International Concrete Abstracts Portal

The use of fibres as the main reinforcement for concrete pipes is recognized as an attractive alternative to steel bars, especially for smaller diameters. Nevertheless, the use of fibre reinforcement has not been consolidated yet. The lack of more reliable procedures for systematic quality control of the steel-fibre-reinforced concrete pipes is one of the reasons for that condition. In addition, there are no reliable technical data to support durability evaluation. A numerical simulation of the pipe crushing test can be used, with the goal of reducing this lack of knowledge. This paper presents a new solution based on a numerical simulation of steel-fibre-reinforced concrete pipes using constitutive equations derived from Barcelona tests. An experimental campaign was carried out to confirm the suitability of both the model and the constitutive equation to simulate the pipe response subjected to crushing test. This is an important tool to support evaluations of the ultimate limit state and serviceability limit state of fibre-reinforced concrete (FRC) precast elements. This is of particular relevance when the new concepts of the fib Model Code for Concrete Structures 2010 are taken into account.

In tunnels built by means of a tunnel boring machine, the use of fibre-reinforced concrete (FRC) in the precast segmental linings lead to several advantages. On one hand, FRC improves the mechanical behaviour of the segments during construction, reducing the incidence of damages. On the other hand, it contributes to a decrease of the global costs by reducing the amount of conventional passive reinforcement required. The objective of this paper is to present a real experience regarding the use of fibres as the main reinforcement in precast segmental linings in the metropolitan area of Barcelona. First, the design approach presented in the fib Model Code for Concrete Structures 2010 for fibre-reinforced concrete is adapted taking into account the mechanical requirements of the precast segments. Then, this methodology is used to assess the minimum fibre content for this application, estimated in 60 kg/m³. These results are compared with the obtained in an experimental program performed with real-scale segments simulating the critical load condition found during the most critical transient stage (stocking). The study shows an example of optimized design of FRC tunnel segments that might be useful for future tunnels.

The application of steel-fibre-reinforced concrete (SFRC) with or without additional steel bars has been growing recently in structural engineering. To accurately predict both holistic load-deflection curves and the redistribution of stresses in statically indetermined systems, calculation methods that take non-linear stress-strain relations and tension stiffening into account are favoured. Here, a moment-curvature-based approach for SFRC is proposed. In contrast to conventional tri-linear moment-curvature relations, all four stages: initially uncracked concrete, crack formation, stabilized cracking and yielding of reinforcement, are incorporated explicitly employing optimization methods. On the cross-sectional level, general and dimensionless diagrams have been derived to read the strain state w.r.t. bending moment and mechanical reinforcement ratio, two factors to account for the fibre effectiveness and the ratio of concrete cover to the effective depth. Thereby, tension stiffening is considered adapting a modified stress-strain relation of rebar. Since crack spacing of SFRC elements with additional reinforcement compared to conventionally reinforced concrete is reduced, the effective tension area and the tension stiffening bond factor have been modified. To verify the new approach, experimental load-deflection curves from literature are recalculated by numerical integrations of the obtained moment-curvature relations. The results are in good accordance. The paper summarizes the major findings of the contribution “Non-linear analysis of SFRC elements bearing capacities accounting for tension stiffening by means of modified moment-curvature relations” by the same authors published in (Heek/Mark, 2014).

Thin webbed roof girders are sensitive to fire. The first part of our study was directed to the optimization of concrete composition for real-scale roof girders to improve fire resistance.

The application of small polymeric fibres and selecting appropriate filler for the selfcompacting concrete resulted in adequate fire resistance. Experimental results on real-scale elements showed an increase in fire resistance from 12 minutes to 71 minutes. These experiments demonstrated the potential of concrete mix optimization to increase fire resistance as well as decrease sensitivity for spalling.

The purpose of the second part of our experimental study was to analyse the effectiveness of polymeric as well as steel fibres in reducing surface cracking and in improving compressive behaviour subjected to fire. Compressive strength tests were carried out on cubes with 150 mm sides. The concrete compressive experimental strength range was 60 to 75 N/mm². The test variables were concrete composition and maximum temperature (20, 50, 150, 300, 500 and 800 °C). The specimens were tested at room temperature after the heating process and a 2-hour exposure to temperature.

Our test results indicated that the advantageous influence of polymeric fibres in concrete subjected to high temperatures is mainly available for thin fibres and not for thicker fibres. Our test results also indicated that, if steel fibres are used, improvement in fire resistance can be achieved if small diameter fibres with relatively short lengths are used.

Large concrete hydraulic structures exhibit high temperature strains due to significant heat generation at early ages and to seasonal variations of water and air temperatures. When subjected to restraint, these structures are prone to extensive cracking and leakage problems at service level. The size effect phenomena generally related to concrete softening has notable influence in these large and lightly reinforced structures and contributes to strength reduction at ultimate level. The use of steel-fibre reinforcement is numerically investigated in this study, for the example of a semi-spiral case hydraulic structure. Two different sizes with a geometrical similitude are considered. Two alternative designs using steel-fibre-reinforced concrete (SFRC) and ultra-high-performance fibre-reinforced concrete (UHPFRC) are presented and compared to the conventional reinforced concrete (RC) solution. Benefits of fibre reinforcement are shown at both service and ultimate levels.