Sprouts says that the concrete mix design and selection of raw materials used in the mix is important to the result. If there isn't enough cement paste, aggregate pop-outs result. If the cement content is too high, it will be difficult to achieve the aggregate exposure desired. Admixtures commonly used in concrete, like water reducers and air entrainers, will generally have little effect on the outcome when used properly. Sprouts says that the concrete supplier or the admixture manufacturer should be consulted for recommendations on the use of these products in exposed aggregate mixes.

Clean aggregate is also important, including both coarse and fine aggregate. Aggregate should be protected from dirt contamination and from chemicals such as efflorescence-producing salts.

Kati Hazrati, the North American product manger for Grace Construction Products, says that the water-cement (w/c) ratio of a concrete mix affects the results. Generally speaking, the higher the ratio, the deeper the reveal. And conversely, the lower the w/c ratio, the more difficult it is for the retarder to work its way into the surface. She adds that European contractors tend to use higher w/c ratios for their exposed aggregate slabs. Consider using a w/c ratio of 0.45.

Points to consider when planning for exposed aggregate work

The list of conditions that can affect the outcome is large. Both Hazrati and Sprouts stress the need to construct jobsite mock-up panels first. Do them under the same weather and jobsite conditions that the work will be installed under. If you decide to change any of your procedures during a project, do another mock-up to be sure of the result. Here's a list of conditions that can affect the final appearance of your work:

  • Hazrati stresses that the surface retarder be sprayed on at the right time. Wait until the bleed water goes away. If you apply the retarder too soon or too late, the expose will not be uniform. The concrete surface shouldn't appear dry before placing the surface retarder—timing is everything.
  • It's important for ready mix to arrive on the jobsite in good condition. When ready-mix temperatures are over 85° F, adding a retarding admixture to the concrete at the batch plant can help to insure that a slab isn't hydrating too fast for the surface retarders to react properly.
  • When a polyethylene cover is used, it must make good contact with the concrete surface. Concrete in areas that are exposed to air dry out faster, resulting in less exposure.
  • Changing weather conditions will influence your work. If it is hot one day and cool the next, for instance, you must still expose the slab when the concrete is at the same hardness. Again, timing is everything.
  • Sprouts says that surface retarders work by penetrating the top surface and temporarily retarding the setting of the concrete at the desired depth. Cement paste below this layer sets normally and bonds the aggregate to the concrete matrix. The window of opportunity for exposing the aggregate (depending on temperature, weather and other conditions) begins at final set or as soon as the surface can support the weight of a person. In some cases this window may be as long as 24 hours, depending on the type of retarder selected.
  • Whether you expose the aggregate in a concrete mix or seed aggregate on the surface, finishing steps should ensure that cement paste surrounds the aggregate without depressing it much below the surface of the slab. Obviously, using a jitterbug is not a good practice.
  • The application of surface retarders is performed by spraying it over the surface of a slab. Do not perform any further finishing processes after the application. Plan on using one gallon every 100 to 300 square feet (check manufacturer instructions). Apply until the color of the retarder is uniform on the surface.
  • The retarded mortar matrix can be removed with water and a stiff bristle broom, but a pressure washer is a more effective and efficient method of removal. The amount of pressure needed to expose the aggregate can vary, but typically 1000 to 1500 psi is sufficient, and the skill of the operator is very important in producing a consistent depth of exposure