Shear failure of reinforced concrete beams occurs abruptly without pre-warning indicators and could lead to calamitous results. Fiber reinforced polymers laminates have emerged as substitutes for the traditional materials and techniques adopted for strengthening of concrete structures. Though expensive, the fiber reinforced polymer laminate technique is still favorable for use in reinforced and pre-stressed concrete strengthening due to its numerous advantages. Most of the current research work has concentrated on the shear capacity and response of simply supported reinforced concrete beams that are strengthened in shear with fiber reinforced polymer sheets. As of now, limited research addresses response of continuous reinforced concrete beams strengthened in shear with fiber reinforced polymer laminate.
In a recent paper published in Composite Structures, researchers led by professor Ezzeldin Yazeed Sayed-Ahmed at The American University in Cairo in Egypt used carbon fiber reinforced polymer for shear strengthening of reinforced concrete beams in zones of combined shear and normal stresses. They also investigated the applicability of the bond reduction factor in regions of combined shear and normal stresses.
In their experimentation, eight continuous reinforced concrete beams of rectangular cross sections and definite spans were selected. The beams had flexural reinforcement of 16 mm bars and 8 mm shear stirrup reinforcements. Near the middle support the stirrup spacing was increased to 200 mm from 100 mm in order to ensure failure occurs at the regions of combined shear and normal stress.
The beams were then grouped into four pairs based on the carbon fiber reinforced polymer sheets configuration. One pair was reserved as the control experiment. The other beams were wrapped with 500 mm wide carbon fiber reinforced concrete polymer sheets at different points and depths. The beams were then loaded monolithically to failure by applying the load symmetrically 550 mm away from the middle supports. Bending moments and shear forces were calculated. Strain gauges were placed near the middle support stirrups and to the carbon fiber reinforced polymers near the same locations. Deflections at the mid-spans were also monitored.
From the empirical analysis conducted the authors of this paper noted that in all the tested beams shear failure occurred with diagonal cracks extending from the point of load application and propagated toward the middle support. For the control beams a sudden increase in strain in the middle span stirrups was observed and at a lower load than all the other groups. In the other beam groups, the first diagonal shear crack appeared at higher loads where sudden increase in the carbon fiber reinforced polymer was also observed.
The researchers realized that the bonded laminate increased the failure load depending on the laminate point of placement on the beam. Continuous wrapping on the full depth of the beam yielded the greatest contribution thereby con-curing with the ACI 440.2-08 code of practice.
From the test, it is clear that the carbon fiber reinforced polymer laminate has significant contribution to the shear strength for beams wrapped along the entire length. It is also noted that post cracking strength decreases as the amount of carbon fiber reinforced polymer laminate increases. This study confirms the bond reduction factor adopted by the ACI 440.2-08 code, which at first was derived for zones of high shear stress only, is now applicable to regions of high shear and normal stresses.
Noor Akroush, Tariq Almahallawi, Mohamed Seif, Ezzeldin Yazeed Sayed-Ahmed. Carbon Fiber Reinforced Polymer shear strengthening of reinforced concrete beams in zones of combined shear and normal stresses. Composite Structures volume 162 (2017) pages 47–53.
Construction Engineering Dept., The American University in Cairo, AUC Avenue, P.O. Box 74, New Cairo 11835, Egypt.Go To Composite Structures