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Bond and anchorage of ribbed bars in high-strength concrete
Abstract
For structural utilisation of high-strength concrete, a good understanding of the bond and anchorage behaviours of ribbed bars in this specific material is essential. The research project aims to increase the knowledge of bond and anchorage behaviours of ribbed bars in high-strength concrete. Furthermore, the validity of the design method in Model Code 1990 aimed for normal strength concrete is investigated. An extensive experimental study was carried out to examine the bond and anchorage behaviour in high-strength concrete compared with that in normal strength concrete. Tests on four types of specimens were conducted. Detail tests and all major anchorage regions (support regions, cut-off regions and splice regions) are treated. For each high-strength concrete specimen tested, a reference specimen of normal strength concrete with the same detailing was also tested. Both simple numerical modelling based on schematic bond stress-slip relationships and more complex non-linear finite element modelling were used to complement the experiments. The study showed that the bond and anchorage behaviours were similar in both concrete types, and governed by the same phenomena. The bond strength in well-confined concrete increased linearly with the concrete compressive strength. Furthermore, high-strength concrete gave a much stiffer local bond stress-slip relationship than normal strength concrete. If the yield capacity of the reinforcing steel was reached, the local bond resistance decreased substantially, especially in high-strength concrete. Schematic local relationships between bond stress and slip are proposed for anchorage in confined normal strength and high-strength concrete; these take into account the bond deterioration when the yield capacity of the reinforcement steel is reached or exceeded. Although the anchorage capacity of a high-strength concrete specimen was higher than that of a comparable normal strength one, the increase was not in proportion to the rise in compressive strength, unlike the local bond strength under well-confined conditions. However, the high-strength concrete showed less capacity for stress redistribution, both along anchorage or lap lengths and between bars anchored in the same region. This can result in premature and brittle failures, unless compensated by a sufficient amount of stirrups. The design method examined usually gave results on the safe side when compared with test results. However, the scatter was large and there was a tendency for the safety to decrease with greater concrete strength.
... due to the presence of the concrete cover and internal transverse steel). Nevertheless, in the majority of conventional laboratory-based bond tests, different sources of confinement are present and cannot be readily decoupled [30][31][32][33][34]. In the current work, a new bond testing configuration was adopted, which allowed for internal confinement effects to be isolated and a particular stress-state to be achieved in the concrete. ...
... Attempts have been made to remove such pressure from the bonded region of pull-out bars by adopting debonding sleeves over the supports [30,32,33] or effectively suspending the specimens with indirect supports above the bonded length [34]. However, it was reported that the effects were not completely removed [30,31,33,34]. This has been highlighted as a drawback of existing experimental setups. ...
... This new geometry allows for the development of a stress field in the concrete that is more representative of that on the underside of flexural beams, or the bottom of a halfjoint, without the parasitic effect of support pressure. On the loaded (active) side of the tested bars, the specimens had an inclined face, similar to the geometry proposed by Magnusson [31]. In addition to replicating the shape of a bending-shear crack of a conventional beam, thus inducing a more relevant stress field, this geometry allowed for the length of debonding sleeves to be minimised [26,30,33]. ...
The strength assessment of existing reinforced concrete structures is challenging when deterioration and detailing deficiencies are present. The anchorage of the internal steel reinforcement can be critical under particular confinement conditions of external pressure, concrete cover and transverse reinforcement. The interaction between different confinement contributions and their effects on bond were investigated with a novel experimental set-up that removed the parasitic support pressure. With significant transverse reinforcement and varying concrete cover, maximum bond strengths were obtained for intermediate cover-to-diameter ratios. The transverse reinforcement contribution depended on the occurrence of splitting cracks. These findings can lead to improved assessment and maintenance strategies.
... The specimens had two ø8 stirrups in the support region. The tests are described more in detail in (Magnusson, 2000). The finite element mesh and boundary conditions adopted for the analyses are shown in Fig. 6a. ...
... Particular emphasis was placed on the effects of concrete strength, confining stirrups, and splice length. The beams were part of a larger study (Magnusson, 2000). ...
... The beam with the stirrup index of confinement equal to 0.041, HSC 58, displayed a typical gradual failure with a high residual capacity. This failure mode was similar to a splitting-induced pull-out failure that was observed in anchorage regions at supports (Magnusson, 2000). ...
... The yield capacity of the steel was approximately 500 MPa. For more information about the properties of the reinforcement, see Magnusson (2000) and Lundgren (1999). The compressive and the transverse reinforcement were modelled as "embedded" reinforcement, meaning that perfect bond between the concrete and the reinforcement was assumed. ...
... A total of sixteen beams were tested; six of these were analysed and will be discussed in greater detail here. For more information about the tests, see Magnusson (2000). The beams had tensile reinforcement spliced in the mid-span, and the effect of transverse reinforcement along a fully lapped splice was studied. ...
... The study was carried out using two concrete types, a normal-strength and a high-strength concrete with cylinder compressive strengths of about 25 MPa and 100 MPa respectively. For more detailed information, see Magnusson (2000). ...
A model describing the bond between deformed reinforcement bars and concrete was developed, which considers the splitting stresses of the bond action. The model was previously verified against pullout tests of various geometries. In this study it is used in 3D finite-element analyses of different anchorage situations. One application was lapped reinforcement splices in beams and in frame corners. Another application was bars anchored in beam-ends with varying support conditions. When no additional confinement from the support reaction was present, splitting cracks appeared that reduced the anchorage capacity both in the analyses and in the tests. Analyses and tests of the anchorage at end regions of simply supported beams showed that the main part of the tensile force transfer between reinforcement and concrete takes place above or close to the support. It can be concluded that the bond model could describe the behavior of the different anchorage situations for both normal- and high-strength concrete in a satisfactory way, and thus can be used for further studies of different anchorage situations.
... Normal strength concrete is commonly reinforced by steel reinforcing bars to serve integrally resisting the applied loads through composite action. Consequently, efficient bond between concrete and steel bars improves the overall performance of the composite material under different types of loading including load carrying capacity, deformation and stiffness [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Therefore, it is very essential to achieve sufficient bond for satisfactory performance of reinforced concrete structures. ...
... The latter is primary for deformed bars (See Fig. 1). Chemical adhesion has minor effect, which disappears at the first slippage of rebar leaving the transfer of loads between rebar and surrounding concrete occur through the interlocking [2][3][4]9,10,17,18]. Factors affecting bond strength comprises concrete strength, concrete cover thickness, reinforcement geometry and size, reinforcement position, bonded length and amount of transverse reinforcement [15,19,20]. ...
Earlier research efforts related to concrete-to-steel rebar bond behavior predominantly focused on common parameters such as concrete strength, rebar diameter, bonded length and concrete cover, disregarding the actual/ real state of concrete that comprises tension cracked regions. This investigation aims to study the concrete-steel rebar bond behavior through bending stressed specimens, in order to evaluate the influence of concrete cracking coupled with other parameters to achieve a realistic simulation. Accordingly, beam-end specimen(s) have been modified to yield the possibility of conducting pull out testing under certain bending stress level. In this study, ten test specimens were prepared and tested to cover some parameters including concrete strength, rebar diameter , bonded length, concrete cover thickness and confining stirrups in conjunction with bending stress level. Test results proved remarkable influence for stress level on bond behavior whereas, unstressed specimens showed higher bond strength than partially and fully stressed specimens by 40 and 63%, respectively. New models are proposed to predict bond strength and describe bond stress-slip behavior for stressed members. The proposed models demonstate adequate agreement with the experimental results.
... Analyses of pull-out tests with differing geometries and with both monotonic and cyclic loading showed that the new model is capable of dealing with a variety of failure modes, such as pull-out failure, splitting failure, and the loss of bond when the reinforcement is yielding, as well as dealing with cyclic loading in a physically reasonable way. Results from Magnusson (2000) also indicate that the effect of outer pressure is well described by the improved version of the model. ...
... Such parameter studies can serve as a basis for design codes. The model is calibrated for normal strength concrete, but analyses by Magnusson (2000) show that this calibration also gives satisfactory results for high strength concrete. Hence, it is likely that the calibration would also be useful for other types of concrete, e. g. light weight concrete or fibre reinforced concrete. ...
A new theoretical model which is especially suited for detailed 3D analysis of bond in reinforced concrete was developed. In the new model, the splitting stress of the bond action and on the radial deformation between the reinforcement bar and the concrete is included. The model was used in detailed 3D analysis of frame corners. The tests and analysis showed that splicing the reinforcement in the middle of the corner has advantages over splices placed outside the bend of the reinforcement. It also indicated, in agreement with previous work, that there are no disadvantages in splicing the reinforcement within the corner of a frame subjected to closing moment.
... Pull-out tests carried out by Magnusson 8 and Balázs and Koch 10 were analysed. Magnusson had concrete cylinders with a diameter of 300 mm and an embedment length of 40 mm; Balázs and Koch had concrete specimens with a quadratic cross-section 160 3 160 mm and an embedment length of 80 mm. ...
... This also reflects the behaviour of the model presented. A pullout test similar to Magnusson's one with short embedment length, 8 but with a reduced cover, was analysed both with and without a confining outer pressure. The cover in these analyses was 30 mm. ...
The bond between ribbed bars and concrete is influenced by a number of parameters, such as the strength of the surrounding structure, the occurrence of splitting cracks, and yielding of the reinforcement. A model for three-dimensional analyses was developed earlier by the author, where the splitting stresses of the bond action were included, and the bond stress depended not only on the slip but also on the radial deformation between the reinforcement bar and the concrete. The bond model, however was shown to generate energy for some special loading-unloading sequences. This undesirable effect has led to a change in the formulation of the bond model. With the modification as described here, the model becomes equivalent to the Coulomb friction, complemented with a yield function describing the upper limit. Pull-out tests were analysed, using the modified bond model and non-linear fracture mechanics for the concrete. The tests were selected to show various types of failure: pull-out failure, splitting failure, pull-out failure after yielding of the reinforcement, rupture of the reinforcement bar and cyclic loading. The results show that the modified model is capable of predicting splitting failures, and the loss of bond if the reinforcement is yielding, as well as simulating cyclic loading in a physically reasonable way.
... These transverse micro-racks are also called bond cracks (Lundgren 1999). (Magnusson J., 1997) Further bond is also provided by the friction and wedging action between the cement paste and the pitting of reinforcing bars (Wei Yu 2006). Over the slipping length, only the friction drag remains, and the highest adhesive, bond stress can act only close to this slipping portion (Gan 2000). ...
The seismic retrofitting of existing multi-storey multi-bay reinforced concrete (RC) frame buildings by the conversion of selected bays into new RC infilled walls is the subject of this research work. The parametric study of the contribution of dowels that connect a new RC infill wall to the surrounding RC frame members was performed through nonlinear dynamic analyses of a numerical finite element (FE) model. The FE model was simulated in DIANA finite element analysis (FEA) software in order to study the effectiveness of the seismic retrofitting of existing structures with the conversion of selected bays into new infilled RC walls for the retrofitting of a multi-storey multi-bay RC frame building. A two-dimensional (2D) frame was modeled, and nonlinear transient analyses were performed to calibrate it using the experimental results obtained from a full-scale experiment found in the literature. The description of the experimental results and of the FE model simulation of the test specimen is provided along with a comparison between the experimental results and the numerical ones. Based on the preliminary results it was concluded that the number of dowels used in the experiment resulted in a monolithic behavior of the RC infilled frame. In order to complement the experimental results and to study the interaction between RC infills and the bounding frame both at the global and local level, numerical simulation experiments were performed by reducing the number of dowels starting from a spacing of 100mm (monolithic) to no dowels. For each scenario, nonlinear response-history analysis was performed and an evaluation of the numerical results of each of these scenarios was subsequently performed. The parametric study of the number of dowels connecting the wall to the bounding frame is presented and conclusions are drawn. The parametric results provide a basis for the development of a general model for the design of RC infills in existing RC frames, particularly regarding the connection details of the new RC infill walls to the existing bounding frame.
... As can be observed, ascending and descending branches exist [446,465]. Initially, the bond between a bar and surrounding matrix depends on adhesion, as can be seen in Part A of the curve. When the pull-out force increases with increasing slip, the adhesion will undergo damage (Part B). ...
Currents flowing along paths not being elements of a purpose-built electric circuit, are called stray currents. Various types of reinforced concrete structures (such as viaducts, bridges and tunnels) in the neighborhoods of railways may be subjected to stray current leaking from the rails. In these cases the concrete pore solution acts as an electrolyte, and the reinforcing rebars (or pre-stressed steel wires) embedded in concrete act as conductors, which can “pick up” the stray current and can corrode.The understanding of stray current-induced corrosion of steel rebar in concrete still remains unclear, as it is challenging to inspect in detail the full scale of steel rebar, as embedded in concrete. Most of previous understanding and preventive measures for stray current corrosion refer to investigations or field tests on pipelines. Besides, it is difficult to rebuild or repair the structures under or near rail transits. All above reasons reflect that stray current corrosion of reinforced concrete structures is in need of more in-depth investigation and understanding.As an expansion of the current body of knowledge of stray current corrosion of steel rebar in cement-based materials, this research aims to be a step forward towards for a better understanding of stray current corrosion mechanisms, a basis of feasible preventive measures for stray current-induced corrosion of reinforced concrete structures.
... To the authors' knowledge, no such tests have been conducted on specimens from existing structures. Another challenge with beam-end tests is the support pressure acting on the anchorage zone, which substantially increases the anchorage capacity (Magnusson 2000). Beam-end specimens are therefore commonly produced with a bond-free zone above the support (Zandi et al. 2011). ...
Mimicking natural deterioration in accelerated tests is challenging; a highly relevant alternative option is to use deteriorated specimens from decommissioned structures. This paper describes a methodology to select and design tests of the bond and anchorage between reinforcement and concrete in such specimens, with the aim of providing general information, needed when developing methods for assessing structures in general. The methodology includes the following steps: (1) choice of existing structure for samples, (2) choice of test method, (3) design of test setup, and (4) design of test programme. Each step is discussed in detail and comments are made on considerations and challenges arising specifically due to the use of specimens from existing structures. As the scatter of test results is typically large, a suitable test method should enable a large number of tests by being robust, quick and affordable. It is recommended to keep track of the position of the specimens in the original structure, to document cracks, and to take samples also of uncorroded bars. These can then be used for reference in quantifying the corrosion level of corroded bars. This methodology is exemplified in the design of three test series on edge beams from two bridges; two series resulted in beam test setups and one in direct pull-out tests. The methodology described strongly highlights that careful investigations are required to design experiments which generate reliable data. Acquiring data from decommissioned structures will improve our understanding of the structural behaviour of existing structures and thus enable improved assessment methods.
... The bond inside reinforced concrete is made up of chemical adhesion, friction and mechanical interlocking (Magnusson, 2000). For plain bar, the bond resistance is only made up by adhesion and friction between bar and surrounding concrete. ...
A study about bond strength of normal strength concrete (NSC) and Steel Strap Confining Technique (SSTT) confined NSC was presented. A series of 8 specimens pull-out test were carried out to investigate the bond strength of short embedment (5Db) in SSTT confined NSC. The concrete compressive strength was about 45 MPa meanwhile the 12mm diameter reinforcement tensile strength was about 500 MPa were used in the pull-out specimens. In order to determine the effects of lateral confinement pressure of steel strapping, three groups of different steel strap gap distance pull-out test were conducted and compared with control specimens and previous theoretical bond stress equation. It was found that SS-B pull-out specimen exhibited highest bond strength and about 40 percent higher compared with Cont-B specimen as lowest bond strength pull-out specimen in this study.
... Depending on, for instance, the size and orientation of the elements used and what load case is studied, one cannot be sure that the cracks will localise within one element even though accurately modelling the bond-slip relation; see for example Johansson (1997) and Lundgren (2000). Neither will the difficulty automatically be resolved by using the embedded crack model (Magnusson 2000), since it will encounter the same type of problem as the smeared crack approach, perhaps making it necessary to adjust the stress-strain relation used here too. To summarise, if the crack extension is assumed as the mean crack spacing and this length is larger than the length of the elements used, the results obtained will probably be somewhat too brittle. ...
From a safety point of view it is important that a concrete structure, apart from necessary load capacity, also is able to show ductile behaviour that allows redistribution of forces so that a local failure not lead to total collapse of the structure. A structure’s ability to exhibit such behaviour is highly dependent on the reinforcement detailing of the joint connections between its independent members. Accordingly, to obtain sound structural behaviour, the joints should ideally be as strong as the structural members connected to them and show ductile behaviour in the ultimate limit state. Even though fulfilling these requirements, the reinforcement detailings in frame corners previously prescribed in the Swedish Shelter Regulations for the design of civil defence shelters were difficult to apply correctly. Therefore, a simpler method, by which all reinforcement bars are spliced within the corner region, was proposed and evaluated.
To examine the effectiveness of the new detailing when subjected to a positive (opening of the corner) or a negative (closing of the corner) moment, a combination of a literature survey, static full-scale tests and static non-linear finite element analyses has been carried out. The main parameters varied were the reinforcement detailing and the reinforcement ratio, and it was found that the new alternative is suitable to use. However, since a civil defence shelter is designed to withstand transient loading, such as blast waves from a nearby explosion or the impact of falling masses from a collapsing building, this also has to be taken into consideration.
When subjected to such loads, the response of a structure may be quite different from that of static loading. The peak load for transient loads is often several times higher than the “equivalent” static load used in the design of the shelter. Further, if the load is applied fast enough, it is possible that part of the structure will still “not be aware” of the loading when another part reaches failure. Therefore, non-linear finite element analyses based on explicit integration were carried out to increase the knowledge of how a structure behaves when subjected to such transient loads. It was found that even though the global structural behaviour may differ widely there are still large similarities in the local behaviour between a static and a transient load case. The results imply that the conclusions made in the previous static studies are valid also for a transient loaded structure.
... Most of these parameters are chosen as in CEB (1993). The only exception is the maximum bond strength, max , for confined concrete, which is chosen as suggested in Magnusson (2000), to better correspond also for high-strength concrete. In Figure 1, the bond-slip curve resulting from the reformulated equation (8) is compared to the one given in the CEB-FIP Model Code; as can be seen, the difference is small. ...
... Shear failure in reinforced concrete beams is principally indicated by propagation of diagonal shear cracks in the shear span zone. The occurrence of diagonal shear crack affects the distribution of tensile force along the longitudinal reinforcement and a significant quantity of tensile force develops at the support [1][2][3]. In his extensive report, Mylrea [4] suggested that extending the bar past the support may Tensile force on stirrups at yield result in a better tensile force distribution and provide higher bond capacity. ...
This paper presents a study on the effect of end anchorage length and stirrup ratio on bond and shear capacity of concrete beams reinforced with Carbon Fiber Reinforced Polymer (CFRP) bars. This study was carried out using test data on nine simply supported reinforced concrete beams with stirrups. The
beams were subjected to two point monotonic loads and the test variables were the length of end anchorage and the stirrup ratio. Theoretical equations for calculating bond strength and shear capacity obtained from literature were applied and then compared with experimental values. Beams with inadequate end anchorage length showed premature bond failure even when there was
sufficient stirrup ratio. Beams with adequate end anchorage length failed in shear or flexure modes depending on the stirrup ratio. A numerical model for bond stress and slip in response to pullout forces was also used to determine analytically the bond stress distributions along end anchorage. The behavior of tensile force acting on the stirrups was also examined. Finally, a simple model
for predicting tension force acting on the stirrups was proposed.
... Due to this it seems incorrect to consider this a splitting failure. Instead a combined failure mode formulated by Magnusson (2000), was adopted; splitting induced pull-out failure. For Series 1.0a and 1.0b, the cracks keep open symmetrically around the specimen, and the specimens fail in pull-out, see Figure 6.10Figure 6., which shows results for Series 1.0b. ...
... Furthermore, it is well known that adhesion, mechanical interaction, and friction are considered the three main stress transfer mechanisms between concrete and steel bar in RC elements [46,47]. Chemical adhesion is created by chemical bonds and stresses developing during the curing process of concrete, prevailing in plain bars and it disappears when initial cracks occur in the concrete/steel interface. ...
To investigate bond-slip behavior of reinforcement in high volume fly ash concrete (HVFAC), 189 pull-out specimens are studied under monotonic static load in this paper. The main research variables involve the volume of fly ash, the type and diameter of the steel bars and the water-to-cement ratio (w/c). The tensile loading in this study is applied to steel bar, which increases stably by controlling the gradual increase of steel bar’s slip until end of the tests. For each specimen, the complete relationship curve to bond stress and slip are collected. Results indicate that the bond strengths of steel bars increased along with the decrease of the w/c ratio and decreased when the diameter of steel bar increased. Other results also show that the type of steel bar has a significant influence on bond and slip behavior and similar bond-slip relationship curves are presented in HVFAC, compared to conventional concrete (CC). To assess the feasibility of existing bond strength models in HVFAC, predictions from the models are compared with experimental results in the study. Based on the analyses and comparative results, a revised ultimate bond strength model and bond–slip relationship model are proposed to evaluate behavior of deformed steel bar in HVFAC. The first model is affected by the volume of fly ash and could evaluate the bond strengths well, the second one could monitor complete bond-slip curve reasonably. In addition, using the above revisions to the bond behaviors of bar in HVFAC, a simple calculation method for the embedment length of deformed steel bar in the concrete is recommended, because it has a stable design safety reserve.
... After the occurrence of inclined cracks, the tensile force in the reinforcement in the shear span increases. This tensile force in the reinforcement must be transferred to the concrete by bond stresses, and this is called the anchorage (Magnusson, 2000). Based on this definition, the available anchorage length was measured from the crack patterns that appeared after the tests: it was measured from the point where the inclined shear crack met the tensile reinforcement bars to the end cross-section (see examples in Figure 8). ...
The bond properties of naturally corroded reinforced concrete members were experimentally investigated. Thirteen specimens were taken from the northern edge beam of Stallbacka Bridge, a girder bridge in Sweden. The specimens exhibited different levels of corrosion-induced damage, including concrete cracking and cover spalling. The damage was carefully documented and the specimens were tested in suspended four-point bending tests. Their general behaviour was monitored through measurements of applied loads and vertical deflections. At the same time, the local anchorage behaviour was recorded at the end regions. The test results showed around 5% lower anchorage capacity for damaged specimens compared with the reference ones. The residual bond strength was studied with respect to the observed damage. These tests, and artificial corrosion results from the literature, indicate that the bond strength decreased with increasing maximum splitting crack widths. However, the bond capacity was higher for the naturally corroded specimens. The results thus show an obvious difference between artificial and natural corrosion, and furthermore indicate that the provisions given in fib Model Code 2010 are on the safe side. These test results contribute to further knowledge regarding the structural behaviour of corroded reinforced concrete structures.
... Experimental results demonstrate that under general corrosion bond strength can be reduced by 50% when loss of bar cross section is only 10-12%. Magnusson [2000] developed a local bond stress-slip relation as shown in Figure 5 for a bar with good bond condition and other bond conditions. The same model is used in the present study for corroded bar. ...
This paper presents the mathematical modeling techniques for nonlin-ear finite element analysis of RC structure to incorporate uniform corrosion effects. Effect of corrosion has been simulated as reduction in effective cross-sectional area of reinforcing bar, reduction in bonding phenomena and as reduction in material properties of reinforcing bar such as yield strength and elastic modulus. Appropriate constitutive laws for (i) corroded rebar elements and (ii) bond slip with corroded bar have been described. Procedure has been outlined to determine the global damage indicator by secant stiffness based approach. A corroded RC beam has been analysed to validate the proposed model and results have been compared with experimental response. A RC chimney has been analysed by considering the uniform corrosion effects. The result of corroded chimney shows the growth of damage with respect to increase in age of the structure. The results will give an insight for the maintenance and repair measures to be taken during the service life.
... However, the improvement was less significant when the bar was subjected to cyclic loading (Lee and Hwang 2013;Xin et al. 1992). Thus, the use of HSC appears to be advantageous as far as bond degradation under monotonically increased loading is concerned (Magnusson et al. 2000). Unless a significant column axial load was present, joint cores in a seismic-resistant ductile frame should be designed on the assumption that no shear force could be resisted by the concrete. ...
This paper presents the experimental findings of high-strength concrete (HSC) interior beam-column joints under column axial compressive loading. Eight full-scale interior beam-column joints with varying degrees of reinforcement detailing were subjected to a constant column axial compressive load and quasi-static horizontal cyclic load. The test revealed that HSC improved the bond condition of bars with a larger diameter due to the higher achievable bond stress, thereby allowing for the use of longitudinal beams of a larger diameter. Parametric studies via finite-element (FE) modeling were performed to study the influence of various parameters on the strength and bond of HSC beam-column joints. The study confirmed that reinforcement of larger diameter can be used in HSC beams with the improvement in the bond condition. The presence of axial compressive load improved the bond stress of HSC beam-column joint but a threshold limit should be applied. Last, two standards were reviewed and improvements to the design equations were proposed.
... The experimental evidence to support a reduction in bond length with a portion of bars continuous is limited to specimens with short bond lengths which showed a gain in strength and ductility that would not be realised with longer bond lengths. Referring to the experimental results presented by Magnusson [7], the bond length adopted, equal to 7.5 the bar diameter, was too short for practical purposes and sufficient to develop around only 35 7 55 % of the yield strength of the bars. Consequently, significant enhancements in flexural strength were observed with increasing proportions of continuous reinforcement. ...
Most Design Codes encourage the staggering of lapped splices in a section, such that a portion of the bars in the cross section remains continuous throughout the lap zone. A penalty on lap length may be imposed if all bars are lapped at one section. In the literature, however, almost all tested specimens have had all reinforcement lapped at the same section. This paper presents results of tests carried out to shed some new light on the behaviour of lap splices where only a portion of bars is lapped. To this end, four point bending tests were carried out on 24 full-scale beams with all or part of the longitudinal reinforcement lap spliced at mid-span. The beams were reinforced with either 16 mm or 20 mm diameter rebars, included two grades of concrete and various lap splices configurations, all confined by links. All the beams were designed with the same concrete cover and with the minimum amount of transverse reinforcement permitted by MC2010, equal to 50% of the area of the lapped bars. The resistance and the residual strength of the splices were measured and compared with the results of specimens with continuous bars and with all bars lapped. The results show that lapping only a portion of bars at a section impairs splice strength, although some post-peak strength is maintained by the continuous bars. These outcomes raise questions over the validity of EC2 and ACI 318-11 provisions which allow a reduction in lap-length when splices are staggered.
... The geometry of the eccentric pull-out specimens was similar to that used by Magnusson (2000), which had the shape of a beam-end after inclined shear cracking; see Figure 3.1. The behaviour of the eccentric pull-out test shares some similarities and dissimilarities with a beam-end region. ...
There is a growing need for reliable methods of assessing the load-carrying capacity and remaining service life of corroded structures. Previous research has been mainly concerned with lower corrosion levels leading to cover cracking. Moreover, the main focus of the available knowledge concerns the corrosion of the main reinforcement; while the corrosion of the stirrups is often overlooked. Therefore, these two uncertainties ; i.e. high amount of corrosion leading to extensive cover cracking and spalling and the effect of corroding stirrups, were investigated in an experimental program. Pull-out tests were carried out on eccentrically reinforced specimens with long embedment length to study the anchorage capacity of a corroded bar. The influences of the location of the anchored bar, i.e. middle or corner placement; the presence or absence of transverse reinforcement; the corrosion level of longitudinal reinforcement and the corrosion of transverse reinforcement were studied. The specimens were of three types in relation to the reinforcement arrangement and corrosion: specimens without stirrups, where the main bars were corroded (type A); specimens with stirrups where the main bars were corroded and the stirrups were protected by insulating tape (type B); and specimens with stirrups where the main bars and stirrups were corroded (type C). All of the specimens were subjected to accelerated corrosion, with an average current density of 100 µA/cm2, for three time spans that caused a rebar weight loss up to approximately 20% in the main bars and 35% in the stirrups. All of the specimens showed longitudinal cracks along the main bars for relatively low corrosion levels. The corrosion level at first cracking was about 0.6% 1.0% corrosion weight loss; the cracks widened with increased corrosion levels. Crack patterns formed depended on the presence or absence of stirrups and whether the stirrups were corroded. The crack patterns showed differences between specimens with or without stirrups and when stirrups are corroding or not. The tests showed an important effect of the cover cracking in terms of loss of confinement. They also indicated that the bond behaviour and the failure were strongly governed by the position of the anchored bar, i.e. corner or middle positions, and the level of the corrosion attack. Stirrups played an important role after cover cracking, as they then became the primary source of confinement. The knowledge gained in this study contributes to better understanding of the effects of deterioration on structures.
... Examples of test set-ups: a pull-out test; a beam-end test; a directly supported four point bending test; and an indirectly supported four point bending test. Figures fromMagnusson (2000). ...
... The geometry of the eccentric pull-out specimens was similar to that used by Magnusson (2000), which had the shape of a beamend after inclined shear cracking; see Figure 1. The behaviour of the eccentric pull-out test shares some similarities and dissimilarities with a beam-end region. ...
Eccentric pull-out tests were carried out to study the influence of severe corrosion leading to extensive cover cracking, and the effect of corroded and non-corroded stirrups on the anchorage of deformed bars. The specimens were subjected to a low-rate electrochemical corrosion process for three time spans that caused a rebar weight loss up to approximately 20% in the main bars and 35% in the stirrups. Pull-out tests were then carried out in each specimen, on either the two corner bars or the middle bar, to measure the bond capacity. The effects of corrosion and the mechanical testing were simulated with non-linear finite-element analysis. The combination of tests and analyses gives a better understanding of the effect of high corrosion penetrations and the presence of corroded Stirrups on failure modes. The presence of stirrups, corroded and non-corroded, was found to significantly change the behavior of an anchorage region, namely the corrosion-induced crack pattern, the failure mode and the bond capacity.
... Eccentric pull-out tests were carried out to investigate the anchorage capacity of a severely corroded bar. The geometry of the eccentric pull-out specimens was similar to that used by Magnusson (2000), which had the shape of a beam-end after inclined shear cracking. (Fig.1) The specimens (Fig.2) were of three types with respect to the reinforcement ar-rangement and corrosion of main bars and stirrups: (a) without stirrups, main bars were subjected to corrosion; (b) with stirrups, only main bars were subjected to corrosion; and (c) with stirrups, main bars and stirrups were subjected to corrosion. ...
A recent experimental and analytical research program (Zandi et al., 2011-a, b) provided bond strength values for highly corroded bars. The test program included both main reinforcement and stirrup corrosion. These results must be compared to the rather wide but scattered database of test results in this field. Two relevant sources for comparison are the indications of FIB MC2010, based on the existing literature, and some recently published test results (Regan and Kennedy Reid, 2009). The latter simulated the delamination of the concrete cover by casting the concrete either flush to the bar surface or at mid-barrel i.e. the bars had either no cover or even were partly external to the concrete; the reinforcement was not corroded. The paper summarizes the tests by the authors and develops the comparison with the other studies. The conclusions discuss the choice of bond strength values for the assessment of existing structures and future research outlooks.
... In particular, corrosion-induced cracking and spalling around the reinforcement in the anchorage region precluded direct support against these surfaces. Moreover, a direct support applies an active confinement [13] which is expected to influence the anchorage capacity, for more information see [14]. In addition, the reinforcement bars in question were bundled, which made gripping of reinforcement for direct pull-out tests complicated. ...
p>Corrosion of reinforcement is a significant cause of deterioration in existing structures. While this topic has been extensively studied using artificially corroded test specimens, little is known about the actual response of naturally corroded structures. Overall, the literature suggests that drastic reduction of the corrosion time, i.e. from years to days, in accelerated corrosion process, might markedly influence the bond and the anchorage capacity of a reinforced concrete member.
Consequently, the anchorage capacity of specimens with naturally corroded reinforcement was investigated. Since a large scatter was observed in the tests of the first series, see [1, 2], fourteen more specimens were tested in a second round using samples from the other side of the same bridge. The specimens were carefully documented regarding crack patterns, crack widths, and spalled regions before testing. In all cases, diagonal shear cracks preceded a splitting-induced pull- out failure; i.e. anchorage failure was achieved as expected. Subsequent work will combine results from both test series, such as load versus deflection, free end-slip and crack patterns; and compare these with the predictions from detailed nonlinear finite element modelling, using the available bond and corrosion models. These will result in a better understanding of the structural behaviour of corroded reinforced concrete structures to field conditions.</p
... Note, however, that this series showed the lowest compressive strength, f cm.95d = 55 MPa compared with 63-65 MPa for the other series. When normalizing the bond stress with the compressive strength as suggested by Magnusson (2000), it is seen that all the series show nearly identical initial stiffness and capacity; see Figure 5b. ...
In this study, pull-out tests of specimens with short embedment length and varying fibre content were carried out. The results showed no effect from the fibres on the bond-slip behaviour before peak load when normalized with respect to the compressive strength. After peak, the fibre reinforcement provided extra confinement, changing the failure mode from splitting to pull-out failure. The test results were used to calibrate a frictional bond model in non-linear finite-element analyses. The model proved to yield results in good agreement with the experimental results regarding failure modes, load-slip relation and splitting strains on the surfaces of the pull-out specimens. The tests and analyses in combination confirmed that the fibre reinforcement neither disturbed nor improved the bond properties at the interface layer between reinforcement steel and concrete; i.e. the fibres only provided confinement to the surrounding structure.
... s in model code 2010, the case of splitting with stirrups was also considered in order to obtain an estimate of the amount of stirrups that would yield the same confinement as the fibre reinforcement in these two series. It can be seen that the peak stress and the stiffness before peak, for SCSFRC material, are underestimated by the proposed model. Magnusson (2000) experienced the same when comparing experimental results for high-strength concrete with model code 90 (CEB-FIP, 1993).Figure 15 shows that the peak stress, in the SCSFRC material tested here, is better described by the confined model with good bond conditions. Possibly the stiffness of the ascending branch, and the maximum bond stress, ...
Crack control, one of the main benefits of using fibre reinforcement, depends to a large extent on the concrete– rebar bond. Pull-out tests of specimens with short embedment length were carried out and the results showed no effect from the fibres on the normalised bond–slip behaviour before peak load. After this, the fibre reinforcement provided extra confinement, changing the failure mode from splitting to pull-out failure. The test results were used to calibrate a finite-element bond model that considers both tangential stresses and stresses in the radial direction from the rebar. Splitting cracks may be thus considered in the finite-element analyses. The model proved to yield results in good agreement with the experimental results regarding failure mode, load–slip relation and splitting strains on the surfaces of the pull-out specimens. The analyses revealed that two types of action were active in the cracking process. In addition, the confinement effect of the fibre reinforcement was compared with the confinement of conventional stirrups using the bond model in CEB-FIP model code 2010.
... The detailed 3D bond and corrosion model, developed by Lundgren, was used to analyze test specimens with severe corrosion. The type of the specimen was similar to the ones tested by Magnusson [5], in which the specimens had a shape of a beam-end after inclined shear cracking. The details concerning the specimens, electrochemical corrosion and test set-up are presented in a companion paper subtitled "Part 1. Crack Initiation, Crack Propagation and Cover Delamination". ...
There is a growing need for reliable methods of assessing the loadcarrying capacity and remaining service life of corroded
structures. In an ongoing research by the authors, issues that have not been investigated in the methods and models available
today to calculate the remaining load-carrying capacity of the corroded structures are identified. Two main issues; i.e. high
amount of corrosion leading to cover spalling and the effect of corroding stirrups, were investigated in an experimental program.
Pull-out tests were carried out on beam-end specimens with long embedment length to study the anchorage capacity of a corroded
bar. The specimens were subjected to electrochemical corrosion process leading to different corrosion penetrations prior to
mechanical loading. Details concerning electrochemical corrosion setup, corrosion-induced cracking and numerical modelling
of a corroding bar are presented in a companion paper subtitled “Part 1. Crack initiation, crack propagation and cover delimitation”.
Three types of specimens, with stirrups, without stirrups and with corroding stirrups, were subjected to pull-out test. The
test results showed a significant influence of stirrups not only on corrosion-induced cracking but also on anchorage capacity
and failure mode in the pull-out test. Finally, the corrosion and mechanical testing phases were simulated in a finite element
model using the corrosion and bond models earlier developed by Lundgren [1,2]. The outcomes of the numerical modelling help
to further understand the effect of high corrosion penetrations and presence of stirrups on failure modes observed in the
experiments.
... Most of these parameters are chosen as in CEB (1993). The only exception is the maximum bond strength, max , for confined concrete, which is chosen as suggested in Magnusson (2000), to better correspond also for high-strength concrete. In Figure 1, the bond-slip curve resulting from the reformulated equation (8) is compared to the one given in the CEB-FIP Model Code; as can be seen, the difference is small. ...
Corrosion of reinforcement affects the bond mechanism between reinforcement and concrete, and thus the anchorage. Reliable models describing this are needed especially for assessment of the load-carrying capacity of existing structures. This paper presents an analytical one-dimensional model for bond-slip response of corroded reinforcement. The proposed model is an extension of the bond-slip model given in the CEB-FIP Model Code 1990, and is practically applicable for structural analyses to determine the load-carrying capacity of corroded structures. Furthermore, the anchorage length needed to anchor the yield force is calculated from the bond slip, using the one-dimensional bond-slip differential equation. Results of the proposed model are compared with experimental results as well as results from an advanced three-dimensional finite element model. The suggested model is shown to give results that are consistent with the physical behaviour.
This study was aimed at developing a probabilistic model using Bayesian linear regression approach to examine bond strength behavior between concrete and GFRP rebars. This probabilistic model presents a quantitative description of the epistemic uncertainties of model parameters and model error, which decrease as new information is received. By using the problem mechanics, variables were selected and explanatory functions were developed. Then, based on an observational database including 405 beam test specimens, 47 candidate models were selected based on different combinations of explanatory variables, and the fittest model was selected based on prediction quality, error non-normality, scattering non-homogeneity, residual correlation, and nonlinearity. Afterward, Bayesian regression was used to obtain the probability distribution of the model parameters, and model error was obtained using Bayesian regression. The paper continues by an in-depth explanation of the observation selection process, model development, model diagnostic, step-by-step model reduction process with the removal of ineffective parameters. The analysis results specified the bar position and transverse reinforcement ratio parameters as ineffective parameters. In addition, a comparison between deterministic and probabilistic models showed that contrary to the deterministic model, the probabilistic one could capture uncertainty in the model.
This review paper outlines the recent research works on bond properties of reinforced concrete specimens. Bond strength depends mainly on corrosion of the main bar and stirrups, type of concrete, concrete cover and corrosion rate. The environmental effects also play a dynamic role in the degradation of bond stability within reinforced rods and concrete. To integrate the bond behaviour amidst reinforced rods and concrete, several testing methods have been used and the pull-out experiment is used worldwide among them due to its simplicity. The study is gone through bond toughness amid reinforcement and concrete in different environments in different types of concretes such as high strength concretes, fibre-reinforced concretes, reinforcement corroded concretes, lightweight concrete and recycled aggregate concrete. Bond-slip behaviour is also studied for the lightweight concrete, and the graph visualizes the fact that the curve of bond-slip in lightweight concrete is comparable with the standard concrete. From the literature study, it is remarked that very few investigations have organized on the behaviour of bond in corroded reinforced steel rods. The span of cracks on the surface of concrete that occurs due to corrosion performs a prominent part in estimating the bond stability. Also, the bond toughness varies in real-time corroded structures compared with laboratory corroded techniques. Still, there is a shortage of study in those areas, and advanced tests and investigations are required.
Reinforced concrete (RC) structures constitute a major proportion of the built environment and society relies continuously on their service. Many of these structures were built in the era following the Second World War and are thus approaching the end of their intended service life. The likelihood of deterioration increases with time and so damage caused by, say, corrosion is not uncommon. Also, increased demands are often laid on the load-carrying capacity of existing bridges, aimed at increasing utilisation of the road network by allowing heavier vehicles. Simply dismantling and re-constructing all bridges at the end of their designed service life, or taking needless strengthening measures, is unsustainable. Rather, improved methods of assessing the capacity of existing infrastructure are needed.
The current work has aimed to develop improved, reliable assessment methods. Its focus areas were structures with reinforcement corrosion and structures with cracks from previous loading. Both simplified and advanced methods of evaluating anchorage capacity were developed for concrete structures with corroded reinforcement. The simplified method modifies the bond stress-slip relationship and is calibrated against a large database of bond tests, with the safety margin ensured by deriving partial safety factors. The advanced method is based on finite element (FE) analysis, with tensile material properties altered for elements positioned at the splitting cracks along the reinforcement. The latter method was also investigated for RC without corrosion damage but with cracks from previous loading. Design results from advanced nonlinear FE analyses (meaning results with a proper safety margin) are obtained by applying a “safety format”. The current work investigated whether safety formats available in fib Model Code 2010 also ensured reliable design capacities for structures with somewhat complicated load application and geometry; in this case, a concrete frame subjected to vertical and horizontal loads.
The results indicate that the anchorage capacity may be reasonably well estimated by using the simplified method. The proposed partial safety factors also provided sufficient safety margin. Furthermore, in the advanced anchorage assessment, the capacity could be estimated solely from weakened tensile properties located at the position of the splitting cracks and without input concerning the corrosion level. Moreover, by including cracks from previous loading in advanced modelling, improved predictions of the failure mode, ultimate capacity and ductility were demonstrated. Lastly, in the investigation of safety formats for nonlinear FE analysis, the method of estimating a coefficient of variance of resistance (ECOV), did not reach the intended safety level. However, the global resistance factor method (GRF) and partial factor method (PSF) did.
This work has the potential to improve both simplified and advanced assessment methods, providing more sustainable infrastructure management in the future.
The viability of employing fibre reinforcement to improve the durability performance of RC structures by delaying and/or reducing rebar corrosion and by mitigating the structural impact of corrosion-induced damage have been investigated. Given the enhanced crack control of FRC, it could be advantageous to use fibres in civil engineering structures to decrease the ingress of corrosion-initiation substances. However, the combined use of both types of reinforcement in chloride environments raises questions regarding the potential influence that fibres may have on the corrosion process of conventional rebar.
Long-term experiments were carried out featuring naturally corroded RC elements subjected to different loading conditions and varying crack widths. Complementary short-term experiments were carried out to isolate the influence of fibres on individual parameters governing the process of reinforcement corrosion, such as chloride diffusion, internal cracking and electrical resistivity, as well as on corrosion-induced damage, such as cracking and spalling of the cover.
From the experiments it was found that the ingress of chloride ions into concrete, assessed through migration and bulk diffusion tests, was not significantly affected by the presence of fibres. The internal crack pattern of conventionally RC beams subjected to bending loads revealed a tendency for crack branching and increased tortuosity when fibres were present, which can potentially decrease the permeation of concrete and promote crack self-healing. The time to corrosion initiation, evaluated through half-cell potential monitoring, for fibre reinforced beams were similar or longer than the plain concrete ones. However, the effect of fibres was minor compared to the difference between cracked and uncracked specimens, thus highlighting the importance of cracks for the initiation of corrosion. The DC resistivity was found to be unaffected by steel fibres, indicating that they do not pose a risk for increased corrosion rates. Gravimetric steel loss measurements showed that the corrosion level of reinforcement bars embedded in FRC beams was similar or even lower than for plain concrete beams. Moreover, the examination of the corrosion patterns and a detailed analysis of individual corrosion pits revealed a tendency for more distributed corrosion with reduced cross-sectional loss in FRC. Corrosion-induced cracking of the cover was somewhat delayed by fibre reinforcement, particularly for small cover thicknesses, which was attributed to the additional source of passive confinement provided by the fibres. Thereafter, corrosion-induced cracks were effectively arrested by fibres, which resulted in an enhanced bond behaviour of SFRC with no apparent loss of bond strength and high residual bond-stresses. Fibres also had a positive effect on the residual flexural capacity of corroded beams, which generally displayed a slightly increased load-carrying capacity and rotation capacity compared to plain concrete beams with corroded reinforcement.
The promising results obtained in this study indicate that FRC may be effectively used to extend the service life of civil engineering structures by delaying and reducing reinforcement corrosion as well as by mitigating the structural effects of corrosion-induced damage.
Deterioration and ageing of concrete structures and the increased traffic intensities and loads are some of the major problems facing civil engineers, industry and researchers. Corrosion of embedded steel reinforcement is one of the main causes of deterioration. Deterioration of reinforced concrete may lead to a number of undesirable consequences such as loss of serviceability, loss of load carrying capacity and reduction in safety of structures and traffics. Steel bar corrosion affects the reinforcement itself, the surrounding concrete and the composite action between steel and concrete. The most critical effect is probably the reduction in cross sectional area of the affected bar. If corrosion is allowed to propagate over a sufficiently long period of time and proper interventions not are made, loss of bond may also lead to impaired stiffness and strength of affected structural components.
The main objective of this PhD project is to develop methodology for assessment of deteriorating concrete structures based on nonlinear finite element simulations. For the numerical simulations to be accurate it is required that constitutive and kinematical models represent realistic approximations to the true behaviour of reinforced concrete structures attacked by reinforcement corrosion. Hence, these models must be based on existing experimental evidence of the structural consequences of reinforcement corrosion. However the available experimental data on steel bar corrosion have been obtained on small scale specimens corroded and tested in the laboratory and showed considerable scatter in the published results.
The first part of the thesis deals with presentation of reliable information about change in the maximum bond strength between reinforcement and surrounding concrete and bond-slip behaviour due to corrosion. Available data from published laboratory investigations are collected and compared. Some analytical models for calculation of the residual bond strength and some bond-slip models for uncorroded and corroded bars used in the numerical simulations are presented and discussed in the thesis. An attempt to analyse the effect of the current density on the bond strength and on the residual load-carrying capacity of the concrete beams with corroded reinforcement is also performed.
The second part of the PhD project is focused on validation and verification of the nonlinear finite element analysis of deteriorated and repaired concrete structures through a numerical test program. As a first step, simulations of laboratory experiments on small scale beams are performed to verify the finite element model proposed in this project. Accordingly, loss of steel bar section, and reduced bond between deteriorated concrete and corroded rebar are accounted for in the present work. The load-deflection behaviour obtained from the finite element simulations are generally in good agreement with the test data.
As a second step in the numerical program, analyses of a full scale beam are carried out. A close approximation to the true sequence of loading, deterioration, unloading, repair and reloading until failure, a so-called service life cycle of concrete beams, are simulated using the phase analysis approach in the finite element software DIANA. The results of the phased finite element simulation showed that the failure load of the repaired beam was larger than the experimental and estimated probabilistic values. The central deflection of the concrete beam from the finite element analysis was significantly lower than in the experimental study at both the corrosion and repair stages.
Next, numerical simulations of the service life cycle of a concrete beam without strengthening, where time-dependent effects such as creep and shrinkage were included in the constitutive models for concrete and repair material, were carried out. The results of the simulations showed that the failure load is in good agreement with the failure load corresponding to the upper bound of the 95 % confidence interval obtained in a previous probabilistic study. Initial strains due to shrinkage of the concrete and the repair material were implemented in the numerical analyses according to a cross section type model, and a model for development of free shrinkage strain in beams exposed to one-side drying. Total strain variations across the cross-section of the beam calculated from the finite element analyses are compared to data available from the experimental study.
Finally, effect of some parameters influencing the results of the numerical simulations and the agreement between the laboratory and numerical results are presented.
The paper reports on an experimental investigation into the performance of lapped joints in structural concrete. The investigation focuses primarily on concrete covers close to or below the minimum set for structural reasons in design codes such as Eurocode 2 or ACI 318. Two other parameters are also varied in the tests reported here, namely the proportion of bars lapped at a section and the relative size of the bars within a lapped pair. It is found that a reduction in minimum cover from 1·2 to 0·6 times bar diameter would require an increase in lap length of 21% to develop the same stress in a lapped bar. It is verified that provisions for lapped joints in the fib Model Code 2010 are valid for minimum covers down to 0·5 times the diameter of a lapped bar. It is also found that lap strength does not change significantly when laps are staggered. The finding raises questions over the validity of the α 6 coefficient in Eurocode 2, which is dependent on the proportion of bars lapped at a section. Results demonstrate that, where lapped bars differ in diameter, lap length may safely be based on the diameter and covers to the smaller diameter bar.
Resumo: Esta pesquisa propõe-se estudar a viabilidade de um ensaio de aderência aço-concreto apropriado (Appropriate Bond Test - ABT) para estimativa da resistência à compressão axial do concreto, objetivando empregá-lo como complemento ao controle de qualidade do concreto armado em campo. Originalmente os autores Lorrain e Barbosa (2008) apresentaram a utilização de um ensaio de aderência apropriado, denominado APULOT, para estimar a resistência à compressão do concreto, aumentando as possibilidades de controle tecnológico do concreto armado em canteiros de obras. Os mesmos propõem uma adaptação do método pull-out test (POT) tradicional, normalizado pela RILEM CEB/FIP RC6:1983, por ser este um ensaio de baixa complexidade e de custo reduzido. Para viabilizar o uso de um ensaio de aderência apropriado como ensaio de controle tecnológico do concreto em canteiro de obras é necessário definir um padrão para o mesmo e adaptá-lo da prática experimental do laboratório para o campo. O presente trabalho buscou avaliar os seguintes parâmetros: (1) Tipo de carregamento do ensaio (pull-out e push-in); (2) Influência do tipo de configuração geométrica das barras de aço; (3) Análises da preparação, moldagem, cura e estocagem dos corpos de prova do ABT; (4) Análises quanto à execução do ABT referentes à idade de ruptura, taxa de carregamento e tipo de ruptura; (5) Implementação do ABT em canteiro de obras, avaliando a sua potencialidade de efetuar estimativas da resistência à compressão a partir dos dados da tensão de aderência. Para tanto, foram ensaiadas 26 composições de concreto de classes distintas, com idades entre 3 e 28 dias. Foram, ainda, testadas 8 configurações distintas de barras de aço com diâmetros nominais de 8 e 12,5 mm. Os resultados obtidos mostram que, sob condições padronizadas de ensaio e adotando os coeficientes adequados, a correlação entre a tensão máxima de aderência e a resistência à compressão do concreto é satisfatória, fortalecendo o propósito de consolidar este ensaio como uma alternativa complementar para controle de qualidade do concreto armado.
Abstract:This research proposes a study on the feasibility of bond test steel-concrete appropriate (Appropriate Bond Test - ABT) to estimate the compressive strength of concrete, aiming to use them as a supplement in the quality control of concrete in situ. Originally the authors Lorrain and Barbosa (2008) proposed the use of a modified bond test, called APULOT to estimate the compressive strength of the concrete, increasing the possibilities for technological control of reinforced concrete on construction sites. They propose an adaptation of the traditional method pull-out test (POT), normalized by the CEB / FIP RC6: 1983, because it is a test of low complexity and low cost. To enable the use of the test as a test technological control of concrete in construction site is necessary to define a standard for yourself and adapt it practice experimental laboratory to the field. This study evaluated the following parameters: (1) Type of load test (pull-out and push-in); (2) Influence of the type of geometric configuration of steel bar; (3) Analyses of preparation, molding, curing and storage of specimens of ABT; (4) Review of the implementation of ABT on age rupture, loading rate and type of fracture; (5) Implementation of ABT in construction site, evaluating its potential to make estimates of compressive strength from the data of bond stress. Therefore, 26 different compositions of concrete classes, aged between 3 and 28 days, were tested. Were also tested 8 different configurations of steel bars with nominal diameters of 8 and 12.5 mm.
The results indicate that, under standard testing and adopting the appropriate coefficients conditions, the correlation between the maximum bond stress and the compressive strength of concrete is satisfactory, strengthening the purpose of consolidating this test as a complementary alternative to quality control reinforced concrete.
Many studies on the structural effects of corrosion in reinforcement have been conducted. However, most of them are based on artificially corroded test specimens. Thus, the knowledge available entails one major uncertainty, i.e. whether the results are reliable enough to be used for naturally corroded structures. The purpose of this study was to develop a test method and carry out experiments on naturally corroded specimens taken from an existing structure to investigate the anchorage capacity. Beam specimens were taken from the edge beams of a bridge at repair. The specimens showed corrosion-induced damage to a varying extent from no sign of corrosion to extensive cracking and spalling of the concrete cover. A four-point bending test indirectly supported by suspension hangers was chosen. The beams were strengthened with transverse reinforcement around the suspension hangers to avoid premature failure. Eight successful tests were carried out; in all these tests, diagonal shear cracks preceded a splitting induced pull-out failure; i.e. anchorage failure was achieved as intended. The results showed around 10 % lower capacity for the corroded specimens than for the reference ones. The average bond stress in the anchorage zone was estimated based on the applied load and available anchorage length. The stress was about 16 % lower in the beams with corrosion cracks, and 9 % lower in the beams with cover spalling compared to the reference specimens; there was also a larger variation among the damaged specimens than for the reference specimens. The results extend our knowledge concerning the structural behaviour of corroded reinforced concrete structures during field conditions.
This paper presents the results of an experimental and analytical study on the residual strain behavior of non-prestressed reinforcement in partially prestressed concrete (PPC) beams under fatigue loading. For the strain measurements, fiber Bragg grating sensors were used. Two beams were initially tested under monotonic loading in order to determine their ultimate load-bearing capacity. Six were tested under fatigue loading conditions. The experimental results show that fatigue failure mode of PPC beams is fracture of the non-prestressed reinforcement. Moreover, residual strain in the non-prestressed reinforcement gradually increases during the fatigue life and the corresponding residual strain evolution can be divided into three stages. Finally, an analytical model for estimating the residual strain of non-prestressed reinforcement in beams is developed. Comparisons of the predicted curves with the experimental data indicate a good agreement. The results of investigation in this paper furnish the experimental basis for the fatigue design and analysis of PPC beams.
The paper describes the changes to design provisions for embedded steel reinforcement in the fib Model Code 2010. The changes introduce new coefficients for steel grade and for clear spacing between bars, and extend the range of concrete strengths covered. The way in which the contribution of hooks or anchorages is determined calculated has been revised and the contribution of end bearing to laps and anchorages of compression bars is recognised. The revised rules represent a move away from a distinction between laps and anchorages per se towards a distinction based on the presence or absence of transverse pressure perpendicular to the bar axis within the bond length. The benefits of staggering of laps with only a proportion of bars lapped at a section are reviewed. Finally, the potential impact of performance of laps and anchorages on structural robustness is discussed, and it is concluded that this can only be achieved if bar yield precedes splitting mode bond failures.
Staggering lapped joints increases the complexity of detailing and steel fixing, and may require additional resources and slow construction on site. Major design codes encourage staggering lapped joints in tension by imposing a penalty on lap length depending on the proportion of bars lapped at the same section. There are, however, inconsistencies in the value of the coefficients to be applied, and little evidence is available for validation. A programme of 17 physical tests found no evidence of an increase in strength when laps were staggered, and when allowance is made for increases in transverse spacing, staggering was found to reduce lap strength. Differences in the distribution of bond stress through a lap joint and in the share of the tension force taken by continuous and lapped bars are demonstrated to be responsible for the reduction.
A growing concern for better assessment of existing concrete structures has revealed a need for improved understanding of the structural effects of deterioration. The two most common causes of deterioration in concrete structures are freezing of the concrete and corrosion of the reinforcement. The aim of this study is to deepen the understanding of the structural effects of deterioration with special attention to the bond between deformed bars and concrete. The effects of freezing on the material properties of concrete and the bond behaviour of bars were investigated through experiments. A significant influence of frost damage was observed on the stress-strain response of concrete in compression, tensile stress-crack opening relation, and bond-slip behaviour. Based on this, a set of methods was introduced to predict the mechanical behaviour of reinforced concrete ?structures with a measured amount of frost damage. The ?methodology was applied to frost-damaged beams using non-linear finite element analysis at the structural level. The results? indicated that the changes in failure mode and the effect on failure load ?caused by internal frost damage can be predicted by modelling at the structural level. Corrosion of reinforcement leads to volume expansion of the steel, which can cause cover cracking and spalling; this weakens the bond of the reinforcement. The bond-slip model given in Model Code 1990 was extended to include corroded reinforcement. Analysis of corroded beams using the methodology gave results which are on the safe side. However, for large corrosion penetrations that lead to extensive cover cracking, more detailed modelling of the surrounding concrete and stirrups is required. Under such conditions, when wide cracks develop, the favourable effect of rust flowing through the cracks becomes significant; this decreases the splitting stress around the bar. A previously developed corrosion model was extended to include this phenomenon. The volume flow of rust through a crack was assumed to depend on the splitting stress and the crack width. The splitting stress was evaluated from the strain in the rust, and the crack width was computed from the nodal displacements across the crack. The extended model resulted in more corrosion cracks with smaller crack openings, which better corresponds to the measurements on specimens tested. Eccentric pull-out tests were carried out to study the influence of cover cracking and stirrups on the bond of corroded bars. The extended corrosion model was used in detailed three-dimensional analyses of the tests. The tests and analyses showed an important effect of the cover cracking in terms of loss of confinement and the flow of rust through the cracks. They also indicated that the bond behaviour and the failure were strongly governed by the position of the anchored bar, i.e. corner or middle positions, and the level of the corrosion attack. Stirrups played an important role after cover cracking, as they then became the primary source of confinement. Furthermore, corrosion of stirrups led to a more extensive cover cracking for a relatively low level of corrosion attack. The knowledge gained in this study contributes to better understanding of the effects of deterioration on structures, and can be used primarily for assessment of the load-carrying capacity of existing structures.
Abstract A growing need for better assessment of existing structures has revealed the need for improved understanding of structural effects of deterioration. The two most common causes of deterioration in concrete structures are corrosion of reinforcement and freezing of concrete. Previous research has been concerned primarily with the causes and mechanisms of corrosion and frost deterioration; relatively little attention has been given to the important practical problem of assessing the residual load-carrying capacity of damaged structures. This study offers a methodology to analyze the mechanical behaviour of reinforced concrete structures damaged by frost or the corrosion of reinforcement. It is proposed that the effect of damage be modelled by reducing material properties, such as compressive strength, tensile strength, elastic modulus, as well as by modifying the bond properties and geometry. For frost-damaged concrete, the effect of freezing on material properties was gathered from the literature. It was proposed that the effects of internal frost damage and surface scaling can be modelled as changes of material properties and geometry, respectively. The methodology was used in analyses of concrete beams affected by internal frost damage, by using non-linear finite element analysis at component and structural levels. Comparing the results with available experimental data indicated that the change in failure mode and the decrease of load-carrying capacity due to frost damage can be predicted by using the proposed methodology. For corroded structures, the decrease in ductility and area of the corroded reinforcement and the behaviour of cracked concrete around corroded reinforcement were extracted from previous publications. The one-dimensional bond-slip model given in the CEB-FIP Model Code 1990 was extended to include corroded reinforcement. Furthermore, a method to calculate the anchorage length from the bond-slip relation was developed. The proposed methodology was used in analyses of beams, using both non-linear finite element analyses and analytical methods. Comparisons with results, taken from the literature, showed that the methodology could reasonably well estimate the load-carrying capacity and failure mode of corroded beams. The thesis provides a guideline, for the assessment of concrete structures damaged by freezing or corrosion, which can be applied in engineering practice. The change of material properties for the types of damage is quantitatively described. However, some uncertainties exist: the main ones for frost-damaged concrete are the elastic modulus and bond properties; for corroded bars they are the ductility and the bond properties when the cover has spalled off.
In this work, an axisymmetric plasticity model is used to simulate the concrete-steel interface behavior. A nonlocal correction is here introduced in order to capture the degradation of the bond due to splitting cracks. Damage of the interface is also modeled as a function of the rib spacing, allowing application of the model to different bar diameters. The model is able to capture the transition from splitting to pull-out failure and to yielding of bars with the same set of predefined interface parameters, showing the predictive character of the model. The development of macroscopic cracks is also correctly simulated.
The oxide formed during corrosion has an expansive character which induces the cracking of concrete cover. The properties
of this oxide layer are focussing the interest due the need to introduce its mechanical characteristics in the models to calculate
crack width. In a previous paper it was suggested that the oxide behave as “water” and in consequence it has to be modelled
so, which fitted very well in the experiments and model associated an that time. The attribution to the water was based in
the observation that the rust has not mechanical consistency and was like a suspension. However, much recent papers insist
in attributing other much higher young modulus to the rust. Present paper tries to insist in the original concept by illustrating
the formation of the rust in solution and in mortar. The oxides are of mixed colours evolving in function of the time to contact
to open air and they have not mechanical consistency, being apparently hydrophilic by retaining large amount of water.
The volume increase that takes place when reinforcement in concrete corrodes causes splitting stresses in the concrete. Thereby, the bond between the reinforcement and the concrete is influenced. This effect has been studied both experimentally and theoretically by many researchers. In the current paper, the effect of corrosion on the bond between reinforcement and concrete is investigated and described in a systematic way. Literature studies of experimental work are combined with finite element analyses of different cases. In this way, modelling is used as a tool to give the basic understanding, and the results are compared with experiments. An overview of the effect depending on the reinforcement type, existence of transverse reinforcement and confinement owing to concrete and boundaries is given. This overview is intended to be of help in understanding the phenomena as well as in assessment of existing structures.
The effect of loss of bond for longitudinal reinforcement on the structural behavior of RC beams is investigated in a test series of six beams where nominal length without bond is varied. Loss of bond is created with plastic tubes surrounding the longitudinal reinforcement leaving short bonded lengths over support and at positions where stirrups are crossing the longitudinal reinforcement. Using a bond-stress slip relationship made it possible to see that the total bond force could be calculated as the sum of local bond forces. Maximum bond force occurs at maximum load and when the available bond force decreases the load carrying capacity is also reduced. Even for significant loss of bond, the reduction in shear capacity was moderate, implying that short bond lengths are sufficient to create high bond forces. Local areas with bond contributes efficiently to create anchorage for the longitudinal bars. It was also found that the loss of bond was compensated by increased utilization of stirrups, and that loss of bond does not lead to more brittle types of failure.
The bond of ordinary steel reinforcement in concrete depends on many factors, such as the pullout resistance, the geometry of a concrete member, the placement of a bar in the member cross section, the cover splitting, the confinement caused by concrete and the surrounding reinforcement, the order of bond-crack appearance, and the bond-stress distribution along the bond length. The bond of FRP reinforcement depends on even a greater number of factors. Moreover, the types of FRP bars are numerous. Their surface is weaker than that of steel bars and may fracture by bond forces. The surface of FRP bars is softer and does not create as high local stress concentrations in bond contact points to concrete as the harder steel bars do. This fact often delays the appearance of cover splitting cracks along the bars. However, the load necessary for developing the crack pattern of ultimate splitting failure in concrete is then very dependent on whether the bar surface is glossy or rough. The FRP reinforcement can also be used for external shear and/or flexural strengthening of existing members. For this application, FRP bars are placed in grooves cut on the surface of the member to be strengthened and are fixed there with a cement mortar or epoxy paste. In such an application, the performance of bond between the FRP rod and the mortar or resin and then between the mortar or resin and concrete is critical for the effectiveness of the technique. The presence of two interfaces increases the number of parameters needed to characterize the global joint behavior and introduces new possible failure modes. The fundament for the bond resistance estimation should be an accepted bond philosophy linked to appropriate models. A system of bond tests should provide necessary coefficients for the models.
The bond behavior of prestressing strands is of great importance for the capacity of precast prestressed concrete structures. In the present study, the bond behavior of three-wire strands, and some influencing parameters, were examined by means of steel encased pull-through and push-in tests. The three mechanisms: adhesion, friction and other mechanical actions were found to be present at the strand-concrete interface at different slip values. The results from the experiments showed that the micro roughness of the strand surface strongly affected the initial bond response of the strand, that is the adhesion in the interface. The maximum bond capacity of indented three-wire strands was found to be directly connected to the geometric properties of the strand indents. The influence of the concrete strength on the bond capacity of the strand was hard to interpret. However, the density of the concrete matrix was found to be a better parameter for determine the influence of the concrete rather than the compressive strength.
Results are presented from a RILEM Round Robin Investigation. It deals with tension stiffening of reinforcement bars embedded
in concrete tie elements. Seven groups of researchers have performed some 50 tests and analyses. Parameters discussed are:
cover thickness, crack spacing, bar size, tension stiffening of naked reinforcing bars, and influence of concrete strain softening.
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