. SHUTTERING FORMWORK
When concrete is placed with its upper surface below finished ground level, as is the case with foundations for walls, the sides of the excavation itself provide the necessary containment for the concrete, define the plan shape of the concrete and act as shuttering or formwork. The levelling of the top surface of the concrete is simply achieved by positioning timber pegs in the bottom of the excavation, projecting out by an amount equal to the thickness required for the concrete. The pegs are spaced at no more than 1500mm centres along the centre line of the foundation and are levelled with a spirit level and straight-edge or a Cowley level (see Chapter 6). When the concrete is poured it can then be levelled to the top of the pegs.
Whilst this chapter does not concern itself with the details of excavation methods, it is worth noting that the process of positioning and levelling pegs to provide both the level and the thickness of the concrete will also give an indication as to whether the bottom of the excavation has been dug out level. If there are any high spots, further grading out or additional excavation may be necessary, otherwise the effective thickness of the concrete will be reduced. Dips or hollows will obviously fill with concrete but excessive low spots should be minimized during the excavation process as additional concrete is an expensive way of compensating for excavating too deeply in places.
When concrete is placed with the upper surface above ground level, extra temporary provision has to be made to
contain and form the shape of the concrete as it hardens. The easiest and quickest solution takes the form of shuttering or formwork. For one-off concreting projects such as a shed base or concrete path, this is made from timber; it is bespoke for each project and is not necessarily intended to be reused.
Fig. 46 Timber shuttering/formwork.
A typical timber formwork arrangement (see Fig 46) is held together with double-headed nails so that it can be dismantled once the concrete has hardened, although the use of screws is probably more common. Angled bracing timbers are used, designed to resist the lateral pressure of the wet concrete and to prevent the formwork distorting or bulging outwards. Where the concrete being cast is comparatively thin (around 100mm) then the formwork and its bracing need not be quite so elaborate.
Fig 47 shows the casting of a shed base with the formwork constructed from 100mm x 20mm lengths of timber. In this case, the timbers have
been screwed together at the outside corners. Additional support has been provided by 20mm x 30mm section vertical timber pegs positioned at the corners and along the sides, with screws inserted through the pegs into the formwork. Additional support to resist deformation during placing of the concrete has been provided by the use of dry bricks.
All formwork is constructed with its upper edge at the same height as the top surface of the finished concrete and is positioned so that it is level all the way round. This assumes of course that the finished concrete is required to be level; falls to the top surface can be easily created by constructing the formwork accordingly. This means that the wet concrete can be levelled as it is placed and compacted with a tamping board. Where necessary, the tamping board can be ‘shuffled’ from side to side as it is drawn forward along the top of the formwork in order to grade off any excess wet concrete and to obtain a flat surface. Generally, the timber straight-edge used as a tamping board must be long enough to extend 100mm beyond both sides of the formwork in order to be effective. For more on the placing, compacting and finishing of concrete, see Chapter 2.