Delamination is characterised by thin layers of surface mortar becoming detached from the main body of the slab. Concrete Advice Sheet 18 provides an overview of the causes of delamination in concrete floors.
Powdering at the surface of a concrete slab is called dusting. It is characterised by a light-powdery surface that may weather to expose the aggregate. Dusting surfaces powder under any kind of traffic and can be easily scratched with a nail or even by sweeping.
Reinforcement ripple is the name given to a surface irregularity that sometimes occurs on the surface of large areas of flat concrete slabs. It takes the form of shallow troughs over the line of the reinforcement after the concrete has been finished. In some cases this is just in line with the upper bars in the top mat but in the worst cases ripples are formed in both directions, giving a quilted effect. The probable cause is vibrations set up in the reinforcement, which results in additional compaction of the concrete around the bars in the finished areas, resulting in the depressions seen.
The only certain way to avoid plastic shrinkage cracks is to reduce the rate of evaporation from the surface of fresh concrete by early curing.
The tendency for plastic settlement cracks to form may be reduced by adjusting the concrete mix, for example by avoiding gap-graded fine aggregate and reducing the water content, and by appropriate workmanship and control of vibration. Particular care will be required for tall elements. In some cases, plastic settlement cracks can be eliminated, rather than prevented, by careful re-vibration of the concrete after they have formed. However, it is important to ensure that the concrete is not over-vibrated.
Avoiding the following can reduce the occurrence of crazing:
– over-rich or excessively wet mixes
– over-vibration
– excessive finishing.
Drying shrinkage occurs within the cement paste fraction of the concrete. Therefore the occurrence of drying shrinkage cracking can be reduced by using a concrete with a lower water content or by increasing the aggregate volume where possible to minimise the cement paste volume. Good curing will also help, particularly on slabs, as it will enable the concrete to gain greater tensile capacity before drying out occurs. The extent of shrinkage cracking can be reduced at the design stage by providing movement joints and sufficient crack control reinforcement. The aim is to produce a large number of fine cracks rather than a few wide ones.
Localised rust coloured stains on the concrete surface may be caused by small amounts of contaminants in the aggregate, such as pyrites. These can be avoided by careful selection of the aggregate and control of its production and storage prior to use. Similarly local stains can be caused by pieces of tying wire left in the formwork prior to casting the concrete. In both cases this is a cosmetic problem only and will not affect the long-term performance of the concrete structure.
Blow holes are individual, generally rounded, cavities on vertical surfaces of concrete, generally less than 10 mm across. They are caused by air in the concrete being trapped against the form face, sometimes due to insufficient vibration. Some blow holes are almost inevitable unless a permeable formwork material is used. The occurrence of blow holes can be minimised by the use of a suitable release agent on the surface of the formwork and the use of adequate vibration. In addition the concrete should have adequate workability. Blow holes are a cosmetic problem only and will not affect the long-term performance of the concrete structure.
Honeycombing is the term used to describe areas of the surface that are coarse and stony. It may be caused by insufficient fine material in the mix, perhaps due to incorrect aggregate grading or poor mixing. This can be corrected by increasing the sand and cement content of the mix and by proper mixing, placing and compaction. Alternatively, honeycombing may be caused by leakage of grout or mortar fraction from the concrete at construction or formwork joints. The obvious solution here is to ensure that joints are well sealed and leak-free.