Ground bearing floor slabs

Refinements in concreting technology have resulted in an ability to lay large areas of concrete in single pours. However, since the drying of concrete is usually accompanied by shrinkage of the cement paste, some method of movement control is needed to reduce the effects of cracking. There are a variety of placement methods but generally movement will be controlled by the use of formed or induced joints (at about 6m centres) or by the use of steel fibres in the mix. See Types of floor.

Cracking due to early drying shrinkage is common but can be reduced by careful mix design. The cracks that develop are not particularly serious and can often be ignored. See Surface regularity.

Induced joints are formed by cutting the surface of the slab into bays. The saw cuts should penetrate into the surface of the slab by 25% of its depth. Contamination of the joints is usually prevented by applying a soft, flexible sealant and then replacing this with a harder variety after 12 months or so when most of the shrinkage has taken place.

Delamination of power floated floor slabs can create a serious serviceability issue. Sheets of thin surface concrete ranging from a few square metres to several square metres can delaminate and break up under traffic loads.

To be successful, power floating of a floor slab needs to be carried out at the optimum time - usually when no bleed water or sheen is visible on the newly placed concrete and, as a rule of thumb, when the top surface allows an indentation underfoot of around 4-6mm. If the floating operation is carried out when the concrete is too wet, it will simply tear off the surface; too dry and it will not smooth the high spots.

By failing to level off the concrete sufficiently before power floating, the float will tend to draw concrete from high spots into hollow areas. Since a degree of stiffening will have already occurred, the 'new' layer in the hollow area may not bond effectively, creating a plane of weakness causing delamination later.

In certain circumstances, the surface of the concrete may set before the process of bleeding has been completed. If this happens, the presence of trapped bleed water could again give rise to a plane of weakness. Such a problem could even occur prior to power floating. Water trapped below the surface could be disturbed by the action of the paddle blades from the floating machine dragging over the surface, extending the water below the surface and pulling up a blister. The blister may not be noticed, leaving a thin crust of concrete that subsequently breaks down.