How to Raise Aggregate Concrete

Exposed aggregate, in which the aggregate is raised to the surface, is an attractive, decorative finish for concrete. Raising the aggregate highlights the color, form and texture of the stone, bringing out a distinctive visual quality in concrete work. A variety of objects can be used for aggregate, including marbles, seashells and recycled glass, but local aggregate is often the best choice. Aggregate is exposed -- raised -- by removing surface mortar before it hardens, to reveal the top 1/3 of stones. Washing and brushing the mortar away is a standard technique. Apply an acid wash and sealer for a high-gloss finish.

Materials for form, place and finish of concrete
Chemical retarding agent
Sprayer or pressure washer
Stiff medium-bristle broom
Safety glasses
Rubber gloves
Rubber boots
Respirator
Acid wash -- 1 hydrochloric acid to between 10 and
20 parts water
Clear plastic spray container
Transparent concrete surface sealer, nonyellowing, UV-resistant type
Short-nap roller brush or airless sprayer

Instructions

- 1
  Choose aggregate for the concrete mix according to decorative qualities or specify to the concrete supplier the quality of the desired aggregate. Prepare the subgrade. Form, place, screed and bullfloat concrete as usual.
- 2
  Spray the retarding agent on the concrete once finishing is completed, following the manufacturer's directions. Cover the surface uniformly.
- 3
  Wait for the concrete to stiffen to the point at which the aggregate won't be dislodged or removed when the surface mortar is removed. Test for readiness by lightly brushing the surface mortar with the broom.
- 4
  Fill the spray or pressure-washer tank with clean water. Spray the mortar from the surface. Sponge concrete from small sections.
- 5
  Use fine spray on sections with small aggregate -- 3/8 inch or smaller. Don't let water build up to a point where stones are exposed more than 1/3 of their depth or are washed out completely.
- 6
  Cure the concrete for a minimum of seven days.
- 7
  Thoroughly wet the area with water. Let it dry completely. Don safety glasses, rubber gloves and respirator. Mix acid wash in the plastic spray container. Apply as directed by the manufacturer, removing the fine cement film from the surface. Rinse completely with clean water.
- 8
  Apply the transparent sealer with the short-nap roller brush or airless sprayer, according to directions.

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