Vitor M. C. F. Cunha

Pullout Behavior of Steel Fibers in Self-Compacting Concrete

In steel fiber-reinforced composites materials, fiber and matrix are bonded together through a weak interface. The study of this interfacial behavior is important for understanding the mechanical behavior of such composites. Moreover, with the outcome of new composites materials with improved mechanical properties and advanced cement matrices, such in the case of steel fiber reinforced self-compacting concrete, the study of the fiber/matrix interface assumes a new interest. In the present work, experimental results of both straight and hooked-end steel fibers pullout tests on a self-compacting concrete medium are presented and discussed. Emphasis is given to the accurate acquirement of the pullout load versus end-slip relationship. The influence of fiber embedded length and orientation on the fiber pullout behavior is studied. Additionally, the separate assessment of the distinct bond mechanisms is performed, by isolating the adherence bond from the mechanical bond provided by the hook. Finally, analytical bond-slip relationships are obtained by back-analysis procedure with an interfacial cohesive model.

The influence of fibre orientation on the post-cracking tensile behaviour of steel fibre reinforced self-compacting concrete

Adding fibres to concrete provides several advantages, especially in terms of controlling the crack opening width and propagation after the cracking onset. However, distribution and orientation of the fibres toward the active crack plane are significantly important in order to maximize its benefits. Therefore, in this study, the effect of the fibre distribution and orientation on the post-cracking tensile behaviour of the steel fibre reinforced self-compacting concrete (SFRSCC) specimens is investigated. For this purpose, several cores were extracted from distinct locations of a panel and were subjected to indirect (splitting) and direct tensile tests. The local stress-crack opening relationship (?-w) was obtained by modelling the splitting tensile test under the finite element framework and by performing an Inverse Analysis (IA) procedure. Afterwards the ?-w law obtained from IA is then compared with the one ascertained directly from the uniaxial tensile tests. Finally, the fibre distribution/orientation parameters were determined adopting an image analysis technique.

Bond-slip mechanisms of hooked-end steel fibers in self-compacting concrete

The experimental results of hooked-end steel fibers pullout tests on a self-compacting concrete medium are presented and discussed in this work. The influence of fiber embedment length on the fiber pullout behavior is studied. The role of the end hook of the fiber on the overall pullout behavior is also investigated by carrying out tests with fibers without its end hook, in order to separate the contribution of the frictional bond component from those derived from the mechanisms provided by the end hook of the fiber. Finally, the experimental bond-slip relationships are modeled by an analytical model.

Tensile behavior of steel fiber reinforced self-compacting concrete

The tensile behavior of a self-compacting concrete (SCC) reinforced with two hooked ends steel fiber contents was assessed in this paper by performing stable displacement control tension tests. Based on the stress-displacement curves obtained, the stress-crack width relationships were derived, as well as the energy dissipated up to distinct crack width limits and residual strengths. The number of effective fibers bridging the fracture surface was determined and was compared with the theoretical number of fibers, as well as with the stress at crack initiation, residual stresses and energy dissipation parameters. In general, a linear trend between the number of effective fibers and both the stress and energy dissipation parameters was obtained. A numerical model supported on the finite element method was developed in this paper. In this model, the fiber reinforced concrete is assumed as a two phase material: plain concrete and fibers randomly distributed. The plain concrete phase was modeled with 3D solid finite elements, while the fiber phase was modeled with discrete embedded elements. The adopted interface behavior for the discrete elements was obtained from single fiber pullout tests. The numerical simulation of the uniaxial tension tests showed a good agreement with the experimental results. Thus, this approach is able of capturing the essential aspects of the fiber reinforced composite’s complex behavior.

Earth-based Render of Tabique Walls – An Experimental Work Contribution

ABSTRACT A research work focused on studying earth render for tabique application purposes is presented. Initially, a brief description of the tabique building technique is provided. The relevance of the application of this traditional building technique is also highlighted. Different compositions of earth render are experimentally analyzed and the respective performance is evaluated. Flexural and compressive strengths, workability, drying shrinkage cracking, and water resistance are the material properties assessed. A simple earth render is selected as being adequate for tabique building applications and it is applied on the manufacturing of a tabique wall sample. This wall sample is monitored in terms of thermal insulation ability and its thermal transmission coefficient is estimated. Taking into account that there is still a lack of published technical information related to this topic, this article may contribute to solve this limitation and to give some guidance in future repairing processes of tabique construction. The technological benefit of adding lime or cement with earth is researched. Real tabique timber structure samples are applied in order to validate the obtained experimental results.

Time-Dependent Flexural Behaviour of SFRSCC Elements

This work presents and discusses the long-term behaviour of pre-cracked steel fibre reinforced self-compacting concrete (SFRSCC) laminar structures of relatively small thickness. One hundred and twelve prismatic specimens were extracted from a SFRSCC panel. These specimens were notched with different orientations regarding to the expected SFRSCC flow direction, and were tested under four-point flexural sustained loading conditions. The influence of the following parameters on the creep behaviour was studied: initial crack opening level (0.3 and 0.5 mm), applied stress level, fibre orientation/dispersion, and distance from the casting point. Moreover, to evaluate the effect of the long-term residual crack opening on the flexural post-cracking strength, as well as on the secondary stiffness, a series of instantaneous monotonic and cyclic tests were carried out, and the corresponding force vs crack tip opening displacement (F–CTOD) curves were compared to the ones obtained by assembling the F–CTOD curves determined in the pre-crack monotonic tests, creep tests and post-creep monotonic tests. Finally, based on the results obtained from the creep tests, an equation was proposed to predict the creep coefficient for the developed SFRSCC.

Reactive powder concrete reinforced with steel fibres exposed to high temperatures

The authors would like to acknowledge the Zhejiang Boen Company and MAPEI Company for providing gratuitously, respectively, the steel fibers and micro silica fume. The first author would also like to acknowledge the grant obtained under the scope of the Erasmus Mundus - Marhaba project. The third author wishes to acknowledge the grant SFRH/BSAB/114302/2016 provided by FCT.

Numerical simulation of galvanized rebars pullout

The usage of rebars in construction is the most common method for reinforcing plain concrete and thus bridging the tensile stresses along the concrete crack surfaces. Usually design codes for modelling the bond behaviour of rebars and concrete suggest a local bond stress - slip relationship that comprises distinct reinforcement mechanisms, such as adhesion, friction and mechanical anchorage. In this work, numerical simulations of pullout tests were performed using the finite element method framework. The interaction between rebar and concrete was modelled using cohesive elements. Distinct local bond laws were used and compared with ones proposed by the Model Code 2010. Finally an attempt was made to model the geometry of the rebar ribs in conjunction with a material damaged plasticity model for concrete.