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    Assessment of self-sensing capability of Engineered Cementitious Composites within the elastic and plastic ranges of cyclic flexural loading
    (Elsevier Sci Ltd, 2017) Al-Dahawi, Ali; Yildirim, Gurkan; Ozturk, Oguzhan; Sahmaran, Mustafa
    Engineered Cementitious Composites (ECC) are emerging construction materials with proven mechanical and durability characteristics. These outstanding properties make the material an attractive choice for different infrastructure types. But the growing use of ECCs brings about the need to monitor the health of the structures that employ them. This study therefore focused on the self-sensing capability of ECC with different carbon-based materials (multi-walled carbon nanotubes [CNT], graphene nanoplatelets [GNP], carbon black [CB] and carbon fibers [CM when subjected to repetitive loading and unloading cycles within the elastic and plastic ranges. Tests were conducted on beam specimens loaded and unloaded under four-point bending loading. Within the elastic range, 30% of the ultimate flexural strength was decided for application on the tested specimens. For the plastic range, prismatic specimens prepared for cyclic flexural loading at high levels were loaded up to 70% of their ultimate flexural strength. Experimental findings showed that for all proposed ECC mixtures, self-sensing of imposed damage (i.e. loading) was successful for both elastic and plastic ranges, while the self-sensing of load removal (i.e. unloading) in the elastic range was not achieved as successfully. This was most probably due to very small imposed damage and continuously increasing intrinsic electrical resistivity of individual carbon based materials under loading, which cannot be discharged upon unloading. Compared to other carbon-based materials, CF utilization during ECC production was the most effective method for self sensing of cyclic loading and unloading in the elastic and plastic ranges. Improving the proven superior mechanical and durability properties of ECC materials with self-sensing characteristic will multiply the benefits for truly sustainable infrastructures. (C) 2017 Elsevier Ltd. All rights reserved.
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    Electrical percolation threshold of cementitious composites possessing self-sensing functionality incorporating different carbon-based materials
    (Iop Publishing Ltd, 2016) Al-Dahawi, Ali; Sarwary, Mohammad Haroon; Ozturk, Oguzhan; Yildirim, Gurkan; Akin, Arife; Sahmaran, Mustafa; Lachemi, Mohamed
    An experimental study was carried out to understand the electrical percolation thresholds of different carbon-based nano- and micro-scale materials in cementitious composites. Multi-walled carbon nanotubes (CNTs), graphene nanoplatelets (GNPs) and carbon black (CB) were selected as the nano-scale materials, while 6 and 12 mm long carbon fibers (CF6 and CF12) were used as the micro-scale carbon-based materials. After determining the percolation thresholds of different electrical conductive materials, mechanical properties and piezoresistive properties of specimens produced with the abovementioned conductive materials at percolation threshold were investigated under uniaxial compressive loading. Results demonstrate that regardless of initial curing age, the percolation thresholds of CNT, GNP, CB and CFs in ECC mortar specimens were around 0.55%, 2.00%, 2.00% and 1.00%, respectively. Including different carbon-based conductive materials did not harm compressive strength results; on the contrary, it improved overall values. All cementitious composites produced with carbon-based materials, with the exception of the control mixtures, exhibited piezoresistive behavior under compression, which is crucial for sensing capability. It is believed that incorporating the sensing attribute into cementitious composites will enhance benefits for sustainable civil infrastructures.
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    Piezoresistive behavior of CF- and CNT-based reinforced concrete beams subjected to static flexural loading: Shear failure investigation
    (Elsevier Sci Ltd, 2018) Yildirim, Gurkan; Sarwary, Mohammad Haroon; Al-Dahawi, Ali; Ozturk, Oguzhan; Anil, Ozgur; Sahmaran, Mustafa
    Self-sensing property of concrete is mostly assessed using small specimens without reinforcement, which may be misleading for real-time structures. To better simulate the actual field conditions, this study examined self-sensing of damage in large-scale reinforced concrete beams tested under four-point bending. During flexural testing, special attention was paid to the self-sensing capability of shear failure, since this type of failure occurs suddenly and catastrophically. Inadequate shear reinforcements were used to increase shear failure possibility of 100 x 150 x 1000 mm(3) (width x height x length) reinforced large-scale beams and beams were produced with a high shear span (a= 350 mm) to effective depth (d = 125 mm) ratio of 2.8. To increase the electrical conductivity of large-scale beams, chopped carbon fibers (CF) and multi-walled carbon nano tubes (CNT) were used. Instantaneous self-sensing recordings were made using brass electrodes embedded in different shear spans of beams. In addition to conducting self-sensing evaluations, researchers also investigated the effects of CF and CNT particles on the mechanical properties/structural behavior of large-scale beam specimens with the proposed reinforcement configuration. Results showed that compared to CNT, CF usage significantly improved the load carrying capacity and ductility, resulting in bending mode of failure even with indaquate shear reinforcement. Shear damage was successfully self-sensed in all tested beams, although all CF-based specimens started self-sensing from the beginning of loading with significantly higher changes in electrical resistivity results, unlike CNT-based specimens. Conductive network of CF-based specimens seemed to be disturbed more easily at high load levels. CF usage seems like a better option compared to CNT given its lower cost and easier mixability. (C) 2018 Elsevier Ltd. All rights reserved.