SUB-BASES FOR CONCRETE
Where concrete is placed closer to finished ground level – for example, for paths, patios and bases – it is vital that the soft top-soil, with no bearing capacity, is excavated and removed and that excavation is continued to a depth where sub-soil of a reasonable bearing capacity can be found. The subsequent reduced level is then brought back up using layers of well-compacted hardcore, thus saving on concrete, but also providing a key load-bearing and distribution layer between the concrete and the sub-soil. Hardcore can be clean, crushed rubble or Type 1 MOT stone and, if properly
placed, prevents settlement of the concrete and distributes loads through to the sub-soil beneath. Failure to excavate down to a satisfactory reduced level and then provide a hardcore sub-base will undermine the strength of the concrete, resulting in cracks and failure when subjected to significant loading.
Fig. 44 T ype 1 MOT stone.
The most commonly used material for constructing a sub-base is graded MOT Type 1 stone. Its correct technical description is ‘Granular sub-base material to Type 1 of the Department of Transport Specification for Highway Works (Clause 803)’. In theory, Type 1 MOT stone is meant to have a maximum particle size of 37.5mm, but typically it is a crushed rock, usually granite or limestone, with a granular structure of 40mm chunks down to dust.
Sub-base material should be laid and compacted in layers, with each layer being thoroughly compacted before the next layer is placed, until the desired overall thickness is achieved. The rule of thumb is that the thickness of each individual layer should be no less than twice the thickness of the largest grain size. On this basis, Type 1 MOT stone should be compacted in layers no less than 75mm thick (in fact, 100mm layers are typical). The idea of this is that ‘point loads’ within the sub-base are avoided because no individual stone can be in direct contact with both upper and lower surfaces at the same time and is, instead, ‘cushioned’ by smaller particles above and/or below. This approach ensures that the layer evenly spreads the loads placed on it and any ‘rocking points’ within it are eliminated. Sub-base materials using particles of a larger size will, of course, require compacting in thicker layers.
Ideally, when forming a sub-base, mechanical compaction in the form of a ‘whacker plate’ or roller should be employed in order to ensure maximum compaction of each hardcore layer. This will result in an extremely hard surface that is ready to receive the concrete on top.
Recommended minimum thicknesses of sub-bases for different applications are shown in the following table. These values assume that excavation down to a satisfactory load-bearing sub-soil has been achieved. A sub-base thicker than the minimum stated may be required where a deeper-excavated reduced level needs to be brought back up, in the event that a satisfactory sub-soil is located at a much lower depth. The minimum thicknesses indicated are, therefore, not to be taken as indicating the maximum amount to be excavated beneath the underside of the proposed layer of concrete, as a sub-base cannot be placed on just any type of sub-soil!
Hardcore Sub-Base Thicknesses
Patios, garden paths etc. Driveways, public footpaths etc. Heavy uses Highways