Building pathology: Screeds

The failure of cement–sand levelling screeds can be highly disruptive, dangerous and costly, owing to the unavailability of floor space and the downtime needed to put it right. Repairs often need to be carried out quickly, which is all the more reason to carry out a rigorous investigation to determine the cause of failure and propose a remedial solution that resolves the defect.

Failures may relate to deficiencies in design, workmanship, materials, interfaces with other elements and use. Often more than one factor plays a role. Failures may follow shortly after installation or not manifest themselves for many years. Recent trends for increased thermal insulation under screeded ground floors and the use of pumped screed are associated with common defects.

Collapse or crumbling of a screed

This typically occurs with cement–sand screeds where the screeds are poorly mixed, for example with pockets of cement, or if they are inadequately compacted or laid too dry. Failures occur if loads are higher than the local crushing strength of the screed – typically under loads such as furniture legs or areas of heavy traffic. The floor covering is as likely as the screed to become damaged.

The investigation may include testing the impact strength of the floor with the BRE screed soundness tester. The screed should be at least 14 days old and applied floorings will have to be removed. Testing may be carried out randomly, once per 10m2 or on a 2-5m grid for example, and should include all areas of heavy traffic. The smaller the indentations that occur when testing, the heavier the traffic the screed can withstand. Take care at buried services and sub-structure joints.

Repair options range from local patch repairs to complete replacement. More difficult or specialist remedial solutions include impregnation with a resin system or overlaying with a flowing renovation screed.

Curling

This is a high-risk defect in unbonded or floating levelling screed construction. Water moisture in the screed evaporates from the surface, which dries and shrinks at a faster rate that the low parts of the screed. This results in the upward curling of the screed. Curling typically occurs in the first year after laying. Bonded screeds may curl at daywork joints and edges where bonding is inadequate.

Risk of curling is best addressed in the design and workmanship stages. There are many factors that may cause curling:

• Thickness. The British standard on screeds BS 8204 recommends at least 50mm for an unbonded screed and 75mm for a floating screed (65mm is acceptable for domestic levelling screed). Steel fabric reinforcement also helps minimise curling
• Water content. This should be just enough to ensure good compaction
• Drying. Screeds should be allowed to cure and dry out slowly.

Curled screeds may “uncurl” over time when covered with water containing smoothing compounds or adhesives, or if moisture is redistributed when the screed is covered by impervious flooring. The uncurling process causes the floor covering to ripple upwards.

Repair options include cutting out curled sections and replacing. Resin injection may be possible followed by the grinding down of lifted screed.

Cracking

Drying shrinkage will invariably lead to cracks in cement–sand levelling screeds. Defects arise where the cracking has been inadequately controlled or not allowed for in services or subsequent construction, generally floor finishes. Here are some of the key issues to look out for:

• Structural movement joints in the base slab are not reflected in the screed layer or the floor covering
• Inadequate stress relief joints in levelling screed. These should be straight, vertical, sunk to at least to mid-depth and cut into the screed within three to four weeks of it being laid
• Inadequate drying out. Cracking may occur where the screed is dried too quickly. And if it is not done adequately, floor finishes may be damaged by too much moisture. Particular care is needed when drying out pumped screeds as they tend to include more moisture than traditional cement–sand ones
• The screed is laid in hot weather without adequate overhead covering
• The services are embedded in the screed without adequate cover. There should be a minimum 25mm above pipes. There may also be a lack of reinforcement; this should stand at least 150mm on either side of the pipework
• Underfloor heating can lead to problems if it is turned on too early or is too hot. The European Standard BE EN 1264–4 for floor heating recommends 21 days before heating cement screeds; temperature should be kept to 20-25ºC for the first three days.

Thermal and acoustic insulation laid under the screed should be continuous and have adequate bearing capacity. Problems can arise if rigid foam insulation is laid on an uneven substrate, as it bridges between high points and might collapse in time.

Investigation process

Further information may be found in publications from CIRIA, BRE, Concrete Society and BSRIA. The BS EN 13892 series provides methods of tests.

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