Written by Nelson Birch
Introduction to Resin Bound Surfacing
Resin bound aggregate is a permeable, durable monolithic surface course that is suitable for both pedestrian and vehicular areas.
The system has been around in one form or another for well over 30 years.
The earlier products mainly originating from the USA relied on epoxy resins as the stone binder.
Although epoxy resin is extremely strong, it’s also not flexible.
This led to cracking of the resin screed when subjected to any thermal or ground movement, and with the epoxy binders lack of UV resistance, and the suggested screed depth recommended being 12mm, it gained a poor reputation for premature failures among the early adopters.
Later two-part Polyurethane Resins became the preferred choice of stone binder.
It’s combination of relatively high strength and most importantly flexibility, coupled with the option of Aliphatic UV resistant versions, transformed the product from a niche offering to a durable and aesthetically pleasing permeable surface.
Now the typical Resin bound aggregate system offered by most manufacturers today follows the same basic ingredients as its predecessors but has been refined and strengthened to enable it to stand the various rigours of the modern world’s requirements.
It is usually comprised of a selection of finely graded, kiln dried natural aggregate, the common size blend being 25% of a 1-3mm aggregate and 75% of a 2-5mm aggregate.
These aggregates are usually held in place by an Aliphatic (UV stable) 2k polyurethane binder.
There are Aromatic (non-UV) Resin binders available, however these will darken under sunlight and as such non UV is usually limited to Darker aggregate blends or commercial projects, where aesthetics are less of a priority.
A quartz sand is the final ingredient to the mix, which improves the strength and anti slip properties of the surface, whilst still maintaining permeability.
The most popular method of mixing is by a forced action or Pan mixer.
The pre-weighed bags of aggregate are emptied into the Pan mixer, and allowed to mix thoroughly.
Usually whilst this is happening, another member of staff will be mixing the resin binder.
The Resin is mixed by pouring the contents of the part B tub (Hardener) into the part A tub (Base or Polyol)
They are then mixed until homogeneous by a paddle mixer or a drill with whisk attachment.
Once mixed, the contents are then poured into the loaded Pan mixer to mix with the aggregate.
Once a consistent mix is achieved, the Quartz sand is added and left to mix further, again until the sand has been distributed consistently throughout the mix.
The batch is now ready to be laid.
The most common methods of levelling Resin bound are usually by Rake/ Spazzle or Screed box (Sledge)
The material requires a Trowelled finish to ensure proper compaction and a flat and uniform finished surface.
Hand trowelling is still the most common method, but with the new specialist Resin powertrowels now available, machine trowelling is getting increasingly popular.
Once the mix has been levelled and trowelled to the desired finish, an anti-slip powder is broadcast on surface which provides more grip for the end users. These powders are usually crushed glass, Aluminium Oxide or fine grade sand.
Each have slightly different properties, and can change the aesthetic of the final appearance.
Resin bound aggregate is usually laid on a fixed base, the most popular being no fines Concrete and open graded Asphalt.
Other options are a plastic grid system filled with clean aggregate and the most recent innovation, a resin base.
The resin base is in essence a beefed-up version of the resin bound surface, but uses larger aggregate and glass fibre mesh to create a strong and stable base.
Under this base, a permeable sub base should be installed.
The most common sub base known as Type 1, is not the ideal material for a permeable system.
The high percentage of fines retain a large amount of moisture, and can suffer from settlement and swelling in periods of wet and dry weather.
A Type 3, or Type 1X sub base should always be the preferred option, allowing surface water an unhindered route to percolate into the substrate.
These bases should always be laid upon a permeable geotextile membrane, which will stop contamination of soil into the sub base, and provide a stable footing for all of the subsequent build up layers.
