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Liquid Polymer Admix v Engineered Screed

A question we get asked frequently is whether it is better to use a polymer based liquid admix in the screed or to use a fully engineered screed.


Our Pump Trucks have the ability to either dose a liquid admix into a traditional sand cement screed or mix a fully engineered screed. Both are possible, but we have a clear preference for and strongly recommend a fully engineered screed, every time.


What's the objective?

In general, liquid additives or engineered screeds both seek to achieve a number of benefits over traditional sand cement screeds including:


  • Higher compressive strength - normally >25 MPa

  • Higher flexural strength - normally >3-5MPa

  • Faster drying time

  • Reduced shrinkage or risk of cracking


All of these are great objectives for a screed, so what is the problem?


Engineered Screeds - the gold standard

In our experience, there are very few problems with engineered screeds and many advantages.


All the special additives are already blended in with the cementitious powder binder in the correct proportions. A lot of the risk is taken out of the process and all that we have to do is mix to the correct ratio and water content.


We find that will fully engineered screeds, there is a high degree of consistency and very rarely unexpected installation problems.


Liquid Admixes - not our recommended approach

Liquid admixes can also deliver the benefits of engineered screeds, however we see a much higher incident rate. In particular, the problems that we see with liquid admix dosing into screeds includes:


  • a higher incidence of delamination or drummy screed

  • sticky screeds that are hard to work with

  • screeds drying too fast

  • incorrect admix dosing rates


So why does this happen?


If you look at the technical data sheet for most liquid admix products, they will normally specify 2 things which are problematic. The first is a very high sand cement ratio, often 3 parts sand to 1 part cement. The second is a very high dose rate for the liquid admix.


High cement dose requirement

The high cement dose rate is problematic in the first instance. A 3:1 sand cement ratio will dry fast, be hard to work with an have a higher risk of delamination as the screed is at risk of drying faster than the adhesive. The admix guidance normally specifies such a strong cement mix in order to make sure that the screed achieves a high strength, but this comes with the cost of the problems mentioned above.


Compare this with a product such as Mapei Topcem, an excellent engineered screed that we routinely mix as part of our ScreedMax Pro range. Topcem is mixed at 6:1 which means that immediately, you are starting with a product that is less sticky and therefore easier to work with and is not subject to overly rapid drying.


A 3:1 cement dosage rate is very strong and that is before any of the liquid admix has been dosed into the screed


High liquid admix dose rate

In addition to the high cement dose rates, the technical guidance for most liquid admix products requires an admix dosing rate of 1 part admix for every 3 parts water (or similar - each product is different). So if a cubic meter of screed would use approximately 100L of water (about 10% moisture content at the time of mixing is typical), this means that 30L of water is replaced with 30L of admix.


Firstly, this is a lot of admix to put into a screed (which is expensive) but because water has been replaced with polymer the screed often becomes very sticky, hard to work with and because of the reduced water content, dries very fast - which can dramatically increase the risk of delamination.


The screed itself will be rock hard, but the trade offs include:

  • Sticky screed that is hard to work with

  • Strong curling and lifting effect as the polymer enriched screed dries

  • Increased risk of the screed delaminating from the substrate as the screed dries and lifts faster than the adhesive dries and forms its bonds


What about lower dosage rates

So what we then see in practice is installers disregard the recommended dosage rates and often will install only a fraction of the recommended liquid admix dose. For example, installers will instead of putting a full litre into a mixer, will put a 100ml cup - 1/10th of the dose.


It then acts more like a placebo. The installer can say with their hand on their heart that they have added a polymer admix and because of the low dosage rate, they won't have the sticky, rapid drying problems, but they won't achieve the compressive or flexural strength and they certainly won't be able to rely on the product warranty.


Ah, good point - so what about warranties?

An engineered screed is fully guaranteed. So long as its mixed correctly and well installed it comes with the peace of mind of a comprehensive guarantee.


But with an admix, whilst the admix product itself would be warranted by the manufacturer, as they can't control what cement it is used with, in what dosages, if something goes wrong there are a lot of variables and lots of areas for things to go wrong before the admix supplier will accept responsibility.


Conclusion

We try to avoid liquid polymer admix dosing into our screeds. We will always try to direct and encourage our clients to shift to a fully engineered screed wherever possible.




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