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Fatigue behaviour of concrete has become an increasingly popular topic in the last century, especially with the development of railway bridges. Fatigue loading may appear in various forms, from physically applied loads, to indirect loads, including corrosion and thermal fatigue among others. These fatigue loadings may occur independently or in conjunction with the applied fatigue loadings, which could exacerbate the fatigue process and likely decrease the lifespan of the structure. The mechanisms of fatigue failure in concrete may be divided into three phases: (1) crack initiation, (2) progressive growth of micro-cracks, and (3) convergence of micro-cracks to form macro-cracks. In fibre reinforced concrete (FRC), energy is dissipated in the wake of the crack tip, which increases the load carrying capacity, thereby providing post-cracking ductility. Unlike ferrous materials, concrete was found to exhibit no fatigue limit after 2 million load cycles. However, its performance may be influenced by stress levels, load frequency, boundary conditions, matrix composition, and number of applied cycles. In this paper, the flexural fatigue behaviour of pre-cracked steel fibre reinforced concrete was investigated. Various pre-cracks and load levels were considered. X-ray Computed Tomography (CT) scans were implemented to determine the extent of damage to the fibres within the concrete matrix after fatigue loading.

In fibre reinforced concrete (FRC), energy is dissipated in the wake of the crack tip through the actions of fibre bridging and fibre pull out. This is the main mechanism which inhibits crack growth, thus increasing the load carrying capacity of FRC by providing post–cracking ductility. Furthermore, the same mechanism is present when FRC undergoes fatigue loading. Typical applications for FRC which undergo significant fatigue loading during their service life include paving applications such as bridge decks, highways and industrial floors. The continuous exposure to cyclic loading results in a decrease in apparent stiffness of the material, which may lead to fatigue failure [1, 2]. Fatigue failures are almost always unexpected, and can have a catastrophic outcome [3, 4]. Thus, the fatigue characteristics become vital performance and design parameters [1]. In this paper, the mechanisms of fatigue failure of pre-damaged hooked-end steel fibre reinforced concrete (SFRC) are investigated at a single fibre level. An initial pre-damage was applied to the fibres before the cyclic loading commenced. The pre–pull out ranged from 0.6 mm to 2.5 mm, and the cyclic loading was applied at 70% and 85% of the maximum static pull out capacity of the fibre embedded in the concrete.

The interaction between superplasticisers and cement in concrete is complex and can result in unpredictable and unwanted concrete behaviour. It is known that the positively charged tricalcium aluminate (C3A) component of the cement does not only react with the sulphate (gypsum) present in ordinary Portland cement, but also absorbs the superplasticiser. However, the exact interaction is still not fully understood. This study aims to identify compatibility issues between superplasticisers when exposed to gypsum, fine sand dust and fly ash using the Marsh cone test. The results showed that especially gypsum, which is used in the production process of cement, influence the flow time of the concrete and affects the interaction between the C3A component and the superplasticiser. The more gypsum added, the more superplasticiser is needed to have the same effect. This indicates that the ratio between C3A, gypsum and superplasticiser can result in compatibility issues. Particle size and shape, as in the case of the fly ash, was found to play a role in the effectiveness of the superplasticiser. When fly ash is used to increase the flowability of a concrete mix, the superplasticiser does not result in the same significant improvement in flowability than without fly ash.

In the past, industrial waste and by-products have successfully been used to improve the properties of concrete. Used engine oil is a waste product which is burdensome to discard of and, due to frequent replacement, is produced in high quantities in the construction industry. The utilisation of used engine oil in concrete has shown potential as an admixture by reducing slump and increasing air-content. The main disadvantage is a reduction in long term compressive strength. This study investigates used engine oil (UEO) and used hydraulic oil (UHO) as admixtures to concrete, focusing on its effect on the rheological properties. Slump, air-content, compressive strength and rheometer tests are conducted for concrete containing different dosages of UEO and UHO. Adding low dosages of UEO and UHO have no noteworthy effect on the compressive strength, although increasing air-content and altering the rheological properties significantly. UEO and, to a lesser extent UHO, reduced the energy required to initiate flow (static yield stress) as well as decreased the plastic viscosity. Adding UEO has a similar effect on the static yield stress and plastic viscosity as increasing water content or substituting cement with a proportion of fly-ash. In conclusion, UEO shows potential as an air-entrainer or water-reducing admixture.

The influence of different supplementary cementitious materials (SCM) and superplasticisers on the rheological properties of concrete was investigated to identify potential compatibility issues. Superplasticisers and SCM often have unexpected interaction with certain cementitious compounds, resulting in concrete that is difficult to place in the fresh state due to poor rheological properties. Various mixes were designed containing different superplasticisers and/or SCM in different quantities. Slump, slump flow and concrete rheometer tests were conducted to determine the yield stress, plastic viscosity and thixotropic behaviour of the concrete. Obtained results showed that the specific Sulphonate Naphthalene Formaldehyde (SNF) and Polycarboxylic Ethers (PCE) superplasticisers used, reduced the yield stress, thixotropic behaviour and plastic viscosity of concrete. Modified Acrylic Polymer (ACR) superplasticiser showed a similar effect except for the plastic viscosity which increased at higher dosages. The addition of fly ash and slag to concrete containing superplasticiser had little effect on the rheology and showed similar results as mixes only containing superplasticiser. The use of superplasticiser in conjunction with silica fumes caused a decrease in yield stress and thixotropic behaviour while plastic viscosity increased. The use of superplasticiser in conjunction with higher than normal dosages of gypsum also caused a decrease in yield stress and thixotropic behaviour but had negligible effect on plastic viscosity. It was also found that the use of PCE superplasticiser in conjunction with gypsum, used to control the set of concrete, can cause potential slump loss issues.