Locating and fixing the steel

Press, 8 October 1987, Page 32

Locating and fixing the steel

Bars or mesh should be cut to the correct length using bolt-cutters or hacksaws. It is necessary to secure individual bars in their correct positions while pouring and compacting the concrete. This is usually done by placing bars at right angles and tying them together at their intersections. Soft steel tie-wire is used for this purpose. There are many different ideas on the configuration of the wire loop, but the only important thing to have in mind is that the steel bars must be held in their correct position. Your wall will not fall over because the tie wires are not strong enough, but you may have a problem if the steel rods are in the wrong place.

If possible single length bars should be used, and since they are available up to 6M or more in length this is not usually a problem.

However there are times when two shorter pieces must be used to make up the full length. The overlap at the junction should be not less than 400 mm for 10mm diameter bars, nor less than 500 mm for 12mm diam. Adjacent sections of steel mesh should overlap by one full square. To prevent rusting of the steel it must be surrounded by an adequate thickness of good dense concrete.

For normal outdoor work the steel should be at least 50mm from the nearest face of the concrete, and this should be increased to 75mm for foundations, etc. where the concrete is poured against the ground. In a typical situation the steel reinforcing bars would be spaced at about times the thickness of the wall or slab — say 150 mm spacing for a driveway 100 mm thick. As a maximum the spacing should not exceed twice the thickness.

Where your main problem is likely to be from concrete shrinkage as in a drive, path or patio, HRC

mesh or DIO bars will usually be sufficient

Where loading may be more intense, as in the case of a retaining wall of about Im high, Dl2 bars would be preferred. As a general guide on individual slab sizes, an unreinforced driveway 100 mm thick could be constructed with joints spaced at not more than 3m intervals.

The spacing could be increased to 6m if the slab were reinforced with HRC 665 mesh. This of course requires that good concrete practices of batching, placing, and curing are followed. If the steel bars are to resist tension it,is necessary that they are placed in that part of the concrete which will be in tension.

They will contribute nothing if they are in the wrong place, and the area of concrete which is in tension will still crack anyway.

The example of the driveway is fairly straightforward.

Tension arises from shrinkage of the concrete as a whole, and to resist

this the steel should be placed at mid-depth. The same principle will apply to most walls and fences with the reinforcing being correctly located at the centre.

Occasionally the handyman may encounter a situation where the steel would be more effective if placed off-centre. Remember it should be in the tension zone. A somewhat crude method of identifying the tension zone is to imagine the shape if the concrete member was drastically deformed by the loading.

Slabs over a drainage channel would tend to deform downwards under load and the steel bars should be located towards the bottom.

If a concrete decking slab projects beyond its support the deflected shape would take up a reverse curve. Here the steel should be located towards the top. A small retaining wall would tend to deflect outwards under the effect of the horizontal soil pressure so the reinforcement should be placed towards the back.