Influence of surface preparation method on the bond behavior of externally bonded CFRP reinforcements in concrete

materials Article Influence of Surface Preparation Method on the Bond Behavior of Externally Bonded CFRP Reinforcements in Concrete Sérgio Soares, JoséSena-Cruz *, JoséRicardo Cruz and Pedro Fernandes Institute for Sustainability and Innovation in Structural Engineering (ISISE), Department of Civil Engineering, University of Minho, Azurém Campus, 4800-058 Guimarães, Portugal; [email protected] (S.S.); [email protected] (J.R.C.); [email protected] (P.F.) *Correspondence: [email protected]; Tel.: +351-253-510-200 Received: 18 November 2018; Accepted: 28 January 2019; Published: 29 January 2019   Abstract: In last decades significant investigation has been carried out in order to predict the bond strength of externally bonded reinforcement (EBR) systems with carbon fiber reinforced polymer (CFRP) materials in concrete and, as consequence of that, many analytical expressions can be found in the literature, including in standards. However, these expressions do not account for the influence of several parameters on bond behavior such as the type of surface preparation which is a mandatory and critical task in the strengthening application. The present work gives contributions to reduce this lack of knowledge. For this purpose, an experimental program composed of single-lap shear tests was carried out, the main parameters studied being: (i) the type of concrete surface preparation (i.e., grinding and sandblasting) and (ii) the bond length. Prior to the application of the EBR CFRP system, the roughness level provided by the different methods of surface preparation was characterized by a laser sensor. Test results revealed that sandblasting concrete surface preparation yielded higher values, in terms of maximum shear force and fracture energy. Finally, existing expressions in standards were upgraded in order to account for the concrete surface roughness level in the estimation of the bond strength. Keywords: EBR; CFRP laminates; concrete surface preparation; roughness characterization; bond behavior; analytical predictions 1. Introduction In last decades, fiber reinforced polymer (FRP) materials have been increasingly proposed to strengthen reinforced concrete (RC) structures [ 1 – 3 ]. This approach has been demonstrated as an effective alternative solution to the traditional strengthening methods which typically employ reinforced concrete and/or steel. FRP materials have shown to be a useful and efficient solution due to their good mechanical properties, lightness, resistance to corrosion, low thermal conductivity, good fatigue behavior and easy installation procedures [ 4 , 5 ]. Different strengthening techniques have been developed for applying FRP. The first one, Externally Bonded Reinforcement (EBR), appeared in the 1980s [ 6 , 7 ], where the FRP laminate is glued onto the tensile face of the RC element. A few years later, other techniques emerged such as the near-surface mounted (NSM), where the FRP laminate or bar is inserted onto a groove previously cut in the concrete cover. According to the literature, when FRP are used as reinforcement material, bond behavior between this and concrete substrate is generally a critical issue for both EBR [ 8 – 19 ] and NSM techniques [ 13 , 16 , 20 – 24 ] that directly influence the effectiveness of the structural reinforcement. The strengthening performance of these techniques depends significantly on the resistance of the concrete cover, which is normally Materials 2019,12, 414; doi:10.3390/ma12030414 www.mdpi.com/journal/materials Materials 2019,12, 414 2 of 20 the most degraded concrete region in the structure and the concrete surface condition. As a result, premature failure of FRP reinforcement can occur and, generally, its full mechanical capacity is not mobilized, mainly when adopting the EBR technique. In attempt to overcome these issues, other strengthening techniques with composite materials have emerged such as Mechanically Fastened and Externally Bonded Reinforcement (MF-EBR) [ 4 , 25 ], which uses multi-directional carbon fiber laminates, simultaneously glued and anchored to concrete, and Mechanically Fastened FRP (MF-FRP) [ 25 – 29 ] using only steel anchors to fix the laminate to the substrate. Despite their potential, these techniques also reveal some shortcomings such as (i) the possible concrete damage during anchoring, (ii) the limited opportunity of anchor’s installation in the presence of congested internal reinforcement in the members to be strengthened and (iii) other problems related to the stress concentration and bearing failures of the systems leading to slippage of FRP and loss of bond. The present work focuses on the bond behavior characterization between EBR CFRP system reinforcement and the concrete surface. As many researchers verified in their experimental works when this technique was applied, the bond strength depends on several factors, including mechanical and physical properties of concrete, composite material and epoxy adhesive which prevent the fully FRP tensile strength to be attained due to the premature debonding failure that typically occurs [ 12 , 13 , 15 ]. Moreover, it is well known that the concrete surface preparation and bonding technique are crucial tasks for adequate installation of an EBR FRP system to delay the brittle debonding failure. Therefore, it is a key issue to proceed a preparation method that ensures the cleanliness, the soundness and a proper concrete surface roughness level [ 11 , 12 ]. More specifically, the concrete surface preparation process includes the removal of unsound concrete, strength verification and opening of the pore structure. The methods that have been most used by the scientific community are sandblasting, grinding, brushing, scarifying, steel shot blasting and bush hammering with different advantages and disadvantages associated, in terms of roughness level provided, cost, processing time and application difficulties. Through bond characterization tests between concrete surface and FRP, some authors demonstrated that the bond strength is higher when an effective surface preparation method is used due to the higher surface roughness level that provides better adhesion along the system interface [11,12,18,19]. In terms of standard guidelines, the influence of the concrete surface roughness on the EBR system performance and bond strength has been recognized in a comprehensive manner [ 1 , 2 , 30 , 31 ]. According to the International Federation for Structural Concrete (fib) [ 2 ], the concrete substrate should be roughened and made laitance and contamination free, in such a way that the concrete quality can be utilized in an optimum way. This guideline recommends that the surface preparation is done preferably by means of high-pressure blasting (sand, grit, water jet blasting) or grinding. In turn, the ACI Committee 440 [ 31 ] proposes that at least the application of abrasive or water blasting techniques for surface preparation should be performed. On the other hand, the Italian National Research Council [ 1 ] highlights the importance of a quality control that includes the determination of concrete conditions, removal of any deteriorated or loose concrete, cleaning and protection from corrosion of existing steel reinforcement, and finally substrate preparation for receiving the selected FRP reinforcement. In accordance with this standard, once the quality control of the substrate had been performed, the deteriorated concrete had been removed, the concrete cross section had been restored, and the existing steel reinforcement had been properly treated, sandblasting of the concrete surface shall be performed. Although the main guidelines recognize that concrete surface preparation prior the installation of the EBR FRP is an important task, the analytical formulations provided to predict the system bond strength still do not consider the influence of the use of different methodologies to do that. Possible reasons for this reside on the fact that different concrete preparation methods provide different roughness levels of the surface. Additionally, sometimes it is difficult to obtain a uniform surface roughness level, because this depends on human skills in the execution of the surface preparation method and it becomes a critical aspect when specimens under the same test conditions are produced. Therefore, a good scientific development would be to quantify the roughness level of the concrete Materials 2019,12, 414 3 of 20 surface before the reinforcement procedure and to understand how this influences the bond behavior of the EBR FRP system, when different methodologies of surface preparation are used. Then, the next step would be to introduce this influence on the bond strength prediction formulations. To measure the concrete surface roughness level after the application of different methods of surface preparation, several authors have used lasers profilometers [ 11 , 32 , 33 ]. This approach has proven to be very effective for quantifying the roughness level of the concrete surface, and can be the key to correlating this result with the bond behavior of the strengthening system. The use of this technology is something that is feasible in fieldwork, namely in a real application of the EBR FRP system. Nowadays, there are already commercial and patented systems that allow the assessment of the quality control of the concrete surface preparation. Thus, concrete surface roughness should be a requirement at the execution stage of the intervention and subject to quality control before proceeding to the concrete element strengthening. The present paper aims at contributing to a better understanding the influence of the use of different concrete surface preparation methodologies on the bond behavior of CFRP EBR systems. For this purpose, an experimental program composed of single-lap shear tests was carried out. This was performed by adopting an experimental set-up developed by the authors and the main parameters studied were the type of concrete surface preparation (SB—sandblasting and GR—grinding) and the bond length between CFRP and concrete surface. Prior to the application of the EBR CFRP system, the roughness level provided by the different methods of surface preparation was characterized by a laser sensor, in order to make it possible to correlate these measurements with the results obtained in the bond tests which characterize the interface behavior between concrete and CFRP laminate. Then, the formulation proposed by the Italian National Research Council [ 1 ] was updated in order to account for the roughness level of the concrete surface on the estimation of the bond strength. 2. Experimental Program 2.1. Test Program The main objective of the present investigation was to characterize the bond behavior of concrete elements with EBR CFRP systems, considering the following study variables: (i) the concrete surface preparation method and (ii) the bond length. The program was composed of 24 single-lap shear tests divided into two main groups according to the concrete surface preparation methodology used: (i) GR—grinding and (ii) SB—sandblasting. The selection of these two methods of concrete surface preparation was based on the fact that these two have been the most used in structural strengthening with the EBR technique. In each group, three different bond lengths (L b ) were considered: (i) 150 mm, (ii) 200 mm and (iii) 250 mm. According to some provided analytical formulations [ 1 , 2 , 8 ], the theoretical value of the effective bond length ranges between 191 and 217 mm. This means that the L b values adopted represent, respectively, a lower, an approximate, and a higher value than the theoretical effective bond length. Thus, it is possible to perform an analysis of the influence of the L b adopted on the EBR CFRP system bond behavior. Consequently, six series were adopted, each one composed of four specimens in the same conditions in order to ensure reliable results. The generic denomination of the test specimens is X_LbY_Z, where X represents the concrete surface preparation method (GR or SB), Y is the bond length in millimetres (150, 200 or 250) and Z is the specimen order number tested under the same conditions (1, 2, 3 or 4). 2.2. Specimen’s Geometry, Experimental Set-Up and Instrumentation Figure 1depicts the test set-up and instrumentation adopted in the experimental program for the case the specimen with a L b of 200 mm. Concrete blocks of 400 × 200 × 200 mm 3 were used, where two CFRP laminate strips were applied in opposite faces (parallel to the casting direction), according to the EBR technique. Pultruded CFRP laminate strips with 50 mm of width and 1.2 mm of thickness were used. To avoid premature failure by concrete rip off ahead of the loaded end, the bonded lengths Materials 2019,12, 414 4 of 20 started 100 mm apart, from the extremity. The concrete specimen was placed horizontally on a steel plate (support S1) with 70, 300 and 550 mm of thickness, width and length, respectively. Support S1 was fixed to the stiff base of the testing steel frame system through eight M16 steel threaded rods. To assure negligible horizontal displacements of the concrete block in the loading direction during the test, a steel plate (support S2) was placed in the bottom front part of the concrete block, which acted as a reaction element at a height of 50 mm. To minimize vertical displacements during the test, a steel plate (support S3) of 50 × 70 × 300 mm 3 was placed in the rear top part of the concrete block. Both support S2 and S3 were fixed to the support S1 through two M20 steel threaded rods. The tests were performed using a servo-controlled equipment (INEGI, Porto, Portugal). The applied force was measured through a load cell of 200 kN maximum carrying capacity (linearity error of 0.05% F.S.), placed between the actuator and the grip used to fix the CFRP laminate during the test. The relative displacement between the CFRP and the concrete (slip) at the loaded end section (s l ) was computed as the average of displacements measured by the linear variable displacement transducers (LVDT) 1 and 2 with a stroke of ± 5 mm (linearity error of 0.24% F.S.). The tests were performed under displacement control at the loaded end through LVDT2 (rate of 2 µ m/s). Additionally, a series of four strain gauges TML BFLA-5-3-3L (SG1 to SG4) were placed along the CFRP laminate centerline to measure the longitudinal strains during the loading process (Figure 1). Materials 2019, 12, 414 4 of 20 assure negligible horizontal displacements of the concrete block in the loading direction during the test, a steel plate (support S2) was placed in the bottom front part of the concrete block, which acted as a reaction element at a height of 50 mm. To minimize vertical displacements during the test, a steel plate (support S3) of 50 × 70 × 300 mm3 was placed in the rear top part of the concrete block. Both support S2 and S3 were fixed to the support S1 through two M20 steel threaded rods. The tests were performed using a servo-controlled equipment (INEGI, Porto, Portugal). The applied force was measured through a load cell of 200 kN maximum carrying capacity (linearity error of 0.05% F.S.), placed between the actuator and the grip used to fix the CFRP laminate during the test. The relative displacement between the CFRP and the concrete (slip) at the loaded end section (sl) was computed as the average of displacements measured by the linear variable displacement transducers (LVDT) 1 and 2 with a stroke of ±5 mm (linearity error of 0.24% F.S.). The tests were performed under displacement control at the loaded end through LVDT2 (rate of 2 μm/s). Additionally, a series of four strain gauges TML BFLA-5-3-3L (SG1 to SG4) were placed along the CFRP laminate centerline to measure the longitudinal strains during the loading process (Figure 1). (a) (b) Figure 1. Experimental set-up and instrumentation: (a) scheme; (b) photo. Note: units in [mm]. 2.3. Specimen’s Preparation According to the test program, two different methods of concrete surface preparation were studied. The surface preparation consists of removing the superficial concrete layer with inappropriate characteristics in order to obtain a clean and healthy surface with absence of the laitance layer and with an adequate roughness level, showing the aggregates. The aim was to understand the influence of the different methodologies in terms of providing concrete surface roughness and its consequence on the bond behavior of the EBR system. The two concrete surface preparation methods used in the experimental program are presented in Figure 2: (i) GR—grinding and (ii) SB—sandblasting. Both surface preparation methods used are relatively easy to apply. The GR is a mechanical method of surface preparation and consists of removing the unsound concrete layer through rotating shafts with sprockets that progressively wear out the surface layer. In this case, a grinding wheel was used until the aggregates had become visible (see Figure 2a). In turn, SB is a process of surface preparation based on particles’ impacts, being one of the most used techniques. This consists of the projection of small particles against the surface in order to cause surface abrasion and wear due to the high speed of projection (see Figure 2b), allowing a faster concrete surface preparation than the GR method. However, the SB method application requires professional equipment and skilled labor. After surface preparation, the treated surface was cleaned by highpressure water jet (Figure 2c) to remove the debris and dust resulting from the treatment applied, ensuring a good adhesion between the concrete surface and the reinforcing system. The next steps of preparing the specimens were performed after the specimens became dry. Fl Lb100 LVDT 1 / 2 (loaded end) 200 70 350 50 50 70 505070 10 SG1SG2SG3SG4 CFRP laminate projection 50 Support S1 Support S2 Support S3 Concrete block CFRP (free end) Figure 1. Experimental set-up and instrumentation: (a) scheme; (b) photo. Note: units in [mm]. 2.3. Specimen’s Preparation According to the test program, two different methods of concrete surface preparation were studied. The surface preparation consists of removing the superficial concrete layer with inappropriate characteristics in order to obtain a clean and healthy surface with absence of the laitance layer and with an adequate roughness level, showing the aggregates. The aim was to understand the influence of the different methodologies in terms of providing concrete surface roughness and its consequence on the bond behavior of the EBR system. The two concrete surface preparation methods used in the experimental program are presented in Figure 2: (i) GR—grinding and (ii) SB—sandblasting. Both surface preparation methods used are relatively easy to apply. The GR is a mechanical method of surface preparation and consists of removing the unsound concrete layer through rotating shafts with sprockets that progressively wear out the surface layer. In this case, a grinding wheel was used until the aggregates had become visible (see Figure 2a). In turn, SB is a process of surface preparation based on particles’ impacts, being one of the most used techniques. This consists of the projection of small particles against the surface in order to cause surface abrasion and wear due to the high speed of projection (see Figure 2b), allowing a faster concrete surface preparation than the GR method. However, the SB method application requires professional equipment and skilled labor. After surface Materials 2019,12, 414 5 of 20 preparation, the treated surface was cleaned by high-pressure water jet (Figure 2c) to remove the debris and dust resulting from the treatment applied, ensuring a good adhesion between the concrete surface and the reinforcing system. The next steps of preparing the specimens were performed after the specimens became dry. Materials 2019, 12, 414 5 of 20 (a) (b) (c) Figure 2. Concrete surface preparation: (a) grinding; (b) sandblasting; (c) surface cleaning. 2.4. Concrete Surface Roughness Characterization Considering the different preparation methodologies of the concrete surface (GR and SB), the roughness of these distinct surfaces was measured and analyzed. The main purpose was to relate the different methodologies to the provided concrete surface roughness and with the bond between CFRP laminate and concrete substrate. The procedure involved the use of a laser sensor, whose main characteristics are presented in Table 1. Three longitudinal profiles (with 20 mm apart from each other—transverse direction) were analyzed by the laser sensor in each of the 24 surface areas where the EBR CFRP system was installed. The roughness measurement through the laser sensor allowed to obtain qualitative and quantitative information of the different surfaces. The configuration of the surface roughness measuring system is also stated in Table 1. The laser sensor was connected to a metallic plate, which was part of a mechanism conceived to produce a slow displacement at a constant rate, as well as to allow the displacement of the laser sensor during scanning at a parallel and rectilinear path relatively to the scanned surface. The displacement rate was 8.8 mm/s and the data acquisition rate was 34 Hz, leading to consecutive readings spaced at 0.3 mm. Representative roughness profiles provided by the two methods of surface preparation and obtained by the laser sensor along 50 mm of the bonded length are shown in Figure 3. The results clearly show that the SB method produced a higher level of surface roughness than the GR method. Figure 2. Concrete surface preparation: (a) grinding; (b) sandblasting; (c) surface cleaning. 2.4. Concrete Surface Roughness Characterization Considering the different preparation methodologies of the concrete surface (GR and SB), the roughness of these distinct surfaces was measured and analyzed. The main purpose was to relate the different methodologies to the provided concrete surface roughness and with the bond between CFRP laminate and concrete substrate. The procedure involved the use of a laser sensor, whose main characteristics are presented in Table 1. Three longitudinal profiles (with 20 mm apart from each other—transverse direction) were analyzed by the laser sensor in each of the 24 surface areas where the EBR CFRP system was installed. The roughness measurement through the laser sensor allowed to obtain qualitative and quantitative information of the different surfaces. The configuration of the surface roughness measuring system is also stated in Table 1. The laser sensor was connected to a metallic plate, which was part of a mechanism conceived to produce a slow displacement at a constant rate, as well as to allow the displacement of the laser sensor during scanning at a parallel and rectilinear path relatively to the scanned surface. The displacement rate was 8.8 mm/s and the data acquisition rate was 34 Hz, leading to consecutive readings spaced at 0.3 mm. Representative roughness profiles provided by the two methods of surface preparation and obtained by the laser sensor along 50 mm of the bonded length are shown in Figure 3. The results clearly show that the SB method produced a higher level of surface roughness than the GR method. Table 1. Properties of laser sensor SICK OD2—N50W10U0 and roughness measurement system. Parameter Value Roughness Measurement System Measuring range 40 to 60 mm Materials 2019, 12, 414 6 of 20 Table 1. Properties of laser sensor SICK OD2—N50W10U0 and roughness measurement system. Parameter Value Roughness Measurement System Measuring range 40 to 60 mm Resolution 5 μm Repeatability 15 μm Measuring frequency 2 kHz Light spot dimension (distance) 0.5 mm × 1 mm (50 mm) (a) (b) Figure 3. Roughness profile when different surface preparation methods were applied: (a) grinding; (b) sandblasting. To quantitatively characterize the roughness level provided by each of the surface preparation methods several statistical indicators were considered, such as the mean roughness coefficient (Rm) and the root mean square (Rq). These indicators can be determined using Equations (1) and (3), respectively. In these equations, the parameters l, z(x) and n are the evaluation length, the profile height at position x and the number of scan readings performed by the laser sensor, respectively. R=1𝑙 |𝑧󰇛𝑥󰇜−𝑧  |   .𝑑𝑥 ≈ 1𝑛 |𝑧−𝑧  |   , (1) z= 1𝑙 𝑧󰇛𝑥󰇜 .𝑑𝑥 ≈ 1𝑛 𝑧󰇛𝑥󰇜    , (2) 0 1020304050 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Z [mm] Length [mm] GR 0 1020304050 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Z [mm] Length [mm] SB Resolution 5 µm Repeatability 15 µm Measuring frequency 2 kHz Light spot dimension (distance) 0.5 mm ×1 mm (50 mm) Materials 2019,12, 414 6 of 20 Materials 2019, 12, 414 6 of 20 Table 1. Properties of laser sensor SICK OD2—N50W10U0 and roughness measurement system. Parameter Value Roughness Measurement System Measuring range 40 to 60 mm Resolution 5 μm Repeatability 15 μm Measuring frequency 2 kHz Light spot dimension (distance) 0.5 mm × 1 mm (50 mm) (a) (b) Figure 3. Roughness profile when different surface preparation methods were applied: (a) grinding; (b) sandblasting. To quantitatively characterize the roughness level provided by each of the surface preparation methods several statistical indicators were considered, such as the mean roughness coefficient (Rm) and the root mean square (Rq). These indicators can be determined using Equations (1) and (3), respectively. In these equations, the parameters l, z(x) and n are the evaluation length, the profile height at position x and the number of scan readings performed by the laser sensor, respectively. R=1𝑙 |𝑧󰇛𝑥󰇜−𝑧  |   .𝑑𝑥 ≈ 1𝑛 |𝑧−𝑧  |   , (1) z= 1𝑙 𝑧󰇛𝑥󰇜 .𝑑𝑥 ≈ 1𝑛 𝑧󰇛𝑥󰇜    , (2) 0 1020304050 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Z [mm] Length [mm] GR 0 1020304050 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Z [mm] Length [mm] SB Figure 3. Roughness profile when different surface preparation methods were applied: ( a ) grinding; (b) sandblasting. To quantitatively characterize the roughness level provided by each of the surface preparation methods several statistical indicators were considered, such as the mean roughness coefficient (R m ) and the root mean square (R q ). These indicators can be determined using Equations (1) and (3), respectively. In these equations, the parameters l,z(x) and nare the evaluation length, the profile height at position xand the number of scan readings performed by the laser sensor, respectively. Rm=1 l l Z 0 |z(x)−z|·dx ≈1 n n ∑ i=1 |zi−z|, (1) z=1 l l Z 0 z(x)·dx ≈1 n n ∑ i=1 z(x), (2) Rq=s1 n n ∑ i=1 z2 i. (3) The statistical indicators used were then computed for the two different types of surface preparation and the results from the roughness assessment are presented in Table 2. The comparison between the value of the parameters R m and R q provided by each surface preparation method is established in the Figure 4. Table 2. Results from the roughness assessment (average values). Series Roughness Parameter [mm] Mean Roughness Coefficient (Absolute Value), RmRoot Mean Square, Rq GR_Lb150 0.137 (21.8%) 0.260 (32.7%) GR_Lb200 0.120 (10.3%) 0.181 (11.5%) GR_Lb250 0.165 (13.4%) 0.329 (29.4%) Mean value 0.141 0.256 SB_Lb150 0.533 (17.9%) 0.655 (21.8%) SB_Lb200 0.460 (14.8%) 0.570 (9.4%) SB_Lb250 0.491 (31.1%) 0.691 (32.7%) Mean value 0.494 0.639 Notes: the values in parentheses are coefficients of variation (CoV). Materials 2019,12, 414 7 of 20 Materials 2019, 12, 414 7 of 20 R= 1𝑛 𝑧   . (3) The statistical indicators used were then computed for the two different types of surface preparation and the results from the roughness assessment are presented in Table 2. The comparison between the value of the parameters Rm and Rq provided by each surface preparation method is established in the Figure 4. Table 2. Results from the roughness assessment (average values). Series Roughness Parameter [mm] Mean Roughness Coefficient (Absolute Value), Rm Root Mean Square, Rq GR_Lb150 0.137 (21.8%) 0.260 (32.7%) GR_Lb200 0.120 (10.3%) 0.181 (11.5%) GR_Lb250 0.165 (13.4%) 0.329 (29.4%) Mean value 0.141 0.256 SB_Lb150 0.533 (17.9%) 0.655 (21.8%) SB_Lb200 0.460 (14.8%) 0.570 (9.4%) SB_Lb250 0.491 (31.1%) 0.691 (32.7%) Mean value 0.494 0.639 Notes: the values in parentheses are coefficients of variation (CoV). (a) (b) Figure 4. Roughness level comparison provided by each surface preparation method (GR and SB) in terms of: (a) mean roughness (Rm); (b) root mean square (Rq). Note: the values in parentheses represents the percentage increase when SB was applied instead of GR. The results of each statistical indicator were obtained by the average value of the four test specimens with the same test conditions, i.e., the specimens with the same bond length and the same concrete surface type. According to the results, it is possible to verify that the SB method allowed to obtain considerably higher roughness levels than the GR. In terms of Rm and Rq, the SB method allowed an average increase of 250% and 150%, respectively, in relation to the surface preparation with GR. It is important to note that the values of coefficient of variation (CoV) are sometimes significant due to the difficulty of obtaining a uniform surface roughness level. This depends on the human skill in the execution of the surface preparation method, as well as on the homogeneity of the material, and becomes a critical aspect when specimens under the same test conditions are produced, in which, theoretically, the same levels of roughness would be desired. 0.0 0.2 0.4 0.6 Mean roughness, R m [mm] L b =150 [mm] L b =200 [mm] L b =250 [mm] GR SB L b =200 [mm] L b =250 [mm] (+289%) (+283%) (+198%) 0.0 0.2 0.4 0.6 0.8 Root mean square, R q [mm] L b =150 [mm] L b =250 [mm] GR SB L b =200 [mm] (+152%) (+215%) (+110%) Figure 4. Roughness level comparison provided by each surface preparation method (GR and SB) in terms of: ( a ) mean roughness (R m ); ( b ) root mean square (R q ). Note: the values in parentheses represents the percentage increase when SB was applied instead of GR. The results of each statistical indicator were obtained by the average value of the four test specimens with the same test conditions, i.e., the specimens with the same bond length and the same concrete surface type. According to the results, it is possible to verify that the SB method allowed to obtain considerably higher roughness levels than the GR. In terms of R m and R q , the SB method allowed an average increase of 250% and 150%, respectively, in relation to the surface preparation with GR. It is important to note that the values of coefficient of variation (CoV) are sometimes significant due to the difficulty of obtaining a uniform surface roughness level. This depends on the human skill in the execution of the surface preparation method, as well as on the homogeneity of the material, and becomes a critical aspect when specimens under the same test conditions are produced, in which, theoretically, the same levels of roughness would be desired. 2.5. Material Characterization The material characterization included the assessment of the mechanical properties of the materials involved in this experimental program, mainly: (i) concrete, (ii) CFRP laminate strip and (iii) epoxy adhesive. Table 3shows the obtained results. Only one batch was used for casting all the test specimens. The used concrete had the following characteristics: class C25/30; exposure class XC2; maximum aggregate size of 12.5 mm; slump S4. The average Young’s modulus (E cm ) and average compressive strength of concrete (f cm ) were assessed using five cylinders with 300 mm of height and 150 mm of diameter and following the recommendations NP EN 12390-13:2014 [34] and NP EN 12390-3:2011 [35], respectively, at 28 days of concrete age. The CFRP laminate strips used in the experimental work (type: S&P Laminates CFK) consists of unidirectional carbon fibers held together by an epoxy vinyl ester resin matrix. These were produced by S&P ® Clever and Reinforcement with the trademark CFK 150/2000. According to the manufacturer, this type of CFRP laminate presents a smooth external surface and the fiber volume content is about 70%. Pultruded CFRP laminate strips with 50 mm of width and 1.2 mm of thickness were used. The tensile mechanical properties included the average Young’s modulus (E f ), the average tensile strength (f fu ) and the average strain at the peak stress ( εfu ) were assessed using five samples and following the recommendation ISO 527-5:2009 [36]. The epoxy adhesive (type: S&P Resin 220 epoxy adhesive), produced by the same supplier as for the CFRP laminate, was used as bond agent to fix the reinforcing material to the concrete substrate. This adhesive is available in the form of two components (Component A—resin and Component B—hardener) that need to be mixed. According to the manufacturer, the ratio A:B in the mix should be 4:1. In the scope of the present experimental work, the epoxy adhesive was not characterized, and its Materials 2019,12, 414 8 of 20 mechanical properties were collected from a previous work performed by some of the authors of the present work [ 21 ]. In this context, six samples were prepared and tested following the recommendation ISO 527-2:2012 [ 37 ]. These results are also presented in Table 3, in terms of average tensile strength (f a ), average Young’s modulus (Ea) and average strain at the peak stress (εa). Table 3. Material characterization (average values). Concrete Age of curing fcm (MPa) Ecm (GPa) 28 days 33.4 (4.33%) 30.8 (2.84%) CFRP Cross-section geometry (mm2)ffu (MPa) Ef(GPa) εfu (%) 50 ×1.2 2222.4 (4.7%) 176.4 (2.0%) 1.2 (2.2%) Adhesive Type of adhesive fa(MPa) Ea(GPa) εa(%) S&P Resin 220 epoxy adhesive a22.0 (4.5%) 7.2 (3.7%) 0.36 (15.2%) Notes: the values in parentheses are coefficients of variation (CoV); aResults collected from [21]. 3. Results and Discussion 3.1. Main Results In this section the main results obtained in the single-lap shear tests are presented. Table 4 summarizes the average results for each series obtained in the experimental program through several parameters that characterize the bond behavior between the CFRP and the concrete, mainly: Fl,max is the maximum shear force; s l,max is the loaded end slip at F l,max ;G f is the total energy up to 0.3 mm of loaded end slip (common s l value attained in all the tests); εfmax is the CFRP normal strain at F l,max ; and, finally, FM represents the failure mode observed in each specimen tested. Table 4. Main results obtained from the single-lap shear tests (average values). Series Fl,max (kN) sl,max (mm) Gf (kN·mm) εfmax (%) FM GR_Lb150 23.8 (6.9%) 0.34 (11.7%) 5.08 (5.4%) 0.22 D [4] GR_Lb200 23.8 (4.7%) 0.34 (18.0%) 5.37 (6.6%) 0.22 D [4] GR_Lb250 26.8 (7.9%) 0.35 (19.2%) 5.89 (6.2%) 0.25 D [4] SB_Lb150 27.2 (6.7%) 0.31 (12.5%) 5.79 (3.0%) 0.26 D [4] SB_Lb200 30.2 (9.7%) 0.41 (8.2%) 5.92 (3.7%) 0.29 D [4] SB_Lb250 31.3 (5.7%) 0.52 (24.0%) 6.07 (2.9%) 0.30 D [4] Notes: the values in parentheses are coefficients of variation (CoV); D = cohesive debonding in the concrete; the values between brackets [ ] are the number of specimens with the specified failure mode. From a general analysis, a significant influence of the surface preparation method used on the bond behavior is verified. In fact, the SB method proved to be more efficient than GR because it allowed an increase in the bond strength against debonding failure. Therefore, when the same bond length was adopted, the SB method provided an increase of F l,max ,s l,max , and G f in relation to the GR method. In turn, the L b adopted also showed to have influence on the bond behavior. With the Materials 2019,12, 414 9 of 20 increase of L b , a bond strength increase was observed, translated by the increase of the necessary shear force to induce debonding failure of the EBR system. Furthermore, when the same surface preparation method was used, the increase of Lbprovided an increase of sl,max and Gf. In all tests, the debonding of the CFRP laminates occurred with a thin layer of concrete. The shear force vs. loaded end slip curves (F l -s l ) obtained in all tests carried out in this experimental work are presented in Figure 5. In each graph, the responses corresponding to the four specimens tested under the same conditions are gathered. Later, a more detailed analysis about the influence of the study variables is performed. Materials 2019, 12, 414 10 of 20 (a) (b) (c) (d) (e) (f) Figure 5. Shear force vs. loaded end slip obtained in each series: (a) GR_Lb150; (b) SB_Lb150; (c) GR_Lb200; (d) SB_Lb200; (e) GR_Lb250; (f) SB_Lb250. 3.2. Failure Modes As shown in Table 4, cohesive debonding in the concrete failure mode was observed in all the single-lap shear tests carried out, regardless of the surface preparation method used and bond length adopted. After debonding failure, a concrete layer attached to the epoxy adhesive and CFRP laminate was analyzed. This failure mechanism was also verified in others investigation works, e.g., [11,12]. During the loading process, inclined microcracks arose on the external concrete layer since the concrete tensile strength is lower than the corresponding adhesive’s strength. The emergence of these microcracks occurred from the loaded end to the free end section, accompanying the debonding process of the EBR CFRP system that starts when the bond strength is reached. Therefore, the bond strength is gradually lost in the successive loaded end sections during the loading process, and then other zones of bonded length are activated, providing its contribution to the stress transfer. While the 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 10 20 30 40 Shear force, F l [kN] Loaded end slip, s l [mm] GR_Lb150_1 GR_Lb150_2 GR_Lb150_3 GR_Lb150_4 0.00.10.20.30.40.50.60.70.8 0 10 20 30 40 Shear force, F l [kN] Loaded end slip, s l [mm] SB_Lb150_1 SB_Lb150_2 SB_Lb150_3 SB_Lb150_4 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 10 20 30 40 Shear force, F l [kN] Loaded end slip, s l [mm] GR_Lb200_1 GR_Lb200_2 GR_Lb200_3 GR_Lb200_4 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 10 20 30 40 Shear force, F l [kN] Loaded end slip, s l [mm] SB_Lb200_1 SB_Lb200_2 SB_Lb200_3 SB_Lb200_4 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 10 20 30 40 Shear force, F l [kN] Loaded end slip, s l [mm] GR_Lb250_1 GR_Lb250_2 GR_Lb250_3 GR_Lb250_4 0.00.10.20.30.40.50.60.70.8 0 10 20 30 40 Shear force, F l [kN] Loaded end slip, s l [mm] SB_Lb250_1 SB_Lb250_2 SB_Lb250_3 SB_Lb250_4 Figure 5. Shear force vs. loaded end slip obtained in each series: ( a ) GR_Lb150; ( b ) SB_Lb150; (c) GR_Lb200; (d) SB_Lb200; (e) GR_Lb250; (f) SB_Lb250. Materials 2019,12, 414 16 of 20 line in Figure 11a. In Equations (4) and (5), b f ,t f and E f are the width, thickness and Young’s modulus of the CFRP material, whereas ΓFm is the fracture energy. In turn, γRd is a corrective factor with a value of 1.25 and f b represents the mean bond strength obtained as a function of the parameter ΓFm . The fracture energy ( ΓFm ) is obtained by Equation (6) where k b is a geometrical corrective factor obtained as a function of the ratio between the width of the CFRP laminate (b f ) and the width of concrete specimen (b) (Equation (7)) and k G is an additional experimental corrective factor which, according to CNR guideline [ 1 ], for the case of pre-cured FRP systems the average value is 0.063 mm. The parameters fcm and fctm represent the average compressive and tensile strength of concrete, respectively. le=max(1 γRd·fbrπ2·Ef·tf·ΓFm 2, 200 mm), (4) Fmax =bf·p2·Ef·tf·ΓFm, (5) ΓFm =kb·kG·pfcm·fctm, (6) kb=s2−bf/b 1+bf/b≥1i f bf/b≥0.25. (7) Analyzing Figure 11a, it is remarkable that the formulation presented by the CNR [ 1 ] can be considered as a good prediction of bond strength when the concrete surface preparation method used has a limited efficiency. By comparing the prediction of F max provided by the guideline with the experimental values of maximum shear force obtained in SB specimens, it is clear that improvements in the formulation are needed. Therefore, in order to improve the analytical formulation taking into account the influence of the concrete surface roughness prior to the EBR system installation and allow a better prediction of the bond strength when a more effective surface preparation method is used, the following procedure was adopted: (i) using the Equation (5) and considering the value F max equal to the maximum shear force obtained from experimental tests, the average experimental values of ΓFm were calculated; then, (ii) using the values of ΓFm inside Equation (6) and all the other known parameters, the experimental values of k G were obtained. Figure 11b shows the relation between the experimental k G values and the mean roughness coefficient obtained in the concrete surface roughness characterization (R m ) through a linear tendency line (R 2 = 0.8). In the same figure, the k G value proposed by the Italian guideline is represented by a red line. As concluded by the accuracy of the maximum shear force prediction, it is also verified that the k G mean value suggested by the CNR [ 1 ] can accurately predict the experimental values obtained when the concrete surface preparation method adopted does not provide high levels of roughness. The same observation cannot be drawn when a more effective surface preparation method is used. In the SB specimens, the k G value obtained proved to be higher than that suggested by the formulation included in the CNR guideline. To improve the F max prediction (Equation (5)) maintaining the k G value suggested by the guideline, a new parameter (k R —roughness corrective factor) was introduced in the calculation of the average interface fracture energy ( ΓFm ), which is defined as a function of the mean roughness coefficient (R m ), as demonstrated by Equations (8) and (9). From GR to SB, values of k R ranged between 0.8 and 1.7, respectively. According to the obtained results, the CNR guideline [ 1 ] considers in its formulation (k R = 1) a surface preparation method slightly more effective than the GR (k R = 0.9, in average terms) and markedly less effective than the SB (k R = 1.4, in average terms). In the absence of any information about the concrete surface roughness, it is recommended to use a value of k R equal to 1, as suggested in the CNR standard. In order to assess the accuracy of the proposed analytical formulation, the average values of maximum shear force for each test series, obtained from the analytical model and from the experimental tests are compared with the prediction provided by the CNR guideline [ 1 ] in Figure 12a. The values of maximum shear force for each specimen, obtained from the analytical model and from the experimental Materials 2019,12, 414 17 of 20 tests are also compared in Figure 12b. From these figures, it is possible to verify the high accuracy of the proposed analytical model. The difference between analytical and experimental results was determined by Equation (10). Through the inclusion of the roughness corrective factor (k R ) on existing CNR formulation for predicting the F max , the error values between the analytical formulation results and the experimental results were reduced from 11.8% to 4.4%. kR=0.09Rm+0.04 0.063 =1.4 Rm+0.7, (8) ΓFm =kb·kG·kRpfcm·fctm, (9) % RPE =Fl,max,exp −Fl,max,anal Fl,max,exp ×100. (10) Materials 2019, 12, 414 17 of 20 surface preparation method is used. In the SB specimens, the kG value obtained proved to be higher than that suggested by the formulation included in the CNR guideline. (a) (b) Figure 11. Relationship between the following parameters: (a) Fl,max vs. Rm; (b) kG vs. Rm. To improve the Fmax prediction (Equation (5)) maintaining the kG value suggested by the guideline, a new parameter (𝑘R—roughness corrective factor) was introduced in the calculation of the average interface fracture energy (ГFm), which is defined as a function of the mean roughness coefficient (Rm), as demonstrated by Equations (8) and (9). From GR to SB, values of kR ranged between 0.8 and 1.7, respectively. According to the obtained results, the CNR guideline [1] considers in its formulation (kR = 1) a surface preparation method slightly more effective than the GR (kR = 0.9, in average terms) and markedly less effective than the SB (kR = 1.4, in average terms). In the absence of any information about the concrete surface roughness, it is recommended to use a value of kR equal to 1, as suggested in the CNR standard. In order to assess the accuracy of the proposed analytical formulation, the average values of maximum shear force for each test series, obtained from the analytical model and from the experimental tests are compared with the prediction provided by the CNR guideline [1] in Figure 12a. The values of maximum shear force for each specimen, obtained from the analytical model and from the experimental tests are also compared in Figure 12b. From these figures, it is possible to verify the high accuracy of the proposed analytical model. The difference between analytical and experimental results was determined by Equation (10). Through the inclusion of the roughness corrective factor (𝑘R) on existing CNR formulation for predicting the Fmax, the error values between the analytical formulation results and the experimental results were reduced from 11.8% to 4.4%. 𝑘=0.09R+0.04 0.063 =1.4 R+0.7, (8) Γ=𝑘.𝑘.𝑘  𝑓 . 𝑓 , (9) % RPE=𝐹,,−𝐹,, 𝐹,, ×100. (10) 0.0 0.2 0.4 0.6 0.8 20 25 30 35 40 F l,max = 17.7 R m + 23.0 R 2 = 0.8 (27.1 kN) SB Maximum shear force, F l,max [kN] Mean roughness, R m [mm] CNR-DT 200 (2013) GR 0.0 0.2 0.4 0.6 0.8 0.00 0.05 0.10 0.15 K G = 0.09 R m + 0.04 R 2 = 0.8 GR k G (CNR) = 0.063 k G [mm] Mean roughness, R m [mm] SB Figure 11. Relationship between the following parameters: (a)Fl,max vs. Rm; (b)kGvs. Rm. Materials 2019, 12, 414 18 of 20 (a) (b) Figure 12. Comparison between the experimental results, the analytical results and the prediction provided by National Research Council [1] (a) and the accuracy of the proposed analytical model (b). 4. Conclusions This paper presented and analyzed the obtained results from an experimental program composed of 24 single-lap shear tests in prismatic concrete specimens with CFRP laminates applied according to the EBR technique. The aim of this work was to study the influence of some important parameters on the bond behavior between concrete and the CFRP laminate, namely: (i) the type of concrete surface preparation method used before the application of EBR system that provides different levels of surface roughness (GR—grinding and SB—sandblasting) and (ii) the bond length (150, 200 and 250 mm). For this purpose, the roughness of the distinct concrete surfaces was assessed by using a laser sensor in order to allow the identification of its influence on the bond strength. Additionally, the formulation proposed by the Italian guideline CNR [1] for the prediction of maximum shear force of EBR FRP systems in concrete was updated/recalibrated, since the actual formulation does not account for the effect of concrete surface roughness. Thus, the following main conclusions can be highlighted: • Based on the measurements performed with the laser sensor, the results clearly showed that the SB method produced a higher level of concrete surface roughness than the GR. In terms of mean roughness coefficient (Rm) and the root mean square (Rq), the SB method yielded to an average increase of 250% and 150%, respectively, in relation to the surface preparation with GR; • Different roughness levels, provided by the different preparation methods of the concrete surface, resulted in different levels of bond strength; • Cohesive debonding in the concrete was the governed failure mode observed in all tests carried out, regardless of the surface preparation method used and bond length adopted. After debonding failure, a concrete layer attached to the epoxy adhesive and CFRP laminate was observed. In SB specimens, this concrete layer was considerably thicker than the layer observed in GR specimens, which reflects the better bond behavior provided by the SB method; • The analysis of the Fl-sl curves allowed to verify that the different preparation methods of the concrete surface did not significantly change the initial stiffness of the strengthening system. However, the debonding process started at higher shear force values in the case of SB specimens compared to the GR specimens. Thus, in turn, the increase of Lb value did not influence the Fl-sl curves shape neither in the initial branch nor in the stiffness degradation phase and the debonding process; • The use of the SB method instead of GR provided an increase of maximum shear force (Fl,max) and total energy (Gf), allowing a better use of the tensile strength capacity of the CFRP laminate; • The use of higher Lb values provided an increase of maximum shear force (Fl,max) and total energy (Gf); • Based on the obtained results from the experimental program, the influence of the surface roughness was included in the analytical formulation for predicting the maximum shear force 0 10 20 30 40 Maximum shear force, F l,max [kN] Analytical result Experimental result CNR-DT 200 (2013) (27.1 kN) GR_Lb200 GR_Lb250 SB_Lb200 SB_Lb250 10 20 30 40 50 10 20 30 40 50 Maximum analytical shear force, F l,max,anal [kN] Maximum experimental shear force, F l,max,exp [kN] Figure 12. Comparison between the experimental results, the analytical results and the prediction provided by National Research Council [1] (a) and the accuracy of the proposed analytical model (b). 4. Conclusions This paper presented and analyzed the obtained results from an experimental program composed of 24 single-lap shear tests in prismatic concrete specimens with CFRP laminates applied according to the EBR technique. The aim of this work was to study the influence of some important parameters on the bond behavior between concrete and the CFRP laminate, namely: (i) the type of concrete surface Materials 2019,12, 414 18 of 20 preparation method used before the application of EBR system that provides different levels of surface roughness (GR—grinding and SB—sandblasting) and (ii) the bond length (150, 200 and 250 mm). For this purpose, the roughness of the distinct concrete surfaces was assessed by using a laser sensor in order to allow the identification of its influence on the bond strength. Additionally, the formulation proposed by the Italian guideline CNR [ 1 ] for the prediction of maximum shear force of EBR FRP systems in concrete was updated/recalibrated, since the actual formulation does not account for the effect of concrete surface roughness. Thus, the following main conclusions can be highlighted: • Based on the measurements performed with the laser sensor, the results clearly showed that the SB method produced a higher level of concrete surface roughness than the GR. In terms of mean roughness coefficient (R m ) and the root mean square (R q ), the SB method yielded to an average increase of 250% and 150%, respectively, in relation to the surface preparation with GR; • Different roughness levels, provided by the different preparation methods of the concrete surface, resulted in different levels of bond strength; • Cohesive debonding in the concrete was the governed failure mode observed in all tests carried out, regardless of the surface preparation method used and bond length adopted. After debonding failure, a concrete layer attached to the epoxy adhesive and CFRP laminate was observed. In SB specimens, this concrete layer was considerably thicker than the layer observed in GR specimens, which reflects the better bond behavior provided by the SB method; • The analysis of the F l -s l curves allowed to verify that the different preparation methods of the concrete surface did not significantly change the initial stiffness of the strengthening system. However, the debonding process started at higher shear force values in the case of SB specimens compared to the GR specimens. Thus, in turn, the increase of L b value did not influence the F l -s l curves shape neither in the initial branch nor in the stiffness degradation phase and the debonding process; • The use of the SB method instead of GR provided an increase of maximum shear force (F l,max ) and total energy (G f ), allowing a better use of the tensile strength capacity of the CFRP laminate; • The use of higher L b values provided an increase of maximum shear force (F l,max ) and total energy (Gf); • Based on the obtained results from the experimental program, the influence of the surface roughness was included in the analytical formulation for predicting the maximum shear force proposed by CNR [ 1 ], through the inclusion of the roughness corrective factor (k R ) defined as a function of the mean roughness coefficient (R m ). The proposed analytical model proved to be able to accurately predict the bond strength of the EBR FRP system taking into account the roughness level of the concrete surface where the CFRP laminate is subsequently installed. Author Contributions: Conceptualization, validation and formal analysis, S.S. and J.S.-C.; methodology and investigation, S.S., J.S.-C., J.R.C. and P.F.; writing—original draft preparation, S.S.; writing—review and editing, S.S., J.S.-C., J.R.C.; supervision, J.S.-C.; funding acquisition, J.S.-C. Funding: This work was carried out in scope of the project FRPLongDur POCI-01-0145-FEDER-016900 (FCT PTDC/ECM-EST/1282/2014) funded by national funds through the Foundation for Science and Technology (FCT) and co-financed by the European Fund of the Regional Development (FEDER) through the Operational Program for Competitiveness and Internationalization (POCI) and the Lisbon Regional Operational Program and, partially financed by the project POCI-01-0145-FEDER-007633. Acknowledgments: The authors also like to thank the S&P Clever Reinforcement Ibérica Lda. company for providing the materials. The first author wishes also to acknowledge the grant SFRH/BD/131913/2017 provided by FCT. Conflicts of Interest: The authors declare no conflict of interest. Materials 2019,12, 414 19 of 20 References 1. Council, N.R. Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Existing Structures; CNR-DT 200R1; Advisory Committee on Technical Recommendations for Construction: Rome, Italy, 2013. 2. Fédération Internacionale Du Béton. Design and use of externally bonded FRP reinforcement for RC structures. Bulletin 2001,14. 3. Bank, L.C. Composites for Construction: Structural Design with FRP Materials; John Wiley & Sons: Hoboken, NJ, USA, 2006. [CrossRef] 4. Sena-Cruz, J.M.; Barros, J.A.; Coelho, M.R.; Silva, L.F. Efficiency of different techniques in flexural strengthening of RC beams under monotonic and fatigue loading. Constr. 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Testing Hardened Concrete. Part 3: Compressive Strength of Test Specimen; Instituto Português da Qualidade (IPO): Caparica, Portugal, 2011. 36. ISO 527-5. Plastics—Determination of Tensile Properties. Part 5: Test Conditions for Unidirectional Fibre-Reinforced Plastic Composites; International Organization for Standardization (ISO): Genève, Switzerland, 2009. 37. ISO 527-2. Plastics—Determination of Tensile Properties. Part 2: Test Conditions for Moulding and Extrusion Plastics; International Organization for Standardization (ISO): Genève, Switzerland, 2012. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).