Building owners often wait until their concrete problems have gotten scary before deciding to consult an engineer. When big chunks of concrete are falling off or a whole structure seems about to collapse, the owner knows it’s time to seek professional help, authorize a thorough investigation, and request a complete repair design.

When a contractor is being asked to address less dire problems, it’s still important to figure out what exactly has gone wrong and why. You need to know what caused the damage or deterioration, and what loads and conditions the concrete must withstand, in order to choose materials for an effective and durable repair. Repairing a concrete surface generally means constructing a composite system that differs somewhat from the original concrete. This composite is made up of the original concrete substrate, the new repair material, and the interface between the two. Many time, the interface is the critical component.

The International Concrete Repair Institute (ICRI) publication “Guide for Selecting and Specifying Materials for Repair of Concrete Surfaces” describes a process and provides a series of checklists to facilitate the selection. The “Owner Requirements” checklist covers such factors as expected service life, desired appearance, the need to use the structure during rehabilitation, and budget constraints. The “Service Conditions” checklist deals with loading requirements, weather exposure, and chemical environments that will affect the repair site. The “Application Conditions” list includes weather expected during the work, the configuration and orientation of the repair, site access, and application methods to be used.

Following the ICRI guidelines or taking a similarly methodical approach will help you identify the properties needed in the repair material. Then you can choose the material or system that offers the best balance of performance, risk, and cost factors.

Desirable properties

While the specific requirements will vary from one project to another, certain properties or characteristics are always desirable in a repair material. One of these is compatibility with the substrate. The repair material should be similar to the substrate in terms of its stiffness or flexibility (modulus of elasticity), its thermal expansion properties, water absorption, and water vapor diffusion.

Another characteristic to look for is durability—the ability to produce a lasting repair with little or no change in performance over its expected service life. Some factors that contribute to durability include resistance to UV exposure, resistance to alkaline environments, and physical properties that remain constant over a wide temperature range.

Other important properties include good adhesion, low permeability, and low shrinkage. The durability of a repair depends on proper bond with the substrate and between layers of repair material. Low permeability helps protect against carbonation and chloride ion penetration. Because most repairs are made to older concrete that will not be experiencing drying shrinkage, using repair materials with low shrinkage can help prevent cracking and loss of bond. Also consider freeze/thaw resistance, if the climate and application call for it.

Finally, to ensure that the repair work can be done with reasonable ease and efficiency, most good repair product should offer excellent workability.

Classifying repair materials

Choosing from among hundreds of repair materials is a daunting task; it helps to recognize some of the basic types and understand their typical uses. To keep things a little simpler, we’ll leave aside extensive full-depth repairs and reconstruction projects where the typical goal is to reproduce the original concrete as closely as possible.

The most basic repair materials available are cementitious patching compounds. These are one-part, normal-setting products that are ready to use with the addition of water. Modern repair materials, though, have enhancements that make them easier to use and the repairs more durable.

By adding powder or liquid latex polymers to cement-based patching compounds, manufacturers have developed products with enhanced performance characteristics, such as increased bond strength and reduced shrinkage. These one- or two-component, polymer-modified cementitious repair mortars are available in pre-proportioned kits for easier mixing and application.

Some commercially available repair mortars are further modified by the addition of silica fume, which increases mortar strength and durability. The extremely fine silica particles react within the mix to increase the mortar’s density, which reduces its permeability and offers greater protection against chloride ingress in high chloride environments. Some mortars also contain corrosion inhibitors to help protect embedded steel and further extend the service life of repairs.

Polymer concretes and mortars represent another category of repair products. These are not cement-based, but rather thermosetting plastic materials, such as epoxies, polyesters, vinyl esters, and methyl methacrylates, usually with aggregate fillers. Epoxies and other polymer mortars offer high strength, durability, and chemical resistance. They work well in small annular spaces around rebar and in narrow cracks, but they can’t be used to fill large cracks and holes. Epoxies also will weaken or burn when exposed to heat, so they can’t be used for structural repairs without additional fire protection.

Application methods

The nature and scale of concrete deterioration on a particular project will influence your approach to repairs, and the application method you choose also affects the selection of a repair material. For example, if the deterioration is extensive, or if material will need to be placed overhead, applied in thick layers, or formed into complex geometric shapes, you probably want to consider mechanical application, such as wet or dry process shotcrete. Shotcreting calls for a material with excellent adhesion and sag resistance. If, on the other hand, the material will be poured or pumped into large or hard-to-access voids, high flowability will be more critical to ensuring a successful repair.

Informed compromises

The ICRI Guide describes the material selection process as “arriving at informed compromises.” It’s seldom possible to find a repair material that will fill voids completely without shrinking, while also responding to loads and temperature or moisture changes in ways identical to the substrate. Therefore, establishing repair priorities early on is important, since this then allows you to focus on the most critical issues when considering necessary tradeoffs.

For example, in a structural repair, the primary requirement is likely to be for the repair material to carry the loads intended by the engineer. In that case, you want a material with modulus of elasticity close to that of the substrate, and with very low compressive creep and drying shrinkage. Otherwise, the repair might relax under creep and lose volume, reducing its ability to carry compressive loads. As long as these criteria are met, other factors can be considered secondary.

On another project, such as a road repair, fast turnaround time may be crucial. Finding a repair product that gains strength quickly would then be a top priority, so that the road can be reopened to traffic as soon as possible.

In a third case, the owner may be most concerned about the appearance of the finished repair, placing a premium on a good color match with the existing concrete, or on avoiding any visible cracking. To achieve that result, you’d seek out a product with very low drying shrinkage and low surface water during placement.

Whether or not they are top priorities, both expected service life and repair cost are also factors to be considered in every project. They too are subject to tradeoffs, when you’re determining the best repair solutions.

Seeking guidance

Manufacturers’ literature is a good source of information on the properties and recommended uses of repair materials. ICRI advises consulting manufacturer data sheets, product evaluation reports, and test results when considering a product’s suitability, and to request specific information on key properties if it doesn’t already appear in published documents. Working directly with knowledgeable manufacturers’ reps can also help you find the best product for a particular project. CC

- Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.