What is screed made of?

I’m Andy Parkin, Managing Director of the Multi-Award Winning Speed Screed. I’m here to talk about what is screed made of.

Screed consists of a number of set ingredients but the mix proportions are varied according to where and how the screed will be used.  The final quality of the screed will depend on certain factors like aggregate quality and how the screed is stored.

The usual standard mix for levelling screed is 1:4:5 of cement, sand and aggregate. Here are the constituent parts and other factors you will need to consider:

Cement

Different types of cement are suitable for different types of screed.  The correct type of cement must be used because this will directly affect the quality and strength of the screed.

Aggregates

The aggregate has a major impact on how the screed looks and how well it performs.  It is vital to make sure the size and shape of the aggregate are appropriate for where the screed will be used.

Aggregates should avoid large amounts of coal, iron pyrites or lignite because these can affect the finished surface of the screed. Here are the aggregates considered most suitable for specific use cases:

  • Ordinary levelling screed – 0.4mm washed sharp sand.
  • Fine concrete screed – mix cement, a single sized aggregate and sand at a ratio of 1:1:3.
  • Heavy-duty fine concrete levelling screed – use aggregates between 6-10mm.

Avoid:

  • Fine sands including brick-laying sands.
  • Crushed rock-fines
  • Any sea-dredged sand containing a large amount of flat shell.

Water

Water can be adjusted according to the mix required, for example, to allow for pumping, pouring, trowelling and compaction.  Water should always be running and potable. Consistency can be perfected by using a ‘Snowball Test’ – screed should be able to be squeezed tightly without losing water.

Fibres

Some screed will be reinforced with polypropylene (PP) fibres which help to reduce cracking but these are not appropriate for use in damp areas.

Admixtures

These are added to the screed or used to replace cement to prevent shrinking or curling, or to help with drying times.  In some cases, there is a need to reduce the ratio of cement to water. All of the types of additives must be used according to the manufacturer’s instructions because they all vary.  It is important to understand how they work and interact before you mix them.

Storage

The cement must be protected from damp so it should always be kept covered and off the ground.  It is a very vulnerable product that must be treated carefully. In cold weather cement and aggregate should be kept in an area that does not fall below freezing.  If the weather is wet you may need to adjust the ratios of aggregate and water in the screed.

Protecting Your Screed

Levelling screed must be protected against damage by other building operations; it is not designed to take heavy wear.  It is also important that the surface of the screed should be at least 5oC for a minimum of 4-5 days.  It should be covered with plastic sheets or tarpaulin laid on a framework above the surface.  This works well for protection against cold, wet and hot weather.

I hope this has been helpful in answering some of the questions you may have when asking “what is screed made of” Please get in touch for further information.

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