Really this category should have been positioned at the start of this series as cleaning, or considering to clean any struck surface should be the first of any potential intervention once the formwork has been removed. The reason why I have dedicated so much time to this subject in the blog is that I often see unplanned aggressive cleaning of struck concrete that permanently alters the appearance of the surface. Well considered cleaning will improve the appearance of most surfaces dramatically. For this reason it should be the first item on the list of any ‘post finishing’ work and the concrete should not be judged before cleaning is finished or a sampled area offered up.
Bear in mind that once the surface is touched it will be very difficult to return it to it’s original state should any undesirables occur. For this reason cleaning should not be undertaken as a matter of course and should not be interpreted as a non-risk activity either. Instead it should be controlled and considered with the chosen method trialled beforehand. Give the concrete time to come up to strength and start with the least aggressive method that will have the most positive impact. Always keep the method consistent across the whole surface and never scrub at specific areas to remove stains. The result will be far worse than the stain itself as the photo here shows.
When I clean concrete I prefer to do it dry, not wet. When wet you cannot see what’s happening to the real concrete tone. There should be no logical reason to wash the concrete if the face is not caked in mud and even less reason to use a jet wash! If any water is used (as localised stains can be reduced by water) then a test patch should be carried out in a discreet location and thoroughly buffed and then dried to see what tonal change occurs.
To clean dry, begin very lightly using diamond pads (not sandpaper). Some flexible soft diamond pads such as 3M Scotchbrite (Sienna) are a really good start and can be easily fitted to a small palm sander. More aggressive diamond pads such as nickel encrusted diamond can also be used with care. Any mechanical cleaning of this sort will alter the surface sheen so be really careful with shiny phenolic surfaces. Always step back at least 3 metres from the surface to review and catch it in the light to see if and how the sheen has changed. Always clean panel by panel and dust off the surface with a clean cotton cloth to get a true picture of the final tone.
As a last resort stains can sometimes be reduced or removed chemically but an acid is really not a good idea unless an acid wash is specified. Also, acids are horrible. Mild alkaline cleaners should not etch the surface but a strong alkali will dull it. Natural and artificial stone bleaching agents are also good for organic and tannin stains. Rust is usually best reduced mechanically as normally these stains are from nail and rebar rust on the formwork face. Rust can be reduced chemically but I’ve found that it can turn the concrete purple. Grease can also be greatly reduced with stronger alkaline cleaners or a solvent poultice. Expect to treat grease several times and do not rub at the stain! Meths also works well but don’t use white spirit.
If stains, dirt, marks, graffiti or anything else remains on the surface it will have to be masked and colour matched to the surrounding concrete.This is not part of the cleaning process and will require specialist input.
Stuff stuck to the surface such as grout runs, tape, silicone etc can be removed using sharp razor blades. Mild steel tools may blacken the concrete so razor blades are a good choice. Bear in mind that anything stuck to the concrete when it is young and green will probably leave some sort of mark which can only be lost through masking as described above.
In a nutshell:
Don’t clean “just because”
Test your method first
Keep any method consistent
Use the least aggressive way with the biggest positive impact