Page 34 - Build 152

Adding 10 litres/m3 only represents around 5% of the total water content of concrete and replaces the water lost through evaporation.
Adding water in an uncontrolled manner is inviting problems with high shrinkage and low strength.
Concrete needs early nurturing
Concrete is vulnerable until it has gained
some strength. The hydration of cement is a chemical reaction affected by temperature
and the presence of moisture. As soon as the fresh concrete starts to dry to the atmosphere, shrinkage will commence and strength gain will also slow.
The critical areas to focus on are nishing, curing, saw cutting and mesh placement. Finishing
A slab is very exposed to the weather when being placed, nished and cured. Avoid rain, strong drying winds (particularly in spring), hot sunny conditions and large temperature drops overnight. These can all cause cracking.
Download a localised weather forecast – some concrete suppliers offer this as part of their service. If conditions are unfavourable for concreting, delay the pour rather than taking
a risk.
Curing
Curing ensures that the concrete does not dry out prematurely and will gain strength from an early age. If the commencement of curing is delayed, you cannot make up for it by curing later – you will lose around 50% of the potential concrete strength.
Curing is achieved by ooding the concrete or water sprinkling the entire surface.
Alternatively, a plastic sheet can be laid on the surface and weighed down with sand or the like so that wind cannot get under the sheet. Curing
50 mm down to R12 dowel
300 mm
bond breaker joint formed with building wrap or bituminous coating
grade 500E seismic mesh reinforcing terminated both sides of bond breaker
30 mm cover to reinforcing mesh
100 mm concrete slab
one side of R12 reinforcing bar in a plastic sleeve to allow relative movement
Mesh placement
For slabs designed to NZS 3604:2011, the spacing of saw-cut joints, shrinkage movement joints and location of the reinforcing mesh are all important in avoiding shrinkage cracking. The reinforcing mesh needs to be at 30 mm cover to the top of the slab (see Figure 2). Its main function is to prevent shrinkage cracks occurring between saw-cut joints.
To ensure the mesh is at the correct height, support it on mesh chairs. Reinforcing mesh required is grade 500E (seismic mesh) 2.27 kg/m2 welded wire mesh.
The maximum distance between saw-cut joints is 6 m, and every fourth joint should be a free movement joint at a maximum spacing of 24 m (see Figure 2). Where parts of the oor are exposed, such as for a coloured or polished oor, the saw-cut spacing should be reduced to 5 m.
R12 reinforcing bars @ 300 mm centres (600 mm long)
100 mm
300 mm
Figure 2
Free movement joint.
should begin the day after pouring in summer, before the concrete surface has had a chance to dry out.
Unfortunately, adequate curing is often ignored in practice yet it is critical to achieving concrete strength gain and avoiding shrinkage cracking.
Saw cutting
NZS 3604:2011 Timber-framed buildings requires saw cutting to take place the day after pouring in summer and 2 days after pouring in winter (see Figure 1). This timing is critical, and the occur- rence of uncontrolled cracking due to saw cutting too late is far too common in slabs.
Once uncontrolled shrinkage cracks have formed, the saw-cut joints will not act as
the weak link for shrinkage cracking to occur. Movement will always take place preferentially at the existing cracks.
32 — February/March 2016 — Build 152