Rubber-Gravel Surfacing, which is known in the trade by the acronym SBRA (Styrene Butadiene Rubber Aggregate – our old friend SBR plus aggregate!) or simply ‘resin bound rubber crumb’, is a comparatively low-cost option for projects that require a sort-of ‘mid-point’ solution between loose or self-binding aggregate and the more expensive resin bound materials, providing a bound, stable and reliable surface without the cost of expensive decorative aggregate…and by including a significant proportion of recycled rubber, it’s actually a sound and sustainable choice.
Rubber-Gravel Surfacing is porous, so ideal for suds-compliant projects, and that means it requires no additional drainage provision such as point gullies or linear channels. It directs surface water straight back into the ground.
For existing paved areas, it can be installed as an overlay to other substrates, including macadam, concrete or even modular paving.
Consequently, both installation time and overall costs are kept to a very attractive minimum.
It has excellent slip-resistance properties, too, making it an ideal choice for surfaces where a little extra surety of grip is a very welcome feature.
- Slip Resistant
- No base required
- No edge restraint necessary
- Low maintenance
Rubber-Gravel Surfacing is ideal for projects with very little or no vehicular traffic and is proving to be increasingly popular for footpaths, cycleways, countryside trails, tree-pits as well as private courtyards, patios and garden areas.
The following Case Study project features a popular canalside towpath near Leeds, regularly used by walkers, joggers, cyclists and anglers, but which, over the years, had deteriorated to an uncomfortable mish-mash of muddy sections interspersed with bare trackway and short stretches that had been crudely stoned-up. The belief was that by providing a more suitable and accessible surfacing, more users would be encouraged to take advantage of the pathway, explore the open countryside of an old colliery site, and, in the process, avail themselves of invaluable exercise and fresh air.
DCM Surfaces were selected by the Sustrans charity to carry out the work due to their unrivalled experience with precisely this type of specialist surfacing and to do so on large commercial projects which present awkward logistical challenges – it’s not easy to get tonnes and tonnes of surfacing, skilled workers and essential equipment along narrow canal towpaths.
They also had a short window in which the work could be carried out – over the winter months when the number of users was at its lowest and therefore disruption would be minimised. Of course, having fewer people wanting to use the path on which you’re working is all well and good, but at the expense of working through the cold, wet, short days of winter brings its own problems. However, DCM Surfaces rose to the challenge with aplomb.
The sub-base has been prepared in advance by a groundworks contractor. 150mm of a limestone DTp1, levelled out to roughly 3m width and compacted with a heavy double drum roller.
The path width varies along the length of the project but aims for a 3m width wherever possible. When passing beneath bridges the width is necessarily reduced, sometimes significantly, but the intended usage, as a pedestrian footpath and cycleway, requires a reasonable breadth as far as practical.
Other than clearing any litter or shifting the odd bit of mud that have found their way onto the bare sub-base in between its installation and the arrival of the surfacing team, here’s no special preparation required for the surface of the sub-base, no need for geo-synthetics, geo-grids or any other membrane. The surfacing material is laid directly over the compacted stone.
What is required is a basic setting-out procedure to establish a consistent path width given there are no edgings or other edge restraint to define the width. This is simply a matter of marking out the required 3m width on the prepared sub-base, a task undertaken by the gangerman, setting-out each section just prior to laying the surfacing.
The installation team:
The surfacing team comprises five operatives, including the gangerman. He’s actually the main laying operative, spreading and levelling the surfacing material as it arrives at the leading face. He’s supported by a roller operative, responsible for smoothing and closing the freshly laid material. There’s also a support operative performing a number of functions, from barrowing in the freshly-mixed material, to float-finishing the free edges.
At the other end, a pair of operatives are responsible for mixing the material. One loads the bagged aggregate into the mixer, while the second adds the resin binder and empties the mixed material into the lined barrows ready for wheeling up to the laying face.
Note that the operative looking after the resin binder wears special protective gloves. The binder can cause skin irritation if direct contact occurs and therefore all operatives handling the binder or mixed material ensure their arms are fully covered (no short sleeved shirts), as are the legs (full length extra-thick working trousers) and a double layer of gloves: standard work gloves beneath a secondary pair of Nitrile Solvent protective gloves.
There are two basic materials: the blend of crumb rubber and gravel which we’ll refer to as “The Aggregate”, and the resin used to stick it all together, which is "The Binder”. There is an optional third constituent, an Accelerator, which can be used to speed-up the curing process if necessary, but it is not essential and it is used in very small quantities, typically a few millilitres per mix.
The Aggregate comes pre-bagged, 25kg at a time, so the consistency of the blend and the quantity of aggregate added to each mix is uniform. There are 20 bags per pallet and these are positioned at a mixing station to the rear of the area to be surfaced.
The Resin Binder is supplied in 205 litre drums, which are used one-at-a-time, with a drum laid on its side and a measured quantity emptied into a bucket via a tap fitting affixed to each drum as it is opened.
The Mixing Process:
Mixing is a controlled process. The aggregate operative lifts and empties three of the 25kg bags into the forced action mixer while it is stopped. The binder operative measures out a fixed quantity of the resin which is added to the mixer drum after the aggregate but before the drum starts rotating.
The speed of curing for the binder can be controlled, to some degree at least, by adding a measured does of accelerator to the binder resin. On some projects, a slower, longer cure may be desirable and therefore no accelerator will be required, but on this project, in the open on a popular footpath alongside the canal, where it is difficult, if not impossible, to keep the public out of the working area, a reasonably quick cure is preferred and so a small syringe is used to measure out the required volume. It’s surprising how little accelerator is actually required – only 20ml or so to 5 litres of binder – a ratio of 250:1!
In colder weather, the binder can thicken and prove difficult to pour from the 205 litre drum in which it is supplied. Careful application of heat by means of a hand-held burner for just a few minutes will render the binder less viscous, easier to accurately measure and pour, and, most importantly, easier to mix properly with the aggregate.
Once the aggregate and the binder have been added to the mixer drum, the lid is closed, the drum starts to rotate and the fixed paddles stir through, thoroughly blending the ingredients and ensuring all of the aggregate is completely coated by the binder. This process takes 30 to 60 seconds – much faster than is the case when stone aggregate is used in a resin bound system. It is the experienced eye and discretion of the operatives to judge when the mixing is complete, whereupon the lid of the mixer is lifted, raising the paddles from out of the mixed material, and the drum can be tipped to empty the prepared material into a waiting wheelbarrow.
Note that the wheelbarrows are lined with polythene film salvaged from the wrapping provided with the bagged aggregate. This prevents the mixed material accumulating within the tray of the barrow, reducing volume and making it increasingly difficult to effectively tip-out the material at the laying face.
The polythene film does get covered with sticky material, but it is easily ripped out from the barrow’s tray when necessary and replaced with new. A typical polythene liner might serve for 1-2 hours before needing replacement.
The mixed aggregate is wheeled to the leading face and, directed by the laying operative, tipped out into smaller, easier-to-handle piles.
The required laying thickness, following compaction, is 35mm, and so the laying operative uses a gauged profile timber that is roughly 40mm deep, positioned at the free edge, and another 40mm timber running transversely about 750mm or so from the established surface. This enables the operative to spread the surfacing material roughly and then use one of the timbers as a short screeder board to level-out the material so that it is even and consistent in level with the previously laid material.
This particular laying operative prefers to carry out the initial spread using double-gloved hands, as previously described. He finds this gives him better control over the surfacing material and allows him to push or pull it to where its required. Others will use a float dipped in thinners or a separate short screeder/spreader. There is no defined method: whatever suits the laying operative, as long as it is safe and gives satisfactory results.
Once the surfacing has been spread and levelled to the satisfaction of the laying operative, the steel roller is brought into play. This has been washed with thinners to prevent it sticking to the binder-coated aggregate and possibly plucking it from the surface, creating an uneven finish. This hand-pushed roller weighs around 7.5kg and only light pressure is applied. It is more of a smoothing and tightening-the-surface operation rather than one of compaction, although, obviously, so light compaction does take place and the depth of the spread material is compressed from 40mm or so to the target 35mm as specified in the design.
Repeated washing and re-coating of the steel roller with suitable thinners is critical to ensuring a satisfactory finish, but there should never be so much thinners that the binder is over-diluted or washed off the aggregate.
Tidying and Finishing:
Similar for the third operative at the laying face: the float guy. This operative is responsible for tidying the free edges using a short steel float dipped in thinners to prevent sticking and plucking, and for smoothing out any minor unevenness left by the steel roller. The longitudinal edges are trowelled to a rounded profile that softens any trip hazard and eliminates a square arris that could be vulnerable to crumbling.
Work proceeds continuously as far as practical, so that the number of transverse seams or day joints is kept to a minimum. When laying has to stop temporarily, for meal breaks or at the end of a shift, the material is laid against a temporary timber restraint to create a straight line, transverse joint that is full depth and not feathered.
Working this way, the five-operative team expects to lay approximately 6 tonnes (240 bags @ 25kg) of mixed material per day, sufficient to cover around 200 square metres or so.
The surfacing takes around an hour to harden but it is cordoned off for at least the first 24 hours to ensure a full cure can take place without some clueless member of the public trudging across it or ploughing it with bike tyres (there’s always one!).
The finished pathway is an attractive and hard-wearing surface, that gives a slight but comfortable cushioned feel when walked across, and a wonderfully smooth ride for cyclists.
Tel: 01772 440340