International Concrete Abstracts Portal

High-strength concrete (HSC) is commonly used for columns that need to withstand significant compressive forces and bending moments. However, the brittleness that characterized HSC behaviour negatively affects the ultimate strength and ductility of columns. Thanks to fibres, high-strength fibre-reinforced concrete (HSFRC) and ultra-highperformance fibre-reinforced concrete (UHPFRC) have a rather ductile behaviour in tension and compression and may be used to improve the structural response of highly stressed columns, thus leading to thinner sections and a reduction of ordinary reinforcement. This paper presents the results of a preliminary test campaign on HSFRC and UHPFRC columns under bending with normal force. Several arrangements of ordinary reinforcement were tested, including columns without any longitudinal or transversal reinforcement. The observed behaviour is interpreted by means of a mechanical model. Based on test results and parametrical analysis, the contribution of fibres to the structural response and the possibility to replace ordinary reinforcement with fibres are discussed.

The strengthening of existing reinforced concrete (RC) structures, built according to the Italian construction practice of the ’60-‘70s, has become an important and urgent issue. The reasons for strengthening are the need for: increasing strength to withstand underestimated loads; increasing the load-carrying capacity; and fulfilling the seismic code requirements. A new technique to strengthen existing RC structures, based on the application of a thin highperformance fibre-reinforced jacket, is investigated herein. The application of this technique has been studied for the strengthening of beams, columns and beam-column joints by full scale experimental tests which results showed that the application of a high-performance fibrereinforced concrete (HPFRC) jacket leads to a significant increase in strength of the elements, also reaching an adequate level of ductility. Subsequently, the experimental results are compared with analytical evaluations of the load carrying capacity of the retrofitted elements, showing that the analytical formulations are consistent with the experimental evidence.

In recent years important efforts have been devoted to develop new types of polypropylene (PP) macro fibres able to provide significant toughness and ductility to concrete. These PP fibres, which are now widely available in the market, present a series of advantages: a significantly number of fibres per unit volume that allows less variability of experimental results (hence, higher characteristic values for specific mean values); a higher number of fibres intercepting cracks and controlling their propagation (of outmost importance for early-age shrinkage cracking); last but not least, no corrosion stains at the concrete surface. However, even if several experiments available in literature showed that fibres, if provided in sufficient amount and with an adequate toughness, are significantly effective as shear reinforcement, just few of them focused on the shear behaviour of elements made of polypropylene-fibrereinforced concrete (PFRC). In this context, structural applicability of PP macro fibres, adopted as shear reinforcement, is investigated in this paper. Experimental results of full scale tests on fourteen wide-shallow beams (WSBs) and nineteen deep beams in reinforced concrete (RC) and PFRC are presented. These results show that, in both beam typologies, the addition of PP fibres significantly enhances both the shear bearing capacity and the ductility. PP fibres can completely replace and reduce the conventional shear reinforcement in WSBs and deep beams, respectively.

In a research project funded by the Austrian Research Foundation (FFG), ultra-highperformance fibre-reinforced concrete (UHPFRC) mixtures were developed and optimized. Compression tests, uniaxial tension tests and several small-scale four-point bending tests were performed and the full constitutive law was derived. A series of 10 large-scale beams with a span of 3 m and an I-shaped cross-section was subsequently produced and tested in a fourpoint setup, where the reinforcement amount and the fibre content were varied. The tensile strength of the longitudinal reinforcement was 800-1100 MPa. Several circular openings were foreseen in order to facilitate potential installation crossings. In the tests with flexural failure at ultimate load level, a pronounced ductile behaviour was observed. Cracking and crack widths were controlled in a satisfying way by the longitudinal reinforcing bars and the fibres as well. Apart from the mostly observed bending failure mode, two beams with increased longitudinal reinforcement and 2% fibres exhibited diagonal tension failure in shear. Regarding the ultimate load, in the tests with shear failure the fibre contribution was much more significant than in the tests with bending failure.

Pretensioned precast concrete girders are mainly designed to resist high bending moments. Although limited shear forces act on these girders, a minimal amount of web reinforcement should be added in order to meet design code requirements. Since the cutting, bending and placing of stirrups is a labour-intensive job, a possible alternative can be the replacement of stirrups by adding fibres to the concrete. By using steel-fibre-reinforced concrete (SFRC), the production process of precast elements can be shortened and costs reduced significantly. In order to investigate the shear capacity of full-scale prestressed fibre-reinforced concrete elements, an experimental programme was carried out on 20 m span precast girders. Additionally, standard prisms were cast from the same concrete batch to derive the postcracking behaviour of the SFRC. The ultimate shear capacity of all the girders is evaluated with respect to the current shear design provisions for SFRC according to the fib Model Code for Concrete Structures 2010.