Weekend Project

Justin Newcombe's tips on outdoor DIY projects

Weekend project: Ready? Get set

By Justin Newcombe

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Concrete is so versatile, it's a wonder we don't use it more often, writes Justin Newcombe.

Justin Newcombe with his finished concrete slab. Photo / Natalie Slade
Justin Newcombe with his finished concrete slab. Photo / Natalie Slade

Concrete is amazing stuff. You can mix it like porridge, move it around, flatten it, mould it, decorate it and then it sets like rock into whatever shape you like. It becomes a long-lasting substance ideal for walking or driving on or for something more structural.

So why is it we tend to shy away from concrete for those smaller projects around the home? The tendency for most people is to go for pavers. Nothing wrong with that, I'm not anti-paver. I just think sometimes we could come up with something a little more unique and there is more chance of that happening if we use concrete.

Of course things can go wrong: once it's set, that's it baby. But if you're considering upgrading those steps from pebbles to something hard or making a solid surface under the clothes line then decorative concrete may be just the thing. Concrete can be coloured using oxide, it can be textured, or the surface setting can be what is known as retarded (meaning it doesn't set with the rest of the slab) so that the surface can be washed to reveal the aggregate beneath. Decorative pebbles and stones can be introduced randomly or in a pattern like a mosaic.

In this project I've also installed a little bit of drainage, using a new DIY product from Bunnings that makes thing easy. And because the slab is thin, at 60mm, I'm also using a bagged concrete which is self-reinforcing. The slab may still crack but it is worth a shot and at the end of the day it's not the front landing, it's just the back yard.

Step 1

Clear and excavate the area. Be thorough with the edges and heights. I've dug down 100mm but ideally you should go for a depth of 150mm. It all depends on what the ground is like and how heavy the traffic will be going over the surface.

Step 2

In my case I don't need to do any boxing because all the edges of the slab I'm going to pour already butt up to hard edges, but if you do, now is the time. Depending on the depth of pad you are going to pour you may require boxing of different dimensions. Most paths are formed using 100 x 20mm rough sawn, untreated pine, which comes in random lengths. Measure and cut the boxing, then arrange it in place. Drive your pegs into the ground on the outside of the boxing and screw the boxing to the pegs from the pegged side. The screws should be as long as possible without going right through the boxing. I use 63mm 8g screws. Screws make removing the boxing much easier later on. Make sure the boxing slopes to create a sloping surface on the finished concrete pad so water can drain away and won't pool on the surface.

Step 3

Install base course, flatten and compact. For my base course, I've used recycled old, broken up concrete topped with some left over builders' mix, then compacted it with a sledge hammer. Spray the base with water before you place your concrete.

Step 4

Mix the concrete following the instructions on the bag. I coloured the concrete by adding black oxide to my mixture. It's important that you measure each amount of oxide so that the colour of each barrow of concrete remains the same each time.

Step 5

Working in meter-square sections, pour and spread the concrete using a screed or long piece of timber, then trowel the surface so the top seals over with slurry. While the concrete is still wet press in any stones or other decorative material you want. The concrete should form a wet seal around the object, which slopes upward. If it slopes downward, the concrete is too dry and the object (pebble etc) will eventually pop out leaving an ugly gap.

Step 6

Once the concrete is installed and all edges are tidy, spray the surface with a retardant, after which you can't work on the surface again.

Step 7

Follow the instruction on the bottle of retardant to work out when the concrete is ready to wash down. Once the concrete has set, wash off the surface and collect the residue in a bucket. You don't want that stuff in your drains and the fish don't want it in the harbour. Dispose of the slurry you collect in at the nearest landfill or concrete recycling plant - if it's a small amount you can put it in the bin.

Step 8

After three or four days remove any boxing and wash the surface with a diluted solution of mineral salts. Make sure you wear all the protective clothing necessary.

- NZ Herald

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