The Regs: Geoff Wilkinson discusses hardcore
This week I am discussing hardcore – a subject that is rarely covered in the technical press, but can often cause problems for contractors, and not just if they have tried googling it on the office laptop.
The problems include chemical reaction between the hardcore and the concrete, settlement due to poor compaction, and swelling or consolidation due to changes in water content. Because of these past failures, Approved Document C requires ‘well-compacted hardcore, no greater than 600mm deep, of clean, broken brick or similar inert material, free from materials including water-soluble sulphates in quantities that could damage the concrete’.
The use of oversite concrete and hardcore developed soon after the War as timber was in short supply and builders needed to find an alternative method of forming floors. In the early days of its development, inappropriate materials (typically colliery spoil/slag) were selected, and as a result there were structural failures of the slabs. These problems have for the most part been eradicated.
Readers should be aware that BRE has issued guidance on the selection of materials following the introduction of new European standards on the specification of aggregate materials. The guide BRE digest 522 is divided into two parts – part 1 concentrates on the new standards and part 2 focuses on more recent problems.
These issues arise from the fact that the hardcore will tend to compress with time if it has been inadequately compacted. One reason for this compression is the wetting of previously dry material from groundwater, which can weaken particles in the hardcore.
As a result the floor slab loses support over part or all of its area and settlement may occur, if it is not adequately reinforced. Typically, gaps appear between the floor and skirting board, the slab starts to crack and any embedded services fracture.
Ensure adequate compaction of hardcore material by use of an appropriate specification
Another typical flaw is that the hardcore is specified as the layer on which the thermal insulation, DPM or radon barrier are placed. To prevent damage to this membrane the top of the hardcore must be given a level and smooth finish by ‘blinding’ it with a thin layer of fine sand. In high risk areas ‘full radon protection’ is needed, including a radon sump and a gas extraction system. For the installation to be effective, radon must be able to migrate freely to the sump, ie the hardcore must be gas permeable, but should not contain excessive fines’.
The choice of hardcore requires some consideration. The most often used materials are Type 1 and Type 2, which contain a substantial proportion of fines and may fail to offer sufficient permeability when well compacted. Type 3 may be specified as an alternative, but this is coarser than Type 1 and Type 2 so it is not as readily compacted in 100–150mm placement layers. The digest recommends that Type 1 and Type 2 are specified for the lower layers while the topmost layer uses a crushed rock coarse aggregate, graded 4/20 Gc 85/15 to the new BS EN 13242.
The new standard has complicated methods of grading hardcore so most practices should review the guidance and update their standard specification notes accordingly.
Here is a summary of BRE Digest recommendations:
• Ensure adequate compaction of hardcore material by use of an appropriate specification, including the maximum thickness of layers and the type of compaction plant.
• Total compacted hardcore thickness should generally
be in the range of 100–600 mm. For greater thickness specify a suspended floor or seek expert advice.
• If you use Type 2 as a hardcore material, ensure the supplier declares the density and optimum water content.
• If using Type 1, you can compact the material adequately for use as hardcore without reference to its optimum water content as long as the material is visibly damp but not wet.