The effectiveness of concrete structures reinforced by the high strength steel wire mesh-polymer mortar depends on the bond-slip behavior of the interface between the polymer modified cement mortar layer (referred as mortar layer) and the concrete. The areas of the mortar layer tested in the previous studies are generally very small and the obtained conclusions are with limited applications. In this study, the experimental approach was used to analyze the slip behavior of the interface between the concrete block and a larger scale mortar layer. 30 concrete blocks retrofitted with mortar layers were fabricated and the single shear tests on these specimens were conducted. The applied shear loads, slip displacement between the concrete block and the mortar, as well as the strain of the mortar were monitored during tests. The experimental results exhibit the specimens have two representative failure types: debond with and without cracks. Then the bonding performance of the interface was systematically explored by analyzing the failure modes of the specimens and the mechanical property of the mortar. Meanwhile, the slip behavior of the interface influenced by the bond area, the thickness of the mortar layer, the mortar strength and the roughness of the interface were discussed. It was presented that the interface roughness treatment and increasing the mortar strength could significantly improve the behavior of the interface.
Construction and Building Materials 228 (2019) 116751 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat Large scale experimental study on bond behavior between polymer modified cement mortar layer and concrete Weizhang Liao ⇑, Hongwei Wang, Miao Li, Chao Ma, Bo Wang Beijing Higher Institution Engineering Research Center of Civil Engineering Structure and Renewable Material, Beijing Advanced Innovation Center for Future Urban Design & Beijing University of Civil Engineering and Architecture, Beijing 100044, China h i g h l i g h t s Tests on large-scale concrete blocks retrofitted with mortar layers Two typical failure types of crack or debond Layer thickness, bond length, mortar strength and interface roughness influencing the shear strength of interface a r t i c l e i n f o Article history: Received 10 September 2018 Received in revised form June 2019 Accepted 16 August 2019 Keywords: Mortar layer Interface Single shear test Bond-slip Debonding a b s t r a c t The effectiveness of concrete structures reinforced by the high strength steel wire mesh-polymer mortar depends on the bond-slip behavior of the interface between the polymer modified cement mortar layer (referred as mortar layer) and the concrete The areas of the mortar layer tested in the previous studies are generally very small and the obtained conclusions are with limited applications In this study, the experimental approach was used to analyze the slip behavior of the interface between the concrete block and a larger scale mortar layer 30 concrete blocks retrofitted with mortar layers were fabricated and the single shear tests on these specimens were conducted The applied shear loads, slip displacement between the concrete block and the mortar, as well as the strain of the mortar were monitored during tests The experimental results exhibit the specimens have two representative failure types: debond with and without cracks Then the bonding performance of the interface was systematically explored by analyzing the failure modes of the specimens and the mechanical property of the mortar Meanwhile, the slip behavior of the interface influenced by the bond area, the thickness of the mortar layer, the mortar strength and the roughness of the interface were discussed It was presented that the interface roughness treatment and increasing the mortar strength could significantly improve the behavior of the interface Ó 2019 Elsevier Ltd All rights reserved Introduction Retrofitting approaches were proposed in order to repair, strengthening and updating existing reinforced concrete (RC) structures to resist higher loads, improve load-carrying capacity of structures [1–5] The commonly utilized systems are generally made of fiber sheets embedded in an epoxy matrix (i.e., fiber reinforced polymers, FRP) due to the high strength to weight ratio, high corrosion resistance, convenience application and minimal section area change [6,7] The effectiveness of the FRP systems in retrofitting RC structures has been proved by numerous experimental and theoretical studies Good bond behaviors of CFRP sheets attached ⇑ Corresponding author E-mail addresses: liaoweizhang@bucea.edu.cn (W Liao), machao@bucea.edu.cn (C Ma) https://doi.org/10.1016/j.conbuildmat.2019.116751 0950-0618/Ó 2019 Elsevier Ltd All rights reserved to concrete have been validated [8] The externally bonded reinforcement on grooves techniques could improve the FRP-concrete bond strength [9–13] and eliminate the debonding failure of the interface [14] However, due to the poor fire resistance of the organic matrix, poor thermal compatibility with concrete, susceptibility to radiations, the FRP is inapplicably applied on wet surfaces and at low-temperatures [15] Relatively, the high strength steel wires mesh-polymer mortar (HSWM-PM) composite layer as another structural strengthening and retrofitting system has been developed rapidly because of its good durability, material compatibility, adhesive properties and high-temperature resistance [16] Moreover, the effectiveness of the HSWM-PM system retrofitting columns [17–21], beams and other structures [22–33] were also evaluated The HSWM-PM system consists of the high strength steel wires, mortar layer [34,35] and the interfaces, which are between the W Liao et al / Construction and Building Materials 228 (2019) 116751 mortar layer and the concrete as well as between the steel wires and mortar layer Generally, the high strength steel wires play two roles in this system: like the longitudinal bars in RC structures carrying the tensile forces, like hooping bars in RC structures carrying shear forces and providing confining loading to improve the shear strength of the mortar The load carrying capacity of the structures retrofitted by the HSWM-PM system mainly depends on the bonding behavior of the interfaces, and debond failure of the interfaces would result in the loss of the reinforcement effect [36,37] The bonding behavior of the interface between the steel wires and mortar layer influenced by the strength of mortar-based matrix [38], the diameter and anchorage length of the steel strand [39,40] and the density of the steel strand mesh [16] has been studied in detail, and a series of valuable conclusions were acquired Such as, the mineral mortar with natural kaolin and bauxite can be used to enhance the bonding force of the interface The grid density of the steel wire mesh is not directly proportional to the bond strength of the interface The critical anchorage length of the strand has a linear relationship with the diameter of steel strand [40] Accordingly, to ensure enough bonding force, the bond length of the strand should be greater than the critical anchorage length [41] Theoretical and numerical approaches were also used to analyze the stress distribution rule [41,42] and the bond-slip distribution of the prestressing strands, furtherly, to analyze the transmission and anchorage lengths of prestressing steel strands in different mortar-based matrixes [43] The reasonable arrangement of the steel strand will lead to a well-distributed bond stress [16] These experimental data and theoretical analyses present that the interface between the steel wires and mortar layer exhibits a soundness behavior [38–44] On the other hand, the reinforcement effect of the composite layer also depends on the bond performance between the mortar layer and the concrete The bonding performance between the mortar layer and concrete not only involves the strength of the bonding interface, but also involves the durability of the interface The bond-slip mechanism of the bonding interface between the mortar layer and the concrete is the same as the interface between old concrete and new concrete [45,46], this is because the bonding action in these interfaces is mainly achieved through the mechanical interlock between the inorganic matrices However, there are still differences in mechanical properties and microscopic molecular structures between the mortar and concrete, such as the age between new and existing concretes and the relative stiffness between the substrate concrete and added concrete layer Consequently, the achievements of the bonding mechanism of the concrete-concrete interface [47] could not be suitable for the interface between the mortar and concrete directly To analyze the bonding mechanism of the interface between the mortar layer and the concrete, experiments [48–55] were conducted on the mortar-based system Experiment data presented that crack [56] and debond [57] of the mortar layer were the main failure modes in the mortar-concrete interface Moreover, the curing time, strength of the concrete and mortar, interface roughness and repair position also greatly influence the bonding behavior of the interface The mechanical occlusion of the mortar layer and the concrete interface are the important guarantee for the reinforcement effect of the whole composite layer The bonding behaviour between the magnesium potassium phosphate cement and the concrete substrate was evaluated based on the pullout tests [58] Besides, an excellent bonding property between the magnesium potassium phosphate cement and the old concrete was verified [59] by the splitting tests For example, the Alkali-activated mortar could also develop a fast and good adherence strength (30 MPa after 24 h) to the concrete substrate [60–64] However, during the past studies, the bond area of mortar layer was usually less than 200 mm  200 mm and the deformation of mortar layer was small and almost with no damage occurring before the interface producing failure In fact, the contribution of the thickness and bond area of the mortar matrices to the bond performance need to be investigated Moreover, shear tests on the interface between mortar layer and concrete at small area cannot be used to adequately explain the interface bonding mechanism, and the corresponding conclusions as the design reference directly in actual engineering are defective In order to demonstrate the damage process of the mortar layer, and to analyze the difference of bonding performances influenced by the bond length, mortar strength, interface treatment and the thickness of mortar layers, tests on the mortar layer and the concrete interface were conducted in this study Then the bond mechanism and failure mode of the larger bond area were systematically studied based on the experimental data Test program 2.1 Test specimen preparation 30 large-scale plain concrete blocks retrofitted with mortar layer, which sketch is shown in Fig 1, were designed and tested to investigate the bond-slip behaviors influenced by the bond area, thickness and strength of the mortar layer, as well as the roughness of the interface between the mortar layer and concrete Each specimen consists of a plain concrete block with the size of 300 mm  300 mm  900 mm and a mortar layer There are three steps to fabricate the concrete specimens: plain concrete block preparing, concrete surface chiseling and mortar layer laying The plain concrete blocks were fabricated using concrete with grade of C30 and the largest aggregate size of 25 mm Then after 28 days curing, manually chiseling was conducted at the surface of the concrete blocks to increase the mechanical occlusion between the concrete block and the mortar layer The chiseled concrete block is illustrated in Fig 2(a) Finally, the mortar layer was then equably laid on the concrete blocks after the chiseling procedure, the fabricated concrete specimen is shown in Fig 2(b) To prevent the evaporation of the water, the specimens were covered by plastic sheets during the curing of the mortar layer, shown in Fig 2(c) Note that three blocks were not chiseled as the comparative cases The detailed information about the 30 specimens is listed in Note: In the experiment, the thin layer of mortar fell off before the W1-1 and W4-1 specimens were loaded, therefore, the final quantity of test specimens is only one Fig Sketch of the retrofitted concrete block with mortar layers W Liao et al / Construction and Building Materials 228 (2019) 116751 (a) Interface treatment (a) Concrete blocks after chiseled (b) Laying standard sand (b) Mortar layer (c) Measuring standard sand Fig Evaluation of interface roughness (c) Specimen during curing Fig Preparation process of specimens In the process of the surface chiseling, the maximum depth is controlled as 12 mm The sand filling method was used to estimate uniformity of the interface roughness Fig shows procedure of the sand filling, and the roughness is calculated by the Eq (1) r¼ V A ð1Þ where, r is the roughness at the bonding interface, Vmm2 is the volume of standard sand filled in the chiseled holes, A is the area of the bonding interface The standard sand particle size ranges from 0.08 mm to 2.0 mm The calculated roughness is around 1.27 mm with variance of only 0.036 mm Moreover, in order to analyze the discreteness of interface chiseling, specimens with band area of 350 mm  300 mm presented in Table were designed The mixture ratio of the concrete is presented in Table 2, and the average compressive strengths of 25.8 MPa and 36.1 MPa were respectively obtained from cubic tests after days and 28 days casting The major ingredients of the applied mortars are the high-performance cement, 20–40 mesh sand, 40–70 mesh sand, chopped fiber, additives and water The mortars are referred to as the commercially TCPM polymer mortar with the strength grade of M30, M40, M50 The component proportions of the polymer mortar are shown in Table and the measured compressive strengths of the mortars are listed in Table The interface treating W Liao et al / Construction and Building Materials 228 (2019) 116751 Table Test specimen mortar layer details Number Mortar layer thickness (mm) Bonding surface length (mm) Bond surface width (mm) Mortar strength grade Chiseling or not Quantity W1-1 W1-2 W1-3 W1-4 W2-1 W2-2 W2-3 W3-1 W3-2 W4-1 W4-2 20 20 20 20 25 25 25 25 25 25 25 250 300 350 400 400 250 400 300 350 250 350 300 300 300 300 300 300 300 300 300 300 300 M50 M50 M50 M50 M30 M40 M50 M50 M50 M50 M50 yes yes yes yes yes yes yes yes yes no no 2 2 4 Table Mixture ratio of concrete (Unit: kg) Material P.O42.5 cement Water Sand Crushed stone BM-PM001 water reducer F kind of Fly ash grade I Blast furnace slag powder S95 Consumption 247 103 866 1024 7.60 57 76 Note: the moisture content of sand is 7.8%, and the moisture content of crushed stone is 0.2% Table Component proportion of polymer mortar Mortar strength grade Water (g) Cement (g) Emulsion powder (g) Early strength agent (g) Silica fume (g) Cellulose ether (g) Fly Ash (g) Sand (g) M30 M40 M50 198 216 252 770 815 770 27 45 45 9 90 45 90 0.9 0.45 40 40 40 1350 1350 1350 2.2 Test setup Table Strength properties of TCPM high performance mortars Mortar strength grade 3d compressive strength (MPa) 7d compressive strength (MPa) 28d compressive strength (MPa) M30 M40 M50 24.21 28.7 38.47 26.87 31.41 46.01 32.05 40.08 58.77 agent was used in the concrete surface commercially known as YT302 type two-specimen treatment agent The 14-day shear bond strength and tensile strength of the agent obtained from the manufacturer are 1.7 MPa and 0.9 MPa, respectively Tests were conducted by applying quasi-static single shear loading at the interface between the concrete blocks and mortar layer Considering the areas of the mortar of the specimen are rather large, to finish the tests smoothly, a hydraulic jack operated by the manual pump (shown in Fig 4) was used to provide the single shear loading by the horizontal thrust between the lifting jack and the concrete block The specimens were placed between the lifting jack and the force-transmitting steel plate The hydraulic jack and the force-transmitting steel plate are connected by four steel bars The force-transmitting steel plate is welded with the four steel bars, and the hydraulic jack is also fixed with the steel bars The movement of the mortar layer was restricted by the steel Fig Experimental setup W Liao et al / Construction and Building Materials 228 (2019) 116751 plate During tests, the loading device and specimens were placed horizontally as shown in Fig 5, the horizontal thrust compels the relative horizontal displacement and provides the single shear force between the concrete block and mortar layer In the following, the end of the concrete block close to the hydraulic jack is referred as the loading end, the end of the mortar layer close to the force-transmitting steel plate is referred as the constraint end and the other end of the mortar layer is referred as the free end, as shown in Fig It should be emphasized that the applied shear loading should be concentric with the longitudinal axis of the specimen in case of avoiding the eccentric effect During the quasi-static loading, Fig Setup of the experimental device and specimen (a) Sketch of measuring points arrangement (b) Layout of measuring points arrangement Fig Layout of displacement gauges and strain gauges (Unit: mm) 6 W Liao et al / Construction and Building Materials 228 (2019) 116751 the loading procedure will be stopped when any one of the following conditions occurs: (1) obvious misalignment or mortar layer slip occurs, (2) the local mortar layer is obviously cracked or crushed; (3) the load bearing capacity of specimens decreases obviously due to other uncertainties 2.3 Test observations A total of Sz120-100AA-type strain gauges were bonded at the surface of the mortar to monitor the surface strains during testing The gauges were glued along rows transversely and columns longitudinally Locations and labels of the strain gauges are illustrated in Fig The gauges are glued 100 mm and 200 mm away from the constraint end and with the transverse spacing of 150 mm The shear load was monitored by the hydraulic pressure conversion Two rod-type displacement meters with the range of ±25 mm were arranged on the upper left of the concrete block and the free end of the mortar layer to respectively monitor the horizontal displacements of the mortar layer and the concrete Displacement meter is used to measure the displacement of the concrete block, and displacement meter number is used to measure the deformation of the mortar layer Then the difference between the two measured displacement is the sliding displacement of the interface (a) Failure mode of W1-2-2 (b) Failure mode of W1-4-2 Fig Failure modes of complete debond with no cracks Test results and parameter studies (a) Failure mode of W1-2-1 (b) Failure mode of W2-1-1 Fig Failure modes of complete debond with cracks According to the test observations, the failure types of all the specimens could be classified into two categories: uncracked and cracked types The uncracked type is defined as that there is no visible crack in the mortar layer during the loading process, and the cracked type is defined as that there are visible cracks in the mortar layer during the process These two failure types of the mortar layer mainly include five failure modes: (1) The failure mode of the complete debond with cracks is that the mortar layer experiences the formation and development of cracks and eventually produced debond failure, shown in Fig (2) The failure mode of the complete debond with no cracks is that the mortar layer was gradually debond but the mortar layer did not produce cracks, given in Fig (3) The failure mode of the local crushing with expansion is that crushing failure occurred in the mortar layer with expansion shown in Fig (4) The failure mode of the local debond is that the mortar layer produced local debond as shown in Fig 10 (5) The failure mode of the local debond with the crush is that the mortar layer produced local debond with the crush at the constraint end as given in Fig 11 In the uncracked type, noticeable compression deformation, local crushing can be obtained during loading as shown in Figs and 11 Besides, complete debond and local debond without cracks of the mortar layer can be observed as shown in Figs and 10 In the cracked type, the mortar layer cracking includes three stages: the formation of cracks, W Liao et al / Construction and Building Materials 228 (2019) 116751 Fig 11 Failure mode of local debond with crush of W3-2–7 (a) Failure mode of W2-2-1 load, and the minimum value of the shear stress and strain depends on the bond length of the mortar layer Particularly, if the mortar has enough length, there might be no stress and strain at the free end Even so, the applied load and the horizontal displacement between the mortar layer and the concrete could be applied as the indicators to discuss the shear performances of the bonding interface Obviously, the means shear stress along the interface could also be a reliable indicator to explore the shear strength of the interface The failure modes and peak loads of each specimen in the tests are summarized and analyzed in Table The local bond stress s (MPa) is the average bond stress between the mortar layer and the concrete It can be calculated by dividing the applied force by the bond area as follows: s¼ (b) Failure mode of W2-3-1 Fig Failure modes of local crushing with expansion F A ð2Þ where, F is the load applied at the bonding interface between mortar layer and concrete The failure mode of the mortar layer and the interface bonding performance are greatly affected by the strength of the interface bond area, the thickness of the mortar layer and the roughness of the interface These factors will be discussed respectively in the following sections 3.1 Effect of the mortar layer thickness on the bonding performances Fig 10 Failure mode of local debond of W2-2–2 the development of cracks and cracking failure Mortar embedded in the concrete blocks can be seen in Fig 12, it proves that the characteristics of mortar in concrete interface after chiseling are obvious According to the loading scheme, the shear stress and strain at the interface should not be even as presented in Fig 13 The maximum value of the shear stress and strain depends on the applied The failure loads of the specimens of Groups W1, W2, W3 and W4 with mortar layer thickness of 20 mm and 25 mm are contrastively presented in Fig 14 With the increase of the layer thickness, the ability of the mortar layer to resist the shear force will be enhanced when the strength grade of mortar and bond area are same Therefore, increasing the thickness of the mortar layer can result in the enhancement of the bond force and the reduction of the occurrence of the complete debond failure of the mortar layer Fig 14 also presents the shear load differences of two specimens with same area and different layer thickness, which are values of 7.8%, 1.6% and 1.4% They are calculated by ðF 25 À F 20 Þ=F 20 , herein, F 20 is the failure load of the interface with the mortar layer thickness of 20 mm, and the F 25 is the failure load of the interface with the mortar layer thickness of 25 mm As presented, the differences are no more than 8.0% Hence, when enhance a structure, the mortar layer with thickness of 20 mm might be enough 3.2 Effect of the bond length on the bonding performances Fig 15 presents the relation between the bond length L and the bond stress s Fig 15(a) and (b) present all the experimental data W Liao et al / Construction and Building Materials 228 (2019) 116751 (a) W2-1-2 (c) W2-3-3 (b) W1-2-1 (d) W4-2-1 Fig 12 Failure mode of the interface on the mortar layer of 20 mm and 25 mm It exhibits the stability and feasibility of the tests Fig 14(c) presents the relationship À s and bond length L As presented, À when the bond thickness is 20 mm, s equals 2.47 MPa, with maxbetween the mean bond stress imum value of 2.49 MPa and minimum value of 2.45 MPa The maximum difference value is only 0.044 MPa and 1.78% deviation À Similarly, the average value of s is 2.55 MPa when the bond thickness is 25 mm, with the maximum value of 2.64 MPa and the minimum value of 2.50 MPa The maximum difference value is only 0.14 MPa and 5.6% deviation Experimental results indicate that the bond stress at the interface changes slightly when the bond length is in the range of 250 mm to 400 mm Therefore, the uneven distribution of the bond stress can be ignored when the bond length is larger than 250 mm to reinforce a concrete structural component with the HSWM-PM 3.3 Effect of the mortar strength on the bonding performances Essentially, the shear strength of the bond surface is the bond strength between the concrete and mortar, obviously, the mortar strength partly determines interfacial bond strength Fig 16 presents the average strength when the bond thickness is 25 mm Experiment results show that with the increase of strength grade of the mortar, the shear strength of the interface increases remark- ably Therefore, the mortar with high strength results in the bond interface with high shear capacity and reducing the occurrence of the interfacial debond failure On the other hand, the failure modes of the mortar layer with different strength grades are diverse The specimens retrofitted with mortar with lower mortar have the complete debond failure mode, as shown in Fig 7(b) On the contrary, the specimens retrofitted with the higher strength mortar have the crushing and local crushing failure modes, which is shown in Fig 9(b) 3.4 Effect of the interface roughness on the bonding performances The bond strength between the concrete and mortar layer is also affected by the mechanical occlusal interaction between the mortar layer and the concrete Interface treatment can change the interface roughness; therefore, interface treatment can significantly improve the bond strength of the interface Certainly, interface treatment can also improve the frictional strength of the interface Interface treatment influencing the frictional strength will not be discussed in this study As presented in Table 5, the average peak shear force of Group W3-2 with the interface treatment is 263.75 kN, the average shear force of Group W4-2 without interface treatment is 161.62 kN This proves that interface treatment improves 63.2% shear load-carrying capacity of the interface Moreover, the specimens without interface treatment failed with W Liao et al / Construction and Building Materials 228 (2019) 116751 Loading complete debond mode, it also proves that the interface without treatment has lower shear strength from another perspective Free end end Load-slip response 4.1 Interface bond-slip relationship τmax τmin Rod-type displacement meters were used to measure the displacements of the loading end and free end, which are respectively the deformations of the concrete blocks and the mortar The difference value of the monitored displacements is the relative displacement between the free and constraint ends Because the maximum load applied during tests is about 300 kN, correspondingly, the maximum axial stress of the concrete block is smaller than 3.4 MPa which is far below the average compressive strength of 36.1 MPa This confirms that the deformation of the concrete block is very small and can be ignored Further, the relative displacement between the free and constraint ends can be treated as the slip displacement between the mortar layer and the concrete block Fig 17 shows the load-slip relationship of representative specimens, and the behavior of each specimen can be divided into three stages L (a) Distribution of the shear stress Loading end Free end εmax εmin (1) Preloading stage During this stage, the deformation of the mortar layer increases quickly with the tight compress between the steel plate and the mortar layer of the constraint end because the constraint end is not absolutely smooth However, the shear load hardly increases during this stage The load-slip curves of this stage are nearly parallel to the horizontal axis L (b) Distribution of the strain Fig 13 Sketch of shear stress and strain distribution at the interface Table Results of shear test Number A = L  b (mm  mm) t (mm) Mortar strength grade F (kN) À sp = Fp/A(MPa) À s (MPa) Failure mode W1-1 W1-2–1 W1-2–2 W1-3–1 W1-3–2 W1-4–1 W1-4–2 W2-1–1 W2-1–2 W2-2–1 W2-2–2 W2-3–1 W2-3–2 W2-3–3 W2-3–4 W3-1–1 W3-1–2 W3-1–3 W3-1–4 W3-2–1 W3-2–2 W3-2–3 W3-2–4 W3-2–5 W3-2–6 W3-2–7 W3-2–8 W4-1 W4-2–1 W4-2–2 250  300 300  300 300  300 350  300 350  300 400  300 400  300 400  300 400  300 250  300 250  300 400  300 400  300 400  300 400  300 300  300 300  300 300  300 300  300 350  300 350  300 350  300 350  300 350  300 350  300 350  300 350  300 250  300 350  300 350  300 20 20 20 20 20 20 20 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 M50 M50 M50 M50 M50 M50 M50 M30 M30 M40 M40 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 M50 186.73 220.11 2.490 2.416 2.475 2.512 2.432 2.415 2.522 1.869 1.796 2.091 1.907 2.543 2.201 2.808 2.460 2.777 2.707 2.562 2.499 1.611 2.122 2.269 2.003 3.051 2.518 2.009 2.371 1.474 1.735 1.343 2.490 2.446 complete debond with cracks complete debond with cracks complete debond with no cracks complete debond with no cracks complete debond with cracks complete debond with cracks complete debond with no cracks complete debond with cracks complete debond with no cracks local crushing with expansion local debond local crushing with expansion complete debond with no cracks complete debond with no cracks complete debond with cracks local debond with crush complete debond with no cracks complete debond with cracks local debond with crush complete debond with no cracks complete debond with cracks complete debond with no cracks complete debond with no cracks complete debond with no cracks complete debond with no cracks Serious local debond with crush complete debond with no cracks complete debond with no cracks complete debond with cracks complete debond with no cracks 259.56 296.24 219.89 149.94 300.37 237.25 263.75 110.57 161.62 À 2.472 2.469 1.832 1.999 2.503 2.636 2.512 1.474 1.539 Note: t denotes the thickness of the mortar layer; FP denotes peak load; F denotes mean failure load; sp denotes bond shear strength; strength À s denotes mean value of bond shear 10 W Liao et al / Construction and Building Materials 228 (2019) 116751 350 2.8 25mm 296.24 300.37 300 1.4% Shear force (kN) 259.56 263.75 250 220.11 200 237.25 1.6% 7.8% 186.73 150 100 50 Load bearing capacity (MPa) 20mm 2.48 2.42 2.49 2.4 2.52 2.42 2.51 2.43 2.0 1.6 1.2 0.8 0.4 0.0 250 300 350 400 200 Bond length (mm) 3.2 Load bearing capacity (MPa) The shear load decreases quickly after reaching its peak value, and the interface loses the load bearing capacity during this stage Moreover, the monitored displacement in this stage is mainly the slip displacement between the mortar and concrete Meanwhile, the specimens fail to various failure mode, including local debond, complete debond, local crushing, etc 350 400 450 (a) Relationship between the bond length and load bearing capacity of Group W1 (2) Bonding performing stage (3) Failure stage 300 Bond Length L(mm) Fig 14 Shear force of bond with thickness of 20 mm and 25 mm During this stage, the bonding effect between the mortar layer and concrete plays its function The shear force between the mortar and concrete is resisted by the bond force of the interface With the increase of the applied load, the displacement monitored by the rod-type displacement meters also increases Because the interface does not lose the bond strength, the shear load could increase to its peak value 250 3.05 2.78 2.71 2.56 2.50 2.8 2.4 2.81 2.54 2.46 2.20 2.52 2.37 2.27 2.12 2.01 2.00 1.61 2.0 1.6 1.2 0.8 0.4 0.0 250 300 350 400 450 Bond length L(mm) (b) Relationship between the load bearing capacity and bond length with the W2-3 group and W3 group 2.7 As described above, all the specimens were instrumented by strain gauges bonded to the surface of the mortar layer at predetermined locations as shown in Fig Figs 18 and 19 present the relation between the shear forces and surface strain of the representative specimens which failure modes belong to the cracked and uncracked types The solid lines represent the curve of the strain close to the constraint end and the dash lines represent the curve of the strain close to the free end As shown in Figs 18 and 19, during the early stage of loading, although the stress and the strain are not even, the differences between each curve shown in Figs 18 and 19 are not too large With the increase of the applied horizontal load, the compressive strains of S4, S5 and S6 increase more rapidly than those of S1, S2, S3, then the uneven distribution of the stress and strain becomes more and more pronounced The initial stage of the shear force versus strain relation curves shown in Figs 18 and 19 is not parallel to the horizontal axis, which is rather different from the bond-slip curve shown in Fig 17 This confirms that the initial stage of the tests is the preloading stage The shear force versus strain relation curves can be divided into two groups: strain only increasing, strain increasing firstly and then decreasing The shear force versus strain relation curves of S1, S2 and S3, which locate far away from the constraint end have no strain decreasing stage Whereas, the shear force versus strain relation curves of S4, S5 and S6, which close to 2.6 Load bearing capac ity(MPa) 4.2 Surface strain response 0.14MPa 2.5 0.044MPa 2.4 Difference value < 5.6 % 2.3 20 mm 25 mm 2.2 2.1 2.0 200 250 300 350 400 450 Bond length, L (mm) (c) Relationship between the load bearing capacity and bond length Fig 15 Relationship between the load bearing capacity and bond length the constraint end, have both the strain increasing and decreasing stage Moreover, the shear force versus strain relation curves of S4, S5 and S6, which specimens have crack failure type, have a remark- 11 W Liao et al / Construction and Building Materials 228 (2019) 116751 2.5 Shear force (kN) Load bearing capacity (MPa) 3.0 2.0 1.5 1.0 20 30 40 50 Mortar strength grade (M) 60 260 240 S2 220 200 180 160 140 120 100 80 60 40 20 -500 S1 S1 S2 S4 S5 S3 S6 -400 -300 -200 Strain ( ) S4 -100 (a) W1-2-1 Shear force (kN) Fig 16 Relationship between the load bearing capacity and mortar strength grade Shear force (kN) Fig 17 Typical curve of load-slip relationship able strain decreasing stage The strain decreasing stage of the shear force versus strain relation curves of S4, S5 and S6, which specimens have debond or no crack failure types, is nonsignificant Fig 18 shows the typical shear force versus strain curves of the specimens with cracked failure type, which typical failure pictures are shown in Figs 7(b) and The shape of the shear force versus strain curves depends mainly on the position of the strain gauge When the loads carrying by the mortar reach its strength and the bond interface still works in its load carrying capacity, then the mortar layer close to the constraint end is crushed and produced cracking failure firstly Cracks result in the mortar layer expanding, but the applied load imposed on the specimens will not decrease Consequently, the mortar close to the constraint end, especially near the surface, changes from compression state to extensile state, i.e., the value of the strain decreases Therefore, the shear force versus strain curves of the mortar close to the constrained end exhibit a remarkable strain decreasing type On the contrary, because the mortar close to the free end does not lose its load carrying capacity, there is no crack generated and the shear force versus strain curves of the mortar exhibit a continuously increasing type Fig 19 shows the typical shear force versus strain curves of the specimens with debond failure type, which typical failure pictures are shown in Fig When the load carrying by the mortar reaches S6 S5 S3 240 220 200 180 160 140 120 100 80 60 40 20 -500 S2 S1 S5 S3 S6 S4 S1 S2 S3 -400 S4 S5 S6 -300 -200 Strain (με) (b) W3-2-2 -100 260 S2 S6 S5 240 S1 220 S3 200 S4 180 160 140 120 100 80 S4 S1 60 S5 S2 40 S3 S6 20 -700 -600 -500 -400 -300 -200 -100 Strain (με) 0 100 (c) W3-1-3 Fig 18 Curves of mortar strain - interface shear force of the cracks type to its strength, the bond interface also performs its cohesive functionality absolutely At this time, the mortar close to the constrained end has no visible crack and expands inconspicuously W Liao et al / Construction and Building Materials 228 (2019) 116751 Shear force (kN) 12 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 -250 Conclusions S4 S2 S5 S1 In this paper, large scale single shear tests were carried out on the plain concrete blocks reinforced by the high strength steel wire mesh-polymer mortar, aiming to study the bond mechanism of the interface between the concrete block and mortar layer Mortar strength, bond area, thickness of mortar layer, treatment of concrete bonding interface influencing the failure mode of the mortar layer and shear strength of the interface were comparatively analyzed The main achievements are listed as follows S3 S6 S1 S2 S3 S4 S5 S6 -200 -150 -100 -50 Strain ( ) 50 Shear force (kN) (a) W1-4-2 280 260 S5 240 S6 S1 S3 S2 220 200 180 160 140 120 100 80 S4 S1 60 S2 S5 40 S6 S3 20 -350 -300 -250 -200 -150 -100 Strain ( S4 -50 50 ) Shear force (kN) (b) W3-2-8 240 220 200 180 160 140 120 100 80 60 40 20 S5 S4 S6 S3 S2 (1) 30 large-scale concrete blocks retrofitted with mortar layers were tested The failure modes of the specimens can be divided as the uncracked and the cracked types The uncracked failure type includes the complete debond without cracks and local debond failure modes, and the cracked failure type contains the complete debond with cracks, local crushing with expansion and local debond with crush failure modes (2) Base on the experimental data, four influencing factors affecting the bonding performance were discussed The thickness of mortar influences the failure mode of the specimen greatly but the failure load When the bond length is in the range of 250 mm to 400 mm, the mean bond stress at the interface has no significant change Improving the mortar strength can greatly increase the shear strength of the bond interface Comparing with other factors, the roughness has the largest impact on the shear strength of the interface (3) There is a big distinction between the deformations of the mortar layer at the free and constrained ends Due to the crack, the mortar layer at the constrained end might expand, which results in the decrease of the strain Comparatively, the strain of the mortar at the free end increases monotonously to its maximum value (4) Comparing with the compressive test, tensile test might provide a mean stress distributing along the interface between the concrete block and mortar, therefore, tensile test should be conducted to explore the bond behavior between polymer modified cement mortar layer and concrete in the future works S1 Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper S1 S2 S3 -1000 Acknowledgements S4 S5 S6 -800 -600 Strain ( -400 ) -200 (c) W3-1-4 Fig 19 Curves of mortar strain - interface shear force of the no cracks type Therefore, the strain decreasing stage of the Gauge S4, S5 and S6 are not pronounced Similarly, with the crack type, the shear force versus strain curves of the mortar close to the free end exhibit a continuously increasing type Due to lost load carrying capacity of the bonding interface, the mortar slips along the surface of the concrete Finally, the specimens fail in a debond type gradually The authors gratefully acknowledge the financial support from the Beijing Municipal Natural Science Foundation, China (Grant No 8162015) and the National Natural Science Foundation of China (Grant No 51878028, 51378046) This research is also funded by the support of the Beijing Higher Institution Engineering Research Center of Civil Engineering Structure and Renewable Material, Beijing Advanced Innovation Center for Future Urban Design, Pyramid Talent cultivation plan of BUCEA References [1] R Eid, P Paultre, Compressive behavior of FRP-confined reinforced concrete columns, Eng Struct 132 (2017) 518–530 [2] A Kashi, A.A Ramezanianpour, F Moodi, Durability evaluation of retrofitted corroded reinforced concrete columns with FRP sheets in marine environmental conditions, Constr Build Mater 151 (2017) 520–533 W Liao et al / Construction and Building Materials 228 (2019) 116751 [3] O Ali, D Bigaud, H Riahi, Seismic performance of reinforced concrete frame structures strengthened with FRP laminates using a reliability-based advanced approach, Compos B Eng 139 (2018) 238–248 [4] C Pellegrino, M Vasic, Assessment of design procedures for the use of externally bonded FRP composites in shear strengthening of reinforced concrete beams, Compos B 45 (1) (2013) 727–741 [5] S.A Hadigheh, R.J Gravina, S.T Smith, Effect of acid attack on FRP-to-concrete bonded interfaces, Constr Build Mater 152 (2017) 285–303 [6] C.E Bakis, A Ganjehlou, D.I Kachlakev, et al., Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures, Proc Int Symposium on Frp Reinforcement for Concrete Structures (2002) 440 [7] J.F Chen, S.T Smith, L Lam, et al., Behaviour and strength of FRP-strengthened RC structures: a state-of-the-art review, Struct Build 156 (1) (2003) 51–62 [8] Majid Ghorbani, Davood Mostofinejad, Ardalan Hosseini Experimental investigation into bond behavior of FRP-to-concrete under mixed-mode I/II loading, Construct Build Mater 132 (2017) 303–312 [9] Niloufar Moshiri, Amir Tajmir-Riahi, Davood Mostofinejad, Christoph Czaderski, Masoud Motavalli Experimental and analytical study on CFRP strips-to-concrete bonded joints using EBROG method, Composer Part B 158 (2019) 437–447 [10] Ardalan Hosseini, Davood Mostofinejad Effect of groove characteristics on CFRP-to-concrete bond behavior of EBROG joints: experimental study using particle image velocimetry (PIV), Construct Build Mater 49 (2013) 364–373 [11] A Hosseini, D Mostofinejad, M Emami, Influence of bonding technique on bond behavior of CFRP-to-clay brick masonry joints: Experimental study using particle image velocimetry (PIV), Int J Adhes Adhes 59 (2015) 27–39 [12] M Davood, H.M Mohsen, H Ardalan, et al., Investigating the effects of concrete compressive strength, CFRP thickness and groove depth on CFRPconcrete bond strength of EBROG joints, Constr Build Mater 189 (2018) 323– 337 [13] Majid Ghorbani, Davood Mostofinejad, Ardalan Hosseini, Bond behavior of CFRP sheets attached to concrete through EBR and EBROG joints subject to mixed-mode I/II loading, J Compos Construct 21 (5) (2017) 04017034 [14] A Hosseini, D Mostofinejad, Experimental investigation into bond behavior of CFRP sheets attached to concrete using EBR and EBROG techniques, Compos B Eng 51 (2013) 130–139 [15] A D’Ambrisi, L Feo, F Focacci, Bond-slip relations for PBO-FRCM materials externally bonded to concrete, Compos B 43 (8) (2012) 2938–2949 [16] S.D Santis, G.D Felice, Tensile behaviour of mortar-based composites for externally bonded reinforcement systems, Compos B 68 (2015) 401–413 [17] S.H Kim, D.K Kim, Seismic retrofit of rectangular RC bridge columns using wire mesh wrap casing, KSCE J Civ Eng 15 (7) (2011) 1227–1236 [18] H Sezen, Repair and Strengthening of Reinforced Concrete Beam-Column Joints with Fiber-Reinforced Polymer Composites, J Compos Constr 16 (5) (2012) 499–506 [19] I Ortega, T.M Pellicer, J.M Adam, et al., An experimental study on RC columns repaired on all four sides with cementitious mortars, Constr Build Mater 161 (2018) 53–62 [20] A Cascardi, F Longo, F Micelli, et al., Compressive strength of confined column with Fiber Reinforced Mortar (FRM): New design-oriented-models, Constr Build Mater 156 (2017) 387–401 [21] P Colajanni, F.D Domenico, A Recupero, et al., Concrete columns confined with fibre reinforced cementitious mortars: Experimentation and modelling, Constr Build Mater 52 (2) (2014) 375–384 [22] B.T Truong, T.T Bui, A Limam, et al., Experimental investigations of reinforced concrete beams repaired/reinforced by TRC composites, Compos Struct 168 (2017) 826–839 [23] L Lanzoni, M Soragni, A.M Tarantino, et al., Concrete beams stiffened by polymer-based mortar layers: experimental investigation and modeling, Constr Build Mater 105 (2016) 321–335 [24] J.A Thamboo, M Dhanasekar, C Yan, Flexural and shear bond characteristics of thin layer polymer cement mortared concrete masonry, Constr Build Mater 46 (2013) 104–113 [25] Z.X Guo, L Wang, Z.L Chai, et al., Flexural behavior of stone slabs strengthened with reinforced mortar, Constr Build Mater 144 (2017) 158– 167 [26] S.M Raoof, L.N Koutas, D.A Bournas, Textile-reinforced mortar (TRM) versus fibre-reinforced polymers (FRP) in flexural strengthening of RC beams, Constr Build Mater 151 (2017) 279–291 [27] H.M Elsanadedy, T.H Almusallam, S.H Alsayed, et al., Flexural strengthening of RC beams using textile reinforced mortar - experimental and numerical study, Compos Struct 97 (2) (2013) 40–55 [28] X Yang, W.Y Gao, J.G Dai, et al., Flexural strengthening of RC beams with CFRP grid-reinforced ECC matrix, Compos Struct 189 (2018) 9–26 [29] O Awani, A.E Refai, T El-Maaddawy, Bond characteristics of carbon fabricreinforced cementitious matrix in double shear tests, Constr Build Mater 101 (2015) 39–49 [30] J.A Thamboo, M Dhanasekar, Characterisation of thin layer polymer cement mortared concrete masonry bond, Constr Build Mater 82 (2015) 71–80 [31] A Younis, U Ebead, K.C Shrestha, Different FRCM systems for shearstrengthening of reinforced concrete beams, Constr Build Mater 153 (2017) 514–526 [32] F.G Carozzi, P Colombi, G Fava, et al., A cohesive interface crack model for the matrix-textile debonding in FRCM composites, Compos Struct 143 (2016) 230–241 13 [33] O Rabinovitch, Dynamic debonding in concrete beams strengthened with composite materials, Int J Solids Struct 49 (26) (2012) 3641–3658 [34] L Fenu, D Forni, E Cadoni, Dynamic behaviour of cement mortars reinforced with glass and basalt fibres, Compos B Eng 92 (8) (2016) 142–150 [35] L Ombres, Analysis of the bond between Fabric Reinforced Cementitious Mortar (FRCM) strengthening systems and concrete, Compos B 69 (2015) 418–426 [36] B.W Russell, M.D Brown, Evaluation of test methods in assessing bond quality of prestressing strands Final report, State University, Stillwater, OK: Oklahoma, 2004 [37] Q Meng, H Hao, W Chen, Laboratory test and numerical study of structural insulated panel strengthened with glass fibre laminate against windborne debris impact, Constr Build Mater 114 (2016) 434–446 [38] C.N Dang, W.M Hale, J.R Martí-Vargas, Quantification of bond performance of 18-mm prestressing steel, Constr Build Mater 159 (2018) 451–462 [39] J.R Martí-Vargas, P Serna, J Navarro-Gregori, et al., Bond of 13 mm prestressing steel strands in pretensioned concrete members, Eng Struct 41 (3) (2012) 403–412 [40] C.N Dang, C.D Murray, R.W Floyd, et al., Analysis of bond stress distribution for prestressing strand by Standard Test for Strand Bond, Eng Struct 72 (2014) 152–159 [41] J.R Martí-Vargas, W.M Hale, E García-Taengua, et al., Slip distribution model along the anchorage length of prestressing strands, Eng Struct 59 (2) (2014) 674–685 [42] Cao Jun, Wang Zhihao, Experimental study on bond anchorage properties of high strength stainless steel wire in polymeric mortar building structure (2006) 37–38 (in Chinese) [43] M Sahmaran, Investigation of the Bond between Concrete Substrate and ECC Overlays, J Mater Civ Eng 26 (1) (2013) 167–174 [44] J.R Martí-Vargas, P Serna, J Navarro-Gregori, et al., Effects of concrete composition on transmission length of prestressing strands, Constr Build Mater 27 (1) (2012) 350–356 [45] E.N.B.S Júlio, F.A.B Branco, V.D Silva, Concrete-to-concrete bond strength Influence of the roughness of the substrate surface, Constr Build Mater 18 (9) (2004) 675–681 [46] D.S Santos, P Santos, D Dias-Da-Costa, Effect of surface preparation and bonding agent on the concrete-to-concrete interface strength, Constr Build Mater 37 (37) (2012) 102–110 [47] A.K Pandey, Factors Affecting Bond between New and Old Concrete, ACI Mater J 109 (3) (2012) 389–390 [48] L.H Sneed, T D’Antino, C Carloni, et al., A comparison of the bond behavior of PBO-FRCM composites determined by double-lap and single-lap shear tests, Cem Concr Compos 64 (2015) 37–48 [49] R.S Olivito, R Codispoti, O.A Cevallos, Bond behavior of Flax-FRCM and PBOFRCM composites applied on clay bricks: Experimental and theoretical study, Compos Struct 146 (2016) 221–231 [50] T Trapko, M Musiał, PBO mesh mobilization via different ways of anchoring PBO-FRCM reinforcements, Compos B Eng 118 (2017) 67–74 [51] C Wu, V.C Li, CFRP-ECC hybrid for strengthening of the concrete structures, Compos Struct 178 (2017) 372–382 [52] T D’Antino, L.H Sneed, C Carloni, et al., Effect of the inherent eccentricity in single-lap direct-shear tests of PBO FRCM-concrete joints, Compos Struct 142 (2016) 117–129 [53] A D’Ambrisi, L Feo, F Focacci, Experimental and analytical investigation on bond between Carbon-FRCM materials and masonry, Compos B 46 (3) (2013) 15–20 [54] H Huang, B Liu, K Xi, et al., Interfacial tensile bond behavior of permeable polymer mortar to concrete, Constr Build Mater 121 (2016) 210–221 [55] J Wu, X Liu, S.H Chew, Parametric study on cement-based soft-hard-soft (SHS) multi-layer composite pavement against blast load, Constr Build Mater 98 (2015) 602–619 [56] S Ahmad, A Elahi, S.A Barbhuiya, et al., Use of polymer modified mortar in controlling cracks in reinforced concrete beams, Constr Build Mater 27 (1) (2012) 91–96 [57] L Ombres, Debonding analysis of reinforced concrete beams strengthened with fibre reinforced cementitious mortar, Eng Fract Mech 81 (2012) 94–109 [58] M.G Sierra-Beltran, H.M Jonkers, E Schlangen, Characterization of sustainable bio-based mortar for concrete repair, Constr Build Mater 67 (2014) 344–352 [59] Y Li, W Bai, T Shi, A study of the bonding performance of magnesium phosphate cement on mortar and concrete, Constr Build Mater 142 (2017) 459–468 [60] R.H Geraldo, O.G Teixeira, S.R.C Matos, et al., Study of alkali-activated mortar used as conventional repair in reinforced concrete, Constr Build Mater 165 (2018) 914–919 [61] T Phoo-Ngernkham, S Hanjitsuwan, N Damrongwiriyanupap, et al., Effect of sodium hydroxide and sodium silicate solutions on strengths of alkali activated high calcium fly ash containing Portland cement, KSCE J Civ Eng 21 (6) (2016) 2202–2210 [62] T Phoo-Ngernkham, V Sata, S Hanjitsuwan, et al., High calcium fly ash geopolymer mortar containing Portland cement for use as repair material, Constr Build Mater 98 (2015) 482–488 [63] T Phoo-Ngernkham, S Hanjitsuwan, C Suksiripattanapong, et al., Flexural strength of notched concrete beam filled with alkali-activated binders under different types of alkali solutions, Constr Build Mater 127 (2016) 673–678 [64] T Phoo-ngernkham, A Maegawa, N Mishima, et al., Effects of sodium hydroxide and sodium silicate solutions on compressive and shear bond strengths of FA–GBFS geopolymer, Constr Build Mater 91 (2015) 1–8 ... between new and existing concretes and the relative stiffness between the substrate concrete and added concrete layer Consequently, the achievements of the bonding mechanism of the concrete- concrete... roughness on the bonding performances The bond strength between the concrete and mortar layer is also affected by the mechanical occlusal interaction between the mortar layer and the concrete Interface... mechanism of the bonding interface between the mortar layer and the concrete is the same as the interface between old concrete and new concrete [45,46], this is because the bonding action in these