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40 years of expertise in permeable paving

The advent of Sustainable Urban Drainage Systems (SUDS) has seen architects, civil engineers, surveyors being presented with an increasing a variety of permeable paving alternatives as part of the clamour for greener construction products. Grass Concrete Ltd are one UK firm with a 40 year take on the subject.

Grass Concrete Limited have supplied the building and civil engineering industry for forty years, so successful is their best known product; Grasscrete that it has become a generic reference for permeable grass paving.

All manner of permeable solutions exist today and for grass pavers; Grass Concrete can offer both concrete and plastic varieties. In addition to its cast insitu Grasscrete system, pre-cast types are on offer with the Grassblock range. Plastic paving is also represented with the honeycomb cell Grassroad units that can integrate with the Grasskerb dry fix edge units. 40 years on and Grass Concrete is still evolving with new products continuing to be introduced. One such product is the Grassroof green roofing system that can be retro-fitted to existing flat roofs.

In addition to paving opportunities earth retaining walls are also provided for with the Betoconcept dry-build range. These interlocking units include fully plantable solutions capable of scaling heights up to 22 metres.

Managing Director Bob Howden has been around for much of Grass Concrete’s environmental journey and says:
“Many early Grasscrete sites are now reaching 40 years of age, giving us a proven track record with sustainable grass reinforcement products. Growing environmental awareness has provided a new stimulus not just in the UK but also in the 47 countries now served by our network of 20 international partners.”

Our “Fit for Purpose” ethic means that each project no matter how large or small is fully considered relative to use before concrete or plastic alternatives are offered. At the specification stage we’ll try to match intended use to the most suitable product. Occasionally this can lead to a recommendation away from a particular requested system or type. Working closely with specifiers, architects, contractors and environmental agencies we consider the demands placed on the application and the product to offer what we consider to be the best solution.

Plastic grass paving varieties such as the company’s Grassroad achieve a structure in partnership with grass cover that binds with it to form a tensile matrix. They can achieve a grass cover of 95% that can be attractive to customers in comparison say to concrete types that feature lower grass percentages. Care should be taken however to fully consider aspects of use so as not to disappoint.
Plastic paving is being specified on projects calling for regular and often heavy traffic with often little investigation into the ability to provide suitable performance levels. Load bearing claims are often impressive but in reality many are based solely upon laboratory bench tests that bear little relationship to conditions experienced on actual applications in use.

Under controlled specification a plastic paver such as Grassroad will offer a fully grassed solution as well as the ability to withstand infrequent load. Where over-trafficked however, the surface can often slurry under wet conditions, on drying this sets hard into a crust, destroying grass growth in the process. Where vehicle braking is a significant factor such as to highway lay-bys then plastic pavers are not perhaps not advisable; as the grass sward develops it rises above the upper level of the plastic to become the running layer, rapid braking can be problematic for both the system and the driver and re-entry onto the carriageway can also see surface slurry deposited onto running lanes.

Where more regular or concentrated use is called for then concrete systems are the most durable alternatives. Perhaps a little less grass cover, following installation but down the line a greater likelihood of maintaining a consistent grass cover and most importantly structural integrity.”

Concrete options come in both pre-cast and cast on-site types.
Grass Concrete offers a pre-cast system known as Grassblock; on the market since the early 1970’s, it has remained largely unchanged since then. Machine pressed blocks connect together using a system of lugs around which the subsequent introduction of soil and grass forms to create skin friction. As with the top sanding process used with interlocking paving, this serves to place the structure in tension, enabling it to resist compressive loads.

Pre-cast options are well accepted and perhaps less prone to becoming de-stabilised under daily use than plastic alternatives but again some care should however be taken when specifying.

Pre-cast units can suffer under heavy or regular as vibration across the surface will tend to sieve out the soil between the joints leaving blocks singularly exposed to load. Concentrations of load can then induce the blocks to rock in a pumping action that draws underlying layers through to the surface. In access roads where wheels run across a consistent point this can lead to “elephant tracking” and the onset of a progressive breakdown.

Given the structural limitations of plastic and pre-cast types many specifiers prefer a system cast on site as a fully reinforced structure. This system; Grasscrete remains a market leader; proven for accepting traffic loads up to 40 tonnes and water flow up to 8 metres per second with none of the settlement difficulties of its siblings.

Essentially a cellular reinforced concrete structure the grass plays no part in the structural equation. With this the surface will accept load equally as well with gravel fill or indeed with no fill whatsoever a feature that has been adopted for large scale vehicle import storage areas laid to locations with high water tables. Here the water rises and falls within the voided structure with no impact on performance.

With depth ranges up to 150mm Grasscrete will accept point loads up to the maximum permissible on highways. The level upper surface will also result in levels of vibration lower than would be found with pre-cast types.

Unlike plastic or pre-cast systems the surface can be trafficked almost immediately. The cruciform shape of the soil pockets are designed to enable tyres to span from concrete to concrete without intruding into the pocket. The continuous surround to the pocket and its conical section also enables gravel fill to be used without fear of it spilling out.

Grasscrete drainage qualities really make the difference. As a permeable paving layer grasscrete can drain at up to 90% the rate of the ground it replaces. With a large open area at the base of each pocket the system can attenuate surface water until re-charge into the ground below can be accommodated. Further enhancements can be made with the introduction of a drainage blanket of low fines granular material below the sub-base to enable rainwater harvesting. All key factors that have made Grasscrete a forerunner of what we know today as “Sustainable Urban Drainage (SUDS)” technology.
For spillways and embankments Grasscrete is equally at home in slope protection applications, where it can progressively vent hydro-static pressure. The enclosed ribbed profile of the soil pocket also helps to anchor roots helping it to prevent washout under heavy flow across the surface. Grasscrete has been successfully flow tested at a test limit velocity of 8 metres per second.

Grass Concrete’s success has been based around proven experience and installations in a vast range of applications from car parking, embankments, and river spillways to airports and lay-bys.

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