Exposed aggregate concrete is a popular choice for driveways, patios and pavement areas. It tends not to show tyre marks in high traffic areas whilst has the warmth of the natural stone appearance. In combination with oxides which alter the cement paste colour between the aggregate many possible combinations are possible.
Exposed aggregate concrete is a finish achieved by laying a concrete with the chosen type of aggregate (either naturally rounded or crushed) and applying a “surface retarder” to the concrete once placement has been finished which prevents the top 3mm or so from setting. After a period of time (perhaps hours in summer, or overnight in winter) once the concrete has stiffened sufficiently the surface of the concrete is washed off carefully exposing the underlying aggregate. The depth of exposure is greatly controlled by the initial stiffness of the concrete and the climatic conditions during laying, the concrete layer will judge the appropriate time to apply the surface retarder and to wash off using his experience. After a period of one to two weeks a process of “acid washing” is carried out to remove any “lime bloom” from the surface of the concrete (white or milky patches) as well as cleaning any cement from the surface of the aggregate.
As with any concrete project the same basic principals apply which are; well compacted base course (dampened prior to placement), well positioned reinforcement (if mesh is used), sensible concrete thickness (we recommend 100mm min. for residential works), consistent water curing as soon as possible, well positioned joints (tooled joints may be required at high risk areas), saw cutting as soon as is practical.
Saw cutting into squares of 1.0-1.6m greatly enhances the appearance of coloured concrete pavements, acid washing will remove the fine cement slurry from the surface of the concrete providing a fine “sandpaper” textured finish providing more grip but will also darken the concrete colour so take this into account when choosing your colour. Sealing the surface of your new exposed aggregate concrete will prevent staining and will enhance the natural beauty of the aggregate by giving it that “wet look” appearance.
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