Serviceability failures of sand-cement screed flooring are, unfortunately, fairly common. Typical problems include de-bonding, cracking, crazing, softness, poor abrasion resistance and unacceptable surface finish.
In all cases the failures can be traced to one or more of the following:
- Incorrect application
- Inadequate specifications
- Poor materials selection
- Incorrect mix proportions
- Poor site practice and bad habits
Sand-cement screed flooring is suitable for light duty use only. The most common application is as a levelling layer under a type of covering, for example:
Sand-cement screeding is not suitable for use under abrasive traffic or heavy point loads. Concrete toppings are recommended for use under abrasive conditions.
Table 1 of SABS 0109-2:1992 “Finishes to concrete floors”, gives detailed recommendations on the intricacies of screed flooring.
What is not generally realised is that, in terms of material costs, a 30 MPa concrete topping is often cheaper than sand-cement screeding.
Traditionally, sand-cement mixes have always been specified in terms of mix proportions by volume and not by performance. This is still almost universal practice in South Africa.
SABS 0109 specifies mix proportions for sand-cement screed flooring as being one 50kg bag of cement to 130 litres of sand measured damp and loose. The reason for this is that sands bulk appreciably when damp and serious inconsistencies will occur if the sand is ‘batched’ by volume without due regard to its moisture content.
For example the mass of 130 litres of dry sand is about 210 kg, while the mass of the sand component of 130 litres of damp sand is about 150 kg. This Code of Practice (SABS 0109) gives detailed recommendations for finishes for concrete floors and is well worth consulting.
The following cements are suitable for use in sand-cement screed flooring:
- CEM I
- CEM IIA
- CEM IIB
- CEM IIIA
Strength grade should be 32,5N MPa or higher, bearing in mind that the lower grade cements have lower early strengths.
The cements are listed above in order of increasing sensitivity to curing.
Chemical Admixtures And Additives:
Generally speaking, admixtures are not commonly used in screeding. Sometimes bonding aids, or water-proofing agents, or pigments are used.
The use of pigments is becoming more common and it is strongly recommended that the manufacturer’s instructions are closely followed.
Bonding aids must also be used in accordance with the manufacturer’s instructions.
Sand should be a well graded concrete sand of average to low water requirement.
Plaster sands should not be used as they tend to have higher water requirements - the higher the water requirement the weaker the mix and the greater the drying shrinkage.
Mix proportions in the literature vary from 100 to 130 litres of damp sand per 50 kg bag of cement. As mentioned above, SABS 0190 recommends 130 litres of sand.
Sufficient water should be added to make a plastic, workable, cohesive mix – a little drier than mortar or plaster (drier mixes may be used if mechanical compacting equipment is used).
Poor site practice is the cause of many screeding problems and should be avoided to ensure project endurance and quality. Some of these practices are:
- Incorrect use of bonding aids
- Poor surface preparation, dirty concrete
- Making the mix too dry and not being able to compact the screed fully
- Floating a cement-water slurry into the surface of the screed to “improve” the finish
- Floating neat cement powder into the surface to dry it and “improve” the finish
- Mixing too much screed mix at one time. Screed mix should be used within an hour of mixing
- Inaccurate and inconsistent batching
Testing Floor Screeds
Because of the problems experienced with sand-cement screed flooring, a unique test method was developed by the Building Research Establishment (BRE) in the UK.
They developed the “BRE Screed Tester” which is a penetrometer type device where a 4-kg mass is dropped 4 times at the same spot from a height of 1 metre on to a circular foot piece.
The penetration of the foot piece into the screed is measured and compared to various acceptance limits.