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Last edited 12 Apr 2021
How to clean concrete
Concrete needs to be cleaned periodically both for reasons of aesthetics and to extend its lifespan. As dirt and grime can build up gradually on concrete surfaces, it is sensible to schedule cleaning at regular intervals. The technique for cleaning concrete depends on the type of surface. Specific stains may require cleaning chemicals, but water is usually sufficient for removing normal dirt.
The most common method for cleaning external concrete (such as for the removal of oil and grease stains) is by using a power washer, which can often be rented. It is usually advisable to spray the surface with a detergent, or alkaline degreaser, and leave for 15-20 minutes before spraying with water. The degreaser has the effect of emulsifying the oil, allowing it to be flushed from the surface using hot water.
Power washers with a pressure rating of at least 3,000 psi and a flow rate of at least 4 gallons per minute should be used. However, difficult contaminants such as paint and heavy tyre-skid marks may require higher pressures. Power washers with a rotary nozzle instead of a standard fan nozzle can increase the cleaning speed. Use of a hand-held lance can direct the spray more accurately.
Once the power washing is complete, the area should be checked, and any adjacent walls, windows, or other surfaces which have been dirtied should be washed down; usually without the high-pressure function, as normal mains pressure will suffice. Drainage points, gullies or linear channels should be checked for any blockages or silting from wash-off.
Depending on the chemicals used in the power-washing, water runoff may not be allowed to enter storm sewers. In addition, any detritus or dirt that accumulates should be disposed of off-site rather than allowed to wash into sewers.
More difficult stains, such as rust stains which have penetrated deep into the concrete surface, dried grout, or graffiti, may require muriatric acid or oxalic acid, albeit with extreme caution and protective equipment as they are toxic materials.
To tackle graffiti, several proprietary chemical strippers are available, of which, citrus-based solvents are the safest and have the least stringent disposal requirements, although they are the least aggressive and may not work on all types of paint. Products containing potassium hydroxide, which is soaked into the concrete surface for several hours, followed by the application of an acid neutralizer, can also be an effective solution.
A less aggressive approach is required for cleaning internal concrete surfaces. It may be sufficient to use a mop and a bucket of water mixed with a mild cleaning product, followed by rinsing with a mop and clean water. Difficult stains can be removed by using trisodium phosphate (TSP) mixed with water and scrubbed onto the area with a brush.
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