New small-diameter CFRP material for flexural strengthening of steel bridge girders
Introduction
Due to the appealing benefits of Fiber Reinforced Polymer (FRP) materials, their use for strengthening of concrete structures and bridges has gained wide acceptance worldwide and became a common practice. This is mainly attributed to availability of FRP materials with higher elastic modulus relative to concrete and the extensive research conducted in the field during the last two decades which led to the development of the design guidelines and several international codes [1].
The use of FRP for steel structures has not been very successful in the past due to the low elastic modulus of FRP relative to steel. Recent production of Carbon FRP material with a similar or higher modulus than the elastic modulus of steel offers a promising alternative for flexural strengthening of steel structures and bridges. Most of bridge design codes require designing for higher vehicular live loads in comparison to that used in the initial design of existing bridges. Accordingly, existing bridges, even without any signs of distress, may require flexural strengthening to meet the current design standards.
Currently, flexural strengthening systems for steel structures consist of welding or bolting steel plates and/or bars as reinforcement for steel members. However, heavy weight of steel plates, high cost of construction in addition to being receptive to corrosion are serious disadvantage of this strengthening method. These problems encouraged researches to explore other alternatives [2]. Several researches explored the use of Fiber Reinforced Polymer (FRP) materials due to their well-recognized advantages including high strength to weight ratio and resistance to corrosion. The use of FRP for strengthening concrete structures has become well established technique and documented in several national and international codes. However, few researches explored the use of FRP materials for strengthening steel structures [3].
Recently, numbers of researches have explored the use of CFRP materials for flexural strengthening of steel members and structures. Majority of the research are focused on rehabilitation of naturally or artificially deteriorated steel beams. Miller et al. (2001) [4] examined the strengthening of steel beams using low elastic modulus CFRP. Shaat & Fam (2008) [5], Kim & Brunell (2011) [6], and Galal et al. (2012) [7] presented the results of experimental and analytical studies on the repair of artificially damaged steel beams using adhesively bonded CFRP sheets. Deng & Lee (2007) [8] and Linghoff et al. (2010) [9] investigated the effectiveness of CFRP strengthening systems, by retrofitting undamaged steel beams. Furthermore, flexural strengthening of steel-concrete girders using high modulus CFRP laminates, investigated by Schnerch et al. (2007) [10], showed clearly that the governing mode of failure was de-bonding of the CFRP laminates. The low bonding capacity of the CFRP laminate was attributed to the fact that the laminates were bonded to the substrate from one face only. In a research program conducted by Al-Saidy et al. (2007) [11], three steel-concrete composite beams were strengthened using two elastic modulus CFRP materials and test results indicated significant increase in the ultimate flexural capacity. Tavakkolizadeh & Saadatmanesh (2003) [12], El-Hacha & Ragab (2006) [13], and Schnerch & Rizkalla (2008) [14] studied the behavior of steel-concrete composite girders strengthened with high-modulus CFRP materials. Research findings indicated that the use of high-modulus CFRP significantly increased both the ultimate flexural capacity and the stiffness of the steel-concrete girders.
The recent development of small-diameter CFRP strands, shown in Fig. 1, introduced a promising alternative for strengthening steel structures (Hidekuma et al. (2011) [15], Hidekuma et al. (2012) [16], Jiao et al. (2014) [17]). The CFRP strands are approximately 1.0 mm (1/25 in.) in diameter and are stitched together leaving a gap between the strands. The gap between the strands allows the adhesive material to penetrate and cover the entire perimeter of each strand, resulting in a better bond mechanism in comparison to the use of laminates which are bonded to the substrate from one face only. The new CFRP strands are produced with a wide range of elastic modulus equal or higher than that of steel.
This paper summarizes an experimental program which investigates the use of these small-diameter CFRP strands for flexural strengthening of steel structures and bridges. The program included scaled steel-concrete composite beams strengthened at the tension flange of the steel beams to increase the flexural capacity. The parameters considered in the experimental program were the type of CFRP, reinforcing ratio and type of loading. The two types of CFRP materials that were examined for strengthening are low-modulus (LM) and intermediate-modulus (IM) CFRP material. The three types of loading investigated were static, cyclic and fatigue loadings. Test results of static loading were used to determine the most effective strengthening system and the selected system was tested under fatigue and cyclic loading.
Section snippets
Material properties
The mechanical and bond properties of the two types of CFRP strands used for flexural strengthening were determined using two test methods. The first method was used to determine the tensile strength and the elastic modulus of the two types of CFRP strands in accordance to ASTM D3039 [18] and Test Method L2 of ACI 440.3R-04 [1]. Test results are summarized in Fig. 2 for the two CFRP materials used in the experimental program.
The second method of testing was used to evaluate the development
Flexural test specimens
A total of eight scaled steel-concrete composite beams were tested in this study. Six beams were tested under static loading, one beam was tested under incremental cyclic loading and one beam was tested under fatigue loading. All of the test beams were 3350 mm (11 ft) long and were tested in a simply supported configuration as shown in Fig. 4. The loads were applied using four-point bending configuration with a clear span of 3050 mm (10 ft). The applied two point loads were 610 mm (2 ft) apart at mid
Behavior under static loading
The results of the six beams tested under static loading conditions are given in Table 2. The measured loads at failure and percentage increases of the load carrying capacity in comparison to the control beam are given. The measured loads at the initiation of steel yielding and their percentage increases to the control beam are also given. The pre-yielding and post-yielding stiffness, measured by the slope of the load-deflection response, and their percentage increases are presented. The
Summary and conclusion
This paper presents a new strengthening system for steel structures and bridges. The proposed system introduces the use of new small-diameter carbon Fiber Reinforced Polymer, CFRP, strands which are believed to have quite an advantage in comparison to current strengthening scheme using CFRP sheets or laminates. The research findings are based on an experimental program conducted to investigate the effectiveness of the new system. The strengthening program comprised of testing eight scaled
Acknowledgments
The authors would like to acknowledge Nippon Steel & Sumikin Material Co., Ltd, Composites Company, Japan and the National Science Foundation (NSF) Center of Integration of Composites into Infrastructure (CICI), NCSU for their financial support. Thanks are also due to the staff of the Constructed Facilities Laboratory (CFL), NCSU for their help throughout the experimental program.
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