Capping Layer
Summary
Capping Layer - A capping layer is installed to reduce the effect of a weak subgrade on the structural performance of a ground supported concrete floor.
Technical Information
A capping layer is laid over a poor quality subgrade, ensuring that it will provide stiffer resistance to the loads from the structure above. It also reduces the chance of differential settlement in the slab by supporting it more homogeneously than an unimproved subgrade, which may contain ‘soft spots’.
Another way in which a capping layer benefits external slabs is that it forms a permeable layer directly beneath, which helps reduce the chances of pumping. It is also much easier to compact a sub-base on a capping layer than it is to compact it on saturated clay, meaning that by installing a capping layer, delays in the construction of the sub-base due to wet weather can be reduced.
The materials most commonly used in a capping layer are crushed rock, hardcore or a hydraulically bound mixture. Because the grading of the material in a capping layer doesn’t need to be particularly tightly controlled, the material can usually be sourced locally. This means that capping layers are usually relatively cheap to install.
They are constructed by:
- Laying a relatively thin layer of material (max. 250mm).
- Thoroughly compacting the layer.
- Repeating until the required thickness has been reached.
Related Definitions
Subgrade , Fabric Mesh Reinforcement , Slip Membrane , Plate Bearing Test , ApronResources
Civil Engineering Portal - What is the purpose of using capping layers in pavement construction?Paving Expert - Sub-grades
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