Imprinted Concrete Installation

Imprinted Concrete Installation – Site Preparation

Although proposed dates may have been agreed at the time your order was confirmed, a number of things can affect the actual date your imprinted concrete installation commences.

Installation of pattern imprinted concrete is dependant on the weather and if heavy rain or a deep frost is forecast, it would be unreasonable not to expect a delay. Additionally, as much as experience should tell a competent installer that a job should take say, three days, the unexpected can happen which takes it into day four. A caring, professional installer would keep you informed of any delay and reschedule the work.

Apart from a return visit to wash the surface down and seal it, once your imprinted concrete installation starts your installer should stay with it through to completion, even if that means the next job in his schedule is delayed. That way, you will know that once your job commences, the installation team will not be leaving to move to another one, with your imprinted concrete installation half finished.

On the first day of the imprinted concrete installation, the site foreman should check the relevant details of the project with you and confirm the likely job duration, taking account of a recent weather forecast for the area. The installer should also arrange temporary access to your home, especially if it is the front driveway which is being replaced. They should also confirm that you have already removed any prize shrubs or plants which may otherwise end up in a skip.

For many domestic driveways of a reasonable size, anything between 10 and 25 tons of spoil can be removed before the imprinted concrete installation commences.  Most jobs will involve a mechanical digger rather than being excavated by hand; though this may not be the case where access is poor, such as for a rear patio area.

Some installation companies will have their own trucks to take the spoil straight to a landfill site but others may use skips.  If that is the case, skips should be positioned so as not to create a hazard or an obstruction; especially if left overnight.  If a skip is left on the road overnight, permission should have been sought from the local authority and it should have lights attached.

Imprinted Concrete Installation – Excavation and Sub-base

Taking care of the position of any pipes and cables running under the area, the site should be excavated to a minimum of 200 mm below the proposed finished surface. This should be 150 mm below the damp proof course of the host property.

The 200 mm excavation depth allows for 100 mm of sub-base (unless excavation reveals an existing suitable sub-base) and 100 mm of concrete.

Sub-base for an imprinted concrete installation

The new sub-base material for your imprinted concrete installation should be crushed rock or brick rubble conforming to the grading requirements of the Ministry of Transport’s Type 1 MOT.  This should be specified with a particle size of 20 mm to dust.

In an imprinted concrete installation the sub-base material should be compacted using a heavy roller or a vibrating plate in two layers of 50 mm each so that adequate compaction is achieved without air voids, especially in the lower sections of it.

The area covered by the sub-base in your imprinted concrete installation should be slightly larger than the proposed pattern imprinted concrete area. This is to prevent the concrete cracking on the edges as the sub-base falls away below it. The site foreman should be satisfied that the entire sub-base installation is satisfactory.

Although the sub-base should be flat, to plus or minus 10 mm, it will not necessarily be level as in any imprinted concrete installation it would need to follow the contours of the land and take surface water to drainage channels where required.

Use of a Slip Membrane in an Imprinted Concrete Installation

If, in your imprinted concrete installation a slip membrane is to be installed, the sub-base should be blinded with a fine material such as sand so that any sharp stones in it do not puncture the membrane.

The use of a slip membrane is a subject of contention between some installers of pattern imprinted concrete. The argument for is that a polythene sheet between the sub-base and the hardened concrete acts as a slip membrane allowing the concrete to move independently of the sub-base making it less likely to crack.

Also, on hot days, because the polythene prevents water in the concrete from bleeding into the sub-base, the concrete will set more slowly, giving the installer more time to obtain a better quality, more uniform printed pattern.

The alternative argument is that the polythene slip membrane can hold water in the concrete for too long and the surface of the concrete will be too wet to print well. The latter group argues that it is down to the judgement of the foreman to order concrete with the appropriate water content for the job.

The foreman will take into account the weather on the day of imprinted concrete installation and use experience to judge whether a slip membrane should be installed. Even so, it would be appropriate for you to ask whether a slip membrane is to be used in your imprinted concrete installation and if not, why not.

Imprinted Concrete Installation – Drainage

Planning permission for non-permeable paving to the front of houses

Since October 2008 homeowners have required planning permission to install a new non-permeable front driveway which will direct surface water onto the public highway or in to the household drainage system. However, to avoid a planning application for your imprinted concrete installation, drainage systems should be installed which deal with the surface water within the boundary of the property.

This normally involves installing a soakaway or rain garden, or allowing water to drain straight to a border if it is adequate for the expected volume of surface water.

The Flood Water Management Act 2010, which became law in 2012, effectively makes SUDS (Sustainable Urban Drainage Systems) mandatory by introducing legislation to address water quality and scarcity, water flow and the threat of flooding.  For householders it extends the responsibility of dealing with surface water responsibly, to all parts of the property not just the front driveway and garden.

Position and specification of drainage systems in an imprinted concrete installation

The position and specification of the drainage system for an imprinted concrete installation is dictated by the natural contours of the land, unless extensive excavation is carried out and retaining walls are constructed on steeply sloping gardens.

Gullies, channels, and manhole covers should be set on concrete and haunched (supported on both sides with concrete) to the appropriate level and fall, 5 mm below the final concrete surface level.

Gullies and drainage channels should be installed at low points where water would otherwise pond and they should divert surface water through underground drainage pipes to soakaways where required. If the driveway slopes towards the house, a linear drain (aco drain) should be installed to prevent surface water pooling against the house wall, potentially causing a damp problem. Similarly, if the driveway slopes towards the public highway, a linear drain can be installed across the entrance to the driveway.

In both cases the surface water should be taken to a soakaway, though any excess water which the soakaway is incapable of dealing with can be legitimately piped to the household drainage system. The number of soakaways required in an imprinted concrete installation will be dictated by the size of your new pattern imprinted concrete driveway or patio and the degree of slope.  A standard soakaway should cope with approximately 60 square metres of pattern imprinted concrete driveway under most circumstances.

Aco Drainage positioned along movement joints

Linear drainage systems have another use on large or long pattern imprinted concrete driveways where a joint needs to be cut in to help reduce cracking. Concrete has a natural tendency to crack as it contracts or moves and various types of movement joints can be cut into the surface to control where the cracking takes place.

On a very long sloping driveway for example where a number of joints need to be made across the driveway, these can be disguised by installing linear aco drainage across the driveway adjacent to the line of the cut.  The cuts become hidden and the linear drainage takes surface water at intervals from the sloping driveway into appropriate underground drainage.

Imprinted Concrete Installation – Concrete Specification and Concrete Laying

Concrete Specification

The specification of concrete used for pattern imprinted concrete driveways and patio areas is critical and is referred to as prescribed, by ready-mixed concrete suppliers.

That is, it is prepared to a particular formulation which gives it the required properties in terms of its handling, consistency, workability, strength and durability. An experienced installer of pattern imprinted concrete will probably have a good relationship with the ready-mixed concrete supplier. There needs to be a good deal of trust that the particular type of concrete required for the job is both ordered and supplied to the correct specification.

Even so, prior to taking delivery of the concrete the site foreman should check the delivery note, and to avoid potential future disputes between the customer, installer and concrete supplier, fasten a copy of it to both copies of the customer order forms.

To minimise the risk of being supplied with poor quality or wrongly prescribed concrete, most reputable pattern imprinted concrete installers would only use ready-mixed concrete from QSRMS accredited suppliers (Quality Scheme for Ready Mixed Concrete).

Cement Content of the Concrete

The recommended specification for concrete used in pattern imprinted concrete driveways and patios is dependent on the time of year the job is done.  In part, this refers to the amount of cement (Ordinary Portland Cement) in each cubic metre of concrete.

In warm summers it should be 320 kilograms per cubic metre, in spring and autumn 360 kilograms and in winter up to 400 kilograms. The higher cement content increases the strength of the concrete in your imprinted concrete installation but also causes it to harden more quickly. However, more water can be added to the concrete to give the installer more time to work it, though doing this can weaken the concrete mix.

Also, concrete used during the summer months would normally have aggregates with a particle size up to 10 mm, whereas in winter it could be up to 20 mm. This enables your installer to print the concrete earlier.

Fibre Reinforcement and Air Entraining Agent

Other characteristics of the ‘prescribed’ concrete are that it should contain fibre reinforcement and an air entraining agent (though the latter should be without a plasticiser which it is sometimes supplied with).

The fibre reinforcement consists of polypropylene fibres which makes the concrete stronger, with a better impact resistance.  Fibres also slow water bleed to the surface of the concrete, preventing pooling which could spoil the quality of the printed pattern.

The presence of air entraining agents means that up to 5 or 6% of the concrete volume consists of millions of microscopic air bubbles. These bubbles are too small to hold water (which could otherwise freeze and cause minute fractures within the concrete slab) but they do allow the concrete to expand into them, reducing flaking of the pattern imprinted concrete surface.

Concrete Slump Test

One way of checking the consistency or workability of fresh concrete is by carrying out a slump test which measures how concrete flows and is used to indicate, how wet the concrete is.

A small amount of concrete from a fresh batch is formed into a cone on the ground, using a metal slump cone around 300 mm high and wider at its base than at the top.  The amount by which the concrete falls when the slump cone is removed, is the measure of slump.  The more it falls, the wetter it is and concrete suitable for use in summer months would normally be permitted to slump further than that suitable for a winter project.

Laying the Concrete

Once the sub-base, formwork and slip membrane (if one is to be installed) is in place and the surrounding buildings and gardens have been protected, the concrete can be laid.

Most ready mixed concrete is delivered by trucks with a capacity of around six cubic metres. Since the concrete should be a minimum of 100 mm thick, this suggests that up to sixty square metres can be covered from one delivery.

Most pattern imprinted concrete installers have a minimum order value because concrete companies charge a premium rate for small deliveries of less than six cubic metres and also due to the set up costs associated with all jobs, regardless of size. Large areas would need more than one delivery of concrete and for exceptionally large front driveways for example, the imprinted concrete installation could go into a second or even third day.

Preparing the Concrete for Colouring and Printing

Concrete would normally be unloaded into wheelbarrows, but could be pumped through a pipe directly from the delivery truck, and is positioned around the area to be covered. From there it is distributed over the prepared sub-base and raked to the approximate levels required, level with the top of the shuttering.

Using a screeding tool and a large spirit level, the concrete is screeded, or levelled, to the required falls for drainage and then rolled with a roller tamp, or roller bug.  This is to depress the stone aggregate into the concrete slab and even out the concrete surface.

To achieve the perfect surface for applying colour surface hardener, the concrete is then floated with large magnesium floats, often more than a metre wide, which are sometimes attached to the end of long poles which enable installers reach all areas of the imprinted concrete installation.

When the concrete surface has been floated it should be a slightly wet, shiny, flat, familiarly grey concrete coloured slab, ready to accept the colour surface hardener.

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.

Imprinted Concrete Installation – Joint cutting to control cracking of the concrete

Concrete has a natural tendency to crack because it is weak in tension and as a result any forces which develop as the concrete shrinks can cause cracking.

The most common joints cut into concrete are therefore referred to as contraction joints, not expansion joints as they are often wrongly named.

In any imprinted concrete installation, joints should be cut into the concrete as early as possible after it has been laid to control cracking due to shrinkage as it dries and hardens.  Cuts are only made partially through the concrete slab. If the concrete shrinks afterwards, the cut acts in the same way as an electrical fuse so that if the concrete is going to crack, it will crack at its weakest point; that is, the remainder of the concrete slab below the depth of the cut.

Contraction joints, which are cut into the concrete with a diamond saw blade, enable the installer to pre-determine the location of any cracking.  Otherwise the concrete could crack randomly over its surface, spoiling the appearance of your imprinted concrete installation.

A soft cut on the day of the pour should be between a quarter and a third of the depth of the concrete and a saw cut into hard concrete should be between one third and a half of the concrete depth.

Minimise potential cracking of the concrete

To minimise potential cracking of concrete, individual slabs within the whole of the pattern imprinted area should be no more than 20 m² for concrete 100 mm thick.  They should also have a length to width ratio of no more than 2:1, depending on the strength of the concrete mix.

This is particularly important on long narrow footpaths and suggests that a one metre wide footpath should have a contraction joint cut across its surface for every two metres of its length.

Another way to minimise random cracking of your pattern imprinted concrete is to install reinforcement across any expected line of cracking, such as at external corners of buildings.  A typical ‘L’ shaped driveway which is wide over the front garden area and narrow down the side of the house, is likely to crack from the front external corner of the house and steel reinforcement at this point can help.

Also, where the pattern imprinted concrete area is designed to take heavier loads than standard cars, concrete up to 200 mm thick would perform better.  Thicker concrete will reduce the potential for random cracking because of the addition strength of the pattern imprinted concrete slab.

Once all of the contraction joints have been cut into the surface of the pattern imprinted concrete, all the concrete dust needs to be brushed and washed away. For this reason, and to save time, some contractors will make the saw cuts prior to cleaning the antique release agent from the surface but there is no hard and fast rule here.

Imprinted concrete installation – Sealing the surface of pattern imprinted concrete

Applying sealant to the imprinted concrete

The concrete surface should be completely clean and dry before a solvent-based acrylic sealant can be sprayed on. The sealer should be applied without allowing it to pool and several thin coats are better than one thick one.

The sealant should also be applied in warm conditions so that it penetrates into the surface of the concrete and dries before the next coat is applied. On warm summer days this may be done on the same day as the crack control joints are cut in and the surface is washed.  However, unless the surface is completely dry the sealant will not bond to the surface of the concrete and risks delamination.

If the concrete sealant is applied too early or on a less than dry surface it can be cleaned off with a proprietary cleaner containing xylene and then resealed once the surface has properly dried. Some chemicals used in the cleaning processes, both of antique release agent and acrylic solvents can be particularly aggressive and care should be taken not to splash or spray them onto surrounding areas. Needless to say, protective clothing and goggles should also be worn.

Protecting the surface of the pattern imprinted concrete

Solvent-based acrylic sealants will protect the surface of the pattern imprinted concrete from spills and stains, as well as helping to protect the colour surface hardener and antique release agent pigments from fading under Ultra Violet light.

Acrylic sealants are also micro-porous which allows the concrete to breathe and lose its moisture content. Acrylic sealants are not particularly tough though and under the normal domestic conditions of pedestrian and vehicle use, pattern imprinted concrete surfaces will probably require re-sealing after a period of three to five years. To do this properly, the old sealant must be removed with a mild acid wash, the surface washed with clean water and dried fully before new sealant is applied.

Types of concrete sealant

We have referred to solvent-based acrylic sealants used to protect the surface of pattern imprinted concrete but water-based sealants are also available. These are completely ineffective and should not be used.

Sealants are available in gloss or matt finishes and though the high gloss finishes can give a false impression of the natural stone or cobble which pattern imprinted concrete is attempting to replicate, the shine does diminish after a while.

Photographs of shiny pattern imprinted concrete driveways and patio areas in brochures and in the collections used by surveyors to help customer choose the pattern and colour finish are invariably due to the photographs having been taken immediately a job has been sealed.

Improve surface skid resistance of pattern imprinted concrete

When pattern imprinted concrete is first installed, the shiny surface sealant can also cause it to be quite slippery and this can be a particular problem for sloping driveways and on steps. As the shine diminishes so does some of the slippery characteristics of the surface.

In order to temporarily improve skid resistance in the first week or so after an imprinted concrete installation, small particles of micro-polyethylene (a type of plastic) can be added to the sealant to provide more grip.

Imprinted Concrete Installation – Final inspection, cleaning and clearing the site

Now that your imprinted concrete installation is complete it is time to clear the site of all unused materials, tools, and boards & sheeting which have been used to protect your house and garden.

Manhole covers should be checked to ensure that they lift freely and are not held down by over spilled concrete.  Prior to leaving, the site foreman should walk around the newly paved area with you to check that it has been completed satisfactorily.

Some pattern imprinted concrete installers carry out a further inspection several weeks or months after completion of the job to ensure that there are no underlying problems with the imprinted concrete installation which could cause more serious problems later. The state of the surface sealant and effective drainage are two of these issues but so is customer satisfaction.