Giovanni Plizzari

Mechanical Properties of Corrosion-Damaged Reinforcement

Corrosion of embedded reinforcement is the most prevalent form of degradation of reinforced concrete structures, and may impair structural capacity through loss of bar section, loss of bond between reinforcement and concrete as a result of longitudinal cracking, or loss of concrete cross section. The effect of corrosion attack on mechanical properties of reinforcement is investigated through physical tests on bars with simulated and real corrosion damage and through a simple numerical model. Bars subjected to local or pitting attack may suffer a relatively modest loss of strength but a significant loss of ductility, and this is related principally to the variability of attack along the length of the bar. The numerical model supplements experimental work through a parametric study on the influence of steel characteristics. Finally, guidelines on assessment are suggested that are derived from results reported in the paper and from elsewhere in the published literature.

Steel Fiber Concrete Slabs on Ground: A Structural Matter

Steel fiber-reinforced concrete (SFRC) is used in industrial pavements, roads, parking areas, and airport runways as an alternative to conventional reinforcement with reinforcing bars or welded mesh. This article reports on a study of the structural behavior of slabs on ground made of SFRC. Several full-scale slabs reinforced with different volume fractions of steel fibers having different geometries were tested under a point load in the slab center. A hybrid combination of short and long fibers was also tested to optimize structural behavior. The authors found that steel fibers significantly enhance the bearing capacity and the ductility of slabs on ground. The nonlinear behavior of these SFRC structures is well captured by performing nonlinear fracture mechanics analyses where the constitutive relations of cracked concrete under tension were experimentally determined. The authors conclude by proposing a simplified analytical equation for predicting the minimum necessary thickness of SFRC slabs on ground.

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.

Infill Walls with Sliding Joints to Limit Infill-Frame Seismic Interaction: Large-Scale Experimental Test

This article presents the results of an experimental campaign on the in-plane behavior of masonry infill walls. Four tests were performed on large-scale specimens to evaluate the infill deformation capacity and damage for different drift levels. A design solution adopting horizontal sliding joints was proposed for reducing the infill-frame interaction, and its performance was compared with the configuration without joints. Both hollow-fired clay bricks and adobe were tested. The experimental tests showed the effectiveness of the sliding joints in reducing the infill wall damage and a higher performance of adobe infills with respect to hollow-fired clay specimens.

NEW DESIGN APPROACH FOR STEEL FIBER-REINFORCED CONCRETE SLABS-ON-GROUND BASED ON FRACTURE MECHANICS

In concrete pavements, steel fibers are particularly suitable both for limiting shrinkage effects and for increasing their bearing capacity and fatigue resistance. In the present paper, the fracture behavior of steel fiber-reinforced concrete (SFRC) slabs-on- ground is studied by using nonlinear fracture mechanics (NLFM) finite element (FE) analyses. The fracture mechanics method has been validated by results from experiments performed by the authors as well as by test results available in the literature. Experiments concern tests on full-scale slabs on an elastic soil subjected to a load in the slab center. The comparison shows that NLFM analyses closely predict crack development, ultimate load, and the collapse mechanism of the slab-on-ground. The numerical results can be summarized in abaci for design purposes that provide the minimum slab thickness and the maximum crack opening once the material properties and the applied loads are known.

POSTPEAK BEHAVIOR OF FIBER-REINFORCED CONCRETE UNDER CYCLIC TENSILE LOADS

High-strength concrete (HSC) and fiber-reinforced concrete (FRC) are being increasingly used in bridges, pavements, off-shore platforms, and water-retaining structures. These structures are subjected to fatigue loadings caused by vehicles, waves, machine rotations, or even earthquakes. In this study, the role of steel and carbon fibers on fatigue behavior of normal strength concrete (NSC) and HSC is investigated. Experimental results from uniaxial tensile tests on cylindrical specimens and four-point bending tests on beam specimens allowed a comparison between material and structural behavior. Geometrically similar beams with different sizes were adopted. Experiments focused on precracked specimens in which a fracture process zone was present. To study the material behavior, both inner loops and cycles on the envelope curve were applied. It was found that the envelope curves obtained from cyclic tests match the curves obtained from the static tests on cylinders of HSC and FRC quite well. This, however, does not occur in beam specimens because the fatigue damage strongly depends on the fracture process zone development, which is influenced by specimen size and loading history. Furthermore, the effectiveness of steel fibers in improving fatigue life is greater in HSC than in NSC cylindrical specimens. More specifically, the presence of steel fibers in HSC resulted in a smaller crack opening increment per cycle. The number of cycles to failure depends on the beam size because of different fracture process zone development. Finally, a comparison between experimental results and an analytical model for crack increment during inner loops again demonstrated the influence of fibers on fatigue behavior of concrete.

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.

On the influence of uplift pressure in concrete gravity dams

Abstract Cracks are often present in concrete dams, and may have relevant dimensions. The evaluation of the safety factor of old dams under higher flood levels has been investigated through fracture mechanics in previous years. Crack stability in concrete dams can correctly be predicted when uplift pressures are accurately modelled. Current models consider a uniform uplift pressure distribution, but recent experimental results show that it varies along the crack faces. In the present paper, uplift pressure effects in cracked concrete gravity dams are studied. A parametric study on the influence of uplift pressure on stress intensity factors and crack-propagation angle is performed. Furthermore, uplift pressure effects on the LEFM scale law for the maximum water level carried by cracked gravity dams are examined.

Experimental and numerical studies on the behaviour of concrete sandwich panels

ABSTRACT Precast concrete panels are often used for the façades of modern warehouses and commercial malls. During the last two decades, they have generally been made of two concrete layers with interposed thermal insulating polystyrene boards. Traditionally, perimeter concrete ribs allow the weight of the external concrete layer to be transferred to the internal thus causing unavoidable thermal bridges which reduce the energy performance of the building. In the sandwich cladding panel, the two concrete layers can be linked by low-conductivity shear connectors crossing through the insulation layer, thus ensuring the overall thermal efficiency of the building. In this paper, the results of a wide numerical study on the behaviour of concrete sandwich panels realized with glass fibre-composite pultruded connectors are presented, focusing on the stresses and deformations caused by dead load, thermal actions and shrinkage.

Disturbed Stress Field Model for Unreinforced Masonry

AbstractThe traditional smeared crack macromodels for the analysis of masonry structures consider masonry as a homogeneous material with the effects of mortar joints included in an average sense. This approach, suitable for the analysis of large structures, implicitly excludes the possibility of representing local elastic and inelastic mechanisms involving mortar joints. In this study, an innovative formulation based on the disturbed stress field model (DSFM) is proposed for the analysis of unreinforced masonry structures. The advancement introduced by the model lies in the possibility of simulating the global average behavior of the composite material in combination with the local nonlinear shear slip response of both bed and head joints. This paper describes the formulation of the model; as well, it presents results obtained from the simulation of tests performed on shear walls demonstrating the ability of the DSFM to reproduce the structural response of masonry structures.