Q.: I live in Wanaka, which is in the south island of New Zealand, and recently laid 150 square meters of exposed aggregate paths and driveway. After laying and during the washing, we had problems getting rid of the water (all but one side of the house is bounded by walls). In addition, the slope was not correct so the water ponded. We kept brushing the water, with the end result that too much of the cement paste was removed, exposing the aggregate, which continues to break loose. Some have suggested that the rough areas can be fixed by plastering and then washing off. I consider this a quick fix that won't last in our severe climate. Others have said to cut out the affected areas and relay, or apply some type of compound over the top, or, most drastically, rip it out and do it again. Can you offer any suggestions?
A.: You have at least two issues-your slab doesn't drain properly and the exposed aggregate is not staying in place and is uneven. Without seeing it, we think that the best remedy may be complete replacement or the application of an overlay. Patching will not work to restore proper drainage. And patching exposed aggregate is very difficult to do in a way where the patch doesn't show. However, it is likely that the underlying concrete is sound; just the surface is damaged.
The best way to create exposed aggregate concrete is to place and finish the concrete, and spray a retarding agent formulated for this purpose over the entire surface. Then cover the slab with plastic. The next morning, wash off anything that will wash off. The top surface of the concrete will still be soft, while the rest of the slab will be hard. The stones are all held in place and the 'exposure' is even. Take care to remove all the washed off cement because it will eventually harden and cause more problems.
For an overlay, there are polymer-based cements on the market now, which are designed for toppings over sound concrete. You could replace the exposed aggregate finish by building up a 1/4-inch overlay and then applying another type of decorative finish. With this method, colored concrete, stamped concrete, acid-etch stained finishes, or textured surfaces can be achieved. This would save you the aggravation of removing otherwise sound concrete. Overlay processes are not cheap, however, ranging from $3.50 to $8.00 (in U.S. dollars) per square foot, depending on the finish.