The flexural fatigue performance of plain and fibrous concretes

If fatigue cracking is going to occur in concrete structures then it is more likely due to repeated flexural loadings rather than direct compression or tension. Typical examples are road and airfield pavements, bridges, offshore constructions and structures likely to experience earthquakes. Also many of these loadings have a dynamic character and a knowledge of material behaviour at rapid stressing rates as an essential preliminary requirement to understanding flexural fatigue performance. Therefore since flexural loadings are frequently encountered in practical situations then the flexural test is probably one of the most useful types to be used in the examination of both the fatigue behaviour of concrete and its ultimate strength developed at rapid loading rates. Although the designer regards concrete as an homogeneous material, it consists of two phases, the active hardened cement phase, which is the binding material to the inert phase, the aggregate. Concrete behaviour can therefore be complex and aspects interpreted at either the macroscopic, microscopic or molecular level. The composite nature of concrete may be further complicated by the introduction of steel reinforcement. Over the last few years there has been a general research interest with the incorporation of small quantities of fibres in concrete. Steel fibres of dimensions 10mm to 60mm in length, 10m to 60 m in diameter, with material properties ranging from 150 GN/m ₂ to 200 GN/m ₂ in elastic modulus and 700 N/mm₂ to 200 N/mm₂ in tensile strength may be introduced at the mixing stage in proportions between 0.5% and 3.5% of concrete volume. The consequent enhancement in flexural strength is substantial and the likelihood of a similar improvement in fatigue performance needed to be demonstrated. It has also been common practice when conducting modulus of rupture tests, to use 500mm × 100mm × 100mm specimens for mixes incorporating fibres. When casting such specimens there is a danger of fibres not being randomly orientated. Thus for this investigation, beams of 1500 times 200 times 200mm have been used. The selection of larger beams also makes instrumentation more manageable and the dimensions may be more comparable with those associated with most practical applications. Some fifty fatigue tests have been completed supported by a further five hundred strength and control tests. A complete series of strain, deflection and crack development histories have been observed. An appropriate form of S-N curve has been adapted for the examination of results. Models explaining the behaviour of both plain and fibrous concretes have been proposed and a method of flexural fatigue performance prediction has been formulated.