Whereas reconstructed stone finishes are typically achieved by acid etching, removing surface laitance to reveal the natural colour of the fine aggregate or sand, our exposed aggregate finishes additionally reveal its coarser aggregate.
This may be done by a variety of techniques with each requiring specialist expertise and experience in order to achieve a consistent appearance, both within and between each individual precast panel.
Grinding the surface of a concrete panel by machine, typically to a depth of 3 mm, produces a smooth terrazzo-like appearance. This may be honed to a dull eggshell polished finish or progressively ground using a sequence of finer abrasive wheels to achieve a high-gloss finish.
Exposed aggregate finishes may otherwise be achieved by removing that cement matrix that surrounds the coarse aggregate at the face of the concrete panel. Depending on the concrete mix, the shape of the panel and the degree of aggregate exposure, we use the following techniques:
For mixes with coarser aggregate up to 20mm, our preference is to cast each panel face-down using proprietary retarder in order to “kill” the setting of the cement at its seen face. On demoulding, the panel is washed with a high-pressure water spray to remove the “dead” cement, resulting in a coarse textured surface as the coarse aggregate in the concrete mix is exposed. The choice of retarder determines the depth of aggregate exposure.
For coarser textured surfaces with more deeply exposed aggregate finishes, there is the option of casting each panel face-down with largest aggregate hand-placed in a sand bed before the casting of the concrete. On demoulding, the panel is washed with a high-pressure water spray to remove the sand, resulting in a coarse and deeply textured surface. This technique may be varied by the use of gelatine instead of sand. Using 4” and 6” knapped flint, the cladding panels for the West Quay shopping centre and car parks in Southampton were produced this way.
Coarser textured surfaces with more deeply exposed aggregate finishes may alternatively be casting face-up with largest aggregate “seeded” and stamped into the concrete panel before its surface is carefully sprayed with water to remove the cement matrix. Using nom 40mm and nom 30mm aggregates, the cladding panels for the Clifton Cathedral in Bristol, the European Investment Bank in Luxembourg and the IBM Marketing Centre on the South Bank in London are examples of this technique.
As further options to expose the coarse aggregate in concrete panels, their surfaces can be grit-blasted, bush-hammered or needle-gunned.