Effect of the particle size ratio on macro- and mesoscopic shear characteristics of the geogrid-reinforced rubber and sand mixture interface

As a new type of material for civil engineering projects, the rubber and sand mixture is widely used in roadbed fillers, offering environmental benefits over traditional tyre disposal methods. This study uses a large-scale direct shear apparatus to examine the interface shear properties of the geogrid-reinforced rubber and sand mixture, considering different particle size ratios (r), rubber contents, and normal stresses. Based on indoor tests, direct shear models of the mixture with different values of r are established in PFC^3D, revealing the meso-mechanical mechanism of the mixture in the direct shear process. The results show that when r is greater than 1, incorporating a certain amount of rubber particles can increase the shear strength of the mixture. The r values of 15.78, 7.63, and 3.98 correspond to an optimal rubber content of 30%, 10%, and 20%, respectively. When r is less than 1, mixing rubber particles can only reduce the shear strength of the mixture. When the rubber content is low, the smaller the value of r, the greater is the thickness of the shear band. Furthermore, the normal and tangential contact forces are greater. The fabric anisotropy evolution law of the mixture is consistent with the change in the contact force distribution.     

Exploiting carbon nanotube networks for damage assessment of fiber reinforced composites

An approach for damage inspection of composite structures utilizing carbon nanotubes (CNT) networks is investigated. CNT are dispersed in an epoxy using a processing technique compatible with commonly employed composite manufacturing techniques and subsequently used as matrix for a structural glass fiber reinforced composite. The developed electrical conductivity of the composite system is verified experimentally. The electrically conductive CNT network within the GFRP is exploited through distributed electrical voltage measurements to sense and, ultimately, locate damage in the plane of the composite plate. Damage in the form of cracks or delamination interrupts the continuity of the CNT network separating and isolating regions of the conductive network. Employing electric potential fields these changes can become measurable and can provide information for inversely locating the damage. Electrical Resistance Tomography (ERT) is formulated and experimentally applied to measure changes in the potential fields and deliver electrical conductivity change maps which are used to identify and locate changes in the CNT networks. These changes are correlated to capture the damage in the composite. Different damage modes are studied to assess the capabilities of the technique. The technique shows sensitivity to very small damages; less than 0.1% of the inspected area. The solution of the inverse ERT problem delivers a conductivity change maps which offers an effective localization with nearly 10% error and an inspection area suppression of around 75%. The proposed methodology to create CNT networks enables the application of ERT for Non-Destructive Evaluation of composite materials, previously not possible due to lack of conductivity, thus offering damage sensing and location capabilities even in-situ.     

Dynamic particle analysis for the evaluation of particle degradation during compounding of wood plastic composites

A dynamic image analysis method was applied for particle characterisation to study the effect of different process conditions during twin-screw compounding of WPC. The use of distributions based on different types of quantity is discussed with respect to their sensitivity to reveal the effects of different process conditions on particle degradation. Distributions based on length proved to be most suitable to represent the initially broad length distribution of the particles before processing. Sensitivity was strong enough to show differences in particle size after processing depending on process conditions. Particle size was reduced by more than 97% compared to initial size. Degradation was stronger with increasing wood content and when the screw design contained more mixing elements. The effect of screw speed and feed rate was dependent on filler content and screw design.     

A systematic investigation into a novel method for preparing carbon fibre–carbon nanotube hybrid structures

By electrospraying solvent dispersed carbon nanotubes (CNTs) with a binder onto carbon fibre (CF), hybrid structures, with an end aim to improve interfacial bonding in composites, were formed. The electrospray parameters controlling the modification of the CNT morphologies were studied. High-speed camera observations found applied voltage was critical for determining spray mode development. Electric field simulations revealed a concentrated electric field region around each fibre. Both voltage and distance played an important role in determining the CNT morphology by mediating anchoring strength and electric field force. The forming mechanism investigation of different surface morphologies suggested that binder with appropriate wetness gives freedom to the CNTs, allowing them to orientate radially from the CF surface. Linear density (LD) measurements and thermogravimetric analysis revealed that a 10 min coating increased the LD of a single CF filament by up to 31.7% while a 1 h treatment increased fibre bundle mass by 1%.     

Interlaminar fracture of CF/EP composite containing a dual-component microencapsulated self-healant

We present an experimental study of the self-healing ability of carbon fibre/epoxy (CF/EP) composite laminates with microencapsulated epoxy and its hardener (mercaptan) as a healing agent. Epoxy- and hardener-loaded microcapsules (average size large: 123 μm; small: 65 μm) were prepared by in situ polymerisation in an oil-in-water emulsion and were dry-dispersed at the ratio 1:1 on the surface of unidirectional carbon fabric layer. The CF/EP laminates were fabricated using a vacuum-assisted resin infusion (VARI) process. Width-tapered double cantilever beam (WTDCB) specimens were used to measure mode-I interlaminar fracture toughness of the CF/EP composites with a pre-crack in the centre plane where the microcapsules were placed. Incorporation of the dual-component healant stored in the fragile microcapsules provided the laminates with healing capability on delamination damage by recovering as much as 80% of its fracture toughness. It was also observed that the recovery of fracture toughness was directly correlated with the amount of healant covering the fracture plane, with the highest healing efficiency obtained for the laminate with large capsules.     

Centrifuge modeling of the geotextile reinforced slope subject to drawdown

Geotextile is an effective reinforcement approach of slopes that experiences various loads such as drawdown. The geotextile reinforcement mechanism is essential to effectively evaluate the safety of geotextile-reinforced slopes under drawdown conditions. A series of drawdown centrifuge model tests were performed to investigate the deformation and failure behaviors of slopes reinforced with different geotextile layouts. The deformation and failure of unreinforced and reinforced slopes were compared and the geotextile reinforcement was indicated to significantly increase the safety limit and the ductility, reduce the displacement, and change the failure feature of slopes under drawdown conditions. The slopes exhibited remarkable progressive failure, downward from the slope top, under drawdown conditions. The progressive failure was induced by coupling of deformation localization and local failure based on full-field measurements of displacement of slopes subjected to drawdown. The geotextile reinforced the slope by decreasing and uniformizing the slope deformation by the soil-geotextile interaction. Through geotextile displacement analysis, the geotextile-reinforced slope was divided into the anchoring zone and the restricting zone by a boundary that was independent of the decrease of water level. The geotextile restrained the soil in the anchoring zone and the soil restrained the geotextile in the restricting zone. The reinforcement effect was distinct only when the geotextile was long enough to cross the slip surface of the unreinforced slope under drawdown conditions.     

Field trial of a reinforced landfill cover system: performance and failure

The geotechnical stability of an inclined multilayer capping depends on the shear strength available along the various interfaces. If the slope is very steep an additional reinforcing geosynthetic may be used to obtain a safer condition. Full-scale field trials can provide better resolution data on the reinforcement behaviour than conventional calculation methods based only on laboratory tests. The paper deals with a field trial carried out on multilayer capping, reinforced with a geogrid, in an Italian landfill. The geogrid behaviour was monitored for a month using displacement sensors and pressure cells located along the slope and in the anchor trench. Subsequently, the cover system was led to collapse by cutting the reinforcement and an analysis of the reinforcement behaviour and its relevance in the system stability were studied. This paper discusses in detail the setup of the field trial and the experimental data recorded during installation, monitoring, and failure phases of the system. The deformation behaviour of the geogrid during the entire test was recorded and analysed. The resulting data highlight the effects of the construction process on the geogrid behaviour including the contribution of geogrid creep characteristics until the failure.     

The effect of geotextile reinforcement and prefabricated vertical drains on the stability and settlement of embankments

The construction of four dikes on deep strata of very soft clay has required the application of several measures to improve the performance of the foundation, such as very wide berms, basal geotextile reinforcement and prefabricated vertical drains (PVDs). In order to control the rate of construction, the foundation and the dikes have been monitored with settlement plates, topographic stakes, inclinometers and piezometers. The use of back-analysis has allowed finding the adequate material model, the smearing of drains and the coefficient of secondary compression necessary to attain a good agreement between the measurements supplied by the instrumentation and the calculated values obtained with an elastic-viscoplastic (EVP) finite element (FE) program. Both the geotextile reinforcement and the PVDs produce an important increase in the safety factor (SF). The PVDs produce a significant acceleration in settlements, but the influence of the geotextile in the settlements is negligible. The combined use of the geosynthetic reinforcement and PVDs enhances embankment performance substantially more than the use of either method of soil improvement alone. The importance of flow in the results has been established.     

Evaluation of water content gradient using a new configuration of linear array four-point probe for electrical resistivity measurement

Electrical resistivity is used in this study to assess the moisture content of concrete, because of their good correlation. A new process using four point probes to measure the electrical resistivity of concrete is described in this paper. It is based on the exploitation of the short circuit induced by the reinforcement in the concrete structure. A numerical model is also made to simulate the resistivity measurements on concrete by taking into account the electrochemical parameters of the reinforcement. The obtained results have shown the ability of the new measurement process to assess the gradient of the moisture content of the concrete cover.