掺石墨高铝水泥基复合材料的物理性能研究

选用石墨作导电填料,高铝水泥作胶凝材料,两者复合制备高铝水泥基导电复合材料,对这类导电复合材料进行了基础的开发性研究。通过测试高铝水泥基复合材料的物理性能,得到高铝水泥基复合材料的体积电阻率、抗压强度和表观密度随石墨掺量的变化规律,并采用无限网链理论描述了高铝水泥基复合材料的导电机理。上述高铝水泥基复合材料的体积电阻率可在106—102Ω.cm之间变化,所得结果具有一定的实用价值。     

聚乙烯醇纤维强化水泥基复合材料的抗盐冻性能

通过氯盐环境中的快速冻融试验研究了纤维体积率、冻融循环次数、粉煤灰、硅粉对聚乙烯醇(PVA)纤维水泥基复合材料抗盐冻性能的影响。通过扫描电镜观察内部微观结构随盐冻作用的变化规律、PVA纤维在水泥基体中分布情况和界面结合状况。试验结果表明:PVA纤维的掺入可明显改善水泥基复合材料的抗盐冻性能;PVA纤维在基体中分散性较好,且与水泥基体界面结合状况较好;而粉煤灰、硅粉的掺入未明显改善PVA纤维水泥基复合材料的抗盐冻性能。     

水泥基粒状复合材料的数值仿真

为研究水泥基粒状复合材料的物理力学性能,采用一种自编的特殊算法生成任意形状的单个复合材料骨料颗粒,利用HADES堆积程序来形成材料结构(HADES是模拟任意形状的颗粒碰撞和堆积的程序).在此基础上加入骨料颗粒与水泥基之间的界面过渡区(ITZ),并采用专门的网格划分工具对粗骨料、水泥浆以及界面过渡区进行网格划分,最后进行有限元计算来得到水泥基复合材料的力学性质.以混凝土作为一种典型的水泥基复合材料进行模拟分析,得到不同骨料颗粒成份下混凝土弹性模量值为14.3GPa到22.9GPa,结果表明上述数值模拟方法可有效的对三相材料(水泥基、界面过渡区、骨料颗粒)构成的混凝土弹性模量进行预测.     

碱硅酸反应对水泥基材料力学性能的影响

通过对比活性不同的两种砂制备的砂浆试样在碱硅酸反应中的膨胀率,结合三轴抗压测试研究了碱硅酸反应前后水泥基材料的力学性能变化,分析了碱硅酸反应后材料微观结构演变与力学性能间的关系。结果表明碱惰性砂制备的砂浆试样中骨料周围局部区域形成少量针状凝胶相,由于碱硅酸反应速率较慢且凝胶相较少,因而其膨胀率较小且对水泥基材料力学性能的影响较小。碱活性砂制备的砂浆试样中,骨料周围易生成大量针状凝胶层,且随着碱硅酸反应的进行凝胶相膨胀加剧,促进水泥基材料中骨料周围的水泥浆体中形成大量裂纹,进而明显影响水泥基材料的力学性能。研究表明,三轴抗压测试中,除膨胀率外,极限抗压强度和应变量也可以作为评价水泥基材料碱硅酸反应水平的参量。     

纳米炭黑水泥基复合材料的压敏性研究

纳米炭黑水泥基复合材料具有导电性和压敏性.试验结果表明,电阻率随炭黑掺量的变化呈现渗滤特征,渗滤区炭黑体积含量为0.6%-1.5%.处于渗滤区和超过渗滤区的纳米炭黑水泥基复合材料的电阻率具有明显的压力依赖性.在受到压应力时,电阻率减小,卸载后电阻回复具有不可逆性.炭黑水泥基复合材料的压敏性与3个因素有关:试件本身的极化效应、隧道效应和渗流导电.当炭黑掺量在渗滤区以前时,极化效应占主导地位;当炭黑掺量在渗流区内时,隧道效应占主导地位,试件体现出明显的压敏性;当炭黑掺量继续增加时,炭黑聚集体接触的渗流导电占主导地位.     

碳纳米管在水泥基复合材料中的分散性研究

通过试验研究了碳纳米管在不同掺量、超声波分散以及表面活性剂作用下的分散性及其对水泥基复合材料力学性能的影响;采用SEM观测了碳纳米管改性水泥基复合材料的微观结构,讨论了影响碳纳米管在水泥基复合材料中分散性的因素;综合分析了分散性与力学性能的关系。结果表明:合理的碳纳米管掺量在超声波分散和PVP表面活性剂共同作用下能够显著提高碳纳米管在水泥浆体中的分散性,使水泥/碳纳米管复合材料的抗压强度提高14%。     

机敏水泥基复合材料导电相材料的研究

测定了炭黑-水泥体系、铁粉-水泥体系及碳纤维-水泥体系的导电性,对炭黑、铁粉和碳纤维作为水泥的导电相材料与水泥的适配性进行了研究.结果表明：碳纤维作为水泥基机敏材料的导电相较合适,它不仅可提高水泥基体的电导率,而且可提高复合材料的力学性能,并用SEM对碳纤维-水泥基复合材料的微观结构进行了分析.     

水泥基材料早期水化特性与体积稳定性的关系

采用五路裂缝测定仪和非接触式电阻率测定仪，分别测试了水泥砂浆在干燥条件下约束收缩开裂的初裂时间与相应水泥浆体早期水化24h内的电阻率变化，分析研究了水泥浆体在非常早期水化中的电阻率极小值与相应砂浆的初裂时间的关系，结果发现二者之间具有较好的线性相关性（相关系数〉0．90）。机理研究表明在水泥类型和骨料确定的条件下，胶凝材料体系初始水化的液相特征是影响早期收缩开裂的关键；不同化学与矿物外加剂及微量组分（可溶碱）的添加将通过改变水化的液相特征而影响水化物的初始形成速率、形貌、分散性及其生长能力等，最终影响材料的体积稳定性。     

炭黑/低密度聚乙烯/乙烯-醋酸乙烯共聚物复合材料对交联聚乙烯空间电荷注入的影响

采用熔融共混及热压交联的方法制备了炭黑(CB)/低密度聚乙烯(LDPE)/乙烯-醋酸乙烯共聚物(EVA)半导电电极复合材料。研究了CB含量的变化对半导电电极的电阻率的影响以及对绝缘层空间电荷的屏蔽效果。结果表明:半导电电极中CB含量增加时,电阻率降低,当CB含量(质量分数)超过25%后,电阻率减小不明显,表明试样在该CB比例下已接近过导电电阻的逾渗值;空间电荷测试时,随着电场强度的增大,绝缘层的空间电荷密度先增大后减小;半导电电极CB的含量增加时,绝缘层的空间电荷含量先增加后减小。     

整体式钢纤维导电砼力学及导电性能研究

为提高整体式不锈钢纤维导电混凝土的力学性能及导电性,促进其在实际工程中的应用,利用不锈钢纤维良好的导电性,制备成整体式不锈钢纤维导电混凝土,并对其特性进行相关的试验研究。研究发现:掺入适量不锈钢纤维的混凝土的结构的力学性能有效加强,包括抗压强度和弯拉强度。结合复合相导电材料渗滤阀值理论来分析,整体式不锈钢纤维导电混凝土的导电性能与掺入的钢纤维量有着紧密的联系,随着掺量的增加,电阻率会出现谷值,而后出现回弹。     

碳纤维水泥基复合材料电阻变化规律研究

研究了碳纤维水泥基复合材料在水化期间及固化后的电阻变化特性。研究发现：当碳纤维体积分数较低时，试样的电阻率随着固化时间的增大而明显增大；反之，当碳纤维体积分数超过某一个范围后，会出现电阻（率）随水化时间的延长而变小的规律。此外，对于已固化好的碳纤维水泥基复合材料，随着电流（电压）的增大，电阻会出现一定程度的降低，碳纤维体积分数越低（即电阻率越大时），电阻下降的趋势越明显。     

Early age compressive strength of pastes by electrical resistivity method and maturity method

The compressive strength development of Portland cement pastes was investigated by the electrical resistivity method and the maturity method. The experiments were carried out on the cement pastes with different water-cement ratios at different curing temperatures. The results show that the application of the maturity method has limitation to obtain the strength. It is found that both of the compressive strength and the electrical resistivity follow hyperbolic trend for all the mixes. The hyperbolic equation of each mix is obtained to estimate the ultimate resistivity value which can probably be reached. The relationship between electrical resistivity and compressive strength of the cement pastes is established based on the test results and interpreted by the empirical Archie equation and a strength-porosity equation. The relationship between the electrical resistivity after temperature correction and the compressive strength was linear and independent of curing temperature and water-cement ratio.     

Influence of Water Content on Conductivity and Piezoresistivity of Cement-based Material with both Carbon Fiber and Carbon Black

The influence of water content on the conductivity and piezoresistivity of cement-based material with carbon fiber (CF) and carbon black (CB) was investigated. The piezoresistivity of cement-based material with both CF and CB was compared with that of cement-based material with CF only, and the changes in electrical resistivity of cement-based material with both CF and CB under static and loading conditions in different drying and soaking time were studied. It is found that the piezoresistivity of cement-ba...     

Piezoresistivity of carbon fiber graphite cement-based composites with CCCW

The electrical conductivity and piezoresistivity of carbon fiber graphite cement-matrix composites(CFGCC) with carbon fiber content(1% by the weight of cement),graphite powder contents (0%-50% by the weight of cement) and CCCW(cementitious capillary crystalline waterproofing materials,4% by the weight of cement) were studied.The experimental results showed that the relationship between the resistivity of CFGCC and the concentration of graphite powders had typical features of percolation phenomena.The percol...     

Electrical measurement to assess hydration process and the porosity formation

The change of electrical resistivity with time at early ages was used to investigate the hydration process and the porosity development. Porosity reduction process of cement-based materials hydration was developed by a proposed method. The porosity reduction is fast at the setting period. The results find that the pore discontinuity occurs faster at lower water/cement ratios than at higher water/cement ratios which is similar to the results of the Percolation method.     

水泥基材料隐身性能改性试验与数值模拟

以水泥为基体,添加硅灰和粉煤灰、石墨和碳纤维、纳米TiO2和钢纤维为吸波剂的试样,对8～18GHz频段内的隐身性能进行了试验分析,结果表明,硅灰和粉煤灰、纳米TiO2和钢纤维与水泥复合制成的吸波材料具有良好的隐身性能,石墨和碳纤维与水泥复合制成的吸波材料的隐身性较差,掺合材对水泥基材料吸波性能的影响必须综合考虑,并不是越多越好,掺合材超过一定极限后,材料的透波能力增强,吸波性能便会下降.在设计时对试件实验数据进行计算机模拟仿真,得到水泥基复合吸波材料,材料的电导率、复介电常数、复磁导率、介质损耗角正切等是评价吸波材料的主要参数.当掺合材作为复合吸波剂应用时,两种材料的介电损耗共同发挥衰减作用,可以明显提高试件的吸波性能,而且可以拓宽频段,比单一吸波剂的添加有着明显的优势.     

Characteristics of Resistivity-temperature for Carbon Fiber Reinforced Concrete

The resistance response to temperature change of carbon fiber reinforced cement-based composites （CFRC） is reported, which shows some outstanding phenomena of positive temperature coefficient （PTC） of resistance and negative temperature coefftcient（NTC） of resistance during the temperature rising. The influences of carbon fiber, cement-based matrix and thermal cycles on the characteristics of temperatare-resistivity for the system were also discussed. Because of the special characteristics for temperature-resistivity, carbon fiber cement based composites can be useful in structure with the function of alarm for fire.     

Experimental and Numerical Analysis of Restrained Early Age Cracking based on Electrical Resistivity Using Eccentric Sample

Crack potential and hydration processes of the cement pastes were monitored using an upto-date eccentric steel cracking frame(ESCF), associated with the non-contact electrical resistivity apparatus, independently. The objective of employing the ESCF is to give a new method determining cracks of concrete at early age. The findings indicate that the lowest water-cement ratio paste reveals highest resistivity values, compasses an earlier inflection point and obtained higher stress. The eccentric restrained cracking test exhibited that lower water-cement ratio paste cracked at the earliest time, accordingly confirms cracking tendency is the highest. Tensile strength test and stresses utilizing ABAQUS simulation was performed. The crack initiation ages obtained are consistent with the experimental program results, which indicates that ABAQUS numerical analysis can well be utilized to predict the crack tendency of cement.     

Effect of Waste Paper Fiber on Properties of Cementbased Mortar and Relative Mechanism

The aim of this work was to investigate the effect of waste paper fiber on the properties of cement-based mortar and the relative mechanism. The cement-based mortars with various contents and mixing way of waste paper fibers were prepared and the slump flow, setting time and strength developments of all mortars were tested. Besides, Ca(OH)_2 content in hardened pastes at different ages and the microstructures of all mortar at 90 d were observed by scanning electron microscopy. The experimental results showed that in the process of mixing, more superplastizier was consumed to maintain the workability because of the absorption of water and superplastizer on waste paper fiber. With more waste paper fiber being added, longer setting time is available for the pastes with it because of the carbohydrate dissolving and its retarding to the cement hydration. Waste paper fiber is adverse to the early and later strength of cement-based mortar, but it increases the mortar strengths at 7 and 28 d. The strength, Ca(OH)_2 content and microstructure are related to the content and mixing way of waste paper fiber. Waste paper fiber helps produce the Ca(OH)_2 at 7 and 28 d, but this case is reverse at ages of 1 and 3 d. Overall, waste paper fiber leads to the appearance of more pores in the hardened paste. However, it increases the toughness of cement-based mortar.