Curing Polished Concrete Floors
What is curing and why cure a floor?
Curing concrete is the process by which newly placed concrete is kept moist so it can keep gaining strength. For industrial flooring this is especially important in terms of increasing surface wearing capability and reducing overall maintenance costs. For the domestic scenario wearing capacity becomes less important and the aesthetic quality of any surface finish becomes increasingly key which can inform the decision of how to cure.
Types of polished concrete flooring
There are two main types of polished concrete flooring namely diamond ground polished concrete and power trowelled polished concrete. It is important to differentiate between the two types as methods of curing may be informed by the two types.
Diamond ground polished concrete will remove the top surface, expose the aggregates and refine the finish by using ever increasing grades of diamond pads. A chemical densifier is normally added during the polishing process closing the pores, allowing for a better shine and making the surface less permeable.
Power trowelled polished concrete floors derive their finish from floating and steel (usually) trowelling the concrete to a fine smooth and dense finish. Trowelled floors will not expose aggregates (although some ‘peppering’ may occur) and the surface will not have an even tone throughout due to the trowelling process. An additional method to the trowelled floor is to lightly diamond polish the surface prior to handover to further refine the finish.
Methods of curing
There are many ways to prevent moisture from escaping newly laid concrete but for the domestic scenario where visual concrete is concerned my opinion is that there are really only three practical options: Using a spray on curing compound, wet cure under a proprietary wet curing blanket or air cure under controlled conditions. Each method has advantages and disadvantages which this blog aims to explore.
Curing compounds are sprayed directly onto the freshly finished concrete and are either solvent or water based such as acrylic resin polymer, water dispersed acrylic emulsion or wax emulsion. Many curing compounds act as sealants for the concrete surface at the same time. The curing compound should have a 95% efficiency and allow moisture to slowly escape from the slab. The advantage of this system is that (with care) they are easy to apply and offer the contractor a cost effective speedy curing regime. If the floor is to be a diamond ground polished floor then using a curing compound is a no-brainer as the diamond grinding will completely remove the compound anyway. However, if not, the issue I have with this method is that cure seals leave the surface with a plasticky finish that is liable to scratches. I have also seen in some instances that it can wear away leaving the floor surface patchy and darkened in areas where the film has ‘delaminated’. The other big disadvantage is that curing compounds are almost impossible to remove (as far as I am aware). There are some that break down with UV light but this is not practical for aesthetic finishes. In my search for the perfect curing system I have found a company in the states that has developed a removable curing compound using a two product system. The compound must not get wet though or it will be very difficult to remove making external application difficult. Their technical dept. also recommended trials on visual concrete so perhaps it has not been fully brought to market for aesthetic applications. I will update on any further information when available but this could be a very exciting find.
One simple cheap way of curing concrete is to cover the surface in plastic. This is not recommended with polished concrete floors. Plastic will quickly promote (possibly in less than 24 hours) the development of differential curing marks in areas where the material is not in full contact with the concrete. In addition to this premature drying out of localised areas on the surface will remove the benefit of curing in the first instance.
A development in the states is that of a wet curing blanket using super absorbent fibres to trap and retain moisture under a poly backing. The blanket should be laid using a squeegee to force out entrapped air. From the outside any creases can be easily seen as the blanket is translucent white. This curing method allows for 100% moisture retention for 7 days. However, it is more labour intensive than spray on compounds and I note that the manufacturers declare that it reduces discolouration of visual concrete floors. I have not seen the elimination of discolouration which is where the ultimate goal must lie. Also I have not found a distributor in the UK yet but there may be someone out there who knows.
Controlled Air Curing
This method relies on creating a draft free environment to slow down the moisture loss as much as possible. The advantage is that the floor will be left completely in a natural state to dry out as slow as possible. The disadvantages are that air curing can decrease the strength of the concrete by up to 40-50% and with external slabs air curing can open up many issues with excessive drying and lack of protection. In my previous life as a producer of floors we ended up air curing as the informed consideration was that potential reduction in strength was a much lesser evil than compromising on surface aesthetics. In reality we encountered few issues with air curing in terms of strength, cracking and curling and through air curing eliminated aesthetic issues we had with other methods. For the main contractor on site air curing can be a pain as temporary protection must be laid and then taken up each day for about 10 days so as not to discolour the floor.
Well the conclusion is that most definitely curing decisions are key ones to discuss with the contractor and that whatever system is decided upon will need to be informed in terms of the trade off between aesthetics and strength. The problem with curing concrete, as with many other aspects of visual concrete, is that products are often sold from the perspective of specific benefits they may possess which may not necessarily cover all the bases in terms of strength, ease of application and aesthetic quality.
If anyone out there wishes to contribute to this discussion in my quest to find the perfect curing solution for polished concrete floors I would be more than willing to share this information out on future blogs.