Ground bearing floor slabs
Types of floor
While ground bearing slabs are essentially similar, their method of construction varies according to the likely end use and the accuracy of building required. On one hand there are what may be termed conventional slabs with steel reinforcement mesh. On the other there are fibre (usually steel) reinforced slabs designed to reduce the need for movement control joints.
Concrete floors behave in similar ways in that they are all subject to drying shrinkage. If this is not provided for during construction, the slab would crack in a random fashion. By introducing fabric reinforcement or steel fibre, cracking can be controlled and, while not entirely eliminated, at least made to occur in set locations.
The following table summarises the principal types of ground bearing floor construction.
| Type | Characteristic |
| Large area jointed floor | Fixed formers at 50m centres (typically). Usually mesh reinforced and subdivided into 6m bays by the cutting of sawn induced joints. Shrinkage at perimeters or formed joints usually in the order of 4-5mm. |
| Large area jointless floor | As above, but steel fibres used to control shrinkage so no sawn joints. Shrinkage at perimeters or formed joints usually in the order of 20mm. |
| Wide bay floor | Same as large area, but greater accuracy achieved by placing side rails at 12-15m centres. |
| Long strip floor | As large area jointed, but rails placed at 6m centres to obtain greater accuracy. Bays can be cast hit-and-miss or sequentially if permanent rails are used. Sawn induced joints at 6m centres. |
| Two layer floor | Two layers of concrete slab laid one over the other. Side rails are fixed to the lowest slab before placing the second slab. This method achieves the greatest accuracy. |
Movement control joints can be either formed or induced. A formed joint is a distinct break in a slab usually created by a pair of metal plates, while a sawn joint is usually cut to a depth of 25% of the floor thickness and filled with soft sealant. Upon contraction the floor should crack along the line of the saw cut. For the sealant to operate correctly it should be placed against a polyethylene backing rod - this ensures that only the facing sides of the cut are bonded to the sealant.
Since most shrinkage takes place during the first 12 months or so of the life of the building, any sealant placed in the induced joint will need to be flexible enough to accommodate the expected strains and hard enough to prevent the shoulders of the cut breaking down under wheeled traffic. It is normal to use sealant of shore hardness below 30 during the defects liability period and then to specify its removal and replacement with a material of shore hardness 60 or so after about 12 months.