Do-it-yourself concrete questions answered
When handling and using cement or fresh concrete, avoid skin contact. Wear suitable protective clothing.
This information has been prepared to provide answers to commonly asked questions about concrete. It will also help the householder to purchase, mix and use concrete with confidence.
Concrete, which consists of a combination of cement, sand, stone and water, is a remarkable material that can be cast into almost any shape. Placed, finished and cured correctly, once hardened it is practically indestructible. Concrete is rotproof and non-toxic once cured, and can be used in a variety of applications around the home, from driveways and paths to floors and entertainment areas.
The ingredients of concrete may be measured and mixed by the homeowner but it is more convenient and economical to use pre-mixed concrete available from local suppliers. Use of pre-mixed concrete ensures quality control. If concrete is required for a small job around the house such as short lengths of paths, garden edgings, fishpond, etc. Dry Mix concrete may be the answer.
Dry Mix consists of the dry ingredients of a concrete mix (cement, sand and stone) measured and bagged and available from hardware stores.
More practical advice can be obtained by purchasing Concrete Basics published by the Cement and Concrete Association of Australia.
Planning a Concrete Paving Project
Points to be considered:
1. Decide on the area to be concreted. Draw up a plan and write in all measurements. Peg out the site. Stretch cord lines supported on the pegs
to define the area. Now remove all turf and vegetable matter.
2. Determine the finished levels of the work; write them on the plan.
Check on drainage required - allow a minimum slope of 20 mm per metre (where drainage is necessary); indicate this on the plan.
Ensure that water will not run:
- towards house footings
- into garage or carport
- into neighbour's property.
3. Determine thickness (depth) of concrete; write it on the plan, eg: a depth of 100 mm is normal for drive-ways, carports and garages used by passenger vehicles but not for trucks. Calculate volume of concrete required.
4. Concrete must be contained by formwork placed along the edges of the area to be formed. Timber boards are generally used and should be properly
braced so that the formwork is sufficiently strong to support the weight of the concrete. Joints between boards should be watertight to prevent leakage of cement slurry.
Allow for drainage and any other requirements shown on the plan and make sure that the ground is firm and even, particularly under the formboards.
Spread a 10-mm layer of sand or fine gravel over the entire area to level it and act as a slip joint between the ground and the concrete.
5. Decide on the surface finish - remember that a steel-floated, trowelled finish produces a smooth surface and is best used internally, externally a wood float or broomed finish is advised.
6. Decide on reinforcement requirements and/or the position of control joints; write them on the plan. Remember, when using reinforcement, it is generally discontinued at the joints to provide crack control.
7. Assess labour requirements; how many able-bodied helpers will be needed? Determine the delivery time of the concrete to the site. Will the
concrete be mixed on site or will pre-mixed concrete be used? Will a concrete pump be required? Remember, if using pre-mixed concrete and/or a concrete pump, check with the pre-mixed concrete supplier the recommended grade of concrete for the particular application, and the maximum time allowed for trucks to stay and discharge the concrete. If the allocated time is exceeded, a surcharge may be incurred.
1 m = 1000 litres. A 50-litre-capacity wheelbarrow will mean 20 loads each weighing 130 kg. It is better to half fill a wheelbarrow- even though you
increase the number of loads.
8. Assess equipment requirements. Basic tools include shovels or spades, screed board, float, edging tool, jointing tool, wheelbarrows and either plastic sheet or proprietary compound to cure the concrete after it is placed and finished.
When mixing the concrete on site, a mechanical mixer is recommended and can be hired from most hire companies. Be sure to order and arrange delivery in advance. Be sure to order materials, eg cement, sand and coarse aggregate, in advance and determine the time and date for delivery.
9. Ensure that the volume of concrete required to cover the area to be concreted can be handled by the assembled team of workers in one day. If not, form a joint by placing a form board to ensure that concreting stops at a formed vertical face.
10. Under a floor slab for a garage, house or covered patio and whenever the floor slab is to be enclosed and have floor coverings, you should install a suitable vapour barrier (purpose-made plastic sheet) on the ground. The vapour barrier should be overlapped a minimum of 200 mm each time it is joined together - then securely taped to prevent separation. The vapour barrier should also be taped around services such as drainage pipes.
Vapour barrier material can be purchased from most builders' supply outlets and major hardware retailers.
11. Determine reinforcement requirements, where it will be positioned and where you will make control joints during the finishing stage. Place reinforcement correctly on bar chair supports.
12. Determine the extent of the work - refer to the plan and pegged out site and ensure that all aspects of the work have been considered.
It may be worthwhile, and will cost very little, to form a 1-m x 1-m trial area to test placing, finishing and curing techniques. Furthermore, a small practice strip of unreinforced concrete can be readily removed and disposed of if the desired result is not achieved.
If after this assessment of the scope of the work, the job appears too large to handle, call in a concrete contractor and use the plan to instruct him about the work required, otherwise proceed.
13. Order concrete, if using pre-mixed concrete.
14. Place, finish and cure the concrete.