Imprinted Concrete Installation – Colouring and Printing the Concrete
Colouring, antiquing and printing of pattern imprinted concrete is not an exact science and much relies on the skills of the installation crew to achieve a relatively even colour and texture over the new concrete surface.
There can be minor imperfections and differences in colour consistency and pattern depth which should be accepted as part of the overall appearance of the driveway or patio area. Many of these can be minimised by adopting certain precautions in the colouring and printing procedures but a completely natural stone or slate driveway would also show differences in its appearance across the surface.
CSH hardens the surface of the concrete – the technical bit!
The first coloured pigment to be applied is Colour Surface Hardener (CSH) which is a mixture of a coloured powder to stain the surface and cement to harden it – hence ‘colour surface hardener’.
Dealing with the hardening qualities first, we have already made the point that increasing the cement content of concrete will make it stronger. The hardness of a substance can be measured in Newton metres, Nm or Newtons per square metre (1 Nm = 1 Pascal). A Newton (named after Sir Isaac Newton) is the net amount of force required to accelerate a mass of one kilogram at the rate of one metre per second, per second.
Regardless of the complexity of the description, Nm is a unit in the measurement of hardness and in laboratory conditions colour surface hardener has been shown to increase the hardness of concrete from 35 Nm to 68 Nm. Colour surface hardener can make the surface of pattern imprinted concrete almost twice as hard as ordinary concrete.
Applying Colour Surface Hardener
Colour surface hardener is supplied in drums and the chosen coloured powder is scattered across the surface of the floated concrete by hand until the entire surface is covered.
There should be no standing water on the concrete surface when the CSH is applied and the powder should be allowed to hydrate on the wet concrete before being trowelled into the surface.
Colour surface hardener powder is not especially expensive in terms of the overall cost of the imprinted concrete installation but some installers will still try to cut corners by not applying enough to achieve a well coloured and hardened surface. Although it is possible to colour the concrete surface with as little as 1½ kilograms of CSH per square metre, a much better result will be achieved if 2½ kilograms per square metre, or even more, is applied.
After trowelling the CSH into the concrete surface, more of the powder should be scattered and trowelled in so that the colour spread is uniform rather than uneven and patchy. In certain conditions a third application may be required and the ‘rule of thumb’ is that an installer carries on applying CSH almost until the concrete won’t take any more, that is, all the surface water is soaked up by the CSH.
In high use areas of your imprinted concrete installation such as a driveway entrance, in front of garage doors and near steps, a greater density of colour surface hardener will help with future wear. Around the edges of the new driveway or patio area and around manhole covers, a Harris trowel is used.
When your installer is happy that the concrete has acquired the correct colour consistency, a large Blue Steel trowel (up to a metre long) is used to smooth and close the surface, removing any trowel marks.
Applying Antique Release Agent
Before the concrete surface can be printed, Antique Release Agent (ARA) powder in a complementary colour to the colour surface hardener is applied to the soft, wet concrete.
The ARA powder is normally scattered over the coloured concrete surface with a soft, dry brush but it is not trowelled in.
ARA has two parts to play in an imprinted concrete installation, its primary one being to prevent the printing mats from sticking to the still moist concrete, as a result of suction. The ability to be able to lift the printing mats cleanly from the concrete surface will help to ensure a cleaner, sharper print for the driveway or patio.
The second job of the coloured antique release agent powder is, together with the colour surface hardener, to create a two-tone effect to the paved surface. This gives it a more realistic and natural appearance than just the base colour surface hardener would provide.
Colour Co-ordinate the Colour Surface Hardener and the Antique Release Agent
When finalising the design, printed pattern and colour of your imprinted concrete driveway or patio area it is important to choose the colours of both the CSH and the ARA so that they complement each other.
In most cases the CSH would be a lighter shade than the ARA so that when the imprinted concrete installation has been completed the darker antique release agent pigment will stain the concrete in the joints and creases of the cobblestone surface to create a more realistic and natural effect. A good example of this would be platinum grey colour surface hardener with highlights of charcoal antique release agent.
When ARA is initially applied, it sometimes looks as though too much has been used and the concrete surface is much darker than first chosen. A day or two after the imprinted concrete installation however, your driveway will be washed off prior to a sealant applied. The concrete surface takes on the blend of colours it was intended to be, with highlighted, antiqued textures on the base CSH colour. There is a major caveat here though, as colours from brochures and photographs can only be taken as an approximation to the end result.
Printing the Concrete
Moulded printing mats are used to create your chosen stone, slate, tile, timber or brick pattern into the semi-firm concrete before it dries.
Printing mats are available in both hard or flexible versions. Hard printing mats are used for the majority of the surface and flexible ones where just a section of the mat is required.
For each pattern, mats are usually available in sets of around six hard printing mats and one flexible one and installers should have at least enough mats to span the narrowest part of the driveway. Flexible printing mats allow the installer to access awkward areas where a rigid printing mat would not fit.
If a soldier course border is to be imprinted around the perimeter of the driveway or patio, this is normally done first, with the main pattern infilling the internal space.
To create the pattern, mats are laid onto the surface of the concrete, each one interlocking with the next and the installer steps onto the mats to press them down firmly. This gives rise to the terms pressed concrete or stamped concrete, both of which tend to be used more in North America than the familiar, pattern imprinted concrete in the United Kingdom.
Order of Printing the Concrete
For a driveway, the pattern would be generally printed from the house towards the pavement unless it is on a slope, in which case the print direction would be up the hill. On very warm days however, it may be prudent to print the areas in direct sunlight before the shaded areas just in case the concrete hardens too quickly under the bright sun.
On very hot days some installers may rig up a tarpaulin on poles over the area to be printed, in order to keep it cooler and the concrete workable for a longer period of time. The choice of pattern can affect the point at which an installer will imprint the concrete, as a deep jointed pattern such as a cobblestone can be pressed into softer / wetter concrete than a flatter design such as Ashlar Slate or Pacific Boardwalk.
Once the entire area has been printed, the installer’s job is almost done for the day and it is time to tidy up and leave the concrete to cure over the next day or so.
Concrete laid in large slabs has a natural tendency to crack but this problem can be vastly minimised if crack control joints are cut into the slab soon after it has been cast.
Crack control joints are dealt with lower down on this page in more detail but those areas which may be at most risk from cracking should be dealt with on the day the concrete is printed. These are referred to as soft cuts as the concrete is cut prior to it hardening off.