Exposed aggregate finish is a decorative finish applied to concrete slabs usually for aesthetic reasons. The aggregate includes larger stones and pebbles usually worked into the interior of the concrete slab. When exposed, it gives a pebble-like finish to the concrete. As an added benefit, the exposed aggregate produces a rougher surface that reduces the possibility of slips or falls. While an exposed aggregate finish looks good and provides better footing, the process adds little to the cost of the concrete project. Most do-it-yourselfers with the tools to accomplish a concrete pour will also have the ability and equipment to produce an exposed aggregate finish.
Build the concrete form in the usual manner for any concrete slab. Make sure the forms are level or provide the proper slope if drainage is needed. Pour the concrete into the form in the standard manner. Use a screed, a straight board that reaches across the forms, to level the concrete. Work the screed back and forth to smooth the concrete. Use a bull float, a wooden trowel, to work the finish of the concrete.
Apply a retarder to the surface of the concrete. A retarder slows the chemical action that causes the concrete to harden. Follow label instructions for mixing and applying the retarder. The retarder slows the curing of the surface of the slab while allowing the interior portions of the slab to cure.
Wash the top level of cement off the slab when the concrete has reached a firm enough set that it can be walked on without the base or interior of the slab moving. A footprint may be left in the cream or paste of the surface of the interior of the slab should be firm. Use a water hose and, if necessary a brush, to work the surface cement paste free and wash it away from the slab. Work carefully to not displace any of the aggregate. Only wash away the paste like material at the top of the concrete slab.
Seal the exposed aggregate slab with a curing compound and a concrete sealer. Apply according to manufacturer’s instructions.
Specify the type of aggregate to be used in the concrete. If the slab will be finished as an exposed aggregate finish the uniformity of the aggregate is more important than a slab that will have a normal finish.
Timing is everything in an exposed aggregate project. Wash the slab early and the aggregate will not be set firmly enough and may shift. Wait too long and the surface may set too firmly and be difficult to wash away. There is no formula to determine how long to wait. Each slab cures differently. Determining when to wash the surface is a judgment call that comes with experience.