Reinforced concrete beams, which are commonly used in the construction of buildings and infrastructure, are designed to support loads throughout their service life. However, over time, these beams can deteriorate due to factors like environmental exposure, applied loads, and physical impacts. When this occurs, the beam’s ability to carry loads decreases, often necessitating a rehabilitation or strengthening process to restore its structural integrity. In some cases, even newly constructed beams may fail to meet design specifications due to errors in the design or construction processes. This article explores one of the modern techniques for rehabilitating reinforced concrete beams: the Near Surface Mounted (NSM) Fiber Reinforced Polymer (FRP) method, as well as the different failure modes that can occur when beams are strengthened using this technique.
I. Introduction: The Need for Beam Rehabilitation
Reinforced Concrete Beam Deterioration
Reinforced concrete beams can face significant wear and tear over time. Exposure to harsh environmental conditions, such as moisture, temperature fluctuations, and chemicals, can cause corrosion of the steel reinforcement. In addition, external loads—such as traffic loads on bridges or heavy equipment in buildings—can cause beams to gradually weaken. If a beam is not properly maintained or rehabilitated, its load-carrying capacity may diminish to the point where it can no longer safely support the intended structural loads. In such cases, rehabilitation is required to restore the beam’s strength and extend its service life.
The Need for Flexural Repair in Newly Constructed Beams
Interestingly, even newly constructed beams can require strengthening. This typically happens due to errors in the design or construction phases. For instance, a beam may be undersized or incorrectly reinforced, making it unable to support the expected loads. In such cases, the strengthening or rehabilitation of the beam is necessary to ensure it meets the required performance standards.
II. NSM FRP Technique for Strengthening Reinforced Concrete Beams
Overview of Near Surface Mounted (NSM) FRP Method
The Near Surface Mounted (NSM) technique is a modern method used to strengthen reinforced concrete beams by improving their flexural strength. This technique involves cutting grooves into the underside of the beam and embedding Fiber Reinforced Polymer (FRP) bars into these grooves. The FRP bars are then bonded to the concrete using a strong adhesive, typically an epoxy-based material.
FRP materials are chosen for their high strength-to-weight ratio, corrosion resistance, and durability, making them ideal for use in structural strengthening. The NSM technique offers a number of advantages over traditional methods, such as the external bonding of FRP plates, including less disruption to the structure and better aesthetic integration, as the bars are embedded within the concrete.
Effectiveness of the NSM FRP Technique
The NSM FRP method is particularly effective in improving the flexural (bending) strength of reinforced concrete beams. By adding additional reinforcement in the form of FRP bars, the technique allows beams to carry higher loads without significant increases in their weight or size. The bonded FRP bars work in tandem with the original concrete and reinforcement, improving the beam’s resistance to bending and helping to restore or enhance its overall performance.
III. Failure Modes of NSM FRP Strengthened Beams
While the NSM FRP technique has proven to be effective in strengthening concrete beams, failure modes can still occur when the beam reaches its ultimate load-carrying capacity. These failure modes are typically divided into two broad categories:
- Composite Failure (Good Bond Between Materials)
- Premature Failure (Loss of Composite Action)
Each category includes various sub-types of failure, which we will examine in detail below.
A. Composite Failure Mode
In composite failure, the FRP and concrete materials remain well-bonded and act as a unified system, but failure occurs when one of the materials reaches its limit.
- Rupture of FRP Bars or Strips
This failure occurs when the FRP bars or strips, which provide additional reinforcement, are stretched beyond their tensile strength and rupture. The rupture of the FRP material under stress can lead to the beam losing its ability to carry loads. - Crushing of Concrete
Another possible mode of failure in composite action occurs when the concrete itself fails in compression. After the FRP material has reached its limit, the concrete beneath it may fail due to excessive compression, leading to crushing and structural damage.
B. Premature Failure Modes
Premature failure occurs when the bond between the FRP and the concrete weakens, leading to a loss of composite action before the beam reaches its ultimate strength. Premature failure can happen through various mechanisms, including debonding and cracking.
- Interfacial Debonding Between Epoxy and Bar
This failure mode occurs at the interface between the FRP bar and the epoxy adhesive used to bond it to the concrete. The bond may fail due to flexural cracks crossing the epoxy layer. When this occurs, the FRP bars may lose their connection to the concrete, causing a loss of strength. This type of failure is particularly dangerous when the bars have been sandblasted or treated, as this can reduce the bond strength. - Splitting of Concrete Cover
Debonding cracks can also develop at the bottom of the beam, particularly where the FRP bars are embedded. These cracks propagate upward, splitting the concrete cover and causing the beam’s outer layers to separate. In severe cases, these cracks may extend to the steel reinforcement, compromising the structural integrity of the beam. - End Cover and Bar Splitting
When the FRP bars extend too far from the support, failure often starts at the location where the bar terminates. Cracks then propagate toward the middle of the beam, further weakening the structure. - Cover Separation Due to Flexural Crack
In this mode, bond cracks occur across the entire length of the NSM FRP reinforcement, particularly in areas experiencing high bending moments. These cracks are often located in the region of maximum moment and shear, causing the concrete to lose its bond with the FRP bars. - Localized Cover Separation
A localized separation of concrete can occur in the region where bond cracks, shear cracks, and flexural cracks intersect. This often results in the detachment of a trapezoidal or triangular section of concrete. Such separation is typically seen before other forms of failure occur and can be an indication of structural weakening. - Beam Edge Cover Splitting
In cases where the FRP bars are placed near the edge of the concrete beam, the concrete cover may separate along the beam’s edge. This failure is usually associated with poor positioning of the FRP reinforcement and can lead to significant structural damage if left unchecked. - Failure at Concrete-Epoxy Interface
If the groove cut for the FRP bar is not smooth or level, failure may occur at the interface between the concrete and the epoxy adhesive. This is particularly dangerous when the FRP bars are poorly embedded or when the adhesive bond is weak. Hassan and Rizkalla (2003) highlighted that strips with inadequate embedded length are prone to failure, often starting at the ends of the FRP strips.
IV. Conclusion
The NSM FRP technique is an effective method for strengthening reinforced concrete beams, offering significant improvements in flexural strength without adding excessive weight or size. However, like any strengthening method, it is not immune to failure. Understanding the different failure modes—ranging from composite failure (rupture of FRP bars and crushing of concrete) to premature failure (debonding, cracking, and separation)—is critical for ensuring the longevity and effectiveness of the rehabilitation process.