Exposed Aggregate Concretes

There are two basic ways of constructing an exposed aggregate finish to a concrete; the first, Surface Dressing, acheives the effect by pressing the decorative aggregate into the body of the freshly poured concrete, whilst the second, Washed to Expose, which is preferred for heavy-duty or commercial use, is brought about by stripping away the matrix from the top 2-6mm of the concrete to reveal the constituent aggregate.

This method is used to keep costs to a minimum and is popular on smaller jobs such as garden paths and patios. Also, the skill level required is relatively low and it's the sort of job that can be handled by the keen DIYer. However, because the decorative aggregate is just the depth of a single layer of stones, any loose aggregate might be flicked out by wear or trafficking, revealing the plain, boring, bog-standard concrete beneath.

Construction follows the principles described on the Concrete Hardstanding page, in that a sub-base is prepared, a damp proof membrane laid and the concrete placed. The concrete, which is typically a C20-C35 mix depending on planned use, is tamped to a level approximately 4-9mm below finished paving level. This may be achieved with a hand tamp (ie, long, straight, plank of wood) a vibrating beam screed or a bug-roller as used for decorative and Pattern Imprinted concretes.

Once the freshly-placed concrete has been tamped, the decorative aggregate is introduced and spread evenly over the entire surface. A hand tamper is then brought in and is used to 'pat down' the decorative aggregate, forcing it into the surface of the concrete and simulatenously forcing up sufficient 'fat' to hold the stones in place.

The degree to which the deco-agg is forced into the concrete depends on the expertise of the operative, the size of the aggregate itself, the condition of the concrete, the desired finish and the planned use. Forcing the deco-agg down well into the concrete and the subsequent pumping of fat to the surface may require extensive 'cleaning' of the finished surface, but will ensure that the deco-agg is held securely. Conversely, just settling the deco-agg into the concrete will reduce, or even eliminate, the requirement to spend time cleaning-up, but the deco-agg isn't held as securely.

The amount of cleaning required will determine the best method. Where only a light cleaning of fat/matrix is necessary, it can be scrubbed off or rinsed off with water, but if there is extensive staining, treatment of the surface with a retarding agent may be a better option, as it allows the cleaning to be done a few hours later, when the concrete has cured somewhat and is holding the deco-agg securely enough to withstand a hose, a brush or even a power washer. Generally, a clean, angular gravel is preferred for this type of work. The size is usually in the range 6mm-25mm, and a single size is often used, although a graded size range, say 10-15mm may be used to achieve a particular look. The stone itself can be almost anything, but the most popular choices are richly coloured rock types, such as black basalts, red granites or white flint. There have even been experiments using tumbled, recycled coloured glass.

Selected pebbles and larger flattish cobbles set into a reddy-brown mortar matrix were used to create the look of a dry river bed in the outback of Australia in a garden exhibited at the RHS Chelsea Flower Show in 2011 .

Using a coloured mortar over a standard concrete base as the 'background' for a surface dressed application opens limitless new possibilities in terms of design.

This second method of producing an exposed aggregate concrete is the more popular for heavy-duty applications. While the aggregate is usually held more firmly than with surface dressed types, the vast majority of what may be a relatively expensive aggregate never gets to see the light of day, as it is permanently encased within the body of concrete itself, and so, where expensive aggregates are required, they may be applied as a thin 'top coat' over a base of plain concrete or 'backing concrete' as it is known. This two layer construction is considered later in this section.

The case study illustrated below follows the creation of an exposed slate aggregate concrete we've recently developed for a project in N.Wales. The concept was to create a natural looking pathway through an important tourist area, using local materials and local skills, that would provide easy access for wheelchairs and other disabled visitiors, and be capable of withstanding the annual flooding of the adjacent river.

The concrete was first developed and tested to allow us to see how the aggregate behaved during mixing, during transit to the site and during placement. Several trial batches were prepared to determine an aggregate grading envelope that provided us with a good selection of sizes but would not jam in the feed hopper or sink out of sight during placement. Once the grading envelope was determined, and colour choice finalised, a concrete mix was prepared to give a 28-day strength in the C30-C35 range. Test panels were prepared for the client to ensure the finished surface was suitable aesthetically, but also to determine the most appropriate placement method and to provide strength testing samples.

Client:

National Trust Wales, Craflwyn, Beddgelert - 01766 890659

Contractor:

McCarter Group, Caergeiliog, Anglesey - 01407 742791

Concrete Supplier:

Hogan Concrete, Caernarfon Road, Bangor - 01248 353595

This procedure can be undertaken without the use of a retarder, although timing of the wash off then becomes more critical and the amount of skill required is considerably increased. The freshly placed concrete will need to be repeatedly hosed down and brushed off from approximately one hour after placement until such time as the desired effect is achieved. Obviously, the time span will depend very much on ambient temperature and atmospheric humidity, and this can lead to problems, particularly with concrete laid in the late afternoon when the wash-off process might not be completed before dark. Also, great care is needed to ensure that the amount of deco-agg that is removed is kept to a minimum, and only the matrix is washed off.

As mentioned above, where the deco-agg itself is expensive, it is possible to use a two-layer construction, placing a relatively thin layer of the deco-agg concrete on top of a cheaper backing concrete.

This two-layer construction is the basis of the Face Mix Blocks that are extremely popular in Ireland and continental Europe, and it has been combined with the 'wash-to-expose' methodology described above to enable the manufacture exposed aggregate block pavers , which bring all the aesthetic apeal of an exposed aggregate concrete in an easy-to-lay modular form.

With a cast in-situ slab, the thickness of the deco-agg layer varies depending on use and size of aggregate involved, but will usually be in the range 10mm to 30mm. It is essential that the deco-agg layer is laid over the backing concrete as soon as the backing concrete is in place, in order to create a monolithic slab and avoid the creation of a construction joint or plane of weakness at the interface between the backing and the deco-agg layer.