463 research outputs found
Effect of anti-corrosive coatings on the bond between corrosion-damaged rebar and concrete repair materials
Reinforcement corrosion is the largest contributor to the deterioration of existing RC structures worldwide. The aggressive electrochemical process causes destruction to the steel by a loss in cross section and rib height, which in turn affects the bond between steel and concrete and consequently the structural performance of reinforced concrete structures. During a repair strategy, when the patch repair method is considered, one of the steps usually involves the application of a protective coating to the corroded and subsequently cleaned steel. It is well accepted that protective coatings offer corrosion resistance to the steel in reinforced concrete repaired elements. However, information on how well the coated rebar bonds to the new surrounding repair concrete is limited. Hence, developing an understanding on the bond performance of repaired coated rebar is important for engineers in the construction repair industry. One commercially available epoxy-modified, cementitious coating material was considered in this study, Sika ® Armatec ® -110- EpoCem, and applied with one or two coats of 0.6 mm each. Rebar corrosion damage levels of 0%, 10% and 20% (of steel mass loss) were simulated by mechanical grinding of sound rebar samples, attempting to represent the condition of cleaned corroded rebar. Two steel bars, Y12 and Y16, of different diameters were considered. Three repair materials with CEM I 52.5N were considered in this study, which included one concrete with a w/b ratio of 0.45 and two mortars with w/b ratios of 0.47 and 0.65, referred to as C45, M45 and M65, respectively. Pull-out testing was conducted on a total of 108 specimens to assess the effect of coating thickness on the bond with respect to the different parameters of this study. The results indicate that the w/b ratio of the mortars had a significant influence on the bond and the addition of stone showed no difference on the failure loads obtained. There was no significant effect of the rebar diameter on the pull-out force for uncorroded rebar, although with cleaned corroded rebar the effect was significant. Specimens with 10% mass loss had larger reductions in pull-out force compared to those with 20% mass loss. While coatings reduced the bond of uncorroded and cleaned corroded rebar, the thickness of the coating had practically no influence on the pull-out force. Using C45-12 with two coats on 20% corroded steel, the pull-out force was 80% compared to before corrosion. Failure of uncorroded specimens was dominated by splitting, and slip was mostly seen for cleaned corroded specimens. With coatings applied to uncorroded bars, slip coating failure was evident with uncorroded steel while splitting with coating failure was evident especially with Y16. From the corroded coated specimens, there appeared to be good contact between the cleaned corroded steel and repair material
Flexural strengthening by means of a arc overlay in the tension zone
Dissertação para obtenção do Grau de Mestre em
Engenharia Civil – Perfil de EstruturasReinforced concrete structures may be flexurally strengthened in the tension zone by the addition of a new concrete layer with embedded steel reinforcement. However, the
interface between the new and old concrete creates a weak area that may cause
failure to occur for loads inferior to the designed. Therefore the success of this
technique relies on the interface capacity to transmit stress.
Surface treatments and cleanliness are the two main aspects that affect adhesion
between different age concretes cast against each other. Nevertheless, the amount of
steel reinforcement crossing the interface is a factor that may overcome adhesion and
improve shear resistance. In the last years, several studies have been elaborated on
this theme.
In the present dissertation, an experimental study was conducted in order to evaluate
the performance of this strengthening solution, characterize failure modes and shear resistance at the interface. Simply supported beams with a bonded concrete overlay in the tension zone were subjected to a three point bending test, and interfacial failure was forced. Three different solutions for stress transmission across the interface are analyzed. In the first solution, shear transmission relies solely on adherence, whereas the two remaining solutions were developed with the aim of evaluating the contribution of reinforcement crossing the interface. Regarding the strengthening solutions with
reinforcement crossing the interface, the difference between the effect of reinforcement concentrated near the borders of the new concrete layer or equally
distributed along the interface is studied.
During the tests, ultimate load, deflections, steel strains, cracking and rupture modes were analyzed. The obtained values were compared among each other and with the provisions of EC2 and MC2010
Comparison of bond stresses of deformed steel bars embedded in two different concrete mixes
Catenary action in a precast concrete building structural system is one of the ways to avoid progressive collapse. The key for catenary action to work successfully depends on the strength performance of longitudinal ties, which closely depends on the bond performance between the ties and concrete. This paper investigates the effectiveness of deformed steel bar as catenary tie in precast concrete beam-column connection under column removal scenario. The main objective of the experimental work is to improve the bond performance between deformed steel bar and concrete topping. The parameter considered in the tests is the types of concrete for the topping. The different concrete mixes are normal concrete of Grade 40 and steel fiber reinforced concrete (SFRC). A series of pullout test specimens are conducted to investigate the bond behavior between the steel ties and the surrounding concrete. The results show the comparison of bond stresses of embedded deformed steel bars in two types of concrete mix. The deformed steel bar with concrete fiber provides higher bond strength as compared to bond in normal concrete. Therefore, it is more suitable for effective catenary tie in precast concrete beam-column connection for maximum efficiency and deformability in order to minimize progressive collapse
Experimental investigation on the shear characteristics of GFRP reinforcement systems embedded in concrete.
To mitigate the deterioration of steel-reinforced concrete members, a fiber-reinforced polymers (FRPs) system has been introduced and has increasingly been used to replace the conventional steel reinforcing bar. However, questions remain about the performance of the Glass Fiber Reinforced Polymer (GFRP) reinforcing bar in concrete with varied stress orientation and shape. The GFRP reinforcement is an anisotropic material that possesses low strength for the transverse direction. This paper presents the results of the shear performance of GFRP reinforcement crossing varied crack angles. Fifteen push-off specimens were tested to investigate the shear characteristics of the GFRP and steel reinforcement. Tests were performed with three varied orientations of steel and GFRP reinforcement embedded in concrete: 90, 45, and 135-degrees with respect to the shear crack plane. In addition, the group-effect of GFRP reinforcement is also investigated with two reinforcing bars. Results indicate that the contributions of aggregate interlock and GFRP reinforcement are significantly varied depending on the bar orientation. Varied orientation of the GFRP bar across the crack plane allows for different failure modes of the reinforcement and absorbed energy capacities. Maximum shear capacity is obtained in specimen with 135-degree orientation accompanying with minimized crack width. This indicates that 135-degree orientation promoted higher aggregate interlock and sufficient development of strength in the reinforcement
Behaviour of Helical Pile connectors for New Foundations
Despite the wide application of the connections between slender pile types, which end with a mono steel bar at the ground level (e.g. helical piles and micro piles), and new reinforced concrete foundations with limited width (e.g. RC grade beams) in the piling industry in North America, neither a clear understanding of the connections\u27 behaviour nor a specific design criteria for their implementation is presented.
The main goal of this research was to clearly understand the behaviour of these connections and their failure mechanism under monotonic and cyclic loadings. The research methodology involved conducting experimental tests on 33 full-scale pile-foundation connections subjected to tension, compression, and shear loadings. The experimental results were used to calibrate a three-dimensional nonlinear finite element model that accurately simulated the structural behaviour and captured the possible failure modes of these connections. Based on the findings from the experimental and numerical investigations, analytical equations were developed to determine the connection capacity.
Both the experimental and the numerical investigations confirmed that it is unsafe to ignore the connection capacity in the foundation design considering only the grade beam capacity. It was shown that the connection behaviour under tension and compression loadings can be represented by the behaviour of the reinforced concrete beams subjected to indirect shear loading, while the connection behaviour under shear loading can be represented by the behaviour of cast-in-place headed anchors subjected to shear loading. The connection behaviour was mainly affected by the concrete compressive strength, the pile embedment depth, the beam\u27s reinforcement, the pile cap configurations, and some other variables depending on the type of loading. Cyclic compression loading had a limited effect on the connection behaviour, while alternating cyclic shear loading had a major effect on the connection behaviour.
The developed connection design equations took into consideration the main factors affecting the connection behaviour under different cases of loading including cyclic loading and they were consistent with the recorded results from the experimental and numerical investigations.
Finally, the research objectives were achieved by providing a design aid and design precautions for helical pile-RC grade beam connections design
Seismic Performance of Reinforced Concrete Beams Susceptible to Single-Crack Plastic Hinge Behavior
Following the 2010/2011 Canterbury and 2016 Kaikoura earthquakes, a number of reinforced concrete (RC) beams in high-rise structures developed a single primary crack at the beam-column interface without the formation of distributed secondary cracks along the beam length. Detailed assessments showed that these beams have conforming longitudinal steel ratios and the single-crack mechanism may be due to design and/or construction practices for beam-column joints in the 1980s. In order to investigate the seismic behavior of reinforced concrete beams with detailing that inhibited the spread of flexural yielding, an experimental program was carried out on RC beam specimens, having similar reinforcement detailing to that of beams that developed a single crack at their ends during the Kaikoura earthquake to understand their seismic behavior, postearthquake repairability, and residual low-cycle fatigue life. Experimental results showed that the beams were able to undergo significant inelastic drift demands without loss of lateral resistance and have sufficient residual drift capacity following moderate and large earthquake demands. The response of the beam specimens was dominated by hinge rotation via the bond-slip mechanism. Comparisons showed that the measured drift capacities of the beams exceeded the predicted drift capacities computed using state-of-the-practice procedures
Role of Force Resultants Interaction on Fiber Reinforced Concrete
Ultra-high performance concrete (UHPC) is a recently developed concrete gaining a lot of interest worldwide, and a lot research has been conducted to determine its material properties. UHPC is known for its very high strength and high durability. Association Francaise de Genie Civil (AFGC) has defined UHPC as a concrete exhibiting compressive strength greater than 150 MPa (22 ksi). To utilize the full compressive strength of UHPC, complementary tension reinforcement is required. A recent research study to find light weight yet high strength alternative deck systems for Florida movable bridges demonstrated that a composite UHPC and high strength steel (HSS) reinforcement deck system is a viable alternative. However, failure modes of the deck system observed during experimental testing were shear failures rather than flexural failures. Interestingly, the shear failures were ductile involving large deformations and large sectional rotations. The purpose of this research is to quantify the sensitivity of UHPC structural member mechanical response to different shear and normal stress demands, and investigate the underlying failure modes. An experimental investigation on small-scale prisms without reinforcement, prisms reinforced with ASTM Grade 60 steel, and prisms reinforced with high strength steel was carried out to capture load-deflection behavior as well as modes of failure of the UHPC specimens. Numerical analysis based on modified compression field theory (MCFT) was developed to verify experimental results at the section level, and further verification using continuum methods was performed using MCFT/DSFM (disturbed stress field method) based finite element analysis software (VecTor2). Results from the numerical analysis could reasonably predict the load-displacement as well as the failure modes of the experimental specimens. Obvious flexural failure was observed on unreinforced UHPC specimens where wide crack opening gradually widened at the bottom fiber of the concrete to the loading position. Whereas UHPC-Grade 60 steel specimens experienced ductile flexural failure with similar wide crack opening after the rebar yielded. On the other hand, UHPC-MMFX specimens largely failed in shear from a diagonal tension crack and crush of concrete top fiber
Effect of FRP Anchors on the FRP Rehabilitation of Shear Critical RC Beams and Flexure Critical RC Slabs
The use of fiber-reinforced polymer (FRP) composites as a repair and strengthening material for reinforced concrete (RC) members has increased over the past twenty years. The tendency for FRP sheets to debond at loads below their ultimate capacity has prompted researchers to investigate various approaches and designs to increase the efficiency of FRP strengthening systems. Various anchors, wrapping techniques and clamps have been explored to postpone and/or delay the debonding process which results in premature failure. FRP anchors are of particular interest because they can be selected to have the same material properties as the FRP sheets that are installed for strengthening or repair of the RC member and can be done so using the same adhesives and installation techniques.
This research study aimed to investigate the effectiveness of using commercially manufactured FRP anchors to secure FRP sheets installed to strengthen and repair RC beams in shear and RC slabs in flexure. Twenty one shear critical RC beams were strengthened in shear with u-wrapped FRP sheets and FRP anchors. Eight RC one-way slabs were strengthened in flexure with FRP sheets and FRP anchors. The test variables include the type of FRP sheets (GFRP,CFRP), type of FRP anchors (CFRP, GFRP) and the strengthening configuration.
The test results of the shear critical RC beams revealed that the installation of commercially manufactured FRP anchors to secure externally applied u-wrap FRP sheets improved the shear behaviour of the strengthened beam. The installation of FRP anchors to secure u-wrapped FRP sheets provided an average 15% increase in the shear strength over companion unanchored beams and improved the ductility of failure experienced with the typical shear failure in beams. The use of FRP anchors allowed the FRP sheets to develop their tensile capacity. Premature failure by FRP debonding was eradicated with the presence of FRP anchors and the failure modes of the strengthened beams with FRP anchors was altered when compared to the companion unanchored beam. Additionally, as the width of a u-wrapped FRP sheet was increased; larger increases in strength were obtained when FRP anchors were used.
The test results of the flexure critical RC slabs revealed that the installation of commercially manufactured FRP anchors to secure externally applied u-wrapped FRP sheets improved the behaviour of strengthened slabs. Installation of FRP anchors to secure flexural FRP sheets provided an average 17% increase in strength over companion unanchored beams. The use of FRP anchors allowed the FRP sheets to develop their full tensile strength. Premature failure by CFRP debonding was not eliminated with the presence of FRP anchors; rather the critical failure zone was shifted from the bottom soffit of the slab to the concrete/steel rebar interface. The failure modes of slabs with FRP anchors were altered for all specimens when compared to the companion unanchored slab.
The effective strain in the FRP sheet was predicted and compared with the experimental results. The efficiency of FRP anchors defined as the ratio of effective strain in the FRP sheet with and without anchors was related to the increase in strength in beams and slabs. A good correlation was established between the FRP anchor efficiency and the increase in strength. A step-by-step FRP anchor installation procedure was developed and a model to predict the number of FRP anchors required to secure a FRP sheet was proposed.
This is the most comprehensive examination of beams and slabs strengthened with FRP sheets and FRP anchors conducted to date. This study provides an engineer with basic understanding of the mechanics, behaviour and failure modes of beams and slabs strengthened with FRP sheets and anchors
Seismic behavior of lightly reinforced concrete squat shear walls
This thesis addresses the seismic evaluation of existing buildings. In particular, it focuses on the seismic behavior of lightly reinforced shear walls that are not designed to withstand earthquake actions. A shear strength envelope for the assessment of deformation capacity of these non-ductile walls is presented. The approach is the result of experimental investigations and analytical modeling. Existing models for plastic hinges in beams are enhanced in order to determine drift capacity of lightly reinforced concrete shear walls. The static-cyclic behavior of non-ductile, reinforced concrete shear walls is investigated by testing four small-scale specimens of shear span ratio equal to 0.8. The design of the specimens includes reinforcement ratios, and axial force levels in existing shear wall buildings. Although the specimens were expected to fail in brittle shear, low to moderate ductile response is obtained. The deformation capacity, not the shear strength, is found to be restricted by shear failure. It is observed that inherent shear strength of concrete and the concrete compression zone are the principal contributors to the shear capacity of lightly reinforced shear walls. It is also observed that low reinforcement ratios and moderate levels of axial force can efficiently prevent brittle response in shear. The analytical model consists of a plastic hinge over the entire height of the low-rise shear wall. Proposals are made for the strain distribution inside the plastic hinge. Explicit relationships between drift and base shear are established and it is found that the model accurately predicts the envelope curve of static-cyclic loading. The shear strength envelope is formulated by using the analytical model. Criteria for the failure modes of diagonal tension, of concrete crushing, and of sliding enclose the shear strength envelope. In addition, inherent shear strength forms the lower bound of this envelope. The contributions of reinforcement and concrete to shear capacity are formulated in terms of initial strength and strength decay. Accurate prediction of both the ductility supply and the drift capacity obtained in static-cyclic tests is observed. Validation of the shear strength envelope on full-size walls prevalent in existing buildings shows potential for further application. The proposal contributes to more realistic evaluation of shear strength in selected situations where available methods are too conservative. Hence, it allows for both avoiding costly seismic strengthening in such situations and better allocation of resources where they are really needed
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