Load Transfer Efficiency Research Articles

Improved Backcalculation Procedure for Continuously Reinforced Concrete Pavement

Falling weight deflectometer (FWD) testing is effective in evaluating the structural response of in-situ concrete pavements through the backcalculated pavement layer parameters. Specifically, the FWD data can be used to backcalculate the foundation layer and concrete stiffness or the soil layer stiffness, effective slab thickness, and slab–base interface condition. Since continuously reinforced concrete pavement (CRCP) has closely spaced transverse cracks, the traditional backcalculation assumption of an infinite slab can lead to significant errors in the backcalculated results. In this paper, solutions for backcalculated modulus of subgrade reaction ( k-value), elastic modulus of concrete ( E), and effective thickness ( heff) for different crack spacing have been derived from 2-D finite element analysis. AASHTO sensor configuration (0, 12, 24, 36 in.) was recommended for CRCP with crack spacing ≥6 ft, and an alternative solution for crack spacing of 4 and 5 ft was proposed with AREA24. Crack load transfer efficiency (LTE) across transverse cracks had limited impact on backcalculated results if the LTE was >80%. As expected, the backcalulation values were sensitive to the load plate’s longitudinal position relative to the transverse crack especially for crack spacings smaller than 8 ft. The proposed backcalculation method was applied to a field CRCP test section with different crack spacing, reinforcement ratio, and base types.

Enhancing the heat and load transfer efficiency by optimizing the interface of hexagonal boron nitride/elastomer nanocomposites for thermal management applications

Amine-terminated butadiene-acrylonitrile (ATBN) was covalently grafted on a hexagonal boron nitride (BN) surface to enhance the interface affinity between BN and a carboxylated styrene-butadiene rubber (XSBR) matrix for preparing thermal interface materials with robust thermo-physical properties. The strong attraction between the amino groups on the ATBN-functionalized BN (ABN) and the carboxyl groups on the XSBR chains can further enhance the interfacial adhesion between the fillers and polymer matrix. The mechanical properties, thermal stability, thermal conductivity, thermo-physical properties, and interfacial filler–matrix network of the prepared XSBR/ABN nanocomposites were studied. The XSBR/ABN nanocomposites exhibited better mechanical and thermo-physical behavior compared to the XSBR/BN nanocomposites over the entire range of filler concentration, representing an enhanced interfacial interaction owing to functionalization. Moreover, the XSBR/ABN nanocomposites exhibited superior thermal conductivity as compared to the XSBR/BN nanocomposites, with the former exhibiting approximately 490% higher thermal conductivity than neat XSBR, whereas the latter exhibited 395% higher thermal conductivity than neat XSBR at a filler loading of 90 phr.

Interface structure and strengthening behavior of graphene/CuCr composites

The strong interface is an essential requirement to ensure the effective load transfer of graphene/Cu composites. Here we attempted to improve the interface adhesion and mechanical properties of reduced graphene oxide (RGO)/CuCr composites by matrix-alloying with ∼0.2 at.% Cr. It was found that a trace amount of Cr7C3 layers/nanoparticles was in-situ formed at the RGO-CuCr interface, which contributed to the dramatically improved interfacial bonding of the composites. The 2.5 vol% RGO/CuCr composite exhibited a tensile strength of 352 MPa, 82% and 19% higher than that of unreinforced CuCr and 2.5 vol% RGO/Cu composite without Cr alloying, respectively. The enhanced strength of RGO/CuCr composite was ascribed to the dual role of Cr7C3 layers/nanoparticles that not only enhanced the load transfer efficiency, but also promoted the dislocation strengthening ability of RGO itself. Furthermore, we proposed the possible Cr7C3 formation/evolution mechanism that involved the four steps of amorphous carbon formation, Cr7C3 nucleation in amorphous carbon, Cr7C3 growth and Cr7C3 coalescence. The formation of medium sized Cr7C3 layers/nanoparticles at 1053 K resulted in the highest strength of RGO/CuCr composite with a satisfactory strength-ductility combination. This study provides new insights into the interface structure, strengthening mechanism and carbide formation/evolution mechanism of graphene/CuX composites.

Fabrication of CNT/Cu composites with enhanced strength and ductility by SP combined with optimized SPS method

Mono-dispersed and homogeneous carbon nanotube (CNT)/copper (Cu) composite powders were fabricated by spraying pyrolysis (SP) method successfully. Subsequently, CNT/Cu composites were obtained by spark plasma sintering (SPS) at sintering temperature ranged from 1023 to 1223 K (70–90% melting point of pure Cu). The mechanical properties, micro-morphology and interface microstructure of CNT/Cu composites were measured and characterized. The results reveal that Cu nano-particles (NPs) coated on the CNT surface are conducive to improving the wettability of CNT within matrix and reducing the interface energy between matrix and CNT, and finally stable interface structure is established. The traditional trade-off tendency between strength and ductility is balanced in the CNT/Cu composites, which are sintered at 1223 K. The ultimate tensile strength (UTS) shows 344 MPa and the elongation is 19.6%, which owing to the improvement of CNT–Cu interface bonding. At last, the role of interfacial bonding extent played in determining the load transfer efficiency of CNT is discussed.

Ni Nanobuffer Layer Provides Light-Weight CNT/Cu Fibers with Superior Robustness, Conductivity, and Ampacity

Carbon nanotube (CNT) fiber has not shown its advantage as next-generation light-weight conductor due to the large contact resistance between CNTs, as reflected by its low conductivity and ampacity. Coating CNT fiber with a metal layer like Cu has become an effective solution to this problem. However, the weak CNT-Cu interfacial bonding significantly limits the mechanical and electrical performances. Here, we report that a strong CNT-Cu interface can be formed by introducing a Ni nanobuffer layer before depositing the Cu layer. The Ni nanobuffer layer remarkably promotes the load and heat transfer efficiencies between the CNT fiber and Cu layer and improves the quality of the deposited Cu layer. As a result, the new composite fiber with a 2 μm thick Cu layer can exhibit a superhigh effective strength >800 MPa, electrical conductivity >2 × 107 S/m, and ampacity >1 × 105 A/cm2. The composite fiber can also sustain 10 000 times of bending and continuously work for 100 h at 90% ampacity.

Variability Analysis of Efficiency and Output Power of an Inductive Power Transfer Link

The wireless power transfer (WPT) technique plays an important role in powering remote devices. The power delivered to the load (PDL) and power transfer efficiency (PTE) are both obtained by knowing the parameters of the WPT system. The knowledge of such parameters and its variability can aid the designer to establish the most important parameters for maximizing the PDL or PTE. This paper presents a variability analysis of PDL and PTE when the WPT parameters change their values. Each parameter can be measured with an associated variability range. The variations in the PDL and PTE functions are computed by using this range of variability of the input parameters. The sensitivities of PDL and PTE with respect to each parameter are also identified with the presented analysis. Experimental measurements were taken on an inductive link, which was specially designed for a recharging battery system of a wireless sensor node with a 5-mm gap between the transmitting and receiving coils. The results pointed out that only small ranges on the input rated values can be tolerated to avoid significant distortions on the output distribution and thus the deviation of the optimal operation point.

Rapid evaluation of a transverse crack on a semi-rigid pavement utilising deflection basin data

To evaluate the transverse crack on a semi-rigid pavement rapidly by a falling weight deflectometer (FWD), three-dimensional dynamic finite-element (FE) models were developed to calculate the deflection response of a transverse-cracked semi-rigid pavement under an FWD load. Models were validated by deflection basin data acquired from in situ FWD tests. Then, the effects of thermal cracking and reflective cracking on deflection basin data were analysed. Four loading locations and seven crack widths were considered in the analysis. Results showed that deflection basins calculated by dynamic FE models coincided well with that measured by in situ FWD test. The largest per cent difference between calculated deflections and measured values was less than 9%. When the distance between loading centre and crack was within 1050 mm, load transfer efficiency (LTE) indexes showed a good performance for distinguishing a reflective crack from a thermal crack. A practical field test method placing the transverse crack between sensor S300 and sensor S600 was recommended and LTE300–600 was proposed as the evaluation index for the rapid determination of crack type.

Ultralow-Carbon Nanotube-Toughened Epoxy: The Critical Role of a Double-Layer Interface.

Understanding the chemistry and structure of interfaces within epoxy resins is important for studying the mechanical properties of nanofiller-filled nanocomposites as well as for developing high-performance polymer nanocomposites. Despite the intensive efforts to construct nanofiller/matrix interfaces, few studies have demonstrated an enhanced stress-transferring efficiency while avoiding unfavorable deformation due to undesirable interface fractures. Here, we report an optimized method to prepare epoxy-based nanocomposites whose interfaces are chemically modulated by poly(glycidyl methacrylate)-block-poly(hexyl methacrylate) (PGMA-b-PHMA)-functionalized multiwalled carbon nanotubes (bc@fMWNTs) and also offer a fundamental explanation of crack growth behavior and the toughening mechanism of the resulting nanocomposites. The presence of block copolymers on the surface of the MWNT results in a promising double-layered interface, in which (1) the outer-layered PGMA segment provides good dispersion in and strong interface bonding with the epoxy matrix, which enhances load transfer efficiency and debonding stress, and (2) the interlayered rubbery PHMA segment around the MWNT provides the maximum removable space for nanotubes as well as triggering cavitation while promoting local plastic matrix deformation, for example, shear banding to dissipate fracture energy. An outstanding toughening effect is achieved with only a 0.05 wt % carbon nanotube loading with the bc@fMWNT, that is, needing only a 20-times lower loading to obtain improvements in fracture toughness comparable to epoxy-based nanocomposites. The enhancements of their corresponding ultimate mode-I fracture toughnesses and fracture energies are 4 times higher than those of pristine MWNT-filled epoxy. These results demonstrate that a MWNT/epoxy interface could be optimized by changing the component structure of grafted modifiers, thereby facilitating the transfer of both mechanical load and energy dissipation across the nanofiller/matrix interface. This work provides a new route for the rational design and development of polymer nanocomposites with exceptional mechanical performance.

Interlaminar shear stress function for adhesively bonded multi-layer metal laminates

A new analytical model is proposed for the estimation of interlaminar shear stress in adhesively bonded metal laminates consisting of an arbitrary number of layers. The interface shear stress in the laminates is related to the difference in average axial strain and elongation between adjoining layers through a newly proposed interlaminar shear stress function (ILSSF). The parameters of the ILSSF are determined from finite element simulations using a data fitting procedure. The accuracy of the model is investigated by comparing experimental measurements of average elongation in three-layer aluminum laminates to values obtained using the model. Good agreement with the experimental results is achieved for several types of adhesives and for different ratios of adhesive-to-layer thicknesses. The influence of Young's modulus of the adhesive on the efficiency of load transfer in three-layer laminates is investigated.

An electrodeposition approach to obtaining carbon nanotubes embedded copper powders for the synthesis of copper matrix composites

CNTs/Cu composite powders were fabricated using an electrodeposition process, and consolidated by hot pressing followed by cold rolling. The electrodeposition is a promising method to obtain homogeneously dispersed CNTs/Cu powders in terms of effective control of CNTs content and particle size of powders during the process. The effects of Cu ions concentration in electrolyte on the dispersion and content of CNTs in powders, particle size, and properties of composites were studied. The electrodeposited CNTs/Cu powders show large amount of embedded CNTs that are strongly bonded with the Cu matrix, and a CNTs network architecture formed in the Cu grains. The particle size 52.72 μm of powders is obtained at Cu ions concentration of 10 g/L in electrolyte. The tensile strength and electrical conductivity of consolidated CNTs/Cu composites are about 418 MPa and more than 90 ± 0.16% IACS, respectively. The CNTs network is beneficial to load transfer efficiency and bridge linking for improving strength, but also provides electron transport pathways for high electrical conductivity.

Investigation of load transfer efficiency in jointed plain concrete pavements (JPCP) using FEM

Owing to heavy traffic loads, rigid pavements encounter various types of failures at transverse joints during their lifetime. Three-dimensional finite-element method (3D-FEM) was used to assess the structural response of jointed concrete pavement under moving tandem axle loads. In this study, 3D FEM was verified using an existing numerical model and field measurement of the concrete slab traversed by a moving truck. This paper also investigated the effects of multiple parameters: material properties, slab geometry, load magnitude and frictional status of the slab and base layer on load transfer efficiency (LTE) of the transverse joints. Further study has been done to investigate the slab performance without the dowel bars which occurs when parts of the pavement needed to be repaired using precast slabs. The aggregate interlock between the new slab and the existing slab is simulated by frictional interface. In 3D FEM model, the load transfer efficiency has been improved by increasing the elasticity modules of the concrete slab and the base layer or increasing the slab thickness. This can decrease the joints' deflections, reduces the damages on pavement joints. Removing dowel bars adversely affected the load transfer.

Effects of noncovalently functionalized multiwalled carbon nanotube with hyperbranched polyesters on mechanical properties of epoxy composites

In order to improve the dispersibility and interface properties of multi-walled carbon nanotubes (MWCNTs) in epoxy resin (EP), aromatic hyperbranched polyesters with terminal carboxyl (HBP) and aromatic hyperbranched polyesters with terminal amino groups (HBPN) were used for noncovalent functionalization of MWCNTs. Epoxy composites reinforced by different types of MWCNT were prepared. The effects of noncovalent functionalization of MWCNTs on the dispersibility, wettability, interface properties and mechanical properties of epoxy composites were investigated. The results show that the dispersibility and wettability of MWCNTs are significantly improved after noncovalent functionalization. A large number of terminal primary amines (NH2) on noncovalently functionalized MWCNT with HBPN (HBPN-MWCNT) form covalent bonds with EP matrix, and thus the interfacial adhesion is enhanced significantly, resulting in high load transfer efficiency and substantial increase in mechanical properties. The interface with covalent bonding formed between the flexible hyperbranched polyester layer on the surface of HBPN-MWCNT and the EP matrix promotes plastic deformation of the surrounding EP matrix. The toughening mechanisms of HBPN-MWCNT are MWCNT pull-out and a large amount of plastic deformation of the surrounding EP matrix.

Field investigation of low-temperature cracking and stiffness moduli on selected roads with conventional and high modulus asphalt concrete

High Modulus Asphalt Concrete (HMAC) was introduced in Poland as a one of the solutions to the problem of rutting, type of deterioration common in the 1990s. After first encouraging trials in 2002 HMAC was widely used for heavily loaded national roads and motorways. However some concerns were raised about low-temperature cracking of HMAC. This was the main reason of the studies presented in this article were started. The article presents the comparison of performance of pavements constructed in typical contract conditions with the road bases made of HMAC and conventional asphalt concrete (AC). The field investigation was focused on the number of low-temperature cracks, bearing capacity (based on FWD test) of road sections localized in coldest region of Poland. Also load transfer efficiency of selected low-temperature cracks was assessed. FWD test confirmed lower deflections of pavements with HMAC and two times higher stiffness modulus of asphalt courses in comparison to pavements constructed with conventional AC mixtures. Relation of stiffness of asphalt layers and amount of low-temperature cracks showed that the higher stiffness modulus of asphalt layers could lead to increase of the number of low-temperature cracks. FWD test results showed that the load transfer efficiency of low-temperature cracks on pavements with HMAC presents very low values, very close to lack of load transfer. It was surprising as section with HMAC road base were aged from 2 to 5 years and presented very good bearing capacity.

Optimisation of borehole-jack fracturing technique for in situ stress measurement

The technology of stress measurement by the borehole jack is a modification of the hydraulic fracturing method aimed at the determination of the stresses in permeable rocks. The standard procedure for the application of this technique is to form fractures by a borehole jack, which is not always possible at great depth due to the high rock pressure. To address these problems, we propose a two-step technique that involves the formation of extended fractures in the isolated interval of the borehole by working fluid and the subsequent determination of the number, orientation, and reopening pressure of fractures by a special directional loading tool. The developed tool has a higher load transfer efficiency on the borehole wall in comparison with existing analogues. The article describes the results of numerical simulations and laboratory tests.

Microstructure and mechanical behaviour of aluminium matrix composites reinforced with graphene oxide and carbon nanotubes

Aluminium (Al) matrix composites reinforced with either 0.5 wt% graphene oxide (GO) or 0.5 wt% carbon nanotubes (CNTs) were hot extruded from ball-milled powders. A control, pure Al bar was also fabricated. Microstructural examination, including Raman mapping, showed a relatively poor dispersion of the carbon nanomaterials within the Al matrix, particularly in the case of the CNTs. Consequently, while the mean grain size of the Al matrix remains invariant with the addition of CNTs, the Al/GO composite exhibits reduced grain size compared to pure Al due to the pinning effect of the reinforcement. Moreover, the addition of both carbonaceous materials resulted in a slight decrease in the typical extrusion duplex + fibre texture intensity. This weakening of the texture was more pronounced in the Al/GO composite, partly due to the pinning effect of the reinforcement. In agreement with their relative mean grain sizes, the Al/GO composite shows an improved mechanical performance over pure Al. Despite the similarity of the mean grain sizes, the Al/CNT composite displays comparable hardness and a decreased compressive yield stress relative to the pure Al. In the absence of chemical reactions at the interfaces, this was attributed to a low efficiency of load transfer from the Al matrix to the reinforcement resulting from the large extent of agglomeration of CNTs.

Graphene welded carbon nanotube crossbars for biaxial strain sensors

Aligned carbon nanotube (CNT) arrays are promising candidates for strain sensors owing to their scalable preparation and excellent conductivity and stretchability. However, aligned CNT arrays are limited by low strain sensitivity and buckling deformation. In addition, cross-stacked CNT array films layer-by-layer assembled on soft substrates exhibit anisotropic mechanical behavior due to their asymmetric layered structures. In this work, we introduced a chemically hybridized CNT-graphene (G/CNT) film in which CNT crossbars are effectively welded together by graphene. The hybrid films demonstrate enhanced isotropic mechanical properties and strain sensitivity with a gauge factor of ∼3, together with a high stretchability of more than 20%. The enhanced electromechanical properties are attributed to the improved load transfer efficiency among CNTs by graphene hybridization, as confirmed by Finite Element Analysis (FEA). Biaxial strain sensors based on the hybridized G/CNT films have been applied for sensitive detection of both minute vibrations caused by sound waves and large deformations from finger bending. The sensors were further integrated into a tactile sensing array to map the spatial distribution of the surface pressures.

Influence of Interface Bond on the Performance of Bonded Concrete Overlays on Asphalt Pavements

Bonded concrete overlays on asphalt pavement (BCOA) offer a rehabilitation method for moderately distressed asphalt pavements. A performance review of the BCOA projects constructed in different parts of the United States suggests that the main distresses, such as longitudinal and corner cracks, develop primarily as a result of debonding of the asphalt layer from the concrete overlay. Debonding, which reduces the contribution from the underlying asphalt layer, increases the stress in the concrete layer, leading to the development of cracks. In this study, a finite-element analysis was performed to investigate the influence of the interface bond on the load-induced stress for 76-mm (3-in.), 102-mm (4-in.), and 152-mm (6-in.) concrete overlays. It was found that debonding can increase the magnitude of the load-induced stress in the concrete by up to 40–55%. Maintaining high joint load transfer efficiency (LTE) reduces the tensile stress at the interface that contributes to debonding. Relationships were developed between LTE and debonding stress for a range of overlay designs. The findings from this study can be used to facilitate the integration of debonding models in the mechanistic-empirical design procedure for BCOA. The results from this investigation can also be used in establishing a reasonable range of the target values for interface bond strength needed when constructing BCOA to ensure good performance.

Influence of Number of Geosynthetic Layers on the Performance of Geosynthetic-reinforced Pile-supported Earth Platforms on Soft Soil: Numerical Study

Geosynthetic-reinforced pile-supported earth platforms have been increasingly used to support infrastructures, such as embankments, retaining walls, and storage tanks constructed over soft soils around the world. On the top of piles, one or more layers of geosynthetics are usually placed. However, the effect of number of geosynthetic layers on the behavior of geosynthetic-reinforced pile-supported earth platforms has not been well explored so far. Based on a well-instrumented large-scale test, a threedimensional (3-D) numerical study was conducted to investigate the behavior of geosynthetic-reinforced pile-supported earth platforms with different number of geosynthetic layers. The numerical results showed that the inclusion of geosynthetics enhanced the efficiency of load transfer. The critical heights of soil arching were almost constant with different numbers of reinforcement layers. The maximum tensile strain in the lower reinforcement was on the edge of pile caps while the maximum tensile strain in the upper reinforcement was close to the pile center. When the square caps were installed in a triangular pattern, the maximum strain in reinforcement is concentrated in the strip bridging two adjacent cap corners.

Mechanical Sensitivity Analysis of BFRP Reinforced CRCP under the Void below Concrete Slab Condition

This study proposes a finite element model to simulate traffic loads on continuously reinforced concrete pavement (CRCP) by using basalt fiber reinforced polymer (BFRP) bars, considering the void below the concrete slab and the load transfer efficiency (LTE) at the transverse cracks of the slab. Sensitivity analysis of mechanical responses and LTE at the transverse cracks of the BFRP-reinforced CRCP slab under the void below concrete slab condition is conducted using the following parameters: slab thickness and elastic modulus, base elastic modulus, elastic modulus and design of BFRP bars, and foundation modulus. Results show that (1) the mechanical condition of the concrete slab can be significantly improved by increasing the thickness, not the elastic modulus, of the slab. (2) In the case of wide width of the void area below the concrete slab, the mechanical condition of the slab may be worsened by increasing the base elastic modulus. (3) The mechanical condition of the concrete slab can be slightly improved by increasing the elastic modulus of BFRP bars and the percentage of longitudinal reinforcement. (4) The maximum transverse tensile stress on top of the loading slab increases, whereas LTE at the transverse cracks significantly decreases with increasing foundation modulus; these phenomena damage the concrete slab near the transverse cracks in BFRP-reinforced CRCP.

The best features of diamond nanothread for nanofibre applications

Carbon fibres have attracted interest from both the scientific and engineering communities due to their outstanding physical properties. Here we report that recently synthesized ultrathin diamond nanothread not only possesses excellent torsional deformation capability, but also excellent interfacial load-transfer efficiency. Compared with (10,10) carbon nanotube bundles, the flattening of nanotubes is not observed in diamond nanothread bundles, which leads to a high-torsional elastic limit that is almost three times higher. Pull-out tests reveal that the diamond nanothread bundle has an interface transfer load of more than twice that of the carbon nanotube bundle, corresponding to an order of magnitude higher in terms of the interfacial shear strength. Such high load-transfer efficiency is attributed to the strong mechanical interlocking effect at the interface. These intriguing features suggest that diamond nanothread could be an excellent candidate for constructing next-generation carbon fibres.