Laying it down

Mix the concrete for your home.

Mixing

There are two types of mixing techniques you can use, either by hand or by using a concrete mixer. Concrete mixers can be purchased and if you intend on working on more than one project using concrete over time, then it may pay to invest in this labour - saving machine. However, for small projects, mixing by hand is perfectly acceptable.

Hand mixing

If you need to mix a small amount of concrete, this is a perfect technique and not too difficult. Rustle up a shovel and a couple of buckets, plus a hose and a hard surface on which to work. Measure out the sand and the gravel into a heap on your hard surface and make a crater in the centre of the heap. Into the crater, add your cement and then turn the whole pile with your spade until the colour becomes uniform.
Once you have completed this, make another crate in this mixed pile and add water. Begin to turn the material into the centre from the edge of the pile and keep adding water as required. The mixture is ready when its neither too crumbly nor sloppy; it needs to be pliable and easy to work.

Run the back of your shovel firmly across the mix to test it and ensure the mark left by your shovel holds and the surface of the mix is uniform and not showing an excess of cement and water.

Machine mixing

To ensure the accuracy and consistency of your mix, measure all of your materials out in containers as per the Golden Bay cement table on following page.

Follow these How To steps for best results:

1. Turn on the mixer, ensuring you are using a residual current device or isolating transformer, as you will be using water.

2. Flush the inside of the concrete mixer with clean water - this will remove any particles from previous use - then drain.

3. Place approximately half the measured water into the mixer.

4. Then place 75% of the measured aggregate into the mixer.

5. Place all of the measured cement into the mixer.

6. Place the remaining aggregate into the mixer.

7. Allow the mixture to turn for two or three minutes, while you slowly add the remaining water.

8. Keep an eye on the consistency of the mix as you add this water and stop to check if the consistency is firm, but quite plastic.

9. Once you have your desired consistency, note the amount of water used as a guide for the next batch.

Laying concrete

As per the previous instructions in the May issue of How To magazine, you will have all your groundwork in place, including your boxing, formwork and reinforcing.

Slightly dampen the ground where you are to place your concrete, as this will prevent puddles forming. Now spread your mixture evenly between the forms and rake smooth to a level 15mm above the finished height you desire. Make sure the concrete gets into all the corners and tap the form with your shovel to force out any air bubble or cavities.

Now it is time to screed the slab and although there are tools available at Mitre 10, for a one - off project about 1.5m, a length of 50mm thick timber on edge will do the job. Shuffling the beam from side to side and slowly forward using the form as a guide will level the concrete. Look out for low spots in the concrete. Re - spread the mix and repeat this process.

Finishing concrete

Now that you have a reasonably smooth concrete layer with a slightly rippled surface, you may want to add texture to avoid any slip issues in the future. To do this, you can simply draw a stiff garden broom evenly across the surface of the concrete while it is still wet or use a softer broom for a finer finish.

Look out for bleed water! This will normally dry up on its own through evaporation, however. if you drag your broom through the water. you will end up with a dusty surface when the concrete hardens. Use a steel float to achieve a smooth finish if thats what youre after, but be mindful that if it is an outdoor path, you may create a dangerous, slippery surface.

Exposed aggregate concrete

You can expose the aggregate to create a unique finish. Removing the top skin of the mortar before the slab dries achieves this look. Brush the slab whilst using a fine mist setting on the hose to wash away the fine material to expose the aggregate - try it on a small section first to make sure you have the technique correct.

You may also want to add coloured aggregate to the top layer of the still - wet concrete mix. Use your float to make sure the surface is even, then leave it for a while until the concrete hardens further and then apply the broom and fine mist technique.

To help with exposing aggregate, use Rugasol C from Sika immediately after the initial bleed water has disappeared from the concrete and before setting has commenced. Apply it to the surface using a garden sprayer to achieve uniform coverage.

Rugasol C will retard the set of the top surface layer of concrete to a depth of approximately 3 mm - 6 mm. It is essential that the soft cement surface be removed before this retardation period has expired, otherwise the surface will be difficult to remove.

In order to achieve the desired exposed aggregate finish, it is essential that you monitor the concrete to determine when the soft surface layer of cement should be removed.

The length of the retardation period will vary depending on the cement content of the concrete, ambient air temperature, the amount of Rugasol C applied and the time it is left on the surface before removal.
This surface layer should be removed using a hose or low - pressure water - blaster and a soft broom, until a good clean exposed aggregate finish is revealed.

Curing concrete

Concrete needs to be cured - it is an essential part of ensuring the finished surface lasts and is strong. As soon as the concrete has dried to a point where it is hard enough not to be marked, it is time to help the concrete cure.

Curing should be carried out for seven days by spraying water on the surface to keep it continually moist. The addition of a curing compound like Sika Antisol, sprayed directly onto newly laid concrete, will not affect the normal setting action of the concrete; a thin film barrier is formed to slow down the rate at which water evaporates from the concrete surface during the first few crucial days of curing.

Curing compounds will assist in:

Reduction of shrinkage and cracking.

Improved strength and durability.

Reduction in surface dusting.

Increased abrasion resistance.

Reduction in the permeability of the concrete.

This enables more water to be available for the hydration of the cement. The concrete can then cure more efficiently, eliminating the need for curing methods such as wet hessian, polythene sheeting and continuous watering.

Visit your local Mitre 10 store and talk to the team about your next concrete project - you will get all the help you require. With the great products available from Golden Bay cement and Sika, your concreting career is only just beginning.