New technology, new systems, and creativity are changing the look and life of integrally mixed or seeded exposed aggregate hardscapes. A few short years ago, vintage exposed aggregate driveways approached boring; the surfaces were uneven and the stones popped out too frequently. Sometimes people were even told to expect exposed aggregate concrete to lose its decorative stones over time. But today exposed aggregate finishes are emerging as beautiful and durable architectural paving including patterned reveals, multiple stones like marble, shells, or glass, and finishes from sand size upward.
Whether you are installing a driveway, patio, pool deck, or spa, begin with a good subgrade. Provide compacted subgrade to diminish settlement and provide for evenly thick concrete placements. Make sure your layout and grades are accurate to provide good drainage and avoid ponding. Properly reinforce the concrete.
Consider the selection of raw materials for the mix design. If there isn't enough cement paste, for instance, you can get aggregate popouts. If the cement content is too high, it's harder to achieve the desired aggregate exposure.
All aggregate, large and small, must be clean. For integral exposed aggregate mixes, increase the amount of stone you would normally use so that the stones are closer together, or exchange the aggregate for more decorative stone. Pay close attention to control joint layouts-owners are less willing to tolerate cracking in decorative surfaces than in regular concrete.
For the new multiple stone look, plan to seed the surface. "The key is to get a good matrix surrounding the seeded material," says Byron A. Klemaske II, executive vice president of T.B. Penick & Sons, San Diego. "Most people put rock into the surface and pound it down. You need to slowly but surely work the matrix up around the rock. Be meticulous, and do not pour too much at a time," says Klemaske.
When it comes to the reveal, designers today specify different depths of exposure depending on the installation's purpose and design. Some require low depth reveals to expose sand particle sizes for a slip-resistant surface. Others call for removing enough material to see a 1.5-inch aggregate for a visual punch. Because of the range, manufacturers make retarding admixtures for light, medium, or deep exposures. Choose the retarding agent according to the amount of exposure desired. For an even reveal, apply it by spraying with a garden sprayer uniformly across the surface after the last finishing step. Remove the retarded cement paste the same day or the next. For next-day removal, cover the slab with plastic. You can remove the retarded matrix with water and a stiff bristle broom, or for more efficient removal use a pressure washer. Skill with the pressure washer is vital for an even consistent depth of exposure. Fine-tuning involves artful rock selections, stone color, and gradation choices. Finish with a good acrylic or penetrating sealer.
FINDING THE ROCK
Fister Quarry, Batavia, Ill., carries special sands, aggregates, and pigment. "Contractors tell us what they hope to achieve, and we send rock samples to let them develop a mix design," says Chris Fister, owner. The range of stone to choose from is astounding-quartz, marble, granite, feldspar, basalt, and obsidian, to name a few. Aggregates are grouped by name and by color. The design opportunity knows no bounds if you consider the different stone combinations and gradations and the choices of white Portland or an integral color. Fister also manufactures pigments that are formulated to perform well with exposed aggregate surfaces and carries aggregate from companies that like to sell in bulk, although they will ship 100-pound bags. Using regional quarries also makes economical sense because prices depend on weight and shipping distance.
"At Schmidt Design Group we like to use local aggregate, but we do not restrict ourselves," says Glen Schmidt, principal of the San Diego landscape architectural firm. He adds, "We have local rock suppliers that supply us with samples and keep us up-to-date with distributors from Arizona and around California. So we have a palate of stone we can specify from and know that it is available."