Slab Placement Tips

Preparation:
Remove turf or loose soil. Where the ground is soft, spread sand or ashes to make a firm base. Fill any holes with stones, broken bricks or clean fill.

The formwork must be deep enough to hold the thickness of concrete and prevent slurry escaping underneath. Secure the forms in position by driving pegs into the ground inside and outside the forms. Hammer pegs down to below the height of the forms and nail the outside pegs only. The inside pegs will need to be removed as the concrete is placed.

For easy removal of the forms afterwards, lightly coat them with thin oil or form oil from the hardware store.

Final levelling of the base can be done with sand. Compact the area by rolling, walking or driving over it, and hosing it lightly. The more it settles the better the foundation will be.

The ground should be thoroughly soaked the night before and dampened again before pouring. Concrete needs to set or “cure” slowly. This procedure is especially important on a hot, dry day, when the concrete can flash-set due to its moisture being absorbed by the dry ground.

If the truck’s discharge shute (3m long) won’t reach your area, have plenty of helpers ready, at least two with wheelbarrows and one to spread the concrete.

Placement: Truck drivers allow 30 minutes to unload (about 5m³), so ensure you are ready to start when the truck arrives. Pour mounds of concrete, starting at the lowest point, and work progressively up the slope.

Use a shovel to spread the concrete. Remove the inside pegs. Work the shovel up and down in the concrete to compact it, especially alongside the formwork, to remove air bubbles along the edges.

When the form is filled to a little higher than the top edge, pull a screed board back and forward across the top of the side forms, edging the concrete along.

Some water normally rises to the surface after this screeding process. Some will run off or evaporate, the rest of the water will be drawn back into the concrete when the “setting process” commences.

If you want a smoother finish than the screeded surface, use a wood float. “Floating” agitates the surface, compacts the material, and brings enough mortar to the surface to fill the voids. After floating, cut the control joints and round off the edges with an edging tool.

Curing: Concrete develops its strength by the reaction of water with cement. The slower it dries, the stronger it gets.
As soon as the surface is hard enough to resist marking, cover the concrete with plastic sheeting or waterproof building paper and secure it along the edges to keep the moisture in.

Alternatively, spread hessian or sand over the surface and keep moist. Concrete will continue to gain strength for at least one month provided it is kept continually moist for that time. After a week of curing, strip the forms away. Do not use screwdrivers or similar tools to pry the forms off because they may cause damage.

Block-fill Placement Tips

Preparation: Blockwork that is to be filled with concrete grout requires careful preparation. The first course of blocks should be a “clean-out” course. These are either open-faced blocks or have an opening cut big enough to remove mortar droppings.

When blocks are tapped down to the string-line, some mortar is squeezed out of the joint. The mortar that squeezes outwards is struck off by the blocklayer but the mortar that squeezes in either hangs in the core or drops off, settling on horizontal steel or the footing.

This mortar must be knocked off so that it does not restrict the flow of the grout. It must then be cleaned off the footing through the “clean-out” course. Mortar that reached the footing must be chipped off and removed also as it is unlikely to be more than 5MPa and if left, would form a layer of weakness between the 20MPa grout and footing.

Concrete needs to set or “cure” slowly. On a hot, dry day, the blockwork should be dampened so it does not rapidly absorb moisture from the grout and allow it to flash-set.

Prop a covering board against the clean-out blocks.

Placement: Concrete should be “placed” into the cores, not “dropped”. Dropping would segregate the concrete and reduce its strength.

The Australian Masonry Manual suggests filling only 1.2m high (6 courses) at a time. The grout is then consolidated, preferably with a pencil vibrator. This is repeated around the site until the starting point is reached by which time the grout in the first core should have firmed and another 1.2m lift can be placed without doubling the hydrostatic pressure.