exposed aggregate concrete

The exposure of the coarse aggregate in concrete, by removal of the surface smooth layer formed in contact with the formwork, produces a concrete with a more durable finish and better weathering characteristics, which is frequently aesthetically more pleasing. Smooth, profiled and deeply moulded concrete can all be treated, with the visual effects being largely dependent upon the form and colour of the coarse aggregates used. While gap-graded coarse aggregates may be used both in precast and in situ exposed aggregate finishes, precasting gives additional opportunities for the uniform placement of the aggregate. In face-down casting, flat stones can be laid on the lower face of the mould, which can be pretreated with retardant to slow the hardening of the surface cement. In face-up casting, individual stones can be pressed into the surface either randomly or to prescribed patterns without the use of retardants. Alternatively, a special facing mix may be used on the fairfaced side of the panel, with the bulk material made up with a cheaper standard mix. The aggregate has to be exposed by washing and brushing when the concrete has cured sufficiently to be self supporting. The use of a retarder applied to the form – work face enables the timing of this process to be less critical. The surface should be removed to a depth of no more than one-third of the thickness of the aggre gate to eliminate the risk of it becoming detached. An alternative method of exposing the aggregate in both precast and in situ concrete involves the use of abrasive blasting. Depending on the size of grit used and the hardness of the concrete, a range of finishes including sculptural designs can be obtained.

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