Failure criteria and triaxial behaviour of HPFRC containing high reactivity metakaolin and silica fume Saeed Karim Babanajad, Yaghoob Farnam ⇑ , Mohammad Shekarchi Construction Materials Institute, School of Civil Engineering, College of Engineering, University of Tehran, Tehran, Iran article info Article history: Received 5 October 2010 Received in revised form 10 August 2011 Accepted 16 August 2011 Available online 24 November 2011 Keywords: Triaxial strength HPFRC High reactivity metakaolin Silica fume Failure criteria abstract In this paper, the triaxial compressive behaviour of high performance fibre reinforced concrete (HPFRC) containing two types of pozzolans namely high reactivity metakaolin (MK) and silica fume (SF) is studied. Standard triaxial tests with four different confining pressure levels were performed on 75  150 mm cylindrical concrete specimens containing 2% steel fibres by volume. Companion uniaxial direct tensile tests are also performed on dog-bone shaped specimens to ascertain the strain-hardening behaviour of these composites. The results show that an increase in confining pressure leads to an increase in the peak additional axial stress in both SF and MK specimens, and the increase is about the same for the two types of composites. However, the peak additional axial stresses of MK specimens occur at higher axial strains thus showing a more ductile behaviour with higher energy absorption capacity. To establish an empirical relation for prediction of axial strength as a function of confining pressure, three constitutive models i.e. Mohr–Coulomb two-parameter, Hsieh–Ting–Chen four-parameter and William–Warnke five-parameter models are used and calibrated with the test results. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction With the increasing demand for modern civil engineering mate- rials and their practical utilization in structural applications, there is increasing need to develop reliable constitutive models for high performance concretes. They include the effects of adding steel fi- bres as well as supplementary cementitious material to the con- crete. Enhancing the structural resistance and performance and also reducing cracking and spalling phenomena is possible by increasing the concrete’s toughness, ductility, tensile strength and flexural strength under various kinds of loading. Fibre rein- forced concrete (FRC) is one of the commonplace engineering materials, employed in several structural applications. High and ultra high strength concrete, named as HSC and UHSC with a very high compressive strength values remain principally a brittle material. Adding adequate additives such as appropriate pozzolans and fibres improves tensile strength, flexural strength, energy absorption, and impact resistance and also provides ductil- ity and intensifies their performance [1–13]. The fibres volume fraction of conventional FRC and high performance fibre-reinforced concrete (HPFRC) is generally in the range of 1–3%. As shown in many research studies, the addition of pozzolans improves the mechanical properties of concrete [1–13]. Therefore, by combining steel fibre and suitable pozzolans, the mechanical properties of concrete are expected to be improved. To the authors’ knowledge, there are no published comprehensive studies investi- gating the properties of HPFRC containing pozzolans and subjected to triaxial loading. Hence, in this research, the behaviours of HPFRC under triaxial tests containing two different practical pozzolans i.e. high reactivity metakaolin and silica fume are investigated and compared. Silica fume is often introduced as a super-pozzolan that reacts with the lime produced from hydrated cement reducing the volume of larger pores commonly found in cement paste. Likewise, high reactivity metakaolin is a highly active and effective pozzolan, which can be used as a cementitious material in partial replacement of cement in concrete. As known, energy absorption and toughness of HPFRC increases in relation to adding the high-reactivity materi- als to mixture [1,6]. Generally, adding convenient pozzolans to the concrete may enhance the mechanical properties i.e. compressive strength, bending strength, and splitting tensile strength and also its performance, durability and toughness. Therefore, the use of var- ious mineral admixture is now becoming commonplace in concrete for structural applications especially in the place where both en- hanced durability and high toughness are required. In relation to the recent published researches, Kim et al. [4] stud- ied HSC specimens using metakaolin from the aspects of strength and durability properties. From this research, it is understood that metakaolin constitutes promising material as a substitute for silica fume. Gutierrez et al. [5] inspected the effect of pozzolans on the performance of fibre-reinforced matrixes. Experimenting three 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.08.094 ⇑ Corresponding author. Tel.: +98 21 88973631. E-mail address: yfarnam@ut.ac.ir (Y. Farnam). Construction and Building Materials 29 (2012) 215–229 Contents lists available at SciVerse ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat