QUESTION: About 2 months ago, we finished placing a concrete foundation for a large commercial building and now the owner is complaining about surface bug holes. The bug holes are fairly small (less than ½ inch wide) but occur fairly frequently in some places. Until the painters started painting the walls, everyone was pleased with our work.
Now, the owner claims the bug holes are unacceptable because they distract from the overall appearance of the painted walls. The owner claims the bug holes are defects and we are responsible for fixing them.
Are bug holes defects? Are we responsible for repairs?
ANSWER: ACI 347-04, “Guide to Formwork for Concrete,” defines bug holes, or surface voids, as small regular or irregular cavities, usually not exceeding about 5/8 inch (or 15 mm) in diameter, resulting from entrapment of air bubbles in the surface of formed concrete during placement and consolidation.
Bug holes commonly occur in vertical cast-in-place concrete such as walls, columns, beams, etc. Both the number and size of bug holes vary and depend on many factors. These include form facing material and condition, release agent type and application thickness, concrete mix characteristics, and placing and consolidation practices.
The permissible number and size of bug holes in a concrete surface is not defined in either ACI 301-05 or ACI 301-10, “Specifications for Structural Concrete.” In fact, ACI 301-05 does not limit either the number or size of surface voids in as-cast surface finishes.
ACI 301-05 specifies two as-cast surface finishes: rough-form and smooth-form. While it does list other requirements for rough- and smooth-form finishes, it does not limit the permissible number or size of bug holes.
Rough- and smooth-form finishes have been dropped in the more recent 2010 version of ACI 301. Now, as-cast surface finishes are SF-1.0, SF-2.0, and SF-3.0. Like ACI 301-05, ACI 301-10 does not limit either the number or size of bug holes in as-cast surface finishes.
However, ACI 301-10 does limit the maximum size of surface voids to 1½ inches wide or ½ inch deep for SF-1.0, and 3/4 inch wide or ½ inch deep for SF-2.0 and SF-3.0. According to ACI 301-10, bug holes or other surface voids exceeding these dimensions for the specified surface finish are defects and must be repaired. Otherwise, bug holes are allowed and considered surface imperfections, commonly referred to by ACI as surface effects.
The specifications for the project may limit the size of bug holes by specifying maximum widths and/or depths of surface voids. Review Section 03300 Cast-in-Place Concrete of the specifications for the project, especially the sections on Form-Facing Materials, Formwork, Finishing Formed Surfaces, and Concrete Surface Repairs.
Whether your bug holes are defects or not depends on:
- The version of ACI 301 specified;
- The size of the bug holes (the width and depth); and
- The void size limitations, if specified by the contract.
If ACI 301-05 was specified and the contract documents did not limit the size of surface voids, bug holes are not defects but surface effects. If ACI 301-10 applies and SF-1.0, SF-2.0 or SF-3.0 was specified, or the specifications established a maximum void size, then only the bug holes that exceed the maximum allowable void size are defects. Otherwise, bug holes are surface effects. Only agree to repair those classified as surface defects.
If the owner wants a bug hole-free surface, then a smooth-rubbed or grout-cleaned rubbed finish as defined in ACI 301 should be specified. Create a smooth-rubbed finish by rubbing the wetted concrete surface with a carborundum brick within a day of formwork removal. This generates paste to create a uniform surface color and texture.
For a grout-cleaned rubbed finish, apply a grout consisting of 1 part by volume portland cement and 1½ parts sand with enough water to produce the consistency of thick paint. For both rubbed finishes, surface voids are filled. CS
Troubleshooting is written by Kim Basham, PhD, PE, president of KBE Engineering, who specializes in concrete construction, troubleshooting, nondestructive testing, forensics, and repair. E-mail firstname.lastname@example.org.
How to Repair Surface Defects
1. Outline the surface defect with a ½- to ¾-inch-deep saw cut.
2. Remove defective concrete down to sound concrete.
3. When chipping, leave chipped edges perpendicular to surface or slightly undercut. Do not feather edges.
4. Clean substrate and remove all loose material, dust, etc.
5. Dampen patch area plus 6 inches around patch area.
6. Prepare a scrub coat by mixing equal parts of portland cement, sand (damp loose volume), and sufficient water to produce a thick cream.
7. Thoroughly brush scrub coat into substrate.
8. Mix the repair mortar using the same materials as the concrete to be patched but without the coarse aggregates. If necessary, use a blend of gray and white portland cement so the mortar matches the color of the surrounding concrete. Use only enough mixing water to create mortar with a stiff consistency but with sufficient workability for handling and placing.
9. When the scrub coat begins to lose the water sheen, apply the repair mortar and thoroughly consolidate into place.
10. Strike off mortar, leaving the patch slightly higher than the surrounding surface to compensate for shrinkage.
11. Leave the patch undisturbed for 1 hour, then finish to match the texture of the surrounding concrete.
12. Keep the patch damp for 7 days.