Ground bearing floor slabs

While failures with applied cementitious screeds are well documented, problems with the delamination of power floated slabs are perhaps less frequently encountered. Indeed, with no applied finish as such it is at first sight difficult to understand why delamination should occur in the first place. However, the problem is not unknown, with areas extending from a few tens of millimetres to several square metres in area and of a thickness ranging from microns to 8mm or thereabouts. For a modern, highly specified high-bay warehouse floor, the consequences of delamination can be severe - not only creating a health and safety hazard due to rocking of fork lift trucks but also causing damage to the trucks themselves. In extreme cases, blisters of concrete can be formed during laying, thus putting the stability and durability of the floor into question.

The problem can affect floors of conventional concrete mixes as well as those constructed from lightweight aggregates such as expanded clay.

Aside from corrosion problems, there are 2 possible mechanisms of delamination failure - both occur during placing.

• The first is due to overlayering - drawing fresh layers of concrete over concrete that has already achieved an initial set.
• The second is a debonding process caused by bleed water.

Overlayering

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 surface is too wet, the floating machine will do no more than tear up the surface. If it is too dry, it will not be able to trim high spots and fill lows effectively. Exposure to sun, wind or rain and temperature and humidity changes during placing can affect the optimum time.

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.

Sometimes a contractor may be tempted to add water and cement powder to the surface of a slab that has already began to stiffen. This is bad practice: although it works up a slurry coat (which enables a measure of smoothing off), it also creates a possible zone of weakness between the main body of the concrete and the slurry, leaving it vulnerable to delamination under traffic or as a result of shrinkage.

Adjacent areas of concrete can sometimes stiffen at different rates, for example, at the perimeter of an area laid at a slightly different time or due to variations in workability. One area may be ready for floating before a neighbouring area, in which case there will be a time interval between floating operations. In some cases the boundary between the two areas can be merged successfully, but if the concrete becomes too hard this may not be possible. So, there is a temptation to float the softer area before it is ready, and thus to draw a layer of softer cement over harder areas.

Overlayering is therefore the process of drawing one layer of concrete over another, with the risk that the bond between the two may not be adequate.

Identification of overlayering:

• The contact surface of the delaminated area may be open in texture but reflect the profile of the underlayer, i.e. it is moulded to suit it.
• The surface of the underlayer will have a grey mortar finish with markings and other characteristics that were present at the time that it was overlayered.
• The surface of the underlayer may have a slight lime bloom.
• In the case of overlayering due to added surface water, the delaminated layer will probably be fine grained and weak.
• With other modes, unless the layer is very thick, it will be finer grained than the main body of the concrete.
• In the case of thicker layers, the delaminated section might be stratified, with a less compacted layer beneath the surface.

Bleeding

The particles of cement and aggregate in freshly mixed concrete are (or should be) distributed evenly through the mix. Once it has been placed, the effects of gravity mean that aggregates gradually settle down until they bridge one another (achieve point to point contact), whilet the smaller cement particles continue to settle between the aggregate particles. The gradual settling displaces water - bleed water - which rises to the surface.

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 burst, releasing some of the water and letting air in.

Investigations by Klieger attempted to create conditions in a laboratory to reproduce the effect - primarily by trying to achieve a surface set by exposure to higher temperatures and drying wind velocities over the surface. Klieger's experiments were unsuccessful, but it is generally recognised that concrete with a propensity to bleed slowly over a long period of time is more susceptible to this type of defect given changing climatic conditions. Low humidity and breezy conditions will cause rapid evaporation and this will result in stiffening if the bleed rate is low. The nature of the aggregate and size of particle can also have an influence on the process. (See Klieger, P., Effect of Atmospheric Conditions During the Bleeding Period and Time of Finishing on the Scale Resistance of Concrete, Journal of the American Concrete Institute,Vol. 27, No. 3, November 1955)

The table below summarises the factors that could affect the risk of delamination due to bleed water.

Factors that could affect bleeding	Factors that could affect the rate of setting at the surface
The temperature of the subgrade	Exposure to high wind or air currents
The existence of a DPM or plastic slip layer membrane beneth the slab	Low relative humidity
A thick slab	Exposure to sunlight
A high water/cement ratio	Rising air temperature
Entrained air or water reducing admixtures

Identification of delamination due to bleeding:

• Some aggregate surfaces will be clean owing to the presence of water.
• The surface of the underside of the delaminated layer will be granular in nature and in some cases particles of aggregate will be covered with cement paste.
• There may be lime bloom and silty material on the surface of the exposed face.
• Shrinkage and/or curling may have occurred.

Problems with air-entrained concrete

Air-entrained concrete was developed in the 1930s with a view to improving resistance to freezing water and de-icing chemicals. By using air entraining Portland cement or by introducing air-entraining admixtures, millions of microscopic voids are formed in the concrete. These voids can often account for 5% of the volume of the concrete and create small expansion chambers thus relieving the effects of the expansion of water when it freezes. (See Air-entrained concrete, Portland Cement Association.) Besides the increase in freeze-thaw resistance, air-entrained concrete is more workable than non-entrained concrete, while bleeding and segregation is less than with conventional mixes.

However, the use of air-entrained concrete for floors that are to be power floated is problematic and best avoided. The reason for this is that the surface of the concrete can become very dry, while a few millimetres below the surface the concrete behaves normally, with gradual stiffening taking place in the way you would expect. As the paddle blades are drawn across the surface, they create a shearing action, which in turn tends to unite the small air bubbles into a much larger, elongated void. These voids can extend to form quite large sheets of debonded concrete surface, which fail a few days or weeks later as a result of drying shrinkage or mechanical action.

Identification of delamination in air-entrained concrete:

• The flat surfaces of the bubble may be visible.
• The surfaces along the line of failure will be grey and granular in nature.