Understanding failure and stress-strain behavior of very-high strength concrete (>100 MPa) confined by lateral reinforcement
Introduction
High strength concrete (HSC) (50–100 MPa) and very-high strength concrete (VHSC) (>100 MPa) are specified more regularly due to their higher strength, stiffness, lower deformation and durability. Columns made with VHSC should have a ductility that meets seismic demands. Therefore, understanding brittle behaviour and improving ductility using transverse steel is vital for ductility design of VHSC columns because adequate ductility is provided by confining the core using lateral steel reinforcement. The analytical programs, which were developed to predict the flexural and ductile behaviours of HSC and VHSC columns, require a full range of stress-strain curves for confined and unconfined VHSC. Once this stress-strain behaviour is established, the analytical program can predict the flexural behaviour and ductility of the VHSC columns using the full-range moment-curvature relationship [1], [2], [3], [4], [5], [6], [7]. Subsequently, suitability and amount of lateral steel for a column can be determined considering the anticipated ductility levels specified in the standards and codes of practice. Therefore, the confined stress-strain behaviour can be considered as an important characteristic to evaluate the suitability of VHSC columns in seismic and non-seismic regions and this is the main focus of the study.
Experimental programs have been conducted by researchers [8], [9], [10], [11], [12] to understand the compressive behaviour of confined and unconfined HSC and VHSC columns. There are some experimental programs in the literature which cover strengths up to 125 MPa [8], [12], [13], [14], [15], [16]. Shin et al. [17] studied the influence of headed crossties on the confinement of ultra-high strength-concrete columns with a compressive strength of 200 MPa. However, experiments from 125 MPa or greater are scarce in the literature. Analytical models based on extensive experimental programs are well-established to predict the stress-strain behaviour of normal strength concrete (NSC) and HSC. Stress-strain models for unconfined and confined VHSC subjected to uniaxial compressive loads are specified by researchers [1], [9], [13], [14], [18]. However, those models are only applicable and experimentally tested for strengths less than or equal to 125 MPa. Thus, the stress-strain behaviour of confined and unconfined VHSC from 125 MPa or more cannot be predicted using state-of-the-art models and has not been captured thoroughly through any experimental program due to scarcity in the experimental results. Therefore, limited information is available to understand the mechanical behaviour and properties of confined VHSC up to 150 MPa, or greater, even though VHSC can be produced up to 150 MPa or more [20]. None of the codes around the world cover VHSC up to this range of compressive strengths [20]. Therefore, understanding the confined behaviour, failure and improvement of ductility due to the confinement of VHSC is vital to increase the strength range of concrete provided by codes and in practice up to 150 MPa or greater.
An experimental program consisting of 22 confined samples with compressive strengths ranging from 120 MPa to 160 MPa was carried out to understand the mechanical behaviour, properties and failure mechanisms of confined VHSC. This paper investigates the mechanical behaviour and presents the properties of confined VHSC, including the failure mechanisms, peak stress, elastic modulus, peak strain, residual stress and stress-strain behaviour. The failure mechanisms of confined VHSC (cover spalling, crushing of the core, buckling of reinforcement and hoop fracture) were investigated using suitable test set-ups and instruments. The samples were confined using lateral stirrups for lower confinements of up to 2.1 MPa, which is consistent with columns used in practice.
Section snippets
Mix design
Three different commercial concrete mixes consisting of two types of aggregates, namely basalt (BAS7) and granite (GRA7), silica fume (SF) and nano-silica fume (NSF) were used to achieve very high strengths. A water-to-binder ratio of 0.22 was used for the three mixes. Superplasticisers were used to obtain the required workability. Due to the higher binder content and lower water-to-binder ratio, VHSC mixes are generally stiff and dry, thereby resulting in a less workable concrete. Therefore, a
Test results and discussion
Table 5 shows the average compressive strength and elastic modulus of three mixes for different ages including the standard deviation (STD).
The confined concrete showed the following failure mechanisms: cover spalling (Fig. 6a); and crushing of the concrete core, buckling of longitudinal steel, and hoop fracture (Fig. 6b). Figs. 7 and 8 show the strain gauge and transducer results for the hoop reinforcement, longitudinal reinforcement, cover concrete and confined concrete of two samples;
Understanding and modelling the behaviour of confined VHSC
In this section, the stress-strain behaviour of confined concrete is predicted using a novel approach based on the failure mechanisms.
Conclusions
Confined VHSC samples showed four key failure mechanisms during uniaxial loading: 1. cover spalling, 2. crushing of core concrete, 3. buckling of longitudinal steel, and 4. hoop fracture. The stress-strain behaviour of confined VHSC has five major branches in the stress-strain diagram: 1. a linear ascending elastic branch up to the failure of cover concrete, 2. a nonlinear parabolic branch up to the peak stress, 3. a descending branch due to the formation of a shear failure plane, 4. stress
Conflict of interest
None.
Acknowledgements
We would like to acknowledge Mr Shushant Menon and Mr Jacques Tessier from Holcim Australia for facilitating trials, and providing aggregates for the experimental program. We would like to thank Mr Shane Clee, Dr Damith Mohotti, Dr Jinghan Lu, Dr Madhuwanthi Rupasinghe and Dr Amitha Jayalath for assisting with the experimental program. We would also like to acknowledge the Materials Characterisation and Fabrication Platform for carrying out the nanoindentation tests.
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