The logistical and economic advantages presented by pre fabricated slabs have made pre-cast concrete planks a convenient choice in floor construction. However, there are certain downsides to these pre-fabricated concrete planks which make it vital to carry out a level survey and visual inspection of the installation condition of the planks on site, well in advance of starting the installation of screed, insulations or acoustic layers.
The pre-cast planks are manufactured off-site and designed to provide a quick structural flooring process, allowing projects to progress rapidly. They are manufactured at concrete plants based on a set of design drawings which span the void layouts of brick, block and steel supports.
The off -site methodology removes the need for structural decking, shoring and time elapse waiting for the curing of the concrete, giving logistical advantages at the construction site. The pre-cast planks are generally delivered to site by artic lorry and the flooring is lifted into place by on-site towers or mobile cranes.
However, the advantage presented by concrete planks comes with another set of problems that require consideration. Pre-cast planks are pre-stressed with cambers that do not always follow the same curvature. The effect is a stepping in the floor surface which is not suitable for the installation of insulations and acoustic layers.
Under such circumstances, a sacrificial levelling layer may be required to rectify this difference. It is recommended that a full level survey is carried out to ensure there is an adequate zone to encapsulate the entire flooring. It is important to evaluate the flooring surface to estimate the amount of pre-levelling required. This helps to ensure that insulations and/or acoustic layers can sit flat on the base. Voids in the layer can result in failure problems in the future as the screed and insulation could give way under load.
The detailed information regarding this is critical to the floor screeding contractor so the correct price can be submitted to allow for minimum, maximum and nominal (average) screed depths. This also gives them an insight into the amount of pre-levelling, if required. A design drawing that has a section showing 100mm maximum and 55mm minimum can be very different in reality. This is often the case when the nominal depth is not included in the M10 specification.
We have found that pre-cast concrete planks can have a huge variance in levels. There have been instances where the camber has been seen to be the other way round and is sometimes lower in the centre of the length.
A recent experience demonstrated that approximately 30% extra screed was required on a project to achieve datum levels and the client had not allowed for such variance in the construction budget. We were under pressure by the on site project manager to get the job done without any extension of time and to discuss the depth at the end of the project. The programme did not allow for any time to reconsider the options and progress had to be made. CSC Screeding Ltd provided the support to our client and completed the work on time. We had three teams of screeders and pumps on site, installing approximately sixty tonnes of screed every day for nine weeks.
Please see the photographs below of some projects recently found to have considerable variance.
A view from the underside
Steps in the floor surface require a sacrificial levelling process
Seriously out of level!