There are very few houses in the UK which do not include some concrete in their construction or in the garden. Concrete, as with any other building material, will suffer from the effects of damp and weather if it has not been treated properly (and sometimes even when it has).
Cleaning Concrete
Concrete on driveways or the floors of garages often gets stained with oil and grease. Soak up fresh spills straight away and sprinkle dry sand over the top to absorb as much of the liquid as possible. Sweep the sand away after a day or two and then wash out the stain with white spirit or a degreasing solution.
Sealing concrete
Newly laid concrete has a very high alkali content and efflorescence can form on the surface as it dries. Let the concrete dry completely before attempting to apply any sort of surface finish. Concrete dries very slowly, indeed, in perfect conditions 4" of concrete could take 4 months to completely dry. Remove any efflorescence as you would with brickwork and then treat the surface with a clear sealant solution or general purpose primer.
Dusty Concrete
If the surface of concrete is over-trowelled as it is laid, a dusty and loose layer can form on top of it as the cement is drawn out. In the event that this happens, you should let the concrete dry and then treat it with two coats of a PVA bonding agent (1 part PVA to 5 parts water). The same solution can be used to prime any very pourous concrete surface.
Repairing Cracks or Holes
Clean out the crack with a stiff brush to remove any debris. A very thin crack (5mm or less) should be opened up slightly with a chisel so that any filling will hold. If you can, undercut the edges to form a lip that will help to hold any filler. Primer the inside of the crack with a bonding agent, diluted as instructed by the product manufacturer. When this is dry, apply the appropriate filler (depending whether the crack is outside or inside the house). If you are filling a larger hole, mix up a standard concrete mix using a fine aggregate. Brush out and prepare the hole as you would with a crack, and then fill with the concrete mix.
Spalled Concrete
External concrete is prone to spalling. This happens when moisture invades the surface of the concrete and then freezes. As the frozen water expands it can cause the surface of the concrete to break away. If the concrete contains steel reinforcement, this effect can be accelerated as the exposed steel corrodes. You can fill the hole or crack in the same way as described above, but you will need to treat any exposed metal with a primer to stop the corrosion continuing. To prevent further spalling, treat the concrete with a bitumen undercoat and then paint over with a reinforced emulsion.
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