Liquid Floor Screed

What is flowing (pumped) screed?

Flowing screed is a new generation screeding product. It gives more benefits to specifiers and also increases design flexibility.
A flowscreed floor screed is based on calcium sulfate whereas traditional screeds are based on Portland cement. It offers a great many advantages over traditional screeds

• Speed of application
• Surface finish
• Attainable accuracy of placement and finishing
• Reduction in thickness
• Elimination or substantial reduction of movement joints

Using a super flow screed means a far less labour intensive operation and may generally be laid substantially thinner than conventional materials due to the lack of voids and its high flexural strength. 35mm of flowing screed will replace 75mm of conventional material.

Due to the reduced thickness, and the efficieny of the delivery method, one gang of four men can place easily 1000m2 or more of screed accurately in a day. This is five to ten times more than a screeder could do when working with conventional screed.
Flow screeds are virtually self-compacting and flows easily into place, filling all voids and irregularities. It is precisely levelled by laser to an accuracy of equal to or better than surface regularity SR2 (5mm deviation under a 3m straight edge).
This accuracy can only be achieved by careful preparation beforehand. Although the screed is often referred to as a self-levelling screed this is not really the case. The screed must be placed with a high degree of accuracy to achieve a level surface in accordance with the surface regularity descriptions above. To achieve this requires a large number of markers or tripods placed in the work area to be screeded, so that when the screed is pumped the operative has a clear indication of the amount of screed needed in each area (typically one to two square metres) and can pump the screed accordingly. If left to its own devices a pumped screed would not truly self-level due to the aggregate it contains, as this would heap under the surface and leave other areas considerably weaker.
Compared with traditional sand and cement screeds, shrinkage is minimal and is unlikely to occur. Therefore, curling is entirely eliminated as there is no differential shrinkage. It can be applied in all dry locations and in most situations where a conventional bonded, unbonded, floating or heated cement screed could be used.
With underfloor heating screeds can be assisted in their drying which is something that cannot be done with a traditional semi-dry screed. However, car must be taken to follow the suppliers recommendations regarding floor temperature and time. In some European countries flowing screeds now account for half of all screeds applied.

Liquid Screed in Small Areas - Can I use it?

Lliquid screeds become increasingly cost effective the larger the area to be screeded. Typically an area of 80 square metres or more is required to be a viable project when judged purely in financial terms. This is because the cost of getting the plant to site, and the set-up costs, will be the same whether 80 square metres or 800 square metres of screed are required.
However, there are other considerations that can offset the apparent financial disadvantages of screeding a small area. In particular where access to the site is severely limited, and the delivery of traditional screed would be either impossible or extremely difficult. Some situations that we have encountered include:

• Screeding in a basement of 60 m2 where the only access was via a small lightwell, or by barrowing screed through the house and down a circular staircase.
• Screed for a cottage of 68 m2 that could only be accessed via a footbridge.
• Screed over Lewis Sheets in a loft, where the loft was effectively the fourth floor of 54 m2, the screed was needed over underfloor heating.
• Screeding a basement in a pedestrian area where obtaining permission to permit vehicles in the area would have delayed the project by around 4 weeks.

In all of the above cases trying to convey that amount of sand and cement screed to the work area would have required a disproportionate amount of labour or caused damage to the rest of the property. The apparent higher cost of using liquid screed actually represented a saving when these factors were taken into account.
Screed can be pumped into most locations providing that a concrete mixing truck can reach an area of level ground, where our pump can be sited, and that this is within 100 metres* of the furthest point of the area to be screeded.
* Distances greater than 100 metres can be accomodated by arrangement.

Liquid Screed Application

The best time to screed when using a liquid floor screed is during the early part of the construction. This will give the advantages of:

• More time being available between screeding and covering the floor with the final floor finish
• Easy alignment of electrical sockets and switches to a level datum, namely the floor
• An accurate level base for measurement of all internal joinery such as staircases, skirtings, door linings and all external doors and windows
• Safer working environment

If screeding over underfloor heating loops then the liquid floor screed should be applied as soon as practically possible once the underfloor heating loops have been laid. This will minimise the risk of damage to the floor insulation, underfloor heating pipe fixings, the underfloor heating loops themselves and eliminate the trip hazard to other trades working on site.

Liquid Screed Curing

In order that the necessary steps be taken to minimise excessive water loss in the first 24 hours, any unglazed or missing windows or doors should be temporarily blocked using plastic sheeting or a similar material. However, after 48 hours all windows and doors should be fully opened. Dehumidifiers may be used to force dry the product. Direct sun should always be avoided during early life.

Hardening and Drying of Liquid Screed

When covering a gypsum based screed for curing, it is essential that the material is dry. It is classed as “dry” when it has a moisture content of less than 1% if a permeable covering is to be applied (such as carpet), and less than 0.5% for any other covering.
The fact that reduced screed thickness is achieved by using flowing screed leads to shorter drying times overall. The surface may take light traffic after 1 or 2 days, depending on drying conditions.

Authority for Liquid Screed

The screed complies with the requirements of European Standard prEN CCCC-2, Screed material and floor screeds, Part 2: screed material properties and requirements (draft).

Performance of Liquid Screed

Excellent resistance to impact is provided by flowing screed and it exceeds the requirements of the most exhaustive test value requirement. Its resistance to impact easily exceeds that offered by alternative traditional systems. The screed is self-compacting as it flows into its position, so that voids and poor compaction can become a thing of the past.

Liquid Screed Composition

A calcium sulfate (CaSO4) binder and selected aggregates are precisely weigh batched in the composition of the screed. Admixtures are used in the formulation to enhance the plastic properties in order that ease of placing is aided, as is surface finishing.

Resiliance of Liquid Screed

Abrasion, impact and indentation
Complies with Building Research Test and indentation requirements of BS 8204, and compared with traditional screeds offers good resistance to abrasion and impact.

Shrinkage Performance of Liquid Screed

No drying shrinkage occurs, so movement joints are rarely necessary.

Liquid Screed Fire Protection Rating

Non-combustible as defined by BS 476: Parts 4 and 8.

Effect of Frost on Liquid Screed

Before final strength is achieved suitable precautions should be taken against frost.

Liquid Screed Durability

As calcium sulfate screed is not a wearing course, it requires covering with a suitable surface finish. It is not recommended for use in continuously wet areas, nor should it regularly be wetted. Therefore it should not be used for communal baths or showers, changing / washing areas of sports centres, abattoirs, external yards or similar.

Application of Bonded Floor Covering to Liquid Screed

The screed must be primed in order to prevent drying due to suction when bonded floor coverings are applied directly. This will make the application of adhesive much easier and will result in a more uniform layer, which is much smoother. The appropriate primer to use depends on the chemical make-up of the adhesive. For example, a calcium sulfate based adhesive will require the use of an acrylic, epoxy or similar primer should be used. For cementitious adhesives, the two materials need to be separated by an impermeable layer and so a polymeric sealer must be used.

Liquid Screed Technical Properties

Flow (DIN 1060 test) - 240-260mm
Plastic density - 2060-2130kg/m2
BRE impact test - Less than 2mm
Flexural strength - 4-6N/mm2
Drying shrinkage - Less than 0.02%
Time to light foot traffic - 1 to 2 days
*Drying time - 1 day/mm
Dry density - 1950-2050kg/m2
Fire rating - Non combustible
Thermal expansion coefficient - 0.01mm/mK
PH - 11-12
Setting time - Not less than 3 hours
*at 60% relative humidity +20°C