A significant number of facilities in the United States were constructed during the first half of the 20th century using reinforced or prestressed concrete materials. Now, at the beginning of the next century, many of these buildings have reached the end of their planned service life, and deterioration in the form of steel corrosion, concrete cracking, and spalling is observed frequently. In addition, many of these structures were built to carry loads that are significantly smaller than the current needs. Because of these factors, many structural engineers are faced with the challenge of evaluating and implementing effective and economical repair and strengthening programs.
Unfortunately, there is no single solution that offers a simple, straightforward method for all repair and strengthening projects. Further, the processes of repair and retrofit of existing structures are complicated because most of these structures are occupied, and much of the mainstream construction community’s expertise is centered on new construction. However, success can be achieved if the repair and strengthening systems are tailored to serve a structure’s intended use without interfering with its occupants or function. The key to success is a combination of the different design skills and application techniques—structural strengthening and structural repair—necessary for such projects. As such, the engineer must relay his or her expertise in using mechanical and structural behavior principles to develop a comprehensive retrofit solution.
Concrete experts commonly use the terms structural repair and strengthening to describe building renovation activities. Although the two terms sound similar, they refer to slightly different concepts. Structural repair describes the process of reconstruction and renewal of a facility or its structural elements. This involves determining the origin of distress, removing damaged materials and causes of distress, as well as selecting and applying appropriate repair materials that extend a structure’s life.
Structural strengthening, on the other hand, describes the process of upgrading the structural system of an existing building to improve performance under existing loads or to increase the strength of structural components to carry additional loads. For upgrade projects, design engineers must deal with structures in which every element carries a share of the existing load. The effects of strengthening or removing part or all of a structural element—such as penetrations or deteriorated materials—must be analyzed carefully to determine their influence on the global behavior of the structure. Failure to do so may overstress the structural elements surrounding the affected area, which can lead to a bigger problem and even localized failure.
Structural concrete repair
Although durable, buildings constructed using reinforced and prestressed concrete have a finite service life. When exposed to harsh environments, de-icing salts, and other chemicals, these structures may experience significant deterioration, which typically occurs in the form of steel corrosion, concrete spalls, delamination, and cracks.
Interestingly, one of the most severe and widespread problems in concrete is the internal damage caused by the corrosive action of external chlorides on reinforcing or prestressing steel embedded in concrete. Corrosion problems are caused by a corrosion-process by-product (rust) that expands up to eight times its original volume. This expansion creates internal pressure, which causes the concrete to crack and spall, resulting in a reduction of the effective area of steel reinforcement and reduced structural capacity of the affected member. If not addressed at early stages, corrosion will continue to grow rapidly, ultimately creating a safety issue of falling concrete and loss of strength.
The assessment, design, and implementation of a durable repair to an existing structure are indeed more complex than for new construction. In addition to the unknown state of existing structural materials, the degree to which repair materials and the existing material will act as a composite and share loads must be addressed. Prior to establishing a repair strategy, the concrete-repair expert must diagnose the problem’s root cause, which enables prescribing repairs that are long-lasting and durable. Failing to follow this process may result in a frustrating, but common, cyclical outcome known as “repairing the repair.”
Photo 1 shows an unsuccessful concrete repair project. The repair materials started to delaminate just a few short months after the first repair was completed. Adequate surface preparation could have prevented this repair failure. See “Structural repair techniques for reinforced concrete” at www.gostructural.com/V5N4/concrete.htm for specific repair steps and diagrams.
Structural concrete strengthening
Many buildings that originally were constructed for a specific use now are being renovated or upgraded for a different application that may require higher load-carrying capacity. As a result of these higher load demands, existing structures need to be reassessed and may require strengthening to meet heavier load requirements.
In general, structural strengthening may become necessary because of code changes, seismic upgrade, deficiencies that develop because of environmental effects (such as corrosion), changes in use that increase service loads, or deficiencies within the structure caused by errors in design or construction. The structural upgrade of concrete structures can be achieved using one of many different upgrading methods such as span shortening, external composites, externally bonded steel, external or internal post-tensioning systems, section enlargement, or a combination of these techniques. Similar to concrete repair, strengthening systems must perform in a composite manner with an existing structure to be effective and to share the applied loads. The following gives a brief description of these methods and case-study applications.
Span shortening—Span shortening is accomplished by installing additional supports underneath existing members. Appropriate materials for span shortening include structural steel members and cast-in-place reinforced concrete members, which are simple to install. Connections can be designed easily using bolts and adhesive anchors. An example of this upgrading method is shown in Photo 2. The structural steel system shown was installed on a parking deck to shorten the span and carry part of the load, transferring it to the existing supporting system. On the down side, such applications may result in loss of space and reduced headroom.
Composites—Fiber reinforced polymer (FRP) systems are high-strength, lightweight reinforcement in the form of paperthin fabric sheets, thin laminates, or bars that are bonded to concrete members with epoxy adhesive to increase their loadcarrying capacity. These systems have been used extensively in the aerospace, automotive, and sport-equipment industries, and now are becoming a mainstream technology for the structural upgrade of concrete structures. Important characteristics of FRPs for structural repair and strengthening applications include their non-corrosive properties, speed and ease of installation, lower cost, and aesthetic appeal.
As with any other externally bonded system, the bond between the FRP system and the existing concrete is critical, and surface preparation is very important. Typically, installation is achieved by applying an epoxy adhesive to the prepared surface, installing the FRP reinforcement into the epoxy and, when required, applying a second layer of the epoxy adhesive. After curing, the FRP composite will add capacity to the element because it has a tensile strength up to 10 times that of steel.
Figure 1 shows a schematic for the structural strengthening of a utility tunnel at a university in South Florida. The utility tunnel roof originally functioned as a pedestrian walkway. A new dormitory structure required the walkway to be the primary access for emergency vehicles. Analysis of the tunnel’s top slab revealed that it did not have adequate strength to carry loads from fire trucks and other emergency vehicles; the school needed an innovative approach to strengthen the tunnel slab. A structurally efficient, easy-to-install, and cost-effective strengthening option was achieved by using externally bonded FRP sheets.
The strengthening solution consisted of carbon FRP sheets bonded to the bottom of the slab, serving as additional bottom tension reinforcement. In addition, the overhanging portions of the slab were strengthened using carbon FRP bars epoxy-bonded in grooves made on the slab’s top side. The latter technique is more appropriate than FRP sheets since the bars were bonded below the surface, thereby avoiding traffic damage to the externally bonded reinforcement.
In addition to FRP, steel reinforced polymer composites (SRP) may be used as externally bonded reinforcement. This steel-based, innovative strengthening system (known as Hardwire) was developed by Hardwire LLC and first introduced to the market in 2002. Hardwire is a low-cost, reinforcement system consisting of ultra-high-strength steel wires that are twisted together to form reinforcing steel cords approximately 0.035 inches in diameter. The steel wires have a tensile strength equal to 450 kips per square inch (ksi), about 10 times more than the strength of conventional structural steel, and the same elastic modulus of 29,000 ksi. This strengthening system can be applied using epoxy or cementitious materials and can be used to increase the shear and flexural capacity of structural elements.
Bonded steel elements—Strengthening concrete members by using bonded steel plates was developed in the 1960s in Switzerland and Germany. With this method, steel elements are glued to the concrete surface by a two-component epoxy adhesive to create a composite system and improve shear or flexural strength. The steel elements can be steel plates, channels, angles, or built-up members.
In addition to epoxy adhesive, mechanical anchors typically are used to ensure that the steel element will share external loads in case of adhesive failure. The exposed steel elements must be protected with a suitable system immediately following installation. And regardless of the specified corrosion protection system, its long-term durability properties and maintenance requirements must be considered fully.
The administration of an elementary school in New Jersey wanted to install skylights on the existing roof. The roof consisted of prestressed concrete, hollow planks. Installation of the skylights required cutting openings in the planks that would reduce their load-carrying capacity. This issue was resolved by designing a hybrid strengthening system composed of FRP fabric and steel elements. The externally bonded FRP strengthened the planks adjacent to the one to be cut, while the steel elements tied the plank to the adjacent ones, thus creating a new unit consisting of three planks with adequate capacity. In addition to the fast application of this system, this was a cost effective solution that also was aesthetically pleasing (see Photo 3).
External post-tensioning—The external post-tensioning technique has been used effectively to increase the flexural and shear capacity of both reinforced and prestressed concrete members since the 1950s. With this type of upgrading, active external forces are applied to the structural member using post-tensioned (stressed) cables to resist new loads. Because of the minimal, additional weight of the repair system, this technique is effective and economical, and has been employed with great success to correct excessive deflections and cracking in beams and slabs, parking structures, and cantilevered members.
The post-tensioning forces are delivered by means of standard prestressing tendons or high-strength steel rods, usually located outside the original section. The tendons are connected to the structure at anchor points, typically located at the ends of the member. End-anchors can be made of steel fixtures bolted to the structural member, or reinforced concrete blocks that are cast into place. The desired uplift force is provided by deviation blocks, fastened at the high or low points of the structural element. Prior to external prestressing, all existing cracks are epoxy-injected and spalls are patched to ensure that prestressing forces are distributed uniformly across the section of the member.
Figure 2 illustrates an external post-tensioning system used to strengthen prestressed double tees damaged by vehicular impact. Four double tee stems on an overpass located on the premises of a university in Washington, D.C., were damaged when the driver of an over-height truck failed to observe the posted height restriction. The four stems suffered excessive concrete cracking and spalling, and damage occurred to some of the internal prestressing steel.
Proposed solutions included replacing the damaged double tees with new ones and installing a steel frame underneath for support. Both options would render the overpass out of service for a longer-than-desired period. The option of an external post-tensioning system was more economical, required less time to complete, and allowed for a strengthening system that provided active forces. Therefore, it was more compatible with the existing construction. After all cracks were injected, the sides of the stems were formed and new concrete was cast to restore the integrity of the stems. The strengthening system then was installed, and—after the concrete cured—the external strands were stressed according to the engineer-specified forces.
Section enlargement—This method of strengthening involves placing additional “bonded” reinforced concrete to an existing structural member in the form of an overlay or a jacket. With section enlargement, columns, beams, slabs, and walls can be enlarged to increase their load-carrying capacity or stiffness. A typical enlargement is approximately 2 to 3 inches for slabs and 3 to 5 inches for beams and columns.
Figure 3 depicts details of a section enlargement used to increase the capacity of a main girder in a university parking garage. The girder was re-evaluated because of a change in the required loading and found to be deficient in flexure and shear. To correct the deficiency, additional flexural and shear steel were added. The entire beam then was formed and a 4-inch jacket of concrete was cast to enlarge the section.
Regardless of the experience and experimental knowledge gained in more than 100 years of reinforced concrete construction, structures require repair and/or strengthening because of natural causes, human error, and change in loading conditions. Further, it is important to recognize that concrete repair and strengthening is a scientific art form that involves the use of conventional, cementbased materials, as well as new techniques and materials.
It is crucial that structural engineers recognize that strengthening assessment and design is infinitely more complex than new construction. Typically, challenges arise because of unknown factors associated with the structural state—such as continuity, load path, and material properties—as well as the size and locations of existing reinforcement or prestressing. The degree to which the upgrade system and the existing structural elements share the loads also must be evaluated and addressed properly in the upgrade design, detailing, and implementation procedure. The importance of detailing and its direct effects on the effectiveness and durability of structural upgrades cannot be overemphasized. In fact, inadequate detailing is one factor that can lead to the total failure of a structural repair system.
In addition, engineers should consider the procurement process for specialty repair and strengthening projects to be different from new construction services. Engaging contracting firms that are familiar with all of the critical aspects highlighted here will ensure the most cost-effective and long-lasting results. Although it may appear that there is an up-front financial benefit to obtaining these specialty services from firms with experience in new construction, the real risk is that the repairs will cause an endless “repair of repairs” cycle, resulting in additional disruption and expenditure to owners. When it comes to structural repair and strengthening, the mantra “do it right the first time” pays dividends.
Tarek Alkhrdaji, Ph.D., is a design engineer with Baltimore-based Structural Group. He has been involved in numerous projects involving structural repair and upgrade, as well as fullscale, in-situ load testing. He can be reached via e-mail at firstname.lastname@example.org. Jay Thomas is vice president for Structural Preservation Systems Inc., a unit of Structural Group. He has 21 years of experience in structural repair, strengthening, and protection of concrete and steel. He can be reached via e-mail at email@example.com. Both authors are active members in the American Concrete Institute’s committees 440 (FRP) and 437 (Strength Evaluation), as well as members of the International Concrete Repair Institute.