In the last couple years, there has been a revival of interest in exposed aggregate, in part due to the introduction of micro-exposed finishes. Surface retarding agents have changed in recent times as well. It's much easier to use them and for contractors to achieve quality results with less risk. Some products now include polymer resins to help reduce movement of the material once they're applied. Dennis Baugh, architectural product specialist for WR Grace, Cambridge, Mass., says that these resins cause the retarder to “dry,” forming a crust on the surface that can protect fresh concrete even from intermittent rain. They also restrict moisture loss from fresh concrete during the initial set period. Covering a slab with poly plastic after applying surface retarders isn't necessary anymore.
Companies sell products with different profile ratings. Chris Forgey, an industry market manager for WR Grace, says their product, Top Cast, comes with 11 different profile ratings; the least profile etch being classified as a “micro etch” producing a 1 millimeter reveal and the largest profile producing a 11/2-inch reveal. Each of the 11 profiles comes in a different color which also helps the applicator spray it evenly.
Before work is started, you should plan to do a sample. Baugh says it's important to use the mix you intend to install at full cross-section thickness.
The only two pieces of equipment necessary include a dependable sprayer and a pressure washer to do the exposing work afterward. Sprayer types can be either hand pump or airless. The important consideration is the tip. For these more viscous retarding agents, tips can be either conical or fan in design, but must have hole sizes large enough to spray half a gallon per minute. When it's time to spray, it must work properly so there are no time delays. It's wise to pass retarding fluids through a fine mesh screen beforehand. Those used to screen paints will work. Run a test before concrete is placed to be sure there are no problems.
When there is more than one load of concrete involved it's important that the water-cement (w/c) ratio be the same for every load—preferably a lower w/c ratio. This helps to achieve uniformity between placements. It also means that you should be careful about jobsite water additions.
When to expose is related to concrete temperature so it's wise to check concrete temperatures when the ready-mix truck arrives. Electronic infrared thermometers are now inexpensive to buy and will instantaneously record the temperature. Baugh says that as a general rule, concrete that arrives over 80° F can be exposed in as little as four hours but suggests the use of a hydration stabilizing admixture if left overnight during higher temperature placements. When concrete temperatures are lower, exposing can proceed later, generally without issue.
Timing is important. Retarding agents should be applied as the water sheen on the surface of the concrete is disappearing. If there is too much water on the surface, retarders won't penetrate to the proper depth. If the application is too late, the profile depth will also be less than specified due to reduced rates of penetration.
When spraying surface retarders, even coverage is important. If the material is colored you should look for an even colored appearance and in the case of the WR Grace product, you won't get a greater profile reveal if there is a build-up of retarder in some areas and not in others. It's not a good idea to over-spray the retarder onto work from previous placements as removal is difficult. Baugh says using citrus or mild alkali cleaners for removal is the best approach.
Using a hand brush and water, check a small area of your work to see if the concrete is ready to be exposed. You can also tap the side edge of the slab to judge the hardness of the concrete. Using a 2000 psi pressure washer if very adequate to wash off the retarded cement paste and expose the aggregate.
The market for exposed aggregate
Forgey says micro etches or “sand finish” concrete have become the largest market for exposed aggregate concrete now. Seeding special aggregates, including glass aggregates, are also increasing.