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What has caused my sand cement screed to break-up?
There are two aspects; is failure due to cracking or crumbling?
Crumbling is caused by a lack of compaction or low cement content. A screed tester (in accordance with BS 8204) is likely to show that the screed falls short of the lowest Category C (indentation of 5 mm). An analysis to assess the proportion of sand to cement might indicate a low cement content. Visual inspection and density check of the screed may suggest poor compaction.
Cracking is expected in all sand cement screeds (Clause 6.9.1 BS 8204) unless sufficient joints are provided. Cracks are not considered a problem under thin floorings e.g. vinyl sheet/tiles or carpet, unless the screed has curled at the crack. See clause 8.4 in BS 8204-1
The general recommendation would be that unsound areas of screed should be cut out and replaced. Proprietary systems can be used to upgrade an unsound screed, which may be by impregnation with a low viscosity resin or by laying a fibre-reinforced, flowing cementitious screed. Trials of repair systems should be undertaken using test methods such as the BRE Screed tester.
If the overall quality of the existing screed is found to be similar to the areas tested, it may prove more cost effective to remove and replace.
Acknowledgement: The Concrete Society
Other references:BRE Defect Action Sheet 51 Floors
BRE Defect Action Sheet 52 Floors
BRE Digest 104 Floor screeds
BRE Good Building Guide GBG 28, Part 2 Domestic floors
BRE Good Building Guide GBG 28, Part 4 Domestic floors
BS 204 Screeds, bases and insitu floorings