Exposed aggregate affects the surface of the concrete. The surface of the concrete is decorated with coarse aggregate, usually of interesting color and or shape. Exposed aggregate finishes are very popular for sidewalks and driveways. Commercially, they are commonly used to allow the sidewalk finish to match or accent an exposed aggregate finished tilt-up or pre-cast building panel.
There are two methods of producing an exposed aggregate surface.
1 - Wash away a portion of the concrete from embedded rock. The concrete mix typically includes a gap-graded or special colored coarse aggregate. During placement and finishing of the concrete, an ample amount of cement paste is floated to the surface and then sprayed with a surface-retarding agent. Once the concrete has reached initial set, the surface is washed with a high-pressure washer or special nozzle to wash away the paste and expose the embedded aggregate.
2 - "Seed" the decorative aggregate into the fresh concrete. A standard concrete mix is placed, screeded, and floated in the usual manner. The entire surface is then uniformly seeded with wet, clean aggregate. This seeded aggregate is tapped into the surface of the plastic concrete and then embedded by the use of a bull float, hand float or darby until the appearance of the surface is similar to that of a normal slab after floating. Once the aggregate is securely embedded, and the initial set of the concrete is obtained (when the concrete can bear the weight of a person on kneeboards without making an indentation), the surface is brushed with a stiff-bristle broom to remove excess mortar. The brush is then dampened and the surface brushed again. It is then flushed with water several times. This procedure is repeated until the flush water runs clear and no noticeable cement film is left.
Thorough curing of the concrete must follow both of these methods. In most cases, a sealer is applied after curing has been completed.