Effects of cement content, polypropylene fiber length and dosage on fluidity and mechanical properties of fiber-toughened cemented aeolian sand backfill

Using aeolian sand(AS)for goaf backfilling allows coordination of green mining and AS control.Cemented AS backfill(CASB)exhibits brittle fracture.Polypropylene(PP)fibers are good toughening materials.When the toughening effect of fibers is analyzed,their influence on the slurry conveying performance should also be considered.Additionally,cement affects the interactions among the hydration products,fibers,and aggregates.In this study,the effects of cement content(8wt%,9wt%,and 10wt%)and PP fiber length(6,9,and 12 mm)and dosage(0.05wt%,0.1wt%,0.15wt%,0.2wt%,and 0.25wt%)on fluidity and mechanical properties of the fibertoughened CASB(FCASB)were analyzed.The results indicated that with increases in the three aforementioned factors,the slump flow decreased,while the rheological parameters increased.Uniaxial compressive strength(UCS)increased with the increase of cement content and fiber length,and with an increase in fiber dosage,it first increased and then decreased.The strain increased with the increase of fiber dosage and length.The effect of PP fibers became more pronounced with the increase of cement content.Digital image correlation(DIC)test results showed that the addition of fibers can restrain the peeling of blocks and the expansion of fissure,and reduce the stress concentration of the FCASB.Scanning electron microscopy(SEM)test indicated that the functional mechanisms of fibers mainly involved the interactions of fibers with the hydration products and matrix and the spatial distribution of fibers.On the basis of single-factor analysis,the response surface method(RSM)was used to analyze the effects of the three aforementioned factors and their interaction terms on the UCS.The influence surface of the two-factor interaction terms and the three-dimensional scatter plot of the three-factor coupling were established.In conclusion,the response law of the FCASB properties under the effects of cement and PP fibers were obtained,which provides theoretical and engineering guidance for FCASB filling.

Single-factor analysis and interaction terms on the mechanical and microscopic properties of cemented aeolian sand backfill

The use of aeolian sand(AS)as an aggregate to prepare coal mine cemented filling materials can resolve the problems of gangue shortage and excessive AS deposits.Owing to the lack of research on the mechanism of cemented AS backfill(CASB),the response surface method(RSM)was adopted in this study to analyze the influence of ordinary Portland cement(PO)content(x_(1)),fly ash(FA)-AS(FA-AS)ratio(x_(2)),and concentration(x_(3))on the mechanical and microscopic properties of the CASB.The hydration characteristics and internal pore structure of the backfill were assessed through thermogravimetric/derivative thermogravimetric analysis,mercury intrusion porosimetry,and scanning electron microscopy.The RSM results show that the influence of each factor and interaction term on the response values is extremely significant(except x_(1)x_(3),which had no obvious effect on the 28 d strength).The uniaxial compressive strength(UCS)increased with the PO content,FA-AS ratio,and concentration.The interaction effects of x_(1)x_(2),x_(1)x_(3),and x_(2)x_(3) on the UCS at 3,7,and 28 d were analyzed.In terms of the influence of interaction items,an improvement in one factor promoted the strengthening effect of another factor.The enhancement mechanism of the curing time,PO content,and FA-AS ratio on the backfill was reflected in the increase in hydration products and pore structure optimization.By contrast,the enhancement mechanism of the concentration was mainly the pore structure optimization.The UCS was positively correlated with weight loss and micropore content but negatively correlated with the total porosity.The R^(2) value of the fitting function of the strength and weight loss,micropore content,and total porosity exceeded 0.9,which improved the characterization of the enhancement mechanism of the UCS based on the thermogravimetric analysis and pore structure.This work obtained that the influence rules and mechanisms of the PO,FA-AS,concentration,and interaction terms on the mechanical properties of the CASB provided a certain theoretical and engineering guidance for CASB filling.

Constitutive model for monotonic and cyclic responses of loosely cemented sand formations

This paper presents a model to simulate the monotonic and cyclic behaviours of weakly cemented sands.An elastoplastic constitutive model within the framework of bounding surface plasticity theory is adopted to predict the mechanical behaviour of soft sandstone under monotonic and cyclic loadings. In this model, the loading surface always passes through the current stress state regardless of the type of loading. Destruction of the cementation bonds by plastic deformation in the model is considered as the primary mechanism responsible for the mechanical degradation of loosely cemented sands/weak rock.To model cyclic response, the unloading plastic and elastic moduli are formulated based on the loading/reloading plastic and elastic moduli. The proposed model was implemented in FLAC2D and evaluated against laboratory triaxial tests under monotonic and cyclic loadings, and the model results agreed well with the experimental observations. For cyclic tests, hysteresis loops are captured with reasonable accuracy.

A constitutive model for evaluation of mechanical behavior of fiber-reinforced cemented sand

The aim of this study is to present a constitutive model for prediction of the mechanical behavior of fiberreinforced cemented sand. For this purpose, a generalized plasticity constitutive model of sandy soil is selected and the parameters of the model are determined for three types of sandy soils using the results of triaxial tests. Next, the proposed model is developed using the existing models based on the physicomechanical characteristics of fiber-reinforced cemented sand. The elastic parameters, flow rule and hardening law of the base model are modified for fiber-reinforced cemented sand. To verify the proposed model, the predicted results are compared with those of triaxial tests performed on fiber-reinforced cemented sand. Finally, the efficiency of the proposed model is studied at different confining pressures, and cement and fiber contents.

Effect of Sand Content on Strength and Pore Structure of Cement Mortar

The effects of four sand contents on the compressive, flexural and splitting-tensile strength of cement mortars were evaluated. Moreover, we experimentally investigated the pore structure of cement mortar brought about by changing the sand content and water/cement ratio. The changes in the pore structure were quantified by measuring the porosity and pore size distribution obtained by using mercury intrusion porosimetry（MIP） technique. The test results show that the strengths of cement mortar increase with increasing sand content. It is also suggested that the traditional water/cement ratio law can be applied to cement mortar with different sand contents, provided that a slight modification is introduced. Sand content is an important parameter influencing the pore structure of cement mortar. Moreover, there is a good relationship between the pore structure and strength of cement mortar.

Influence of zeolite and cement additions on mechanical behavior of sandy soil

It is well known that the cemented sand is one of economic and environmental topics in soil stabilization. In this instance, a blend of sand, cement and other materials such as fiber, glass, nanoparticle and zeolite can be commercially available and effectively used in soil stabilization in road construction. However, the influence and effectiveness of zeolite on the properties of cemented sand systems have not been completely explored. In this study, based on an experimental program, the effects of zeolite on the characteristics of cemented sands are investigated. Stabilizing agent includes Portland cement of type II and zeolite. Results show the improvements of unconfined compressive strength （UCS） and failure properties of cemented sand when the cement is replaced by zeolite at an optimum proportion of 30% after 28 days. The rate of strength improvement is approximately between 20% and 78%. The efficiency of using zeolite increases with the increases in cement amount and porosity. Finally, a power function of void-cement ratio and zeolite content is demonstrated to be an appropriate method to assess UCS of zeolite-cemented mixtures.

Triaxial shear behavior of a cement-treated sand——gravel mixture

A number of parameters,e.g.cement content,cement type,relative density,and grain size distribution,can influence the mechanical behaviors of cemented soils.In the present study,a series of conventional triaxial compression tests were conducted on a cemented poorly graded sandegravel mixture containing 30% gravel and 70% sand in both consolidated drained and undrained conditions.Portland cement used as the cementing agent was added to the soil at 0%,1%,2%,and 3%(dry weight) of sandegravel mixture.Samples were prepared at 70% relative density and tested at confining pressures of 50 kPa,100 kPa,and150 kPa.Comparison of the results with other studies on well graded gravely sands indicated more dilation or negative pore pressure in poorly graded samples.Undrained failure envelopes determined using zero Skempton’s pore pressure coefficient (= 0) criterion were consistent with the drained ones.Energy absorption potential was higher in drained condition than undrained condition,suggesting that more energy was required to induce deformation in cemented soil under drained state.Energy absorption increased with increase in cement content under both drained and undrained conditions.

Review of Proposed Stress-dilatancy Relationships and Plastic Potential Functions for Uncemented and Cemented Sands

Stress-dilatancy relationship or plastic potential function are crucial components of every elastoplastic constitutive model developed for sand or cemented sand.This is because the associated flow rule usually does not produce acceptable outcomes for sand or cemented sand.Many formulas have been introduced based on the experimental observations in conventional and advanced plasticity models in order to capture ratio of plastic volumetric strain increment to plastic deviatoric strain increment(i.e.dilatancy rate).Lack of an article that gathers these formulas is clear in the literature.Thus,this paper is an attempt to summarize plastic potentials and specially stress-dilatancy relations so far proposed for constitutive modelling of cohesionless and cemented sands.Stress-dilatancy relation is usually not the same under compression and extension conditions.Furthermore,it may also be different under loading and unloading conditions.Therefore,the focus in this paper mainly places on the proposed stress-dilatancy relations for compressive monotonic loading.Moreover because plastic potential function can be calculated by integration of stress-dilatancy relationship,more weight is allocated to stress-dilatancy relationship in this research.

Experimental Comparative and Numerical Predictive Stu-dies on Strength Evaluation of Cement Types: Effect of Specimen Shape and Type of Sand

Quality of cement is evaluated via group of tests. The most important, and close to understanding, is the compressive strength test. Recently, Egyptian standards adopted the European standards EN-196 and EN-197 for specifying and evaluating quality of cements. This was motivated by the large European investments in the local production of cement. The current study represents a comparative investigation, experimental and numerical, of the effect of different parameters on evaluation of compressive strength. Main parameters are shape of specimens and type of sand used for producing tested mortars. Three sets of specimens were made for ten types of cements. First set were 70.6 mm cubes molded according to old standards using single sized sand. Second group were prisms molded from standard sand (CEN sand) according to the recent standards. Third group were prisms molded from local sand sieved and regenerated to simulate same grading of CEN sand. All specimens were cured according to relevant standards and tested at different ages (2,3,7,10 and 28 days). Results show that CEM-I Type of cement does not fulfill, in all of its grades, the strength requirements of Ordinary Portland cement OPC specified in old standards. Also, the use of simulated CEN sand from local source gives strengths lower than those obtained using standard certified CEN sand. A limited number of tests were made on concrete specimens from two most common CEM-I types to investigate effect on concrete strength and results were also reported. Numerical investigation of the effect of specimen shape and type of sand on evaluation of compressive strength of mortar specimens, presented in the current study, applies one of the artificial intelligence techniques to simulate and predict the strength behavior at different ages. The Artificial Neural Network (ANN) technique is introduced in the current study to simulate the strength behavior using the available experimental data and predict the strength value at any age in the range of the experiments or in the future. The results of the numerical study showed that the ANN method with less effort was very efficiently capable of simulating the effect of specimen shape and type of sand on the strength behavior of tested mortar with different cement types.

冻融对水泥改良路基土力学特性影响研究

以季冻区路基粉砂土为研究对象进行室内三轴压缩试验,分析水泥改良粉砂土与素土的应力-应变关系,探究冻融循环、水泥掺量和围压对土体抗剪强度的影响。试验结果表明:改良土的破坏以脆性破坏为主;围压增长不改变试样的破坏形式,土抗剪峰值随围压的增加而增大;冻融循环作用会削弱土体的黏聚力,内摩擦角的变化规律表现为在一定范围内波动;在水泥掺量相同的条件下,随冻融次数的增多,土的抗剪强度因水泥持续水化反应故呈先减小后增大再减小;当水泥掺量为2%时,土体已获得较大抗剪强度。

煤矸石机制砂C20混凝土力学性能研究

为探究煤矸石机制砂对低强度等级C20混凝土力学性能的影响,调控煤矸石机制砂掺量和水灰比浇筑混凝土试块,开展抗压和抗折强度试验。结果表明:相同水灰比下,随煤矸石机制砂掺量增加,混凝土的抗压强度先增强后降低;相同取代率下,混凝土的抗压和抗折强度随水灰比增大而降低。相较于煤矸石机制砂取代率,水灰比对混凝土抗折强度的影响更大。取代率不超过30%时,对混凝土的抗压和抗折强度均有利,水灰比为0.55、取代率为25%时,混凝土抗压和抗折强度提高最多。

基于BP神经网络的胶结砂砾石应力-应变关系预测

在前期宏观试验基础之上,采用离散元模拟和BP神经网络相结合的方法获取不同胶凝材料掺量和围压下胶结砂砾石的应力-应变关系。根据前期胶凝材料掺量分别为20、40、60、80、100 kg/m3的胶结砂砾石三轴排水剪切试验结果,开展离散元数值模拟。以试验数据为学习样本,开展BP神经网络模型训练,预测胶凝材料掺量分别为30、50、70、90 kg/m3的胶结砂砾石应力-应变关系,并将预测结果和离散元模拟结果进行对比。研究结果表明,BP神经网络能够实现胶结砂砾石应力-应变关系的预测,并在较低围压下具有较好的精度。

钢纤维-海泡石纤维优化铁尾矿砂-石英砂混合砂浆性能试验研究

为了在保证经济性的同时改善铁尾矿砂-石英砂混合砂浆的性能,选用钢纤维、海泡石纤维优化混合砂浆,研究了钢纤维、海泡石纤维单掺和以不同比例混掺对混合砂浆工作性能、力学性能和自然干燥收缩率的影响。结果表明,在单掺情况下,海泡石纤维对流动性的负面影响高于钢纤维,两种纤维分别以合适掺量掺入混合砂浆中,钢纤维对抗折强度和28 d自然干燥收缩率的改善效果优于海泡石纤维,而海泡石纤维对抗压强度的改善效果更佳;钢纤维-海泡石纤维以0.50%-1.0%和0.75%-1.0%组合混掺对矿浆力学性能和干燥收缩改善效果最佳。从性能改善效果和经济性角度综合考虑,确定了最优纤维掺量组合,即钢纤维、海泡石纤维掺量分别为0.50%、1.0%。在此掺量组合下,混合砂浆3 d、7 d和28 d抗折强度分别提升了15.8%、13.0%和12.5%,混合砂浆3 d、7 d和28 d抗压强度分别提升了16.2%、14.7%和13.9%,28 d自然干燥收缩率降低了32.8%,且经济性良好,每吨混合砂浆仅增加28.8元左右的成本。

透水混凝土力学性能影响因素的研究进展与分析

为保证工程建设的安全性和有效性,充分了解不同因素对透水混凝土力学性能的影响机理,总结了近年来国内外关于水灰比、砂率、掺合料种类及掺量、骨料粒径等因素对透水混凝土力学性能影响的研究成果,分析了现有研究中存在的不足和尚未解决的问题,并基于透水混凝土细观损伤机理和力学行为,指出了未来进一步研究方向。

350℃热采侧钻水平井分级固完井一体化工艺管柱研究

为解决渤海油田疏松砂岩稠油热采老井开窗后固井、防砂工序繁琐、工期长、费用高等问题。根据海上热采井层系复杂、出砂严重、作业日费高等特点,研究了一套适用于350℃热采侧钻水平井分级固完井一体化工艺管柱。该一体化管柱一趟钻入井可实现下部裸眼段的砾石充填作业和上部裸眼段的注水泥固井作业,且满足后续350℃热采注气多轮次冷热交变密封要求。作业后无需钻塞,完井后留井通径大,方便后期开采和修井作业;通过软件模拟其安全性和通过性,一体化管柱下入过程中不会产生屈曲,满足设计要求。该技术可有效解决海上疏松砂岩稠油热采老井开窗固井、防砂作业费用高、工艺复杂,施工周期长等难题。

水灰比、砂率及纳米碳纤维对混凝土微波加热性能的影响

通过调整水灰比和砂率,研究了混凝土基本参数对其微波加热性能的影响,并对比分析了水灰比和砂率对混凝土力学性能及微波加热性能的影响程度。为提高混凝土的微波加热性能,以纳米碳纤维为吸波剂,研究了其对混凝土升温速率、温度分布、除冰效率等的影响。并通过介电常数试验和电阻率试验,分析了纳米碳纤维的影响机理。结果表明:随着水灰比和砂率的增大,混凝土的微波加热性能不断增强。与水灰比相比,砂率的改变对混凝土微波加热性能的影响程度更大。改变砂率对于混凝土微波加热性能的影响比对力学性能的影响更为显著。纳米碳纤维通过增大混凝土的介电常数、降低混凝土的电阻率,从而增强混凝土的介电损耗能力和电阻损耗能力,进而提高混凝土的微波加热性能。纳米碳纤掺量越大,混凝土的微波加热性能越佳。

海水-海砂混凝土研究进展

针对混凝土结构的大规模应用导致淡水、河砂资源短缺的现状,对海水-海砂混凝土国内外研究进展及应用进行综述,包括海水和海砂自身材料特性以及海水-海砂混凝土水泥基材料的水化过程、水化硅酸钙微结构发展、力学性能、耐久性及内部钢筋锈蚀;总结当前海水-海砂混凝土研究成果并分析主要不足,指出海水-海砂混凝土作为一种凝结硬化快速、耐久性较差且易锈蚀钢筋的低成本混凝土材料,未来研究重点是将海水-海砂混凝土配合纤维增强复合材料形成组合结构、完善不同养护方式及条件对海水-海砂混凝土的影响及海水-海砂混凝土在真实复杂条件下的耐久性。

微生物加固钙质砂地基电阻率特性试验研究

微生物加固砂土地基存在加固不均匀的问题,采用自主研发的多通道电阻率测量技术对微生物加固钙质砂地基的电阻率分布规律开展了试验研究,研究了菌液和反应液在注入土体后对电阻率幅值及变化规律的影响,最后根据电阻率分布情况,探讨了微生物加固钙质砂地基空间上加固程度的差异性,以及利用电阻率法监测微生物加固过程和加固效果的可行性。试验结果表明:微生物加固钙质砂地基的电阻率受土体密度,菌液与反应液影响较大,其中,反应液对电阻率的影响程度最高。整体上电阻率随着加固次数的增加呈现先增大后减小的规律,深部土体电阻率显著低于上部土体,第七至第八次加固后,土体电阻率开始增加,这一现象的原因可能为土体中的孔隙在多次加固后逐渐被生成的碳酸钙填充,土体中的离子交换通道减少导致电阻率增加,此时碳酸钙填充孔隙对电阻率的影响占主导地位。

生物矿化固沙长效性分析

荒漠化土地大量分布是沙尘天气频发的根源之一。为有效防沙固沙,需要解决两个关键难题,一是把沙尘固定住,防止沙尘飞起;二是长效固沙,提高固沙的长效性。生物矿化方法是解决长效固沙的一种创新技术。通过室内试验和现场验证,证明生物矿化技术实现长效固沙的可行性和适用性。研究包括脲酶活性试验、钙化试验、固沙风蚀试验等,分析了脲酶活性、胶凝液浓度、pH值、灌注次数、灌注量、温度对生物矿化固沙效果的影响,在此基础上提出了生物矿化-植物-栅栏的联合固沙技术体系,并通过生物矿化固沙风洞试验和现场工程固沙效果进行验证分析,验证表明了联合固沙技术体系的固沙长效性。现场晴天进行生物矿化作业的固沙效果最好,更有利于长效固沙。

水砂—胶结充填隔离矿柱稳定性及宽度研究

某嗣后充填矿山前期水砂充填泌水效果不佳,充填体富水,随着水砂充填区域增大,开采过程中顶板及上盘稳定性变差,为改善开采安全状态,改用胶结充填,水砂充填区与胶结充填区之间的隔离矿柱是保证未来采矿安全的关键。构建了侧壁承压倾斜矿柱理论分析模型,并采用FLAC3D数值模拟软件对10~15 m宽度水砂—胶结充填隔离矿柱和不同水砂充填体静水压力梯度进行了研究。研究表明:①开采扰动会使隔离矿柱内形成应力集中,且未凝固的水砂充填体会进一步增加矿柱内最大主应力;②矿柱内部应力重分布及水砂充填体水平压力使得矿柱水平变形增加,逐步产生破坏形成塑性区,10~13 m的隔离矿柱不足以保障充填体含水工况下的采场稳定性;③考虑到爆破扰动的影响,在现有富水充填情况下,建议隔离矿柱宽度为15 m;④隔离矿柱宽度15 m时,随着静水压力梯度增加,矿柱内部最大主应力和水平变形会有所增长,但塑性区未发生明显增加,15 m宽的隔离矿柱可以保障后续开采安全。