Malawi

Malawi is a coloured, exposed aggregate mix, available in the S.E. Queensland area from Nucon.

The stone aggregate in this mix is primarily white and black stone, at an approximate ratio of 1:1. It is also a coloured concrete mix, which makes the base colour of the concrete a darker black colour. There is a very small amount of yellow or light brown stone in the mix as well.

Exposed Aggregate

This image shows the Malawi mix finished and exposed, with water on top creating a wet look. Often exposed aggregate slabs or driveways will additionally be sealed at the end and a wet look sealer helps increase the colour vibrancy and depth of contrast in the appearance of the slab.

Wet vs. Dry Look

As you can see in the picture above, there’s a pretty vast difference in look of wet vs dry, as is the case for all concrete. This photo was taken on the pour and finish day, so curing and acid wash has not occurred. An acid wash on the top of the slab helps remove any residues and helps clean the stone surfaces to make the colours more vibrant. Acid washing is recommended prior to application of a surface sealer.

Sunlight vs Shade

Finally we have a photo of the mix in sunlight vs. shade.

Concreter’s Perspective

For the concreter laying this mix, this was very nice to work with. This was a pump mix in our instance, and the mix closed up nicely. We bull floated, fresnoed, stick troweled and then sprayed the top with a retardant chemical to allow the same day wash off.

Due to the black oxide, taping up surround slabs and structures is heavily recommended, and always be aware of stepping out of the slab and then walking around with gumboots. Mess will spread everywhere very quickly if all workers are not vigilant about keeping the site clean from this, and the black oxide will make clean up extremely difficult, so ensure all workers know to remove their gumboots if they step out of the slab area to go get something.

The placement and finish time of this mix is a little faster than plain concrete, probably due to the oxide colouring. Our pour was done and dusted in 4 hours flat from when concrete arrived to when we drove away (site entirely cleaned up included), so ensure you have enough labour to keep up with it. We had 3 experienced concrete finishers and 2 labourers for the placement, then the 3 finishers did the finishing, wash off and clean up. The driveway was approximately 50m2.

When washing off the work, ensure gutters to drains are dammed to prevent the concrete wash off and oxide going down the drains, and that workers remove the wash off sediment from the dammed gutters as the wash progresses. Masking along where the slab adjoins existing structures like the gutter or kerb is heavily recommended and having a large strip of black plastic at the road end will help prevent a lot of mess to be cleaned up. Ensure you allow for adequate labour to deal with all of this, as this type of requirement is generally additional to a normal concrete pour. The black coloured oxide (any colour of oxide) contributes a lot to extra clean up requirements.

Summary

The finished Malawi exposed aggregate mix make a very nice, high contrast, black and white option. The black colouring in the mix reduces glare on the eyes, but will increase surface temperatures in hotter weather.

Concreters need to consider the following:

• Oxide colouring will greatly increase potential for mess to surrounding areas/structures – mask up walls/fences, tape down protective plastic to surrounding slabs
• Mix will harden and cure a bit faster than usual. Prepare for the finish work time to be about 35% shorter than plain concrete (about 2.75 hours of finish time compared to 4 hours – we poured on a breezy, sunny day that peaked at about 30C. The mix was 80 slump when it hit the ground.)
• Ensure you have an adequate damming system in place before starting to wash off to prevent anything going down the drains. Councils can and will fine you for washing wash off down the drains.

What do you think?

Do you like the look of this mix? Do you like exposed aggregate or not? You can comment below if you like.