International Concrete Abstracts Portal

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.

Strain-hardening cement-based composites (SHCC) are a group of high-performance materials which shows a high non-linear deformation capability and experiences strainhardening behaviour under tensile loading. This behaviour is achieved by a specific material design which uses micro-mechanical effects to enhance the bridging properties of the fibres. In the presented research work, a series of large-scale uniaxial tension tests was performed to investigate the influence of the steel reinforcement on the load-bearing capacity as well as on the deformation and cracking behaviour of slabs made of SHCC. Both the global deformations of the test specimens and local deformations of the reinforcement bars were measured. The experiments showed that the load-bearing behaviour of the structural elements is characterized by multiple cracking of SHCC. This results in a quasi-elastic, tensionstiffening behaviour of the tension element after the initial cracking strength was reached. For a realistic description of this load bearing behaviour, a model for concrete stress-strain relationship is proposed.

A four-year industrial research project led by Polytechnique Montreal was conducted to develop innovative precast bridge barriers made with high-performance fibre-reinforced concretes (HPFRC). The design of a 2-m long precast barrier in HPFRC anchored with Ushape rebar on a bridge slab was optimized using nonlinear finite element calculations. Specimens were produced at full scale in a precast plant and tested in laboratory under various experimental conditions. Phase 1 of the project included application of static and impact loadings on the barrier. Phase 2 concerned the mechanical behaviour of overhang bridge slabs with precast barriers (with or without longitudinal continuity) and cast-in-place barriers. Experimental results demonstrated that the bridge slab and barrier exceed the CSA static design load requirements with all configurations tested, and utilization of precast barriers with longitudinal continuity improve the ultimate load. Numerical models accurately reproduced the various experimental conditions studied; they were then used to provide information on the impact of barrier length.

An experimental campaign on a full-scale steel-fibre-reinforced concrete (SFRC) flat slab of 70kg/m³ was performed in order to investigate the design methods recently published by fib in the Model Code for Concrete Structures 2010 (MC2010). These experiments were based on uniformly distributed loadings as well as concentrated loadings. By using the concrete’s characterization on small specimens, the ultimate flexural load-bearing capacity of the slab was checked according to the relationships given in the MC2010 with a full plastic approach. The tests confirm the ductile behaviour and the redundancy potential of SFRC in flat slabs. Identification of the worst yield lines pattern remains an important task for the designer as it significantly influences the result. Structural design supported by the yield line theory and laboratory characterization based on three-point bending tests provides a rather conservative result for comparison with the observations of the full-scale test.