Fausto Minelli

On the Effectiveness of Steel Fibers as Shear Reinforcement

An experimental study on steel fiber-reinforced concrete (SFRC) beams subjected to shear loading tested at the University of Brescia in recent years is presented and discussed. A total of 18 full-scale experiments were carried out, aimed at investigating the effect of randomly distributed steel fibers within the concrete matrix on shear behavior. The focus was on the parameters influencing the shear response of members, including concrete class, fiber content, and mixture of different fibers. All tested members contained no conventional shear reinforcement. All SFRCs used were characterized in tension according to the provision included in the fib Model Code 2010. A useful database—with other tests published elsewhere—was developed, linking the shear strength of members to the codified residual strengths of the corresponding fiber-reinforced concrete (FRC) materials. Results show that a relatively low amount of fibers can significantly increase the shear strength and ductility of concrete beams without transverse reinforcement. Moreover, visible cracking and noticeable deflections offer ample warning of impending collapse in FRC members. A critical discussion of two recent analytical models for calculating the shear strength of FRC materials is also provided.

Compression Field Modeling of Fiber-Reinforced Concrete Members Under Shear Loading

Several laboratory experiments have demonstrated the effectiveness of steel fibers in substituting the minimum code-required shear reinforcement in beams, particularly in precast high-performance concrete structures. Despite the large number of experimental results available, only a few numerical studies have been published concerning fiber-reinforced concrete structures. The behavior of different kinds of full-scale steel fiber-reinforced concrete elements is analyzed herein using a finite element code based on the modified compression field theory (MCFT) and the disturbed stress field model (DSFM), and suitably adapted for steel fiber reinforcement. The numerical model is validated against the experimental results obtained on full-scale fiber-reinforced concrete (FRC) structural elements and is shown to adequately simulate the strength, stiffness, ductility, crack pattern development, and failure modes of all specimens tested, including those lightly reinforced or with fibers only.

A New Round Panel Test for the Characterization of Fiber Reinforced Concrete: A Broad Experimental Study

Standard test methods for determining the mechanical properties of Fiber Reinforced Concrete (FRC) are properly defined if they reproduce the actual structural behavior. Among many proposals, a round panel test seems to have all potentials to become an easy-to-use tool and, at the same time, a reliable procedure for the characterization of FRC, in terms of toughness and the post-cracking constitutive cohesive law. A new geometry for the round panel test is herein proposed and discussed in order to make the panel easier to place, handle, and test, therefore avoiding one of the major drawbacks that limit an extensive utilization of the panel test. A comparison between different test typologies for characterizing FRC is reported and discussed in the present paper, with special emphasis on the different scatter that each test produces. Tests are performed on beams as well as on panels. All specimens herein compared have the same concrete mechanical properties and fiber content. The aim of the experimental investigation is to critically discuss the advantages and disadvantages of each testing procedure, focusing on the applicability of the method and on the reliability of results toward a consistent characterization of the structural behavior. Suitable correlations among the different fracture and energy parameters defined in the standards considered are finally reported, and the results are very useful for harmonizing the available standards.

Crack Control in RC Elements with Fiber Reinforcement

SYNOPSIS Durability is nowadays a key-parameter in Reinforced Concrete (RC) structures. Several codes require that structures have a defined service life during which the structural performance must satisfy minimum requirements by scheduling only ordinary maintenance. Durability can be associated to permeability, defined as the movement of fluid through a porous medium under an applied pressure load, which is considered one of the most important property of concrete. Permeability of concrete is strictly related to the material porosity but also to cracking. The former is basically controlled by the water/cement (w/c) ratio while microcracks and cracks are related to internal and external strains or deformations experienced by the RC structures. Shrinkage, thermal gradients and any factor determining volumetric instability, as well as the loads acting on a structure, lead to both microcraking and visible cracking. It is well known that, after cracking, tensile stresses are induced in the concrete between cracks and, hence, stiffen the response of a Reinforced Concrete (RC) member under tension; this stiffening effect is usually referred to as “tension stiffening”. After the formation of the first crack, the average stress in the concrete diminishes and, as further cracks develop, the average stress will be further reduced. When considering Fiber Reinforced Concrete (FRC), an additional significant mechanism influences the transmission of tensile stresses across cracks, arising from the bridging effect provided by the fibers between the crack faces; this phenomenon is referred to as “tension softening”. Fibers also significantly improve bond between concrete and rebars and act to reduce crack widths. The combination of these two mechanisms results in a different crack pattern, concerning both the crack spacing and the crack width. The present paper describes results from a collaborative experimental program currently ongoing at the University of Brescia and at the University of Toronto, aimed at studying crack formation and development in FRC structures. A set of tensile tests (52 experiments) were carried out on tensile members by varying the concrete strength, the reinforcement ratio, the fiber volume fraction and the fiber geometry.

ROUND PANEL VS. BEAM TESTS TOWARD A COMPREHENSIVE AND HARMONIC CHARACTERIZATION OF FRC MATERIALS

Standard test methods for determining the mechanical properties of Fibre Reinforced Concrete (FRC) are better defined if they reproduce the actual structural behavior.

Strengthening of a Bridge Pier with HPC: Modeling of Restrained Shrinkage Cracking

After more than fifty years from the opening of the largely discussed “Autostrada del Sole” Highway in 1964, the infrastructure system in Italy appears marked by the passing of time, similarly to what observed in several other countries worldwide. The great heterogeneity of the Italian landscape has determined a great variety of construction types, such as large span concrete bridges over the northern rivers and large arch concrete bridges over the valleys of the central region. Increment of vehicle traffic and new seismic regulations are setting new requirements to adapt the existing infrastructure, which should be otherwise replaced. Moreover, reinforced concrete (RC) aging and deterioration have led to structural and material degradation, including severe cracking and corrosion. Specialized materials such as High Performance Concrete (HPC) could represent a viable convenient solution for repairing, strengthening and retrofitting of RC structures as both structural capacity and durability can be refurbished. However, alongside high mechanical performance, HPC is characterized by a high cracking sensitivity at very early age, due to its high stiffness and shrinkage. Restrained shrinkage cracking, particularly significant in repaired structures where the existing concrete generates a considerable restraint against the free movement of the repair material, may represent a limit to the effective application of these materials. For this reason, shrinkage compatibility of HPC with the existing concrete substrate needs to be experimentally and numerically assessed. A study is herein presented where, based on experimental tests, different numerical models are developed and compared to assess and eventually minimize the risk of shrinkage cracking in bridge piers strengthened with HPC.

Predicting Uniaxial Cyclic Compressive Behavior of Brick Masonry: New Analytical Model

AbstractA constitutive model for simulating the compressive response of unreinforced brick masonry subjected to cyclic loading is presented and discussed. The developed formulations are consistent ...

Experimental Study of Solid and Hollow Clay Brick Masonry Walls Retrofitted by Steel Fiber-Reinforced Mortar Coating

ABSTRACT An experimental study including reverse quasi-static cyclic tests on solid and hollow Unreinforced Masonry walls strengthened with thin steel fiber-reinforced mortar (SFRM) coating is herein presented. Three types of mortars containing nano-silica and three different typologies of short high-strength steel fibers are used. Compared to traditional coating techniques, the proposed strengthening method adopts a thin layer (25 mm tick) of SFRM anchored on the wall surface by means of steel connectors. Results show that this novel technique provides significant enhancement in terms of strength and stiffness. Critical considerations on the in-plane lateral resistance of masonry walls are reported.

Retrofitting RC Members with External Unbonded Rebars

External unbonded rebars represents a suitable strengthening technique for the retrofitting of existing Reinforced Concrete (RC) members. Advantages regard the ease of installation, a minimum invasiveness and possibility of future inspections. Structurally, increment of flexural stiffness and bearing capacity and enhancement of shear-flexure behavior can be achieved. The presence of both bonded and unbonded bars introduces a change in the way the shear actions are resisted. Unbonded rebars develop an arch action component, with no bond present and constant force in the rebars, in addition to the beam action component, normally developing in presence of bonded bars. The present paper reports the results of four point loading tests on full-scale beam, with the aim of studying the influence of different bond condition. Moreover, the Double Harping Point technique, using external rebars and vertical deviators, is presented, with attention to the definition of the vertical equivalent stiffness of the deviators.

Suitable Restrained Shrinkage Test for Fibre Reinforced Concrete: A Critical Discussion

Concrete performance traditionally refers to compressive strength and workability. Recently, high performance concrete evidenced the possibility of enhancing other material properties. Among these, resistance to shrinkage cracking is gaining more attention among practitioners, due to its strict relation to durability requirements. Shrinkage cracks occur in restrained structures: for this reason, material characterisation should be made on the basis of a restrained shrinkage test. The ring test is an easy-to-use tool since one can measure the time-to-cracking of a concrete mix. Focus of this paper is to critically discuss the actual standard test procedure and then to propose enhancements of the test set-up with the aim of reducing the time-to-cracking and making the test duration more suitable for practical uses. Furthermore, the effect of fiber reinforcement on shrinkage cracking is presented.