Constructing a new concrete floor slab can be quite a complex exercise involving several different trades and sub-contractors. Things can go very wrong, very quickly when you are dealing with large quantities of wet concrete and you are fully exposed to the weather and adverse ground conditions. Therefore, it is very important for the client to have a good understanding of what is involved, what is the correct sequence of works and what to look out for during the process. After completing over 150 concrete slabs in the last 30 years, below are our recommendations for the best practices when constructing a new concrete slab.
A. Preparation and documentation
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Study the “soil report” and review the soil / ground conditions with your builder. Specifically discuss any potential cost variations due to site conditions before the builder starts excavation and slab preparation on site.
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Clarify exactly what type of “termite treatment” system will be used and what is involved in carrying out this system on site. Also, find out what warranty is provided with the “termite treatment”.
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The builder or project manager should prepare a detailed “slab set out plan” which should be laminated and show
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The construction grid lines
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The measurements for all edges of the slab
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The set out for all rebates, shower bases and slab step downs.
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All heights and levels
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All sewer pipe locations
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Hydronic heating details including boiler and manifold locations.
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Do a “site inspection” with the builder prior to starting the slab to identify any potential problem areas and cost variations; such as – ground “soft spots”, drainage issues, large trees and root balls, fence line problems or any issues with the surrounding properties.
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Before starting the underground plumbing or slab preparation, the builder should do a “site cut.” This is when the site is excavated to a flat level of approximately 300mm below the finished slab floor level. This will remove any vegetation and demolition spoil and will get the site ready for the plumbing and concreting contractors.
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The builder must apply for a council “road occupation permit” at least seven days ahead of the date the concrete slab will be poured. This permit is required for the concrete pump truck and several concrete trucks to occupy the roadway during the concrete pour. The builder will need to provide a “traffic management plan” to the council when applying for this permit.
B. Other “Pro-tips” for concrete slab construction
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All the “underground plumbing” for the new sewer and storm water pipes below the house slab should be completed before the concrete contractor starts the slab preparation.
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During the slab excavation, find out from the builder if any “blinding concrete”, “bored piers” or “screw piles” will be required due to the excavated ground conditions and if there will be any variation costs?
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Make sure the builder and concrete contractor are aware of the final floor finish (i.e. exposed concrete, timber flooring, vinyl flooring etc.). Some floor finishes require the concrete slab to be finished in a particular way.
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We recommend that the builder remove any excess soil from site as the slab excavation progresses to avoid ending up with a large pile of soil at the front of the job site which can be dangerous and cause access problems.
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If you are installing “in slab hydronic heating” (which we highly recommend), you should also install “under slab foam insulation” which can greatly improve the efficiency of the slab heating due to minimizing the potential heat loss down into the ground.
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The hydronic heating pipes (“floor coil”) must be installed at a consistent height (approximately 50 to 70mm below the finished slab surface) and should have an additional layer of light weight reinforcing mesh installed over the top of the heating pipes to help control shrinkage cracking in the concrete.
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Make sure the top layer of reinforcing mesh is installed flat and level and is not sagging down between the pads or around the edge of the slab.
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We recommend that concrete slabs are not poured if the ambient temperature is below 5ºC or above 33ºC. If it is necessary to pour concrete in extreme temperatures, special additive chemicals must be used to slow down or speed up the curing of the concrete and must be approved by the consulting structural engineer.
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The form work around the edge of the slab should be removed or “stripped off” the same day as the slab is poured as soon as the concrete is hard enough to walk on.
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The surface of the finished slab should be kept wet or covered in plastic for at least 48 hours after it is poured. Note: the slower the concrete dries and cures, the better and the less shrinkage cracking will occur.
Also, remember that in Victoria, there are two mandatory building inspections by the building surveyor required for a concrete floor slab. Firstly, an inspection of the finished excavation (footing beams and packing sand pads) prior to the installation of the polythene moisture barrier. Secondly, an inspection of the finished reinforcing bars and mesh and the form work (“boxing”) before pouring the concrete slab.
