Many of the factors which help achieve long term durability of a pattern imprinted concrete driveway will also help to minimise the chances of it cracking.
Ensure the concrete is properly specified – The concrete should have a minimum of 320 k/m³ of cement and more in cold weather. It should have fibre reinforcement for durability and strength and an air entraining agent to help prevent the surface from flaking.
Concrete should be correct thickness – In simple terms, regardless of the formulation, a thicker slab of concrete is less likely to crack than a thinner one. For most domestic driveways the concrete should be at least 100 mm, with up to 200 mm if vans are used on the driveway, whilst 75 mm should be adequate for pedestrian-only areas such as patios.
Use a slip membrane if required – A slip membrane is a polythene sheet placed between the sub-base and the concrete allowing the concrete to move or ‘slip’ independently of any movement of the sub-base due to ground conditions. Because the concrete can move ever so slightly, it will reduce the build-up of stress in the slab which means that it is less likely to crack.
Some contractors oppose the use of slip membranes on the basis that they prevent water from seeping from the concrete into the sub-base and the resulting excess water can spoil the print. An experienced contractor will make a value judgement as regards the need for a slip membrane for each individual job.
Position contraction joints correctly – Because concrete has a tendency to crack as it shrinks, cuts should be made partially through the concrete where it is most likely to crack anyway, otherwise the concrete may crack randomly across its surface. These cuts are referred to as crack control joints, movement joints or contraction joints; they are certainly not expansion joints as they often mistakenly called.
Crack control joints are partial cuts through the concrete slab and become the weakest point so that any movement in the slab will cause the remainder to crack underneath the cut where it cannot be seen. Contractors may not be able to prevent concrete from cracking but in this way they can attempt to control where it does crack.
Crack control joints should be cut in positions of high stress such as at the external corners of buildings; to reduce any slab size within the whole paved area to 20 m² and to ensure that any individual slab has a length to width ratio of no more than 2:1, which means that a one metre wide footpath should have a contraction joint every two metres.
Don’t cut the contraction joints too late – Concrete can crack as it shrinks and it can shrink at a faster rate as it is drying out and hardening. It makes sense therefore to cut contraction joints as soon as possible to avoid the risk of the concrete cracking before it can be cut.