An investigation on failure behavior of semi-flexible composite mixture at different temperatures

Semi-flexible composite mixture(SFCM)is a kind of pavement material formed by pouring cement-based grout material into a porous asphalt mixture with air voids from 20%to 30%.SFCM is widely used for its outstanding anti-rutting performance.Its mechanical performance is complicated due to its heterogeneity and interlocking structure.According to the present study,asphalt deforms at different temperatures,whereas cement-based grout has no similar characteristics.Rare research focuses on the temperature-based performance of SFCM.Therefore,the study was on the thermal performance of SFCM by seven open-graded asphalt mixture skeletons with different porosities and two types of grouts with early strength(ES)and high strength(HS).The test temperatures ranged from 10℃to 60℃.The mechanical investigation was performed using the semi-circular-bending(SCB)and beam bending tests.The strain sensor was used for analyzing the thermal performance of SFCM.The results show that the temperature significantly affected the SFCM's performance.The porosity was selected for three sections based on the trend of fracture energy(Gf)curves at 25℃.The turning points were the porosity values of 20%and 26%.The initiation slope during elastic deformation increases with the porosity increase.This trend was more evident at intermediate temperature.The shrink strain of SFCM was lower than that of the usual asphalt mixture(AC).The thermal stress of the SFCM filled with HS(HS-SFCM)was higher than that of the SFCM filled with ES(ES-SFCM)at 10℃.Moreover,the thermal failure characteristics of SFCM were influenced by porosity.

Effect of dry-wet cycles on dynamic properties and microstructures of sandstone:Experiments and modelling

Underground pumped storage power plant(UPSP)is an innovative concept for space recycling of abandoned mines.Its realization requires better understanding of the dynamic performance and durability of reservoir rock.This paper conducted ultrasonic detection,split Hopkinson pressure bar(SHPB)impact,mercury intrusion porosimetry(MIP),and backscatter electron observation(BSE)tests to investigate the dynamical behaviour and microstructure of sandstone with cyclical dry-wet damage.A coupling FEM-DEM model was constructed for reappearing mesoscopic structure damage.The results show that dry-wet cycles decrease the dynamic compressive strength(DCS)with a maximum reduction of 39.40%,the elastic limit strength is reduced from 41.75 to 25.62 MPa.The sieved fragments obtain the highest crack growth rate during the 23rd dry-wet cycle with a predictable life of 25 cycles for each rock particle.The pore fractal features of the macropores and micro-meso pores show great differences between the early and late cycles,which verifies the computational statistics analysis of particle deterioration.The numerical results show that the failure patterns are governed by the strain in pre-peak stage and the shear cracks are dominant.The dry-wet cycles reduce the energy transfer efficiency and lead to the discretization of force chain and crack fields.

Experimental study on unloading induced shear performances of 3D saw-tooth rock fractures

A fractal model governing saw-tooth fractures was first introduced to replicate sandstone samples containing an inclined 3D penetrating rough fracture surface with various joint roughness coefficients(JRC).In conventional triaxial compression,the peak strength for fractured samples increased with both confining pressure and JRC.During the unloading confining pressure process,the normal stress of fractures declined but the shear stress increased,resulting in shear sliding of fractures.The shear displacement of fractures exponentially increased,and the positive normal displacement decreased gradually to negative values under coupling effects of shear contraction caused by normal stress and shear dilation due to climbing effects of fractures.Transition from quasi-static to dynamic sliding of the fractures was identified.The sliding resistance duration increased with confining pressure but decreased with JRC.After prepeak unloading,the fracture surfaces presented a more significant surface wear response and JRC values decreased by 1.70%–59.20%due to more remarkable asperity degradation compared with those after conventional triaxial compression.The theoretical model for shear strength of fractures was established through improving the Ladanyi&Archambault model by introducing the relations between normal stress and surface wear ratios of fractures,which agreed well with the experimental results.

Molecular mechanism of fly ash affecting the performance of cemented backfill material

The great challenge of cemented tailings backfill(CTB)is difficult simultaneously maintaining its excellent mechanical properties and low cost.Fly ash(FA)can potentially address this problem and further replace cement in favor of low carbon development.However,its mechanism on CTB with low cement dosage and low Ca system remains unclear.Consequently,this study conducted uniaxial compression,Xray diffraction(XRD),and scanning electron microscopy(SEM)-energy dispersive spectrometer(EDS)tests to investigate the effect of FA dosage on the mechanical property and microstructure of CTB.A molecular model of FA-CSH was constructed to reproduce the molecular structure evolution of CTB with FA based on the test results.The influences of FA dosage and calcium/silica molar ratio(Ca/Si ratio)on the matrix strength and failure model were analyzed to reveal the mechanism of FA on calcium silicate hydrated(C-S-H).The results show that the strength of CTB increases initially and then decreases with FA dosage,and the FA supplement leads to a decrease in Ca(OH)_(2) diffraction intensity and Ca/Si ratio around the FA particles.XRD and SEM-EDS findings show that the Ca/Si ratio of C-S-H decreases with the progression of hydration.The FA-CSH model indicates that FA can reinforce the silica chain of C-S-H to increase the matrix strength.However,this enhancement is weakened by supplementing excessive FA dosage.In addition,the hydrogen bonds among water molecules deteriorate,reducing the matrix strength.A low Ca/Si ratio results in an increase in water molecules and a decrease in the ionic bonds combined with Ca^(2+).The hydrogen bonds among water molecules cannot withstand high stresses,resulting in a reduction in strength.The water absorption of the FA-CSH model is negatively correlated with the FA dosage and Ca/Si ratio.The use of optimal FA dosage and Ca/Si ratio leads to suitable water absorption,which further affects the failure mode of FA-CSH.

Assessment of Fiber Distribution in Steel Fiber Mortar Using Image Analysis

A new test method was introduced to measure fiber distribution in steel fiber reinforced mortar by using image analysis technique. Through specimen preparation, image acquisition, fiber extraction, and measurement of related fiber parameters, quantitative analysis of fiber distribution could be obtained by two parameters, namely dispersion coefficient and orientation factor. Effect of boundaries, size and steel fiber content on fiber distribution was discussed. Results showed that, steel fiber distribution was affected by boundary effect, which would be weakened with the increase of specimen size. If the length and width remained constant, the specimen height had a significant effect on orientation factor of fiber, while its influence on dispersion coefficient was not so obvious. With the increase of steel fiber content, dispersion coefficient decreased slightly, and orientation factor deviated from 0.5.

A Protocol to Develop Phenolic Emulsifiers Used in Micro-surface by Control Technology

The phenolic emulsifiers used in emulsified asphalt of micro-surfacing, which was the most important tools in the road maintenance, were investigated by control technology. Many factors influencing this reaction were studied and three kinds of phenolic emulsifiers were prepared without catalyst in ethanol. The performance was researched that 2-（{2-[2-（2-Amino-ethylamino）-ethylamino]-ethyl-amino}-methyl）-4- nonyl-phenol （abbreviated as TETA） could be used in micro-surface. With addition of 0.5 % demulsifier, the mixing time was extended to 120 seconds obviously, and the cohesion torque （60 min） was 2.8 N＊m, which satisfied the opening traffic time shorter than 1 h. The wet track abrasion （6 d） was lower than 807 g/m2, with interracial modifier added, but the load wheel was increased with interfacial modifier increasing. When the TETA： demulsifier： interracial modifier =3：1：3, excellent performance was obtained and the experimental results met the International Slurry Surfacing Association （ISSA） standard. The synthesis process of this emulsifier is simple and the performance used in micro-surface was excellent, so this kind of emulsifier could have a better application future.

Effect of raw materials and proportion on mechanical properties of magnesium phosphate cement

Magnesium phosphate cement (MPC) cementitious material is a phosphate cement-based material with strengthformed by a serious of acid-base neutralization reactions among magnesium oxide, phosphate retarder and water,which has a high early strength and a broad application prospect in the field of pavement rehabilitation. Thisreview collects and organizes the latest progress in the field of research on the influencing factors of mechanicalproperties of magnesium phosphate cementitious materials worldwide in recent years, and discusses the possibilitiesof application in airport engineering.The type of phosphate has a great influence on the reaction products, and the strength of the reaction productsof ammonium salt is higher. Borax is the most commonly used retarder, and the retarding effect is related to theratio of boron to magnesium. However, borax retarders have an adverse effect on the strength of MPC. In terms ofthe influence of mineral admixtures on the properties of MPC, fly ash, silica fume and metakaolin, as commonmineral admixtures, have a positive influence on the mechanical properties of MPC, but the mechanism anddegree of the influence of the three materials on the strength of MPC are slightly different;Aggregates can alsoimprove the volume stability and mechanical properties of MPC by forming skeleton structure and slowing downthe exothermic reaction. In fiber reinforced MPC matrix, steel fiber is the most widely used and the bondingperformance between special-shaped steel fiber and MPC matrix is higher than that of straight fiber;basalt fiberhas also been proved to be used to improve the mechanical properties of MPC system.

水化硅酸钙-集料界面交互作用及湿度影响的分子动力学研究

水泥水化浆体与集料的界面性质在很大程度上决定了混凝土的各项性能。本研究利用分子动力学模拟探讨了常用的集料相碳酸钙/二氧化硅与水化硅酸钙(C-S-H)之间的界面相互作用机制,以及湿度对界面性质的影响。结果表明,无论在干或湿条件下,C-S-H/碳酸钙界面都具有较强的稳定性。这是由于碳酸钙中钙离子与C-S-H表面的高极性非桥氧原子具有高强度化学连接的原因。二氧化硅则通过氢键作用吸附于干燥的C-S-H表面,但界面上水分子的存在会大大降低界面亲和度。此外,还利用分子动力学进行了C-S-H/集料界面分离的动态模拟,计算结果趋势符合准静态黏聚法则。而润湿度和拉伸速率对C-S-H/二氧化硅的分离过程有较为明显的影响,较湿润的界面和较小的加载速率均有可能导致黏聚力降低。本研究可为纳米尺度下理解浆体/集料相互作用以及高性能水泥基材料的研制提供新的思路。

Analysis of Compressive Strength Development of Ultra-high Performance Concrete

A numerical procedure was presented for evaluating the compressive strength development of ultra-high performance concrete(UHPC) with cement-silica fume-slag binder.This numerical procedure started with initial packing behavior of designed UHPC using a random sequential packing method.Furthermore,synergistic effect of combined mineral admixtures was addressed with respect to hydration heat.Accordingly,hydration degree of cement and reaction degrees of mineral admixtures were determined based on a blended cement hydration model.Finally,a compressive strength evolution model was proposed and the evolution of compressive strength of three mixes with different binder recipes was compared.The results showed that the both initial packing behavior of UHPC mixes and synergistic effect of mineral admixtures are critical for predicting the properties of UHPC.A remarkable void fraction of 0.2042 was observed for UHPC mix designed by optimization algorithms under random packing.Furthermore,a negative synergistic effect of the combination of silica fume and slag was obtained with regarding to compressive strength.In addition,for a given mineral admixture replacement of 20%,the formulation of mineral admixture should be carefully tailored where the UHPC incorporating 5%silica fume and 15%slag shows the highest compressive strength.

The Doubly Thermo-responsive Triblock Copolymer Nanoparticles Prepared through Seeded RAFT Polymerization

The doubly thermo-responsive triblock copolymer nanoparticles of polystyrene-block-poly（N- isopropylacrylamide）-block-poly[N,N-（dimethylamino） ethyl methacrylate] （PS-b-PNIPAM-b-PDMAEMA） are successfully prepared through the seeded RAFT polymerization in situ by using the PS-b-PNIPAM-TTC diblock copolymer nanoparticles as the seed. The seeded RAFT polymerization undergoes a pseudo-first-order kinetics procedure, and the molecular weight increases with the monomer conversion linearly. The hydrodynamic diameter （Dh） of the triblock copolymer nanoparticles increases with the extension of the PDMAEMA block. In addition, the double thermo-response behavior of the PS-b-PNIPAM-b-PDMAEMA nanoparticles is detected by turbidity analysis, temperature-dependent 1H-NMR analysis, and DLS analysis. The seeded RAFT polymerization is believed as a valid method to prepare triblock copolymer nanoparticles containing two thermo-responsive blocks.

Effect of recycled powder on the yield stress of cement paste with varied superplasticizers

The influence of superplasticizer on the yield stress of cement pastes with recycled powder(RP)was examined in the study.Four superplasticizers were used to obtain the similar fluidity by adjusting the dosage.The results show that the 10%RP decreases the yield stress of paste compared to the reference paste at the same fluidity,but 20%and 30%RP increases the yield stress,ranging from 11 to 599%.The superplasticizer with adsorptive group of phosphate-type minimizes the yield stress of paste than that of polycarboxylate-type,but it made a significant increment in yield stress as the incorporating of RP increased.Besides,the polycarboxylate superplasticizer with the higher molecular weight of side chain and charge density led to lower yield stress.Based on the Yodel model,the yield stress of paste with RP was analyzed by the polymer adsorption and particle packing density of particles to reveal the influence of RP with different superplasticizers on the colloidal interaction and contact network among the particles.The packing density of particles with recycled powder was a little higher than the reference paste,but the higher fraction of fine particles made a stronger PSD effect,which improved the particle contact interaction.On the other hand,due to the higher polymer adsorption of recycled powder than cement,especially for superplasticizer with phosphate group,the average surface coverage was increased,which extended the separation distance,so that colloidal interaction among particles was weaken.