Ground bearing floor slabs

Cracking in concrete is a complex subject and often results from the effects of various faults acting in combination. Leaving aside cracks resulting from structural faults, cracking in floor slabs is generally the result of plastic shrinkage, early or long term drying shrinkage, chemical effects (such as ASR) or corrosion.

Mechanism of bleeding in a concrete floor slab

Second stage of bleeding, plastic shrinkage cracks develop

Plastic shrinkage cracking is rare in power floated floor slabs as the action of mechanical trowelling recompacts the surface and eliminates the cracking.

Cracking in floor slabs is most usually the result of restrained shrinkage of a slab. Early loading may pin the slab to the sub-base and features such as columns, inspection chambers or re-entrant corners can create conditions of restraint that permit the tensile stress to exceed the tensile strength of the concrete. This is not to say that structural cracking does not occur; overloading or ground movement could cause localised failure, although the pattern of cracking will usually suggest that something serious is amiss as opposed to straightforward shrinkage cracking.

Plastic shrinkage cracking occurs within a few hours of placing the concrete and is usually associated with high rates of evaporation. Similarly, plastic settlement often occurs when there are high levels of bleeding and some form of restraint to the settlement of particles, for example reinforcement. Plastic settlement and cracking can only occur while the concrete is still plastic, i.e. before it sets. If the rate of evaporation exceeds the rate at which bleed water rises to the surface, shrinkage will occur, inviting the risk of cracking.

The nature of different crack patterns

To avoid problems of rapid evaporation it is usually preferable to protect a new slab from the drying effects of wind and sun, therefore placement after completion of the roof and walls is desirable. Proper curing is essential, so polythene sheeting and/or a suitable spray on membrane must be used. However, while proper curing is essential for the correct performance of a concrete slab, the curing process does not affect contraction; it merely improves the tensile strain capacity of the concrete. The best means of minimising shrinkage is to keep the water and cement contents as low as possible.

Cracking as a result of drying shrinkage usually takes place within several months of construction and for up to 3-4 years. The rate depends on relative humidity. When concrete is mixed, more water than is needed for the chemical reactions is added to make the mix workable. This residual water is held in capillary pores and gradually evaporates from the surface. Removal of water is accompanied by a reduction in volume, hence shrinkage of the concrete. If the concrete is restrained in some way, cracking will occur; this is usually at right angles to the direction of the restraint. For ground bearing slabs, the cracks are most likely to occur when the length of the panel is much greater than its width; the crack running parallel with the short side and through the centre of the panel or sometimes diagonally at corners.

Shrinkage cracking will usually stabilise, but damage to shoulders of the crack could occur and when it is thought that cracking has become dormant, the cracks can be ground out and filled with a suitable epoxy sealant. If shoulder or arris cracking occurs before the anticipated movement has ceased, a more flexible sealant may be desirable. If the matter is not attended to, deterioration can be quite rapid.

Crazing

Crazing of a power floated surface is a common characteristic of this type of floor and is not normally detrimental to use and serviceability. Crazing is characterised by a map pattern of cracks around 0.1mm in width and extending only a few millimetres into the depth of the slab. The individually linked map 'polygons' are around 50-75mm across.

Crazing will occur wherever there is a change in the properties of concrete close to its surface. The effect of power floating can, if overworked, lead to a thin layer of laitance (a dilute mix of cement paste and fine aggregate particles). The surface tends to dry out quicker than the main body of the slab thus imparting tensile forces in the surface. These forces are relieved by cracking.

Curling

Curling of floor slabs is fairly common and usually occurs at joint positions. The problem is due to different rates of shrinkage between the top and bottom surfaces of the slab. Since the top surface tends to dry more rapidly, it shrinks further; because the bottom is wetter, the slab curls. With induced joints a certain amount of restraint is offered by aggregate interlock, or special sliding reinforced joints if these have been specified. In less severe cases of curling, the raised sections can be ground down (otherwise shoulder damage may occur under wheeled traffic). In more severe cases, grouting under the slab may be needed to restore strength and prevent the floor cracking or rocking.

Deterioration of shoulders to induced joints possibly due to slight curling of the adjacent slabs and joint sealant being too soft, offering insufficient support.

Damage to edge of an induced joint following from curling or raised edges pulled up during the sawing process.

Surface defects

Occasionally, particles of organic material or other contaminants may find their way into aggregates. If near to the surface, the soft particles can break down leading to localised pockets of spalling. Such defects can be cored out and filled with a suitable resin. Similarly, small particles of aggregate can rise to the surface during placing and their subsequent disturbance under traffic can lead to small pockmarks in the surface. These are usually not a problem, although if they are bad enough to affect serviceability, the defective areas can be drilled out and filled.

Organic material (part of an acorn) and the pocket from which it was extracted - typical example of localised damage.

Abrasion resistance

Abrasion resistance of a slab is important and in an area that is in heavy use, severe scratching, scuffing or wear can lead to problems. In aisles, a lack of abrasion resistance can lead to the gradual wearing of shallow grooves into the surface which can affect serviceability and flatness. Severe wear is usually self-evident, but if there is any doubt, an accelerated wearing test is set out in BS 8204-2:2002. The test involves a rotating steel wheeled plate that is held in position by a fixed clamp. The machine is operated for a fixed period of time and the difference in level measured and compared with BS 8204, which sets out various categories of wear according to use. Factors affecting abrasion resistance include the concrete strength, water cement ratio, method of finishing and curing.

Abrasion test in progress

Floor following abrasion test. In this case, the results showed a very good level of abrasion resistance - in excess of the specification. This is surprising given the extent of scuffing and striations apparent on the floor surface

Steel fibres can be found in the surface of a floor and occasionally they may stick proud. Generally, fibres that are exposed in this way are not a major problem, but any that do protrude can be ground or cut away fairly easily. The incidence of fibre exposure is generally less when dry shake finishing methods have been employed. Steel fibres tend to reduce the amount of bleed water as their high surface area impedes the settlement of aggregate particles and thus the displacement of water.

Chemical effects on floors

Aside from the risks associated with sulphate attack, concrete industrial ground floors can be affected by a variety of chemicals contained within common products. Beer, cider, sour milk and sugar, for example, all cause gradual disintegration of concrete, while damp coal can cause rapid deterioration in damp conditions on account of soluble sulphides producing sulphuric acid. Further details of potential harmful chemicals are given in Technical Report 54 published by the Concrete Society.