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Correct preparation and finishing of the concrete can make a huge difference to the final appearance and durabilty. The following is not a complete guide but is intended to provide some helpful advice on items to watch out for.
Key Principles to Manage
The most important thing to understand is 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.
2. Concrete shrinks as it dries.
Sawcuts are used to control where the shrinkage occurs. The more water in the mix the more shrinkage will occur, so avoid placing very wet concrete. Also control how fast the conrete is allowed to dry by curing and get sawcuts done in a timely manner.
3. Allow the concrete to move.
Shrinkage isn't the only cause of cracking - restraint is a major contributor. So sawcut between any restraints (eg footings, foundations, penetrations, slab thickening, sumps, drains, posts, and the like)
4. Steel is more elastic than concrete.
The mesh is used primarily to minimise crack widths, not prevent cracking.
5. Concrete colour is mostly affected by water-cement ratio, and temperature.
Different drying rates will give you a different colour so shading can cause problems, as can pours on different days. Avoid both.
6. Traffic Management is the responsibility of the site manager or owner.
Plan Your Project
» Patios, paths driveways and pool surrounds should be at least 80mm but are better at 100mm
» Consider using DPM (plastic sheet) for patios, paths and driveways: this eliminates moisture movement which can sometimes contribute to colour variation
» Use bar chairs to keep mesh at the right level
» Use 665 mesh. 668 is cheaper but has just over half the restraint of 665
» Plan where cuts are to be made
» Don't use polypropylene/plastic fibres as a mesh replacement. It is our belief that polypropylene fibres help only with early age cracking reduction and are not suitable to replace mesh.
Plan Your Pour
» Check the weather. Hot windy days cause the concrete to dry quickly and so have higher cracking risk.
» In Spring and Autumn, be aware of areas with shading as drying rates between sunny and shaded areas (eg under eaves, fences or trees) can cause set time differences resulting in colour differences are likely.
» Carefully check concrete volume - most important is the depth (if the concrete is 10mm deeper you will use 10% more)
» Try to use a 20mm structural mix: 10mm pump mixes shrink up to 20% more and so have a much higher chance of cracking.
» Polypropylene fibres such as Sika Fibre can be used to help reduce the chance of early age cracking.
» Confirm the type of concrete, slump and volume that is required.
» Work out the speed of supply - how close together the concrete trucks are required?
» Order your concrete and pump at least 2 weeks before. Advise pump company they will not be able to blow back into the concrete truck on-site
» Don't pour a project in a lot of small pours, they will be different colours
Getting Ready for the Pour
» Ensure concrete is an even thickness throughout as thinner sections are more likely to crack
» Saturate ground prior to pouring if not using DPM
» Plan your sawcuts and mark on the formwork where they should be.
» Don't cut every second bar of reinforcement: this weakens the mesh which is supposed to hold the crack together
» Tie mesh sheets together
» Get materials ready to keep the slab wet after finishing is complete (see below for curing options).
» If the job is a cash sale, organise payment
» Ensure there is safe and clear access to the site for concrete trucks
» Pouring the concrete on a hot and windy day can be risky as the cracking risk is much higher due to a higher evaporation rate. Go early in the morning while it is cool and have enough people to get the concrete in quickly, then flood as soon as is practicable after finishing. Plus use Sika Anti-Vap to reduce evaporation. See our Laying in Hot Weather Page for more discussion and options.
» 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 Sika Anti-Vap on hot and or windy days
» 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
» The pumps will not be allowed to blow back any excess concrete into the bowl of the truck
» If there are any breakdowns, Bridgeman trucks are only allowed to be towed out by a specialised contractor that we will organise
Concrete should be protected from early loss of moisture (this loss can cause shrinkage cracking, as little strength development has occurred to withstand the stresses resulting from the volume change from evaporation). Cure your concrete immediately after the finishing process for a minimum of three days, preferably seven.
Methods of curing
» Ponding: build a sand bund around the perimeter and fill with water to cover slab
» Spraying: the use of sprinklers to keep slab continuously wet
» Covering: impermeable covering such as plastic sheet will trap moisture on concrete surface and minimise evaporation
» Curing compounds: apply after finishing when bleed water disappears (use with caution, these products may affect follow on trades like paint, tiles, vinyl, adhesives)
» Ensure curing is even and consistent - do not allow some areas to dry and some to stay wet during curing - different colors will likely result
» Do not store anything on wet or covered concrete as this will change the way the concrete cures beneath, likely colour variation.