Polished Concrete

Polished concrete is a popular choice for floors nowadays, however exposing concrete like this means that any cracks in the concrete are very visible, and these can sometimes be very problematic for the homeowner, especially if the cracks are very large and unsightly.

Unfortunately, concrete often cracks, mostly due to normal shrinkage of concrete as it dries and cures (called ‘shrinkage cracking’). There are many variables and factors that affect the final result, including:

  • Ratio of reinforcement to concrete
  • Concrete mix/time spent in truck
  • Temperature and weather conditions
  • Preparation of subgrade
  • Prevention of water evaporation in wet concrete as it is placed
  • Correct curing methodology and duration of curing

Controlling all of these factors can sometimes be challenging, and it is difficult to guarantee a good result. How much effort and money is spent to attempt to minimise cracking is an individual decision by each homeowner – we can advise on the best practice, but cannot be responsible for a concrete pour.

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There are some fundamental methods to follow when pouring concrete though, to maximise your chances of minimising the number and severity of any cracks. The below information is supplied to help the homeowner or builder plan for a concrete pour, and help make a decision about how much effort and money are going to be spent to try and minimise shrinkage cracking.

Specification of the slab and reinforcement:

  • Slabs for polishing normally are required to be 32MPa for hardness to assist with polishing
  • The primary decision for the builder/homeowner is whether to upgrade the reinforcing mesh to SL81, from a normal SL72 or SL82 mesh. The higher the ratio of steel to concrete in the cross-sectional area of the slab the more the slab is restrained against shrinkage forces. Shrinkage tends to be more spread out in invisible smaller cracks rather than concentrated into fewer bigger cracks that can be seen.
  • Increase the slab thickness to 120-130mm thick (from standard 100mm)
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On the day of the pour and beyond:

Using good practices during the pour and after can be relatively easy and low cost to implement with careful planning.

  • Ensure slump specified is sufficient for workability. Do not allow water to be added to the mix once the concrete has left the batching plant.
  • Ensure reinforcing mesh is placed as near to the top surface of the slab as is practical, given cover requirements.
  • Consider cancelling the pour if weather is too hot.
  • Protect the slab from wind (and preferably direct sun) to minimise evaporation.
  • Apply aliphatic alcohol immediately once concrete has been screeded.
  • Start curing of the concrete immediately after concrete is finished. Wetting the surface continuously and covering with taped plastic with no holes is a fairly secure method, otherwise a wax emulsion curing compound (not great for polishing though). Do not allow the concrete to dry out during the curing process, ever, even for a short amount of time.
  • Curing to continue for as long as practical, 7 days at the very minimum.

Located in Tintenbar on the North Coast of NSW, Lucena offers engineering consulting services. With a team of 10 skilled professionals, we deliver inventive solutions for engineering projects of all scales, ranging from minor upgrades to multi-level constructions.

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