Sub Base
Summary
Sub Base - A sub-base is the aggregate layer directly beneath the concrete floor slab, resting on the subgrade. It acts as a working platform on which the slab is constructed.
Technical Information
A sub-base is constructed from either a stable, well graded granular aggregate or a bound material; such as a hydraulically bound mixture (HBM). It is very important to check fill materials will not potentially expand or react with lime and cement. There is generally no requirement for materials to be frost resistant.
The sub-base is a component of the slab design, just like the concrete floor, so is designed with the working loads taken into consideration (from the base specification) and it won’t allow any movement in the base once compacted.
Sub-bases must be designed in accordance with the appropriate specification and surveyed prior to any concrete being laid to meet the maximum tolerances specified in TR34. Any localised rutting / deformation from plant during construction of the slab must be remediated prior to concrete being poured over. This requires careful and ongoing management during the pour.
The sub-base has three main purposes, as follows:
- To provide a level formation for the construction of the floor slab.
- To provide a firm working platform for construction activity.
- To transfer the loads from the floor slab to the subgrade (in ground supported slabs).
Any trimming of the surface should leave the sub-base homogeneous and well-compacted and cannot make up for deficiencies in the sub-base construction (ridges, cracks, loose material, potholes, ruts etc.). Sand or stone fines may be used solely for closing the surface.
In the UK, if granular material is used for the sub-base it should generally have a minimum thickness of around 150mm; going shallower if finer fillers are used.
Related Definitions
Recycled Aggregate , Shear Stress , Slump Test , Fabric Mesh Reinforcement , Compressive and Flexural StrengthResources
Technical Report 34 – concrete industrial ground floors: A guide to design and constructionHIGHWAYS AGENCY Manual of Contract documents for Highways Work. Volume 1, Specification for Highway Works, Series 800, Road pavements - Unbound materials. The stationary office, London., 2009
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