid pave stones allow vegetation to grow which in turn decreases stormwater runoff and provides a cooling effect to counterbalance urban heat.

While no one debates the value of recent decades' rapid urban development, it has posed new challenges for design professionals and contractors. The expanse man-made surfaces contribute to urban heat and stormwater runoff; the latter which may generate pollutants, erode streams, and create flooding conditions, among many negative environmental drawbacks. Permeable pave stones offer many features to alleviate these problems. Concrete grid pave stones offer another alternative.

"Concrete grid paves stones, like Grasstone, were first used in 1961 for overflow parking," Joey Guedea Commercial Sales Manager, Pavestone Company explained. "Originally, building blocks were placed in the ground; the open voids allowed vegetation to grow and decreased stormwater runoff while providing a cooling effect to counterbalance the heat. Since that early paving method, grid pave stones have been used in the U.S. for overflow parking as well as sloped surface erosion control. Common applications today include lakeside and slope surface stabilization, minor ditch liners, driveways, parking structures, emergency fire lanes, and median crossovers."

With an open-void, or lattice design, grid pave stones have a flat surface that forms a continuous pattern of concrete when installed. The open void allows for on-site infiltration of stormwater. Designs can use topsoil and grass or aggregates in the grid openings depending upon desired effect.

"Because of the hydrologic and pollution abatement benefits, concrete grid pave stones can be considered by state and federal agencies to be a possible best management practice (BMP) for reducing stormwater runoff," Richard Bodie, National Manager, Pavestone Company said. "Since municipalities can regulate the quantity and/or rate of stormwater released into sewers and streams with grid pave stones, they can decrease runoff and pollutants with strategic design objectives."

The Fritz Park/Delaware Creek project in Irving, Texas utilized the Grasstone product to minimize slope erosion between wall terraces adjacent to the creek, specifically in zones most impacted by high velocity water.

"A major tributary in Irving, Texas, Delaware Creek frequently channels high velocity storm drainage to the Trinity River, resulting in steep, eroded banks up to 12-16 feet high above the popular community park," Melanie Vanlandinham, ASLA Associate Principal, Newman, Jackson, Bieberstein Landscape Architecture stated. "Problems such as the loss of large pecan groves due to erosion, safety concerns and high sediment loads were solved creatively with Pavestone retaining walls. Between wall terraces, Grasstone pave stones helped stabilize areas prone to the full force of storm flows along the winding creek channel, while allowing a green approach without concrete ditch construction. Additionally, the utilization of Grasstone reduces costly plant maintenance and replacement, minimized sediment loads and improves fish habitat."

The concrete grid stone reinforces grassy areas that are subject to wheeled or vehicular traffic. Typically an area that undergoes repeat vehicular traffic will become so compacted that the permeability of the soil structure is inhibited to the point that the grass is unable to survive. Because Grasstone enables the traffic to go in areas designated as greenspace, it is a good product for vehicular paths that must remain green such as emergency routes, driveways, historical sites, university and school campuses, and golf courses.

Traditional poured-in-place concrete paving methods can kill trees by robbing them of unnecessary water and oxygen. Using concrete grid pave stones for residential driveways is becoming more popular, especially in historic neighborhoods. The owner of the Dallas, Texas area home shown in the photos here was interested in this product for a variety of reasons. The aesthetics complement the architecture of the home. Although the grid pave stone is generally specified in natural gray or pewter (no pigment added), this homeowner actually chose to have the stone colored to enhance the home's exterior brick. One of the added benefits to this homeowner was the drive-up appeal and distinction the grid pave stone driveway provided her home, allowing simplicity in directions to the residence. An additional benefit is the reduction of heat absorption compared to the standard suburban driveway.

Grasstone is manufactured from a no-slump concrete mix comprised of 1/4-inch washed limestone, coarse sand and Portland cement. Made under pressure and high-frequency vibrations, grid pave stones have an average compressive strength of 4000 psi, with no individual unit less than 3000 psi, a water absorption maximum of 7 percent, no individual stone greater than 10% and a freeze-thaw resistance equal to ASTM C-67.

Maintenance:

"Concrete grid pave stones will require the same maintenance required for lawns such as watering, mowing, weed removal and fertilization," Guedea explained. "While most commonly used for greenspace applications, if the grass cannot be maintained, crushed stone aggregate should be placed in the stone openings. This maintains the stone's permeability while providing mass in the stone weight for applications such as emergency fire lanes."

Another benefit of concrete grids is they may be removed and replaced to accommodate repairs to underground utilities.

Installation of concrete grid stones:

Grid pave stones are generally covered by vegetation, so it is normally specified without color. The project designers at this Texas park designed the grid stone to match the retaining wall in earth tones for an enhanced aesthetic.

Erosion control applications require the slope to be graded uniformly (ideally to a 3:1 or more gentle slope) before bedding the Grasstone stones in a thin layer of sand. In order to prevent the migration of fine granular material it may well be necessary to place a man-made filter on the graded slope before applying the bedding course of sand. If such is the case, the filter fabric should permit revegetation.

In order to support grass or plant growth, the surface openings must be filled flush with the surface or 1" to 2" over the surface with suitable topsoil or mixture of soil and fertilizer. Then the openings are then seeded, sprigged or plugged.

When installed as a pavement, unsuitable, unstable or unconsolidated subgrade material shall be excavated according to the direction of the Site Engineer/Architect/Landscape Architect and compacted. Place and compact a minimum of 6" or as otherwise directed by the above noted Site Authorities a compacted, dense graded aggregate. The grid pave stones are placed on a thin compacted layer of not more than 1/2 inch sand. Revegetation procedures are the same as those described for erosion control applications.

For more information on permeable and concrete grid pave stones, contact Pavestone Company at: www.pavestone.com.