Standing firm - ultra-high performance concrete
Ultra-high performance concrete improves structural integrity under extreme loads. Dr Christoph Mayrhofer and Oliver Millon from the Fraunhofer-Institute for High-Speed Dynamics, Ernst-Mach-Institut, Freiburg, Germany, report
Concrete is one of the most important and efficient construction materials. The increasing demand for taller and more highly loaded constructions, as well as the threat of extreme loads from bomb blasts, impact or natural disasters, makes it essential not only to analyse the behaviour of conventional and high performance concrete, but also to develop new concrete materials with enhanced performance to meet the challenges placed on structures.
Furthermore, the need for cost reductions through material savings and better use of building areas demands higher and more slender constructions.
In the past, concretes of the high strength class with improved strength and stiffness, and an improved resistance could be developed, extending the applications of concrete. Ultra-high performance concrete (UHPC) is one material of this new concrete class.
Taking higher loads
Depending on its composition, UHPC is characterised by a high compressive strength (>150MPa), dense and low porous matrix, and a high stiffness. This is achieved using high-strength coarse and fine aggregates, high-strength cement, a low water/cement ratio and a high amount of fine aggregates. This high-strength material allows steel to be replaced as the only construction material in highly loaded constructions, such as high-rise buildings or bridges.
Constructions in aggressive media can be accomplished more easily because the pore-reduced matrix leads to a lower permeability and better protection of the reinforcement. Non-reinforced UHPC shows brittle behaviour with high strength, however, it also has a low fracture energy.
To increase ductility, the addition of malleable, high-strength fibres is required. Steel fibres are commonly used for reinforcement. Static and dynamic material characterisation must be considered for different loading conditions, and the description of failure mechanisms are important.
Comparisons to standardised concretes, like conventional and high performance varieties, can quantify the potential of new materials. With the help of Hopkinson-Bar experiments, the material properties can be determined. The graph below gives an overview of the dynamic properties of UHPC with different steel fibre contents compared to standard concretes.
The tensile strength of non-reinforced UHPC is 2.5 times greater than conventional concrete, 40MPa, and increases with the addition of steel fibres [by 3.5 times], up to 55MPa.
Similar observations were made for fracture energy. Here, the influence of the fibre reinforcement is considerable. The non-reinforced mixture shows low fracture energy and fails, and so no differences were observed with other brittly materials. With the addition of small fibre content (about one volume per cent), this property increases significantly. With a fracture energy of around 10,000N/m, it is up to 30 times higher than for the non-reinforced materials, as well as 2.5 times higher for fibre-reinforced high performance concrete with the same amount of fibre.
Both the high tensile strength and the high fracture energy of fibre-reinforced UHPC are responsible for higher material resistance against static and extraordinary dynamic loads. To describe the material behaviour under real loading-situations, scaled tests on structural elements are needed to help qualify the damage and failure mechanisms.
In another test campaign, the structural response and the damage behaviour of columns and walls fabricated using fibre-reinforced UHPC have been analysed. The focus was on columns under contact detonation, based on the scenario of a case bomb and blast loading of a wall (scenario detonation at a large distance).
The different loading situations lead to different failure mechanisms of the elements. Large local damage was found in the loading case of contact detonation. Fragmentation on the loading side of the element and extended cracking at the rear side occur. However, compared to experiments on conventional concrete (see images, below), the extent of damage in UHPC is reduced. On conventional concrete, marked compression and tensile damage zones were found.
The effect on fibre-reinforced UHPC is limited to compression damage and only low tensile damage in the form of cracks. The reduced damage of the columns in UHPC is evidence of a better material resistance against dynamic loads, resulting in a residual strength that is four times higher.
Similar results were found for blast experiments on concrete walls and for close-in detonations on concrete façade elements as well (see images, below). This results in lower damages on UHPC elements, expressed by a wide-spread zone of micro-cracks and no fragmentation on the element’s tensile side.
The addition of steel fibres, the combination of fibres and dense matrix, and the good bond between fibres in the matrix lead to the largely extended micro-crack zone. The global deformation can be reduced, leading to less damage, and an increased loading capacity and usability.
Fracture and failure
Fragments and fracture surfaces were analysed for any loading case. Identifying failure mechanisms is important to aid understanding of the microstructural effects during the loading processes.
In conventional concrete, under dynamic loading, the principal failure mechanism is bond failure between aggregate and matrix. In UHPC, computer tomography and microscopic analyses reveal different failure mechanisms. Bond failure between fibre and matrix is one of the most important failure modes, consuming large amounts of energy. Other mechanisms like bond failure between aggregate and matrix – known from conventional concrete – as well as aggregate failure have been observed. Multiple failure of UHPC is greatly different to conventional concrete, strongly influencing the material properties. The good embedment of fibres and coarse aggregates into the matrix leads to high consumption of loading energy and to higher loadability.
Ultra-high performance concrete offers much improved behaviour in its material properties, with higher strength, fracture energy and residual strength.
Dr Christoph Mayrhofer, Fraunhofer-Institute, High-Speed Dynamics, Ernst-Mach-Institut, Safety Technology and Protective Structures, Am Klingelberg 1,79588 Efringen-Kirchen, Germany. Tel: +49 (0) 7628/9050 633. Email: firstname.lastname@example.org. Website: www.emi.fraunhofer.de
Main image © mishmoshimoshi. Licensed under CC BY 2.0 page on Creative Commons (http://creativecommons.org/licenses/by/2.0/).