by Pietro Scalise
Important information about concrete you should know, before making that "concrete decision"...
Preparation and Groundwork of New Concrete
- Make sure all who will be working with concrete placement are trained and well protected
- Measure and form with edge boards the areas that need to be concreted. Work out the falls the driveways or paths must follow to grates or stormwater pits.
- Finished floor levels must not cover any weep holes along brick walls. Water needs to drain away from boundaries and buildings.
- Dig out roots and grass and reach a firm base for your concrete subgrade. A bed of crushed rock should be used where the soil is weak. It is also a good idea to compact the subgrade evenly as this can avoid potential cracking of the concrete slab or paths.
- A vapour barrier is laid over the entire area to be concreted and overlapped a minimum of 200 mm.
- Install steel reinforcement as per structural engineering details and make sure the steel will have enough concrete cover. This will protect the steel from rusting. This detail is often not given enough emphasis whilst a concrete pour is in progress that can have disastrous effects down the track.
- When ordering concrete always allow 10% for wastage.
- The method by which concrete is transported and placed depends heavily on which is the most cost effective and easiest for the job size. A number of methods can be employed to place concrete in a desired location including a concrete truck, a concrete pump that delivers concrete via a boom or connected steel pipes, crane and kibble or a hoist. On smaller works, the most practical methods may be to unload concrete from the chute, located at the back of the mixer truck, directly onto the driveway or path or onto a wheelbarrow and carted to its location.
- When placing, use a vibrating machine to compact the concrete and rid it of any internal air.
- Initial finishing of concrete begins with leveling the surface to the correct levels with an aluminium screed and bullfloating which helps close any minor imperfections on the surface.
- Final finishes consist of floating, steel toweling, edging, jointing and applying patterns to the surface. Special finishes are also applied and discussed later.
- Curing allows the concrete to stay moist. In so doing, the bond between the concrete paste and the aggregates strengthens.
- Curing can begin immediately after the surface has hardened enough not to be damaged. It allows for a harder concrete surface that is less likely to crack.
- Concrete is placed in the standard method.
- The paper stencil is placed on the freshly floated concrete surface and cut to fit the driveway or path with scissors. A stencil roller is used to embed the stencil paper into the surface of the concrete. You can control the depth to which the stencil is to sit with the number of passes of the float. This will cause that portion of the surface that will become the "bricks" to be raised in relief more so than the "joint".
- Wait till the excess surface moisture has evaporated from the concrete then cast the colour by hand evenly over the entire area. Use about two thirds of the manufacturer's recommended amount at this stage. The remaining one third a second time. This will further embed the stencil and allow the "brick" to be raised further in relief. The desired surface finish can then be applied.
- Wait till the surface has gained enough strength to allow pedestrian weight then remove stencil carefully. The areas covered by the stencil paper will now resemble a grout joint between brickwork. Finally, apply a curing compound to the surface.
- After the concrete has been placed and bull floated using standard concrete practices, the concrete colour is applied by scattering it evenly by hand over the surface after excess bleed water has evaporated.
- A powder release agent must be applied prior to stamping of concrete to avoid the stamp tools to pull upwards any concrete.
- Take great care in positioning the first stamp mat. It is critical to the remaining layout of driveway since all the other mats will have to be placed square to it.
- A tamper is used to firmly imprint the texture and pattern of the stamp into the concrete. Keep the stamps fitted tightly together. This is be crucial to create neat grout lines.
- Allow the concrete to set after stamping for several days, then pressure wash the release agent from the surface. Following this, apply two coats of clear sealer with a roller.
- Over new concrete, the only thing required is to allow at least three weeks time for curing before staining. Over old concrete, it is necessary to remove any ingrained dirt, grease sealers or paintwork and even curing agents since these will prevent the stain from absorbing through. As part of planning and preparation, always test a small area first.
Coatings over existing concrete
- Proper preparation of the concrete surface is paramount in successful polymer cement coating. To this end, surfaces need to be degreased and even acid etched. Other surface coatings need to be removed mechanically such as sand blasting.
- This material can be broomed. Apply 2 coats of the polymer cement coat with a steel trowel then run a broom across to give the texture.
- It can be troweled. The coating is first sprayed on over a skim coat then troweled to give a smooth surface finish.
- It can be stenciled. Firstly a layer of polymer cement skim coating applied. A stencil then placed, then coloured coating sprayed over and worked with a trowel. The stencil is then peeled to reveal a desired pattern.
Concretors and the law
- Always demand that the concretors be licensed for the work they're intending to carry out.
- Ask to see a portfolio of similar works the concretor has completed
- For any residential concreting work over $1,000 demand that a written contract be drawn up.
- For concreting work over $12,000 demand that the concretor obtain Home Warranty Insurance on your behalf.
- For any concrete work is more than $20,000 in value only allow for a 5% deposit. Pay only 10% otherwise.