The Use of near surface Mounted Fiber Reinforced Polymer bars for strengthening of reinforced concrete beam:
There are several factors that might decrease ultimate load capacity of concrete structures such as steel corrosion in an aggressive environment, design calculation errors, and poor mix design. Demolition and reconstructing deteriorated structures are uneconomical.
Therefore, it is considerably important to strengthen and improve the peak capacity or regain strength of deteriorated structures. There are several methods and techniques which has been used to improve reinforced concrete elements such as externally bonded plates in which steel plate were employed and then FRP layers.
Near surface mounted fiber reinforced polymer technique is one of the several methods which are employed to improve reinforced concrete elements.
Near Surface Mounted FRP Technique Procedures:
1. Cutting grooves on the cover of the beam along tension side.
2. Use brushes and pressurized air to remove debris in the grooves.
3. Epoxy past or grout cement is inserted in to 2/3 of the groove as binder.
4. FRP bar is pushed into the binder materials until it encircled by the binder agent.
5. Finally, the remaining part of the groove is filled with the epoxy paste.
In this process steel reinforcements should be prevented from cutting otherwise the element will lost all capacity. So, reinforced concrete element should minimum 20 mm cover in order to be strengthened by this method.
Figure 1 Shows FRP Bars Adjustment in The Grooves.
Properties of FRP bars:
There are three types of FRP bars namely, Carbon, Aramid, Glass FRP bars. As shown in figure 2 tensile strength of FRP bars are more than steel reinforcement and lacking yield point is a clear distinction of FRP bars.
FRP bars are corrosion resistant which is a substantial advantage over steel. Due to high tensile strength FRP bars can resist larger loads.
Figure 2: Different Types of FRP and Steel Reinforcement Properties
Advantages of Near Surface Mounted Fiber Reinforced Polymers Technique:
- This method can be handled easily in the field due to the weight of FRP bars.
- It is not prone to corrosion in hostile environment.
- FRP bars are protected from detrimental mechanical impact because the bars are covered by concrete and binder agent.
- After strengthening is completed the structural appearance of the elements are almost unchanged.
Disadvantages of NSM FRP Technique:
- There are no approved codes to use as guidance.
- FRP bars are expensive compared with steels.
- FRP bars have low transverse strength and it can be damaged by fire or vandalism.
Design of Near Surface Mounted Fiber Reinforced Polymers Technique:
There is no specific code to employ for designing that is why numbers of models have developed to estimate ultimate capacity of strengthened elements and expects their failure. It is very crucial to conduct inspection and collect data about deteriorated beams such as:
- Main reinforcement ratio, if the element is over reinforced the strengthening would not be much effective because when applied load is increase the beam can fail in compression which is not acceptable according to ACI Code. Reinforcement ratio can be obtained from previous design calculations.
- Concrete cover thickness should be found out in order to check the feasibility of using this technique. Concrete cover can be obtained from design calculation if available otherwise it can be determined by magnetic rebar locator (cover meter) test or by breaking small part of the cover to reach the main reinforcement.
Factors affecting design of NSM Fiber Reinforced Polymer Concrete:
There are several parameters which affect the design:
- Spacing between grooves.
- Concrete thickness between FRP and steel bars.
- Concrete compressive strength.
- Axial rigidity of fiber reinforced polymer bars
- NSM FRP bars to groove perimeter.
- FRP to steel reinforcement ratio.
- The effect of the distance between beam edge and grooves.
Types of failure in strengthened reinforced concrete beams
There are two major failures:
1. Rupture of FRP bars followed by crushing of concrete: this types of failure occurs when the bond between concrete and FRP is strong.
Fig: Rupture of FRP bars in RCC Beams
2. Premature failure: this type of failure occurs when the bond between FRP bars and concrete is lost and it can be in different forms and mechanisms such as splitting of concrete cover, failure at the interface between concrete and epoxy and interface failure between bars and epoxy.
Figure-3: Concrete Cover Delamination in RCC Beam