BEHAVIOUR OF STEEL FIBRE REINFORCED CONCRETE IN FLEXURAL FATIGUE

Abstract

The addition of small, closely spaced and uniformly dispersed discrete steel fibres to concrete substantially improves many of its engineering properties such as tensile strength, flexural strength, fracture toughness, resistance to fatigue, impact, thermal shock and spalling. The degree of enhancement of the properties of the hardened concrete depends upon the type, size, shape, volume fraction and aspect ratio of the fibres. Interest in fatigue performance of concrete began at the end of the last century with the development of reinforced concrete railroad bridges. Concrete bridges, off-shore structures and concrete pavements are the structures most influenced by fatigue loading. Despite early interest in metal fatigue, interest in non-metallics, particularly plain and steel fibre reinforced concrete, lagged behind for many years. Concrete fatigue investigations, like those on metal, have also been motivated by practical problems. Concrete pavements are usually un-reinforced and the concrete is expected to resist tension in these structures. Consequently, knowledge of fatigue performance in flexure is needed. Concrete structures supporting dynamic machines are also subjected to hundreds of millions of load cycles involving complicated stress states. A critical review of the existing literature indicates that extensive studies have been carried out to investigate the static, impact and fatigue behaviour of plain and fibrous concrete. Most of the static properties of fibrous concrete such as compressive strength, split tensile strength, flexural strength, stress-strain relationship, toughness and investigations of dynamic behaviour in terms of impact resistance and fatigue resistance, have mainly focussed on the type, size, shape, and aspect ratio of fibres. The available information on the mechanical and fracture toughness characteristics of fibre reinforced concrete specimens is very limited. As regards the fatigue behaviour of plain and fibrous concrete, the fatigue-life data usually shows (i) considerable variability at a given stress level. Hence, the concepts of probabilistic analysis have been implemented to ensure adequate resistance against fatigue failure. The Weibull probability law has been extensively examined to describe the fatigue behaviour of plain concrete and to some extent fibrous concrete. It has been established that besides the volume fraction, the aspect ratio of the fibres plays a crucial role in determining the workability and mechanical properties of the composite. If a matrix reinforced with an identical volume fraction of long fibres and short fibres is considered, then for the volume of fibres normally used in cementitious composites, the relatively long fibres give only a small improvement in strength. If fibres are far apart, they have no ability to arrest micro-cracks. However, once the micro-cracks coalesce into macro-cracks, the long fibres can arrest propagation of macro-cracks and substantially improve the toughness of the composite. By combining fibres of varying length (mixed aspect ratio) in a matrix, improvement in both the peak stress and post peak toughness canbe expected. Therefore, the present investigation was planned to study the influence of mixed aspect ratio and volume fractions of steel fibres on static, impact and fatigue behaviour of fibres concrete. The probabilistic analysis of flexural fatigue-life distribution of plain and steel fibre reinforced concrete was carried out for various fatigue stress levels for fibrous concrete specimens with different mixed aspect ratios and volume fractions of fibres. It was also proposed to obtain the two-million cycles endurance limits corresponding to different fatigue stress levels for steel fibre reinforced concrete with different mixed aspect ratios and fibre volume fractions. An extensive experimental investigation consisting of 165 cylindrical specimens for determining the split tensile strength and stress-strain relationships of plain and fibrous concrete; 85 cubes for determining the compressive strength; 672 beams for flexural tests (192 in static mode, 480 in fatigue mode), and 112 beams for impact resistance tests was carried out. Two types of crimped-type flat steel fibres (ii) viz. 25 x 2.0 x 0.6-mm and 50 x 2.0 x 0.6-mm, corresponding to fibre aspect ratios of 20 and 40 respectively, were used with mixed fibre ratios containing 0%, 35%, 50%, 65% and 100% of short fibres, at each of the fibre volume fractions of 0, 1.0, 1.5 and 2.0%. The dimensions of the cube specimens were 150 x 150 x 150 mm, the cylindrical specimens 150 x 300 mm and the beam specimens were of size 100 x 100 x 500 mm. The complete stress-strain relationship of concrete in compression and all of the flexural static and fatigue tests were conducted on a closed loop UTM. The specimens were tested under 3 point flexural loading on a simply supported span of 450 mm. For impact tests, the beam specimens of size 100 x 100 x 500 mm were supported on a span of 400 mm and a drop-weight impact machine was used for conducting the tests. The number of blows required for the first visible crack to form in the specimens and for ultimate failure were recorded and the impact energy calculated. The span and points of loading in the fatigue as well as static tests were kept the same. The flexural fatigue tests were conducted at a constant stress ratio (R=fmin/fmax) of 0.10 and different stress levels (S=fmax/fr) in the range of 0.90 to 0.70 for both plain and fibrous concrete specimens with different mixed aspect ratio of fibres. Constant amplitude, sinusoidal, non-reversed loads were applied at a frequency of 20 Hz. The number of cycles to failure for each specimens under different load condition was noted. The physical properties of the basic constituent materials viz. cement, fine aggregates and coarse aggregates were obtained as per the relevant Indian Standard Specifications. The concrete mix proportion of C:F.A:C.A