I've read all these articles on how great certain concrete repair products are and how they promise miraculous results, but despite these claims, repairs often fail. Why is that?
There are several determining factors that influence whether or not a concrete repair will be successful. Without these issues being addressed, failure is typically the result. The most common is poor analysis of the repair area.
Proper Analysis: Why did the failure occur in the first place?
Larger repair projects require that methods of extreme accuracy which test the quality of the base concrete. This is necessary in order to avoid repeating the costly repairs that fail once again due to a poor quality base concrete. The preferred method is to take in cores of 3-4 inches diameter around the failure areas. These cores should then be analyzed by a qualified petrographer who will microscopically examine the concrete for the precise reason for the original failure and to determine the soundness of the base concrete to be repaired.
Then, a number of questions should first be answered. First of all, one must determine the owner's expectations and at what level of usage the floor is expected to perform. For example: Wheeled traffic, cart traffic, exposure to high abrasion, to impact, to chemical attack, etc. Are aesthetics important? What are the time parameters for turnaround of the repair area and a return to service?
Q: Specifications are often vague on how to properly prepare a surface which is scheduled to receive a repair material. We have a large project which requires repairs from spot patching to sections that require overlay products. What do you recommend for surface preparation on floor repairs?
A: Preparation: Your odds for a successful repair are directly proportionate to the quality of the preparation and the quality of the base concrete. Once the quality of the concrete and the owners' expectations are known, proper techniques must be followed in order to have a long lasting repair. In all cases, the method chosen must remove substandard concrete to expose good quality concrete.
Good practice surface preparations include:
- Where repairs are necessary in the middle of a slab, thickening the edges of the repair area is a must. This requires the contractor to saw cut the perimeter of the area to be patched slightly deeper than the rest of the repair area. The center of the repair area is then bush-hammered down to sound concrete. A minimum of ¼” in depth is the normal standard.
- Larger areas of repair or floor areas requiring complete overlays are best prepared by means of a scarifier if the concrete is of questionable quality. This type of machine employs carbide tipped bits attached to a rotating head, which is powered by either a gasoline engine or electric motor. A good quality scarifier or planer can remove up to ¼” of poor quality concrete per pass. (See photo above.)
- For less aggressive preparation, shotblasting is another alternative. A shotblasting machine propels small steel “shot” at the concrete, removing any loose or weak material. It provides a Concrete Surface Profile of approximately 1/8 inch amplitude, the measurement from valleys to peaks of the concrete surface.
Q: What's the best method to use for bonding small area repairs?
A: For small repairs, after mechanical preparation has been completed, surface saturation with water is the first step in creating a strong bond line. In most situations it is advisable to saturate the repair area the night before, or at least many hours before the product installation by means of pressure washing or ponding of the surface with potable water. After a saturation of water, the area must remain wet. The next day, or just prior to installing the repair material, remove any excess, ponded water. The concrete is now what is referred to as “SSD,” that is: Saturated Surface Damp. This is a very important step in the process as it prevents a thirsty base concrete from pulling the water out of the repair mortar, thus weakening the repair material.
The second step in assuring a proper bond is to prepare and install a bonding slurry. A bond slurry is often times accomplished simply by using a scrub coat of the repair mortar itself. We recommend this procedure when we know that the repair material contains polymer additives, as is the case with L&M’s Duracrete. An alternate method is to create your own bonding slurry simply by mixing one or two parts of Portland cement with one part of L&M’s Everbond, acrylic bonding agent. In either case, the slurry coat should be scrubbed into the repair area by means of a brush or stiff bristle broom. This scrubbing action is quite important to the success of the repair. Do not allow the bonding slurry to dry before application of the patching material. If this happens, reapply more bonding slurry mixture.
Q: Is there a repair mortar that will handle a wide variety of heavy duty floor repairs, including a complete overlay? Can any of these repair materials be used to create a polished concrete appearance?
A: Repair mortars come in a wide variety of types and performance capabilities. The one product that meets most floor repair scenarios you describe is our L&M’s Durafloor HP. This product will rapidly gain strength in just one day and allow the owner full use of the repair area within a short period of time. If the project involves polishing of the concrete, Durafloor HP will polish very nicely the day following placement.
Durafloor HP is an excellent choice for performing large floor overlays where the owner is looking for a completely new surface on the concrete. This product is self-leveling when properly mixed and installed.
Special surface preparation for Durafloor HP applications are required when scheduled for polishing. To these surfaces, the saturation process described above is not recommended. Instead, after a proper mechanical surface preparation of a minimum of shotblasting, apply a single heavy coat of L&M Epoxy primer and bonder. While the epoxy resin is still tacky, broadcast into the resin to saturation kiln dried, coarse sand. After the epoxy bonder has set a minimum of six hours remove by vacuum all loose and unbonded aggregate to produce a dry, sanded surface. To this surface install properly mixed Durafloor HP, leveled using a gauge rake set for the desired thickness. This will result in a finished floor appearance without the need for troweling. Durafloor HP is self curing, but it may be necessary to protect the surface during installation from excessive air movement to minimize plastic shrinkage cracks. After curing overnight, Durafloor HP is ready for the FGS/PermaShine polishing process.
Bill Butler is a sales and tech rep for L&M Construction Chemicals. He has worked in the concrete industry since 1976 and has been involved with ready mix trucks, concrete admixtures and construction products for the concrete industry. His approach to helping contractors and installers “do things right the first time” or when necessary “doing things right the second time” is to ask good questions, be thorough, use common sense and logic.
© 2013 L&M Construction Chemicals, Inc. | ConcreteNews Spring 2013.