Experimental study on behaviour of concrete filled double skin tube columns under lateral impact loading

This paper presents an experimental investigation on the lateral impact response of axially loaded concrete filled double skin tube (CFDST) columns. A total of four test series are being conducted at Queensland University of Technology using a novel horizontal impact-testing rig. The test results reported in this paper are from the first test series, where the columns are pinned at both ends and impacted at mid-span. In the next three series, effects of support conditions, impact location and repeated impact will be treated. The main objectives of the current paper are to describe the innovative testing procedure and provide some insight into the lateral impact behavior and failure of simply supported axially pre-loaded CFDST columns. The results include time histories of impact forces, reaction forces, axial force and global lateral deflection. Based on the test data, the failure mode, peak impact force, peak reaction forces, maximum deflection and residual deflection, with and without axial load, are analyzed and discussed. The findings of this study will serve as a benchmark reference for future analysis and design of CFDST columns.

Steel-concrete-steel composite columns subjected to lateral impact

Concrete-filled double skin tube (CFDST) is a creative innovation of steel-concrete-steel composite construction, formed by two concentric steel tubes separated by a concrete filler. Over the recent years, this column form has been widely used as a new sustainable alternative to existing structural bridge piers and building columns. Since they could be vulnerable to impact from passing vessels or vehicles, it is necessary to understand their behaviour under lateral impact loads. With this in mind, physical tests on full scale columns were performed using an innovative horizontal impact testing system to obtain the failure modes, the time history of the impact force, reaction forces and global lateral deflection as well as permanent local buckling profile of the columns. The experimental testing was complemented and supplemented by developing and using an advanced finite element analysis model. The model was validated by comparing the numerical results against experimental data. The findings of this study will serve as a benchmark reference for future analysis and design of CFDST columns.

Fracture process zone size and true fracture energy of concrete using acoustic emission

Acoustic emission (AE) energy, instead of amplitude, associated with each of the event is used to estimate the fracture process zone (FPZ) size. A steep increase in the cumulative AE energy of the events with respect to time is correlated with the formation of FPZ. Based on the AE energy released during these events and the locations of the events, FPZ size is obtained. The size-independent fracture energy is computed using the expressions given in the boundary effect model by least squares method since over-determined system of equations are obtained when data from several specimens are used. Instead of least squares method a different method is suggested in which the transition ligament length, measured from the plot of histograms of AE events plotted over the un-cracked ligament, is used directly to obtain size-independent fracture energy. The fracture energy thus calculated seems to be size-independent.

Prediction of fatigue strength in plain and reinforced concrete beams

A fatigue crack propagation model for concrete is proposed based on the concepts of fracture mechanics. This model takes into account the loading history, frequency of applied load, and size, effect parameters. Using this model, a method is described based on linear elastic fracture mechanics to assess the residual strength of cracked plain and reinforced concrete (RC) beams. This could be used to predict the residual strength (load carrying capacity) of cracked or damaged plain and reinforced concrete beams at a given level of damage. It has been seen that the fatigue crack propagation rate increases as. the size of plain concrete, beam increases indicating an increase in brittleness. In reinforced concrete (RC) beams, the fracture process becomes stable only when the beam is sufficiently reinforced.

A model to similate the strength and deformation of concrete in compression

A simple model is developed to represent the strength and deformational characteristics of concrete when subjected to a rate of strain or rate of stress or creep or relaxation testing under uniaxial compression.

Design of rectangular reinforced concrete slabs supported on all the sides with a short discontinuous edge

Bending moment coefficients for the design of rectangular reinforced concrete panels supported on four sides with a short discontinuous edge are derived using the strip theory. The moment fields resulting from the use of proposed coefficients are examined in terms of the moment volume for possible savings in reinforcement and compared with other codified procedures. The strip coefficients averaged over the corresponding sides of the panel, besides resulting in considerable savings in reinforcement, are found to be identical with the coefficients predicted by simple yield line theory using an orthotropic layout of reinforcement.

Microcracking in concrete under repeated compressive loads

The nature of microcracks formed in concrete under repeated uniaxial compressive loads are investigated by experiments on prismatic specimens. The distribution and orientation of cracks formed are studied by optical microscopic techniques. The basic failure mechanism of concrete at the phenomenological and internal structural level are examined by the formation and propagation of cracks. The tests have indicated that local tensile failures constitute the dominant mode of fracture, with the bond cracks forming the major percentage of the total magnitude of cracks. Significant differences were observed in the proportion of bond cracks formed under static and repeated load systems.

Markov Chain Model for Cracking Behavior of Reinforced Concrete Beams

In this paper, nonhomogeneous Markov chains are proposed for modeling the cracking behavior of reinforced concrete beams subjected to monotonically increasing loads. The model facilitates prediction of the maximum crackwidth at a given load given the crackwidth at a lower load level, and thus leads to a better understanding of the cracking phenomenon. To illustrate the methodology developed, the results of three reinforced concrete beams tested in the laboratory are analyzed and presented.

Size effect in self consolidating concrete beams with and without notches

The aim of this study is to obtain the fracture characteristics of low and medium compressive strength self consolidating concrete (SCC) for notched and un-notched plain concrete beams by using work of fracture G(F) and size effect model G(f) methods and comparing them with those of normal concrete and high performance concrete. The results show that; (i) with an increase in compressive strength, G(F) increases and G(f) decreases; (ii) with an increase in depth of beam, the decrease in nominal stress of notched beam is more when compared with that of a notchless beam.

Probabilistic Analysis of Cracking Moment of Reinforced Concrete Beams

Probabilistic analysis of cracking moment from 22 simply supported reinforced concrete beams is performed. When the basic variables follow the distribution considered in this study, the cracking moment of a beam is found to follow a normal distribution. An expression is derived, for characteristic cracking moment, which will be useful in examining reinforced concrete beams for a limit state of cracking.

Elasto-plastic Cracking Analysis of Reinforced Concrete

A new elasto-plastic cracking constitutive model for reinforced concrete is presented. The nonlinear effects considered cover almost all the nonlinearities exhibited by reinforced concrete under short term monotonic loading. They include concrete cracking in tension, plasticity in compression, aggregate interlock, tension softening, elasto-plastic behavior of steel, bond-slip between concrete, and steel reinforcement and tension stiffening. A new procedure for incorporating bondslip in smeared steel elements is described. A modified Huber-Hencky-Mises failure criterion for plastic deformation of concrete, which fits the experimental results under biaxial stresses better, is proposed. Multiple cracking at Gauss points and their opening and closing are considered. Matrix expressions are developed and are incorporated in a nonlinear finite element program. After the objectivity of the model is demonstrated, the model is used to analyze two different types of problems: one, a set of four shear panels, and the other, a reinforced concrete beam without shear reinforcement. The results of the analysis agree favorably with the experimental results.

Nonlinear fracture properties of concrete-concrete interfaces

The fracture properties of different concrete-concrete interfaces are determined using the Bazant's size effect model. The size effect on fracture properties are analyzed using the boundary effect model proposed by Wittmann and his co-workers. The interface properties at micro-level are analyzed through depth sensing micro-indentation and scanning electron microscopy. Geometrically similar beam specimens of different sizes having a transverse interface between two different strengths of concrete are tested under three-point bending in a closed loop servo-controlled machine with crack mouth opening displacement control. The fracture properties such as, fracture energy (G(f)), length of process zone (c(f)), brittleness number (beta), critical mode I stress intensity factor (K-ic), critical crack tip opening displacement CTODc (delta(c)), transitional ligament length to free boundary (a(j)), crack growth resistance curve and micro-hardness are determined. It is seen that the above fracture properties decrease as the difference between the compressive strength of concrete on either side of the interface increases. (C) 2010 Elsevier Ltd. All rights reserved.

Comparative Study of Predictions of Flexural Strength of Steel Fiber Concrete

Several methods are available for predicting flexural strength of steel fiber concrete composites. In these methods, direct tensile strength, split cylinder strength, and cube strength are the basic engineering parameters that must be determined to predict the flexural strength of such composites. Various simplified forms of stress distribution are used in each method to formulate the prediction equations for flexural strength. In this paper, existing methods are reviewed and compared, and a modified empirical approach is developed to predict the flexural strength of fiber concrete composites. The direct tensile strength of the composite is used as the basic parameter in this approach. Stress distribution is established from the findings of flexural tests conducted as part of this investigation on fiber concrete prisms. A comparative study of the test values of an earlier investigation on fiber concrete slabs and the computed values from existing methods, including the one proposed, is presented.