An exposed aggregate finish is the name given to concrete where the top layer of cement paste has been removed to reveal the aggregate. This results in an attractive, low-maintenance and slip-resistant surface
Concrete NZ outlines the process for achieving an exposed aggregate finish and provides guidance on how wash water should be managed.
Aggregates
The colours, shape and size (whether rounded or crushed) of the aggregates can be used to create a unique surface texture and appearance.
To discover the options available, talk to your local ready mixed concrete supplier. When selecting the desired aggregate, it is important to consider grading. This refers to 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 course aggregates from a single source to avoid colour variation.
The colour of the cement also affects the colour of the matrix. For more consistent colour, it is advisable to stay with one supplier and cement type.
Adding aggregates
Aggregates are normally added using one of three methods — integral mixing, a topping mix, and by ‘seeding the surface’. In integral mixing, aggregates are added to the mix at the batching plant. The concrete is placed, vibrated, screeded and bullfloated, resulting in approximately
2mm coverage of cement paste over the aggregates.
With a topping mix, the coloured aggregate is placed only in the top upper layer of concrete. The underlying concrete is typically placed to within 25-40 mm below the finished level, and the topping mix is then placed and consolidated while the underlying concrete is in its plastic phase.
The coarse aggregates can also be added to a freshly poured concrete surface 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 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 two millimetres, then surface cement paste can be washed off to expose the aggregate.
A medium-bristle broom and continuous water spray is used. The surface should not be over-broomed, as this can result in dislodging the stones. Mixing water into the underlying cement paste 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 to 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 Concrete NZ – Readymix Sector document On Site Management of Concrete Washwater, which can be downloaded from concretenz.org.nz. The advice is to prevent discharge to waterways by discharging to land or collecting runoff for off-site disposal.
The information provided is intended for general guidance only and in no way replaces the services of professional consultants on particular projects. No legal liability can be accepted by the New Zealand Concrete Contractors Association for its use. Text provided with kind permission by the Concrete New Zealand Inc.
© Concrete New Zealand Inc. 2018