How do you mix a strong screed?
Screed is fundamentally a simple product. Sand, cement powder and water mixed together to make a damp product the consistency of the perfect sand-castle building sand. Damp enough to hold its shape but not too wet to make it sloppy. Admixes can add further complexity, but fundamentally, screed is a simple product.
So why is it so hard to get good quality, strong screed?
A bit like poaching an egg, sometimes what seems simple and should be simple, can be very hard to get perfect, and when its not perfect, its bad with little margin for error.
At ScreedPro after many years of mixing screed, we have identified the key ingredients for getting a perfect mix every time:
1. Good quality sand
We regularly refer to ourselves as "sand nerds". Australia has a huge supply and variety of sands, but not all sands are made equal. Sand provides the filler and light aggregate that is the body and strength of the screed. Too fine, too much clay, particle shape too round or too uniform in shape and the screed will not be strong enough. We spend a lot of time analyzing sands to find the perfect particle distribution and particle shape and clay ratio to deliver the strongest possible screed. A good sand makes all the difference to not only how strong the screed sets but also to how it feels under the trowel, how well it compacts and how easy it is to work with. A good quality sand speeds up your job and gives you a better quality finish and stronger screed.
2. Good quality cement
Not all cement powders are made equal. We have tested many suppliers across Australia and found that cement performance differs not only from supplier to supplier but also from plant to plant. If you are working with a slightly lower quality cement (which you only know from experience and lab testing) you are working with one hand tied behind your back. If your cement is not fresh it also directly impacts performance. Old pallets of cement that have been sitting around slowly absorbing moisture from the atmosphere should not be used for making screed.
3. Strong mechanical mixing
Poor mixing is one of the hallmarks of low strength, traditional screeds and the causes of many failures. Mixing screed on the slab by shovel or in a cement mixer will never result in a strong screed. Mixing in a horizontal planitary screed mixer is better but still doesn't guarantee a perfect mix.
ScreedPro's pump trucks use a number of elements to get a very strong and comprehensive mix including:
vertical paddle mixing that tumbles the mix whilst the paddles break up any dry or wet patches
a high shear speed that cuts through the screed rather than just moving it around
pumping through our hoses which further mixes the screed as it is delivered to the floor which continues the mixing process right up to the point of installation
4. Immediate installation
Time is the enemy of screed. As it is a dry/damp mix product as soon as it is mixed, it starts to cure. Leaving screed sitting around either in the mixer, in a barrow or on the floor waiting installation does it no favours as it starts to cure. Once screed has started to cure, it can't be compacted properly, form a strong bond or seal over properly. Screed needs to be installed promptly as soon as possible after mixing to ensure a strong product.
5. Water content
Water is an essential element of mixing screed, however too much water and it quickly becomes your enemy. Screed is a dry mix or damp product. It needs to be this consistency so that it can be troweled into the correct falls. However too much water also results in a decreased set strength. It can be very tempting for installers who mix by hand to add extra water when mixing or add extra water once the screed has been tipped on the floor whilst awaiting installation to wet it up or "refresh" the screed and extend its life. However, all the extra water really does is overwater the screed resulting in a lower final set strength.
6. Incorrect use of admixes or additives
Most construction chemical companies make latex based admixes for screed which are advertised as increasing strength. The problem with these products is that in order to be able to achieve the strengths that they advertise, the ratio of admix to water/cement and sand is very high. This makes the products very expensive and also makes the screed very sticky to work with and with a strong curling effect when drying that increases the risk of the screed delaminating.
In our experience, most installers don't strictly follow the dosage recommendations and dose significantly less admix than specified. This saves money and leaves the screed easier to work with and have less risk of going drummy, but the low dosage rate means that the strength won't be achieved. It effectively has a placebo effect. They dose a little bit of admix so they can say, "we dosed XYZ admix" but it has little effect because of the low dose rate and they know that if they use the full dose rate, the screed becomes sticky and unworkable (as well as very expensive) so they don't do that.
In general, we discourage the use of latex based admixes for this reason. They are not the best way to achieve high strength screeds and result in sticky screeds that dry too fast and curl resulting in delamination and a low dose which doesn't have these problems, doesn't make a substantial difference to the ultimate screed strength.
7. Compaction, compaction, compaction
Compaction is the most important element of the screed installation process. It doesn't matter how well the screed is mixed, if it isn't compacted well the sand and cement particles cannot bind together, leaving a screed with air voids, a matrix that is not sufficiently dense, that is not well bonded to the slab underneath and has a low surface strength (resulting in dusty or crumbly surface) and is liable to cracking. Compaction is critical to achieving a good screed result and failure to compact a screed normally results in the screed needing to be replaced.
8. Protection once laid
Once the screed has been laid, it is very important to protect the screed from the elements. If the screed is exposed to wind or sun it can dry too quickly resulting in cracks or a dry/crumbly surface. If it is exposed to rain or water the fine cement particles can wash out of the top surface of the screed also resulting in a dusty or crumbly surface and an overall delayed curing. If heavy equipment operates over the screed this can result in cracking. It is important to protect the screed from the elements and heavy traffic until it is fully cured in order to achieve the best results.