The transfer of forces through rough surface contact in concrete

Aggregate interlocking is a phenomenon occurring when the opposite sides of a concrete crack slide relative to each other. Due to surface roughness, protruding material from one crack-side can engage with the opposite one and thus exchange forces. This affects the load-carrying capacity for structures where the failure is determined by the development of large cracks, like beams and slabs without transverse reinforcement. In such cases, aggregate interlocking contributes significantly to the various shear-transfer actions, allowing forces to traverse cracks.

Aggregate interlocking has been studied for several decades, leading to various experimental and theoretical approaches. However, a number of aspects remain unclear, like the development of new cracks from the initial one and the influence of certain concrete properties on the interlocking forces (e.g. aggregates, crack surface roughness). The present research project comprises three scientific publications focusing on the fundamentals of interlocking and force transfer across concrete cracks and other interfaces related to concrete structures. It is based on the results of an extensive experimental campaign performed with an arrangement allowing to apply precise kinematics upon material discontinuities. The tested specimens include pre-cracked concrete prisms subjected to simultaneous crack opening and sliding and steel-to-concrete interfaces characterized by simple geometries (spheres and rebar-surfaces). Particular attention is given to the roughness properties of the tested cracks and interfaces, several of which were scanned at high resolution.

The experimental results are used to develop a new model for estimating aggregate interlocking forces as a function of crack kinematics. The model is based on the approach originally introduced by Li and Maekawa (1987) with the Contact Density Model and allows estimating the contact properties using 2D crack-profiles. Depending on the crack width, two contributions to force transfer are considered. For contacts occurring in wide cracks, contact forces are calculated using an elastic-plastic material law, whereas for small crack opening the effect of residual material soundness is considered. The model can be applied on surfaces corresponding to tests with various failure modes, thus considering different levels of roughness. Finally, the bond between steel rebar-ribs and concrete is investigated using steel-to-concrete interface specimens, and several similarities with aggregate interlocking are discussed. The previously introduced model is extended to the case of interlocking rebar-ribs, where it can estimate the bond and confinement stresses and the decrease in bond strength for cases where cracks develop parallel to the rebars.

The thesis ends with an extensive appendix containing details on the performed tests which can be of interest for future researchers.

Interaction sol-structure dans le domaine des tranchées couvertes

Sylvain Plumey

The present research on cut-and-cover tunnels gives a theoretical contribution to the understanding of the behaviour of these structures up to their ultimate limit state. A simplified method inspired on the convergence-confinement method is investigated and applied systematically to several practical cases. This method, based on the failure mode of the structure, gives an approximate solution of the equilibrium between the soil and the structure. It enables a better understanding of the complex soil-structure mechanisms, typical of these constructions, constituting in this way a design tool. The behaviour of the soil which interacts with the structure comprises typically two main phases. The elastic phase is followed by the progressive yielding of parts of the soil until a plastic mechanism is formed (plastic phase). Depending on the geometrical and the mechanical parameters, the structure takes more or less advantage of the soil contribution in carrying the loads. The main conclusion of this work is the demonstration of the existence of different modes of behaviour. Three main modes of practical interest, defined mainly by the phase of the soil governing the structural behaviour, are distinguished : elastic soil, elasto-plastic soil and completely plasticized soil. The identification of the mode then enables an efficient design and analysis of the structure. The theory of plasticity (upper bound) was applied to two soil-structure systems, a surface strip footing submitted to a centred load and a lateral wall of a frame-type cutand- cover tunnel under construction, aiming at studying their behaviour at the ultimate limit state. This study clearly emphasizes the favourable or unfavourable role played by the soil-structure interaction in the collapse of such structural systems. A proper consideration of the structure failure kinematics is thus fundamental to a proper representation of the ultimate limit state. A new safety format is proposed to define the ultimate limit state when the finite element method is used. This safety format is compatible with the new SIA codes of practice and clarifies the structural design procedure. The research also showed that the structure's ductility plays a major role in the development of the structural soil capacity. Several ductility limits are emphasized for cut-and-cover tunnels. For frame structures, the deformation capacity of the top slab is very small if no shear reinforcement is provided in the zones of significant shear forces. Stirrups are thus recommended in these structures. For arch structures, the spalling of the concrete cover can limit the deformation capacity of the structure. Experimental tests performed within this research showed that this phenomenon was affected negatively by plastic deformations of the steel reinforcement and by reinforcement splices and anchorages. SIA 262 (2003) code of practice design criterion is judged insufficient. The structural design of cut-and-cover tunnels with important plastic redistributions is thus possible only if certain ductility conditions are fulfilled.

EPFL2007

Topologie des champs de contraintes pour le dimensionnement des structures en béton armé

Neven Kostic

The stress field method, which is based on the lower bound theorem of plasticity, provides an unlimited number of solutions allowing to carry the applied loads to the supports for a given structure. This possibility which gives a large freedom to designing, it raises the question of which is the most suitable stress field for the analysis of the structure. The present research provides further contributions to the generation and the optimisation of strut-and-tie and stress field models. Different automatic and manual methods are generalised and developed. Regarding the design of structures, manual methods are based on the graphical statics and on consideration about the structural behaviour, while for the analysis of existing structures they are based on the upper bound theorem of plasticity. When using automatic methods, firstly the optimisation criteria are evaluated. Different optimisation criteria can lead to different layouts of the steel bars in the structure. Thus, the selection of the optimal structure can be defined according to several criteria, which can meet one of the requirements of the structure while the remaining may be fulfilled at least partially. For automatic analysis, the criterion of minimum strain energy is selected as the most suitable amongst those investigated. This criterion is firstly applied in the linear optimisation of an initial truss, and than it is applied using the proposed algorithm which is based on the displacement method respecting also compatibility conditions. According to this algorithm, the optimal solution but also others close to the optimal are obtained. Finally, an algorithm is proposed which allows to transform the optimal strut-and-tie model obtained into a discontinuous stress field. The proposed algorithm, which is based on displacement method, considers not only the equilibrium of the structure but also compatibility conditions. Therefore it can be equally used for the optimisation of continuous stress fields that are based on non-linear finite element analysis. In that way, the position of the required ties and the development of discontinuous stress field models can be easily obtained. The results obtained are compared to classical solutions and to examples recently proposed in the scientific literature. The actual behavior of structures is also assessed by means of both continuous and discontinuous stress field analyses. The main concerns are the need for reducing the concrete strength and the analyses of the different phenomena which influence the structural behaviour. The conclusions withdrawn from this study were verified for the particular case of a corner joint cast in lightweight aggregate concrete which was tested in laboratory during the thesis and analysed with the stress field method. Additional structures containing discontinuity regions, were included in this study. The behaviour at service load is finally studied by selecting the cracking zones in the structures which were previously identified using the stress field method. In addition, tension zones outside the tie zones are analysed in detail. At these regions, it is necessary to ensure the propagation of the tie cracks in order to limit crack openings. Suggested values for the minimal reinforcement and the maximal distance between reinforcement bars are finally proposed.

EPFL2009

Structural Response of R-UHPFRC - RC Composite Members Subjected to Combined Bending and Shear

Talayeh Noshiravani

The addition of a thin overlay of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to Reinforced Concrete (RC) members is an emerging technique to strengthen and protect existing structures and to design durable new structures. Combining UHPFRC with closely spaced, small-diameter steel rebars in Reinforced UHPFRC (R-UHPFRC) layers improves the UHPFRC's strain hardening behaviour. For reasons of practicality, R-UHPFRC layers are cast or glued (in the case of prefabricated elements) on top of RC members, thus changing the latter into R-UHPFRC - RC composite members. The high strength and deformation capacity of R-UHPFRC elements make them a suitable external flexural reinforcement for RC members over intermediate supports, e.g., bridge decks and slabs or beams in buildings. Over reinforcement of RC beams and slabs with tensile flexural reinforcement can result in their shear failure at either a lower resistance or deformation than the associated values for member failure in flexure. A comprehensive experimental program was conducted to study the flexure-shear behaviour of R-UHPFRC - RC composite beams. The program comprises two test series on cantilever beams and continuous beams. The test parameters include shear span-depth ratio (a/d), the amount of transverse reinforcement ( ρν), the amount of longitudinal reinforcement, and the strength and bond condition of the R-UHPFRC rebars. The experimental results reveal the different failure modes of R-UHPFRC - RC composite members and the contribution of the R-UHPFRC elements to the member resistance, ductility and capacity to redistribute the internal stress. It was shown that in R-UHPFRC - RC beams with ribbed rebars and a shear span to depth ratio greater than 2.5 the stresses are carried by beam action. Depending on the degree of longitudinal reinforcement, all but two of the beams with 3.0≤a/d≤3.4 and ρν≤0.17 had a flexure-shear failure; the rest failed in flexure. The flexure-shear failure of the composite beams was at an approximately equal rotation level as their RC reference beam but at a resistance 2.3 times that of the RC beam. This is due to (1) the debonding interface zone between the elements that allows the R-UHPFRC - RC beams to rotate more freely and (2) the out-of-plane resistance of the R-UHPFRC element that contributes to the shear resistance. The internal flow of forces and the structural response of composite members strongly depend on the bond condition between the R-UHPFRC and RC, the UHPFRC and its rebars, as well as the concrete and its rebars. Cracking of the concrete along the interface zone causes bond reduction, i.e., softening of the shear connection, between the two elements. In presence of high shear stresses and diagonal flexure-shear cracks, interface zone softening is observed between the elements prior to the maximum resistance, while UHPFRC is strain hardening. The cause of this softening behaviour is the prying action due to the relative rotational movement of the RC rigid bodies separated by the flexure-shear cracks. Static and kinematic solutions of the theory of plasticity for RC beams are extended to predict the collapse load of R-UHPFRC - RC composite beams at the ultimate limit state. A mechanical model for predicting the structural response of composite beams is proposed. In combination with truss models, the concept of an R-UHPFRC - RC plastic hinge is introduced to calculate the force-displacement response of composite beams. The failure criterion based on the collapse mechanisms (kinematic solutions) sets the limit of the force-displacement response. The model is corroborated by the experimental results. This model provides a tool for analysis of RC members reinforced with an added tensile R-UHPFRC element.

EPFL2012