When sealing the very best time to seal is when the concrete has fully cured and all the moisture has left naturally, this is typically 28 days. As you can imagine this guideline is rarely kept to, therefore if it must be sealed sooner at least 7 days should have passed since printing. This is because the type of sealer used will react with moisture. This also means that there should be no damp patches or evidence of moisture whatsoever, as it can cause the surface to take on a milky appearance.
Also ensure all buckets and brushes used are completely dry and clean.
Pour the sealer either neat or diluted with cellulose thinners or xylene (dependent on weather conditions) into a bucket and brush into the surface with a medium to soft broom with natural fibres. Make sure you brush it well in, to allow it to penetrate the surface of the concrete. Do not allow the sealer to lie on the surface. It can usually be walked on within an hour, but no vehicle should be driven on for a minimum of 48 hours – longer if possible as it actually takes around a month for the sealer to cure properly. Always add a non-slip additive to the sealer for the final coat.
Do not seal in the direct sun around midday – try to do it early in the morning or later in the evening.
Prior to washing down crack control joints should be installed at this stage. These should be cut with a circular diamond-edged blade. Cuts should be at least a quarter of the thickness of the slab deep, and there should be one every 20m2 dependent on shape.
During the summer washing down can usually be done after 24 hours, in the winter it is best left for 48 hours to ensure no damage is done to the finished surface.
The release powder has a waterproofing agent as part of its makeup, it also contains a colour pigment. This has the effect of either antiquing the colour or adding a variation to the base colour, most natural stone, slate etc have more than one colour, therefore, release agents add to the natural finished look.
Power washing is the most effective way of removing the powder and getting the waterproofing agent out without removing too much pigment. It is important that the waterproofing agent is removed as it would inhibit the sealer from getting into the surface of the concrete. Should too much pigment remain in the concrete after power washing, then specialist cleaning formulas such as a pre-wash can be used. A solution of ordinary patio cleaner in water can also be used, only a capful of patio cleaner to a bucket of water will do.
Note that some people will use washing up liquid, while this will also do the job, you must be careful as squirting it onto the concrete neat can stain the surface.
When placing the imprinting mats the first few mats are crucial, not only for the correct alignment but also in withstanding your weight without distortion.
From start to finish the imprinting stage should take no longer than 1 hour for 30 m2 i.e 3 m3 of concrete. In the summer the concrete laid can go from soft to a stage that’s too hard to print in this time, in the winter this can take 3 or 4 times longer.
Once you have begun, ensure that each mat has been printed thoroughly, using a metal stomper. As you lift each mat check for the depth of print and for imperfections. For hard to reach places a combination of a flexible mat, a small texture mat or texture roller and hand tools are used to fill in the gaps the rigid mat cannot print, the last thing you want is any bare patches in the print. When printing is complete double check that all hand tooling and touch ups have been done, as it cannot be done after the concrete has fully set.
In adverse weather conditions such as rain or frost, hessian or visqueen covers should be used to protect the surface after printing.
Printing should never be undertaken during periods of rain unless full protection can be erected.
*** A blog about what to do when a print has been affected by weather or other circumstances will be posted at a later date. ***
Once you have reached the stage where all the colour is trowelled in, time should be available, prior to printing, to clean off equipment and surrounding areas. In normal conditions and having used the correct equipment, you should have reached this stage before the concrete has set. It takes experience to handle large areas even with plenty help, beginners should never exceed loads of 3m3.
Concrete release agents acts as a barrier between the colour and the imprinting mats to stop the mats from sticking to the concrete, it is also coloured to add a further dimension to the look, like an antiquing affect to resemble aged stone. It is also a curing agent which traps moisture in to slow down the setting rate.
Spread a small amount of release agent where you are going to start and place your first mat in position, this should be able to take your weight without distortion, you should feel some give into the concrete. If the concrete is too soft the mat will unduly distort under your weight – a good test prior to laying your first mat is to push your finger into the concrete, should you get little resistance then the concrete is too soft, should you find you are only able to depress the your finger into the surface, then you should be ready to print.
Application of the colour is most commonly done via the broadcast method, where handfuls of t
he colour hardener is thrown over the surface, keeping your hands low as if the colour is being rolled on. Other methods include using an applicator, similar to a colander and shaking over the surface, this can be difficult to get adequate coverage in areas further from the edge.
The colour is usually despatched in bags, the weight and coverage rates should be checked prior, to ensure you have sufficient coverage for the size of the job. Using less than the recommended amount will detract from the strengthening properties of the product, not to mention the final look. Coverage rates can be affected by the weather and also surface water if the concrete. The timing of the application of the colour is critical, apply too soon and the colour will be eaten up by the concrete, too late and you will not be able to trowel it into the surface. When the colour hits the surface of the concrete it will darken, keep applying in layers until the you see the colour stay light. Test by running a steel pool trowel over a treated area, should the grey of the concrete show through, more colour is to be applied.
The whole are should then be trowelled to a smooth finish using a pool trowel and a fresno, also ensure all grids, manholes, shuttered edges etc. are bullnosed using a radius edging trowel.
Ensure you are purchasing a colour hardener, not just a dry shake colour, the difference being that the colour hardener will make the surface tension much greater, so that when the surface is imprinted it will not wear. Any dry shake powder without a hardener in it relies on the concrete to absorb it and colour the surface giving it very little extra surface strength required to pattern the concrete. If in doubt always ask for a copy of the test certificate for the colour hardener, any good supplier should have one to hand.
Specialised tools have been developed not only to create the desired effect, but to help lay and screed the concrete within the time available.
The main two specialist tools for patterning imprinting are:
ROLLER BUG/ROLLER TAMPER
In order for the surface of the concrete to be properly prepared, the aggregate and stone has to be below the surface, large stoney aggregate concrete mixes can be prepared using the roller bug.
This is taken over the surface handle attachments to cover the entire area. The action of the roller bug brings the fines to the surface without bringing up the water, this gives the best surface to print into, it also re-activates the surface whilst the underlying aggregate is setting.
All the concrete surface has to be “closed”, meaning that there is no aggregate showing through the surface, so once the
roller bug has been used, run the bull float over the surface to flatten the roller bug tracks and also take out any bumps.
Using both the roller bug and the magnesium bull float or magnesium hand float, the surface of the concrete is now ready for the colour application.
***There are many more specialist tools used in the imprinting industry, these will be looked at in further detail in a future blog.***
When the concrete is delivered don’t allow the concrete to be tipped away from the job, this would result in the concrete being moved twice, place the concrete direct onto the area to be laid, if this is not possible, chute it into wheelbarrows and move directly to the are to be laid.
There should be at least one person that stays where the concrete is to be tipped, with a rake or placer, their job being to roughly level and screed the concrete sticking to the level pegs previously installed, while others transport it over. Then, using screeds, the concrete should be levelled, ensuring all pre-planned falls are kept to, and the open face of the concrete is then closed using a magnesium float.
Be careful not to tamp the concrete as this causes water to rise to the top and destroy your print. The better the quality of the screeding, the better the overall look of the finished product.
These articles will lay out the general principles of how to lay imprinted concrete from start to finish. If you are an established fitter and looking to expand your knowledge, further blogs will be available focussing in more detail on each aspect of the process, sharing the tips and tricks that the experience we at Elite Crete have picked up over the years. However we advise that everyone read over this as it may still contain new information.
PHASE 1 – PREPARING YOUR BASE
The finished product is designed for driveways taking the loads one would normally expect to have on a residential driveway.
Preparation of the base is the same as for normal concrete, which is to remove vegetation, soil and loose material, and replace with a suitable hardcore – MOT type 1 or similar. Then compact with a roller or vibrating plate, leaving enough room or depth for 100mm of concrete and taking care that should you abut a house or property, the finished level of concrete complies with building regulations and is a minimum of 150mm below the damp proof course.
Prior to the arrival of the ready mix concrete, ensure that all levels and shutters are in place. Make sure that adequate falls are installed to ensure all water will run off to suitable areas and no pooling will occur.
In warm conditions drench the hardcore or lay visqueen over it, this will give you more time before the concrete becomes too hard to print.
A word of warning – normal concrete practice allows for visqueen to be laid prior to concrete being placed, however, this tends to hold water within the concrete, so when the imprinting stamps are applied the stamping action can cause water to rise to the surface and destroy the imprint quality.
When evaporation is slow on a cold winters day, do not lay visqueen under the concrete.
***More detailed information on concrete, and printing in both extremes of temperature will be discussed in further blogs.***