March-April 2007

Testing a Permeable Paving System in a Truck-Washing Area

A fire station in Oceanside, CA, was selected as the site for installation and testing of a new permeable paving system, Soil Retention Products Inc.’s Drivable Grass. A permanent deflection and endurance study was performed on the system.

The fire station’s adjacent truck washing area has been recently paved with the Drivable Grass system. This location offered the ability to control and measure varying loads, drive passes, and the moisture percolation capability of the plantable concrete paving mat. During the course of the study, the mat was actually subjected to a higher load frequency and larger loads than would be expected over the “typical use” service life of the mat.

This study used both qualitative and quantitative measures of the paving system’s performance. Manometer readings and photographic documentation were used to evaluate how the concrete mat performed with the repeated loading of various fire trucks. Test results showed negligible permanent deflection and deformation of the product.

Drivable Grass is a permeable, flexible, and plantable concrete pavement system that is manufactured from poly-reinforced concrete. The pavement mat gains tensile strength by incorporating polymer grids, spaced at 2 inches on center, that connect the concrete muffins (raised concrete cubes in the mat that supports most of the load). The grid maintains its tensile strength from the polymer reinforcing while also allowing large amounts of flexure facilitated by the linear grooves and holes between the muffins. These cracks and holes also allow for root penetration and moisture drainage. Typical areas of installation include emergency and service vehicle access driveways, overflow and recreational vehicle parking, patios, golf cart paths, residential driveways, roundabouts, vehicle wash areas, V-ditches, and other non-driving areas such as bioswales.

This has been the first study to research and collect quantitative data on the long-term performance of the product as it is related to permanent deflection. The study took place at an active fire station where the pavement system was subjected to loads from fire trucks.

Table 1 shows the properties of the mat.

Fire Station No. 5 on North River Road in Oceanside is located adjacent to the San Luis Rey River and walking trail. In the spring of 2005, the San Diego Regional Water Quality Control Board made the Oceanside Fire Department aware that the practice of washing fire trucks on the station’s asphalt pavement was resulting in runoff draining directly into the riverbed. To temporarily mitigate the problem and contain the runoff, the fire station installed a gravel bed with a polyurethane geomembrane liner. The method proved to be ineffective, and other alternatives were considered by the fire department. Soil Retention’s Drivable Grass was chosen based on its ability to handle large vehicle loads and limit runoff by allowing water to infiltrate the pavement surface.

Construction of the fire truck wash area was completed at the end of September 2005. Drivable Grass concrete paving mat was installed underlain by a structural section consisting of 8 inches of Class II aggregate base course placed over a compacted subgrade. The subgrade was scarified to a depth of 6 inches, moisture conditioned, and compacted to at least 90% relative compaction. A separator fabric of MirafiHP-570 was placed between the subgrade and the aggregate base. The aggregate base was moisture conditioned and compacted to 95% relative compaction (Figure 1).

A 1.0- to 1.5-inch topper consisting of sand or sand and organics was overlaid on the aggregate base. The topper with organics was placed to facilitate grass growth through the mat. The grooves and holes in the concrete mat allow roots to penetrate through the mat and reach the topper soil with organics. The topper consisting of sand only (i.e., with no organics) was used as a leveling course where grass was not needed.

The native subgrade consisted of two different soil types:

• 1. Dark olive clayey sand with gravel (SC) with a maximum dry density of 131.0 pounds per cubic foot (pcf), optimum moisture content of 9.5%, and an R-value of 77.

• 2. Dark brown clayey sand (SC) with a maximum dry density of 122.0 pcf, optimum moisture content of 12.0%, and an R-value of 71.

The unit weights and water contents were different for the two soils due to gravel content, but their R-values were fairly similar.

Pavement section design was based on the lowest R-value of 71. The design was completed using the California Department of Transportation (Caltrans) design method assuming a Traffic Index of 5.5. Additionally, extra base material was required by the City of Oceanside. In the design method, the Drivable Grass was disregarded in computing the structural section.

Three different types of infills were used on top of the concrete mat: sandy soil with grass seed, sod, and pea gravel with varying amounts of cement binder. These different infills allowed for direct comparison both quantitatively and qualitatively between each section. It should be noted that the topper with organics was only used below infill areas 1 and 2 (Figure 2).

Following the placement of the sod and seed, the grass areas of the Drivable Grass installation (i.e., Infill Areas 1 and 2) were cordoned off and access was not allowed, because the grass needed time to become established.

Originally, fire trucks were allowed to be driven and washed over the mat installation with pea gravel only. In January 2006, after the grass sections became established, they were opened to normal use, and conclusions were drawn from the acquired data. Figure 13 shows an example of the mat’s response to a load being applied by a fire truck. Table 2 shows the specifications for each fire truck that drove over the area, and Table 3 shows the number of fire truck passes by date. A visual inspection of the pavement surface was completed at each site visit. At no time was any cracking of the concrete muffins, or any signs of permanent deflection, noticed.

To measure any permanent deflection of the concrete mats, manometer surveys were performed. A manometer can measure very small differences in elevation by recording relative water level changes. Measurements were taken on every other muffin cube on marked lines and charted by row number. The first reading was then set as a baseline and relative movement measurement was taken from that baseline. Figure 3 shows the layout of the mats and the location of each of the measurement lines.

The first baseline reading was taken on September 28, 2005, before any driving use was allowed. Measurements were also taken on November 9, 2005; November 30, 2005; December 15, 2005; and February 10, 2006.

Rows 1 and 2, as shown in Figure 3, are the grass-planted sections of the concrete-mat-paved area and therefore were not subjected to as many passes as the other mat-paved areas. The error of the manometer survey is ±0.2 inch. Most of the movement detected falls into the error range (0.2 inch) with potential permanent deflections of less than 0.2 inch occurring where the wheels passed directly over the mat. Drivable Grass with the grass infill areas performed very well considering the organics present underneath the mat.

For Rows 3 and 4, as shown in Figure 3, no discernable pattern of movement could be detected, and all movement was within the error of the survey. As shown in Figure 3, these two rows run perpendicular to the driving direction of the fire trucks. Row 5 runs parallel to the driving direction, and a pattern of deflection can be seen with a maximum deflection of 0.4 inch ±0.2 inch.

This study has been able to quantitatively and qualitatively describe the performance of Drivable Grass with repeat high vehicle loading. During the study, Drivable Grass was subjected to a higher load frequency and/or higher loads than what would be expected over the service life of the product for typical uses. Drivable Grass performed very well in regard to both deflection and structural performance. No cracks in the concrete muffins were found, even in the areas where very minor permanent deflections occurred. Comparison of the different infills has shown that there is no connection found between the type of infill used and permanent deflection or performance.