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Intro

As material suppliers we are very keen to ensure a great result for the client that exceeds their expectations. Consequently we welcome the chance to work with you and your subcontractors to ensure you all do a great job.

In the concrete industry it is common to cut corners to save money. Too often a bad result is blamed on the concrete when in fact bad workmanship is at fault. The guidelines below are designed to help you achieve as good a result as possible.

We highly recommend reading the Residential Concrete Slab-On-Ground Floors construction guide from the Cement and Concrete association.

concrete slab guide for builders

Ordering/Purchasing

Colour: It is usually easiest to buy the colour direct from your local ready mix concrete company.

Auxiliary products (sealer, grout, etc): Order direct from Peter Fell - 0800 422656

Subcontractors

We have good relationships with concrete and sealing contractors all around the country - please get in touch if you would like some names of some reputable contractors.

Getting a Great Result

Client Expectations

The most important thing is to ensure that your client understands that concrete is a natural product with natural variation - it is not a perfect product and so some variation in texture, and colour is to be expected. Cracking is possible, the colour will be motley and and colour will not match the sample exactly should all be well understood by the client. Some contractors get acceptance of this in writing.

Key Principles to Manage

1. Concrete shrinks as it dries. The more water in the mix the more shrinkage will occur, so don't place very wet soupy concrete, and control how fast it is allowed to dry. Plus, put sawcuts in at the right time and in the right place.

2. Shrinkage doesn't cause cracking by itself - restraint is a major contributor. So sawcut between any restraints (eg footings, foundations, penetrations, slab thickening, sumps, drains, posts, etc)

2. Steel is more elastic than concrete - so the mesh is there primarily to minimise crack widths, not prevent cracking

3. Concrete colour is affected by water-cement ratio and temperature. Different drying will give you a different colour so shading can cause problems as can pours on different days

Dos and don'ts

Don't

  • Don't pour the concrete on a hot and windy day - cracking risk is much higher. If you must pour, go very early in the morning, and/or take a lot of precautions such as misting, barriers.
  • Don't pour concrete where you have heavy shading and hence temperature and drying rates like eaves, fences or trees.
  • Don't cut every second bar of reinforcement: this weakens the mesh which is supposed to hold crack together
  • Don't use polypropylene/plastic fibres as a mesh replacement; It is our belief that polypropylene fibres help only with plastic shrinkage cracking and are not suitable to replace mesh.
  • Don't use 17.5MPa concrete - higher strength concrete has more cement and finishes better
  • Don't pour a project in lots of small pours, they will be different shades
  • Don't allow some areas to dry and some to stay wet during curing - different colors will likely result

Do

  • Check with your client what they expect for their colour, texture, cuts grout and sealer.
  • Plan your sawcuts and mark on the formwork where they should be.
  • Use granular fill beneath concrete: it minimises groundwater capillary action and possible resultant colour variation, plus helps to protect concrete from any potential ground movement (especially important on clay)
  • Ensure concrete is an even thickness throughout: thinner sections are more likely to crack
  • Patios, paths driveways and pool surrounds should be minimum 100mm
  • Consider using DPM for patios, paths and drive ways: this eliminates moisture movement as potential creator of colour variation
  • Use bar chairs - they cost 35c each - why wouldn't you!
  • Use 665 mesh. 668 is cheaper but has just over half the restraint of 665
  • Tie mesh sheets together
  • Carefully check concrete volume and over order by 10% - running out of colour is not an option
  • Use a 20mm structural mix: 10mm pump mixes shrink up to 20% more and so have a much higher chance of cracking. It's worth paying for a bigger pump.
  • Use 20MPa minimum, preferably 25MPa: It has more cement so a better and creamier surface will result, plus there is less chance of cracking as it gains strength faster.
  • Reinforce internal corners with two 1.2 m lengths of D10. Also a 1m x 2m diagonally laid extra sheet of mesh can help to stitch the crack together
  • Saturate fill prior to pouring if not using DPM
  • Try not to add water to the concrete truck on site unless absolutely necessary. Wet concrete is easy to place but it shrinks more and so cracks more
  • Use an anti-vap like SIKAFilm on hot and or windy days (http://nzl.sika.com/en/cement-redirect/sika-cement-solutions/02a001/02a001sa04/02a001sa04ssa05.html)
  • If a cold night expected get your sawcuts in same day, or use crack inducers or tooled joints to minimise chance of thermal shock cracking
  • Cure your concrete immediately after finishing process with water or plastic sheets for minimum 7 days.
  • Ensure curing is even and consistent - do not allow some areas to dry.
  • Protect the concrete during the construction process with polythene, and/or carpet
  • Do not store anything on polythene/carpet (like sheets of Gib) as this will change the way the concrete cures beneath, likely colour variation.
  • Allow the concrete to fully dry prior to application of sealer. For interior floors, ensure good ventilation and removal of moisture laden air to ensure good drying. Lift protection as early as practicable to allow drying while protecting it from gib stopping dust, paint, etc.