Figure 1: Load testing of bonded overlay.

Figure 2: Floor joist with FRP shear reinforcement

Figure 3: Installed steel reinforcement.

Figure 4: External post-tensioning of beams.

Figure 5: CFRP reinforcement of slab

Figure 6: Steel frame for two-way span shortening.

The repair and strengthening of concrete structures is a challenging and growing segment of the concrete repair industry for both engineers and contractors. The continued economic growth of the United States confronts this industry with distinctively new trials arising from extensive infrastructure renewal. Repair or strengthening of an existing structure may become necessary due to structural inadequacies that adversely affect the structure or its members. After all, deficiencies are not caused exclusively by distress/deterioration of concrete in the form of spalls, delamination, or cracks. Inadequacies may arise from: 1) code changes i.e. seismic upgrade, 2) deficiencies that developed due to environmental effects such as corrosion, 3) changes in use that increase service loads, 4) deficiencies within the structure caused by errors in design and construction.

Due to increasing economical constraints, the current trend is to repair/upgrade deteriorated and functionally obsolete structures rather than replacing them with new structures.

The successful structural repair or upgrade involves four basic elements: concepts used in system design; compatibility and composite behavior with repair materials; field application methods; and design details. You can never overemphasize the importance of detailing and how it directly affects the durability of structural upgrades. In fact, inadequate detailing is one factor that can lead to the total failure of the structural repair system.

Except in cases where new structural members are installed to share the load with existing members, most retrofits comprise a composite repair/retrofit system. The composite repair/retrofit can be achieved by section enlargement, external post tensioning, externally bonded steel elements or advanced fiber reinforced polymer (FRP) composites, span shortening, or a combination of these techniques. The composite repair system should be tailored to serve the intended use for the designed service life of the structure without interfering with its functionality.

No matter what strengthening technique is used, the ability to perform as an integrated system can be achieved only by providing an adequate bond between the existing concrete member and the externally/internally applied repair/reinforcement to ensure monolithic structural behavior. Stress concentrations resulting from added material that induces a sudden local increase in stiffness may cause a localized failure and should be investigated.

Unfortunately, there are no simple, straightforward design, specification, and execution methods for repair/upgrade projects. Engineers, architects, and contractors should be innovative, and their innovation should be reflected in the design details, specifications, and application.

Conventional strengthening methods

Section enlargement

This method is the oldest strengthening technique known in the concrete construction industry, and involves the placement of additional concrete on an existing structural member in the form of an overlay or jacket. The additional concrete may be:

• structural concrete, which is adequately bonded and reinforced with steel bars or wire mesh and designed to be a load-carrying element, or
  
  
• protective concrete, which is used to fireproof post tensioning steel, FRP cables, or bonded steel elements, and protect them from mechanical and environmental damage.

With this method, columns, beams, slabs, and walls can be enlarged to add load-carrying capacity, or to increase stiffness. In all cases, the designer should incorporate the weight of the additional concrete overlay/jacket in the design of the enlargement. An additional (overlaid) structural concrete slab placed on top of the existing deck slab can increase the structural capacity of the supporting beams or joists by increasing the effective depth of reinforcement at the positive moment region (typically at mid-span). It can also increase the structural capacity at the negative moment region (typically at supports) with the addition of steel reinforcement in the overlay-usually achieved with steel wire mesh.

Sufficient clearance should be allowed between the top of the existing concrete slab and the bottom of the overlay reinforcement to ensure adequate concrete flow around the bars, which depends on the maximum aggregate size. Based on this requirement, and considering the minimum concrete cover requirement for steel reinforcement, a typical reinforced concrete overlay thickness is approximately 51 mm to 76 mm (2-3 in.). Accordingly, this technique is more appropriate for members in which increasing the effective depth of the reinforcement by this magnitude sufficiently increases the capacity to the point where it nullifies the effect of the overlay's weight.

To minimize the additional weight of the overlay, the use of lightweight concrete is recommended. The composite behavior of the existing slab with added overlay can only be taken into account if monolithic structural action is assured. This requires good bond or horizontal shear transfer capacity at the interface that does not prematurely deteriorate under cyclic traffic, and environmental and temperature loads.

In some occasions, the concrete at the surface of the member is weak and does not have adequate strength to ensure sufficient shear transfer. In this case, the weak concrete should be chipped away. Steel shear dowels may be used to enhance the composite behavior. [For adequate surface treatment and preparation, refer to guidelines provided in International Concrete Repair Institute (ICRI) publications.]

The section enlargement method is relatively easy and economically effective, but it does not address the possible corrosion of embedded reinforcing steel, especially in a harsh environment. This can be avoided by using FRP reinforcement, which is immune to corrosion.

Testing was performed using a rapid load test procedure involving hydraulic jacks that produced the same internal forces as those caused by a uniform live load. Reaction forces were supplied by a micro-pile that was driven directly below the upgraded members. Test results indicated that the overlay improved the structural capacity of the floor system, enabling it to carry a live load of 7.2 kPa (150 psf) from a load-verified capacity of approximately 4.8 kPa (100 psf).

Load testing of the structural elements was necessary due to limited information on the original design of the floor system. Relatively uniform cracks were observed on the top of the overlay at the supports indicating adequate shear transfer between the overlay and the existing slab. In addition to the reinforced concrete overlay, carbon FRP reinforcement was externally bonded to the surface of the joist stems to improve shear capacity.

In another parking garage, analysis indicated the beam was deficient in shear and flexure at the support. This deficiency was corrected by doweling additional steel stirrups to the sides of the beam. Compression steel bars were also installed at the bottom of the existing beam. The beam was formed and enlarged, which increased the flexural strength by increasing the effective depth of the steel reinforcement. The concrete surface was prepared by sandblasting prior to forming to remove loose concrete particles. (Remember that code requirements for reinforcement anchorage, development length, spacing, and concrete cover for new construction should be met when performing section enlargement.)

Post tensioning

External prestressing of concrete members was already a mode of construction by the 1950s, and has been effectively used to increase the flexural and shear capacity of reinforced and prestressed concrete members. Here, the members are upgraded by applying external (active) forces to counteract the effect of the design or additional loads. The forces are delivered by means of prestressing tendons located outside the reinforced section. Due to the minimal additional weight of the repair system, this technique is particularly effective and economical for long span beams, and has also been employed with great success to correct excessive and undesirable deflections in existing parking structures. It has also been used to strengthen existing concrete structures against fatigue and cracking.

External prestressing is a simple construction method involving a strand or tendon profile that can be produced easily on-site with few or no problems for grouting, and can be easily replaced if needed. The disadvantage of external prestressing is that it is located outside the structure, rendering it susceptible to corrosion and fire, as well as acts of vandalism. Almost all of these problems can be avoided by encasement in concrete or by using shotcrete. There is also a new fireproofed tendon system on the market that permits external application.

Post tensioning requires accessibility to the sides and possibly the ends of the member. In the case where external prestressing is used as a supplemental reinforcement, fireproofing requirements may be relaxed, as their failure will not jeopardize safety of the structural element.

To achieve external post tensioning, the tendons are connected to the structure at the anchor points, typically located at the member ends. The desired uplift force is provided by deviation blocks, fastened at the high or low points of the structure. A wide variety of deviators has been used in the field, but they typically consist of structural steel brackets or saddles seated on the soffit of the member, or bolted to the stem of the member. Another method involves drilling through the stem of the member to insert a grouted steel tube that protrudes on both sides. In any case, extreme caution should be taken to avoid damaging or cutting the existing reinforcement of the member.

Tensioning of the prestressing element is achieved by hydraulic jacks, and in cases of limited clearance, by turnbuckles, or nuts and threaded rods. For tendons of straight length, high-strength threaded steel rods are typically used, while wire strands are used for tendons profiled along the member.

The designer of the external prestressing system should consider the effect of the newly applied concentrated forces at the location of deviators and anchorage. Prior to external prestressing, all existing cracks should be epoxy injected and spalls patched, to ensure that prestressing forces are distributed uniformly across the section of the member.

The upgrade of a two-way ramp slab and beams of a parking garage for a shopping mall resulted from inadequate detailing of the existing post tensioning system. The ramp's deficiencies were further magnified by the corrosion of the steel tendons. A number of beams were also found to be deficient in shear or flexure. Both time and cost constraints applied to this project, as the ramp provided the only access to the back of the mall. Proposed solutions included demolition of the existing ramp and construction of a new one, or the installation of a steel frame underneath for support. Both options would render the ramp out of service.

The option to install a new post tensioning system was more economical afnd required less time to complete. The system was installed in grooves made in the existing slab, whose depths varied to accommodate the profile of the steel tendons. Tendon anchors were embedded within the slab. This option saved the owner approximately $500,000 in construction and operation costs. The interior beams were upgraded using an external post tensioning system to address flexural deficiencies, and external steel stirrups addressed shear deficiency.

Bonded steel elements

Strengthening reinforced concrete members using bonded steel plates was developed in the 1960s in Switzerland and Germany. In this method, steel elements are glued to the concrete surface by a two-component epoxy adhesive to create a composite system. The steel elements could be steel plates, channels, angles, or built-up members. The bonded steel element is considered a passive reinforcement-that is, the new steel does not become effective until the concrete deflects under additional loads. In the ultimate strength design philosophy, however, both existing and new steel will yield, and the composite section may be assumed to resist the total load. Adequate design, specification, and execution of the job are necessary to ensure the composite action of the repair/upgrade system.

Steel elements (typically plates) bonded to the tension face of concrete beams can increase flexural capacity, and contribute to increases in flexural stiffness, and associated decreases in deflection and cracking. Steel elements bonded to the sides of the member can improve the shear strength of the concrete member. The bonded steel elements are considered both a supplement to the existing embedded reinforcing steel, and a secondary reinforcement that shares the applied forces, thereby bringing the stresses in existing concrete and steel bars to satisfactory levels.

The principle of the method is quite simple: steel plates are glued with a two-component epoxy adhesive to the concrete surface creating a concrete-adhesive-steel composite system. The preparation of all component surfaces, as well as the bonding operation itself, must be executed with great care to achieve the composite action of the system, and the capability of the adhesive to transfer shear stresses. The surfaces to be bonded must be clean (abrasive blasting for the steel and concrete surfaces is preferred). The epoxy's bond strength to the concrete should at least match the concrete's tensile strength of the concrete.

As with any gluing operation, bonding the steel plates to the concrete requires pressing them together. This is achieved by using adhesive anchors. It is strongly recommended to provide some supplemental anchors, especially at the ends of the plate to ensure the bonded steel element will still share some load in case of adhesive failure. Considerable site work is required to accurately locate the existing reinforcement to avoid damaging it while placing the anchors. In addition, elaborate and expensive falsework is required to maintain the steelwork's position during bonding.

The exposed steel elements must be protected with a suitable system immediately following installation. Regardless of the corrosion protection system specified, its compatibility with the other materials and its long-term durability properties and maintenance requirements must be fully considered. Externally bonded steel elements should be used as a long-term solution for structures subjected to aggressive environments. Fire protection is also an important consideration when using bonded steel elements.

Strengthening with advanced composites

Due to their weight and the restrictive length, steel plates can be unwieldy on-site. The steel elements may also need to be spliced, which complicates the design and construction operations. These disadvantages can be overcome with composite fiber reinforced polymers (FRP). Used extensively in industries such as defense, aerospace, automotive, chemical, shipbuilding, etc., the typical FRP composite material comprises reinforcing fibers embedded in a polymer matrix that protects them against mechanical damage, transfers stress between them, and maintains the shape of the composite. Other properties of the matrix such as chemical inertness, thermal stability, and environmental durability are important in many applications. Besides strength, the reinforcing fibers give the composite its stiffness and electric properties.

The most important characteristic of FRPs for structural repair and strengthening applications is speed and ease of installation. The higher material cost is usually offset by reduced costs in labor, heavy machinery, and shutdowns-all of which make FRP strengthening systems competitive with traditional strengthening techniques.

Typical fibers used to create the FRP composite are glass, aramid and carbon, but studies in Switzerland demonstrate that aramid fibers possess inadequate compressive strength, and glass fibers do not resist alkalis. Thus, the most common fiber for concrete strengthening applications is carbon FRP (CFRP) due to its superior properties and durability. The most common types of CFRP are fabrics, plates, and rods.

As with any other composite system, the bond between the strengthening plates and the existing concrete is critical, and surface preparation of both phases of the system-concrete and CFRP-is very important. If CFRP plates are used, they should be grounded on the bonding side, and immediately before the bonding, their surface cleaned. The epoxy glue components should be applied to the plate immediately after mixing. After assembling the plate in the designated position, a slight pressure is applied to squeeze out excessive adhesive. The application to existing concrete members does not require special handling skills and equipment.

Clearly, CFRP plates are limited to certain geometrical shapes-flat surfaces, to be precise. A modification to this strengthening technique involves using CFRP fabric sheets that come in continuous rolls. These sheets can be easily and quickly tailored and wrapped around almost any profile. CFRP fabrics may be adhered to the tension side of structural members (i.e. slabs or beams) to provide additional flexural strength, or adhered to web sides of joists and beams, or wrapped around columns to provide additional shear strength. Wrapping CFRP around columns also increases concrete confinement, thereby boosting the strength and ductility of columns.

Prior to applying the fabric, the concrete surface should be blasted with sand or water to remove loose concrete. It is necessary to remove all chloride-contaminated concrete before CFRP bonding to avoid further corrosion of the reinforcement, which could lead to continued delamination and spalling. The strengthening can only be applied after all corrosion problems have been determined and addressed. In addition, all existing cracks should be injected with epoxy.

To apply the fabric, epoxy-based primer is rolled or brushed onto the prepared concrete surface. The first epoxy coat is applied by roller while the primer is still tacky. CFRP sheets are applied to the fresh epoxy using a ribbed roller to remove air bubbles and ensure impregnation of the fibers with resin. This operation is similar to wallpapering. The sheets may be spliced, if necessary, in the direction of the fiber with a minimum 102-mm- (4-in.) overlap, depending on the product. After the sheet is installed, the second and final epoxy coat is applied and allowed to cure. Externally bonded FRP reinforcement is designed to supplement existing interior reinforcement; should something cause the FRP reinforcement to be compromised, the structure must still be able to carry existing service loads without collapse.

In one particular floor system upgraded with FRP reinforcement, CFRP strips were bonded to the soffit of the joist (to increase positive moment capacity at mid-span), to the soffit, and on top of the existing prestressed concrete girders (to increase their positive and negative moment capacity). FRP strips applied to the sides of the girders in the form of U-wraps were used to increase shear strength. In this case, the FRP solution cost the least of all other options. The capacity improvement of the structural elements was verified through an in-situ rapid load test.

Strengthening using externally bonded FRP is more complicated than with conventional materials, as the linear/elastic behavior of FRP can result in different failure modes (i.e. concrete crushing or FRP rupture). In fact, the contribution of FRP reinforcement to the strength of the member depends on many variables.

Many agencies have developed (or are in the process of developing) material standards and design guides for practitioners. In the United Kingdom, the Concrete Society released Technical Report No. 55 for design guidance in strengthening concrete structures using fiber composite materials.

The American Concrete Institute (ACI) is in the final stages of releasing a similar document (ACI Committee 440), and the Federal Highway Administration (FHWA) is sponsoring research programs to develop model specifications for the repair and strengthening of existing bridges using FRP composites that ensure quality and performance.

Span shortening

Span shortening has been used to reduce the force in overstressed beams, and can also be used to increase the load-carrying capacity of the member. It is accomplished by erecting additional columns some distance away from existing ones. The new columns require footings, which may drive up the strengthening cost considerably. A less expensive approach is to install diagonal braces that extend from the bases of existing columns. Both methods sacrifice some space under existing beams or joists, and when additional beams are required between the knee braces to support intermediate beams or joists, headroom may be reduced.

The best material for these applications are steel structural members, which are quick to install. The fact that they do not shrink allows them to help carry the load on newly installed members. Connections can be easily designed using adhesive anchors, as the steel section mostly carries compression and shear forces. It is important to remember that knee braces exert horizontal forces on columns and could over-stress them in shear or flexure. In addition, supported beams should be investigated for shear to ensure they are capable of resisting reaction forces exerted by knee braces.

Conclusion

The structural repair/upgrade of concrete structures is an art form that has evolved into a complex science. It involves the use of conventional cement-based materials, as well as new techniques that exploit the advanced composite materials commonly used in aerospace and military applications.

Regardless of the experience and experimental knowledge gained in more than 100 years of reinforced concrete construction, structures will continue to deteriorate from natural causes and human error. As practitioners, we must recognize that strengthening assessment and design is infinitely more complex than new construction and should not be treated lightly. In addition to dealing with the unknown actual structural state, the degree to which added new materials and an existing structure share the effects of a repair system must be evaluated and properly addressed.