Curvature-relevant analysis model of eccentrically loaded circular concrete-filled steel tube columns

The existence of curvature can reduce the effective confining pressure within concrete-filled steel tube (CFST) columns under eccentric compression. The existing analysis method ignores such an effect and still treats the confining pressure as constant across the whole section as if it is under axial compression. In this paper, the authors have made significant improvements on the existing model, so that the curvature effect can be taken into account to properly interpret the non-linear behaviour of eccentrically loaded CFSTcolumns. Meanwhile, shortening between two hinges is an important data output in test programmes of this type, which has been completely neglected by the existing analysis method. This improved method also provides calculation steps to track the value of shortening for each load increment. Unknown parameters in this new method are determined on the basis of data fitting of 87 specimens from previous researchers' test programmes. The following parameters are incorporated in this study: effective tube length (660-4670 mm), diameter of tube (76-600 mm), tube thickness (1.52-8.81 mm), yield strength of tube (256.4-517.0 MPa), cylinder strength of concrete (26.42-112.70 MPa) and eccentricity (9.4-300 mm). The load-axial displacement, load-deflection and moment-curvature curves predicted by the new method agree well with their measured counterparts in the tests

Equivalent stress block for normal-strength concrete incorporating strain gradient effect

To account for the different behaviours of concrete under uniaxial compression and bending in the flexural strength design of reinforced concrete (RC) members, the stress-strain curve of concrete is normally scaled down so that the adopted maximum concrete stress in flexural members is less than the uniaxial strength. However, it was found from previous experimental research that the use of a smaller maximum concrete stress would underestimate the flexural strength of RC beams and columns. To investigate the effect of strain gradient on the maximum concrete stress developed in flexure, a total of 12 plain concrete and RC inverted T-shaped specimens were fabricated and tested under concentric and eccentric loads separately. The maximum concrete stress developed in the eccentric specimens was determined by modifying the concrete stress-strain curve obtained from the counterpart concentric specimens based on axial force and moment equilibriums. The test results revealed that the maximum concrete stress increases with strain gradient up to a certain maximum value. A formula was developed to correlate the maximum concrete stress to strain gradient. A pair of equivalent rectangular concrete stress block parameters that incorporate the effects of strain gradient was proposed for flexural strength design of RC members

Improving interface bonding of double-skinned CFST columns

It has been demonstrated that high-strength concrete (HSC) is able to improve the strength-to-weight ratio of reinforced concrete columns and maximise the usable areas of tall buildings. However, closely spaced transverse reinforcement needs to be installed to provide stronger confinement for averting brittle failure of HSC. To resolve the problem, double-skinned concrete-filled-steel-tubular (CFST) columns have been advocated, which eliminates the steel congestion problem for better concrete placing and reduces the concrete arching action thus providing a more uniform confining pressure. Despite these advantages, a major shortcoming of double-skinned CFST columns is that imperfect interface bonding occurs in the elastic stage that reduces elastic strength and stiffness. Thus, the authors proposed to adopt external confinement to restrict the lateral dilation of the outer tube of double-skinned CFST columns. To verify the effectiveness of the proposed external rings, a total of 20 double-skinned normal- and high-strength CFST columns were tested. From the test results, it was observed that the stiffness, axial load-carrying capacity and ductility of ringconfined double-skinned CFST columns were significantly higher than the unconfined columns

Experimental and numerical investigation of restrained shrinkage of concrete

To promote the understanding of shrinkage related behaviour of concrete used for tunnel linings the experimental and theoretical investigation including numerical and analytical approach was performed on ring-shaped specimens. Overall one analytical (an.) and two numerical models, namely (i) and (ii) were also developed. Models (an.) and (i) considered the restraining steel ring to be rigid, thus not exhibiting any deformation. Numerical model (ii) considered the steel ring to be deformable. The experimental set-up consisted of a large concrete ring with an inner diameter of 120 cm, an outer diameter of 160cm and 20 cm in height. The restraining steel ring was 5.5 cm thick. Two concrete rings were made, namely R1 with a low compressive strength of ~26MPa and the other, R2, with medium compressive strength of ~40 MPa. The strain was measured in the hoop direction on the inner circumference of the steel ring and on the outer circumference of the concrete ring. Concrete rings were subjected to circumferential drying. Numerical model (ii) predicted critical time to the formation of the first crack to be between 13 and 14 days. The experimentally determined critical time is found to be 11 to 13 days with cracks gradually opening over several days. This was indicated by changes in measured concrete and steel strain. Modelled concrete strain just before cracking was between -20 and -30 % 10-6 m m-1 however, measured concrete strain was ~150 % 10-6 m m-1. Modelled steel strain was between -30 and -40 % 10-6 m m-1 while measured steel strain was between -10 and 20 % 10-6 m m-1. These discrepancies, in particular the positive steel strain obtained in experiments, require further investigation and improvements of the experimental set-up

Deformability design of high-performance concrete beams

The use of high-performance materials (HPMs) such as high-strength concrete (HSC) and high-strength steel (HSS) is becoming more popular in the construction of beams and columns of tall buildings. These HPMs not only increase the stiffness and decrease the strength-to-weight ratio, but also provide a more sustainable construction method by minimising the construction materials needed. However, HSC and HSS are more brittle than normal-strength concrete and steel, respectively. Therefore, it will adversely affect the deformability of concrete beams. To evaluate the pros and cons of adopting HPM in beam design, the author will investigate the flexural strength and deformability of concrete beams made of HPMs. The deformability in this study is expressed in normalised rotation capacity and investigated by a parametric study using nonlinear moment-curvature analysis taking into account the degree of reinforcement, confining pressure, concrete and steel yield strength. From the results, it is evident that the deformability of concrete beams increases as the degree of reinforcement decreases or confining pressure increases. However, the effects of concrete and steel yield strength depend on other factors. For practical design purpose, charts and formulas are produced for designing high-performance concrete beams to meet with specified flexural strength and deformability requirement

Limited ductility design of reinforced concrete columns for tall buildings in low to moderate seismicity regions

Nonlinear moment-curvature analysis using stress-strain relationships of the constitutive materials and take into account the stress path dependence of longitudinal steel was performed to study the structural parameters affecting the flexural ductility of high-strength reinforced concrete (HSRC) columns. From the analysis, a theoretical equation for designing square-shaped limited ductility HSRC columns was proposed that correlates the volumetric ratio of confining reinforcement within critical region to the cross-section core area ratio, yield strengths of longitudinal and confining reinforcement, area ratio of longitudinal reinforcement, concrete strength and compressive axial load level. The validity of the proposed theoretical equation was verified by testing eight square-shaped columns with concrete cylinder strength varied from 50 to 96 MPa and longitudinal steel ratio from 0.9 to 6.1% that contained the proposed content of confining reinforcement within the critical region of columns. Outside the critical region, the confining steel is designed based on ultimate shear demand. The columns were tested under reversed cyclic inelastic displacements and compressive axial load, whose magnitude was held constant throughout the test. From the test results, it was observed that the ultimate curvature ductility factor obtained for these columns were about 10, which are considered to behave in a limited ductility manner

Multi-sized fillers to improve strength and flowability of concrete

Concrete with a low carbon dioxide footprint (LCDF) contains less cementitious materials (CM) than ordinary concrete and hence less cementitious paste. Besides the reduced carbon dioxide footprint, LCDF concrete offers other advantages such as reduced cost as well as improved dimensional stability due to reduced hydration heat, creep and shrinkage. Nevertheless, decreasing the quantity of CM has a negative impact on concrete workability. To restore workability, multi-sized fillers are advocated to replace aggregates and CM. Generally, fillers can improve the packing density of concrete due to the filling effect and thus more excess water or paste is available to facilitate spread and flow rate. Two types of fillers were examined in this study – limestone with particles smaller than 75 μm and foundry sand with particles of 75–400 μm. The particles of these two fillers are respectively similar and larger in size when compared with cement. Concrete mixes with no fillers and with mono- or multi-sized fillers were prepared and tested for flowability and strength. The results indicate that more superplasticiser is needed to achieve the same flowability when fillers are added. It was also found that, at the same water/CM ratio, fillers can improve concrete strength, and the use of multi-sized fillers can simultaneously improve the flowability and segregation resistance of concrete