Reinforced concrete structures can experience a decrease in their ultimate load capacity due to various factors. Steel corrosion in an aggressive environment, design calculation errors, and poor mix design are some of the significant contributors to this deterioration. Demolishing and reconstructing such structures can be a costly and impractical solution. Therefore, it is essential to find ways to strengthen and improve the peak capacity or regain the strength of deteriorated structures.
Several methods and techniques have been used to improve reinforced concrete elements. One of these methods is the use of externally bonded plates, where steel plates and then FRP layers are employed. However, another technique that has proven to be effective is the near surface mounted fiber reinforced polymer technique. This technique involves the use of fiber reinforced polymer bars that are mounted close to the surface of the concrete to improve its strength.
By using near surface mounted fiber reinforced polymer bars, it is possible to enhance the capacity of reinforced concrete structures without the need for extensive and costly demolition and reconstruction. This method can effectively address the problem of steel corrosion in an aggressive environment, design calculation errors, and poor mix design. Therefore, it is a practical and economical solution for improving the strength and capacity of deteriorated structures.
Near Surface Mounted FRP Technique Procedures:
To strengthen reinforced concrete elements, a process involves cutting grooves on the cover of the beam along the tension side. The grooves are then cleaned using brushes and pressurized air to remove any debris. After that, a binder made of epoxy paste or grout cement is inserted into two-thirds of the groove.
The next step involves pushing an FRP bar into the binder materials until it is fully encircled by the binder agent. Finally, the remaining part of the groove is filled with the epoxy paste. It’s essential to prevent steel reinforcements from being cut during the process as it can cause the element to lose all capacity. To avoid this, reinforced concrete elements should have a minimum 20 mm cover to be strengthened by this method.
Figure 1 Shows FRP Bars Adjustment in The Grooves.
Properties of FRP bars:
FRP bars are classified into three types: Carbon, Aramid, and Glass. These bars have a higher tensile strength than steel reinforcement, as illustrated in Figure 2. One clear distinction of FRP bars is their lack of yield point. Additionally, FRP bars are corrosion resistant, providing a substantial advantage over steel. Due to their high tensile strength, FRP bars are capable of withstanding larger loads.
Figure 2: Different Types of FRP and Steel Reinforcement Properties
Advantages of Near Surface Mounted Fiber Reinforced Polymers Technique:
Fiber reinforced polymer (FRP) bars are a convenient solution in the field due to their lightweight nature. Unlike traditional steel reinforcement, FRP bars do not add significant weight to the structure, making them easy to handle during installation. Additionally, FRP bars are highly resistant to corrosion in harsh environments, which is a common problem with steel reinforcement.
Another advantage of FRP bars is that they are protected from mechanical damage due to their placement within concrete and binder agent. This protective layer helps ensure that the bars remain intact and functional, even under heavy loads or impact.
Finally, when FRP bars are used to strengthen existing structures, the overall appearance of the element remains largely unchanged. This is because the bars are typically installed within the existing concrete, and do not significantly alter the appearance or shape of the structure. Overall, the use of FRP bars is a reliable and effective method for reinforcing structures in a variety of settings.
Disadvantages of NSM FRP Technique:
It appears that there is a lack of approved codes or standards that can be used as a guide for a particular task or activity. This could potentially lead to confusion or uncertainty regarding the best course of action to take.
Additionally, it seems that there is a cost disparity between FRP bars and traditional steel bars, with the former being more expensive. This may be a consideration when deciding which materials to use for a project or construction.
Furthermore, FRP bars may have lower transverse strength compared to other types of bars, which could impact their suitability for certain applications. Additionally, they may be more vulnerable to damage from events such as fire or vandalism. It is important to take these factors into account when evaluating the feasibility and effectiveness of using FRP bars for a given purpose.
Design of Near Surface Mounted Fiber Reinforced Polymers Technique:
Designing a structure requires a careful approach, and several models have been developed to estimate the ultimate capacity of strengthened elements and anticipate their failure. To achieve successful results, it is essential to conduct inspections and collect relevant data about deteriorated beams. This data includes the main reinforcement ratio, which determines the effectiveness of the strengthening process. If an element is over reinforced, strengthening may not be effective as it could fail in compression when subjected to increased loads. This is not acceptable under ACI Code. Obtaining the reinforcement ratio is possible through previous design calculations.
Another critical factor to consider is the thickness of the concrete cover. It is important to determine the feasibility of using strengthening techniques by identifying the concrete cover’s thickness. This information can be obtained from design calculations if available. If not, magnetic rebar locator (cover meter) tests can determine the concrete cover thickness. Alternatively, breaking a small part of the cover to access the main reinforcement can also determine the thickness.
In summary, the success of structural strengthening depends on several factors, including the reinforcement ratio and the thickness of the concrete cover. Collecting accurate data through inspections and testing is crucial to ensure effective strengthening techniques are applied.
Factors affecting design of NSM Fiber Reinforced Polymer Concrete:
The design of a structure can be influenced by various parameters. One such parameter is the spacing between grooves, which can impact the overall strength and stability of the structure. Another important factor to consider is the thickness of the concrete between the fiber reinforced polymer (FRP) and steel bars. This thickness can have an impact on the overall performance of the structure, particularly its resistance to compression.
The compressive strength of the concrete used in construction is also an important consideration. This strength can determine how well the structure is able to withstand loads and maintain its shape under pressure. Another factor to consider is the axial rigidity of the FRP bars, which can affect their ability to transfer loads effectively.
The ratio of NSM FRP bars to groove perimeter is another important parameter to consider. This ratio can impact the overall strength and stability of the structure, particularly in terms of its ability to resist bending and torsion. Additionally, the ratio of FRP to steel reinforcement can have an impact on the overall strength and durability of the structure.
Finally, the distance between the beam edge and grooves can also play a role in the design of a structure. This distance can affect the overall strength and stability of the structure, particularly in terms of its ability to resist shear forces. As such, it is important to consider all of these parameters when designing a structure to ensure that it is able to perform optimally under a range of conditions.
Types of failure in strengthened reinforced concrete beams
One common type of failure that can occur in structures involves the rupture of FRP bars followed by the crushing of concrete. This type of failure is typically seen in situations where the bond between the concrete and FRP is particularly strong. In these cases, the force placed on the FRP bars can become too great, causing them to rupture. When this happens, the concrete surrounding the bars may also begin to fail, resulting in the crushing of the material. This can ultimately lead to significant damage to the overall structure and may require extensive repairs to address the issue.
Fig: Rupture of FRP bars in RCC Beams
Premature failure is a common type of failure that can occur in reinforced concrete structures where fiber reinforced polymer (FRP) bars are used. This type of failure is typically characterized by the loss of bond between the FRP bars and the surrounding concrete. There are various forms and mechanisms that can contribute to this type of failure, including splitting of the concrete cover, failure at the interface between the concrete and epoxy, and interface failure between the bars and epoxy. Each of these mechanisms can lead to premature failure and compromise the integrity of the structure. Therefore, it is important to carefully consider the design, installation, and maintenance of FRP-reinforced concrete structures to prevent premature failure and ensure their long-term durability and safety.
Figure-3: Concrete Cover Delamination in RCC Beam