The near surface mounted strengthening of reinforced concrete beams improves its flexural strength. Failure modes of such beams is discussed.
Reinforced concrete beams are likely to deteriorate during their life service as a consequence of several factors for instance applied loads, environmental effects, and impact. In this case, it would be required to rehabilitate the reinforced concrete element so as to regain its load carrying capacity.
The previous cases are normally prevalent in existing beams, but there are cases in which newly constructed reinforced concrete member need flexural repairing for instance due to errors in the design and construction the beam will not be able to support the design load and hence it will be necessary to carry out rehabilitation process to upgrade the strength of the element.
The use of near surface mounted fiber reinforced polymer technique is novel technique that effectively improve the strength of reinforced concrete beams in flexure. The process includes cutting grooves into the beam underside and placing FRP bars into the grooves and bonding to concrete using proper adhesive agent.
Commonly, the rehabilitated reinforced concrete beams would suffer from failure when its ultimate load carrying capacity is realized. The modes of failure of reinforced concrete beam strengthened with NSM FRP technique, which have been described in the literatures are explained in this article.
Modes of Failure of Reinforced Concrete Beam Strengthened with NSM FRP Technique
There are two main failure modes of strengthened beams, when the composite action between materials are good and in this case the element is fail by rupture of FRP bars or strips and crushing of concrete.
The second type is premature failure which occurs when the materials loss their composite action and it can be in different mechanisms which are explained below.
Interfacial Debonding between Epoxy and Bar
This type of failure occurs between FRP rod and groove filler interface and it is dangerous in the case of sandblasted bar if the opening size is adequate to prevent splitting failure. It is claimed by De Lorenzis and Teng (2007) that this kind of failure is advanced as a result of flexural cracks crossing cover of epoxy.
Splitting concrete cover
This mode of failure occurs due to debonding cracks at the bottom side of the beam and after these inclined cracks propagates and reaches the edge of the element after that it might direct up on the lateral side of the beam whilst preserving 45o inclination and within the thickness of the cover. Then, it develops along the beam at the level of steel reinforcement.
Finally, different shapes of debonding are going to happen depend on the following crack patterns:
End Cover and Bar Splitting
When the extension of FRP bar is ended at a fairly long distance from the support, the failure begins at the part of the element where the bar is finished. After that the cracks develop to the middle of the beam.
Cover Separation due to Flexural Crack
In this type of failure the bond crack takes place virtually at the same time all over the NSM FRP reinforcement. The maximum moment and one of the shear span area is engaged in most of the times.
Localized Cover Separation
In this mode of failure a trapezoidal or triangle piece of concrete is separated due to the combination of bond cracks around the maximum moment area, shear and flexural cracks which the last two types of cracks occur before the first one.
Finally, one or some pieces of concrete will be off the element.
Beam Edge Cover Splitting
The concrete cover might be separated along the beam due to the position of the NSM FRP bar which is settled close to the edge of the element.
Failure at the Interface between the Concrete and Epoxy
When the cutting of the beam is even and level this kind of failure become dangerous and this has been noticed by (Lorenzis et al., 2004) in their investigation for spirally wounded or ribbed bar with small projection on the surface of the bars entrenched into the groove filler.
It is claimed by Hassan and Rizkalla (2003) that for the strips which possess inadequate entrenched length this failure mode starts from the end of the strips.