Lay out the project
Using stakes, twine, a tape measure and a level, stake out the boundaries. Be sure to consider such factors as drainage, nearness to tree roots and sufficient space for forms. Now you are ready to remove the topsoil and level out the area, taking dirt from the high places to fill the low spots. Remove enough soil from around the edges to permit room for slab forms.
Build strong forms
Path or patio forms are usually made with planks set on the edge and braced with heavy stakes on the outside, at 300mm intervals. Spreaders may be staked in to be removed after the concrete is placed.
Reusable forms should be precoated with oil or varnish. Take care not to use coatings which might stain the concrete. For good drainage, pitch the surface 10mm per metre in the desired direction. The sub-grade and the forms should be thoroughly soaked the night before the pour and dampened again before the truck arrives.
Make way for the truck
A 5m 3 concrete truck is 2.4m wide, 3.3m high and loaded weighs about 20 tonnes. The truck is approximately 8m long with a turning circle of 11m.
Before directing a truck onto your property make sure that it will not pass over any dry wells or other hidden weak spots. Protect your lawn or driveway by laying down planks for the truck to pass over. If the truck is to be driven between the forms over the sub-grade, use of planking will help to avoid deep ruts. If you are building a driveway, make sure the forms are wide enough for the truck to drive between them. This will greatly expedite placing and save you the trouble of transporting the concrete in a wheelbarrow.
Have some help available
You will save time and money if you are ready for the truck when it arrives. This means having all preliminary work done and sufficient tools and help on hand to handle the concrete. Among the tools you will need are several shovels, one or two rakes, a 100mm x 50mm piece of timber for screeding, wood floats, steel trowels and a grooving tool.
You will need help shovelling the concrete into place as it comes down the chute from the truck. One or two able-bodied friends will make good assistants and help you reduce truck waiting time charges.
Placing the concrete
Trucks cannot always get close enough to place the material where you want it. A deep tray contractor's wheelbarrow with rubber tyres can be hired for this purpose. Be sure to use planks to roll the wheelbarrow over to avoid jarring the contents and to make your job of pushing it easier. Compact the concrete thoroughly with shovels. Avoid segregation and contamination with the subgrade material. Loads should be placed so that they overlap or flow into each other when tipped. Remember, concrete is heavy. Do not risk hurting yourself by attempting to move more than you can comfortably wheel in the barrow.
When the concrete is placed you are ready to screed it. This is done by running a 100mm x 50mm timber section across the surface in a sawing fashion, preferably with one man at each end of the board. A third person could work ahead of the board filling low spots and removing the concrete which collects as the screed board moves forward. A deeper section of timber (150mm) may be required when screeding wide sections.
The next step in the placing operation is floating. The purpose of floating is to agitate the surface, to compact the material and bring enough mortar to the surface to fill the voids. A float can be home-made (a piece of hardwood 300mm x 100mm with a wooden handle) or purchased from a supplier. Use it with some light pressure and tilted slightly upward. Floating provides a non-skid surface.
Do not attempt surface finishing until the sheen has left the concrete and it has hardened enough to permit a steel finishing trowel to pass over it without digging in or causing water and cement to be brought to the surface. Kneel on a board to reduce pressure on the wet concrete surface. When the job is done, thoroughly rinse your tools so they'll be usable for other projects. If there is a risk of rain or of damage from cold weather, cover the concrete with hessian or plastic sheets, as soon as it can be done without damaging the surface.
The purpose of contraction (and expansion) joints is to provide a plane of weakness so that cracks which appear in concrete due to shrinkage tend to follow these planes and be hidden. To ensure that this occurs it is essential that grooves be no more than 2m apart and that they be cut deeply (normally 1/4 - 1/3 slab depth). Grooving tools are cheap and ensure that your finished job has a professional look.
Curing the concrete
If concrete dries out too quickly it will crack and lose strength. It is necessary to extend the drying out time by keeping the concrete moist for at least seven days. This can be done using a variety of techniques, advice on which can be obtained from your local Holcim Australia office. Curing must be started immediately after the 'set' has taken place, say four to six hours, depending on climatic conditions.
When the concrete has enough strength to support the weight it must bear (seven days) strip the forms away. Do not use a metal tool against the surface to pry off forms because you may cause damage.
How to measure your quantities
The calculator below shows the approximate amount of concrete you will need for your job. To use, measure the length and width of the area to be laid in metres; find the average depth to be filled and read off the calculator to find the volume of concrete in cubic metres that you will need.
As concrete is sold in units of 0.2m 2 the quantities have been adjusted accordingly.
The tools you need
Most of the equipment you need for laying concrete can be hired.
- Deep barrows with rubber tyres
- Planks to wheel the barrows on
- A screed board
- A wooden float
- A mallet
- A steel trowel
- Grooving tool
- Spirit level
- A tamping tool
- Timber for formwork
- A rake
By following the procedures in this publication and exercising proper care you will obtain the best performance from your Holcim concrete. Holcim Technical Bulletins and advice are readily available from your nearest Holcim Australia office to assist in achieving the best possible results.
Note: If reinforcement, membranes, insect treatment sprays, or any special techniques are to be used, seek specialist advice from experts in those fields as this brochure is only a basic guide to laying concrete in DIY projects.
WARNING: Concrete is a highly alkaline substance. Prolonged skin contact can cause severe irritation. The use of gloves and gumboots is strongly recommended.