The most important step in placing concrete is planning. Always plan every step before any concrete is delivered. Proper planning avoids delays, wastage, segregation and problems which develop from these. Proper concrete planning also eliminates problems of Delay, Segregation and Wastage.
Workers on the site should always wear protective clothing, strong boots and, if required, helmets or eye protection. Always avoid direct contact with cement and never kneel in or touch the concrete mix directly. Wear gloves or use barrier creams. Ensure that anyone using heavy equipment, such as screeds or vibrators, has been properly trained. The following steps should be taken before any concrete is placed:
Measure and stake out the area to be concreted and consider how thick the slab must be. The thickness will depend on the weight the concrete must carry (ie driveway carries the weight of a car and needs to be thicker than a garden path).
The Finishing Level
See also : Watch Concrete Finishing Video
Once the thickness of concrete has been established, work out where the concrete will finish.Concrete cannot finish too high against steps or the external house wall and should not cover any part of weepholes in the wall.The finishing level shows how much digging or excavation must be done.Pavements must grade away from buildings and boundaries.
Steps must have even risers.
The ground should be excavated as deep as is required by the finishing levels.Any roots or grass must be dug out until there is firm soil to place on.Always dig the hole wider than needed to allow for the formwork. Try to keep the edges and corners square.
See also Subgrade in Road structure
The soil a concrete pavement or floor rests on is called the subgrade.If the soil is soft or varies in softness, a layer of crushed rock should be used.If there are only a few poor areas these can be dug out, refilled and compacted.It is important that the soil evenly supports the concrete. Many later problems can be avoided by properly preparing the subgrade.
Site Preparation for Concrete
Formwork gives concrete its shape, Formwork must be properly braced so it is strong. It should not flex or move.
Plumbing, heating or electrical services often run through a slab.These must be in place before any concrete is poured.
UNDERLAY AND SERVICES
The underlay, or vapour barrier, is a heavy plastic covering the ground to minimise water vapour rising through the hardened concrete.Always overlap the sheets a minimum of 200 mm and do not tape them.Tape the edges of underlay only around drainage pipes or services which pass vertically through the concrete slab. Termite protection may be required around service penetrations and round the perimeter of the slab.
Reinforcement can be used to increase the strength of concrete and/or to help control cracking. For house floors resting on the ground it is placed in the top 1/3 of slabs and in the bottom of trenches and footings. The reinforcement must be covered by a set amount of concrete which protects the steel from rusting. This is called cover. The amount of cover depends on whether it is inside or outside and is measured to the top or bottom of the outer surface. Reinforcement should be securely held for slab on ground construction.It should overlap a set distance or from one piece of reinforcing bar, or wire fabric, to another and at the corners of a trench.
Clear access must be provided to transport the concrete. If concrete is to be delivered by trucks make sure they have unrestricted access to the site in all weather conditions. PLACEMENT Ensure all planning and site preparation takes into account how concrete will be placed allowing room for trucks, ramps for wheelbarrows, space for a pump etc.
See also Joints in Building Construction
The position, type and number of joints should be planned well before the concrete is placed.
Good planning and site preparation reduces wastage.Reducing wastage can cut costs, since up to 15% of concrete can be lost this way.