Guide – What makes a suitable base for a Resin Bound System? – Part 2

Guide to Concrete Bases

Concrete is a mix of sand, crushed aggregate and a hydraulic binder such as cement. When mixed with water you are left with a dense semi-homogeneous mass.  10-15% is cement, 60-65% is aggregate and 15-20% is water. It’s a very versatile and simple-to-use product. It is so popular that it’s regarded as the second most used material in the world after water.

Its structural integrity is measured with several different grades of strengths.

These grades start with C8/C10 Gen 1 (being the weakest) through to C40 (strongest). C10 and C15 are generally used in residential applications and small projects such as small walls or steps. C8/C10/C15 is not considered strong enough for any structural formations and is not suitable for driveways.

For a resin bound driveway, you need to use either C20, C25 or C30. These grades have the strength for a domestic driveway. C30 can withstand 30 Newtons/28-day strength being suitable for reinforced bases and most domestic requirements.

There are stronger grades, C35 and C40 but these are generally used in areas like commercial buildings and foundations.

The strength of concrete is measured in two ways. It is naturally very strong in terms of compressive strength. Concrete is cast as a cylinder and measured for compressive strength with a psi rating (pressure per square inch). It can vary from 2500psi to 4000psi. 

Tensile strength, however, needs reinforcement incorporated to absorb the tension. This stops the concrete from breaking or cracking. Reinforcement is usually steel rods or grids called ‘rebar’. This is available in multiple sizes, for domestic projects you would usually use #3, #4, and #5. The rebar size #3 is more commonly used for driveways and patios.

Flexural strength is the measurement of concrete being able to withstand bending. It is an indirect measure of tensile strength.

Many other factors affect the strength of concrete from temperature, cement used, and mixing to the amount of water used, the list is quite extensive.

A lean mix is where you have more water and low cement content and is usually C8/C10. It is not considered suitable for driveways or patios.

It is no surprise concrete is the most widely used construction material with 20 billion tonnes used every year.

Once cast the concrete starts to cure and gets harder as time goes on. After the initial 4 weeks, it will reach around 90% of its strength but still gets stronger year on year for literally decades.

Concrete needs to dry at a rate of 1 week per inch before the application of the resin otherwise you may get a chemical reaction (whitening) due to the moisture. Additionally, a high-strength bonding primer must be used on ALL concrete bases. It is applied using a decorator’s roller and waits for 15-20 minutes for the surface to become tacky. The primer C – is a single-part polyurethane-based polymer that seals the porous base and aids adhesion stopping the concrete from absorbing the resin. A 5-litre tin will cover an area of approx. 40m2. Care should be taken when walking on a primed surface so that you do not contaminate or mark other surfaces as the resulting stain is permanent.

Careful consideration needs to be given to the dissipation of rainwater known as surface water management. The UK gets 85.6cm of rain every year. With an average of 133 days that have either rain or snow, it is imperative that you plan for suitable drainage. Failure to do this can result in severe consequences.

The resin bound system is very permeable and water drains through at a fast rate, 820 Litres per 1m2 per minute. When the water reaches the concrete, it follows the pitch of the slab. This can result in pooling/puddling or draining towards the house potentially causing damp problems. A properly designed and planned concrete base will have a pitch running away from the house into either a natural soakaway or a manmade drainage system. Systems may incorporate the use of ‘Aco” channel creating a system feeding into existing surface water drainage. These channels are U shaped with a grate to stop leaves and debris from blocking the channel. The grates can be removed for maintenance purposes.

Most driveway drainage problems can be remedied by either re-directing the water, creating a soakaway, or drilling 3cm drainage holes.

Every driveway is unique so the design will depend on the site conditions and the possibility of improving the existing drainage system.

If water is left to sit on concrete it may eventually damage the slab. Water contains bicarbonate ions and carbon dioxide causes a reaction known as carbonation within concrete.  As the concrete becomes more acidic and as the damage progresses, the acids affect the protective layer of rust on the rebar, which causes corrosion. When the rebar (steel) corrodes it expands. Expansion can also be caused by fluctuating temperatures. When the slab is first cast it will naturally shrink. Expansion usually occurs on the designed expansion gap allowing slabs to move independently. Failure to design expansion gaps and moisture/standing water can cause the concrete to crack in stress locations. This crack is often referred to as a reflective crack as resin bound systems are not very flexible and a crack in concrete often results in a reflective crack in the surface dressing.