Cement sand levelling screeds to receive floorings
These are covered by BS 8204, Screeds, bases and in-situ floorings, Part 1: Cementitious levelling screeds and wearing screeds– code of Practice (2004).
A levelling screed is one that is suitably finished to obtain a defined level and to receive the final flooring; it does not serve as flooring on its own. Most screeds for domestic use will be made from cement and sand only, without additives or reinforcement. Typically the mix proportions are within the range 1:3 to 1:4.5 cement:sand. Water content is kept to a minimum to minimise drying shrinkage of the screed. However, cracking can still occur in screeds laid in large areas.
Differential curling at day joints can be minimised by the introduction of fabric reinforcement across the joint at mid-screed depth.
The adequacy of the laid screed to take the imposed loading is a function of the floors application and the flooring material applied. The in-situ crushing resistance (ISCR) test is used to measure the soundness of the screed by producing an indentation under a standard impact. Depending on the category of use, the soundness of the screed is related to the depth of this indentation.
Screeds should be covered with a suitable curing material (e.g. polythene sheet) for a minimum of four days and preferably seven and the screed should not be trafficked during this period.
Further reading: PYE, PW and HARRISON, HW. Floors and Flooring, BRE, 2003.