Re: Kudos 2 Precracked Mod
Genciana Haggins
Carbon fiber reinforced polymers (CFRP) were widely used in strengthening of reinforced concrete members in the last few years. Experimental and theoretical investigations were carried out to find the behavior of reinforced concrete beams strengthened in shear by CFRP strips. Six beams measured of 200x300x2000mm were investigated. The variables investigated in this work are orientation (vertical and inclined) and the spacing between CFRP strips.
It was found that the strengthening by CFRP strips increased the crack, yield and ultimate load by 10%, 71% and 77% respectively on average. Inclined CFRP strips show a better performance than vertical CFRP strips with same distances and increase the yield and ultimate load by 11% and 13% respectively on average. By covering all faces of specimen with CFRP strips, the yield and ultimate load increased by 82% and 95% respectively. Using the CFRP strips changed the failure model from shear to flexural by increasing the shear strength, so the ductility was increased by 198% on average. CFRP increased the strain in compression face of concrete and the value was greater than (0.003). For all strengthened specimens, there was no effect on CFRP strips.
The maintenance, renovation and improvement of structural members, are probably the most critical problems in civil engineering applications. Additionally, a large number of structures built in the past using the older design codes in different parts of the world are structurally risky allowing the new design codes. Since replacement of such deficient elements of structures suffers a huge amount of community money and time, strengthening has developed the satisfactory way of improving their load carrying capacity and extending their service lives. Infrastructure deterioration caused by early drop of buildings and structure has led to the investigation of some processes for renovating or strengthening purposes.
The external bonding of high-strength carbon fiber-reinforced polymers (CFRP) to structural concrete members has extensively grown acceptance in the recent years, mainly in rehabilitation works and newly built construction. Comprehensive experimental investigations led in the past have shown that this strengthening method has several advantages over the traditional ones, particularly due to its corrosion resistance, high stiffness-to-weight ratio, improved durability and flexibility in its use over steel plates. The use of (CFRP) materials in civil infrastructure for the renovation and strengthening of reinforced concrete constructions and also for new construction has become common practice.
Historically, concrete members have been repaired by post tensioning or jacketing with new concrete in conjunction with a surface adhesive [1]. Since mid 1960s, epoxy-bonded steel plates have been used in Europe and South Africa to retrofit flexural members [2]. However, steel plates have a toughness problem exclusive to this application, because corrosion may happen along the adhesive interface. The technique of strengthening reinforced concrete buildings by externally bonded FRP laminates was started in 1980s and has since attracted many researchers around the world. It was investigated in the USA [3-8], Switzerland [9], Greece, Canada [10, 11], Japan and several other European countries.
Norris et al. (1997) [12], presented the results of an experimental and analytical study of the behavior of damaged or understrength concrete beams retrofitted with thin carbon fiber reinforced plastic (CFRP) sheets. The CFRP sheets are epoxy bonded to the tension face and web of concrete beams to enhance their flexural and shear strengths. The effect of CFRP sheets on strength and stiffness of the beams is considered for various orientations of the fibers with respect to the axis of the beam. Nineteen beams were fabricated, loaded beyond concrete cracking strength, and retrofitted with three different CFRP systems. The beams were subsequently loaded to failure. Different modes of failure and gain in the ultimate strength were observed, depending on the orientation of the fibers. Results show that the CFRP sheets can provide increase in strength and stuffiness to existing concrete beams when bonded to the web and tension face.
Khalif et al. (1998) [13], used a slight modification (Triantafillou's equation 1998) to define shear failure combined with FRP rupture. Meanwhile, the bond model of Maeda et al. 1997 [14], was used to describe shear failure combined with CFRP debonding. The two models were then presented in the AC1318-14 [15] shear design format.
In 2004, Santhakumar and Chandrasekaran [16] carried out a study on the unretrofitted RC beam designated as control beam and RC beams retrofitted using (CFRP) composites with 45o and 90o fiber orientations. The effect of retrofitting on uncracked and precracked beams was studied too. The finite elements adopted by ANSYS [17] were used in this study. The study concluded that numerical modeling helps to track the crack formation and propagation especially in case of retrofitted beams in which the crack patterns cannot be seen by the experimental study due to wrapping of CFRP composites. This numerical study can be used to predict the behavior of retrofitted reinforced concrete beams more precisely by assigning appropriate material properties.
Wang Wenwei and Li Guo in (2005) [18], investigated six reinforced concrete beams strengthened in flexure using (CFRP) laminates subjected to different sustaining loads. The main goal of the test was to study the effects of initial load and load history on the ultimate strength of strengthened reinforced concrete beams by externally bonded CFRP laminates. The main experimental parameters included different levels of sustaining load at the time of strengthening and load history. Test results show that sustaining load levels at the time of strengthening have important influence on the ultimate strength of strengthened reinforced concrete beams.
Zhang and Hsu (2005) [19], investigated eleven RC beams without steel shear reinforcement. After the beams were kept in the curing room for 28 days, carbon-fiber strips and fabrics made by Sika Corp were applied on both sides of the beams at various orientations with respect to the axis of the beam. Results show that the CFRP system can significantly increase the serviceability, ductility, and ultimate shear strength of a concrete beam; thus, restoring beam in shear by using CFRP is a highly effective technique. An analysis and design method for shear strengthening of externally bonded CFRP has been proposed.
Abdel-Jaber et al. (2007) [20] examined experimental results obtained from an earlier study and utilized in this research to present a reasonable model for strengthening. The experiments investigated the shear behavior of reinforced concrete beams strengthened by the attachment of different configurations and quantities of CFRP using. The application of CFRP strips to the shear spans of the beams increased the strength between 19% and 56%.
Nicolae et al. (2008) [21], presented the use of FRP composite materials for new structural members (internal reinforcements) and strengthening of existing members (externally bonded reinforcements). The advantages and disadvantages as well as the problems and constraints associated with both issues are discussed.
Mofidi and Chaallal (2011) [22], studied the shear strengthening of RC beams using externally bonded by FRP. This study reveals that the effect of transverse steel on the shear contribution of FRP is important and yet is not considered by any existing codes or guidelines. Therefore, a new design method is proposed to consider the effect of transverse steel in addition to other influencing factors on the shear contribution of FRP. Separate design equations are proposed for U-wrap and side-bonded FRP configurations. A comparison with current design guidelines has shown that the proposed model achieves a better correlation with experimental results than current design guidelines.
Alferjani et al. (2013) [23], presented the reviews of 10 articles on CFRP strengthened reinforced concrete beams. This study was an attempt to address an important practical issue that has encountered in shear strengthening of beams with CFRP laminate. Also this study proposed a simple method of applying FRP for strengthening the beam with CFRP.
Vuggumudi (2013) [24] investigated the shear performance and failure modes of RC T-beams strengthened with externally bonded GFRP sheets. In order to achieve these objectives, an extensive experimental program consisting of testing eleven, full scale RC beams was carried out. The variables investigated in this study included steel stirrups, shear span-to-depth ratio and GFRP amount.
The experimental results indicated that the contribution of externally bonded GFRP to the shear capacity is significant and depends on the variable investigated. The failures of strengthened beams are initiated with the debonding failure of FRP sheets followed by brittle shear failure. However, the shear capacity of these beams has increased as compared to the control beam which can be further improved if the debonding failure is prevented.
Ibrahim et al. (2015) [25] investigated the effective and practical approaches for strengthening load bearing walls with openings to resist extreme loads. The researcher presented the results of investigation on structural behavior of the load bearing walls of interlocking bricks system. Six specimens were prepared with the same height (1.2m) and width (0.8m), one was made from brick with thickness 0.24m and the other was made from concrete block with thickness 0.2m. Two walls were constructed with rectangular opening, and two walls were opening and strengthened with carbon fiber reinforced polymer (CFRP) strips. The test results clearly demonstrate the efficiency of using CFRP strips as a repair and strengthening technique for unreinforced load-bearing walls to increase the stiffness and ultimate bearing load.
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