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Exposed aggregate is a type of concrete paving in which the gravel used in the concrete mixture remains visible on the finished surface. While traditional concrete has a smooth finish, exposed aggregate offers a highly-textured finish, which reveals the natural beauty of the stones. This type of finish represents one of the oldest forms of decorative concrete, predating modern etched or acid-washed paving.
Exposed aggregate finishes generally require a bit more planning and prep work than standard concrete. Installers must mix the concrete to just the right consistency, using more water than normal. Care must also be taken when choosing the aggregate. While aggregate typically acts as a filler to give concrete its strength, it is primarily chosen for its aesthetic appeal when it comes to exposed finishes. Installers should inspect stone for color, texture, and size in order to achieve the desired look.
This type of finish is best suited to low-traffic applications. Exposed aggregate finishes are often chosen for patios and walkways, as well as concrete porches. They may also be appropriate for some driveways, but are generally not used on roads because they cannot accommodate heavy traffic loads.
The process of laying exposed aggregate starts with pouring a fresh concrete base. The base is then troweled smooth using a wooden float or trowel. Next, the gravel or aggregate mixture is poured over the concrete and spread using a rake or broom. Installers use a piece of lumber or a concrete float to press the stones into the top of the wet concrete, embedding them in place. Finally, a lawn roller is used to smooth out the surface and remove any unwanted lumps or bumps.
Sealing represents one of the most important steps in the exposed aggregate installation process. Unsealed aggregate can quickly become dirty, and may wear away over time due to foot traffic. It's also vulnerable to moisture and other outdoor elements.
Installers can choose between two basic types of sealers, including surface and penetrating products. Penetrating sealers tend to be the most durable, but also the most expensive. These sealers are absorbed into the aggregate, resulting in a matte finish. Surface sealers form a thin film over the aggregate, resulting in a wet, glossy appearance. They are the most affordable option, but also tend to wear away fairly quickly.
Buyers should use caution when choosing a sealer for exposed aggregate. While many prefer a wet, glossy look, this type of sealer often leaves the surface of the concrete fairly slippery. Grit or sand can add texture to the sealer, resulting in better traction and reduced fall risk.