How To Mix Screed
I’m Andy Parkin, Managing Director of the Multi-Award Winning Speed Screed. I’m here today to talk about the subject of how to mix screed.
We are often asked “how to mix screed” how do you mix it? What goes in? What should we do?
Well, there are many types of screed on the market today and it would take a long time to run through every single one. I will concentrate on the UK’s largest selling screed, and that would be a 1:4 fibre screed as defined by the British Standard.
What can you actually mix it in?
I think that is the question first when looking at how to mix screed, have you got to have the right equipment. The following are big “no no’s”
- You can’t mix it by hand. The British Standard doesn’t recommend that you should be mixing it with a shovel on a piece of board and turning the material over. That s not going to give you a sufficient mix. It’s not going to give you good distribution of the materials, cement, fibres, water, etc.
- A free fall mixer, again, this is not going to be a solution for you because that, with it being a semi-dry mix, it ends up balling and you’re not getting the product mixed correctly because you end up with balls of screed.
So what you actually need is a forced action mixer, a paddle mixer (not a the two-handed whisk type). You can get electric/diesel versions or you could use a purpose built screed pump, which mixes in the vessel and then when mixed, it then pumps to area where the screed is then laid.
The mix ratio is one part cement to four parts sand and that is by weight. We start by adding half of the sand into the forced action mixer. The sand should conform to the correct grading, as defined by the British Standard. Sand with high lignite/silt or contaminants that may affect the performance and should not be used. Using such sand will reduce the strength of the product, and lead to possible failures.
Once we have half the sand in the mixer, we then look at putting the correct ratio of cement in. If for example, you placed a 25 kg bag of cement, you would therefore need 100 kgs of sand to give you that one to four ratio.
Water is added next, you need to check the moisture of the sand. In the winter, the sand is quite moist, with a high moisture content. In the summer it is normally fairly dry unless it just comes directly from the washing plant. You need to take a view as to how much water is already in the sand and then the additional water that you are going to add. What you don’t want to do is not put sufficient water in, and you end up with difficulties in placement and also the cement doesn’t get hydrated because of lack of moisture. The converse of this is putting in too much water which will leave the screed too wet to lay, and reduce the strength of the screed.
You would then also look at adding polypropylene fibres, sometimes termed as PP fibres. The PP fibres help to minimise shrinkage cracking. Anything that contains water, as it dries, it shrinks. A typical dosage would be 900 grams of fibre per cubic metre of screed. The fibres are actually added to the water, mixed well and then both added to the mix.
If you have any add mixtures to go in they should be add at this stage with the fibres and water. It could be a plasticiser, it could be something to make the screed gain strength quicker, then that is something you would add at this stage.
The remaining balance of the sand is then added on top of the mix, and then the mixing actually starts. The mixing needs to be thorough, with the cement, sand, water, fibres, etc. being well-distributed. You don’t want any soft spots with pockets of sand etc.
Once that is completed, then the screed is good to go. Then you’re able to pump it in place or to place it into the wheelbarrow and take it directly to where it’s needed.
I hope we have answered some of the questions when asking “how to mix screed”. If you have any further questions, just please get in touch and we would be happy to answer them.