September 3, 2009

Concrete Matters #3 in a series: Concrete Repair

by Kevin Morris

Repair of a concrete substrate is something that is required of almost every project, new construction or rehabilitation. Use of the structure should dictate to what extent the repair is made.

Uncoated Bug HoleSurface imperfections that should be addressed will include:

  • Bugholes
  • Honeycombs
  • Tie Wire Holes
  • Spalling
  • Rebar Corrosion
  • Pop-Outs
  • Abrasion or Erosion
  • Cracking

Coated BugholeThe top three items on this list are probably the most common imperfections and yet they are the ones that are most likely to be addressed improperly. The structures should be opened up during surface preparation to reveal the entire imperfection beneath the surface. Once they are properly opened, you can determine how to treat them based on the use of the structure. I commonly see or hear references that the specification calls for filling imperfections of a given size or greater. While for a concrete substrate that will only receive a coating for aesthetic reasons it may be acceptable, it will cause premature failure in a substrate that will be used for primary or secondary containment of any commodity.

Spalling, rebar corrosion, and pop-outs are typically found in areas of freeze/thaw cycling, de-icing solutions and environments that lower the pH of the concrete below 10.5 so that they no longer passivate the steel. These types of concrete repairs typically require greater depth of placement for repairs. The areas to be repaired should be identified in their entirety and saw cut a 90 degree angle to the surface. If the repair includes corroded reinforcing steel, the substrate should be removed from around the reinforcing steel to a depth allowing for proper cleaning/preparation of the steel via abrasive blasting or mechanical abrasion.

Abrasion or erosion are typical problems associated with wheeled or vehicular traffic and flow of a liquid commodity. These may not be significant enough to warrant a repair greater than proper surface preparation and restoring the surface to its original plane or to a level plane.

Rebar CorrosionCracks pose the greatest risk of repair if not properly identified. You need to consider if the crack is static (Non-Moving) or dynamic (Moving). When cracks are not properly identified they are typically considered static by the coatings applicator and could reappear through the coating due to this misdiagnosis. If time allows, a thin glass plate can be adhered to either side of the crack with an epoxy putty and left to cycle through some temperature variances which would cause the substrate to expand and contract. If the crack is dynamic the glass plate will break. Keep in mind that a test of this nature is a snapshot in time and the conditions may change with greater swings in temperature or weight loads that may not have been experienced during the test.

Products for repairing concrete imperfections/damage can be easily located, but selecting the proper material for your unique situation can be a challenge.  Reinforcing steel should receive a primer/bonding agent to prevent the substrate from flash rusting prior to the placement of the repair material. The actual repair materials for the substrate can then either be cementitious or resinous depending on the need for the project. Material selection should consider:

  • Size of repair
  • Time frame for coating over the repaired substrate
  • Coefficient of Expansion (Compared to the substrate)
  • Shrinkage
  • Depth of repair
  • Bonding Agent or Scrub Coat
  • Freeze/Thaw zone

I listed a factor above that is somewhat controversial in the market, “Bonding Agent or Scrub Coat”. There are many discussions taking place as to¬†the use of a bonding agent under cementitious toppings and the effects that they might have on life cycle performance of repairs due to differences in¬†expansion. This is a great reason to be active in local trade organizations to voice your opinion and hear those of other industry leading professionals.

Next up in the Series: “Application”

Comments (4)

  1. October 20th, 2009 at 1:27 am

    need Proofing membranes n other coating material for new projects

  2. February 11th, 2010 at 4:43 am

    If it’s a major repair then it is likely you need to clear out every loose bit and drill into the surrounding material and put in reinforcement bars.

  3. March 8th, 2010 at 3:59 pm

    I agree that if a major repair is needed then determining the depth of the repair would be critical. Removing all loose and damaged substrate including damaged/corroded reinforcing steel must be accomplished. There are many methodologies for repairing damaged concrete/reinforcing steel and all should be explored to determine the most effective and economical for the intended service

  4. October 3rd, 2015 at 7:20 am

    Hey very good blog!!!! I’m really enjoying reading your posts and learning a lot.

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