Effect of carbon nanotubes on strengthening of RC beams retroﬁtted with carbon ﬁber/epoxy composites Mohammad R. Irshidat a, ⁎, Mohammed H. Al-Saleh b , Hashem Almashagbeh a a Department of Civil Engineering, Jordan University of Science and Technology, Irbid, Jordan b Department of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan abstract article info Article history: Received 4 May 2015 Received in revised form 9 August 2015 Accepted 30 September 2015 Available online 3 October 2015 Keywords: CNTs RC beams Carbon ﬁber Strengthening CFRP Retroﬁtting The effect of carbon nanotubes (CNTs) in improving the strengthening efﬁciency of carbon ﬁber/epoxy compos- ites retroﬁtted reinforced concrete (RC) beams was investigated. A total of sixteen simply supported RC beams were prepared and tested under four-point loading. The incorporation of CNTs within the systems was done by modifying the epoxy resin using CNTs and/or coating the carbon ﬁber sheets with CNT enriched sizing agent. The effects of epoxy modiﬁcation with CNTs, incorporation of CNT enriched sizing agent, anchorage length, and number of retroﬁtting layers were investigated through crack patterns, failure modes, load-deﬂection curves, and scanning electron microscopy (SEM) micrographs of fractured surfaces. Experimental results showed that using CNT modiﬁed epoxy resin enhanced the ultimate load and stiffness of retroﬁtted beams. The enhancement efﬁciency highly depends on the level of dispersion of CNT, anchorage length, and number of retroﬁtting layers. SEM characterization showed that CNTs could improve the adhesion at the concrete/epoxy interface and carbon ﬁber/epoxy interface leading to improvement in the load transfer and ultimate load of the strengthened beams. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Strengthening and upgrading of structural elements such as rein- forced concrete (RC) beams are necessary to extend its service period, overcome the original design limits and to minimize the effect of con- struction or design defects. One of the most common strengthening techniques for ﬂexural RC members is the use of externally bonded ﬁber reinforced polymer (FRP) composites. This method of strengthen- ing is becoming more popular due to the outstanding properties of FRP materials, such as high strength to weight ratio, high stiffness, excellent corrosion resistance, ease of application, and minimal change in the ge- ometry. Over the last three decades, extensive research has been con- ducted on the strengthening of RC beams with carbon FRP (CFRP) composites. It was reported that retroﬁtting RC beams with CFRP sheets signiﬁcantly enhances its ﬂexural properties in terms of strength and ductility. The level of enhancement depends on several factors, namely: type, length, width, and thickness of FRP sheet, number of FRP layers, beam size, concrete cover, and the resin type [1–30]. Despite the men- tioned advantages, strengthening RC beams using FRP composites has some drawbacks regarding the performance of the transition layer be- tween the FRP sheets and concrete [5,15,18,19,21,31,32]. These disad- vantages may include the debonding of FRP sheets and incompatibility of epoxy resins and concrete. These weaknesses make the debonding of FRP sheets the most common failure mode of strengthened beams, which makes the total utilization of the tensile strength of the FRP ma- terials impossible [19,21]. One possible solution to the above problems would be modifying the resin properties for better impregnation of the ﬁbers and binding between the FRP and concrete. Carbon nanotubes (CNTs) may be considered as one of the most promising nanoﬁller that can act as matrix modiﬁers owning to their unique properties. A host of studies have investigated the inﬂuence of CNT addition on the proper- ties of epoxy resin. It was reported that modifying epoxy with CNTs im- proves its tensile strength [33,34], ﬂexural strength [35], toughness [34, 35], fracture strain [33], and young modulus [33,35,36]. However, the enhancement efﬁciency depends on many factors such as CNT disper- sion, alignment, and interfacial adhesion between CNTs and the poly- mer matrix [37–39]. Recently, using CNT modiﬁed epoxy resin as polymer matrix to produce hybrid FRP composites has attracted signif- icant attention. Most of the studies reported on the inﬂuence of using CNT modiﬁed epoxy resin on the mechanical properties of carbon ﬁber/epoxy nanocomposites. The results of these studies indicated that incorporation of CNTs into epoxy resin and use it along with carbon ﬁber reinforcements caused an enhancement in the nanocomposite ﬂexural strength and modulus [40], fracture toughness [41–43], interfa- cial shear strength [44–47], and interfacial adhesion between epoxy matrix and carbon ﬁber [45,46,48]. It is evident that good understanding of the behavior of carbon ﬁber/ epoxy with CNTs has been established. However, very limited studies are focused on using these nanocomposites in strengthening RC struc- tures. For example, Rousakis et al. [49] studied the effect of using CNT Materials and Design 89 (2016) 225–234 ⁎ Corresponding author at: Civil Engineering Department, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan. E-mail address: mrirshidat@just.edu.jo (M.R. Irshidat). http://dx.doi.org/10.1016/j.matdes.2015.09.166 0264-1275/© 2015 Elsevier Ltd. All rights reserved. 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