Using CFRP for Enhancing of Circular Cutout in High Strength RC Deep Beam

Transcription

1 DOI 1.763/s Using CFRP for Enhancing of Circular Cutout in High Strength RC Deep Beam Alaa Alsaad, Abduljalil Sulaiman, and Jaafar A. Mohammed Received 27 Jun 215 Accepted 23 Jul 215 Abstract This paper presents the results of an experimental investigation of the using of carbon fiber reinforced polymer (CFRP) to enhance the high strength reinforced concrete (RC) deep beam in case of creating a circular cutout throughout its web. A set of 5 specimens were grouped and investigated to evaluate the structural behavior in terms of shear strength capacity and deflection. The specimens were 12mm long with rectangular cross-section of x5mm. Two symmetric circular cutouts of 15mm diameter were made at midpoint of each shear span. CFRP strips were installed around the cutouts in three different configurations in order to enhance the cutout zone. The results of this experimental work showed that the diagonal configuration of CFRP strips around the cutout was the best one, as the ultimate shear strength ratio with respect to control solid beam was.97, while it was.86 for control beam with cutouts. A significant positive effect was observed on the deflection for the beams that enhanced with CFRP strips. Also, CFRP sheet oriented diagonally around cutout was the most appropriate for reducing the deflection. Keywords- CFRP; circular cutout; deep beam; high strength concrete be affected [1] and hence, enhancement will be necessary to compensate the reduction of beam capacity. The beam with small openings may fail in two clearly different modes [2, 3]; beam-type failure and frame-type failure. The traditional shear design approach can be used, in the both cases, to predict the nominal shear resistance (Vn) where it is a combination of the concrete shear capacity, Vc, and the shear reinforcement capacity, Vs, crossing the failure plane. (1) Vn = Vc + Vs In case of beam-type failure, the simplified approach of the ACI Code (1995) [4] can be used to estimate the shear strength provided by concrete (Vc), as following : (2) where, bw = width of the web, d = effective depth, do = diameter of the opening. I. INTRODUCTION Reinforced concrete (RC) deep beam is an important structural element in many types of structures especially tall buildings in which it can be in transfer floor or as a link beam. Practically, high strength concrete is more appropriate for using with such type of structures. In order to facilitate the essential services (e.g. utility ducts and pipes), it is common to have openings throughout the beams where a typical steel reinforcement around the openings would be designed and provided. Sometimes, mistakes or some changes in mechanical design, obligates the engineers to make cutouts throughout the web of the cast beams in order to satisfy some requirements. As a result the structural behavior would While the shear strength (Vs) provided by shear reinforcement can be calculated based on the available stirrups across the plane of failure, Figure 1, within a distance of (dv - do). Thus: where, (3) Av = area of vertical legs of stirrups per spacing S, fyv = yield strength of stirrups. DOI: / _

2 B. Specimen details and test matrix A set of five specimens were investigated in order to evaluate the enhancement of circular cutout in high strength deep beam. All the specimens were 12mm long with mm effective span and the cross-section was a rectangular of x5mm. Deformed steel bars with a yield strength of 42 MPa were used for flexural and shear reinforcing with different nominal diameters. A scheme of a typical beam specimen is explained in Figure 2. Figure 1. Shear resistance, Vs, provided by shear reinforcement at an opening [2]. In the past few decades, many types of materials have been developed for the repair or strengthening of RC structures. Fiber reinforced polymer (FRP) is of those materials which has been widely used in construction engineering [5] because of its tensile strength, corrosion resistance, and easier handling, in addition to the economic advantages [6-9]. Many studies have investigated RC deep beams with openings [1-12] and using of FRP composites for strengthening of opening zone [13-15]. However, most of these studies have been conducted testing deep beams with rectangular openings, but no enough attention been paid to circular openings. The main goal of this study is to investigate the best configuration of CFRP strips for enhancing of high strength RC deep beams in case of creating a circular cutout throughout their webs. The purpose is to evaluate the improvement of performance in terms of ultimate shear capacity and deflection. II. EXPERIMENTAL PROGRAM A. Materials Concrete: High strength concrete was produced in the laboratory and used for casting all specimens. Moist curing was used for at least 28 days. The average value of concrete compressive strength at 28 days was 52MPa. The concrete mix proportions were 1:1.5:2 (cement: sand: gravel) by weight, with a water-cement ratio of.4. Crushed natural gravel with a maximum size of 12 mm and natural sand were used as coarse and fine aggregate, respectively. Composite materials: A unidirectional carbon fiber reinforced polymer (CFRP) from the SIKA Company was used for strengthening of specimens, and ASIKAdu33 adhesive was used in the process. Plywood moulds were fabricated to be used for casting of RC beams. The specimens were cast in a horizontal position. To overcome the difficulties of achieving the cutouts in concrete specimens, the web steel mesh at the desired positions were cut prior to concrete casting. Cylindrical pieces of polystyrene were installed to get the cutouts as shown in Figure 3. Two symmetric circular cutouts (15mm in diameter) were made at mid-point of each shear span (a). The specimens were grouped as explained in table I. Group (A) involved the control deep beams; solid beam and DBCN1 beam with cutouts. Group (B) composed of three beams to investigate the enhancement of cutout zones by using CFRP sheet. Three configurations were used for enhancement; (i) inner strip of CFRP over full width of beam, (ii) mm vertical and horizontal strips (both sides) of CFRP sheet around the cutouts, and (iii) mm diagonal strips (both sides) of CFRP sheet around the cutouts. The strengthening of beams followed the instructions provided by the manufacturer (SIKA). The two components of the adhesive material were mixed with a ratio of 1:4 by weight, and applied to the clean dry surface of the concrete using a paintbrush and roller. CFRP sheet was fixed manually with the fiber orientation as per experimental program. An extra layer of adhesive material was then applied on the installed sheet. C. Test setup and instrumentation In order to develop the deep beam action, all tests were conducted under shear span to depth ratio of.7. A testing machine with a capacity of 8 kn was used to load the beam until failure. Two symmetric point loads were applied monotonically. Deflections were measured by using two linear variable differential transducers (LVDTs) with a range of 1mm and accuracy of.1mm. The data were recorded by using a PC-based data acquisition system. Figure 4 shows the test set-up. 16

3 a =35 mm P/2 3 mm P/2 a =35 mm 2φ1 mm Mesh φ6 mm 5 mm 2φ12 mm 2φ16 mm mm mm 12 mm Figure 2. Typical details of specimen Figure 3. Moulds and steel mesh installation. Figure 4. Test set-up. TABLE I. Group Specimen DETAILS OF TESTED SPECIMENS Strengthening configuration Comments Control solid beam Strengthening: No Control beam with cutouts Strengthening: No Beam with cutouts Strengthening: inner strip of CFRP A B 17

4 III. Beam with cutouts Strengthening: mm vertical and horizontal strips of CFRP Beam with cutouts Strengthening: mm diagonal strips of CFRP Typical crack pattern of deep beam under two point loads was observed in control specimen (solid beam), Figure 5, while diagonal cracks at the face of circular cutout towards the support and point load were observed in control specimen (beam with cutouts) and the failure mode was distinctly governed by beam-type failure. For enhanced beams, there was no clear typical mode of failure except for the beam with inner strip of CFRP in which beam-type failure was also observed. RESULTS AND DISCUSSION A. Cracking and ultimate shear strength The results of all groups of beams were presented in table II. The ultimate shear strength of control beams ( and ) were used to evaluate the influence of cutout in deep beam. Ultimate shear capacity of the control beam, (beam with cutouts), was also used as a basis for finding the contribution of CFRP sheet in strengthening the cutout. Furthermore, the shear strengths of control beams were predicted based on simplified approach of ACI, Eq. (1) through Eq. (3), and been presented in table II. The reduction in shear strength due to the influence of cutout was 14%. The diagonal configuration of CFRP enhancement around the cutout () found to be the best one, where the ultimate shear strength ratio with respect to control solid beam was.97. Figure 6 explains the comparison between ultimate shear strength capacities of all specimens. Changing in shear strength capacity (%) Figure 5. Crack patterns and modes of failure. 89 Specimen Figure 6. Comparison between shear capacities of tested specimens. 18

5 (solid beam), which was characterized by an approximately linear response. Also, it has been noticed that the curves for and were more close to the curve of than the once of, while the load-deflection behavior of was more close to particularly near to ultimate load. Also, these trends can be explained in figure (8). The figure presented the deflection curves of tested beams which have been drawn based on the values of deflection that were measured under point load and mid-span of the deep beams at collapse loads. The maximum mid-span deflection of control beam with cutout () increased by 53% compared with the solid beam. While the maximum deflections of enhanced beams increased by 42%, 29% and 24% for, and, respectively. Thus, the diagonal configuration of CFRP sheet around the cutout was the most appropriate for enhancing. B. Load-deflection relations Figures (7a) and (7b) demonstrate the load-deflection behavior for all test specimens at mid-span and under point load. All specimens with cutout exhibited different behavior comparing with control specimen, TABLE II. Group SUMMARY OF TEST RESULTS AND PREDICTIONS. Specimen Experimental shear strength Vnexp. (kn) Analytical shear strength (kn) A B Shear strength Ratio* Maximum deflection at mid-span (mm) Vc Vs Vnanl. Vc Vs Vnanl *With respect to control solid deep beam, Total load (KN) Total load (KN) Deflection at point load (mm) Mid-span deflection (mm) (b) (a) Figure 7. Load-deflection curves. 19

6 x/l (-) Deflection (mm) DB-PR-ST1 DB-PR-ST2 DB-PR-ST3 Figure 8. Deflection curves of tested specimens at collapse loads. IV. ACKNOWLEDGMENT CONCLUSIONS We sincerely thank the Scientific Research Center at the Faculty of Engineering, University of Duhok for supporting the research. Our special thanks also go to Duhok Laboratory for Construction Materials for their assistance. Based on the results obtained from experiments on enhancement of high strength RC deep beams with circular cutouts by using CFRP sheets, the following conclusions can be drawn: No clear typical mode of failure was observed for RC deep beam with circular cutout that enhanced with CFRP sheets except for the beam enhanced with inner strip of CFRP in which a beam-type of failure was observed as same as for the control deep beam. As was expected, the ultimate shear capacity of the RC deep beam was affected due to the influence of circular cutout. A noticeable improvement in shear strength was observed in beams when the cutout enhanced by using CFRP sheet with different configurations. The diagonally oriented sheet around the cutout, in which the shear strength ratio was.97 with respect to control solid beam, was the best configuration. A significant reduction in the deflection of deep beam with circular cutout enhanced with CFRP sheets was observed. The diagonally oriented configuration of CFRP sheets around the cutout was the most appropriate to reduce the deflection. The load-deflection curves of all specimens with cutout have exhibited different behavior with compare to control solid beam, which was characterized by an approximately linear response. The curves for the deep beams enhanced with horizontal/vertical strips and diagonal strips of CFRP were more close to the curve of control solid beam than the once of control beam with cutout. REFERENCES [1] A. Ahmed, M. M. Fayyadh, S. Naganathan, and K. Nasharuddin, Reinforced concrete beams with web openings: A state of the art review, Materials & Design, vol. 4, pp. 9-12, 212. [2] M. A. Mansur, Effect of openings on the behaviour and strength of R/C beams in shear, Cement and Concrete Composites, vol. 2, no. 6, pp , [3] A. Mansur, and K. H. Tan, Concrete Beams with Openings: Analysis and Design: Taylor & Francis, [4] ACI 318, "Building code requirements for reinforced concrete (ACI ) and commentary," American Concrete Institute, [5] C. E. Bakis, L. C. Bank, V. L. Brown, E. Cosenza, J. F. Davalos, J. J. Lesko, A. Machida, S. H. Rizkalla, and T. C. Triantafillou, Fiber-Reinforced Polymer Composites for Construction Stateof-the-Art Review, Journal of Composites for Construction, vol. 6, no. 2, pp , 22. [6] T. W. White, K. A. Soudki, and M.-A. Erki, Response of RC Beams Strengthened with CFRP Laminates and Subjected to a High Rate of Loading, Journal of Composites for Construction, vol. 5, no. 3, pp , 21. [7] T. H. Almusallam, Load deflection behavior of RC beams strengthened with GFRP sheets subjected to different environmental conditions, Cement and Concrete Composites, vol. 28, no. 1, pp , 26. [8] G. Wu, Z. T. Lü, and Z. S. Wu, Strength and ductility of concrete cylinders confined with FRP composites, Construction and Building Materials, vol. 2, no. 3, pp , 26. [9] C. A. Issa, P. Chami, and G. Saad, Compressive strength of concrete cylinders with variable widths CFRP wraps: Experimental study and numerical modeling, Construction and Building Materials, vol. 23, no. 6, pp , 29. [1] K.-H. Yang, H.-C. Eun, and H.-S. Chung, The influence of web openings on the structural behavior of reinforced high-strength 2

7 concrete deep beams, Engineering Structures, vol. 28, no. 13, pp , 26. [11] G. Campione, and G. Minafò, Behaviour of concrete deep beams with openings and low shear span-to-depth ratio, Engineering Structures, vol. 41, pp , 212. [12] H.-S. C. Keun-Hyeok Yang, and F. A. Ashraf, Influence of Inclined Web Reinforcement on Reinforced Concrete Deep Beams with Openings, ACI Structural Journal, vol. 14, no. 5, 27. [13] H. A. Abdalla, A. M. Torkey, H. A. Haggag, and A. F. Abu-Amira, Design against cracking at openings in reinforced concrete beams strengthened with composite sheets, Composite Structures, vol. 6, no. 2, pp , 23. [14] A. R. Mohamed, M. S. Shoukry, and J. M. Saeed, Prediction of the behavior of reinforced concrete deep beams with web openings using the finite element method, Alexandria Engineering Journal, vol. 53, no. 2, pp , 214. [15] T. El Maaddawy, and S. Sherif, FRP composites for shear strengthening of reinforced concrete deep beams with openings, Composite Structures, vol. 89, no. 1, pp. 6-69, 29. AUTHORS PROFILE Dr. Alaa Alsaad obtained his Doctoral Degree (Ph. D.) in Civil Engineering during the year 29. He is a member of several professional societies including the American Society of Civil Engineers (ASCE). Also, he is a member of Global Science and Technology Forum (GSTF) since 212. He is a Lecturer at University of Duhok since 21. He has taught a number of courses to undergraduate and postgraduate in the field of Civil Engineering. He had worked as Head of Structural Department in AH Consultants (Dubai). He is licensed & accredited from Dubai Municipality for Structural Design of High-Rise Buildings. Mr. Abduljalil Sulaiman received his B.Sc. degree in Civil Engineering from the University of Alsulaimaniah, Iraq, and his M. Sc. in Civil Engineering from University of Salahaddin. He is a Lecturer at University of Duhok since He has taught a number of courses in the field of Civil Engineering. Mr. Jaafar A. Mohammed received his B.Sc. degree from the University of Duhok, Iraq. He is a Ph. D. student at VŠBTechnical University of Ostrava. This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited. 21