The base concrete should be kept wet for several hours before the levelling screed is to be laid, e.g. overnight, any excess water being removed before grouting.
Within a period of 30 min before the screed is to be laid (less in hot weather), a thin layer of neat cement grout or slurry of creamy consistency should be brushed into the surface of the base concrete. It is essential that the levelling screed is compacted onto the base while the grout is still wet. A proprietary bonding agent may be used or a proprietary bonding admixture may be added to the grout or slurry in accordance with the manufacturer’s instructions. In these cases the appropriate procedure of 7.4.2 should still be carried out. Proprietary bonding agents should be used in accordance with the manufacturer’s instructions, which usually require the screed to be placed while the bonding agent is still tacky. Polyvinyl acetate (PVA) bonding agents are unsuitable for bonding screeds.
Wetting of the base can be unnecessary in the case of some proprietary bonding agents, e.g. epoxy resin.