by Roya A. Abyaneh, EIT, and Edward S. Breeze, PE
Externally bonded fiber-reinforced polymer (FRP) can be a practical strengthening solution for all concrete structural elements. However, there is little technical literature and design guidance on how one can achieve durable performance from this relatively new repair technique. With the rise of aging buildings and infrastructure, the need for durable reinforcement methods has become more pressing.
Today’s aging infrastructure requires regular maintenance and reinforcement or, in some cases, complete replacement. The 2017 Infrastructure Report Card of America by the American Society of Civil Engineers (ASCE) notes nearly 40 percent of the country’s bridges are more than 50 years old. (For more, consult the 2017 Infrastructure Report Card of America.) Rehabilitation and maintenance are becoming more critical every day, and so is the need for educating the industry in assessment and preservation of existing infrastructure. With the rising popularity of FRP as a means for structural repair, designers require more guidance in design and construction to obtain durable performance.
General-purpose carbon fiber-reinforced polymer (CFRP) has an elastic modulus in the range of 220 to 230 GPa (32,000 to 34,000 ksi), which is close to the stiffness of structural steel at 200 GPa (29,000 ksi). Improved load-carrying capacity through application of CFRP has been reported in various literature. However, its use among practicing engineers is limited when compared with steel and concrete.
One reason for the limited application may be the lack of expertise and familiarity in design with the material. CFRP manufacturers have helped address this issue by offering in-house engineering. In many cases, these services are free in conjunction with purchase of their CFRP products. In such cases, the structural engineer of record (EOR) for the project determines the required additional strength and delegates the task of CFRP design to the manufacturer’s designer (MD). Inherently, the MD will have limited familiarity with the overall structure. This leaves potential risks with the EOR, who must review the design, verify assumptions, and ensure applicability to the project.
For perspective, the standard procedures in design and construction using CFRP are discussed here in application to a typical reinforced concrete parking garage. Parking structures built in accordance with earlier codes, such as the Uniform Building Code (UBC), were designed for different live load requirements than specified in recent codes. In some cases, the older live load specifications may result in less stringent live load requirements when compared with the more recent International Building Code (IBC). This change in live load is in accordance with the heavier traffic and weight of vehicles. Along with deterioration of aging infrastructure, this code change illustrates the importance of regular assessments for maintaining healthy structures and ensuring public safety.
Once distress is observed, as-built conditions must be determined to perform structural analysis, including:
- determining the concrete density by coring specimens;
- scanning the concrete members to locate reinforcement and compare with design drawings; and
- excavating portions of the members to confirm reinforcing bar size, scan readings, and steel grade.
American Concrete Institute (ACI) 562, Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures, specifies the required information, such as the tests discussed above, to assess structural distress. In addition to ACI 562, engineering judgement and pertinent research publications are necessary to determine the capacity of the members. Once the cause of distress is identified and the deficit capacity is determined, the repair method should be selected in consideration of various factors, including service interruption, cost, and design life of the repairs.
CFRP fabric is often a suitable choice, when considering:
- speed and ease of application;
- limited debris from demolition and repair activities;
- minimal disruption of traffic and aesthetic influence; and
- structural efficiency (strength-to-weight ratio).
A variety of CFRP products and application methods exist. This article focuses on the wet layup system with CFRP fabric for the purpose of shear strengthening. CFRP strengthening is permitted only when the un-strengthened structural member has the ability to withstand a reasonable level of dead and live loads as described in ACI 440.2R, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures.