It is rare to see a perfect Pattern Imprinted Concrete (PIC) installation. As PIC is a hand constructed product using concrete laid in various weather conditions it is subject to a number of different types of imperfections, some of which can be improved upon by having knowledge of and using certain techniques.
A brief overview of some of these points of note are as below:
(images used in this article are of good PIC projects and not the points of note)
Colour fluctuations within one pour or between individual pours
Always take 50% extra CSH to site and undertake smaller areas in order to colour more generously if required. Wetter concrete will always require more CSH than drier concrete in order to achieve the same colour.
Also, always acid wash jobs to regulate the degree of antiquing release agent take up.
Avoid puddles or ponding water
Ensure adequate falls of min 1:100 or preferably 1:80, especially for deeper patterns when the concrete requires printing earlier when the concrete is softer.
Consider using staging boards to work from especially around sensitive areas such as outside front doors.
Lack of print detail and/or opening fissures
Getting a good quality print can become difficult when the concrete becomes a bit too set and also the surface can become a bit hard or dry, causing opening fissures to appear when the pattern is imprinted into the concrete.
To get a more defined surface print and less or no opening fissures, always lay on polythene, pour smaller areas and even consider erecting a shade. All of these points become more important in the warmer weather.
Sealer Pluck, caused and highlighted by, excess RA left on the surface of the PIC
Always wash every job with Acid Wash. If in doubt, remove more release agent rather than less. This is so easy and cheap to do it’s amazing not everybody is doing it, often because they are not aware of the importance of the wash off process.
Also, some people think applying acid to the concrete will damage it. In a way they are right. Applying acid incorrectly WILL damage the concrete. Undertaking acid wash process correctly WILL NOT damage the concrete.
Acid washing concrete will clean the surface of the concrete and remove any release agent residue from the surface of the PIC. This enables the sealer to more easily penetrate the concrete and also adhere better to the clean concrete surface.
Sealer issues are dramatically reduced.
Sealer Pluck, caused by a weak cementitious surface
Sealer pluck can also occur when sealer is applied to a weak concrete surface. The weak surface fails and the sealer comes away with it. In the case of PIC, which hopefully will have been acid washed, it is ESSENTIAL that the surface is THOROUGHLY washed with a high pressure water jet, to remove the weakened surface.
If the surface doesn’t fail, neither will the sealer appear to fail.
Reduce the chance of cracking
Construct a quality sub-base and ensure a well compacted, flat base which is prepared to the correct falls.
Always lay concrete on polythene.
Lay a minimum of 100mm thick concrete.
Install Contraction Joints within 24hrs, or sooner if possible.
Always consider the use of soft cut joints when appropriate in high-risk areas.
Whenever possible place contraction joints to incorporate corners of buildings or manhole covers etc. as these points are high risk areas likely to cause cracks. Subsequently create bays of max length to width ratio of 2:1 and max bay size of 20m2 (based on a 100mm thick slab).
Reduce the noticeability of any colour or texture fluctuations between pours on large jobs
A significant method of improving print and colour quality and uniformity is by undertaking smaller areas. Paradoxically this also increases the risk of ANY (even small) fluctuations of pattern or colour being noticeable as the day joints between individual pours are more frequent.
To massively reduce this becoming a potential issue, design interesting features into the project. Not only will this very often improve the aesthetics of the installation but will also make the job significantly easier to install, easier to cover up from rain and will reduce the chances of cracking. A win, win, win scenario.
Concrete setting too quickly
This is particularly relevant in the warmer summer weather and the speed of set can be slowed down by:
Using a lower cement content concrete (minimum 340kgs per m3)
Use a cement replacement / blended cement. NEVER EVER use a CEMIII, although it will often be offered to you. Only ever use a CEMII blended cement.
Set up a shade. Yes, a pain in the backside, but concrete will set significantly slower if it is kept out of the sun and this is the one of the BIGGEST contributors to having more time to print a pattern into the concrete.
Order the appropriate and correct concrete mix
Basically, as we all know, it’s up the cement content in winter, lower it in the summer. Then there’s the possibly to have a ‘blended’ mix in the summer, which replaces some of the cement with a ggbs, which is a cement replacement, consequently slowing the speed of the set and giving more time to work with the concrete.
All sounds great right?!
Well, yes, but remember.
The minimum permissible cement content is 340kgs/m3 (and the concrete MUST also be air entrained). The cement content should not be lower than 340.
There are three main cement options it is possible to order:
CEMI – 100% cement, (although for interest, it may contain fillers and not be 100% cement).
CEMII – A blended mix of ‘cement’ and up to 35% ggbs cement replacement.
CEMIII – A blended mix of ‘cement’ and up to 55% ggbs cement replacement.
NEVER EVER order, or accept, a CEMIII mix. These mixes are ordered periodically, inadvertently, accidentally, mistakenly or for whatever reason and they are very likely to fail quickly and significantly within a short period of time. There are numerous jobs that we at PICS are aware of where the surface of the concrete has worn away within 12 months, looking like grey exposed aggregate concrete. The concrete cures very poorly and is so weak, that is possible to dig into it with a brickies pointing trowel or even a teaspoon.
There is no remedial action which will resolve the problem. The only answer is to dig it up and do it again………using the correct concrete mix.
Ensure freeze thaw resistance
We use a certain concrete mix for decorative concrete, which has a minimum cement content of 340kgs/m3. In the summer we wouldn’t want to increase the cement content as the concrete would set too quickly, so an air entraining agent (AEA) is added to protect the concrete from freeze thaw cycles and de-icing salts.
It is essential that all concrete for PIC includes AEA. Without AEA the concrete surface may be damaged by freeze thaw cycles over a number of years. AEA must be added to the concrete all year round and not just the winter.
If you are looking for assistance with a PIC project, PICS offer aftercare support and would be happy to help. Click to Contact PICS today to discuss any technical queries.