Types of Screed - Estate Agent's Examination Part 1&2

Q.
Describe screeding the concrete floor. What are the types of screed? How to prepare screed?

A.
There are two types of screed that we can use, either a bonded screed that we apply to hardened concrete or a monolithic screed that we lay just after we have cast the concrete slab, before it cures and hardens. When we talk about the screed being monolithic we mean that it is large and solid and becomes part of the concrete, even though the screed is applied after the concrete has been placed and compacted.

The most usual way to screed floors is to lay the screed later so that it bonds to the hardened concrete, so this is the method we’re going to look at.

Mixing screeds

The most usual mix for a floor screed is a 1:4 (cement:sand) mix. We use a common cement, for example a portland cement and good quality building sand. The rule about water is that it must be good enough to drink. Mix it just as you would mix mortar for bricklaying or plastering.

First mix the dry materials together and then add just enough water to make the mixture pliable. Only mix as much as you can use in a couple of hours because otherwise it will dry out and you will have to discard it. Remember that you must never add water to any cement-based mix if it starts to dry out. This will only weaken the mix.

Screeding

Before you, or the people working for you, can lay the screed on the concrete you need to be sure that the concrete surface is absolutely clean. Dust and dirt on the surface of the concrete can stop the screed from bonding to the concrete and so when it has set and dried the screed will start to chip away.

Before you start screeding, it is a good idea to test how absorbent the slab is by spraying water onto the surface to see if the water gets sucked into the concrete quickly.

If the concrete absorbs the water it means the concrete is very dry. If you screed a very dry concrete surface, the water from the screed mix will get sucked into the concrete and the screed mix will not be as strong as it should be. So if the concrete is dry, spray water on it before you start screeding and then let it dry out. When the surface looks dry we call it surface-dry or touch-dry and you can start screeding.

If the dry concrete didn’t suck up the water, you can work on the dry concrete.

To make sure that the screed will bond to the slab we mix what we call a slush or slurry of equal quantities of common cement and clean building sand together with water. It is important to use the slurry within 30 minutes of mixing it so that it doesn’t dry out.

Spread the slurry over the concrete surface and scrub it into the surface with a hard broom or brush. The idea is to leave a very thin coating of the slurry on the concrete. Work on fairly small areas at a time so that the mixture doesn’t dry too quickly and make sure there aren’t any pools of the mixture left on the surface. Once you have scrubbed the slurry into your concrete surface and it has dried, you can start screeding. Mix the mortar and spread it over the surface of the concrete so that it is 25 mm to 30 mm thick. Work in areas that are no bigger than 3 m x 3 m and use a straightedged length of timber or aluminium to scrape any extra mortar off the surface. Then use a wooden float to smooth the surface. If you want an even smoother surface you can also use a steel trowel.

Ref:
Owner Building at http://www.ownerbuilding.co.za/floors-and-flooring/