Design and Application Techniques Key to Successful Structural Strengthening Projects

Authored by:
Publication: Concrete Technology Today
Near surface mounted CFRP rods installed on top of the slab in two directions.
Completing surface preparation, steel and dowel placement and post-tensioning placement
Failed post tensioned beam.
Beams in the Resorts Casino steam tunnel were enlarged by five inches on all three sides.
Bonded Steel Element.
Additional steel supports installed to arrest excessive deflections and further displacement.

Many of today’s facilities have been in service for several decades and during the course of their operation have experienced changes in use, deterioration, structural defects, seismic effects or revisions to building codes. Any of these factors can prompt the need for restoration or additional load carrying capacity. These projects are made even more challenging because the facilities are often inhabited by tenants and the work has to be performed while minimizing disruption to existing operations. In addition to weighing their options for structural strengthening and structural repair, facility managers must carefully choose among a large variety of available materials and techniques that can be utilized to extend the service life of their structures. To  ensure that repaired and strengthened structures are durable, the repair system must be “tailored” to address the intended service life, and the team involved with a project must be knowledgeable and experienced enough to recognize the complexity of their task. 

A basic understanding of the different design skills and application techniques available will assist facility managers when evaluating different strengthening and repair options.

Embracing the Terminology

Concrete experts commonly use the terms structural repair and structural strengthening to describe building renovation activities. Although the two terms sound similar, they refer to slightly different concepts. Structural repair describes the process of reconstructing a facility or its structural elements. This process involves determining the origin of the distress, removing damaged materials and causes of distress, as well as selecting and applying appropriate repair materials that restore the integrity of the structural elements and extend the structure’s useful life.

Structural strengthening, on the other hand, is an upgrade of a building’s structural system designed to improve performance under existing loads or to increase the strength of the existing 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 element surrounding the repair area, which can lead to a bigger problem and even localized failure. With upgrade projects, contractors also must deal with critical issues related to access to the work area, constructability of the repair, noise and dust control, environmental impact, and the selection of construction materials.

In many cases, the original structural drawings are unavailable and an in-situ assessment of the existing structure may be required to establish member strengths and reinforcement details.  Load testing can be used to verify that a structure can safely support the design loads and to verify the capacity of repaired or strengthened members.

Why Repair or Strengthen?

In general, structural deficiencies can result from code changes, seismic upgrade needs, deficiencies that develop because of environmental effects (i.e., corrosion), changes in use that increase service loads, requirements to enhance blast and ballistic resistance, or deficiencies within the structure caused by errors during original design or construction. Once this has happened, two alternatives to fixing the problem are to replace the structure or to repair and strengthen it.  Economically, repair and strengthening are often the only viable solution.

There are a variety of different methods available to repair or strengthen existing buildings, including fiber reinforced polymer composites, span shortening, externally bonded steel, external or internal post-tensioning systems, and section enlargement. The technique or combination of techniques used will depend on the specific requirements of the project in regard to technical concerns (engineering), constructability issues (construction methods), aesthetics (architectural), and economics (ROI). The following gives a brief description of common repair and strengthening methods and case-study applications.

Fiber Reinforced Polymer Composites

Fiber reinforced polymer (FRP) systems are paper-thin fabric sheets bonded to concrete members with epoxy adhesive that significantly increase load-carrying capacity. Installation typically is achieved by applying an epoxy adhesive to the prepared surface, installing the FRP fabric into the epoxy and then applying a second layer of the epoxy adhesive. After curing, the FRP composite will add considerable capacity to the element despite the fact that it is a very thin laminate because the carbon FRP (CFRP) has tensile strength up to 10 times that of steel.

Case Study: Mall Building – Conversion to a Telecommunications Center

A one million plus square foot building located in Miami, Fla., previously operated as a shopping mall was bought by a telecommunication and computer company with the plan of converting it to house Internet and communication equipment. The structural floor consisted of a one-way slab supported by precast prestressed joists. The joists were simply supported on continuous cast-in-place concrete beams with precast/prestressed soffits. To accommodate the 730 kg/m2 (150 psf) telecommunication equipment, the floor beams and joists required an increase of approximately 20 percent in load carrying capacity. The selected strengthening system consisted of multiple plies of CFRP strips attached to the sides and bottom of each joist to increase the bending moment capacity with U-wrap strips at each end to provide anchorage against peeling. For the beams, FRP strips on each side of and centered on columns were installed on top of the slab to increase the capacity at the supports and on the underside of beams to increase the bending capacity at mid-span. Four shear, U-wrap CFRP strips were applied to the beams. To verify the capacity and service behavior of the strengthened members, full scale cyclic (3 hrs) and monotonic (24 hrs) load tests were performed on selected beams and joists. The load tests were performed to assess the structural performance before and after strengthening. In these tests, 85 percent of the factored design load was applied. Although failure was achieved for the unstrengthened members prior to attaining the full test loads, strengthened members demonstrated elastic behavior up to full test load with almost no residual crack width after load removal. 

Case Study: Fire Station – Floor Strengthening for Increased Truck Loading

Fire Station No.1 in Milford, Conn. is a two-story reinforced concrete structure that was constructed in 1928. It consists of reinforced concrete decks supported on concrete encased steel beams and cast-in-place reinforced concrete joists. A site survey and condition assessment study indicated that the floor system was severely overstressed due to the use of heavier fire trucks not in service during the original design of the floor. Flexural cracks were observed on the topside of the slab along several beams. Bottom cracks were observed on the joists indicating a severe case of overstress. Testing of concrete and reinforcing bars indicated that the existing concrete strength was 46 MPa (6,700 psi) and steel yield strength was 227 MPa (33,000 psi).

A hybrid strengthening solution involving externally bonded FRP sheets bonded to the underside of the joists and FRP bar embedded in the concrete surface on the top side of the slab provided the most effective, economical, and least disruptive option. The topside carbon FRP bars were embedded in surface grooves and bonded with epoxy adhesive. Bending upgrade at joist mid-spans was achieved using externally bonded carbon FRP sheets applied to their soffits. In addition to quality control, FRP pull-off tests were used to determine when to open the floor to traffic and allow trucks and other live loads on the strengthened floor. The fast cure of the FRP systems allowed the fire station to be back in service within approximately 48 hours after completion of FRP installation.

Span Shortening

Span shortening is accomplished by installing additional supports underneath existing members to reduce the span length. Materials used for span-shortening applications include structural steel members and cast-in-place reinforced concrete members, which are quick to install. Connections can be designed easily using bolts and adhesive anchors. Span shortening, however, may result in loss of space and reduced headroom.

Case Study: Parking Garage Repair

The slab of a 1,000 space parking garage for a commercial office building in Atlanta, Ga. was distressed due to an inadequately designed post-tensioning system. The garage consisted of four parking levels connected to the building by a link bridge. The deck was 200 mm (8-inch) thick and was constructed of 2-way cast-in-place concrete, post-tensioned flat slab.

In order to resolve the large overstress in the end spans, an external steel-framing system was installed between the columns at the underside of the second floor. This concept essentially reduced the clear span between the columns within the last bay of the garage and eliminated the bending deficiencies in the slab. At each additional support point, a flat hydraulic jack was inserted between the support frame and the underside of the slab and was used to “pick-up” part of the dead load off the column.

Bonded Steel Elements

In this method, steel elements are attached to the concrete surface using a two-component epoxy adhesive to create a composite system. The steel elements can be steel plates, channels, angles or built-up members. Steel elements bonded to the sides or bottom of a structural member can improve its shear or flexural strength. In addition to epoxy adhesive, mechanical anchors typically are used to ensure the steel element will share external loads in case of adhesive failure. The exposed steel elements should be protected with a suitable system immediately following installation.

Case Study: Elementary School Roof Strengthening

Case in point is the strengthening of a roof system on an elementary school in New Jersey. The school administration wanted to install skylights on the existing roof which 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, the solution cost effective and aesthetically pleasing.

External Post-tensioning

With external post-tensioning, active external forces are applied to the structural member using post-tensioned (stressed) cables to resist new loads. 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 in-situ. 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 prestressing forces are distributed uniformly across the section of the member.

Case Study: Parking Structure Repair

During the spring of 2007, external post-tensioning was used to strengthen two damaged post-tensioned beams at a government facility parking structure in the Washington D.C. area. The garage is a six-story, freestanding, post-tensioned structure with approximately 500,000 square feet of parking area. The typical deck consists of one-way post-tensioned slabs supported on long span post-tensioned beams. Failure of two beams was caused by the snow removal contractor who piled a significant amount of snow on a small area of the roof deck above the two beams. The approximately 8-foot pile of snow corresponds to approximately 200 pounds per square foot. The existing post-tensioned beams are 12 inches (305 mm) wide and 36.5 inches (927 mm) deep and support a 5.5 inch (138 mm) thick post-tensioned slab.

The excessive damage to the beams made it impossible to estimate their residual capacity. As such, the design team initially was only considering replacement of the damaged beams. Full replacement created numerous challenges as it would require de-tensioning of the slab above and installation of shoring on multiple levels. The plan would severely disrupt the operation of the garage. A strengthening option that consisted of external post-tensioning placed in new concrete jacket that was 6 inches (150 mm) wide on each side and 4 inches (100 mm) thick on the bottom was recommended. The proposed post-tensioning solution provided adequate repair at half the cost of the removal option with minimal disruption to garage operations.

The external post-tensioning design assumed that the original reinforcement would no longer contribute to the strength of the beams and consisted of ten grade 270 ksi, ½ inch, seven wire low relaxation strands coated with corrosion prevention grease and encased in a continuously extruded polyethylene plastic sheathing. Longitudinal mild steel reinforcement was also provided.

The existing concrete surface was roughened to create profile with ¼-inch amplitude and then cleaned using high-pressure water blasting to produce porous, open substrate. Then #5 dowels were installed to improve the composite behavior of the jacket with the existing member. Number 4 stirrups were placed in the concrete jacket at 18 inches on center and doweled to the underside of the slab using epoxy adhesive.  Additional dowels and stirrups were installed at the cable’s deviation points to allow for proper confinement and force transfer. 

The formwork that was designed to withstand the pumping pressure was then installed and self consolidating concrete (SCC) pumped into the forms. When the concrete strength reached 3,500 psi, the tendons were stressed and all gauge pressures and cable elongations recorded and reviewed for final approval. The newly repaired beams appeared very similar to the original beams with concrete enlargements that have the look of an original cast-in-place concrete element. 

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.

Case Study: Resorts Casino

The Resorts Casino had more to tackle then just the column strengthening for the promenade slab – the casino also had to address severe concrete deterioration in a steam tunnel and the Grease Recovery Unit (GRU) room. In many of the beams, slabs and columns, the steel rebar was fully exposed or completely deteriorated. Not only would these members require repair, they would also have to be strengthened to meet today’s design codes.

To strengthen the beams, the team decided to employ an enlargement technique using a reinforced concrete jacket that would be bonded to each existing beam. The challenge faced from a design perspective was defining the existing capacity of each beam as a starting point of reference for upgrade. For speed and simplicity, the design of the new jacket was based on the worst case capacity found in the 10 existing beams. This plan addressed the upgrade in all the beams without 10 separate designs -- a common strengthening strategy. Also, for efficiency, all calculations were performed without any consideration of contribution from the original steel in the beams.

In the GRU room, the concrete ceiling and three very large columns in the room had deteriorated to a point of structural concern due to a constant combination of high temperatures and humidity. Once the original ceiling structure was completely removed, formwork was placed and a new two-way slab and beam system was cast with new hangers and supports for the piping and the equipment. Then, reinforced enlarged sections were added to repair and strengthen the columns. These innovative repair and strengthening strategies allowed the casino to remain open during repairs and the project to be completed ahead of schedule.

Doing It Right the First Time

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, deterioration, human error and change in loading conditions. But by accepting this reality, building owners can be prepared for the necessity of strengthening and repair and will be able to implement such programs before structural and safety issues become dire.

Likewise, repair contractors should also be prepared for the tasks they will undertake. Challenges usually arise because of unknown actual structural states such as load path, material properties, as well as the size and location of existing reinforcement or prestressing. The degree to which the upgrade system and the existing structural elements will share the loads must be evaluated and properly addressed in the upgrade design, detailing and implementation methods.

To design and select a repair/strengthening method and material successfully, a system concept must be employed. No matter what strengthening technique is chosen, one of the requirements of the composite system is the ability to perform as an integrated system. This goal can be achieved only by providing an adequate bond between the existing concrete and external reinforcement – creating a bond strength such that the composite structure behaves monolithically.

In addition, facility engineers should consider the procurement process for specialty repair and strengthening projects to be different from new construction services. Engaging specialty engineering and contracting firms that are familiar with the critical aspects highlighted within this article will ensure the most cost-effective and long-lasting results. Although it may appear 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.