Hybrid NSE/EB technique for shear strengthening of reinforced concrete beams using FRCM: Experimental study

The externally bonded (EB) fabric reinforced cementitious matrix (FRCM) has successfully been used as a structural strengthening for various applications including flexural and shear strengthening of reinforced concrete (RC) beams, flexural strengthening of RC slabs and column confinement. However, the EB-FRCM system is characterized by poor FRCM/concrete bond leading to premature debonding of FRCM off the concrete substrate, particularly for thicker FRCM. The present paper reports on an experimental study on the efficacy of a pioneer form of hybrid near surface embedded and externally bonded technique using FRCM composites (NSEEB-FRCM) for shear strengthening of RC beams. With such a technique, higher thickness of FRCM composites can be applied with less likelihood of debonding that is normally experienced when using the EB-FRCM system. Thirteen shear-deficient medium-scale RC beams were constructed, strengthened in shear and tested under three-point bending test. The test parameters were: (a) FRCM type (polyparaphenylene benzobisoxazole, carbon, and glass), (b) strengthening configuration (full versus intermittent strips), and (c) number of fabric layers. The percentage enhancement in the shear capacity of the beams ranged from 43% to 114% indicating the successful implementation of the strengthening methods provided. An average enhancement in shear capacity of 83%, 72% and 62% were observed in carbon FRCM, glass FRCM and PBO-FRCM, respectively. The failure mode of the strengthened specimens was sensitive to the type and configuration of FRCM in addition to the number of FRCM layers. The strengthening systems also resulted in higher deflection at failure and energy absorption value of the strengthened beams with an average of 94% and 204% relative to the reference specimen, respectively.This paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors.Scopu

A shear design model for RC beams strengthened with fabric reinforced cementitious matrix

The shear behavior of fabric reinforced cementitious matrix (FRCM) strengthened beams requires more research efforts to be fully understood. A detailed investigation into the shear capacity of FRCM shear-strengthened beams has generally been limited. Therefore, the development of an accurate and reliable mechanics-based design model is required for broader applications of the FRCM system. This research contribution proposes a shear model based on the simplified modified compression field theory (SMCFT). The accuracy of the proposed model was assessed and compared against an experimental database of FRCM shear-strengthened beams available in the literature. The database is representative of various beam geometries, concrete strengths, and internal reinforcement. Moreover, the FRCM strengthening system comprises of different fiber types including carbon, glass, basalt, and polyparaphenylene benzobisoxazole (PBO) applied using different bond schemes. The proposed analytical procedure provides accurate and safe predictions for the shear capacity of the strengthened beams with an average theoretical to experimental shear capacity (Vth/Vex) ratio of 0.96 with a standard deviation of 0.14. Moreover, the prediction capability of the proposed model was compared with other models available in the literature and resulted in a better prediction capability in terms of safety, accuracy, and economic aspects. - 2019 Elsevier LtdThis paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation) and internal grant # QUST-CENG-SPR-14/15-15. The findings achieved herein are solely the responsibility of the authors.Scopu

A study on the bond behavior of different FRCM systems

Fabric-reinforced cementitous matrix (FRCM) composites are usually applied on the concrete surface for the purpose of strengthening reinforced concrete structures. However, the efficiency of FRCM strengthening is notably affected by the bond between the FRCM system and concrete substrate. In view of that, the current paper presents the results of a preliminary experimental study carried out to investigate the bond characteristics between FRCM composites and concrete. Six number of specimens, each consisted of a 150-mm concrete cube with a double-shear connection to an FRCM system, were subjected to direct-shear loading test. The parameters investigated include (a) FRCM material (carbon, polyparaphenylene benzobisoxazole (PBO), and glass); and (b) Bond length (75 mm or 100 mm). The FRCM systems typically included a single layer of fabric with the associated mortar, and the bond width was uniformly taken as 100 mm. The test results revealed that the bond capacity is enhanced with an increase in the FRCM bonded length. The PBO-FRCM showed the highest bond capacity between FRCM composite and concrete substrate among the three systems. The modes of failure observed in carbon-, PBO-, and glass-FRCM bond tests are fabric delamination, FRCM mortar/concrete debonding, and fabric rapture, respectively. The PBOand glass-FRCM bond tests thus exhibited a more brittle behavior at failure than that of the carbon-FRCM counterpart.This paper was made possible by grant # QUST-CENG-SPR-14/15-16 from Qatar University. The findings achieved herein are solely the responsibility of the authors.Scopu

FRCM/internal transverse shear reinforcement interaction in shear strengthened RC beams

This paper presents a study on the efficacy of a shear strengthening technique utilizing fabric reinforced cementitious matrix (FRCM) systems for beams with and without internal transverse shear reinforcement (ITSR) within the critical shear span (CSS). The paper focuses on the FRCM/ITSR interaction, experimentally and analytically. Three different FRCM fabric types were used; namely, glass, carbon and polyparaphenylene benzobisoxazole (PBO). The test matrix consisted of fourteen medium-scale RC beams prepared and tested to fail in shear. The test results indicated a clear influence of the ITSR within the CSS on the gain in the ultimate load carrying capacity (Pu) of the beams. The FRCM strengthening system has enhanced the shear strength of the beams. With regard to the FRCM fabric type, carbon FRCM was the most effective of all in terms of the gain in Pu of the strengthened beams. Moreover, the beams strengthened with continuous strengthening configuration intuitively performed better than those strengthened with discontinuous configuration. A simplified compression field theory (SCFT) model was used for predicting the ultimate load carrying capacity of the beams. This model features two important contributions; namely, considering the effect of FRCM strengthening and accounting for the critical shear span to depth ratio.This paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors. Appendix

Experimental and analytical study on strengthening of reinforced concrete T-beams in shear using steel reinforced grout (SRG)

The use of steel reinforced grout (SRG) has emerged as a promising technique for the strengthening and retrofitting of reinforced concrete (RC) structures, however, the experimental evidence on its application for shear-critical RC beams is rather limited. Accordingly, this study focuses on the structural performance of SRG-strengthened RC beams critical in shear. At this aim, an experimental investigation has been conducted on eleven RC T cross-section beams. Eight beams were strengthened with different types of SRG (different for the density, namely 1.57 and 3.14 cords per centimeter) comprised of ultra-high tensile strength steel fabrics, and three, which were unstrengthened, were used as reference specimens. The test parameters investigated were as follows: (a) steel fabric density, (b) bond scheme, and (c) amount of internal shear reinforcement within the critical shear span, SRG/stirrups interaction. The experimental results revealed the high potential of SRG system for the strengthening of shear-critical RC beams. It has significantly improved the shear capacity and deformation characteristics of the strengthened beams. The percentage increase in the load-carrying capacity over the reference beam of up to 71% was observed. In addition, an analytical model based on the simplified modified compression field theory (SMCFT) has been proposed to predict the shear capacity of SRG-strengthened beams. The SMCFT-based model has resulted in an accurate and safe prediction of the shear capacity of the SRG-strengthened beams with an average ratio of Vth/Vex of 0.92 and a standard deviation of 4.07%. - 2019 Elsevier LtdThis paper was made possible by NPRP grant # NPRP 9-110-2-052 and UREP grant # UREP24-045-2-013 from the Qatar National Research Fund (a member of Qatar Foundation) and Qatar University grant # QUST-CENG-SPR- 14/15-15 . The authors would also gratefully acknowledge Kerakoll SpA for providing the strengthening material. The findings achieved herein are solely the responsibility of the authors.Scopu

A new approach for predicting the shear capacity of FRCM strengthened RC beams in shear

In spite of the availability of different models proposed to predict the shear capacity of reinforced concrete (RC) beams strengthened in shear using fabric reinforced cementitious matrix (FRCM) system, accurately predicting this value remains a challenge. The simplified compression field theory has shown to accurately predict the shear capacity of RC beams. This paper presents an analytical model based on the simplified compression field theory for predicting the shear capacity of RC beams strengthened in shear using FRCM strengthening system. The model has been validated against a data series of over sixty RC beams strengthened in shear using different FRCM types with different strengthening configuration and orientation. The results showed that the model can reasonably predict the load carrying capacity of the beams. The ratio of theoretically predicted and experimental results for the load carrying capacity ranged between 0.67 and 1.33 while average of this ratio was 1.01 with a coefficient of variation of 0.16. � 2018 Institute of Physics Publishing. All rights reserved.This paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors.Scopu

FRCM/stirrups interaction in RC beams strengthened in shear using NSE-FRCM

The use of a recently introduced "near surface embedded" (NSE) technique for fabric reinforced cementitious matrix (FRCM) system is shown to be a viable alternative to the conventionally used externally bonded FRCM counterpart with the potential to mitigate FRCM/concrete debonding. This paper presents a study on the interaction of FRCM and stirrups for RC beams strengthened in shear using the NSE-FRCM system. The experimental program involved eight (8) rectangular RC beams with and without internal shear reinforcement. Two test variables were considered; namely, fabric type (carbon, glass and polyparaphenylene benzobisoxazole) and internal shear reinforcement within the critical shear span (with and without stirrups). The experimental results revealed that the NSE-FRCM can successfully be used to enhance the shear capacity of the strengthened beams. Carbon FRCM was the most effective of all the fabric types. Moreover, an interaction between the stirrups and the NSE-FRCM system has been observed. The percentage gain in the shear strength was reduced from 69% to 38% due to the presence of stirrups within the critical shear span. Moreover, the NSE-FRCM strengthening has reduced the strain in the stirrups owing to the load sharing between the stirrups and the strengthening system. � 2018 Institute of Physics Publishing. All rights reserved.This paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors.Scopu

Pull-off characterization of FRCM/Concrete interface

Fabric-reinforced cementitious matrix (FRCM) composites are usually surface-applied for strengthening reinforced concrete (RC) structures. The efficacy of the FRCM strengthening systems is dependent on the FRCM/concrete bond performance. This paper reports on the experimental results of FRCM/concrete bond characterization through pull-off tests. Six FRCM-strengthened RC slabs ( mm) were prepared and enabled conducting 72 FRCM/concrete pull-off tests. The parameters investigated included: (a) FRCM material (carbon or polyparaphenylene benzobisoxazole (PBO)); (b) level of substrate roughness (no/low/high roughening); and (c) specimen's test age (7, 28, 56, and 84 days). All FRCM systems were single-plied. The study revealed a significance of the surface preparation and test age of specimens on the FRCM/concrete pull-off strength. High-roughness specimens showed an average of 74% pull-off strength increase compared to those without roughening. Also, specimens tested at Day 84 showed 54% strength increase compared to those tested at Day 7, on average. PBO-FRCM system showed slightly higher pull-off strength than that of the carbon counterpart. The specimens showed two distinctive failure types at the (i) fabric/mortar interface and (ii) concrete/matrix interface: the latter was more prominent in carbon-FRCM. Nonetheless, the failure mode was most dependent on the fabric geometry and the substrate roughness. Based on a statistical analysis of the tested specimens, prediction models were proposed for the FRCM/concrete pull-off strength and failure mode.This paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation) and by grant # QUST-CENG-SPR-13/14 from Qatar University . The findings achieved herein are solely the responsibility of the authors.Scopu

Behaviour of RC beams strengthened in shear using near surface embedded FRCM

This paper presents an experimental study on the efficacy of a lately introduced "near surface embedded" technique (NSE) for fabric reinforced cementitious matrix (FRCM) for the strengthening of reinforced concrete (RC) beams in shear; considering two test variables: (a) fabric type (glass and carbon) and (b) strengthening configuration (intermittent versus full). For this purpose, five (5) simply supported medium-scale RC rectangular beams have been tested under displacement controlled monotonic three-point loading. The experimental results showed that the FRCM system is effective in strengthening of shear deficient RC beams. An average of 62% gain in the shear capacity has been achieved due to the FRCM strengthening system. Carbon FRCM owning the higher axial rigidity showed better performance than that of glass FRCM. The average gain in the shear capacity for carbon FRCM strengthened beams was 22.5% higher than that for glass FRCM strengthened beams. Moreover, full strengthening configuration showed better performance than that for the intermittent configuration counterparts confirming the significance of the FRCM continuity and its quantity in enhancing the shear capacity of the strengthened beams. � 2018 Institute of Physics Publishing. All rights reserved.This paper was made possible by NPRP grant # NPRP 7-1720-2-641 from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the authors.Scopu