LIGHTWEIGHT SCREED

In LAF’s case lightweight concrete toppings are predominantly blends of vermiculite aggregates mixed with cements of various grades (depending on the use). The mixes may be used for floor or roof toppings and have great benefits in terms of strength, durability, impact resistance and low weight. Vermiculite grades vary from Coarse to Superfine. In some cases the vermiculite can also be of a waterproof type.

The most appreciated properties for LAF’s concrete toppings are:

  • Lower weight per m²;
  • No spalling under direct impact;
  • High bond and cohesive strength;
  • Non-combustible and very low thermal conductivity;
  • Reduced impact sound transmission - especially impact sound
  • Low water absorption (in some cases can be waterproofed);

 

Light Weight Screed Thickness

The lightweight concrete must be made to bond to the substrate, in order to avoid ‘curling’ which can lead to drummy panels or even break up. The minimum recommended thickness is 50mm, however if lower thickness (down to 20mm) is required we recommend the of Epoxy bonding agents. Alternatively the concrete surface should be scabbled removing a thin concrete layer to improve bond strength. Substrate preparation is important for bonded toppings. The substrate must be clean of oils and other contaminants. If the existing concrete is very smooth, it will require to be scabbled and prior placing the LWC, the surface should be dampened with water to reduce dry-outs.

 

Light Weight Screed Density

The lightweight concrete has densities varying from 600 kg/m³ to 1600 kg/m³ and the type of the lightweight concrete required depends upon the application requirements such as load bearing capacity required, presence of point loads, exposure conditions, surface Finish (Tiles, Carpet, Vinyl Flooring) etc. Limitations on imposed dead loads will determine the density of the LWC to be used. The most common density for floor toppings is 1,200 kg/m3. At this density, the point load resistance of the Lightweight Concrete is sufficient for most commercial and residential applications.

Often, to avoid the need for strengthening an existing floor or roof, densities as low as 600 kg/m3 to 800 kg/m3 may be required. In such cases, closer attention must be given to the choice of surface covering as point load resistance of the concrete is lower. Coverings such as ceramic or terracotta tiles are ideal and they will distribute point loads sufficiently without damaging the Lightweight Concrete.

 

Surface Finish

Because of the nature of Lightweight Concrete, the surface finish is not as smooth as for standard concrete. Quality finished are easily achieved on high density Lightweight Concrete mixes. Due to the presence of vermiculite, the lightweight concrete surface will have lower abrasion resistance than standard concrete. Consequently, such surfaces should not be used as a hard wearing surface and additional topping may have to be employed. In special cases coverings such as carpet, vinyl, tiles or liquid applied wearing coatings might be suitable. For densities around 1,200kg/m3 the Lightweight Concrete has sufficient mortar content to give a close and dense finish. Any surface irregularities may be removed with a light grinding.

For lower densities such as 800 kg/m3 or less, there is insufficient mortar in the mix to obtain a closed surface. Such surfaces are perfectly adequate for ceramic or terracotta tiles in a mortar bed, but other coverings may require that the Lightweight Concrete is topped with a sand/cement mortar or a proprietary topping material, generally at least 5 to a maximum of 10 mm thick.

 

Point Load requirement

As per table 3.1 - AS1170 (References values of imposed floor actions), LAF have developed many types of lightweight floor screeds with different range of application, density and point load capacity.