Behaviour of ultra high performance fibre reinforced concrete subjected to static and impact loads
The collapse of critical elements of any structure when subjected to accidental loading conditions or natural disasters, usually associated with dynamic loads from earthquakes and explosion loads, can cause large scale catastrophic failures. These events produce non-uniform forces in structures which lead in failure mechanisms resulting in progressive collapse. The increasing requirements for improved resilience of civil engineering and military applications led to the development of ultra-high strength concretes with remarkable strength, durability and toughness. Ultra-high performance fibre reinforced concrete (UHP-FRC) has recently emerged as an advanced cementitious material with the potential of enhancing the impact energy absorption capacity of structural elements due to its superior properties. However, major limitations for its wider use and application in critical infrastructure and protection are associated with the limited studies, lack of design standards and guidelines in designing against static and impact loading conditions. Additional challenges preventing the wider use of UHP-FRC in construction include suitable methodologies for testing of the material under static and impact loads and the high cost due to the high amount of cement and steel fibres required. This research presents extensive experimental studies in contribution to the characterisation of the mechanical properties and dynamic behaviour of beams made of UHP-FRC. The dynamic properties of the material are strain sensitive to the loading rate and effect of fibres distributed within the matrix. In the experimental programme, various parameters were investigated including the use of different amounts of fibres and loading cases, assessment of strain rate dependent models and fracture characteristics. Firstly, the influence of fibre content, fibre type and geometrical configuration of beam specimens on the mechanical properties was investigated in terms of the compressive strength, splitting, direct tensile and flexural strengths, modulus of elasticity and stress – strain under compression. Two types of fibres were used: steel fibres (SF) and hooked-end fibres (HF). Steel fibres were added to UHP-FRC at 0.5%, 1.5% and 2.5% of the total volume, whereas 2.5% of hooked-end fibres were used. Tests on beams with different depths (50 mm, 75 mm and 100 mm) and notch/depth ratios (0.2, 0.3 and 0.4) were performed to investigate the flexural strength, fracture energy, size effect and used to calculate the dynamic increase factors of the material. Testing methods to measure the stress versus strain curves in direct tension were developed and the effect of the boundary conditions was investigated. Secondly, a drop-weight impact testing facility was designed and constructed to measure the low-velocity impact characteristics of UHP-FRC. Impact loading cases including repeated, incremental and direct impacts were performed in beam specimens with different fibre contents, to investigate the behaviour of the material under different strain rates and damage. The development of a multi-channel data acquisition system was designed and developed to acquire synchronised signals from load cells, accelerometers, strain gauges and high-speed cameras. This research presents a novel technique for the design, development and use of speckle patterns on the concrete surface of UHP-FRC beams for digital image correlation (DIC) measurements. The proposed method enables the creation of patterns with consistent speckle sizes and density in a simple, repeatable, rapid and inexpensive fashion. The speckle pattern was designed, transferred to the measurement surface and assessed using 2D and 3D stereovision systems under impact loads. The spatial resolution and sub-pixel accuracy of the proposed method was assessed and compared to traditional speckle pattern techniques for DIC measurements. Additionally, the results of an extensive parametric study affecting the DIC measurements including image noise, grayscale distribution of the speckle pattern, subset size, strain gradients in the subset and the interpolation algorithms were investigated. This research work presents an investigation of the static and dynamic behaviour of UHP-FRC beams with different fibre contents and types and its suitability for wider use of the material in defence and protective infrastructure. In comparison to normal strength concrete, UHP-FRC showed superior structural performance in terms of stiffness, strength, fracture energy and strain rate enhancement under both static and dynamic loads. The results of this research can benefit organisations and industries including military, nuclear, civil defence, testing agencies and structural engineers in using UHP-FRC in protective structures where high strength and energy absorption capacity is required.
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