Fatigue bond-slip properties of steel reinforcing bars embedded in UHPFRC: Extraction and development of an accumulated damage law

The occurrence of cyclic loads in RC structures is known to deteriorate the bond between the reinforcing bars and concrete by reducing both the bond strength and stiffness, eventually leading to debonding through large increases in slip. There is much research to quantify this bond deterioration for normal strength concrete but little research has considered UHPFRC, which is the subject of this paper. This research develops a testing approach and analysis procedure to quantify the deterioration in bond as a result of high-cycle fatigue. The procedure has been developed through 18 tests of steel reinforcing bars embedded in UHPFRC with steel micro fibres. A test rig has been developed to directly measure the bond-slip under monotonic and cyclic loads. Procedures are then developed for quantifying the bond stiffness and the incremental set, that is, the increase in slip per cycle, by using the known interaction between the monotonic and cyclic bond-slip already identified by other researchers. It is shown how these procedures can be used to quantify the bond degradation under combinations of fatigue loads and how simply measuring the crack width in a structure can give a very good indication of both the residual fatigue life and bond strength.Barbara Daniela Giorgini Sepulveda, Phillip Visintin, Deric John Oehler

Prestressed fibre reinforced polymer (FRP) laminates for the strengthening of reinforced concrete structures

Recent development in the field of strengthening has seen the application of prestressing of FRP laminate prior to bonding in order to exploit its high tensile strength. The method of prestressing the laminate induces an initial tensile strain in the concrete beams upmost fibre, thus reducing the deflection of the beam throughout the design loads. This alteration of the beams structural characteristics provides advantages in beam serviceability requirements. Structurally the beam can withstand greater ultimate loads, while yielding of internal reinforcement and cracking moments are delayed substantially, compared to unlaminated beams. Extensive experimental investigations have been undertaken by many researchers with variables ranging from anchorage type, number of laminates applied to beam, tensile reinforcement ratio and the initial prestress level of laminates before bonding. Despite the large amount of experimental data in the field, current analytical models generally employ elementary procedures in predicting beam behaviour and as a result the analytical results exhibit poor correlation with the experimental results. This implies the necessity for the development of a generic model that can accurately predict beam behaviour that will be the basis of the present study. The focus of this paper is the development of a new analytical model that can accurately predict the behaviour of an RC beam strengthened with an externally bonded (EB) prestressed fibre reinforced polymer (FRP) laminate. The model will be critically compared to an experimental database for calibration purposes then applied in a parametric study

Concrete-filled square FRP tubes under axial compression

Togay Ozbakkaloglu and Deric J. Oehlershttp://www.hku.hk/apfis07/pdf/APFIS2007-Accepted_Abstracts.pd

Experimental investigation of the influence of fibre content on the flexural performance of simply supported and continuous steel/UHPC composite slabs

The application of relatively low volumes of fibres in normal strength concrete has been shown to be of significant benefit when applied to composite slabs with profiled sheet decking. This paper reports on an experimental study aimed at quantifying further potential benefits that may arise from applying ultra-high performance fibre reinforced concrete. To assess performance six simply supported beams were tested under hogging and sagging loading configurations along with three two span continuous beams. Fibre contents are varied from 0% to 2% and changes in strength, deformation, crack width and moment redistribution are measured. At the serviceability limit state, it is shown that the addition of high fibre volumes can significantly enhance member stiffness and reduce crack widths in all beams. At the ultimate limit state it is observed that a transition from 0% to 1% fibres significantly increases strength but that there is a maximum fibre volume beyond which no further increases in strength are possible. Conversely, member ductility and moment redistribution are shown to be strongly proportional to fibre volume.Sirui Chen, Phillip Visintin and Deric J. Oehler

Bond between very-high and ultra-high performance fibre reinforced concrete and profiled deck sheeting

The behaviour of steel concrete composite slabs at all load levels is controlled by the shear transfer between the profiled steel deck and concrete slab. To apply new concrete technologies, such as high-strength and fibre-reinforced concretes, to steel concrete composite slabs, it is therefore essential to quantify the interfacial bond-slip properties. In this paper, a testing approach based on single-lap shear tests, commonly applied to quantify interfacial shear prop- erties for external reinforcement, is applied to measure the bond between profiled sheets and concrete. The testing regime consists of 6 trial tests used as the basis for developing the test methodology and 48 tests to quantify the impact of concrete strength (very-high and ultra-high- strength), high volumes of steel micro fibres, and coarse aggregate, on the bond between concrete and dovetailed and trapezoidal profile decks. The results show that the concrete strength and the presence of coarse aggregate have limited impact on the bond properties, however the presence of fibres significantly improves bond strength and toughness for dovetailed profiled decks but has limited influence on trapezoidal decks.S. Chen, PhD Candidate, P. Visintin, Associate Professor, D.J. Oehlers, Emeritus Professo

Mechanics of shear failure in fiber-reinforced concrete beams

Abstract not available.A.B. Sturm, P. Visintin, and D.J. Oehler

Design-oriented solutions for the shear capacity of reinforced concrete beams with and without fibers

The inclusion of fibers substantially improves the shear resistance of reinforced concrete beams. Fibers can, therefore, be used as a partial or full substitute for traditional transverse reinforcement. Before replacement of traditional reinforcement with fibers can be undertaken, reliable expressions that incorporate the effect of fibers are required. In a previous study, a mechanics approach based on quantifying the presliding shear capacity of fiber-reinforced concrete beams was developed, broadly validated, and compared with existing design approaches. Although accurate, the numerical solution is too complicated for routine design, and hence, in this paper, simplified solutions are developed. This is achieved by (1) approximating the neutral axis depth at the initiation of shear failure, (2) developing a closed-form solution for the angle of the critical diagonal shear crack, removing the need to iterate, and (3) incorporating a simple approach to estimate the stress in the fibers crossing cracks, removing the need to integrate fiber stresses over a range of crack widths. To validate the simplified solutions, they are used to predict the capacity of tests on 626 reinforced concrete beams without stirrups, 176 reinforced concrete beams with stirrups, and 23 fiber-reinforced concrete beams. Importantly, these simplified solutions largely retain the accuracy of the numerical approach and show an improved fit compared with currently available solutions.A. B. Sturm, P. Visintin and D. J. Oehler

Shear strength of RC beams without web reinforcement

Due to the complexities of the mechanics of shear failure in reinforced concrete (RC) members, most current approaches for predicting shear strength are mainly empirical. Being empirical, these approaches do not physically explain the shear failure mechanism seen in practice and consequently should only be used within the bounds of the testing regimes from which they were derived; this restricts their application to innovative materials such as fibre-reinforced polymer RC beams or those with high-performance concrete. In this paper, a numerical mechanicsbased segmental approach for the shear failure of RC beams with any type of reinforcement and concrete is developed; from the mechanics of the segmental approach, simplifications are made to develop closed form solutions and the resulting design equations are compared to a database of 626 steel-reinforced specimens. Further comparisons to the predictions made by the ACI, AS 3600 and FIB Model Code 2010 approaches show that the proposed approach offers improved accuracy and a reduction in scatter.Tao Zhang, Phillip Visintin and Deric J. Oehler

Blending fibres to enhance the flexural properties of UHPFRC beams

In this paper, the flexural behaviour of ultra-high performance fibre reinforced concrete (UHPFRC) beams reinforced with macro-, micro- and a blend of macro- and micro-fibres is investigated at all limit states. The goal of this study is to investigate whether the benefits of fibre blending that are observed at a material scale translate to the structural scale. To this end, six UHPFRC beams with two different cross-sections and three different mix designs were tested. Standard codified approaches as well as a segmental analysis technique are then applied to predict the measured load-deflection and load-crack width behaviour of the beams and it is shown that while standard approaches can predict serviceability deflections, the segmental analysis is more accurate when predicting crack width, member ductility and ultimate deformations. Following validation for beams with blended fibres, it is then used as the basis for a parametric study to further investigate the influence of beam geometry and reinforcing details.Alexander B. Sturm, Phillip Visintin, Deric J. Oehler