the ACP biocide is fungicide & timber preservative. Wood rot is the work of fungi. The tell tale is the presence of small black dots.
A periodical treatment of the ACP biocide will considerably extend the life expectancy of unpainted timber. Fungi tend to settle at overlapping joints and particular attention should be given to flood the gap of the decking where a cross beam supports. the treatment should impregnate the wood and dry salts remain for further protection. If natural grey is not a desired finish and oil or stain is applied, this needs applying post treatment.
If the decking has been allowed to develop a slime or deeply ingrained soiling the first treatment will be a vigorous brushing with Oleoclear.