Guide – What makes a suitable base for a Resin Bound System? – Part 1

What makes a suitable base?

The answer can be simple yet at the same time complex. Resin bound is a surface dressing. It changes the aesthetics however it gains its strength from the base. You can use open-grade tarmac. This is permeable so deals with water dissipation. Or you can use permeable concrete. Alternatively, you can use standard concrete (not permeable) or a plastic grid system like x grid or eco grid.

All these solutions offer the strength needed but each one has its pros and cons.

Guide to Tarmac & Asphalt Bases

Tarmac (tarmacadam) and Asphalt are two very similar systems and get mixed up all the time. Asphalt is very resistant to wear and is used on approx. 70% of roads. Both systems are stone mixed with a liquid binder – ‘Macadams’. Asphalt is crushed stone mixed with bitumen and is referred to as ‘bitmac’. Bitumen is produced from other petroleum products. Tarmac on the other hand is based on crushed stone mixed with tar, which is often produced from organic matter. Both bitumen and tar are black to dark brown liquids. The density of the liquid is measured as a ‘pen’ rating. This is the depth at which a standard needle will penetrate the bitumen under test conditions, basically, they measure the softness of pure bitumen by forcing a needle into a block (no aggregate). This measures the resistance and gives a grade of viscosity. Pen ratings of 40-50 are the hardest rating meaning the bitumen has the most resistance in the test. A rating of 200-300 is the softest.

For a resin bound system, it is recommended that you use a rating of approx. Pen 125. Aggregate types vary up and down the country however limestone is the favoured aggregate (less movement) as opposed to recycled concrete or granite. The smaller the crushed stone size the denser the macadam. This is classed as close graded and is generally mixed with fines of 6mm or less. The system for resin bound should be open-graded so the aggregate size should be above 10mm to a maximum of 20mm. As a rule the smaller the stone, the density of the base will become tighter and less permeable.  14mm tends to be the optimum size for what is classed as ‘open grade’ and has excellent water dissipation properties.

Guide to Permeable Concrete

Permeable concrete and resin bound share a similar composition. Permeable concrete is based upon aggregate that’s coated in mortar and bonded together. The system uses less water than traditional concrete and very little, in terms of fines. Fines are smaller particles, usually sand. It is the permeability, strength and price that make the system a popular choice for resin bound installations. The system has an open cell structure with an interconnected honeycomb texture (15-35%). It is the structure that allows water to permeate through the system. On average water will dissipate at a rate of 196 – 245 litres per minute per metre square. This ensures the system is SUDs (Sustainable drainage systems) compliant. Suds mimics nature, in the way it dissipates water into the land as opposed to entering a traditional drainage system. The runoff water infiltrates the ground and percolates into the soil. It does not leave any standing water or puddles. It is also environmentally friendly, water that passes through the system gets purified. Rainwater runoff is the leading source of pollutants entering the UK’s waterways. Pollutants vary from dirt and debris, to brake dust from vehicles (heavy metals). The biggest pollutant (90-95%) of hydrocarbons comes from the bitumen used in asphalt. The water passing through the system acts as a filtration process helping to purify the water and protecting sensitive ecosystems. Additionally, aerobic bacteria in the voids help to break down harmful pollutants and chemicals. Natural irrigation helps maintain tree roots and plant life as well as general landscaping.

The light colour of the concrete doesn’t absorb or store heat as much as macadams and will have a lower surface temperature. This reduction in temperature gives installers of a resin bound system longer trowelling times.

The system has been tried and tested since the 1800’s so it is not a new system however, it has become increasingly popular within the UK domestic market during the past 15 years.

The strength of the system is increased if you reinforce this with ‘rebar’, which is a type of textured steel.

Some contractors also add fibreglass fibres to the concrete during the casting process. Both rebar and fibreglass result in making the concrete system stronger – both compressive and tension.

The system is very durable and properly designed systems will last more than 40 Years.

Costs are a little higher when compared to standard concrete however you do not need to wait as long for the system to dry out. With traditional concrete you need to let it dry at a rate of 1 week per inch, therefore a standard concrete slab of 4 inches will mean the resin bound cannot be installed for 4 weeks leading to a longer installation process. The installation of permeable concrete for a domestic drive needs to be a little thicker than traditional concrete. The aggregate used is usually a limestone that has an average graded size of 10-20mm. 

** Expansion joints are usually required as the concrete WILL expand and contract. A ratio of width to length 2:1 needs to be considered with an ideal bay size of <40m2. It needs to be quite equal on the slab. Expansion gaps must be used on pathways laterally also at 2:1 intervals.  Any expansion joint must be replicated with a resin bound expansion trim directly along the joint otherwise you will get a reflective crack. Trims are available in 2.5Mtr lengths and the rubber gasket (in the centre) can be colour matched to the resin bound maintaining a seamless surface. Conventional concrete does not shrink as much, you still get movement. Failure to implement an expansion gap often results in uneven cracks at key stress points.

** Credit must be given to Donna LeCarpentier (Technical Area Sales) Manager at Addagrip Terraco,  who provided information on expansion gap ratios.