Exposed aggregate concrete

An exposed aggregate finish is the name given to concrete where the top layer of cement paste has been removed to reveal the aggregate, resulting in an attractive, low-maintenance and slip-resistant surface – here’s how to achieve one

The colour, shape and size of the aggregates (whether rounded or crushed) can be used to create a unique surface, texture and appearance. Talk to your local supplier to discuss the options available.

When selecting the desired aggregate, it’s important to consider grading – the size distribution of the aggregate particles.

Most conventional concrete mixes use continuously graded aggregates – a well-distributed mix of all particle sizes. However, these mixes can lead to a non-uniform distribution of aggregates when exposed. To achieve a uniform exposure, intermediate aggregate sizes are omitted and a gap-graded blend is used.

The matrix

If the mix is not tinted with pigments, the colour of fine aggregate dictates the overall matrix colour. For a large project, it is advisable to stockpile fine and coarse aggregates from a single source to avoid colour variation.

The colour of the cement also affects the matrix. For more consistent colour, it is advisable to stay with one supplier and cement type.

Adding the Aggregates

Aggregates are normally added using one of three methods:

1. Integral mixing

In integral mixing, aggregates are added to the mix at the batching plant. The concrete is placed, vibrated, screeded and bull-floated, resulting in approximately 2mm coverage of cement paste over the aggregates.

2. Topping mix

With a topping mix, the coloured aggregate is placed only in the upper layer of concrete. The underlying concrete is typically placed to within 25-40mm below the finished level and the topping mix is then placed and consolidated while the underlying concrete is in its plastic phase.

3. Seeding the surface

Course aggregates can also be added to freshly poured concrete by sprinkling them over the surface and fully embedding them by tamping and working with a wooden float. This method is known as ‘seeding the surface’.

Exposing the aggregates

The most common method of exposing aggregates is the ‘wash-off’ method. When the concrete has stiffened sufficiently to support a person whose footprints are no deeper than 2mm, then surface cement paste can be washed off to expose the aggregate.

A medium bristle broom and continuous water spray are used. The surface should not be over-broomed, as this can result in dislodging the stones and mixing water into the underlying cement paste, which can weaken the surface.

While it is not uncommon to wash off the cement paste with a water blaster, high-water pressure should be avoided as it can damage the surface. Proprietary spray-on retarders can be used to prolong the available working time and give a consistent and predetermined depth of exposure.

These retarders are applied with a backpack sprayer, typically with a long spray boom to reach the centre of the slab.

After curing, a mild acid wash (5% hydrochloric acid) is sometimes applied to remove any remaining cement paste that may dull the appearance of the exposed aggregate. The surface is then thoroughly washed and a sealer applied if desired.

Protecting the environment

The wash water used to expose aggregates contains sediment and has a high pH (alkaline) level. If allowed to run into waterways, it can chemically burn fish, insects and plants.

Advice on good practice is explained in the New Zealand Ready Mixed Concrete Association (NZRMCA) document On Site Management of Concrete Washwater, which can be downloaded from www.nzrmca.org.nz.

The advice is to prevent discharge to waterways by discharging to land or collecting runoff for off-site disposal.