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Analytical models which can reliably predict the load–deformation behaviors and the ultimate strength of columns are indispensable tools for the practical design. In this paper, an analytical model is developed to predict the compressive load–deformation behavior and the ultimate capacity of engineered cementitious composite confined concrete encased steel (ECC‐CES) composite columns. The proposed analytical model employs the effective material stress and strain compatibility approaches for the predictions of compressive load–deformation response of ECC‐CES columns. The material constitutive models for ECC, concrete, and steel are first established, and then used in the analytical model. The effect of ECC and steel confinement on the concrete core and the softening behavior of steel column due to compression buckling are also considered. Based on the experimental results, appropriate strength reduction factors are proposed so that the effective material stresses can be calculated. The strength predictions from the proposed analytical methods and different design codes are compared with the experimental and numerical results. It was found that the proposed analytical models using effective material stresses gave better predictions than many commonly used design codes. [ABSTRACT FROM AUTHOR]

Accurate estimation of the in situ strength of concrete at early age is very important as it provides the necessary information required to start subsequent construction operations. Overestimation of the strength may cause serious safety hazards and underestimation may lead to unnecessary costly delays. This study investigates the performance of the maturity method in estimating the strength of in situ concrete subjected to cold weather at early age. Instrumented concrete slabs were subjected to cold weather conditions at early ages and their strengths were measured using drilled core samples from the slab. Sensors embedded in the slabs measured the temperature in the concrete which was used to estimate the strength using the maturity method. The measured core strengths at 24 and 72 h after casting are then compared with the estimated strengths using the maturity method and its performance is evaluated. The core strengths are also compared with the strength of standard cylinders cured at the same condition as the slabs. More than 250 cylinders from two slab thicknesses and four batches of concrete were used in the experiments to obtain statistically significant experimental data. The results show that the maturity method performed much better than the standard cylinder strength. On average the standard cylinder strength underestimated the core strength by more than 40% while the maturity method overestimated the strength by less than 10% with a lower variation. [ABSTRACT FROM AUTHOR]

Although high-strength concrete (HSC) has higher compressive strength than normal-strength concrete (NSC), its application in column construction is often limited by its brittleness and limited post-peak ductility. In this preliminary study, hybrid fibre engineered cementitious composite (ECC) is proposed as a potential encasement material to confine the HSC core and to increase the column's ductility. The behaviours of the proposed ECC-encased concrete stub column under axial compression are studied experimentally. Thirty stub columns, including six NSC/HSC, six ECC and 18 ECC-encased NSC/HSC (ECC–NSC/HSC) are tested under axial compression. The effects of ECC strength, ECC encasement thickness, concrete strength and cross-section shapes (circular, square and rectangular) are then investigated and reported in terms of failure modes, strength ratios, post-peak ductility and energy dissipation capacity of the stub columns. It is found that, when comparing with NSC/HSC stub columns, although no significant improvement in the ultimate strength was observed, the ECC encasement improved the performance of ECC–NSC/HSC stub columns by reducing brittleness and improving the post-peak behaviour during failure. [ABSTRACT FROM AUTHOR]

As high-strength steel box columns are usually fabricated by welding steel plates together, the welding process inevitably introduces residual stress and geometric imperfections and may affect their compressive strength. A detailed numerical investigation was conducted to study the behaviour of high-strength steel built-up box columns under compression. The welding processes were first simulated by a three-dimensional fully coupled thermal-mechanical modelling procedure using the general-propose finite-element package Abaqus. The residual stress distributions in the columns generated by different welding processes were determined. The column strengths under compression were then modelled by incorporating both the computed residual stress and the initial geometrical imperfections of the column as predefined input fields. Furthermore, the effects of different welding processes and heat treatment methods on built-up box column strength were investigated and discussed. [ABSTRACT FROM AUTHOR]