Spalling from insufficient concrete cover requires repairs that increase cover depth to an acceptable amount. Small, isolated areas of chloride-contaminated concrete and carbonated concrete can be repaired by removing and replacing the affected areas. But when large portions or the entire structure is affected by chloride contamination or carbonation, demolition of the structure may be the most economical solution.
Keys to a good, long-lasting repair are to:
- Prepare a good bonding surface
- Utilize compatible repair materials
- Cure the repair thoroughly
Concrete spall repairs start by preparing an adequate bonding surface. Remove unsound areas of concrete – preferably by hydrodemolition or hydroblasting. When an impact method (e.g., chipping hammer) is used, it should be followed with hydroblasting or shotblasting because impact methods can cause microfractured surfaces. Additionally, you should cut a 1-inch deep perimeter around the repair area to avoid tapered areas.
Exposed reinforcing steel should be cleaned with a wire brush or sand blasted to remove rust and scale. If corrosion has reduced the steel reinforcing bar to less than 75% of its original cross-sectional diameter, repairs should be made to the affected bars. Reinforcing steel which has been exposed any appreciable length should have sufficient concrete removed behind to allow the repair material to engage the reinforcing bar.
Repair materials should be compatible with the substrate in thermal, chemical and physical properties to ensure proper bonding. There are 3 common materials used for spall repair on concrete grain structures:
- Replacement concrete. Replacement concrete is regular concrete and an excellent choice for deep spall repairs, since the physical properties are nearly identical to the substrate material. Because replacement concrete often requires the construction of formwork for placement, it’s not frequently used for spall repair on concrete grain structures.
- Shotcrete. Shotcrete is a pneumatically applied concrete and a popular repair material for use with concrete grain structures because it’s versatile and no formwork is needed. Shotcrete is available as a wet or dry mix. The dry mix is often referred to as gunite and favored by contractors. But it’s not problem-free. Problems with shotcrete application include the use of concrete with too high water-to-cement ratio, embedment of rebound material, failure to bond with the substrate, delamination occurring between subsequent layers of shotcrete and the incomplete filling behind reinforcing bars. Because of the potential for problems, you could require all shotcrete applicators, known as the nozzlemen, to be ACI (American Concrete Institute) certified. The certification provides you with evidence of completion of a formal training program and establishes a minimum level of competency for shotcrete application.
- Polymer concrete. Polymer concrete is a repair material that uses a polymeric resin rather than water as a binder. It’s useful for application in shallow repair areas, repairs during cold weather and repairs in areas where a fast cure time is required.
Curing is probably the most neglected process in concrete repair. Because it’s the last step of the repair process, restoration contractors may rush this step. As cementitious materials hydrate, it’s extremely important to properly cure the concrete to ensure the bond between the repair material and the substrate is not compromised. Inadequate curing can lead to excessive shrinkage, which can break the bond between substrate and repair material.
The best concrete curing method is wet curing. But given the scale and the vertical orientation of the repair areas on grain storage facilities, wet curing isn’t always practical. Curing concrete with water prevents excessive loss of moisture when the concrete is covered with a layer of water for a predetermined period of time (28 days cure time is common).
Chemical curing agents are used most often on concrete grain storage structures. It’s important to apply curing compounds before evaporative water losses occur, especially during periods of high temperatures or high winds. Reapplication of curing compounds may be necessary and should be performed in accordance with manufacturer’s recommendations.
Polymer concretes aren’t cured in the manner described above, since water isn’t used in the mixture. In fact, polymer concretes should be protected from contact with water during the curing period. Polymer concretes cure times are largely dependent on ambient conditions and temperatures, and should be cured in strict accordance with the manufacturer’s recommendations.
The article was authored by Ryan Michalek, PE, LEED AP, a licensed structural engineer with the Nationwide Agribusiness Property Engineering Group. The Property Engineering Group is a team of licensed structural, mechanical and fire protection engineers who can assist you with property insurability requirements, building code interpretation, integration of building construction safety features and the development of property asset management programs to help you manage your risk against property loss.
Please contact your risk management consultant or contact the property engineering group directly at 1-800-260-1356.