Analysis of Shear-critical reinforced concrete columns under variable axial load

Older existing reinforced concrete (R/C) frame structures often contain shear-dominated vertical structural elements, which can experience loss of axial load-bearing capacity after a shear failure, hence initiating progressive collapse. An experimental investigation previously reported by the authors focused on the effect of increasing compressive axial load on the non-linear post-peak lateral response of shear, and flexure-shear, critical R/C columns. These results and findings are used here to verify key assumptions of a finite element model previously proposed by the authors, which is able to capture the full-range response of shear-dominated R/C columns up to the onset of axial failure. Additionally, numerically predicted responses using the proposed model are compared with the experimental ones of the tested column specimens under increasing axial load. Not only global, but also local response quantities are examined, which are difficult to capture in a phenomenological beam-column model. These comparisons also provide an opportunity for an independent verification of the predictive capabilities of the model, because these specimens were not part of the initial database that was used to develop it.

Effect of Chain Lengths at Constant Molecular Weights of Water Reducing Admixture on Properties of Cementitious Systems Containing Fly Ash

In this study, the effect of main chain and side chain length of polycarboxylate-ether based high range water reducing admixture (WRA) on the fresh properties, compressive strength and water absorption of cementitious systems containing 0, 15, 30 and 45 wt.% fly ash was investigated. For this purpose, 3 WRAs with same molecular weight but different chain lengths were produced. According to test results, flowability of paste and mortars was negatively affected when the length of the main chain and side chains of the admixture was longer or shorter than a certain value. This adverse effect is thought to be arisen from the weakening of the adsorption of admixture with increase of its chain lengths. However, when the main chain and side chain lengths of the admixture were shorter or longer than a certain value, the time-dependent flow properties of the mortar mixtures improved. The main chain and side chain lengths of the WRAs had not a significant effect on the compressive strength and water absorption capacity of the mortar mixtures. However, irrespective of the admixture characteristics, with the increase of fly ash substitution the flow and time-dependent flow properties of the mixtures were negatively affected but their water absorption decreased.

The role of temperature in the thaumasite formation under the immersion of magnesium sulfate solution

To clarify the role of temperature in the thaumasite formation of cement mortar under magnesium sulfate solution at two different temperature, the corrosion products and microstructure of cement-based materials with different amounts and particle sizes of limestone powder (LP) were quantitatively analyzed by Fourier Transform Infra-Red (FTIR), thermogravimetric analysis (TGA), X-ray Diffraction (XRD), Scanning Electronic Microscopy (SEM) and Energy Dispersive Spectrometer (EDS). At 5oC, the main corrosion product of cement mortar was gypsum and thaumasite. At 20°C, the main corrosion products of cement mortar were gypsum and ettringite. When the temperature increased from 5°C to 20°C, the contents of ettringite, thaumasite and gypsum changed from 0.3%, 12.3% and 64.6% to 4.6%, 0% and 57.0%, respectively. The formation of thaumasite was the combination of direct reaction with ettringite transformation. The incorporation of LP accelerated the corrosion of mortars, and the change coefficient of compressive strength of mortars decreased from 100% to 47.3% when its content increased from 0% to 30%. Low temperature and incorporation of finer limestone powder enhanced the corrosion of magnesium sulfate solution.

Study of Rheological Properties of Carbon Nanotube-Reinforced Ultrafine Cement Grouting Materials

In this study, three different diameters of multi-walled carbon nanotubes (MWCNTs) dispersed by polyvinyl pyrrolidone (PVP) were used to reinforce superfine cement grouting materials. The effect of MWCNTs and polyvinyl pyrrolidone (PVP) on the rheological properties of grouting material were accordingly studied. It was found that the yield stress (τ0) and plastic viscosity (η) were slightly decreased when PVP content was low and increased when the PVP content increased. The effect of MWCNT diameter on τ0 was not found to be clear but was more significant on η. The smaller MWCNT diameter was, the more quickly η increase. It was also found that the thixotropic ring area was increased as the MWCNTs content increased. The addition of PVP and MWCNTs caused an increase in the number of entanglement points in different scales, which was the main reason for the viscosity and thixotropy increase. Therefore, the rheological properties of superfine cement grouting material should be adjusted when MWCNTs were added as a reinforcing component. Due to the wrapping of PVP on cement particles which isolates the contacting part between the water and the cement particles, it slows down the cement's hydration rate thus slows down the fluidity loss of the slurry.

Mechanical properties of high-performance concrete under triaxial compression

It is well known that the mechanical properties of a material are related to lateral confinement. In this paper, 60 cylindrical high-performance concrete (HPC) specimens with different concrete strength grades were cast and subjected to a conventional triaxial experiment to study the mechanical properties of the material. The experimental results indicated that the specimens exhibited longitudinal splitting failure patterns under uniaxial compression and inclined plane shear failure patterns under triaxial compression. The stress–strain curves were divided into three stages: an elastic rising stage, a plastic rising stage and a softening descending stage. The application of lateral confining pressure effectively increased the triaxial compressive strength. As the concrete strength increased, the descending stage of the stress–strain curves became steeper, indicating an increase in brittleness. Based on the experimental results, the failure criterion of the HPC was analysed using the Drucker–Prager yield criterion and Kotsovos failure theory. The parameters of the Drucker–Prager yield criterion were determined, and the applicable range of the Kotsovos failure theory was also obtained.

Compressive behavior of stirrup-confined concrete columns: size effect and a size-dependent stress-strain model

Numerous studies have indicated the existence of size effect on axial compression behavior of stirrup-confined concrete columns. However, most of these studies have been stressed in terms of nominal compressive strength. The investigation on the size effect of axial strain (at peak load) and descending branch was limited. In this study, the size effect behavior of square stirrup-confined concrete columns under axial compression was explored, by using 3-D mesoscale simulation method. Based on the numerical and available experimental results, the influence of specimen size on the peak axial stress (i.e., the compressive strength), the corresponding strain and the softening rate were explored. Moreover, the quantitative relationships between specimen size and the peak axial stress, the corresponding strain and the softening rate for circular and square stirrup-confined concrete columns were derived. Finally, considering the size effect of peak axial stress, the corresponding strain and the softening rate, a novel stress-strain model describing the axial compression behavior of stirrup-confined concrete was developed. The proposed model was verified by comparing with the available experimental results and the existing models provided.

Effects of coupled heat and moisture and load damage on chloride transport in concrete

This paper presents a thoroughgoing research on chloride transport in damaged concrete. Effects of temperature and temperature gradient on chloride transport was investigated along with effects of relative humidity, humidity gradient, concrete damage and exposure time. The higher the temperature and the greater the humidity gradient were, the quicker chloride transport was. Moisture transport increased as concrete damage increased, while chloride transport decreased incrementally. Considering the effect of coupled heat and moisture on chloride transport in concrete, a chloride transport model was established and verified by experiments. Chloride profiles in damaged concrete were related to temperature, temperature gradient, relative humidity and humidity gradient. The chloride attack rate decreased with increasing concrete damage and exposure time. Hence, coupled heat and moisture as well as concrete damage had significant effects on chloride transport in damaged concrete, and effects of concrete damage on chloride transport should be considered when determining chloride profiles in damaged concrete.

Local bond strength prediction of deformed reinforcing bars in concrete considering heterogeneity at interface regions

An analytical model is proposed to predict local bond strength (τf) by incorporating heterogeneity at interface regions for deformed reinforcing bars centrally anchored in concrete. The rib width on the bar surface is introduced as an interfacial characteristic parameter G in the proposed model; this accounts for the heterogeneity. Both τf and the local interfacial fracture energy (GIIf) of each specimen were found to be linked to G and can be determined analytically from the maximum pull-out loads (Fmax) from tests. It was found that the predicted τf was larger than the maximum average bond stress (τavg-max); the discrepancy between the two values reduced with an increase in L/G. Moreover, with an increase in L/G, the predicted τf showed a certain decrease, with the reduction decreasing with stronger interfacial homogeneity. The predicted GIIf was found to be significantly increased because of the weaker boundary effect. The validity of the proposed model was verified using comparisons of predicted Fmax (using the determined values of τf and GIIf) and the experimental Fmax, with the only failure mode being bar pull-out. Moreover, the model can be applied to steel or fibre-reinforced polymer bars and the concrete refers to all types of cementitious materials.