Wengui Li

Simulation Study on the Stress Distribution in Modeled Recycled Aggregate Concrete under Uniaxial Compression

AbstractTo investigate the stress distribution in recycled aggregate concrete (RAC) under uniaxial compression, modeled recycled aggregate concrete (MRAC) was studied by numerical simulation. The mechanical properties of interfacial transition zones (ITZs) of RAC were measured by the nanoindentation technique. A two-dimensional numerical study of the stress distribution characteristics of MRAC under the uniaxial compression is presented. The simulation was verified by experimental results. A parametric analysis is then conducted to study the sensitivity of each phase’s mechanical properties and the amounts of old cement mortar in the MRAC. Simulation results demonstrate that a concentration of tensile stress and shear stress appears around new and old ITZ regions. It is found that when the elastic modulus of natural aggregates increases, the magnitude of tensile stress concentration becomes higher, whereas as the elastic modulus of ITZs increases, the magnitude of stress concentration decreases. It is als...

Influence of Nanolimestone on the Hydration, Mechanical Strength, and Autogenous Shrinkage of Ultrahigh-Performance Concrete

AbstractThe influence of nanolimestone/nanoCaCo3 (NC) on the properties of ultrahigh-performance concrete (UHPC) cured at standard and heat conditions was experimentally investigated. The NC was used at ratios of 1, 2, and 3% as partial mass replacement for cement. Incorporating NC accelerated the hydration reactions of UHPC because of the nucleation effect. On the mechanical properties aspect, a threshold value of the NC content was found so that the compressive, flexural strengths, and flexural to compressive strength ratio of the UHPC were found to increase as the NC content increased towards the threshold content, and then to decrease with the increase of NC contents when the threshold was surpassed. Conversely, replacing cement with NC decreased flowability and increased the amount of autogenous shrinkage of the UHPC. While the NC accelerated the cement hydration process, it also acted as an effective filling material, resulting in enhanced mechanical properties and denser microstructure compared wit...

Structural Behaviour of Composite Members with Recycled Aggregate Concrete - an Overview

A series of investigations on structural behaviour, durability, fire-resistance and seismic performance of composite members with recycled aggregate concrete (RAC) have been carried out in the past 10 years (2005–2014). This paper is consisted of three parts: the first part introduces and discusses the research progress in regard to the structural behaviour of RAC filled steel tubular columns and beams; the second part concentrates on the structural behaviour of steel-reinforced RAC members, including columns, beams, shear walls and slabs; and the third part focuses on the long-term performance of composite members with RAC, involving fire resistance, durability and seismic performance. It seems that RAC composite members with different replacement ratios of recycled coarse aggregate have slightly lower or similar structural behaviour compared to that of normal concrete composite members. Review results reveal that it is feasible to apply steel-RAC composite members as structural applications. This intensive review provides a reasonable knowledge of the structural behaviour of steel-RAC composite members, and recommends further investigations on the failure mechanics and durability of steel-RAC composite members which are needed to promote safe and economic application in the future.

Effects of nanoalumina and graphene oxide on early-age hydration and mechanical properties of cement paste

The effects of nanoalumina (NA) and graphene oxide (GO) on the early-age hydration and mechanical properties of portland cement pastes were investigated in this study. The hydration heat release rate and cumulative heat of cement pastes incorporating different dosages of NA and GO were evaluated using an isothermal calorimeter measurement method. Early-age electrical resistivity development was investigated by a noncontact electrical resistivity technique. The results show that both NA and GO could efficiently accelerate cement hydration. As a physical filler, NA significantly accelerates the hydration of tricalcium aluminate (CA) in cement. On the other hand, GO is able to obviously reduce the dormant period of cement hydration and shift the heat flow peaks to the left by accelerating the hydration of tricalcium silicate (CS) in cement. Compared to plain cement pastes, both the compressive and flexural strengths of cement pastes incorporating NA or GO are significantly increased. However, when NA and GO contents exceed the optimal amounts, improvements in flexural strength tend to decline, which is probably due to particle agglomeration. NA-cement paste exhibited slightly higher electrical resistivity than plain cement paste during hydration acceleration and deceleration stages. But GO-cement paste clearly showed lower electrical resistivity, which might be attributed to iron diffusion caused by GO with large surface areas.

Experimental investigation on quantitative nanomechanical properties of cement paste

Nanoindentation, quantitative modulus mapping, and PeakForce quantitative nanomechanical mapping (QNM) are applied to investigate the quantitative nanomechanics of hardened cement paste at different spatial resolutions. The elastic modulus measured by static nanoindentation is slightly higher than those measured by the other methods. The average elastic modulus and probability obtained by PeakForce QNM are typically consistent with those found by modulus mapping. Both modulus mapping and PeakForce QNM can be used to discriminate different material phases in cement paste at the nanoscale. It concludes that cement paste is a granular material in which the sub-micron scale grains or basic nanoscale units pack together. Moreover, the high resolution Peak-Force QNM can provide an efficient tool for identifying nanomechanical properties, particle sizes, and thickness of the interface between different nanoscale grains.

Effects of Annealing and Training on NiTi Alloy Ring for Clamping Device

Shape memory alloys (SMAs) in the form of rings or tubes with two-way shape memory effect (TWSME) have been applied to high-speed precision micro-mechanical systems. This study investigates the development of NiTi alloy rings with TWSME and analyzes the effects of annealing and thermo-mechanical treatments. The results show there is a maximum in TWSME with respect to annealing temperature, loading deformation, training temperature, and cycle, respectively. The TWSME of SMA rings stabilizes earlier than structure dimension during training. The annealing and training change phase transition temperatures, which has a negative effect on the SMA ring with application to clamping device. A prototype ring made of Ni-49.2 at%Ti was developed and applied to a high-seed precision micro-spindle. Its performance under static and ultra-high speed rotation conditions was tested. The results show the ring developed in this study satisfies the operation and clamping requirements of high-speed precision micro-mechanical systems.

Development of High-Performance Self-Compacting Concrete Applied as the Filling Layer of High-Speed Railway

AbstractThe filling layer of a China rail track system III (CRTS III) type ballastless track structure of a high-speed railway is a complicated structure typically constructed from self-compacting ...

Empirical model of corrosion rate for steel reinforced concrete structures in chloride-laden environments

The corrosion rate of reinforcing steel is an important factor to determine the corrosion propagation of reinforced concrete structures in the chloride-laden environments. Since the corrosion rate of reinforcing steel is affected by several coupled parameters, the efficient prediction of which remains challenging. In this study, a total of 156 experimental data on corrosion rate from the literature were collected and compared. Seven empirical models for predicting the corrosion rate were reviewed and investigated using the collected experimental data. Based on the investigations, a new empirical model is proposed for predicting the corrosion rate in corrosion-affected reinforced concrete structures considering parameters including concrete resistivity, temperature, relative humidity, corrosion duration and concrete chloride content. The comparison between the experimental data and those predicted using the new empirical model demonstrates that the new model gives a good prediction of the corrosion rate. Furthermore, the uncertainty and probability characteristics of these empirical models are also investigated. It is found that the probability distributions of the model errors can be described as lognormal, normal, Weibull or Gumbel distributions. As a result, the new empirical model can provide an efficient prediction of the corrosion rate of reinforcing steel, and the model error analysis results can be utilized for reliability-based service life prediction of reinforced concrete structures under chloride-laden environments.

On 4R Principle in the Rehabilitation of Concrete Historic Buildings

vThe 4R principle refers to reduce, reuse, recycle and regeneration, which should be implemented in the process of concrete treatment. In terms of the special micro-structural properties and self-repairing capacity, concrete is one kind of regenerative construction materials. Through proper handlings, self-repairing (crack close-up) of concrete cracks is possible in the concrete hydration damaged region. Due to the effect of heterogeneous nucleation and subsequent crystal growth of calcium hydroxide, a better bonding strength forms interlocking the new and old concrete interfaces. A proposal on the relationship between the self-repairing, regeneration function of concrete and the rehabilitation of historic buildings is suggested, this is based on the formed process of the bonding strength of the interfaces and the function on concrete repairing. Besides the constitution and features of the microstructure of concrete, the relationship between the micro-structure and the macro-mechanical property is also investigated in this paper