Experimental investigation of evolutive mode-I and mode-II fracture behavior of fiber-reinforced cemented paste backfill: Effect of curing temperature and curing time

The curing temperature-dependent cement hydration causes the nonlinear evolution of fracture behavior and properties of fiber-reinforced cemented paste backfill (CPB) and thus influences the stability of mine backfill materials in deep mines. Therefore, the coupled effect of curing temperature (20, 35, and 45 °C) and cement hydration at different curing times (3, 7, and 28 d) on the mode-I and mode-II fracture behavior and properties of fiber-reinforced CPB is investigated. A comprehensive experimental testing program consisting of semicircular bend tests, direct shear tests, measurement of volumetric water content and matric suction, TG/DTG tests, and SEM observation is carried out. The results show that the coupled thermochemical effect results in strongly nonlinear development of pre- and post-peak behavior of fiber-reinforced CPB. Moreover, the results discover a positive linear correlation between fracture toughness and shear strength parameters and also reveal the vital role played by matric suction in the formation of fracture toughness. Furthermore, predictive functions are developed to estimate the coupled thermochemical effect on the development of K_Ic and K_IIc. Therefore, the findings and the developed mathematical tools have the potential to promote the successful application of fiber-reinforced CPB technology in deep underground mines.     

Centrifuge model studies on the stability of fibre-reinforced cemented paste backfill stopes

Cemented paste backfill (CPB) is used extensively in Australia for providing ground support during underground mining operations. This paper considered the use of polypropylene fibres to reinforce the partial or whole body of CPB models in laboratory centrifuge tests. Specimens were cast as non-reinforced (tailings, cement and water), quarter-height, half-height and full height fibre-reinforced CPB model stopes. The stability of CPB models with vertically exposed faces was investigated by a series of centrifuge tests. The modelling data showed that the prototype height of fibre reinforced CPB stopes could be much higher than that of unreinforced stopes depending on the extent of reinforcing. The vertical displacement and failure mass ratio of CPB models were also compared and discussed. The distinct failure modes showed that fibre reinforcement was effective in preventing the CPB failing into the strong box. Furthermore, virtually no fragments were spalled from the exposed faces of reinforced sections of the stopes. It indicated that the application of fibre reinforcement would potentially reduce ore dilution and recovery costs, because the risks of failure would be lowered and prototype stope sizes be enlarged.     

Experimental study of the shrinkage behavior of cemented paste backfill

Cemented paste backfill(CPB) is largely used in underground mine stopes worldwide.When a CPB is placed in a stope,an important task is to estimate the settlement associated with the shrinkage and selfweight consolidation of the CPB.This is closely related to the volume management to ensure the stability of barricades and tight contacts between the backfill and stope roof.Over the years,shrinkage studies were mostly performed on fine-grained soils(silts and clays),with only a few publications on the shrinkage behavior of uncemented tailings.No study has been published on the shrinkage behavior of CPB.To fill this gap,a series of shrinkage tests has been conducted on CPB with different cement contents,including zero cement content(uncemented paste backfill,uCPB).The results show that the shrinkage response of CPB is very different from that of uCPB.At a given initial water content,CPB exhibits a shorter normal shrinkage stage than uCPB.The unsaturation onset water content and void ratio,shrinkage limit and final void ratio of CPB are generally higher than those of uCPB.At a given cement content,the shrinkage behaviors of CPB and uCPB are significantly influenced by the initial water content.     

Characterization of cemented paste backfill pore structure using SEM and IA analysis

This paper presents a pore structure study conducted on different cemented paste backfill (CPB) samples using scanning electron microscopy and image analysis (SEM–IA). The SEM–IA approach was used to estimate three pore structure parameters: total porosity (n), pore size distribution (PSD), and pore space tortuosity (T). The influence of three binders and three types of water (one de-ionised and two sulphated) was also assessed after 14 and 92 days of curing. The evaluation of n by SEM–IA showed a decrease with curing time that was in accordance with the CPB strength increase. The PSD and the T were only slightly influenced by the water chemistry and the type of binder; the parameters evolved with curing time and were related to a decrease in pore size and an increase in tortuosity. Changes of the pore structure were more significant with sulphated mixing water and the OPC-slag binder. The methods and results presented here will be useful to predict some properties of CPB such as saturated hydraulic conductivity, water retention curve and effective diffusion coefficient.      

Effect of high temperature on strength and microstructural properties of cemented paste backfill

The main purpose of this research is to investigate the influence of high temperatures on the strength and microstructure (e.g. pore structure, porosity) of cemented paste backfill (CPB) through a series of experimental tests. A laboratory experimental setup allowing the simulation of various high-temperature conditions is developed. Different types of CPB specimens are exposed to different high temperatures (100, 200, 400 and 600 °C). The strength, porosity, pore size distribution and water absorption of these CPB specimens are then evaluated by laboratory tests. Thermogravimetric and differential thermal analyses are also performed to study the thermal behaviour of the CPBs exposed to high temperatures. The results show that high temperatures have a significant effect on the properties of CPBs. Generally, increasing temperatures up to 200 °C leads to higher strength in most types of CPB studied. The porosity and pore size distribution of the CPBs change only slightly. Above 200 °C, the temperature reduces the strength of the CPBs. The most notable strength decrease takes place at exposure temperatures exceeding 400 °C. The significant decrease of the strength of the CPB is accompanied by a significant change in the microstructure (porosity, pore size distribution, mineral phases). Moreover, the effect of high temperatures on the strength and microstructure of CPBs depends on the water/cement ratio (w/c ratio) and the tailings type.     

谈粘贴纤维复合材料加固施工方法

介绍了粘贴纤维复合材料加固的施工准备工作,并对施工工艺流程进行了阐述,从基层处理、涂刷底胶、基层整平、粘贴纤维布、表面防护处理等方面总结了一些施工要点,为同类工程的施工积累了经验。     

Chemically induced changes in the geotechnical response of cementing paste backfill in shaking table test

Cemented paste backfill(CPB) is extensively used for underground mine support and/or tailings management.However,CPB behavior under cyclic loadings might be affected by the chemistry of its porewater,which often contains sulphate ions.Till today,no studies have addressed the effect of sulphate on the response of CPB to cyclic loadings by using shaking table technique.This study presents new findings of assessing the effect of the sulphate in the pore water of CPB on its geotechnical response to cyclic loading by using shaking table.CPB mixtures were prepared(with and without sulphate),poured into a flexible laminar shear box,cured to 4 h,and then exposed to cyclic loading using one-dimensional(1D) shaking table.Several parameters(e.g.pore water pressure,settlement,lateral deformation,acceleration,electrical conductivity,effective stress,and liquefaction susceptibility) were monitored or determined before,during,and after shaking.Obtained results indicate that the sulphate-bearing CPB cured to 4 h can be prone to liquefaction under the studied conditions.However,sulphate-free CPB samples are resistant to liquefaction.These results are expected to contribute to a better understanding of the effect of water chemistry on the cyclic behavior of CPB,consequently enhancing the cost-effective design of CPB structures.     

Coupling binder hydration,temperature and compressive strength development of underground cemented paste backfill at early ages

Cemented paste backfill (CPB) is an engineered mixture containing up to 60% solid tailings, and 3–7% binder (often) and water. CBP is used in backfilling underground mine voids. It receives great interest as one of the most commonly used ways in mine backfilling around the world. The usage of CPB greatly contributes to the disposal of mining tailings waste from the surface, increasing working place stability to extract more minerals safely. The key parameter for the design of CPB structure is its strength; namely, unconfined compressive strength (UCS). Knowing the time at which the CPB reaches its reasonable strength is very important for reducing the mining cycle and ensuring the safety of mine workers. As a cemented material, CPB strength is time and temperature dependent, and a function of the degree of hydration. The objective of this paper is to develop a numerical model for predicting the UCS of undrained CPB. Strength development is coupled with temperature and degree of hydration. For validation purposes, the predicted UCS will be compared with three groups of experimental results. The results show a good agreement between the predicted and measured values, and a new formula is suggested for including the effect of temperature into the UCS of CPB.     

Mill tailings based composites as paste backfill in mines of U-bearing dolomitic limestone ore

This paper elaborates on the development of paste backfill using mill tailings generated during the processing of a uranium ore deposit hosted in dolomitic limestone. The tailings have been characterized in terms of the physical, chemical and mineralogical properties. Time-dependent rheological behaviors and geotechnical properties of cemented paste backfill (CPB) are also determined. The studies show that the mill tailing has the potential to form paste and the CPB has adequate strength to provide support to mine pillars, roofs, and walls.     

静压注浆加固台背回填的研究和应用

结合工程实例,分析了静压注浆加固机理,详细介绍了静压注浆施工工艺及施工特点,并总结了静压注浆加固的相关注意事项,通过静压注浆对台背回填进行加固处理,从而达到提高地基承载力,减少路面沉降变形和桥头跳车的目的。     

不排水循环荷载条件下胶结砂土宏微观力学性质离散元模拟研究

天然或人工胶结的存在能够提高砂土的抗液化能力,从宏微观尺度对其动力学性质进行研究具有重大意义.将已有的三维完整胶结接触模型引入到三维离散元程序中,对胶结砂土不排水循环三轴剪切试验进行三维离散元模拟,研究颗粒间胶结、循环应力比对离散元试样宏微观力学性质的影响.研究结果表明,胶结的存在能够抑制轴应变和孔压的发展,提高砂土的...     

Effect of fiber reinforcement and distribution on unconfined compressive strength of fiber-reinforced cemented sand

A series of unconfined compression tests were carried out to examine the effect of fiber reinforcement and distribution on the strength of fiber-reinforced cemented sand (FRCS). Nakdong River sand, polyvinyl alcohol (PVA) fiber, cement and water were mixed and compacted into a cylindrical sample with five equal layers. PVA fibers were randomly distributed at a predetermined layer among the five compacted layers. The strength of the FRCS increases as the number of fiber inclusion layers increases. A fiber-reinforced specimen, where fibers were evenly distributed throughout the five layers, was twice as strong as a non-fiber-reinforced specimen. Using the same amount of fibers to reinforce two different specimens, a specimen with five fiber inclusion layers was 1.5 times stronger than a specimen with one fiber inclusion layer at the middle of the specimen. The fiber reinforcement and distribution throughout the entire specimen resulted in a significant increase in the strength of the FRCS.     

Assessment of fracture process in forta and polypropylene fiber-reinforced concrete using experimental analysis and digital image correlation

This paper aims to characterize the evolution of the fracture process and the cracking behavior in forta-ferro (FF) and polypropylene (PP) fiber-reinforced concrete under the uniaxial compressive loading using experimental analysis and digital image correlation (DIC) on the surface displacement. For this purpose, 6 mix designs, including two FF volume fractions of 0.10%, and 0.20% and three PP volume fractions of 0.20%, 0.30%, and 0.40%, in addition to a control mix were evaluated according to compressive strength, modulus of elasticity, toughness index, and stress–strain curves. The influence of fibers on the microstructural texture of specimens was analyzed by scanning electron microscope (SEM) imaging. Results show that FF fiber-reinforced concrete specimens demonstrated increased ductility and strength compared to PP fiber. DIC results revealed that the major crack and fracture appeared at the peak load of the control specimen due to brittleness and sudden gain of large lateral strain, while a gradual increase in micro-crack quantity at 75% of peak load was observed in the fiber specimens, which thenbegan to connect with each other up to the final fracture. The accuracy of the results supports DIC as a reliable alternative for the characterization of the fracture process in fiber-reinforced concrete.     

Fibre-reinforced cemented soils compressive and tensile strength assessment as a function of filament length

This study aims to develop a dosage methodology based on tensile and compressive strength for artificially cemented fibre reinforced soils considering filament length. The controlling parameters evaluated were the fibre length (l), the cement content (C), the porosity (η) and the porosity/cement ratio (η/C_iv). A number of unconfined compression and split tensile tests were carried out in the present work. The results show that fibre insertion in the cemented soil, for the whole range of cement content studied, and the increase of reinforcement length improve unconfined compressive and split tensile strengths. It was shown that the porosity/cement ratio, in which volumetric cementitious material content is adjusted by an exponent (0.28 for all the fibre-reinforced and non-reinforced cemented soil mixtures) to end in unique correlations for each mixture, is a good parameter in the evaluation of the unconfined compressive and split tensile strength of the fibre-reinforced and non-reinforced cemented soil studied. Analysis of variance (ANOVA) performed on the results of a factorial experiment considering the effect of adjusted cement content, fibre length and porosity showed that all of these factors are significant in affecting the measured changes in split tensile and unconfined compressive strength values. Finally, unique dosage relationships could be achieved linking the unconfined compressive strength (q_u) and the split tensile strength (q_t) of the sandy soil studied with porosity/cement ratio (η/C_iv) and fibre length (l).     

不同胶结砂土力学特性及胶结破坏的离散元模拟

采用离散元法研究不同胶结砂土的宏观力学特性以及微观胶结破坏,其中一种胶结砂土胶结特性由离散元商业软件PFC2D中的胶结接触模型(Contact bond model)进行控制,另一种则采用蒋明镜等提出的无厚度改进胶结接触模型(改进的蒋氏模型)控制.首先,将改进的蒋氏模型引入PFC2D;其次,使用PFC2D对不同胶结强度和不同围压下的上述两种胶结砂土进行双轴压缩试验模拟,对比分析了两种不同胶结砂土的模拟结果.结果表明蒋氏试样(胶结特性由改进的蒋氏模型控制)应变软化和体积剪胀显著,峰值内摩擦角基本随胶结强度增大而增大;而PFC试样(胶结特性由Contact bond model控制)在高胶结强度时应变软化和体积剪胀性比较显著,低胶结强度时表现为应变硬化和体积剪缩,峰值内摩擦角随胶结强度增加而减小.离散元模拟结果在一定程度上与已有室内实测结果相符,蒋氏试样更能反映胶结砂土的主要力学特性.由胶结破坏微观信息统计可知,蒋氏试样中胶结点受拉破坏率远大于受剪破坏率,而PFC试样中两者相当,且蒋氏试样中的总胶结点破坏速率峰值要大于PFC试样.通过对改进的蒋氏模型参数分析可知,低围压条件下,试样宏观力学特性与胶结破坏形式对切向胶结强度与法向胶结强度的比值较为敏感.     

建筑垃圾细料生产流动化回填材料的性能

以灰砂比0.03、0.05和0.08,粉砂比0、0.05、0.1、0.15和0.2为设计参数,对建筑垃圾回填材料进行设计。通过试验对回填材料的流动性(流动度、泌水率)、无侧限抗压强度以及应力应变曲线、本构关系模型和弹性模量等进行研究。研究结果表明:回填材料的流动度受水固比影响较大,两者接近线性关系;流动度在200~250mm范围,泌水率在4%~8%之间;回填材料抗压强度与灰砂比和水固比之间存在很好的幂指数关系;回填材料应力应变曲线形状与普通混凝土的相似,在此基础上提出回填材料的本构关系模型;回填材料无侧限抗压强度与弹性模量之间存在很好的指数关系。     

Experimental research on creep behaviors of sandstone under uniaxial compressive and tensile stresses

The consideration of time dependence is essential for the study of deformation and fracturing processes of rock materials, especially for those subjected to strong compressive and tensile stresses. In this paper, the self-developed direct tension device and creep testing machine RLW-2000M are used to conduct the creep tests on red sandstone under uniaxial compressive and tensile stresses. The short-term and long-term creep behaviors of rocks under compressive and tensile stresses are investigated, as well as the long-term strength of rocks. It is shown that, under low-stress levels, the creep curve of sandstone consists of decay and steady creep stages; while under high-stress levels, it presents the accelerated creep stage and creep fracture presents characteristics of brittle materials. The relationship between tensile stress and time under uniaxial tension is also put forward. Finally, a nonlinear viscoelastoplastic creep model is used to describe the creep behaviors of rocks under uniaxial compressive and tensile stresses.     

基于Hopkinson技术水泥石动态力学性能及其有限元模拟研究

本文通过SHPB试验得到压杆中入射波与透射波应变的时程曲线，求得被测水泥石的本构关系，同时根据有限元程序对Hopkinson杆的一维假定和均匀性假定进行了分析计算。     

高温作用后钼尾矿混凝土单轴受压性能试验研究

为研究钼尾矿混凝土高温后的单轴受压力学性能,进行了不同目标温度（20,200,300,400,600,800℃）条件下钼尾矿混凝土的轴心抗压强度、峰值应变、弹性模量、泊松比、破坏形态及质量变化的试验研究。结果表明：钼尾矿混凝土试件的质量损失率随温度的升高而增加,在800℃时质量损失率平均为6.52%;轴心抗压强度随温度的升高而逐渐降低,800℃时与常温相比平均降低70.04%,且随钼尾矿掺量的增加而降低;而峰值应变随温度的升高先减后增;弹性模量和泊松比都随温度的升高而降低,在800℃时弹性模量和泊松比平均比常温降低88.22%和35.66%。对于弹性模量,大体上随着钼尾矿掺量的增大而减小;而对于泊松比,钼尾矿掺量100%的混凝土略大于掺量50%的混凝土。根据试验结果,建立了钼尾矿混凝土高温后的单轴受压应力-应变本构方程。