A screed is the foundation on which your final floor system is based and is therefore critical to the longevity of your floor. Often large amounts of money are spent on expensive final finishes, with little thought given to the screed.
A weak or poor screed can cause damage and result in the floor having to be replaced. From vinyl and LVTs to epoxies, polyurethane floors, overlays and self-levelling compounds, the screed forms the base of many floor coverings prior to the placement of the floor, which is why it is important to get it right.
A poor screed can result in cracks or even worse, the debonding of the screed from the concrete substrate causing partial or complete damage to the floor. “Poor screeds are generally the result of poor mix design, poor application or just poor contractors. Problems generally show up soon after the screed has been laid, but it can sometimes develop over a period of time leading to a slow deterioration of the bond between the screed and the concrete floor,” says Wayne Smithers from Sika.
Properly laid screeds should outlive the building
Firstly, one needs to determine what type of screed is being laid: a sand and cement floor screed, or a bonded/unbonded screed on a new or existing slab. While floor screeds are unseen, they are a vital part of the floor.
“Whether a screed is sand, cement, bonded or unbonded, if it is correctly applied it should outlive the building. However, if it is applied incorrectly, it can cause the entire floor to fail resulting in the need for extensive repairs and delayed project completion dates,” says Chad Tosen, Flooring specialist at Mapei SA.
A flowing screed is not only responsible for levelling out concrete, it also serves as a retaining system for additional flooring components such as overlays or self-levellers, alternatively resin and polyurethane floor coating applications.
Best practices for pouring screeds on site
Placing the screed
• Place the screed as close as possible to the end position
• Place it into corners, along the edges
• Watch out for segregation: do not throw shovelfuls – rake it into position
• Check for grout leakage
Compact the screed
• Compact the screed until no more air bubbles rise to the surface. 1% air in concrete reduces strength by 5 – 6%
• Vibrate in a pattern with a poker vibrator every 100 – 150mm. Push the poker in with the previous layer. Slowly extract or pull it out, allowing the vibrator to close the hole.
• Use a beam vibrator for floors and screeds. Use a poker vibrator to give sides extra vibration.
• Level the surface as soon as possible after compaction.
Top tips for a successful screed
Prepare the surface
An installation may be compromised as a result of failure or debonding occurring within the subsurface. Start by confirming that the substrate is of significant strength, integrally sound, with no crumbling or cracking, and of a quality and consistency suitable for installation of a screed.
Surface contamination can also lead to flooring failure. It is therefore very important that the substrate be clean, dry and free from all traces of curing agents, surface laitance and contamination before installation of the screed compound. Bituminous compounds must be removed completely, as should organic or fungal growth and the spores killed using an effective fungicide.
Correct surface preparation can never be over-emphasised. An under- or overlay installed over a properly prepared surface will deliver a successful result. Surface preparation includes conducting a floor survey; using a suitable vapour barrier where moisture levels are too high within the substrate, or where time does not allow for a full cure of the substrate due to deadlines on site; and correctly applying priming system for surfaces that are friable, or where the surface is dense and impervious i.e. when applying an underlayment over a power-floated surface or over existing tiles.
Choose the right product
Choosing the correct product is key. The type of underlayment needed will depend on the type of flooring that will be installed on top of it.
“For both underlays and decorative screeds (overlays), factors such as the required application thickness, service and traffic conditions of the installed floor, curing times, etc must be taken into careful consideration,” says Sharon Margon from TAL.
Choose the right bonding material
“During installation of a bonded screed, where a bonding agent is used between the existing substrate and the new screed, correct selection of bonding material lowers the risk of the screed pulling away from the substrate which results in lifting or curling. A cementitious mix that is high in polymer creates a stronger bond over latex based liquid bonding agents,” says Janet van Zyl from Samson Technologies.
Use good contractors
It is recommended that you use a reputable contractor to lay your screed. Good contractors may cost you more but they are going to do a good job and if things go wrong, they are more likely to come back and sort out the problems.
Proper mix design
It is important that good well graded aggregates be used in the mix to ensure that the mix is not susceptible to cracking. A good ratio between cement, aggregates and water will give you a good mix that is easy to lay and compact into a dense, crack free floor finish. There are additives on the market that help to produce good hard screeds, so it is advisable to discuss this with the manufacturer and obtain advice from their technical representative.
Whether placing bonded or unbonded screeds, it is important to try to eliminate unforeseen mishaps and minimise inconsistencies in the screeds. This can be achieved through the correct screed design or by making use of pre-blended screeds which are shrinkage controlled, have well-graded aggregates as well as a special hydraulic binders base.
Introduce saw cuts into the screeds
“Once the screed is laid, it is important to introduce saw cuts into the screed at specific intervals within the specified time frame to allow for shrinkage to take place, and not produce unsightly cracks in a random fashion. Once the screed has cured, these saw cuts have fulfilled their purpose and can be covered,” says Mark Teasdale from iTe Products.
Use the correct admixtures
Elrene Smuts from Chryso SA says that screed strength can be improved and cracking can be reduced by using the right products during the installation.
“Ready-to-use shrinkage reducing admixtures should be used for concrete and mortars because they reduce the capillary tension, therefore impacting the most important mechanism at the origin of the shrinkage. Ready-to-use aqueous phase curing agents should be used on fresh concrete, mortars, other cement based coatings and freshly demoulded concrete. This prevents concrete and mortar from drying out too quickly, which enables a more complete hydration of the cement particles and limits early age cracking and drying shrinkage. Synthetic macro-fibres can be used for structural reinforcement,” says Elrene.
Choose a high-performance self-levelling compound
Installing a fit-for-purpose levelling compound to rectify variances in floor levels prior to tiling will not only reduce costs but can also minimise the risk of an installation failure where tile adhesives are “packed”, or built-up, to compensate for uneven floor levels.
Flooring contractors can also make use of ultra-fast drying self-levelling compounds, ranging from 1 – 40mm with low levels of emissions, organic compounds or VOCs (volatile organic compounds). Such self-levelling systems can be used on substrates, preparing them to receive any kind of flooring system where resistance to load and traffic is needed.
All construction, cold, structural and voided joints in the substrate, as well as structural cracks that are or could become dynamic before and/or after the screed installation, must be extended through to the surface and filled with a suitable product to prevent these joints from being filled with the screed compound. For existing voided joints in the substrate, the full width of the joint should be maintained and extended through the underlayment layer to the surface. These joints need to be aligned with precision.
Vinyl sheeting, laminate flooring systems, stick down carpets and concrete decorative overlays should only be installed over a perfectly smooth and level underlayment. Any surface imperfections in the screeded floor will not only detract from the aesthetic appearance of the final floor covering, it can also affect the longevity of the installation.
Full thanks and acknowledgement are given to za.chryso.com, www.iteproducts.co.za, www.mapei.co.za, www.samson.co.za, www.sika.co.za and www.tal.co.za for some of the information contained in this article.
Main image: TAL
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