New Surface Finishes for As-Cast Formed Concrete

by Kim Basham, PhD PE FACI
Created: January 10, 2011

Allowable formwork fins, other surface projections, surface voids and tolerances are defined in the new ACI 301 specifications.

Rubbed finishes (free of bugholes) consist of either smooth-rubbed, grout-cleaned rubbed or cork-floated finishes. Here a worker is producing a grout-cleaned finish free of bugholes by rubbing a grout...
Rubbed finishes (free of bugholes) consist of either smooth-rubbed, grout-cleaned rubbed or cork-floated finishes. Here a worker is producing a grout-cleaned finish free of bugholes by rubbing a grout into the surface voids.
Do the specifications for your next job reference ACI 301-10 or the latest ACI 301 specification? If so, be sure to read Section 5.3.3 "Finishing Formed Surfaces" in ACI 301-10 and understand the...
Do the specifications for your next job reference ACI 301-10 or the latest ACI 301 specification? If so, be sure to read Section 5.3.3 "Finishing Formed Surfaces" in ACI 301-10 and understand the specific requirements for SF-1.0, SF-2.0 and SF-3.0 finishes. Otherwise, you may end up making costly repairs.
ACI 301-10 requires contractors to remove surface projections exceeding 1 in. for SF-1.0, 1/4 in. for SF-2.0 and 1/8 in. for SF-3.0 as-cast finishes, above . For form-offsets, SF-1.0, SF-2.0 and...
ACI 301-10 requires contractors to remove surface projections exceeding 1 in. for SF-1.0, 1/4 in. for SF-2.0 and 1/8 in. for SF-3.0 as-cast finishes, above . For form-offsets, SF-1.0, SF-2.0 and SF-3.0 uses the standard surface tolerance Classes D, B and A, respectively, below .

Mockup. ACI 301-10 requires a mockup panel for SF-3.0 to show the concrete surface appearance and texture. SF-2.0 also requires a mockup unless otherwise specified. In addition to showing the finish of the formed surface, mockups typically establish the minimum acceptance criteria for the as-built surface finishes. Therefore, mockups should represent the quality of the finish that will likely be achieved during the actual construction.

Concrete contractors should not cast mockups using new formwork but use seasoned formwork that represents the quality of the formwork that will be used on the project. Anticipate formwork wear and tear as the project progresses. Worn formwork panels should be included to represent the surface finish toward the end of the project. Contractors should use the same formwork release agent and application method, concrete placing and consolidation techniques, formwork removal times and curing method as planned for the project. These factors will affect the appearance and texture of the surface finish.

Consider using two different loads of freshly mixed concrete for casting the mockup. One load of concrete should have the lowest anticipated water to cementitious materials ratio (w/cm) and the other the highest w/cm allowed by the project documents. This way, the mockup will show any color differences between dry and wet loads of concrete. Also, use several formwork panels so the mockup will have seams. Make tie hole and surface repairs so the mockup truly represents the finish that will be produced during construction.

Unspecified As-Cast Finishes

When the contract documents do not specify a specific form finish, ACI 301-10 requires the contractor to apply a SF-1.0 for surfaces not exposed to view and a SF-2.0 for surfaces exposed to view. This requirement is similar to the old ACI 301-05 specification that required a Rough-Form Finish for surfaces not exposed to public view and a Smooth-Form Finish for surfaces exposed to public view.

Because ACI 301-05 failed to define “exposed to public view,” post-construction disputes sometimes arose between owners/architects and contractors as to the required surface finish for unspecified as-cast finishes. Changing to “exposed to view” may simplify the criterion, but disputes as to what surfaces are exposed to view and not exposed may still occur. For this reason, concrete contractors should address unspecified as-cast finishes in the bid documents to avoid post-construction disputes.

ACI 301-10 Specifications for Structural Concrete is now available in a hard copy or a downloadable PDF from the American Concrete Institute, www.concrete.org.