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Concrete Grid Pavements

By Alan Starling, Pavestone Company

If a green look is desired, the open cells are filled with topsoil and either hydromulched, sprigged or plugged. If possible, overfill the cells and plant or sod directly over the pavers. The area must receive adequate sunlight, water and nutrients. Furthermore, the grid pavers cannot receive continual, regular vehicular traffic or plantings will not survive due to wear on the grass and excessive compaction of the soil.

Using an economical and versatile concrete grid paving system is well suited to a wide range of applications, including fire lanes, emergency vehicle access routes, overflow parking, slope surface stabilization and as permeable pavement.

Individual grid paver units incorporate a lattice design of flat concrete combined with large openings, or voids, giving the paver surface a honey comb appearance. The open voids occupy fifty percent of the total surface area of the units, allowing for on-site infiltration of stormwater and re-vegetation. These open areas can be filled with topsoil and planted with grass or groundcover for a green effect or backfilled with a variety of aggregates, dependent upon the desired intent.

The BMPs Add Up

The large percent of open area is what makes grid paving so attractive to designers, owners and municipalities. The systems are economical and simple to install. The cells are easy to plant and maintain, letting owners keep large expanses of property green yet still allowing for emergency and maintenance access.

Many corporate business parks incorporate grid pavers into their campus design for a more natural "green" look. By virtue of their open cell structure, grid pavers allow for water and air to freely reach tree roots, making it a good paving choice when trying to conserve valuable trees. These same attributes also make it an ideal pavement to cover gas pipelines where non permeable, traditional pavements will not work. By keeping traditional pavements on a site to a minimum, they help reduce the heat island effect.

The systems are permeable, allowing for recharge of groundwater and preventing excessive stormwater run-off. Permeable pavements can qualify as a best management practice (BMP) by reducing stormwater runoff and can eliminate or reduce in size detention ponds. They can also help reduce non-point source pollutants. Problem slopes can be kept green and erosion free.

On steeper slopes stake or pin every third row of blocks to keep the blocks in place, the blocks at the toe of the slope need to be embedded or "stabbed" into the base of the slope. Grid pavers in slope stabilization programs needs to be re-vegetated. Very often groundcovers such as Asian jasmine, ivies or low growing honeysuckles can be used in difficult to maintain or non-mow areas.

Installation Notes

Design and construction using dense graded aggregates is similar to that of other flexible pavements, including interlocking concrete pavements.

Subgrade soils should be compacted to a minimum of 98 percent of standard Proctor, and the required amount of base material should also be compacted to a minimum of 98 percent standard Proctor. Construction of emergency fire lanes, parking lots and driveways should have minimum base dimensions of eight to ten inches and thicker base sections may be required if the soil subgrade is weak or if extremely heavy axle loads are anticipated. Typically grid pavers are placed on a one inch layer of bedding sand. When compacting the pavers with a vibratory plate compactor, care should be taken to avoid cracking or breaking the individual units. A pad can be placed under the compactor or plywood can be placed over the pavers prior to compaction.

The finished appearance of the grid pavers is dependent upon the requirements of its particular application.

If a green look is desired, the open cells are filled with topsoil and either hydromulched, sprigged or plugged. If possible, overfill the cells and plant or sod directly over the pavers. The area will have the same requirements as any other green space; it must receive adequate sunlight, water and nutrients to survive. Furthermore, the grid pavers cannot receive continual, regular vehicular traffic or plantings will not survive due to wear on the grass and excessive compaction of the soil.

In these cases, the best treatment is to fill the cell areas with crushed aggregate. If the paving surface is receiving daily traffic and the main requirement is for a permeable pavement, the use of a permeable interlocking concrete paver like Uni-EcoStone may be a better choice than a grid paver.

The Base Material

If the grid paving is to function as a permeable pavement, the base material should consist of a minimum of eight inches of open graded base, preferably conforming to ASTM C 33 No. 57 gradation (100 percent passing a 1-inch sieve, 0 to 5 percent passing a No. 8 sieve). This base material is installed in four to eight inch lifts and compacted with a static roller. A maximum of three inches of moist ASTM C 33 No. 8 crushed stone is placed, leveled and roller compacted on top of the 57 stone. The units are then placed directly on the base and can be vegetated as out-lined above or backfilled with aggregate.

If the subgrade soils are unsuitable for infiltration a series of drainage laterals or collector pipes need to connect to either an underground water storage system or to a storm water drain. Many factors contribute to correct permeable pavement design, utilizing an engineer is essential.

Preferably install grid pavers on 3:1 or gentler slopes. Stones are placed over a compacted subgrade. On steeper slopes they should be staked to prevent the stones from moving until vegetative growth takes root. Embed toe of paving into base of slope. Backfill all open areas with top soil conducive to plant growth and hydromulch, seed or sprig.

Using Paver Layers

Mechanical installation of grid pavers is feasible when relatively large, flat areas of paving are required. Long fire lanes and open expanses of parking can be economically and quickly installed, resulting in labor savings of fifty percent or more over traditional hand installation. Hand installation of these type projects average 4000 to 5000 square feet per day. One paver laying machine can average approximately 8000 square feet per day and multiple machines can be utilized on the project.

Grid Pavers can be utilized in surface slope stabilizations and to prevent erosion from stormwater run-off. Ideally, the pavers are placed on 3:1 or gentler slopes. If the slopes are unstable at a saturated condition then the soils need to be either reinforced or replaced with more stable soils.

Typical reinforcement is similar to that used with segmental retaining wall block systems. Layers of geosynthetic reinforcement extend back into the soil between layers of compacted soil. The grid pavers act as a revetment or surface to prevent erosion. Geotextile fabrics should underlie the blocks to prevent migration of soil fines.

Grid pavers can also be used in areas of intermittent, minimal water flow, however the products are not flume tested and their use in areas of measurable water flows should be avoided. Articulated Concrete Block systems (ACB's) are the proper choice when designing in open water channel and dam over topping conditions.

Meeting LEED(TM) Green Building Criteria

The LEED(TM) Green Building Rating System as developed by the U.S. Green Building Council, promotes environmentally friendly construction. Grasstone grid pavers qualify for several LEED(TM) credits, including Credit 6.2, Stormwater Management as a pervious paving surface, Credit 7.1, Landscape and Exterior

Design to Reduce Heat Islands as an open grid pavement system. Other LEED(TM) criteria that grid pavers can meet are credits for local manufacture and recycled content, dependent upon mix design. As more local, federal and state agencies are requiring some degree of LEED(TM) compliance, it is important that owners and the design community become aware of the environmental and economic advantages of grid pavements.

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October 17, 2019, 9:20 am PDT

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