What Is A Bonded Screed
I’m Andy Parkin, Managing Director of the Multi-Award-Winning Speed Screed. I’m here today to talk to about what is a bonded screed, and what that means.
When a screed is described as bonded, this means that it is connected. It is bonded directly to the substrate, which needs to be sufficiently strong enough to be able to take that bond. So, what type of substrate? What is a substrate?
Generally, a substrate will be the structural element of the floor. When talking about the structural substrate, we are talking about in-situ concrete. Concrete that has been poured in-situ so the concrete trucks have arrived, poured into the area, and you are left with structural concrete.
Other systems could be beam and block. The blocks are placed on the beams, leaving a structural element that you could screed onto.
Pre-cast concrete planks are generally hollow core planks manufactured at pre-cast yards, and then shipped on to site, and then manoeuvred into the structure. Screed is then required on top of the planks to give it that final levelling.
One other structural substrate that you might be bonding to is metal decking with your screed on top to give you the final levelling layer.
How Does Screed Bond
How does it actually bond? Now screed naturally, by itself, has a tendency to want to bond to good surfaces so it may be that even if you didn’t use any bonding agents, the screed would naturally bond in some places, and this is described as being semi-bonded, but if you’re looking for a full bond, then a bonding agent will need to be used (sometimes called a primer).
Before starting you have to be satisfied that the surface is able to take not only the bonding agent, but the screed as well. These are some of the things to consider:
- Has the concrete been power floated? If the answer is yes then it is very smooth, and it is going to be very difficult to bond to.
- Is there laitance on the surface? Laitance is the where all the fine material collects on the surface, and can leave a weak finish.
- Surface contamination, oil or grease.
With these scenarios, surface preparation is required.
The top couple of mm of the concrete matrix would need to be ground down to open the surface up, and remove contamination, to allow bonding to take place. This allows it effectively grip the concrete, and then bond with the screed.
Opening up the surface of the matrix could be shot-blasting or grinding. It really depends on what you find, even though the surface may, at first appearances, seem as though it is quite clean, it could contaminated and the surface would need to be prepared, ready to take the bond. If this doesn’t happen, there is a high probability of de-bonding of the screed.
When Do You Need To Bond?
When would we actually use a bonded screed? Theoretically, you could use a bonded screed at any thickness. Generally, you wouldn’t need it at greater thicknesses, and the British standard doesn’t really classify screeds at 75, 85 as technically bonded although they are actually bonded in practise, but you generally look at thinner screeds.
If a screed needs to down to its absolute minimum, then generally, it will need to be bonded, you are looking at thinner screeds. That could be smoothing compounds that are going down from feather edge, or modified screeds, which can go down to 10mm.
The strength of the screed relies on the strength of the substrate so it needs to be a good, sound substrate.
I hope that has been helpful looking at what is a bonded screed. If you have any further questions, please contact us. We love to help!