28th March 2019
When deciding what base to choose for your resin bound gravel installation, it is first important to establish whether there is a suitable existing base that can be utilised. Resin bound can be overlaid onto existing tarmac or concrete surfaces of suitable construction for the expected traffic. They need to be crack free and in good condition. If there is any possibility for movement in the concrete or tarmac, the resin overlay surface can fail.
Generally when installing an overlay, the current base will not be permeable. In most circumstances if a new base is to be installed, contractors can ensure it is permeable and SuDS compliant. Longer guarantees can often be offered when contractors install the full base build up themselves. This is because you know the base build up has been installed in line with best practice for resin bound and there are no nasty surprises at a later stage.
When it comes to resin bound gravel, there is often some confusion with the terminology ‘base’ and ‘sub-base’.
The ‘sub-base’ refers to what is underneath the ‘base’ course.
The ‘base’ is the binder course and refers to what the resin is immediately laid on.
Some people can also refer to the whole base build up structure underneath the resin bound gravel as a ‘base’ so it is important to be clear which part is being referred to when discussing bases.
The sub-base has two functions. One is to provide a working platform for the base and the other to increase the intrinsic strength of the construction of the resin bound system.
The best sub base to use for resin bound gravel is a well compacted bed of non frost susceptible MOT type 3. The primary reason it is the best for resin bound systems, is because it is SuDS compliant. MOT type 3 has a lot less fines in comparison to other sub base materials and so as water permeates through the system, it is less likely to move and cause sinkage.
The thickness of the sub base should be calculated subject to survey and dependant on the condition of the soil below. The general guidelines, however, are that it should be laid at between 100-250mm.
The base provides the binding structure for the resin bound gravel. It creates a smooth surface to lay the resin bound gravel on.
There are different types of tarmac available on the market and there are two different ways to grade tarmac: The hardness and the size of the stone. A ‘pen’ rating is what calculates the hardness of the bitumen. The lower the number of the ‘pen’ rating, the harder the bitumen is and the more brittle it is. If it is too brittle, it will crack. The higher the number of the ‘pen’, the softer the bitumen is. The size of the stone is measured in ‘mm’ and is represented after the ‘AC’, (asphalt content).
When installing tarmac as a base for resin drives, it is all about getting the right balance so that it can withstand hot and cold temperatures without cracking.
You should generally be laying 70mm worth of tarmac as a base coat for when installing resin bound.
AC14 open textured 14mm tarmac is the best tarmac base for resin driveways. This is because it makes the system SuDS compliant by allowing rainfall to permeate slowly through to a soak away. 14mm is the preferred size of stone in asphalt for resin bound surfacing because it is more tightly knitted and there are more contact points, creating a stronger bond.
For the standard resin driveway, the pen rating of the tarmac should be 100-150. This should not soften until the temperature gets to 40°c. If you are based in the south of England in places such as Essex or Kent, a different type of tarmac will need to be installed as a base. This is because warmer temperatures will soften the tarmac. A harder graded tarmac should therefore be used. AC14 open textured should be laid but with a 40-60 pen rating. This will not soften until temperatures reach 50°c.
A limestone based tarmac is the best tarmac to use for resin driveways because it has less thermal expansion. This means that as it starts to get warm and the tarmac begins to expand, the resin is less likely to crack. Limestone based tarmac is more readily available in central England. The north and the south of England have different granites that have a greater rate of expansion so it is always best to check with your asphalt supplier.
As a rule of thumb in average weather conditions, tarmac needs to be left to set for a minimum of 2 weeks in order to let the oils deplete. We do however always recommend that you speak to the tarmac plant before laying your base for resin bound.
Concrete is graded in newtons. The newton graded number of the concrete is placed after the ‘C’ when looking at specifications, for example, ‘C20’. The higher the newton grading, the stronger the concrete.
Concrete as it is, is not permeable and so does not allow water to flow through it as open textured tarmac does. To adhere to SuDS compliance and in order for the resin bound system to be permeable, special permeable concrete must be installed. Some contractors drill holes in the concrete or place vertical piping through the concrete to the sub base to allow for water flow. Whilst this certainly helps with drainage, it is not SuDS complaint. An aco drain can be laid when installing concrete on a slope to collect the water draining through the resin bound.
You should generally be laying 100mm worth of concrete as a base coat when installing resin bound gravel.
The best concrete to use is C20 concrete.
Concrete drys out at different rates depending on the humidity. The general rule of thumb is that you should leave it for at least 4 weeks. This will help it develop to its maximum strength as well. Concrete should be smooth brushed before cure. A primer must be laid in order create better adhesion of the resin bound aggregates.
EcoGrid is a grid made of recycled plastic materials and it comes in different specifications. Doing pretty much as it says on the tin, Ecogrid creates a locked, grid like structure in order to bear heavy loads. It is laid on top of a geotextile membrane which is laid on top of the bed of type 3 sub base. It needs to then be filled with clean stone prior to laying the resin bound aggregates. Regarding the stone it needs to be filled with, the smaller the better.
EcoGrids are 330mmx330mm and arrive on site in grids of 12, so the amount you will lay will depend on the size of your resin bound installation.
The most common one for carrying the load required for resin bound installations is the E50.
The base you choose for your resin bound installation will depend on the specification required for the project. If clients are not too fussed about their surface being SuDS compliant, concrete can act as a suitable base. You will find for the majority of installations however, in order to achieve a SuDS compliant surface, the best base to install resin bound surfacing on to is AC14 open textured tarmac at 70mm. This is because open textured tarmac offers a strong, permeable and SuDS compliant base that is cost effective, with a relatively quick curing time.
The size of the installation will also impact the choice of base. For example, Ecogrid maybe more economical for smaller installations where you don’t have the economies of scale of installing tarmac but ultimately whilst it is more permeable, it is not as strong as tarmac or concrete. Surfaces that will be heavily trafficked should have all of the layers designed according to DoT requirements.
It is important to note that the advice given is based on best practice for resin bound surfacing and that the specifier of project can design different base layers dependant on the ground conditions and the expected use of the surface.
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