Experimental and numerical evaluation of compression confinement techniques for HSC beams reinforced with different ratios of high strength steel reinforcement

A BSTRACT . This work presents experimental and numerical research to evaluate the compression confinement techniques of HSC beams reinforced with different ratios of high-strength steel reinforcement. Twelve specimens of high-strength reinforced concrete beams with two different compression confinement techniques were tested experimentally. The first method is used carbon fiber reinforced polymers sheets (CFRPs) around the compression zone, CF, and the steel fibers reinforced concrete is used in the compression zone by 1% of volume fraction, SF, in the second case. A 3-D finite element analysis was done; using the ANSYS program to simulate and idealize all experimental specimens. The numerical and experimental results of the RC beams were validated and compared in this work. The results showed that there is a good idealization using 3-D finite element models with the experimental specimens. Also, it was found that using the suggested techniques can increase the strength ratio and increase the ductility index depending on the tensile reinforcement ratios. Moreover, the energy absorption and the mode of failure were enhanced. Similar behavior was reported for the values of ultimate load, Pu for the tested beams. The results show that the strengthening techniques can increase the ultimate load by about 23.80 and 52.73 % for CF1 and SF1 respectively. Moreover, for the CFRP confinement technique the values of cracking deflection,  cr, and maximum deflection,  u, were decreased as compared to the corresponding values control beam, C1. On the other hand, for steel fiber confinement technique, the values of cracking deflection,  cr, and maximum deflection,  u, were increased as compared to the corresponding values control beam, C1. The obtained results were approved a highly efficient technique for steel fiber confinement in increasing of ductility and strength of RC 5- For steel reinforcement ratios more than the maximum ratios, using the steel fiber reinforced technique can increase the strength and ductility ratios by 5% and 48% respectively. 6- The numerical results and experimental results in this work were not identical in values but they showed that similar behaviors as found in the results of ductility index and strength ratio with the tensile area of steel reinforcement ratio.


INTRODUCTION
igh strength concrete is widely used in the present days because of the civilization revolution over the world. The high strength concrete has a dangerous brittle behavior and structural elements constructed using HSC usually fail in a brittle manner. The brittle behavior is not favorable condition, so the improvement of high strength concrete HSC behavior is very important. We can improve the failure by strengthening the compression zone with ductile materials by using additional compression steel reinforcement bars. Using highly ductile fiber-reinforced concrete (HDC), characterized by its ability to absorb large ultimate strains, Mingke Deng et al. [1] investigated the effectiveness of HDC in improving deformational characteristics and the failure mode of over-reinforced concrete beams. Concrete beams that are over-reinforced often collapse suddenly due to compressive concrete crushing before the tensile steel reinforcement has yielded. In over-reinforced concrete beams with increased longitudinal reinforcement ratios and thick HDC layers, an increase in ductility of up to 61% can be achieved by adding HDC to the compression zone. By increasing the longitudinal reinforcement ratio, HDC became more effective at strengthening over-reinforced concrete beams. Deesy Gomes Pinto et al. [2] studied the influence of high strength concrete beams confinement with stirrups reinforcement on the flexural ductility. In this study, the only variable is the reinforcement ratio of longitudinal tensile.
The results showed a well positive effect on the flexural ductility due to confinement with stirrups reinforcement. The design ductility of high strength concrete columns and beams was studied in A.K.H. Kwan and J.C.M. Ho [3]. The results showed that the major factors that affect flexural ductility are the steel yield strength, concrete strength, confining pressure, and reinforcement ratio. Also, for columns the major factors affecting the flexural ductility were the steel yield strength, concrete strength, confining pressure, and axial load/stress level. The effect of helically confinement of HSC beams on the displacement ductility was investigated by Muhammad N. S. Hadi and Nuri M. Elbasha [4] . In this work, ten over-reinforced HSC beams were tested. The results showed that the confinement effect is neglected for the helically confined beams when the pitch of the helical is equal to or more than the core diameter. Finally, this study shows that reduce helical pitch can enhance the strength and ductility of HSC beams reinforced with high strength reinforcement steel. Syed Wasim N Razvia and M. G. Shaikhb [5] studied the confinement effect on short concrete column behavior. The results indicated that column with additional confinement ferro mesh gives a 20% increase in axial load compared with regular control column. The results also approved that, columns with ferro mesh with additional stirrups gives better ductility moreover the column wrapped with ferro mesh only fails in ductile manner. The flexural behavior of steel-reinforced, high-strength and prestressed concrete beams were experimentally investigated by Qing JIANG et al. [6] . The failure modes, ductility, flexural strength, and the specimen's crack width were analyzed. The results approved that the failure modes of high strength steel and high strength concrete prestressed beams were similar to the ordinary reinforced concrete. Mehrollah Rakhshanimehr et al. [7] studied experimentally the flexural ductility of RC beams with lap-spliced bars. The compressive strength of concrete, amount of stirrups over the splice length, and the longitudinal diameter bars were selected as the variables. The ductility of the specimens was evaluated based on the ductility ratio. Results showed that the previous variables have a major effect on the beams ductility. J. C. M. Ho et al. [8] investigated the minimum flexural design ductility of high strength concrete beams. Also, the maximum values of tension steel ratio and minimum curvature factor of ductility of the yield strengths steel and various concrete grades have been evaluated. The results concluded that to provide a level of minimum ductility, the minimum ductility factor should be 3·32. 22 beam tests were carried out by Jianwei Zhang et al. [9] to determine the bond behavior between reinforced concrete and recycled aggregate concrete (RAC). A significant increase in normalized peak bond strength is seen by increasing the concrete cover thickness and stirrup ratio. A positive correlation exists between RAC bond strength and f'c 1/2 , which is similar to NAC bond strength and f' c 1/2 . The analytical and numerical approaches were proposed by Jorge Luis Palomino Tamayo and Gabriel Orso Garcia [10] to study the behavior of high strength reinforced concrete beams. Results of nineteen high strength reinforced concrete beams were compared with the proposed approach. It has been found that the model based on a constant ultimate strain at peak stress was acceptable to validate the numerical results with the experimental, so, some codes of practice suggest these parameters. S.H. Chowdhury and Y.C. Loo [11] investigated the damping and cracking in high-strength reinforced concrete beams. Eight full-size high strength reinforced concrete, HSC, beams were tested experimentally. The average and maximum crack widths of all beams have been measured. Also, the beams were subjected to free vibration load. Two formulas were H developed to predict the average and maximum crack widths for reinforced concrete beams and the third formula for damping. Seleem S. E. Ahmad et al. [12] study the confinement effect on the behavior of over-reinforced high strength concrete beams. The testing results of all beams proved that the suggested techniques gave the behavior of a good beam, the first crack and ultimate loads of the beams were increased. Moreover, the mode of failures was changed from sudden failure for control beam to ductile failure for confinement beams. In this work, experimental and numerical works for evaluation the compression confinement techniques of HSC beams reinforced with different ratios of high strength steel reinforcement. Two techniques were used, the first was using carbon fiber reinforced polymers (CFRPs) as confinement strengthening around the compression zone and the second technique was using additional steel fiber reinforced concrete in the half of the beam with a volume fraction equal to 1.0 %. Twelve high-strength reinforced concrete beams with two different compression confinement techniques were tested.

EXPERIMENTAL PROGRAM
ab. 1 presents reinforced concrete beams that were fabricated and tested to investigate the behavior of high strength reinforced concrete beams with different compression confinement techniques, as shown in Fig. 1.a, b, and c. Fig. 2 shows cross section dimensions for control beams and steel bars reinforcement distribution where dimensions and steel bars diameter are in mm. The beams had been loaded up to failure using a load control machine. The experimental work was divided into three groups according to the strengthening technique, the first group was for control beams without any strengthening, the second was for using one layer of carbon fiber reinforced polymers (CFRPs) sheets as a confinement reinforcement in the compression zone, and the third group was for using steel fiber reinforced concrete in compression zone with 1.0% volume fraction as confinement reinforcement.      The high elastic modulus, HM, CFRP fabric used to strengthen the specimens was unidirectional with nonstructural weaves in the transverse direction to hold the fabric together. According to the datasheet provided by the manufacturer, a dry fabric would have a tensile strength of 3.45 GPa, a high elastic modulus of 230 GPa, and elongation at a break of 1.5%, [13]. The fabric was laminated to the specimens with an epoxy resin having a tensile strength of 33.8 MPa and an ultimate elongation of 1.2%, according to Sikadur®-330 epoxy datasheet, [14].

TEST PROCEDURE
he beams were tested to failure by using a universal testing machine with 1000 KN capacity and 0.4% error of calibration while the rate of loading was 0.7 N/sqmm/min. Two rollers support were used for the beams. The load was applied in two points with a 600 mm distance between the two points, and the deflection was measured at the beam mid-span using linear vertical displacement transducers (LVDT) as shown in Fig. 5. All the beams were painted with white color to simplify the cracks observation, and the crack propagation was observed by the naked eye. A load control was used to test the samples, and the load-displacement values were recorded during loading using a data logger system.

NUMERICAL STUDY
3D finite element analysis (FEA) using ANSYS v2021 R1 was done to predict the behavior of high strength reinforced concrete beams confinement in compression zone with two techniques with different ratios of high strength steel reinforcement. The methodology of this work was done by building the 3D model, selecting the type of elements, the models of used materials, contact details, and selecting the type of analysis which considered the displacement control type. Concrete was modeled using a suitable solid element for concrete properties named SOLID 65 with 8-nodes as shown in Fig. 6 a; reinforcement steel and steel fibers were modeled using REINF 264 element, while fiber reinforced polymers were modeled using SHELL 181 element shown in Fig. 6, b and SOLID 185 element was used to model the behavior of rigid the loading and bearing plates. In contrast, the bond slippage behavior between the FRPs sheets and the concrete was simulated using the cohesive zone model (CZM). The maximum shear contact stress was 3 MPa with a maximum debonding gap of 0.01 mm. In comparison, The maximum normal contact stress was 1 MPa with a maximum debonding gap of 0.01 mm [15]. Nonlinear analysis of the reinforced concrete beams was done based on the smeared crack approach with displacement control loading. The displacement load was applied to the top line of the loading plates, and the displacement convergence was included with tolerance value equal to 5%. a) SOLID65 concrete element. b) 4-Node Structural Shell. Concrete was simulated using a multilinear isotropic hardening model. The stress-strain curve was used to simulate the concrete plasticity based on equations 1 and 2 [16]. The concrete material properties are given in a Tab. 3.
where, Ec is Young's modulus for concrete, ε is the concrete strain, and ε0 is the compression failure strain.    The geometry of the experimental specimens was idealized and simulated numerically with the strengthening techniques as shown in Fig. 9.

RESULTS AND DISCUSSION
he experimental and numerical results of the strengthened high strength concrete beams (HSCB) were compared with the control beams results. The comparison includes the yield load, deflection, ultimate load, deflection, ductility index, and failure mode.

Experimental Results
Tab. 4, shows the comparison between the experimental results for control, C, carbon fiber reinforced polymers, CF, and steel fiber reinforced concrete, SF, beams at four steel reinforcement ratios min, avg, max, and max. Fig. 10 shows the experimental relationship between the applied load and mid-span deflection for control beam, C1, CFRPs, CF1, and steel fiber, SF1, techniques at steel reinforcement ratio equals min.  Figs. 11 a, b and c show cracks pattern and failure modes for beams C1, CF1 and SF1 respectively. All beams failed initially due to tension cracks followed by concrete compression failure. The cracks in beam C1 seemed to be less in number and wider as compared to other beams. While CF1 beam showed a debonding failure of CFRP and SF1 beam showed more in number and narrow cracks patterns in tension zone as compared to beam C1.   Experimental relationships between load and mid-span deflection for C2, CF2 and SF2 techniques at steel reinforcement ratio equal to  avg. are given in Fig. 12 Fig. 13 a, b, and c show crack pattern for RC beams using steel reinforcement equal to  avrg . All beams failed initially due to tension cracks followed by shear failure.    Fig. 15 a, b, and c show crack pattern for RC beams using steel reinforcement equal to  max . All beams failed initially due to tension cracks followed by shear cracks up to complete failure.   The results of load and deflection for C4, CF4 and SF4 techniques at steel reinforcement equal to  max are given in Fig.  16. The results indicated that there are similar behaviors as found in Fig. 12 for steel reinforcement ratio equal to max. The maximum loads for beams C4 and CF4 were slightly decreased as compared to C3 and CF3 although increasing in steel reinforcement ratio. This behavior can attributed to the shear failure generated in those beams. On the other hand, using steel fiber confinement technique in compression zone, beam SF4, improves the maximum load of SF4 as compared to SF3. Fig. 17 a, b, and c show crack pattern for RC beams using steel reinforcement equal to  max . All beams failed initially due to tension cracks followed by shear cracks up to complete failure.    Fig. 18 a) and b) show the behavior of ductility index and strength ratio with the increase in steel reinforcement ratio for control beams and confinement beams. The ductility index was calculated as the ratio between maximum deflections to the first cracking deflection. The strength ratio was calculated as the ratio between maximum loads to first cracking load. From data in figures we found that, at min, the CFRP confinement technique enchases the ductility index more than steel fiber confinement technique. While at avg and max the steel fiber confinement technique gives more ductility than CFRP technique. On the other hand, the used confinement techniques were improved the strength ratio at all steel reinforcement ratios with obviously increasing in case of steel fiber.

NUMERICAL RESULTS
ab. 5 shows numerical results of 3-D FE models in this work for control models, C, carbon fiber reinforced polymers, CF, and steel fiber reinforced concrete, SF at four steel reinforcement ratios  min,  avg,  max, and    max. .   Fig. 19 shows the numerical relationship between applied load and mid span deflection for C1, CF1 and SF1 beams at steel reinforcement ratio equals to  min . The numerical results showed that similar behaviors as found in experimental results. The behavior started with linear response up to cracking load, Pcr, and followed by nonlinear response up to the values of maximum load, Pu, and deflection, u. Moreover, SF1 beam showed higher values of Pcr and Pu as compared to C1 and CF1 beams. This behavior can attribute to the effect of steel fiber confinement which prevents the early compression failure and lets the steel reinforcement carry additional loads. The numerical failure modes of these beams are given in Fig. 20 a, b and   The numerical results of load-mid span deflection for C2, Cf2 and SF2 beams are presented in Fig. 21 at avg. Typical behaviors of linear and nonlinear response were found as in case of  min with an increase in the value of Pcr and Pu as compared to min due to increase in the ratio of steel reinforcement. On the other hand, the values of cracking and maximum deflection were decreased as compared to those of min. Fig. 22 a,    As the ratio of steel reinforcement increases, beams of  max and  max , typical numerical behavior for load-deflection was reported as shown in Figs. 23 and 25 respectively. The cracking and maximum load of two beams were going to increase and deflection decreases. The numerical modes of failure for the beams of max and max are given, also, in Figs. 24 and 26 respectively. All beams showed tensile cracks followed by crushing in compression zone. Fig. 27 shows the numerical behavior of ductility index and strength ratio with the increase in steel reinforcement ratio for control beams and confinement beams. The ductility index was calculated as the ratio between maximum deflections to the first cracking deflection. The strength ratio was calculated as the ratio between maximum loads to first cracking load. From data in figures we found that, at min, avg and max the steel fiber confinement technique gives more ductility than CFRP technique. Moreover, the SF confinement technique was improved the strength ratio at all steel reinforcement ratios with slightly decreasing in case of CFRP.     CONCLUSIONS n experimental and numerical study was conducted, including testing and simulating twelve samples of high strength reinforced concrete beams with different compression confinement techniques. The following conclusions can be written from this study: 1-For steel fiber reinforced confinement technique the cracking load value was improved by about 108.4% and the maximum load was improved by about 52.73 % at minimum area of steel reinforcement. 2-The steel fiber strengthening techniques increases the ultimate load by about 48.7 % at average area of steel reinforcement. 3-For maximum steel reinforcement ratios, using the steel fiber reinforced technique can increase the strength and ductility ratios by 8% and 118% respectively. 4-Using carbon fiber reinforced polymers technique has a slight effect on the strength and ductility index. A STEEL STRESSES 5-For steel reinforcement ratios more than the maximum ratios, using the steel fiber reinforced technique can increase the strength and ductility ratios by 5% and 48% respectively. 6-The numerical results and experimental results in this work were not identical in values but they showed that similar behaviors as found in the results of ductility index and strength ratio with the tensile area of steel reinforcement ratio.