Damaged concrete often exhibits cracking, spalling, and scaling. Root cause of damage can vary from reinforcing steel corrosion, post-tension strand failure, freezing-thawing, sulfate attack, or intrinsic issues with the mix or construction, such as alkali-silica reaction, delayed ettringite, chloride content, carbonation, reinforcing steel without adequate cover, etc. Investigations for concrete projects can be complex, but critical for determining the appropriate repairs. Repairs in general remove unsound material, resolve steel damage, and replace with a compatible repair mix. The overarching goal of concrete repair is to develop a repair mix that has similar properties of strength, density, porosity, and water absorption. If the repair mix is too strong and dense in a structure with weaker, more porous concrete, it can cause premature future failures of the historic or existing concrete. 

Matching the color and texture of existing exposed-finish concrete is important, specifically when the structure is historic or important aesthetically. Challenges include: 

• Finding similar existing aggregates that match the existing while meeting current industry standards. 
• Using available cementitious materials, commonly portland cement, that create a hardened paste color similar to the original.  
• Developing techniques to finish the concrete to match the existing weathered surface. Depending on whether formwork or hand-troweled placement are conducted will dictate finishing.  

Crack repair options depend on if the crack is moving or stable, its width, and if it’s structural or limiting water infiltration. If still moving, as determined with a crack gauge, then a flexible material like a sealant may be an appropriate solution. If the crack is no longer moving, then a more rigid repair material, like a mortar or epoxy may be appropriate. Structural crack repairs commonly rely on epoxy injection repairs. Ensure the correct viscosity of epoxy is used for the crack width and project application. Mockups and core samples to confirm epoxy effectively fills and bonds the crack is critical for these types of repairs. 

Much of concrete damage stems from steel corrosion. Water infiltration through cracks, exposure to the environment, carbonated concrete to the depth of the steel, and chlorides used during placement can all lead to corrosion. During repairs, the reinforcing steel needs to be investigated to determine if it has enough structural capacity. Additional or replacement steel may be needed, as determined by the engineer. There are options for protecting the steel, which can include the following: 

• Remove and replace the corroded steel with new coated steel. 
• Clean and coat the steel with corrosion inhibiting coating system. 
• Install galvanic or cathodic protection systems. 
• Apply a coating system to the exterior surface of the concrete (not recommended for historic exposed concrete). 

Coatings are often applied to concrete as added protection from the environment and traffic. Vertical surface coatings can range from acrylic, polyurethane, mineral silicate, silane/siloxane, and cementitious. Horizontal, often high foot or vehicular traffic areas are coated with traffic bearing membrane systems, which can be polyurethane, polyurea, epoxy, methyl methacrylate, and cementitious. Interior concrete flooring surfaces may also use any of the above listed coating types, however, it’s critical that moisture content and vapor drive be tested in conjunction with interior coating applications.

For any concrete re-coating project, an investigation is needed to understand the condition of the existing coating and how well it’s bonded to the substrate. This is performed by pull-off testing. The type of coating must also be known so that the new selected coating system is compatible to the chemistry of the existing coating to prevent compromising adhesion and durability. Mockups, pull-off testing, and quality assessment are recommended during the project.

Concrete structures that are maintained will last longer and have higher resiliency during its life. The maintenance often should focus on limiting water infiltration, limiting salt migration, protecting horizontal facing surfaces, replacing sealant joints at appropriate intervals, conducting spall and crack repairs as needed, and maintaining coating systems. Water exposure exacerbates extrinsic and intrinsic concrete conditions that lead to damage. 

With repair and restoration projects it is important to understand the properties of the existing concrete system, material properties, and reinforcing steel condition prior to selecting repair strategies. The following field and laboratory testing may be needed for concrete structure evaluation. 

FIELD TESTING:

LABORATORY TESTING: