Hello,

My patio (about 25 sq metres) is made of bog-standard concrete slabs laid
about 20 years ago. I have a problem with black stains on the slabs. The
stains are roughly circular, about the size of a small coin, and very
difficult to remove. I suspect they are caused by a fungus or mould in the
slab itself, rather than on the surface.

I've tried patio cleaner solution from a DIY / Garden centre (expensive and
ineffectual). I've tried blasting the slabs with a high-pressure water jet
(shifts the surface dirt nicely, doesn't affect the stains). I've tried
scrubbing with a broom and a variety of scouring / cleaning powders &
liquids (Ajax, Flash, Jif etc).

The only effective method I've found is to scrub the slabs with thick, neat
bleach, leave it for several hours, then rinse it off. It is messy and
time-consuming , requires an awful lot of bleach (about 1 litre of bleach
per sq. metre), and kills the grass at the edge of the lawn (though it soon
recovers). The treatment has to be repeated every year.

Does anyone have any better ideas or suggestions?

Thanks
KK