Frequently asked Questions:

Q. Why do floor slabs crack?

A: Almost all cracks in ground floor slabs are the result of shrinkage, or more specifically, restraint to the drying shrinkage of the new floor slab. The risk of these occurring can be minimized but it is unlikely that any designer would guarantee a crack free floor slab.

Other reasons that may affect this issue are:

• Insufficient local reinforcement due to incorrect dosage of steel fibres or badly placed mesh / bar: This will effectively reduce the flexural strength of the concrete
• Non-uniform distribution of steel fibres throughout the concrete: This typically occurs where the concrete aggregate grading curve is not properly designed to accommodate the fibres or the method integration is not suitable (for example, by hand or from a scaffold tower)
• Inadequate weather protection: This can create an environment during construction where variable drying rates can be created
• Early age loading: the concrete panels are fully loaded before any shrinkage has taken place and the slab is effectively pinned to the sub-base resulting in localised areas of increased stress
• Inadequate detailing of inserts and ‘return’ corners
• Incorrect aspect ratio for jointless panels (e.g., maximum length/width ratio exceeded)
• Poor control of concrete batching quality resulting in a non-uniform concrete mix

Shrinkage cracks can be fully repaired by resin injection. Resin injection will stabilise a shrinkage crack and provide a repair that is stronger than the parent material. It is a relatively simple and quick operation that results in a repair that is visible as a light coloured mark on the surface of the slab until such time as it ‘blends in’ to the surrounding material. It will not result in a loss of surface flatness or abrasion resistance.

 

Q. My floor is slippery; how can I remedy the problem?

A: This has probably been caused by inadequate cleaning of the slab; in effect the sweeping machine and foot traffic has polished the floor leading to it becoming slippery. Once a more vigorous cleaning regime is implemented the floor will return to its original condition. This was demonstrated when we cleaned a small area with a small scrubbing block and detergent.

 

Q. I want a coloured concrete floor slab; should I apply a coating or use a dry shake topping?

A: The Aesthetics, colour and durability of the finish to a new floor are very important and these potentially contentious issues need to be addressed thoroughly by the Client and the Specifier during the design process.

Based on experience, where the Client’s priorities are for a high quality, low maintenance, ultra hard wearing floor finish with reasonable colour consistency, then they should be guided towards a Coloured Dry-Shake Topping; but where a totally uniform, monotone coloured finish is required, they should be guided towards an applied Resin coating or screed.

A cementitious Dry-Shake Topping is ‘added into’, rather than ‘applied onto’ the surface of a new concrete slab, therefore the resultant finish is heavily influenced by any inconsistencies in the concrete supplied, particularly the moisture content as well as the environment in which the floor is laid. Great care should be spent ensuring that both aspects are well controlled.

During the first few months after a Dry-Shake floor has been laid, the finish will reflect the differential drying process that every concrete slab goes through i.e., it will appear patchy. The colour will start to lighten and even out during the first few weeks and months; but it could take anything up to 12 months for the slab to dry through and achieve reasonable colour consistency. This process will be accelerated by a thorough initial cleaning, using a rotary scrubber-drier with an abrasive pad, to remove all surplus curing agent from the surface of the slab.

To the Client, the major benefit of a Dry-Shake Topping is the superior hard wearing surface that it creates, giving an abrasion resistance up to 20 times that of quality power floated concrete and providing a long, maintenance-free life for a floor that will probably outlast the building. A colour can be added to enhance the aesthetics and unlike applied resin coatings, the colour will not wear through. All this is achieved for less than half the cost and a fraction of the time taken to apply an epoxy or similar finish and it will never have to be re-applied.

If cost, durability and low maintenance have the highest priorities and a reasonably uniform colour is acceptable, then a Dry-Shake Topping is the right choice. If the Client’s primary concern is to have a totally uniform, monotone coloured finish, then they should choose, and be prepared to pay the extra cost for a post applied resin system, (and also be prepared to re-apply the same, every few years). The sensible path to a responsible decision is for the Client to view a number of floors and base their choice on experience.

 

Q. Why is the surface patchy in appearance?

A: Concrete is a natural material, sometimes produced with less than scientific precision, meaning that batches of concrete will have minor variations of water content and perhaps even slight aggregate variations.

A dry shake topping material goes some way to minimising this effect, but not negating it. However, the greatest cause of this patchy effect is the variable drying rate of the base slab. Moisture levels may vary across the floor when the concrete is laid. This in turn may prevent consistent timing of the floating and trowelling processes. Good concreting practice then insists that the floor be cured efficiently, which means slowing down the rate of evaporation of the water.

Any areas of the floor with appear darker will contain a slightly higher moisture level than perhaps other areas, and vice versa. These variations will gradually reduce with time as the slab dries to a consistent level. However, it is important that a good housekeeping regime is introduced, as regular cleaning will speed up the improvement.

A well laid and finished coloured dry shake floor will, if cleaned regularly, provide an excellent industrial floor finish that will outlast any form of applied finish. It must be understood though, that a coloured concrete slab will not be as uniform in appearance as a newly painted floor.

 

Q. How important is curing for concrete floor slabs?

A: This is one of the most crucial parts of the floor laying process. The loss of water in fresh concrete can be considerable, even when working inside of a building or warehouse. This can then lead to non-uniform shrinkage and can reduce the durability of a slab. To avoid this, all slabs should be cured immediately after the final finishing process has been completed using a suitably efficient curing compound.

 

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