CFRP-ECC hybrid for strengthening of the concrete structures Chao Wu a,b , Victor C. Li b,⇑ a School of Transportation Science and Engineering, Beihang University, 37 Xueyuan Road, Beijing 100191, China b Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI 48109, USA article info Article history: Received 25 November 2016 Revised 1 May 2017 Accepted 12 July 2017 Available online 13 July 2017 Keywords: Carbon fiber reinforced polymer (CFRP) Engineered cementitious composite (ECC) Bond Strengthening Concrete Beam abstract The strengthening of concrete structures using FRP composite and polymer adhesive has been challenged considering structural fire. This technology is limited to situations with low fire hazards. Though the cementitious adhesive showed improved fire resistance than the polymer counterpart, the former faces challenges of brittleness and high temperature spalling. This paper proposes a CFRP-ECC hybrid system for strengthening of the concrete structures. This hybrid consists of CFRP composite embedded in the ECC matrix. CFRP functions as the main load-carrying element, while ECC acts as the adhesive layer for protecting the CFRP and transferring the load between the concrete structure and CFRP. ECC is ductile in tension and has been shown to have improved fire performance without spalling over normal cemen- titious materials. In this paper, ECC was developed with a tensile strain capacity of 3%. Then direct pull- out tests were conducted to quantify the interface behavior between CFRP and ECC, and to obtain the effective bond length of CFRP. Finally a concrete beam with the CFRP-ECC hybrid strengthening was tested under four-point bending. The experimental results showed that the effective bond length of CFRP in ECC was longer than 170 mm. When the CFRP-ECC hybrid was used for strengthening of the con- crete beam, premature debonding occurred at the interface between CFRP-ECC hybrid layer and the sur- face of the concrete beam, thus the flexural behavior of the concrete beam was barely improved. Therefore, it is recommended that proper surface treatment of the concrete beam should be prepared or innovative anchoring techniques should be developed before the CFRP-ECC hybrid can be used for the strengthening of the concrete structures. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction There has been a considerable increase in the demand of repair and strengthening of the concrete structures worldwide [1–3]. With high strength to weight ratio and desirable resistance to the harsh environment, fiber reinforced polymer (FRP) composites have been widely accepted for the strengthening of the concrete structures [1–8]. Polymer adhesives, like epoxy, are normally adopted as the bonding agent [9–11]. One major issue with the polymer adhesives is that, when the glass transition temperature (T g ) is exceeded, the mechanical properties of the adhesive is reduced and the strengthening effect of the FRP cannot be main- tained [12–17]. Unfortunately, most of the polymer adhesives used for the structural strengthening have a T g less than 100 °C [18].A study in [19] found that the efficiency of the CFRP strengthening of the concrete structures would extensively reduce by 65% at 300 °C [20]. In order to solve this problem, cement based adhesives are introduced for the FRP strengthening of the concrete structures [12,21–23]. Although the bond behavior between the FRP and the concrete is improved under the elevated temperatures, another problem of the cement based adhesives has been observed [21,24,25]. Since most of the cement based adhesives lack ductility, the load cannot be effectively transferred from the concrete to the FRP composite. Also the failure is generally brittle and lack warning [21]. It was found that the FRP-mortar system was generally less effective than the FRP-epoxy system in strengthening. For example, it was concluded that the FRP-mortar system was 45% [26] and 50% [27] less effective than the FRP-epoxy counterparts. Therefore, it is necessary to develop a cementitious adhesive with desired ductility, which can effectively distribute and transfer the load from the concrete substrate to the FRP composite, without deterioration under elevated temperatures. Engineered Cementi- tious Composites (ECC) is a promising candidate for such strength- ening applications [28,29]. ECC was developed based on the micro fracture mechanics with a strain capacity in the range of 3–7% [30], comparing to 0.1% for ordinary Portland cement (OPC) mortar. This http://dx.doi.org/10.1016/j.compstruct.2017.07.034 0263-8223/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: wuchao@buaa.edu.cn (C. Wu), vcli@umich.edu (V.C. Li). Composite Structures 178 (2017) 372–382 Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct