120 research outputs found
Behaviour of steel-fibre-reinforced concrete beams under high-rate loading
Full text linkThe present study focuses on examining the structural behaviour of steel-fibre-reinforced concrete (SFRC) beams under high rates of loading largely associated with impact problems. Fibres are added to the concrete mix to enhance ductility and energy absorption, which is important for impact-resistant design. A simple, yet practical non-linear finite-element analysis (NLFEA) model was used in the present study. Experimental static and impact tests were also carried out on beams spanning 1.3 meter with weights dropped from heights of 1.5 m and 2.5 m, respectively. The numerical model realistically describes the fully-brittle tensile behaviour of plain concrete as well as the contribution of steel fibres to the post-cracking response (the latter was allowed for by conveniently adjusting the constitutive relations for plain concrete, mainly in uniaxial tension). Suitable material relations (describing compression, tension and shear) were selected for SFRC and incorporated into ABAQUS software Brittle Cracking concrete model. A more complex model (i.e. the Damaged Plasticity concrete model in ABAQUS) was also considered and it was found that the seemingly simple (but fundamental) Brittle Cracking model yielded reliable results. Published data obtained from drop-weight experimental tests on RC and SFRC beams indicates that there is an increase in the maximum load recorded (compared to the corresponding static one) and a reduction in the portion of the beam span reacting to the impact load. However, there is considerable scatter and the specimens were often tested to complete destruction and thus yielding post-failure characteristics of little design value and making it difficult to pinpoint the actual load-carrying capacity and identify the associated true ultimate limit state (ULS). To address this, dynamic NLFEA was employed and the impact load applied was reduced gradually and applied in pulses to pinpoint the actual failure point. Different case studies were considered covering impact loading responses at both the material and structural levels as well as comparisons between RC and SFRC specimens. Steel fibres were found to increase the load-carrying capacity and deformability by offering better control over the cracking process concrete undergoes and allowing the impact energy to be absorbed more effectively compared to conventional RC members. This is useful for impact-resistant design of SFRC beams
Application of strut-and-tie models for assessing RC half-joints not complying with current code specifications
Full text linkThe work described is concerned with an investigation of the effectiveness of the use of strut-and-tie models for the structural assessment of half joints. Such elements form a part of many existing bridges which, although not complying with current code specifications, have not as yet displayed any significant signs of distress in spite of the increase in traffic volume and loads over the years. The work is based on a comparative study of the predicted and experimentally established values of load-carrying capacity and location and causes of failure of half-jointed beams with reinforcement layouts that replicate those found in structures designed in accordance with previous code specifications. The results obtained show significant shortcomings of the assessment method as this is found not only to underestimate load-carrying capacity by a margin ranging between 40 % and 65 %, but also to often fail to identify the location and causes of failure. Therefore, there is a need for an alternative assessment method that will be based on concepts capable of both providing a realistic description of structural-concrete behaviour and identifying the causes of failure leading to the loss of load-carrying capacity
Extended P-I diagram method
Full text linkThe pressure-impulse (P-I) diagram method is used in practice (for civilian and military applications) for predicting the level of damage sustained by structures when subjected to blast loads and for assessing the imposed loading regime. Each P-I curve is associated with a certain structural configuration as well as a specific form of blast load and level of damage sustained. When assessing the effect of different parameters (associated with the form of the imposed load and the design of the structure considered) on structural performance, a series of new P-I curves need to be derived. This paper presents an extended P-I diagram method, which is based on derivation of complementary diagrams that can define the effect of two parameters (e.g., the level of axial loading imposed onto a column and the level of damage sustained) on the quasi-static and impulsive asymptotes, thus governing the positions of P-I curves in the diagram plane. The extended P-I diagram method is presented in dimensional and normalised forms. The dimensional form simplifies the derivation of new P-I curves, while the normalised form simplifies the procedure adopted for assessing the behaviour of a certain structure when subjected to a new set of loads. The application of the proposed method is demonstrated in both forms using a typical reinforced concrete (RC) column subjected to a blast load. The column is modelled using finite element analysis capable of accounting for the nonlinear behaviour of concrete and steel. A novel method is proposed for material modelling of concrete. The new material model is validated at both material and structural levels against relevant experimental data. P-I diagrams are initially derived for the axially unloaded column, while complementary diagrams are derived for the column loaded by different axial forces. The framework of the extended P-I diagram method employed for the derivation of new P-I curves and the assessment of the level of damage sustained by the column when subjected to different loading conditions is provided herein.</p
Half-joint beam design based on the CFP theory
No full textThe present work complements recent attempts to investigate the causes of vulnerabilities inherent in half-joint structures. Herein, such vulnerabilities are attributed to shortcomings of the mechanism of force transfer underlying the adopted methods of design, rather than the detailing of the specified reinforcement as widely believed. The work is intended to demonstrate that the forces in half-joint beams are transferred by beam action, and not by the strut-and-tie mechanisms assumed to develop in the presence of the specified reinforcement. Through the use of the compressive force-path method, which has been developed on the basis of a beam mechanism of load transfer, it is shown that the predictions of half-joint beam behaviour correlates closely with the findings of a finite-element analysis package which was at first shown to be capable of successfully reproducing the experimentally-established structural behaviour of such beams
Numerical investigation of the behaviour of RC wide beams under impact loads
No full textThe present work sets out to investigate numerically the dynamic responses of simply-supported reinforced concrete (RC) beams under impact loading. The beams comprise wide sections and thus can be also considered as one-way slab panels often used in pre-cast concrete floor construction. The study was carried out using dynamic Non-linear Finite-Element Analysis (NLFEA) and was validated using published experiential data on RC wide beams tested using a drop-weight at high rate. The numerical predictions obtained show that that the response of the RC wide beams under impact loading differs significantly from that established under equivalent static loading. This change predominantly takes the form of an increase in the maximum sustained load which is primarily attributed to (i) the response of a part of, rather than the whole, structural element and (ii) the development of inertia forces rather than material strain-rate sensitivity. The numerical study is based on the assumption that the effect of high loading rates on the behaviour of structural concrete is mainly linked to the development of inertia forces and not the strain-rate sensitivity of its material properties. Thus, the emphasis is on investigating the effect of loading rate on important aspects of structural response (e.g load-deflection curves, deformation profiles, load-carrying capacity, reaction forces, crack patterns and modes of failure) in an attempt to provide insight into the effect of loading-rate on the mechanics underlying RC structural dynamic response. It is also important to consider that during drop-weight testing it is not easy to correlate the measured response to the actual physical state of the specimens as the maximum value of the contact force generated during impact frequently corresponds to a specimen physical state characterized by high concrete disintegration and low residual strength and stiffness. Therefore, the true load-carrying capacity is likely to be significantly lower than the maximum value of the measured applied load. As a result the validated numerical models developed are employed for conducting a parametric investigation in order to determine the true load-bearing capacity of the examined structural forms under different intensities and loading rates characterising the applied load
Constitutive modelling of concrete behaviour:need for reappraisal
No full textThe present article summarises the fundamental properties of concrete behaviour which underlie the formulation of an engineering finite element model capable of realistically predicting the behaviour of (plain or reinforced) concrete structural forms in a wide range of problems ranging from static to impact loading without the need of any kind of re-calibration. The already published evidence supporting the proposed formulation is complemented by four additional typical case studies presented herein; for each case, a comparative study is carried out between numerical predictions and experimental data which reveal good agreement. Such evidence validates the material characteristics upon which the FE model’s formulation is based and provides an alternative explanation regarding the behaviour of structural concrete and how it should be modelled which contradicts the presently (widely) accepted assumptions adopted in the majority of FE models used to predict the behaviour of concrete
Impact of Systematic Classification and Identification of Treatment Methods of Mine Tailings on Concrete Durability
No full textThe rapid increase in mine tailings generation poses significant environmental challenges, with current disposal methods often unsustainable and leading to catastrophic failures such as tailing pond collapses or toxic slurry spills, resulting in fatalities and severe environmental damage. The urgency for sustainable construction materials is clear, particularly as the global focus shifts toward net-zero emissions and circular economies. Concrete, a primary construction material, traditionally depends on natural resources, making it imperative to explore alternatives that reduce environmental impacts. Mine tailings, with their rich elemental and oxide composition, present a promising option for partial replacement in concrete. Despite this potential, the mechanisms enabling the effective use of tailings remain unclear due to a lack of systematic classification and treatment methods. Literature shows that the chemical and mineral diversity of tailings, coupled with the absence of standardized protocols, has limited their commercial use, unlike established pozzolans. Treatment methods such as mechanical, thermal, and chemical activation are often applied without a clear understanding of underlying mechanisms, resulting in inconsistent outcomes. This study aims to classify mine tailings and investigate how proper treatments affect concrete durability. Through literature review and database analysis, the research evaluates how tailing properties and treatments influence durability. Findings show that appropriate classification and treatment are essential to improving performance, enabling more sustainable construction practices and supporting the circular economy. This work highlights the importance of standardizing treatment approaches and systematically exploring the potential of mine tailings in concrete, advancing environmental sustainability and long-term material viability
Κριτήριο αστοχίας πλακοδοκών σε τέμνουσα:Shear failure criterion for RC T-beams
No full textΠΕΡΙΛΗΨΗ: Αντικείμενο της παρούσας εργασίας είναι η ανάπτυξη ενός κριτηρίου αστοχίας ικανού να λαμβάνει υπόψη τη συμβολή της πλάκας στηναντοχή σε τέμνουσα πλακοδοκών. Το κριτήριο αυτό αναπτύχθηκε εντός του πλαισίου της μεθόδου της τροχιάς της θλιπτικής δύναμης η οποία έχει βρεθεί να βελτιώνει σημαντικά τις προβλέψεις φέρουσας ικανότητας και τιςλύσεις σχεδιασμού κατασκευών οπλισμένου σκυροδέματος, σε σχέση με αυτές που προκύπτουν από τις ισχύουσες κανονιστικές διατάξεις, ικανοποιώντας ταυτόχρονα τις απαιτήσεις δομοστατικής συμπεριφοράς, κυρίως αυτές για πλαστιμότητα και αντοχή. Η εγκυρότητα του προτεινόμενου κριτηρίου επιβεβαιώθηκε μέσω της σύγκρισης των τιμών υπολογισμού με αντίστοιχες πειραματικές και διαπιστώθηκε ότι οι προβλέψεις του υπερέχουν αυτών που προκύπτουν από τους ισχύοντες κανονισμούς.ABSTRACT: The paper is concerned with the development of a failure criterion capable of accurately predicting the shear capacity of reinforced concrete T-beams while correctly accounting for the beneficial effect of the increase of the compressive zone due to the presence of flanges. The development of the subject criterion is based on an alternative design method (the compressive force path method) that leads to predictions of reinforced concrete structural behaviour and design solutions considerably different compared to those of the current design codes without however compromising structural performance requirements (mainly associated with ductility and strength). The validity of the proposed failure criterion is verified through a comparative study of the calculated values with their experimentally-established counterparts obtained from an extensive literature survey. Through this comparative study it is demonstrated that the predictions of the proposed criterion provide a closer fit to the available experimental data than their counterparts obtained from the design codes considered
Numerical modelling of structural concrete under impact loading
No full textThe work is concerned with an investigation of the response of structural concrete to high rates of loading. It is based on a finite-element (FE) program capable of carrying out three-dimensional (3D) nonlinear static and dynamic analyses which has been found capable of yielding realistic predictions to the response of plain- and reinforced-concrete structures under arbitrary static and dynamic actions. The FE model incorporates a 3D material model of concrete behaviour which is characterised by both its simplicity and its attention to theactual physical behaviour of concrete in a structure. In the present context of impact loads, the most significant feature of this model is that it is based on the use of static material properties of concrete, in an attempt to elucidate whether or not the effect of loading rate can be attributed primarily to the inertia of the structure’s mass and not, as is at present widely considered, to the loading-rate sensitivity of the material properties of concrete. By comparing the ensuing analytical (numerical) results with published experimental data, it is shown that this study validates what constitutes a major departure from current thinking as regards material modelling of concrete under high rates of loading
Parametric investigation of factors affecting the behaviour of prismatic concrete specimens under high loading rates of uniaxial compressive loading
Full text linkThis study investigates the response of structural concrete to high rates of loading. The research is based on a finite-element (FE) program capable of carrying out three-dimensional (3D) nonlinear static and dynamic analyses which has been found to be capable of yielding realistic predictions to the response of plain- and reinforced-concrete structures under arbitrary static and dynamic actions. The FE model incorporates a 3D material model of concrete behaviour which is characterized by both its simplicity (fully brittle, with neither strain softening nor load-path dependency) and its attention to the actual physical behaviour of concrete in a structure (unavoidable triaxiality prior to local material failure which is described on the basis of experimental data of concrete cylinders under definable boundary conditions). The most significant feature of this model under impact is that it is based on the use of static material properties of concrete, since it assumes that the effect of loading rate on the specimen behaviour can be attributed primarily to the inertia of the structure’s mass and not, as is at present widely considered, to the loading-rate sensitivity of the material properties of concrete. The existing experimental data, used in order to validate the FE model presently adopted, are characterized by considerable scatter. When reviewing the details of the various experimental investigations carried out to date, it is apparent that a number of parameters (such as the static uniaxial compressive strength of concrete fc, the experimental techniques used for the tests, the shape and size of the specimens, the density of concrete, etc) vary from one experiment to another. Thus, it is the aim of this article to use the FE model in order to investigate the individual and combined effects of these parameters on the response of plain-concrete prismatic specimens under high rates of uniaxial compressive loading and, in so doing, to identify the significance of their contribution to the overall scatter that characterizes experimental data
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