Description of near-tip fracture processes in strain hardening cementitious composites using image-based analysis and the compact tension test

Strain Hardening Cementitious Composites (SHCC) can be distinguished from other types of fiber reinforced cement-based matrix composites by the typical pseudo-strain hardening behavior they develop in tension. The design of Strain Hardening Cementitious Composites (SHCC) towards the development of multiple cracking in tension is based on the micro-mechanisms involved in the cracking process, including the fiber-matrix interaction properties as well as the fiber and the matrix mechanical properties. The cracking mechanisms at the micro-scale assume a key role in the composite behavior of SHCC. Their complexity introduces, however, significant uncertainty in the entire process. The investigation of the cracking processes of SHCC materials at an intermediate level between the micro-scale and the structural length scale is therefore important to further characterize the influence of the fracture processes on the composite tensile behavior. In previous studies the mechanical behavior of SHCC materials, as well as of other strain softening fiber reinforced cementitious composites, was characterized under eccentric tensile loading using the Compact Tension Test (CTT). The present research further extends this investigation, with particular emphasis on cementitious composites reinforced with multiple types of fibers. The experimental tensile load-displacement results are discussed and compared to the numerically derived responses. To numerically predict the tensile load-displacement responses obtained with the CTTs, the cohesive crack model and the tensile stress-crack opening relationships obtained with the Single Crack Tension Test (SCTT) are utilized. Furthermore, the crack initiation and propagation at the early stages of the loading sequence are analyzed. The size of the specimens and the resolution of the digital images acquired allow the detection of relatively small displacements and crack openings. The results are discussed, with special emphasis on the topology of the cracks obtained near the crack tip and on the description of the fracture process zone.Fundação para a Ciência e a Tecnologia (FCT

A FEM-based model to study the behaviour of corroded RC beams shear repaired by NSM CFRP rods technique

This paper presents the main features of finite element FE numerical model developed using the computer code FEMIX to predict the near-surface mounted NSM carbon-fiber-reinforced polymer CFRP rods shear repair contribution to corroded reinforced concrete RC beams. In the RC beams shear repaired with NSM technique, the Carbon Fibre Reinforced Polymer (CFRP) rods are placed inside pre-cut grooves onto the concrete cover of the RC beam’s lateral faces and are bonded to the concrete with high epoxy adhesive. Experimental and 3D numerical modelling results are presented in this paper in terms of load-deflection curves, and failure modes for 4 short corroded beams: two corroded beams (A1CL3-B and A1CL3-SB) and two control beams (A1T-B and A1T-SB), the beams noted with B were let repaired in bending only with NSM CFRP rods while the ones noted with SB were repaired in both bending and shear with NSM technique. The corrosion of the tensile steel bars and its effect on the shear capacity of the RC beams was discussed. Results showed that the FE model was able to capture the main aspects of the experimental load-deflection curves of the RC beams, moreover it has presented the experimental failure modes and FE numerical modelling crack patterns and both gave similar results for non-shear repaired beams which failed in diagonal tension mode of failure and for shear-repaired beams which failed due to large flexural crack at the middle of the beams along with the concrete crushing, three dimensional crack patterns were produced for shear-repaired beams in order to investigate the splitting cracks occurred at the middle of the beams and near the support

End concrete cover separation in RC structures strengthened in flexure with NSM FRP: Analytical design approach

Fiber-reinforced-polymer (FRP) composite materials applied according to the near-surface-mounted (NSM) technique are very effective for the flexural strengthening of reinforced-concrete (RC) structures. However, the flexural strengthening effectiveness of this NSM technique is sometimes compromised by end concrete cover separation (CCS) failure, which is a premature failure before occurring the conventional flexural failure modes. Due to the complexity of this failure mode, no analytical approach, with a design framework for its accurate prediction, was published despite the available experimental results on this premature failure. In the present study, a novel simplified analytical approach is developed based on a closed form solution for an almost accurate prediction of CCS failure in RC structures strengthened in flexure with NSM FRP reinforcement. After demonstrating the good predictive performance of the proposed model, it was used for executing parametric studies in order to evaluate the influence of the material properties and FRP strengthening configuration on the susceptibility of occurring the CCS failure. At the end, regarding to the FRP strengthening configuration, some design recommendations were proposed to maximize the resistance of NSM FRP strengthened structures to the susceptibility of occurring the CCS failure.The authors acknowledge the financial support provided by QREN (through the Operational Program COMPETE) in the scope of the CutInov Project (n. 38780) involving the Clever Reinforcement Company and the Structural Composites Research group of ISISE-Minho University

Development of innovative hybrid sandwich panel slabs: Experimental results

The authors appreciate the collaboration of the following labs: Civitest for developing DHCC materials, PIEP for conducting VARTM process (Eng. Luis Oliveira) and Department of Civil Engineering of Minho University to perform the tests (Mr. Antonio Matos and Eng. Marco Jorge).In this paper, a new generation of composite sandwich slab is proposed as a solution for the rehabilitation of slabs in old masonry buildings. An innovative solution was developed during this research formed by four components: a Deflection Hardening Cement Composite (DHCC) layer on the top compression skin, a glass fiber reinforced polymer (GFRP) skin at the bottom tension surface, GFRP ribs to transfer shear from top to bottom layers, and foam core for thermal-insolation purposes. The DHCC layer contributes significantly for the load carrying and deflection capacity due to its stiffness, compressive strength and toughness, offers resistance to the occurrence of buckling phenomena in the GFRP ribs, improves the performance of this structural concept against impact and fire, and constitutes an excellent medium for the application of finishing materials, like ceramics or timber. Two different hybrid composite slabs were developed and tested, and their behavior was assessed under flexural loading. The results showed that the developed hybrid sandwich slabs accomplish all design requisites for serviceability and ultimate limit states, and assure a stiffness/dead-weight and load-capacity/dead-weight ratios much higher than conventional structural slab systems.FCT - Fundação para a Ciência e Tecnologi

Assessing the effectiveness of embedding CFRP laminates in the near surface for structural strengthening

The authors of the present work wish to acknowledge the support provided by the S&P, Bettor MBT Portugal, Secil, Nordesfer, Ferseque, Casais, Solusel, VSL, Unibetão (Braga) and the colaboration of Cemacom.Near Surface Mounted (NSM) is a recent strengthening technique based on bonding Carbon Fiber Reinforced Polymer (CFRP) bars (rods or laminate strips) into pre-cut grooves on the concrete cover of the elements to strength. To assess the effectiveness of the NSM technique, an experimental program is carried out involving reinforced concrete (RC) columns, RC beams and masonry panels. In columns failing in bending the present work shows that the failure strain of the (CFRP) laminates can be attained using the NSM technique. Beams failing in bending are also strengthened with CFRP laminates in order to double their load carrying capacity. This goal was attained and maximum strain levels of about 90% of the CFRP failure strain were recorded in this composite material, revealing that the NSM technique is also very effective to increase the flexural resistance of RC beams. The effectiveness of externally bonded reinforcing (EBR) and NSM techniques to increase the flexural resistance of masonry panels is also assessed. In the EBR technique the CFRP laminates are externally bonded to the concrete joints of the panel, while in the NSM technique the CFRP laminates are fixed into precut slits on the panel concrete joints. The NSM technique provided a higher increase on the panel load carrying capacity, as well as, a larger deflection at the failure of the panel. The performance of EBR and NSM techniques for the strengthening of RC beams failing in shear is also analyzed. The NMS technique was much more effective in terms of increasing the beam load carrying capacity, as well as, the beam deformability at its failure. The NSM technique was easier and faster to apply than the EBR technique.The first author wishes to acknowledge the grant SFRH/BSAB/291/2002-POCTI, provided by FCT and FSE

Fibre reinforced mortar application for out-of-plane strengthening of schist walls

The aim of the present work is to assess the effectiveness of an innovative strengthening technique for the rehabilitation of masonry buildings deficiently prepared to resist to loading conditions typical of seismic events. This technique is based on the application of outer layers of fibre reinforced mortar (FRM) by spray technology and it is used for increasing the load carrying capacity and deformation ability of masonry elements. For this purpose three almost real scale schist walls prototypes were strengthened and tested. The experimental program is described and the relevant results are presented and discussed. For estimating the properties of the schist walls and FRM taking into account the application conditions, the tested prototypes were simulated with a FEM-based computer program that has constitutive models for the simulation of the nonlinear behaviour of these materials. By using the derived properties, a parametric study was conducted to identify the influence of the FRM properties on the performance of the proposed strengthening system.The author wish to acknowledge CiviTest, Lda (Jesufrei, Portugal) for supporting the experimental program, the sustain provided by INOTEC - Innovative material of ultra-high ductility for the rehabilitation of the built patrimony, QREN project number 23024, and the collaboration of the companies Owens Corning, Exporplas, Sika, Chryso and SECIL for providing, respectively, glass fibres, polypropylene fibres, superplasticizers, Viscous Modifier Agent, and Cement. The authors further wish to acknowledge the Erasmus Plus and Placement Mobility Programs among the University of Ferrara (Italy), the University of Minho (Portugal) and the CiviTest Lda (Portugal) which made this international cooperation possible

A design model for fibre reinforced concrete beams pre-stressed with steel and FRP bars

This paper presents a design oriented model to determine the moment-curvature relationship of elements of rectangular cross section failing in bending, made by strain softening or strain hardening fibre reinforced concrete (FRC) and reinforced with perfectly bonded pre-stressed steel and fibre reinforced polymeric (FRP) bars. Since FRP bars are not affected by corrosion, they have the minimum FRC cover thickness that guaranty proper bond conditions, while steel bars are positioned with a thicker FRC cover to increase their protection against corrosion. Using the moment-curvature relationship predicted by the model in an algorithm based on the virtual work method, a numerical strategy is adopted to evaluate the load-deflection response of statically determinate beams. The predictive performance of the proposed formulation is assessed by simulating the response of available experimental results. By using this model, a parametric study is carried out in order to evaluate the influence of the main parameters that characterize the post cracking behaviour of FRC, and the prestress level applied to FRP and steel bars, on the moment-curvature and load-deflection responses of this type of structural elements. Finally the shear resistance of this structural system is predictedThe study reported in this paper is part of the research program "DURCOST - Innovation in reinforcing systems for sustainable prefabricated structures of higher durability and enhanced structural performance" supported by FCT, PTDC/ECM/105700/2008. The second and forth authors acknowledge the research grant under the project QREN number 3456 "PONTALUMIS", while the third author acknowledges the support provided by FCT Grant SFRH/BD/71934/2010

A model for the prediction of the punching resistance of steel fibre reinforced concrete slabs centrically loaded

With the aim of contributing for the development of design guidelines capable of predicting with high accuracy the punching resistance of steel fibre reinforced concrete (SFRC) flat slabs, a proposal is presented in the present paper and its predictive performance is assessed by using a database that collects the experimental results from 154 punching tests. The theoretical fundaments of this proposal are based on the critical shear crack theory proposed by Muttoni and his co-authors. The proposal is capable of predicting the load versus rotation of the slab, and attends to the punching failure criterion of the slab. The proposal takes into account the recommendations of the most recent CEB-FIP Model Code for modelling the post-cracking behaviour of SFRC. By simulating the tests composing the collected database, the good predictive performance of the developed proposal is demonstrated.Fundação para a Ciência e a Tecnologia (FCT

Time-dependent fibre pull-out behaviour in self-compacting concrete

In the present study, the effectiveness of a fibre as an element for transferring stresses across cracks under a sustained load was assessed. Single fibre pull-out creep tests were performed, in which fibre slip was monitored as a function of the time. The influence of the fibre orientation angle (0, 30 and 60 degrees), as well as pre-imposed fibre slip levels, spr, 0.3 and 0.5 mm on the creep response was investigated. Additionally, instantaneous fibre pull-out tests were carried out on undamaged-bond specimens in order to quantify the effects of the pull-out creep behaviour. The damage introduced by the pre-slip levels in the bond of the fibre/matrix interface influenced the long-term fibre pull-out behaviour and, consequently, accelerated the creep rate. However, the assembled pull-out creep behaviour did not differ considerably from the instantaneous pull-out behaviour for the adopted pre-imposed fibre slip levels.This work is supported by the FEDER funds through the Operational Program for Competitiveness Factors - COMPETE and National Funds through FCT - Portuguese Foundation for Science and Technology under the project 18 SlabSys-HFRC-PTDC/ECM/120394/2010. The authors would like to acknowledge the materials supplied by Radmix and Maccaferri (fibres), SECIL (cement), SIKA and BASF (superplasticizers), Omya Comital (limestone filler), and Pegop (Fly ash).info:eu-repo/semantics/publishedVersio

Shear strengthening of reinforced concrete beams with Hybrid Composite Plates (HCP) technique: experimental research and analytical model

This paper presents a study on the efficiency of Hybrid Composite Plates (HCPs) in enhancing the shear strength and stiffness of reinforced concrete (RC) beams. HCP is a thin plate of Strain Hardening Cementitious Composite (SHCC) reinforced with Carbon Fiber Reinforced Polymer (CFRP) laminates applied to the sides of reinforced concrete beams according to the Near Surface Mounted technique (NSM). Due to the excellent bond conditions between SHCC and CFRP laminates, these reinforcements provide the necessary tensile strength capacity to the HCP. To examine the efficiency of HCPs as a shear strengthening technique, a total of 17 RC beams are tested. Seven of these beams have a rectangular cross-section and ten have a T cross-section. The influence of the percentage and inclination of the CFRP laminates on the shear strengthening effectiveness of HCPs is investigated. Two different processes for applying the HCPs to the beams’ concrete substrate are examined: 1) using epoxy adhesive; and 2) using mechanical anchors in addition to the epoxy adhesive. It is demonstrated that when only epoxy adhesive is used, the shear strengthening contribution of the HCPs is limited by the tensile strength of the concrete substrate of the strengthened beams. When mechanical anchors are applied in addition to the epoxy adhesive, the shear strengthening potential of HCPs is fully mobilized. An analytical model is proposed to predict the shear strength of RC beams strengthened with HCPs. It is demonstrated that the analytical model predicts with good accuracy the shear strength of RC beams strengthened with HCPs.The study reported in this paper is part of the project “PrePam –Pre-fabricated thin panels using advanced materials for structural rehabilitation”, with the reference PTDC/ECM/114511/2009. The first author acknowledges the grant provided by this project