.png)
Exposed aggregate concrete: how the finish is made, and the choices behind it
Exposed aggregate is concrete with the top skin washed away, so the stones in the mix become the surface. It looks simple, and it's decided almost entirely by timing. Here's how two of our jobs were made, and the choices that change how the finish looks.
Two jobs, two looks
Point Chevalier: a 70 m² driveway and path in 20 mm Mangatangi aggregate with black oxide, bordered in Uruti pebbles.
Torbay: a 40 m² front patio in 20 mm Longburn pebble with black oxide, with a curve leading from the street to the house.
How the aggregate is exposed
After the concrete is placed, bullfloated and steel-finished, a retarder is applied to the surface. It slows the curing of the top layer only, so a few hours later that layer can be washed off to reveal the stones underneath while the concrete below keeps curing.
The timing is the whole thing: when to finish with the steel trowel, when to apply the retarder, and when to wash off the top slurry. Too early and the stones loosen; too late and they won't come through. None of it can be redone once the concrete has gone off, which is why experience matters more here than almost anywhere else in landscaping.
Point Chevalier: 20 mm Mangatangi, darkened
The concrete is 110 mm thick, 25 MPa, reinforced with 665 mesh and a D12 border all round. The surface is 20 mm Mangatangi exposed aggregate from Atlas Concrete, with 3 kg of black oxide per cubic metre. The oxide gives a darker tone that blends in better and feels warmer. The borders are Uruti pebbles from Central Landscape Supplies. No sealer was used.
Saw-cut expansion joints went in the next day, at no more than 3 m apart, matched at the corners and evenly spaced for looks.
Two practical details. The mixed aggregate size isn't suitable for most concrete pumps, so the concrete was barrowed into place. And a sucker truck managed and removed the slurry washed off the surface.
6 weeks on site
Torbay: 20 mm Longburn pebble, a curve and no sucker truck
The Torbay front patio is about 40 m² of 20 mm Longburn pebble exposed aggregate with 4 kg of black oxide per cubic metre, 665 mesh with D12 borders, and 3 m saw-cut joints. No sealer. It was made the same way as Point Chevalier, with one difference: there was no sucker truck. The slurry stayed on site and was removed afterwards into a bin.
A curve was built into the concrete to make an inviting path from the street to the front door.
6 weeks on site
What actually decides it
The aggregate. Type and size set the character. Mangatangi and Longburn are both 20 mm, with different stone colours.
The oxide. A few kilograms of black oxide per cubic metre darkens the concrete between the stones: 3 kg at Point Chevalier, 4 kg at Torbay. Playing with aggregate type, size and oxide colour gives a large range of looks.
The borders. A pebble border, like the Uruti at Point Chevalier, frames the concrete and gives water somewhere to go.
The cuts. Saw-cut joints control where concrete cracks as it shrinks. Placed evenly and matched at the corners, they become part of the design instead of an afterthought.
The timing. Everything above can be chosen from samples. The timing of the wash can't. That's the crew.
Where we fit
We pour, finish and cut exposed aggregate driveways, paths and patios ourselves, and we work out the falls and drainage first, because a good surface over bad drainage still floods. More on that in fixing a driveway or patio that floods.