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New Surface Finishes for As-Cast Formed Concrete

Three new surface finishes for as-cast formed concrete are defined in the new ACI 301-10 Specifications for Structural Concrete, which supersedes ACI 301-05. Rough-Form and Smooth-Form As-Cast Finishes have been replaced with Surface Finish-1.0, -2.0 and -3.0 and are referred to as SF-1.0, SF-2.0 and SF-3.0 in the recently revised specification. Surface Finish-1.0 (SF-1.0)

  • No formwork facing material is specified

  • Patch voids larger than 11/2 inch wide or 1/2 inch deep

  • Remove projections larger than 1 inch

  • Tie holes need not be patched

  • Surface tolerance Class D as specified in ACI 117

  • Mockup not required

Surface Finish-2.0 (SF-2.0)

  • Patch voids larger than 3/4 inch wide or 1/2 inch deep

  • Remove projections larger than 1/4 inch

  • Patch tie holes

  • Surface tolerance Class B as specified in ACI 117

  • Unless otherwise specified, provide mockup of concrete surface appearance and texture

Surface Finish-3.0 (SF-3.0)

  • Patch voids larger than 3/4 inch wide or 1/2 inch deep

  • Remove projections larger than 1/8 inch

  • Patch tie holes

  • Surface tolerance Class A as specified in ACI 117

  • Provide mockup of concrete surface appearance and texture

Form-facing material requirements. ACI 301-10 no longer specifies form-facing material requirements. The older version, ACI 301-05, specified no form-facing material requirements for Rough-Form Finish but had specific requirements for a Smooth-Form Finish that required the form-facing material to produce a smooth, uniform texture on the concrete. For a Smooth-Form Finish, ACI 301-05 did not allow form-facing materials with raised grain, torn surfaces, worn edges, dents or other defects that would impair the texture of the concrete surface. ACI 301-05 also required facing materials to be set in an orderly and symmetrical arrangement and required the contractor to keep the number of seams to a practical minimum. The revised specification only requires the form-facing materials to produce the specified appearance and texture as specified for SF-1.0, SF-2.0 and SF-3.0.

Surface voids. ACI 301-05 required tie holes and surface defects to be repaired but failed to define surface defects. This was a major shortcoming of the old specification and typically resulted in costly disputes or repairs for concrete contractors, especially when surface defects were not defined in the project documents.

Because ACI 301-10 specifies the size of acceptable surface voids or more specifically specifies what size voids must be repaired, concrete contractors should be able to avoid costly disputes and unexpected repairs.

ACI 301-05 did not address bugholes or surface air voids that commonly occur on formed surfaces - nether does the revised ACI 301 specification. However, bugholes have been indirectly addressed by specifying the size of surface voids that must be repaired. Therefore, bugholes (and other surface voids) are acceptable for as-cast formed finishes unless voids exceed the specified allowable widths and depths. Specifiers should select a rubbed finish (i.e., smooth, grout or cork-floated) if bugholes are not acceptable because the owner/architect wants an architectural finish or a painted surface.

Projections. Fins or other surface projections created by mortar flowing into the joints between formwork panels or into other spaces in the formwork are limited to 1 inch, 1/4 inch and 1/8 inch for SF-1.0, SF-2.0 and SF-3.0. If projections exceed these heights, ACI 301-10 requires removal. ACI 301-05 required fins exceeding 1/2 inch and 1/8 inch be removed for Rough-Form and Smooth-Form finishes.

Surface tolerances. ACI 117-06 specifies the upper limits for abrupt or gradual surface irregularities for as-cast formed surfaces. Classes A-D correspond with exposure conditions or aesthetic requirements of formed surfaces.

Abrupt irregularities such as form offsets are measured within 1 inch of the irregularity and perpendicular to the surface. For gradual irregularities, the gap between the concrete surface and a 5-foot straight-edge is measured between the contact points. Because the surface tolerances are specified in ACI 117-06, classes of surface and the corresponding tolerances did not change in the revised ACI 301 specification.

  • Class A Surface+1/8 inch surface exposed to public view

  • Class B Surface+1/4 inch surface to receive plaster or stucco

  • Class C Surface+1/2 inch general standard exposed surface

  • Class D Surface+1 inch minimum-quality, rough surface acceptable

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, Formed Surface Finishes in ACI 301-10

  1. Matching Sample Finish

2. As-Cast Finishes

  • SF-1.0

  • SF-2.0

  • SF-3.0

3. Rubbed Finishes

  • Smooth-rubbed finish

  • Grout-cleaned rubbed finish

  • Cork-floated finish

4. Unspecified As-Cast Finishes

  • SF-1.0 for surfaces not exposed to view

  • SF-2.0 for surfaces exposed to view

5.Architectural Finishes

What's Included in ACI 301-10? The new ACI 301-10 Specifications for Structural Concrete lays out the requirements for Surface Finish-1.0, -2.0 and -3.0 and Unspecified As-Cast Finishes. Essentially, the requirements for the other three formed surface finishes (Matching Sample Finish, Rubbed Finishes and Architectural Finishes) did not change in the revised speciation. Here is a review of those specifications.

Matching sample finish. Surface finish required to match sample panel furnished to contractor. Contractor must reproduce a mockup of the sample finish on an area at least 100 square feet. Rubbed finishes

  • Smooth-Rubbed - No later than the day following formwork removal, wet surface and rub it with an abrasive such as a carborundum brick until a uniform color and texture are produced. When insufficient paste can be drawn from the concrete itself, use a cementitious materials grout made from the same source as the concrete.

  • Grout-Cleaned - After removal of all formwork, wet the surface and apply cementitious material grout consisting of 1 part by volume of portland cement and 1½ parts of sand. Mix to the consistency of thick paint and scrub grout into voids and remove excessive grout.

  • Cork-Floated - Remove ties, burrs and fins, wet surface and apply stiff grout consisting of equal parts of portland cement and sand. Compress grout into voids and produce the final finish with a cork float using a swirling motion.

Architectural finishes. Produce specified architectural finish including special textured and exposed aggregate finishes.

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