Influence of the relative rib area on bond behaviour

Steel-to-concrete bond is a basic aspect of the behaviour of reinforced concrete structures both at serviceability and ultimate states. When bond rules were originally developed, experimental results were mainly obtained on normal- strength concrete and a minimum relative rib area (bond index) was required by building codes to ensure good bond properties. The arrival into the market of high-performance concrete and newer structural needs may require different bond indexes. In the present paper, the experimental results of pull-out tests on short anchorages are presented. Several pull-out tests on ribbed bars, embedded in cubes of normal- and high-strength concrete with a concrete cover of 4.5 times the bar diameter, were carried out in order to better understand the influence of the relative rib area and bar diameter on the local bond behaviour, as well as on the splitting crack width generated by the wedging action of ribs. A total of 96 tests were performed on machined bars of three different diameters (12, 16 and 20 mm) with a bond index ranging from 0.040 to 0.105. The results of 55 pull-out tests on commercial hot-rolled ribbed bars of four different diameters (12, 20, 40 and 50 mm) are also presented to confirm that the bond response also depends on bar diameter (size effect). Experimental results provide information concerning the influence of the relative rib area on bond strength and on the bursting force due to the rib’s wedge action. As the minimum measured bond strength of rebars was always markedly greater than the minimum bond strength required by building codes even when low bond indexes were adopted, the test results point out the possibility of reducing the minimum value of the relative rib area required by Eurocode 2 without limiting the safety coefficient of bond. The reduction also allows a higher structural ductility that can be achieved due to a greater strain penetration of the rebars from concrete cracks

Experimental Results on Staggered Lapped Bars in FRC

The paper presents experimental results on lap splices in fiber reinforced concrete (FRC). Four point bending tests were carried out on several 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 and included various lap splices configurations varying the percentage of lapped bars. The behaviour of lapped bars in FRC with a volume content of steel hooked fibres equal to 0.5% was investigated. The results show that the post-peak behaviour of FRC can enhance the strength of staggered lapped splices as well as it can reduce their brittleness, thus allowing a reduction of lap length when only a portion of bars at a section are lapped. The results show also the benefits on the durability of concrete members due to the capability of the fibres to markedly reduce the splitting cracks along the splice at service loadings

Vibrated and self-compacting fibre reinforced concrete: experimental investigation on the fibre orientation

In addition to the fibre type and content, the residual properties of fibre reinforced concrete are influenced by fibre orientation. Consequently, the performance fibre reinforced concrete can be affected by its fresh properties (workability, flowing capacity) and by casting and compaction processes adopted. This paper focuses on the study of the orientation of steel or macro-synthetic fibres in two materials characterized by very different fresh properties: vibrated and self-compacting concrete. Four rectangular slabs 1800 mm long, 925 mm wide and 100 mm high were produced changing concrete and fibre type. From each slab, eighteen small prisms (550 mm long) were firstly cut either orthogonal or parallel to casting direction and, secondly, notched and tested in bending according to EN 14651. Experimental results showed that the toughness properties of a thin slab significantly varies both in vibrated and self-compacting concrete, even if in case of self-compacting concrete this variation resulted higher. Steel fibres led to greater variability of results compared to polymer one, underlining a different fibre orientation. A discussion on the relative residual capacity measured on the prisms sawn from the slabs and the parameters obtained from standard specimens is performed.Facultad de Ingenierí

Effects of new openings on the in/plane behaviour of unreinforced brick masonry walls

Existing brick masonry buildings are frequently modified to satisfy nowadays living demands. Such modifications may include new windows or doors that connect two rooms and require openings to be cut from load bearing walls. In current design practice, these interventions are generally designed and verified for vertical load, but the structural behavior of these altered walls when submitted to in-plane loads (due to seismic actions) is not yet fully understood. Thus, design practice may be inaccurately estimated. The objective of this work is to evaluate, numerically and experimentally, the effects of introducing openings in masonry solidbrick walls subjected to in-plane loading. Three main parameters are considered for the numerical studies: i) walls dimensions, ii) opening type, iii) opening size. As expected, results show that walls with medium-large openings are the most vulnerable case-scenario. These numerical results have addressed the design of a representative wall tested at the University of Brescia. The preliminary results of this experimental program are included in this pape

Experimental and numerical assessment of masonry walls with new openings strengthened with steel frame

The creation of new openings in masonry walls is a frequent intervention executed in existing buildings of unreinforced masonry composed of clay bricks. These openings are widely seen at the street-level, where spaces are modified to create new windows or doors for new stores, garages or offices. Depending on their size and position, these interventions may cause significant decrease of the wall’s original in-plane strength and stiffness, thus, compromising the building seismic resistance. For example, when several garages are created, one after another, the risk of inducing the soft-story mechanism, when earthquake forces arrive, increases. Another example is when a door of significant size is introduced in an originally solid masonry wall, which was a key object to guarantee the box-like behavior of the structure. The opening would reduce the cross section of the remaining piers and spandrel, and thus, weaken the wall’s seismic strength. These changes in the original wall have consequences in the box-like behavior, as during earthquake events, the load demands on the remaining shear walls might be larger than their shear capacity. Therefore, strengthening techniques must restore as much as possible the loss of stiffness and strength. Besides, for masonry structures, the technique must be reversible and respect the compatibility between materials, particularly in the case of protected assets. In an attempt to complying with these requirements, engineering practitioner often introduce steel profiles forming a frame inside the opening. Steel is usually preferred because of its high level of reversibility and the stiffness and strength it can provide to masonry without substantially increasing the building self-weight. The design of this steel frame and the stiffness of the masonry wall with opening is based in the available analytical tools, i.e., the Timoshenko Beam Theory. From these calculations, the loss of stiffness when passing from a solid wall to a perforated wall is about 75% for cantilever boundary conditions and 55% for double-fixed. Thus, very stiff profiles for the steel frame are required. In theory, these profiles are capable of fully restoring the stiffness and resistance. The present work is dedicated to evaluate the effectiveness of this steel frame technique by means of experimental and numerical methods. The experimental program was designed to provide full assessment of the effects of introducing a new door opening in brick masonry walls, from the perforation process to the application of in-plane cyclic loads . A flexible steel frame was designed using numerical tools and consisted in four profiles welded together and tied to the surrounding masonry wall by means of dry-driven dowels. The numerical model was validated against the experimental results, and show that neither a very stiff steel frame nor a more flexible one is capable of restoring the original solid wall’s stiffness. However, both are capable of restoring the in-plane strength and ductility. This paper, also shows that using a very stiff profile might lead to a rather brittle response of the reinforced wall, as the masonry starts cracking before activating the frame. This would not happen with a more flexible profileItalian Ministry of Education, University, and Research (MIUR) for her Doctoral Scholarship is gratefully acknowledged. The Authors also thank the technicians A. del Barba, A. Botturi from laboratory Pietro Pis

The basis for ductility evaluation in SFRC structures in MC2020: An investigation on slabs and shallow beams

The paper presents a synthesis of an extensive experimental campaign on linear and two-dimensional steel fiber reinforced concrete (SFRC) structural elements carried out to check the ductility requirements aimed at guaranteeing limit analysis approaches for the computation of ultimate load-bearing capacity of SFRC structures; special attention is devoted to the role of the degree of redundancy of the structure. In particular, full-scale shallow beams and slabs reinforced with steel fibers (with or without conventional longitudinal reinforcement) were tested in two different laboratories: the Politecnico di Milano (PoliMI) and the University of Brescia (UniBS). In this experimental campaign, two different fiber contents and fiber types were considered. The experimental investigation, carried out within the activities to support Annex L of Eurocode 2, was fundamental also for developing the design rules included in the fib Model Code 2020 and allowed to formulate conclusions regarding optimization of the mix design, ductility, and design prediction at the ultimate capacity

Vibrated and self-compacting fibre reinforced concrete: experimental investigation on the fibre orientation

In addition to the fibre type and content, the residual properties of fibre reinforced concrete are influenced by fibre orientation. Consequently, the performance fibre reinforced concrete can be affected by its fresh properties (workability, flowing capacity) and by casting and compaction processes adopted. This paper focuses on the study of the orientation of steel or macro-synthetic fibres in two materials characterized by very different fresh properties: vibrated and self-compacting concrete. Four rectangular slabs 1800 mm long, 925 mm wide and 100 mm high were produced changing concrete and fibre type. From each slab, eighteen small prisms (550 mm long) were firstly cut either orthogonal or parallel to casting direction and, secondly, notched and tested in bending according to EN 14651. Experimental results showed that the toughness properties of a thin slab significantly varies both in vibrated and self-compacting concrete, even if in case of self-compacting concrete this variation resulted higher. Steel fibres led to greater variability of results compared to polymer one, underlining a different fibre orientation. A discussion on the relative residual capacity measured on the prisms sawn from the slabs and the parameters obtained from standard specimens is performed.Facultad de Ingenierí

Construction methodologies and structural performance of tunnel linings. Optimisation of the structural, technological and functional performance, of construction methodologies and materials, in tunnel linings

This book aims to give a general survey on the main results obtained in the National Research Project (PRIN 2006) on tunnel linings “Optimization of the Structural, Technological and Functional Performance of Construction Methodologies and Materials in Tunnel Linings” (PRIN 2006), financed by the Italian Ministry of University and Research (MIUR). The research was carried out within five different Universities spread all over the Country, namely: Brescia, Lecce, Politecnico di Milano, Parma and Politecnico di Torino, for a total budget of about 250.000 euros. Five Research Units (RU) were involved because their specific experience in the fields of tunnel linings. In particular: (RU1) the Department DICATA of the University of Brescia and the Department of Innovation Engineering of the University of Salento (Lecce) have experience in the field of Fiber Reinforced Concrete (FRC) and in the optimization of structural behaviour of segmental lining; (RU2) Department DITAG of Politecnico di Torino has been working on conventional and mechanized tunnels and on the soil-structure in-teraction; (RU3) Department DISTR of Politecnico di Torino did research on the soil-structure interaction and on the structural optimization of final lining in conventional tunnels; (RU4) Department DIS of Politecnico di Milano has been active in the field of mechanical properties of concrete under high temperatures and on the fire resistance of high performance concrete while (RU5) Department of Civil Engineering of the University of Parma did several research studies on nonlinear analyses of reinforced concrete and developed a specific model for FRC based on fracture mechanics. The book summarizes the main aspects arose during the development of the research project. General issues concerning tunnel linings, both conventional and segmental, were investigated within the main objective of the research that was the improving of the knowledge on the structural behavior and the construction methodologies of these structures. Furthermore, general aspects concerning newer materials for use in tunnel linings are presented with particular reference to Fiber Reinforced Concrete (FRC) and Self Compacting Concrete (SCC)

LEFM applications to concrete gravity dams

The evaluation of the safety factor of old dams under higher flood levels in the last few years has been investigated under the assumptions of fracture mechanics. The extensive need for research in this field was recognized by the U.S. Army Crops of Engineers that now requires a fracture mechanics investigation prior to the rehabilitation of cracked massive concrete structures. In large structures, such as dams, because of the smaller size of the fracture process zone with respect to the structure size, limited errors should occur under the assumptions of linear elastic fracture mechanics (LEFM). In this paper, theoretical considerations and approximate expressions for the evaluation of stress intensity factors and crack propagation in concrete dams are presented. Furthermore, a parametric study of gravity dams under the assumptions of LEFM is performed. Finally, a scale law for the evaluation of the maximum water level carried by cracked dams is propose