Standardizing concrete repair

January 23, 2019

by Tracy D. Marcotte, FACI, PE

Photo courtesy Creative Commons CC0[1]
Photos courtesy Creative Commons CC0

Concrete construction keeps evolving, and so do its governing standards. The American Concrete Institute (ACI), for example, has added an important tool to its toolbox for both architects and specifiers with ACI 563-18, Specifications for Repair of Concrete in Buildings. It complements ACI 562-16, Code Requirements for Assessment, Repair, and Rehabilitation of the Existing Concrete Structures and Commentary (originally released in 2013, then updated in 2016), but 563-18 can also be used alone.

Efforts to standardize concrete repair work have been underway since Vision 2020 (an initiative seeking to establish a set of goals to improve the efficiency, safety, and quality of concrete repair and protection) was created in 2004 at the request of the concrete repair and protection industry. By 2006, the ACI Foundation’s Strategic Development Council (SDC) had identified a list of specific action items in pursuit of these goals, including the creation of a concrete repair code and concrete repair specifications.

Completed in 1976 in concrete, the CN Tower in Toronto, Canada, held the record as the tallest freestanding structure in the world for three decades.[2]
Completed in 1976 in concrete, the CN Tower in Toronto, Canada, held the record as the tallest freestanding structure in the world for three decades.

The need for consensus-based best practices is great. Historically, while clear standards existed for new concrete construction, repair practices were often proprietary or simply based on individual preferences, which were not always in alignment with industry-validated approaches. Moreover, solutions implemented in one geographic region were not necessarily repeated in others, so individual concrete repair efforts could be sub-optimal, necessitating another round of expensive repairs sooner than desired by owners.

Shortly after the development of Vision 2020, the need for guidance in the concrete repair industry became even more urgent. The 2008 recession made repair and reuse of existing facilities an economic imperative because less capital was available for new construction while domestic and international markets and lenders recovered and in the interim, focus turned toward managing assets.

Today, the rehabilitation of existing structures and investment in their repair continues to be a major component of the construction market. The recent global boom in building with concrete also sets the stage for future maintenance and repairs in the coming decades. Further, the trend toward exposed concrete surfaces in modern architectural styles, combined with designers’ efforts at making service buildings such as parking garages a more aesthetically pleasing part of the landscape, means preserving the appearance of concrete as it ages has taken on new importance. Code requirements and concrete repair specifications such as ACI’s provide direction on cost-effective, validated processes for such repairs, which in turn improve the economic viability of restoring and investing in concrete as a building material.

How ACI creates standards

Constructed in the 1940s with reinforced concrete, the Pentagon is the world’s largest office building, spanning 11.6 ha (28.7 acres), with a central courtyard just over 2 ha (5 acres).[3]
Constructed in the 1940s with reinforced concrete, the Pentagon is the world’s largest office building, spanning 11.6 ha (28.7 acres), with a central courtyard just over 2 ha (5 acres).

ACI is a global consensus standards development organization. Its technical committees produce model codes as well as construction and material specifications.

Members of the committee for ACI 563-18 represented diverse viewpoints, coming from different geographical areas and representing different professions within the industry, including contractors, material suppliers, legal experts, and more. Engineers and designers on the team had, collectively, many years of experience with concrete repair projects, ranging from the historic Guggenheim Museum (Frank Lloyd Wright’s famous spiral-shaped building in New York City) to the CN Tower in Toronto, Canada, and a variety of more common structures like multistory residential buildings, parking garages, and office complexes.

Restored between 2005 and 2008, the exterior of the Guggenheim Museum in New York City represented a challenge to restoration experts with its curved architectural concrete exterior.[4]
Restored between 2005 and 2008, the exterior of the Guggenheim Museum in New York City represented a challenge to restoration experts with its curved architectural concrete exterior.

ACI 563-18 addresses numerous variables and contingencies not included in standards aimed at new construction. The as-built conditions of buildings are often not well documented, making it difficult for designers to know what they face when rehabilitating a concrete structure. Additionally, many older buildings might not be fully compliant with the most current building codes for new construction. In such instances, it can be impractical and unnecessarily costly for designers and engineers to apply new-build standards to a repair project when the building is safe and performing well in its current form.

Another challenge is existing buildings are often occupied during repair work, creating a set of safety requirements new-build standards do not address. Unique structural considerations also arise during repair and rehabilitation work. For example, areas in need of repair (such as a balcony or column of a multistory building or a group of columns located in the center of a floor plate) may be isolated. In these situations, shoring and bracing requirements such as those set out in ACI 563-18 are of paramount importance to safely transfer structural loads while the repairs are implemented.

ACI’s new 563-18 specification includes minimum basic requirements to guide architects, engineers, and specifiers. It can be applied to any construction repair and rehabilitation project involving structural concrete and is intended to either be used as a reference or incorporated in its entirety into project specifications. The document includes guidance for creating companion technical drawings and specifications to communicate specific project needs. ACI 563-18 can be tailored for projects of all sizes, whether as small as a curb repair or as large as the Pentagon, as well as for any project delivery type, from traditional design-bid-build to design-build. It is applicable to both private and public projects and a range of aesthetics, such as significant architectural masterworks and more functional parking garages and residential buildings in a marine exposure.

What is in the new document?

Local variations in concrete cover depth can trigger corrosion requiring only localized concrete repairs, as with this column repair work. Photos courtesy CVM[5]
Local variations in concrete cover depth can trigger corrosion requiring only localized concrete repairs, as with this column repair work.
Photos courtesy CVM

As a companion to ACI 562-16, ACI 563-18 provides an ‘on ramp’ for designers and engineers to prepare their work in accordance with code documents. ACI 562 was the first material-specific set of requirements for repair materials and the first ACI code specifically addressing the repair of reinforced concrete. It offers specific criteria for assessment of varying levels of damage, deterioration, or faulty construction, along with information on designing the necessary repairs.

ACI 563-18 provides best-practice information on many of the contingencies associated with concrete repair while still harmonizing with the basics of concrete construction laid out in standards for new builds (notably, ACI 301-16, Specifications for Structural Concrete). For example, details on materials, mixing, typical reinforcement methods, and installation processes remain fundamentally the same.

One section of the new standard, titled “General Requirements,” discusses broad construction requirements for all repair work. It offers a clear delineation of responsibilities along with documentation requirements, preventing surprises in procurement and execution of repair work. Inspection, testing, and quality control (QC) procedures are outlined, with accountability assigned to owners’ testing agencies and/or to contractors. Items to be considered before repair work can be deemed acceptable include dimensional tolerances and appearance.

The section “Shoring and Bracing” focuses on any member(s) to be repaired and addresses sequencing of repair work as the structure is unloaded and reloaded. Specialty engineers are to design all shoring and bracing to maintain stability of the structure during construction, taking into consideration pre-existing unsafe structural conditions as well as load and deflection requirements during repair. Limitations are imposed on concrete or reinforcement removal prior to shoring. The location, spacing, placement, and sequencing of shoring are to minimize impact on building occupants and operations. When repairs involve altering forces in prestressed reinforcement, the change in forces must be considered in the design of bracing and shoring, as it is the bracing that maintains the equivalent of the prestressed force on structural members until the reinforcement can be reimposed.

When well designed and constructed, concrete buildings and parking structures remain safe and durable for decades, even in marine environments.[6]
When well designed and constructed, concrete buildings and parking structures remain safe and durable for decades, even in marine environments.

ACI 563-18 also includes a section addressing concrete removal and preparation of the concrete substrate for repair, with the overarching goal of minimizing damage to the existing structure and producing concrete surface profiles that are free of debris and optimal for bonding. Minimization of damage includes reducing bruising of concrete substrates within or adjacent to the work area. A bruised surface is one weakened by interconnected microcracks in substrates caused by use of high-impact, mechanical methods for concrete removal and surface preparation. The fracture layer typically extends to a depth of 3 to 10 mm (1/8 to 3/8 in.) and, if not removed, frequently results in lower bond strengths compared to surfaces prepared with non-impact methods.

This section also outlines parameters for common concrete removal equipment and methods, including concrete breakers, hydro-demolition, scarifying, scabbling (i.e. removing a thin layer of concrete), and milling/rotomilling. Similarly, it lays out standards for equipment and methods used to prepare and clean concrete surfaces (as well as reinforcement surfaces), including abrasives, compressed air, high- and ultra high-pressure water jetting, low-pressure water cleaning, and vacuum approaches. All equipment must be operated so as to not damage rebar, other embedded items, or adjacent concrete.

Execution of concrete removal must fall within the specification’s requirements for configuring and maintaining the geometry of the removal area, including maximizing the use of right angles, avoiding re-entrant corners, and obtaining uniformity of depth. Assessments performed onsite are critical at this stage of a repair project. Contractors must often perform inspections to identify cracked, delaminated, spalled, disintegrated, and otherwise unsound concrete for removal. They must also conduct site assessments of reinforcement corrosion and use their findings to inform concrete removal work. Before new repair materials are applied to a given area, it must also be verified to be free of any bond-inhibiting materials such as dirt or concrete slurry and have a surface profile suitable for repair materials and coatings.

Surface preparation of concrete for repairs often requires specific surface profiles for coatings and waterproofing systems to be installed properly. Industry-standard tools are available to simplify the verification of the concrete substrate prior to installation.[7]
Surface preparation of concrete for repairs often requires specific surface profiles for coatings and waterproofing systems to be installed properly. Industry-standard tools are available to simplify the verification of the concrete substrate prior to installation.

Another significant section deals with reinforcement and reinforcement supports, with a focus on cutting and/or bending existing reinforcing materials and connecting them with new ones, covering splices, length of splice laps, and details of any mechanical and welded splices. As with mating new concrete surfaces to existing ones, mating new concrete to existing steel reinforcement requires the metal to be free of contaminants deleterious to the bond. When it is necessary to move the reinforcement beyond specified placing tolerances to avoid interference with other reinforcements, conduits, or embedded items, a proposed plan must be submitted to and then approved by the architect/engineer. In general, if concrete cover over reinforcing materials (or other existing conditions) is inadequate when compared to new-build codes, engineers should make owners aware of these pre-existing conditions.

Another important portion of ACI 563-18 is “Proprietary Cementitious and Polymer Repair Materials.” This section addresses properties, proportioning, mixing, and use of proprietary cementitious and polymer repair materials. In addition to requiring architects/engineers work with manufacturers’ representatives in the field, it states submittals must include:

Supplemental testing data and mockups are also required. RMM sheets and other supplied data must be followed throughout the application process and through inspection of post-repair material installation. Manufacturer-supplied data includes information on equilibrating the repair materials and substrate to the proper temperature, controlling moisture conditions, preparing surfaces, and adhering to batching, mixing, placing, and curing requirements.

Ensuring global stability of a structure, especially while occupied, might require shoring of individual or a series of columns.[8]
Ensuring global stability of a structure, especially while occupied, might require shoring of individual or a series of columns.

ACI 563-18 is also one of the few specifications to address workflows including the use of proprietary materials, which are common during any repair project. For example, it requires a manufacturer’s field representative be onsite before proprietary materials or components are mixed or installed. The representative is responsible for training site personnel on product use and is generally required to remain at the jobsite to observe work being performed until such time as the contractor and/or owner is satisfied the crew has mastered the technique of preparing and installing the products acceptably. After that time, the field representative is expected to make periodic visits to review completed work and distribute reports describing workmanship and conformance with the manufacturer’s requirements.

Addressing two other situations unique to concrete repair, ACI 563-18 refers to ACI 503.7, Specification for Crack Repair by Epoxy Injection, and ACI 506.2, Specification for Shotcrete. Remaining sections and paragraphs of the new standard discuss:

Checklists on mandatory as well as any optional requirements are provided to assist the architect/engineer in supplementing the provisions of this specification, as required or needed, by designating or specifying individual project requirements.

ACI 563-18 leaves room for adaptation to individual project circumstances while providing a much-needed framework for design teams faced with concrete repair. As SDC committees and others continue to make progress on Vision 2020’s list of stated goals, codes and specifications will continue to develop, addressing repair materials, performance prediction, education, and more.

Tracy D. Marcotte, FACI, PE, is an expert in the metallurgical and materials engineering field with CVM, based in King of Prussia, Pennsylvania. With more than 25 years of laboratory as well as professional practice, she is past chair of American Concrete Institute (ACI) Committee 563, Specifications for Repair of Structural Concrete in Buildings, and an active member of multiple ACI committees related to corrosion, durability, sustainability, service life, and repair. Marcotte also serves on ACI’s Technical Activities Committee as well as its Board of Directors. She holds degrees in materials and metallurgical engineering from Queen’s University and the University of Waterloo in Canada, with graduate research focused on steel corrosion in concrete. Marcotte can be reached at tmarcotte@cvmprofessional.com[9].

 

Endnotes:
  1. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/01/geisel-library-174106.jpg
  2. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/01/CN-tower-Toronto.jpg
  3. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/01/pentagon-80394.jpg
  4. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/01/guggenheim-museum-2707258.jpg
  5. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/01/column-repair.jpg
  6. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/01/residence-at-beach.jpg
  7. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/01/ICRI-surface-chip.jpg
  8. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2019/01/shoring-of-column.jpg
  9. tmarcotte@cvmprofessional.com: mailto:tmarcotte@cvmprofessional.com

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