Xiangming Zhou

FIBER ALIGNMENT AND PROPERTY DIRECTION DEPENDENCY OF FRC EXTRUDATE

Abstract This paper studies the phenomenon of fiber alignment during the extrusion process. The fiber alignment is largely dependent on shapes of dies and the compression as well as the shear force generated in the extruder. The fiber alignment orientation, of course, would lead to direction dependency of the tensile properties of fiber-reinforced cement (FRC) extrudates. Such a dependency has been investigated in present study. It is found that when fiber volume ratio is small, say 1% of the glass fiber, the majority of the fiber can be aligned in the extrusion direction. As a result, the tensile strength of the thin plate along the extrusion direction is much higher than that along the transverse direction. When the total fiber volume ratio is increased to 2% or 4%, the fiber volume along the transverse direction is largely increased although the percentage of the fiber aligned along the extrusion direction is still higher. Thus, the tensile strength at a transverse direction can be significantly enhanced. In fact, the tensile strength of the samples along the transverse direction is almost the same as that along the extrusion direction when the fiber volume ratio reaches 2%. Furthermore, the strength of the sample at the extrusion direction does not increase proportionally to the fiber volume ratio. For the comparison purpose, plain sheets without any fibers have been prepared by both casting and extrusion. Mechanical properties of the sheets have also been tested and the test results show that extrusion process and fiber addition do have effects on enhancing the tensile properties. Polymer coating on the surface of the samples has also been used to improve the tensile properties of the extrudates with low fiber volume ratio, which shows some promising results.

Shear Strength of Joints in Precast Concrete Segmental Bridges

The behavior of precast concrete segmental box girder bridges at both serviceability and ultimate strength conditions is dependent on the behavior of the joints between the segments. To accurately predict the bridge response throughout the complete range of loading. knowledge of joint behavior is essential. In this study, a series of full-scale joints, flat and keyed, dry and epoxied, single-keyed and multiple-keyed, have been tested under different confining stress levels and epoxy thicknesses. The shear behavior shear capacity, and shear transfer mechanisms of these different kinds of joints have been studied. It was determined that the shear capacity of joints increased as confining pressure increased, and epoxied joints had consistently higher shear strength than dry joints; however; the failure was more brittle than dry joints. The average shear strength for a key in multiple-keyed dry joints was always found to be less than those in single-keyed dry joints due to imperfections in fitting of keys. The shear strength of keys in multiple-keyed epoxied joints, flowever was similar to those in single-keyed joints, indicating epoxy mitigated the fixing imperfections and permitted the shear load to be uniformly distributed. The experimental results obtained in these tests were compared with the AASHTO and other design criterion. It was seen that these relationships tended to underestimate the shear strength of single-keyed joints and multiple-keyed epoxied joints by a value up to 40%, but they always greatly overestimated the shear capacity of dry multiple-keyed joints. Hence, the results indicate that some strength reduction factors should be introduced to the design relationships when applied to multiple-keyed dry joints.

Graphene/fly ash geopolymeric composites as self-sensing structural materials

The reduction of graphene oxide during the processing of fly ash-based geopolymers offers a completely new way of developing low-cost multifunctional materials with significantly improved mechanical and electrical properties for civil engineering applications such as bridges, buildings and roads. In this paper, we present for the first time the self-sensing capabilities of fly ash-based geopolymeric composites containing in situ reduced graphene oxide (rGO). Geopolymeric composites with rGO concentrations of 0.0, 0.1 and 0.35% by weight were prepared and their morphology and conductivity were determined. The piezoresistive effect of the rGO-geopolymeric composites was also determined under tension and compression. The Fourier transform infrared spectroscopy (FTIR) results indicate that the rGO sheets can easily be reduced during synthesis of geopolymers due to the effect of the alkaline solution on the functional groups of GO. The scanning electron microscope (SEM) images showed that the majority of pores and voids within the geopolymers were significantly reduced due to the addition of rGO. The rGO increased the electrical conductivity of the fly ash-based rGO-geopolymeric composites from 0.77 S m−1 at 0.0 wt% to 2.38 S m−1 at 0.35 wt%. The rGO also increased the gauge factor by as much as 112% and 103% for samples subjected to tension and compression, respectively.

Shaking Table Model Test of a Steel-Concrete Composite High-Rise Building

Shaking table tests were conducted on a 1/20 scaled-model of a 25-story steel-concrete composite high-rise building, composed of steel frame (SF) and concrete tube (CT). The seismic behavior of the model was investigated with the increasing of table-input acceleration amplitudes. It has been found that the seismic failure of the model concentrated on the shear walls and corner columns at the lowest story of the CT as well as the joints between the SF and the CT. Even subjected to extremely strong earthquakes, due to effective composite action, the composite model was able to support its weight to prevent collapse.

Study on Utilization of Carboxyl Group Decorated Carbon Nanotubes and Carbonation Reaction for Improving Strengths and Microstructures of Cement Paste

Carbon nanotubes (CNTs) have excellent mechanical properties and can be used to reinforce cement-based materials. On the other hand, the reaction product of carbonation with hydroxides in hydrated cement paste can reduce the porosity of cement-based materials. In this study, a novel method to improve the strength of cement paste was developed through a synergy of carbon nanotubes decorated with carboxyl group and carbonation reactions. The experimental results showed that the carboxyl group (–COOH) of decorated carbon nanotubes and the surfactant can control the morphology of the calcium carbonate crystal of carbonation products in hydrated cement paste. The spindle-like calcium carbonate crystals showed great morphological differences from those observed in the conventional carbonation of cement paste. The spindle-like calcium carbonate crystals can serve as fiber-like reinforcements to reinforce the cement paste. By the synergy of the carbon nanotubes and carbonation reactions, the compressive and flexural strengths of cement paste were significantly improved and increased by 14% and 55%, respectively, when compared to those of plain cement paste.

Fracture mechanisms of rock-concrete interface: Experimental and numerical

The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China under the grant of NSFC 51478083, 51421064, and 51109026, and the fundamental research funds for the Central Universities under the grant of DUT14LK06.

Towards preparation conditions for the synthesis of alkali-activated binders using tungsten mining waste

Partial financial support from the European Commission Horizon 2020’s MARIE Sklodowska-CURIE Research and Innovation Staff Exchange Scheme through the grant 645696 (i.e. REMINE project) is greatly acknowledged. The first author thanks, Thomas Gerald Gray Charitable Trust and Brunel University London for providing fees and a bursary to support his PhD study.

Numerical analysis of shear-off failure of keyed epoxied joints in precast concrete segmental bridges

© 2016 American Society of Civil Engineers. Precast concrete segmental box girder bridges (PCSBs) are becoming increasingly popular in modern bridge construction. The joints in PCSBs are of critical importance, which largely affects the overall structural behavior of PCSBs. The current practice is to use unreinforced small epoxied keys distributed across the flanges and webs of a box girder cross section forming a joint. In this paper, finite-element analysis was conducted to simulate the shear behavior of unreinforced epoxied joints, which are single keyed and three keyed to represent multikeyed epoxied joints. The concrete damaged plasticity model along with the pseudodamping scheme was incorporated to analyze the key assembly for microcracks in the concrete material and to stabilize the solution, respectively. In numerical analyses, two values of concrete tensile strength were adapted: one using a Eurocode formula and one using the general assumption of tensile strength of concrete as 10%f cm . The epoxy was modeled as linear elastic material because the tensile and shear strength of the epoxy were much higher than those of the concrete. The numerical model was calibrated by full-scale experimental results from literature. Moreover, it was found that the numerical results of the joints, such as ultimate shear load and crack initiation and propagation, agreed well with experimental results. Therefore, the numerical model associated with relevant parameters developed in this study was validated. The numerical model was then used for a parametric study on factors affecting shear behavior of keyed epoxied joints, which are concrete tensile strength, elastic modulus of epoxy, and confining pressure. It has been found that the tensile strength of concrete has a significant effect on the shear capacity of the joint and the displacement at the ultimate load. A linear relationship between the confining pressure and the shear strength of single-keyed epoxied joints was observed. Moreover, the variation in the elastic modulus of epoxy does not affect the ultimate shear strength of the epoxied joints when it is greater than 25% of the elastic modulus of concrete. Finally, an empirical formula published elsewhere for assessing the shear strength of single-keyed epoxied joints was modified, based on the findings of this research, to be an explicit function of the tensile strength of concrete.

Mechanical and Thermal Performance of Macro-Encapsulated Phase Change Materials for Pavement Application

Macro-encapsulated phase change material (PCM) lightweight aggregates (ME-LWA) were produced and evaluated for their mechanical and thermal properties in road engineering applications. The ME-LWAs were first characterised in terms of their physical and geometrical properties. Then, the ME-LWAs were investigated in detail by applying the European Standards of testing for the Bulk Crushing Test and the Polished Stone Value (PSV) coefficient as well as Micro-Deval and laboratory profilometry. In addition, the thermal performance for possible construction of smart pavements with the inclusion of ME-LWAs for anti-ice purposes was determined. The crushing resistance of the ME-LWAs was improved, while their resistance to polishing was reduced. Thermal analysis of the encapsulated PCM determined it to possess excellent thermal stability and a heat storage capacity of 30.43 J/g. Based on the research findings, the inclusion of ME-LWAs in surface pavement layers could be considered a viable solution for the control of surface temperatures in cold climates. Road safety and maintenance could benefit in terms of reduced ice periods and reduced treatments with salts and other anti-ice solutions.

Engineering Properties of Treated Natural Hemp Fiber-Reinforced Concrete

In recent years the construction industry has seen a significant rise in the use of natural fibres, for producing building materials. Research has shown that treated hemp fibre reinforced concrete (THFRC) can provide a low-cost building material for residential and low rise buildings, whilst achieving sustainable construction and meeting future environmental targets. This study involved enhancing the mechanical properties of hemp fibre reinforced concrete through the Ca(OH)2 solution pre-treatment of fibres. Both untreated (UHFRC) and treated (THFRC) hemp fibre reinforced concrete were tested containing 15mm length fibre, at a volume fraction of 1%. From the mechanical strength tests, it was observed that the 28-day tensile and compressive strength of THFRC was 16.9 and 10% higher, respectively than UHFRC. Based on the critical stress intensity factor (K_IC^s), and critical strain energy release rate (G_IC^s), the fracture toughness of THFRC at 28 days was also found to be 7-13% higher than UHFRC. Additionally, based on the determined brittleness number (Q) and modulus of elasticity, the THFRC was found to be 11% less brittle and 10.8% more ductile. Furthermore, qualitative analysis supported many of the mechanical strength findings through favourable surface roughness observed on treated fibres and resistance to fibre pull-out.