Effect of curing methods on the properties of plain and blended cement concretes

This paper reports result of a study conducted to investigate the effect of curing methods on the properties of plain and blended cement concretes. The concrete specimens were prepared with Type I, silica fume, and fly ash cement concretes. They were cured either by covering with wet burlap or by applying two types of curing compounds, namely water-based and acrylic-based. The effect of curing methods on the properties of plain and blended cement concretes was assessed by measuring plastic and drying shrinkage, compressive strength, and pulse velocity. Results indicated that the strength development in the concrete specimens cured by covering with wet burlap was more than that in the specimens cured by applying water – and acrylic-based curing compounds. Concrete specimens cured by applying curing compounds exhibited higher efficiency in decreasing plastic and drying shrinkage strain than specimens cured by covering with wet burlap. The performance of acrylic-based curing compound was better than that of water-based curing compound. The data developed in this study indicate that curing compounds could be utilized in situations where curing with water is difficult. Among the two curing compounds investigated, acrylic-based curing compound performed better than the water-based curing compound.     

Effect of electric arc furnace dust on the properties of OPC and blended cement concretes

This paper presents results of a study conducted to evaluate the mechanical properties and durability characteristics of ordinary Portland cement (OPC) and blended cement (silica fume and fly ash) concrete specimens prepared with electric arc furnace dust (EAFD). Concrete specimens were prepared with and without EAFD. In the silica fume cement concrete, silica fume constituted 8% of the total cementitious material while fly ash cement concrete contained 30% fly ash. EAFD was added as 2% replacement of cement in the OPC concrete and 2% replacement of the total cementitious content in the blended cement concretes. Mechanical properties, such as compressive strength, drying shrinkage, initial and final setting time, and slump retention were determined. The durability characteristics were evaluated by measuring water absorption, chloride permeability, and reinforcement corrosion. The initial and final setting time and slump retention increased due to the incorporation of EAFD in both OPC and blended cement concretes. The drying shrinkage of EAFD cement concrete specimens was more than that of concrete specimens without EAFD. The incorporation of EAFD was beneficial to OPC concrete in terms of strength gain while such a gain was not noted in the blended cement concretes. However, the strength differential between the blended cement concretes with EAFD and the corresponding concretes without EAFD was not that significant. The water absorption and chloride permeability, however, decreased due to the incorporation of EAFD in both the OPC and blended cement concretes. The corrosion resistance of OPC and blended cement concrete specimens increased due to the addition of EAFD.     

不同类型混凝土断裂特性研究

通过对国内外现有7种断裂模型的分析总结,结合我国实际国情,引出了由我国学者徐世教授提出的双K断裂模型,参照我国《水工混凝土断裂试验规程》给出的两种常见断裂测试试验方法(楔入劈拉法和三点弯曲梁法)中起裂断裂韧度和失稳断裂韧度的计算过程,总结前人的研究成果,详细介绍了普通混凝土、钢筋混凝土、碾压混凝土、纤维混凝土4种常见混凝土的断裂性能。     

水泥性质对半柔性复合混凝土使用性能的影响

通过室内试验评价了水泥类型对水泥乳化沥青半柔性复合混凝土性能的影响,采用X衍射分析揭示了不同水泥在乳化沥青中的水化机理差异。结果表明,不同水泥对复合混凝土的马歇尔稳定度影响较小,但是对劈裂强度和水稳定性影响较大,传统的马歇尔设计方法无法区分水泥类型对混凝土的影响。为了保证混凝土的强度和水稳定性,宜选择硅酸三钙、铝酸三钙等含量较高的水泥,同时限制水泥中活性掺和材料的含量。另外,增加吸水性较强的熟料含量有助于促进乳化沥青破乳,提高混凝土的早期强度和水稳定性。     

Effect of superplasticizer on plastic shrinkage of plain and silica fume cement concretes

Four types of superplasticizers were used in conjunction with three types of silica fume to prepare cement concrete slab specimens that were utilized to measure plastic shrinkage strain and time to attain maximum strain. The concrete slab specimens were cast and placed in an exposure chamber in which the relative humidity, temperature, and wind velocity were kept at 35 ± 5%, 45 ± 2 °C, and 15 ± 2 km/h, respectively. Results of this investigation indicate that the plastic shrinkage strain varied with the type of superplasticizer and the type of silica fume. Maximum plastic shrinkage strain was measured in the undensified silica fume cement concrete with all superplasticizers. Incompatibility was noted between polycarboxylic ether superplasticizer and plain and two types of silica fume cement concretes.     

Hydraulic properties of single fractures grouted by different types of carbon nanomaterial-based cement composites

In underground projects such as nuclear waste disposal facilities, the requirement of controlling the permeability is very important, and common grouting materials are not suitable. Therefore, creating a composite material to be used for the grouting in engineering projects is essential. The present study characterizes the efficiency of several kinds of carbon grouting composites based on nanomaterials, in terms of the hydraulic properties of single fractures in granites using a rock permeability test system. The pore structure and distribution of the carbon nanomaterials in the grouting pastes were investigated using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The results reveal that the hydraulic properties of grouting samples can be well described by linear Darcy’s law. These grouting composites can decrease the equivalent permeability (K_g) by 31.8–81.0% in different confining pressures. The enhancing effect of the graphene oxide (GO)/multi-walled carbon nanotube (MWCNT)–based ordinary Portland cement (OPC) grouted sample is the best, improving the permeability resistance by 240.3–315.0%. The MIP test proves that mixing carbon nanomaterials will improve the pore characteristics. The GO/MWCNT-OPC grouting presents the optimal decrease in all total intruded volume, average pore diameter and porosity, dropping by 33.8, 40.7 and 22.9%, respectively. Both the average pore diameter and porosity of grouting pastes have zero-intercept linear correlations with the equivalent permeability and can predicate permeability-related properties with considerable accuracy. The SEM images further confirmed that carbon nanomaterials can not only refine the pore structure of grouting materials through the superior properties of the MWCNTs to bridge micro-sized cracks and the interfacial adhesion of GO but also reinforce the flow resistance of the interface between the grouting material and the granite. The findings of this study can help guide the implementation of carbon nanomaterial-OPC grouting in engineering projects.

     

掺MgO膨胀剂水泥浆体不同养护制度下的变形特性

研究了掺MgO膨胀剂水泥浆体在不同养护制度下的变形特性.结果表明,随着养护湿度的降低,MgO膨胀剂对水泥浆体的收缩补偿效果逐渐下降;提高养护温度,Mgo膨胀剂对水泥浆体的收缩补偿增大,膨胀稳定所需时间缩短.     

磨细矿渣对不同细度水泥水化性能的影响

从抗压强度、化学结合水含量、XRD分析和SEM分析等方面研究了磨细矿渣对不同细度水泥水化性能的影响,以期丰富水泥水化基础理论,而且对矿渣的综合利用以及混凝土的制备和应用能产生一定的作用。     

外加剂柠檬酸对硫氧镁水泥性能影响的研究

以柠檬酸为外加剂掺入硫氧镁水泥中,研究柠檬酸含量对硫氧镁水泥力学性能的影响。实验测试了硫氧镁水泥的抗折、抗压力值、抗压强度三个方面的力学性能。随着柠檬酸质量分数的增加,硫氧镁水泥试块的抗折、抗压力值、抗压强度均有所增加,并达到最大值。当超过这个最大值后,随着柠檬酸含量的增加其抗折、抗压力值、抗压强度会逐渐减小。实验结果表明,适量的柠檬酸含量对于硫氧镁水泥的力学性能有很好的改善作用。     

Properties of polymer modified steel fiber-reinforced cement concretes

Polymer modified steel fiber-reinforced concretes were produced with addition of both steel fibers and a styrene butadiene rubber emulsion (SBR). Both flexural and compressive strength of the composites after 28 days curing were tested. Microstructures of the composites were analyzed by using scanning electron microscope and mercury intrusion porosimetry. Results show that the addition of steel fibers increases both flexural and compressive strength of the composites. The flexural strength increases significantly when containing 3–10 wt.% SBR. The optimal use of SBR is 5 wt.%. However, the compressive strength may decrease with the addition of SBR. When the addition arrives 10 wt.%, a 16% reduction is observed. The overall porosity and pore size distribution of the composites vary with SBR content. The addition of 3 or 5 wt.% SBR can refine the pore size distribution. Interweaving polymer films were observed in the composites.     

Effect of varying cement content and curing conditions on the properties of sarooj (artificial pozzolana)

This paper describes research carried out at Sultan Qaboos University on the use of the local pozzolanic material, known as sarooj, with Portland cement. Sarooj was the major constituent and cement was used as an additive with various percentages, as used in restoration of historical monuments around the country. Seven mixes with the proportion of sarooj=1.0, sand=0.4, cement=0.2 and water=0.5 were cured for different durations in air and water to establish the best curing conditions suitable for the hot humid conditions of the Gulf region (28–45°C, R/H 60–80%). In addition, four groups of mixes (sarooj, sand, cement, water), each containing 11 mixes with cement content varying from 0 to 100% by mass of sarooj were tested to study the effect of varying the amount of cement on the strength characteristics. It was found that curing in the sun produced the highest strength results. The cement/sarooj ratio by mass was about 0.6.     

无熟料水泥混凝土的物理力学特性

无熟料水泥混凝土(NSC)是用水淬高炉矿渣(GBFS)和激发剂的混合物作为胶凝材料而配制的新型混凝土,其性能取决于GBFS的碱度、化学成分、玻璃化率以及激发剂的种类和数量。为此,以磷石膏作为GBFS的激发剂来配制NSC后,采用与普通水泥混凝土(简称OPC)对比的方法、进行了和易性、强度及其与钢筋黏结强度的试验探讨。结果表明:NSC的和易性优于OPC;早期强度主要来源于钙矾石并接近OPC,长期强度则主要来源于C-S-H水化物且远远超过OPC。     

谈不同基材等条件对水泥土抗压强度的影响

利用不同品种水泥及不同的土进行配合比试验,通过物理力学性能的检验结果初步分析了材料对水泥土性能的影响,指出不同基材及水灰比、龄期等因素对水泥土强度的影响较大,在工程中应结合具体情况进行选材。     

水泥增效剂对水泥基材料性能的影响

研究了水泥增效剂对水泥及混凝土相关性能的影响。试验结果表明:水泥增效剂的掺入,显著提高了水泥基材料的力学性能:水泥胶砂试块1 d时的抗压强度和抗折强度分别增加149%和176%,28 d时分别增加29%和44%;混凝土材料28 d时的抗压强度和抗折强度分别提高34%和42%,折压比达到12.8%。水泥增效剂在水化早期快速水化形成钙矾石,具有促凝作用,针状钙矾石交叉连接形成网络状结构,有利于胶凝材料力学性能的改善。增效剂对水泥后期水化过程的影响作用甚微。     

机制砂中石粉对不同强度等级混凝土性能的影响研究

机制砂生产中不可避免地产生一定数量的粒径小于75 μm的石粉颗粒.针对国家标准GB/T 14684-2001<建筑用砂>对强度等级低于C30、C30-C60、高于C60的混凝土用机制砂石粉含量限值分别为7%、5%、3%的规定,试验研究了超过国标规定上限的石粉含量分别对C30、C60、C80机制砂混凝土工作性、强度、干缩及氯离子扩散系数、抗冻等性能的影响,据此提出了混凝土用机制砂石粉含量限值可以进一步放宽的建议.     

Interaction diagrams to assess SCC mortars for different cement types

The present paper provides a comprehensive procedure for the design of mortar mixtures which are adequate for self-compacting concrete (SCC). A central composite design was carried out to mathematically model the influence of four mixture parameters and their coupled effects on deformability, viscosity and compressive strength of mortar mixtures. The derived models and a numerical optimization technique were used to determine the range of mortar mixture parameters where deformability and viscosity coexist in a balanced manner. Interaction diagrams are suggested to represent the optimized solutions. Six different types of cement were assessed in combination with limestone filler and a polycarboxylate type superplasticizer. Each type of cement has unique properties that interact with other constituents, resulting in different interaction diagrams and mix proportions in the concrete mixture. The utility of numerical models and optimized solutions for quality control, tailor-made concrete mixtures and selection of constituent materials is highlighted.     

水泥种类对隧道用硅气凝胶防火砂浆性能影响的试验研究

为研制隧道高性能混凝土专用防火涂层砂浆,试验探讨了普通硅酸盐水泥(PO)和高贝利特水泥(HBC)对SiO_2气凝胶防火砂浆性能的影响。基于体积法设计砂浆配合比,其中气凝胶与砂的总体积固定为60%,气凝胶体积取总体积的0、20%、40%、60%、80%。结果表明:不同取代率下,PO气凝胶砂浆抗压强度、抗折强度、吸水率、分别比HBC气凝胶砂浆的对应值高12.8%、41.8%与29.8%,而软化系数与导热系数则分别低2.5%与27.9%。采用扫描电镜SEM法对两类砂浆的微观结构进行了分析,发现PO气凝胶砂浆具有更适于防火保温的微观结构。     

不同环境条件下水泥土的力学特性研究

基于水泥土的实际工程应用环境,通过室内对比试验,研究了水泥土在密封养护（基准养护）、室温清水中浸泡、硫酸钠溶液中浸泡等3种条件下强度等力学性能的变化规律,得出相应的试验结论,为进一步明确工程应用中水泥土的强度等力学性能参数的应用条件及水泥土耐久性能的改善提供了技术基础。     

Neural networks for predicting properties of concretes with admixtures

Backpropagation neural networks were used to predict the strength and slump of ready mixed concrete and high strength concrete, in which chemical admixtures and/or mineral additives were used. Although various data transforms were tried, it was found that models based on raw data gave the best results. When non-dimensional ratios were used, arranging the ratios such that their changes resulted in corresponding changes in the output (e.g. increases in ratios to cause increases in output values) improved network performance. The neural network models also performed better than the multiple regression ones, especially in reducing the scatter of predictions. Problems associated with models trained on non-dimensional ratios were uncovered when sensitivity analyses were carried out. A rational approach was used for carrying out sensitivity analyses on these mix design problems by constraining the sum of input values. These analyses, using the raw data based model, showed that the modelling had picked up not only the fundamental domain rules governing concrete strength, but also some well-known second order effects.     

微生物水泥净浆的自修复性能研究

研究了微生物矿化对水泥基材料的自修复作用,试验测定了微生物水泥净浆的裂缝自愈合、抗压强度、吸水率、孔结构等性能,并采用X射线衍射对矿化产物进行成分分析。结果表明微生物的掺入使普通硅酸盐水泥净浆和硫铝酸盐水泥净浆的抗压强度增加,吸水率下降;试件裂缝处自愈合形成了一条连续、饱满的灰白色矿物填充带。矿化生成的方解石型碳酸钙和菱镁矿型碳酸镁沉淀填充于试件内部缺陷、裂缝,使水泥净浆的总孔隙率及其增长率下降微细孔隙比例增加促进了材料的自修复,改善了其孔结构。