International Concrete Abstracts Portal

Use of fibre-reinforced concrete for pavements, though widely established, lacks comprehensive standardization with respect to both design as well as implementation. Most widely used design methods have adopted the established techniques used for slab-on-grade, mostly due to lack of studies specific to pavements. This is a major drawback since the failure of slabs-on-grade is predominantly caused by static loading whereas the failure of pavements, more often than not, occurs at much lower stresses due to fatigue. In this context, a comprehensive design methodology exclusively for FRC pavements is proposed here including the effect of fatigue in conjunction with inelastic analysis. The static load design equation is based on Meyerhof’s ultimate load (yield line) analysis. The material capacity is estimated using the flexural capacity factored by a stress reduction factor for FRC in the cracked and the uncracked state obtained from suitable S-N relations to account for the fatigue stresses. The material parameters incorporated in the design equations include the flexural strength and the equivalent flexural strength of FRC obtained using the unnotched prisms as per Japanese standards (JSCE-SF4).

This paper enumerates the reasons that motivate the use of finite elements simulations for the analysis of cracking behaviour in the design of SFRC structures. These reasons are mainly due to the irrelevancy of current design recommendations, which do not adequately take into account certain technical problems, such as the evaluation of the crack openings in SFRC structures under serviceability loads, the analysis of the non-linear behaviour of concrete structures under shear and punching, and the analysis of the non-linear behaviour of indeterminate concrete structures.

This paper also proposes the main criteria that a relevant numerical model of SFRC cracking needs to respect in order to simulate cracking. Finally, an example of such a relevant model is presented along with its validation in the context of the shear behaviour of a large SFRC beam.

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.