Photo: Berglund Construction
Photo: David Harmantas
Historic and existing concrete primarily comprises coarse aggregate, fine aggregate (sand), a cementitious binder, and water; ultimately forming a hardened structure. Concrete is strong in compression, with reinforcing steel added for tensile strength. Aggregates provide strength and stability to the mix and are selected for their size, shape, aesthetics, and properties. However, the component that has largely changed through concrete’s history is the cementitious materials.
Since the early 20th century, portland cement has been used as concrete’s primary cementitious binder. Other cementitious materials historically include natural cement, lime (non-hydraulic and hydraulic), along with possible pozzolans, now referred to as supplemental cementitious materials (SCMs). Concrete properties vary based on the binder and mix design, and for portland cement, the changes with manufacturing.
Concrete can be cast-in-place, precast, or applied as shotcrete. Historic concrete can have lower compression strength and density, and higher porosity and water absorption. Conversely, modern concrete can have very high compression strength and density, and low porosity and water absorption. It is critical to understand the year and process the concrete was formed and its subsequent properties, so that concrete repair mixes are selected with similar compatible properties.
For successful historic and existing projects, it’s important to prioritize repairs based on the structure’s conditions and project team goals. Project strategies differ per project and can be selected from a variety of repair options. Although geared toward masonry, similar overall best practices apply for concrete structures. Concrete cast-in-place column and slab construction may be similar to mass wall, and precast panels may be similar to barrier or cavity walls.
Refer to our restoration best practices page for additional information, details, and resources. Below, find concrete repair and restoration options for consideration on your project.
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:
Strengthening of existing concrete may be needed due to damage, deficiency, increased load, seismic upgrade, or other conditions that trigger code. There are a wide range of possible structural options, like carbon/fiber reinforced polymer (FRP), steel supports, external post-tensioning, post-tension strand replacement or repair, and new shear walls, to list a few. Structural engineers need to review and design these solutions.
Applying fresh concrete or mortar mixtures to a substrate through a nozzle with pneumatic pressure is known as shotcrete. Mixes can be applied both pre-mixed with water (wet-mixed) and dry materials mixed with water at the nozzle (dry-mixed). Historic equipment, referred to as a “cement gun” was patented by the Cement Gun Company who coined the term gunite for which shotcrete is sometimes known.
Shotcrete can be an effective solution for repairs and stabilization projects, including for seismically upgrading unreinforced masonry walls. Although the mix design should be designed to be appropriate for the project and compatible to the existing concrete/shotcrete, installation should be performed by a qualified applicator. The pressure, distance from the substrate, angle direction, and moisture content are all critical in making sure that the mortar or concrete material achieves consolidation, bonds to the substrate, and creates long-term durability. Mockup shotcrete panels are critical for these projects to meet the project goals.
Precast concrete manufacturing processes exist and have been modified since the early 1900s, both wet- and dry-cast. Popularity grew in the mid-1900s, with early precast technologies such as Mo-Sai and Schokbeton leading the way. Panels include reinforcing steel, steel mesh, or pretensioned strands. Anchorage of the panels for gravity and lateral support are typically edge or back installed. Most precast concrete panels/units are formed horizontally. For exposed aggregate or specialty finish, in many cases these were produced face down. Repairs to precast units can be challenging since repairs are performed vertically.
Photo: Berglund Construction
IMI’s free project support, technical assistance, and education is here to help you at any stage in your building’s lifecycle.
Our multidisciplinary team draws on decades of experience developing solutions for high-performing masonry and tile projects.
Why settle for anything less than the best when it comes to the installers on your project? BAC craftworkers train throughout their careers to become building enclosure experts and masters of their craft.
When you want to have confidence that you’re working with qualified, experienced crews on your project, you can specify any number of our training programs.
Here are some of the training, certificate, and certification programs you may want to specify for concrete restoration.
Traditional craft skills and contemporary repair techniques are critical to the preservation of historic buildings and structures. This in-depth certificate gives BAC craftworkers integrated knowledge of historic masonry preservation.
Proper flashing is one of your building’s best defenses against moisture. BAC craftworkers stay up-to-date on the latest techniques and building code requirements with IMI’s Flashing Upgrade Training Program.
Air barriers help prevent air and moisture from entering the building envelope. This course teaches BAC craftworkers how air barrier assemblies function, including how to evaluate materials, substrate conditions, and the continuity of components.
Here are some additional resources that focus on concrete and restoration. For a more comprehensive list of repair and restoration resources, please refer to the restoration page. For additional guidance, contact IMI.
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