Method to determine mixture proportions of workable ultra lightweight cement composites to achieve target unit weights

Ultra lightweight cement composite (ULCC) is a type of composite characterized by low unit weights <1500 kg/m³ and high compressive strengths of up to about 65 MPa. The low unit weight is achieved by incorporating cenospheres as micro-lightweight aggregates in cement paste. As the cenospheres are by-products from coal burning power plants, their properties may vary greatly. This results in great difficulty in determining the mixture proportions of the ULCC. In this research, a new design method is developed and proposed that can be used to effectively determine the mixture proportions of workable ULCC to achieve a desired target unit weight with minimal laboratory trials. The relationship between the spacing among spherical cenospheres and the water-to-cementitious materials ratio (w/cm) needed to achieve the target workability is the basis of this design method. For a given cenosphere sample, if its particle size distribution and particle density are determined, it is possible to design a mixture of ULCC using this method.     

Effect of shrinkage reducing admixture on flexural behaviors of fiber reinforced cementitious composites

The use of shrinkage reducing admixture (SRA) at various concentrations was investigated in fiber reinforced cementitious composites. Both mortar and high strength concrete (HSC) matrices were tested. Two types of fibers—steel and polypropylene—were assessed. The effect of SRA was measured on the fundamental properties such as surface tension of the bulk fluids and the contact angle developed between the fibers and the bulk fluids, on the fresh properties such as the air content and the density, and finally on the hardened mechanical properties, specially the flexural behaviors. It was noted that SRA enhances the wettability of fibers and reduces the air content of fiber reinforced cement mortars, while critical SRA concentrations are existing. SRA with critical concentration can significantly improve the flexural toughness and residual strength of steel fiber reinforced cement mortar. In the case of polypropylene fiber, SRA is not as effective in enhancing the flexural behaviors as it is in the case of steel fiber. SRA is generally ineffective in reducing the air content of HSC and the properties of steel fiber reinforced HSC with SRA are inferior to those without SRA.     

碳纤维水泥基材料电阻的非线性研究

研究了碳纤维水泥基材料(CFRC) 非线性电阻的伏安特性,并着重讨论了不同的碳纤维掺量和温度对CFRC材料电压-电阻关系的影响.结果表明:在较小外加电压下(＜2V)CFRC材料的电压-电阻曲线会出现明显的平台区,随着电压的进一步增大,其电阻逐渐降低呈现非线性特性.相同纤维掺量的水泥基材料随温度的升高其电压-电阻曲线下降斜率基本保持不变,但初始电阻值下降;而在温度保持不变时,随碳纤维掺量的增加,电阻随电压下降的趋势逐渐减缓.     

Enhancing the performance of calcium sulfoaluminate blended cements with shrinkage reducing admixture or lightweight sand

This study investigates the effect of using shrinkage reducing admixture (SRA) or lightweight sand (LWS) on enhancing the performance of calcium sulfoaluminate (CSA) cement in combination with ordinary Portland cement (OPC). Of special interest is the efficacy of the SRA or LWS in modifying the expansion/shrinkage and compressive strength characteristics of OPC-CSA systems in the absence of adequate duration of water curing, which is critical for the expansive reaction of CSA cement and its ability to mitigate shrinkage. Hydration kinetics, autogenous and drying deformation, thermogravimetry, and scanning electron microscopy (SEM) are used to evaluate the effect of SRA or LWS on the performance of the OPC-CSA systems. Test results indicate that the OPC-CSA system can exhibit similar drying shrinkage to that of the plain OPC mixture when no moist curing is applied. In the presence of LWS or SRA, the OPC-CSA systems exhibited lower shrinkage or higher extent of expansion compared to the corresponding OPC-CSA mixture alone. This is attributed to delay of the drop in internal relative humidity and promoting hydration of the OPC-CSA system which can enhance the ettringite-generating potential of CSA cement. The use of LWS was found to be highly effective in enhancing compressive strength of OPC-CSA system. SEM results at 91 days confirm the higher density and lower porosity for the paste surrounding LWS particles compared to the corresponding mixture made without LWS. In the case of inadequate moist curing, the presence of LWS or SRA is shown to enhance the overall performance of OPC-CSA system. For a given overall desirability value of 0.65 determined by multi-objective optimization, the incorporation of 1% SRA or 10% LWS was found to enable the reduction the required period of moist curing from 6 days to 5 and 3 days, respectively.     

配筋聚乙烯醇纤维增强水泥复合材料梁的曲率延性

聚乙烯醇纤维增强水泥（Polyvinyl alcohol fiber reinforced cement,PVA/C）复合材料具有优越的受拉应变硬化特性,可显著提高结构的变形能力。本文以PVA纤维体积分数和受拉钢筋配筋率为研究参数,对6根配筋PVA/C梁和2根普通混凝土梁（RC）进行四点弯曲试验,并对其曲率延性进行了试验研究和理论分析。试验研究表明:配筋PVA/C梁的荷载-挠度（P-δ）关系曲线所包围的面积是C梁的1.64~2.43倍,证明配筋PVA/C梁有较好的持荷变形能力;在PVA纤维体积分数一定的情况下,试验梁的曲率延性系数随受拉钢筋配筋率的增大而减小;在受拉钢筋配筋率一定的情况下,配筋PVA/C梁的曲率延性系数是C梁的1.56~2.02倍,证明掺入PVA纤维显著提高了试验梁的延性。建立了配筋PVA/C梁曲率延性系数的计算公式,并分析了PVA纤维体积分数对受压区高度系数和曲率延性系数的影响,试验结果与计算结果吻合较好。      

Effects of coarser fine aggregate on tensile properties of ultra high performance concrete

This experimental research investigates the mechanical properties and shrinkage of ultra high performance concrete (UHPC) incorporating coarser fine aggregates with maximum particle size of 5 mm. To adequately design UHPC mixtures using various sizes of solid constituents, particle packing theory was adopted. UHPC mixtures containing either dolomite or basalt, and four fiber volume fractions up to two volume percent were investigated. Uniaxial tension test was performed to evaluate the first cracking tensile strength, ultimate tensile strength, tensile strain capacity and cracking pattern. The UHPC mixtures with dolomite and steel fibers with more than one volume percent achieved more than 150 MPa of compressive strength at the age of 56 days, and showed strain hardening behavior and limited decrease in tensile strength compared to typical UHPC without coarser fine aggregates. The experimental results highlight the potential of dolomite used as coarser fine aggregate in UHPC.     

Effect of mineral admixtures and steel fiber volume contents on the behavior of high performance fiber reinforced concrete

High Performance Fiber Reinforced Concrete (HPFRC) is a structural material with advanced mechanical properties. The structural design of HPFRC members is based on the post-cracking residual strength provided by the addition into the mix of the fibers. Moreover, the addition of different types of mineral admixtures influences the overall behavior of this material. In order to optimize the performance of HPFRC in structural members, it is necessary to evaluate the mechanical properties and the post-cracking behavior in a reliable way. As a result, an experimental study on six different sets of HPFRC specimens was carried out. The main parameters that varied were the fiber volume content and the types of mineral addition. The behavior in compression, in flexural tension and the shrinkage properties were evaluated and critically analyzed in order to give a guide for structural use.The results showed that by adding high fiber volume content and the Algerian blast furnace slag into the mix, the HPFRC material obtained has a very good performance and it is suitable for use in practice.     

Flexural behavior of geopolymer composites reinforced with steel and polypropylene macro fibers

Like ordinary Portland cement concrete, the matrix brittleness in geopolymer composites can be reduced by introducing appropriate fiber reinforcement. Several studies on fiber reinforced geopolymer composites are available, however there is still a gap to understand and optimize their performance. This paper presents the flexural behavior of fly ash-based geopolymer composites reinforced with different types of macro steel and polypropylene fibers with higher aspect ratio. Three types (length-deformed, end-deformed and straight) of steel fibers and another type of length-deformed polypropylene fiber with optimum fiber volume fraction of 0.5% are studied. The effects of different geometries of the fibers, curing regimes (ambient cured and heat cured at 60 °C for 24 h) and concentration of NaOH activator (10 M and 12 M) on the first peak strength, modulus of rupture and toughness of the geopolymer composites are investigated. The quantitative effect of fiber geometry on geopolymer composite performance was also analyzed through a fiber deformation ratio. The compressive strength, splitting tensile strength and flexural toughness are significantly improved with macro fibers reinforcement and heat curing. The results also show that heat curing increases the first peak load of all fiber-reinforced geopolymers composites. End-deformed steel fibers exhibit the most ductile flexural response compared to other steel fibers in both heat and ambient-cured fiber reinforced geopolymer composites.     

涤纶长丝-UHMWPE/木材复合结构设计及胶合性能研究

采用涤纶长丝和超高分子量聚乙烯(UHMWPE)纤维编织斜纹布,并与木材复合实现增强的效果,研究了斜纹布与木材复合结构的胶合性能,探讨了UHMWPE在木质材料领域应用的可行性。从斜纹布铺装宽度比设计入手,研究了该复合结构的胶合性能,并对其相关机理进行分析。结果表明,由于木材自身尤其是针叶树材剪切强度(ILSS)较低,PUR木材层压胶可用做斜纹布/木材复合的基体树脂;在3种UHMWPE产品形式中斜纹布的综合性能最优;在拉伸强度允许情况下,采用适当降低斜纹布铺装宽度比的方法可显著提高斜纹布/木材复合结构的ILSS值,并从胶层厚度变化入手对影响机理进行了分析。     

Mix proportion and properties of fire-resistant wet-mixed high-strength polypropylene fiber-reinforced sprayed polymer cement composites

This study examined the optimal mix proportions and performance of a fire-resistant wet-mixed high-strength fiber-reinforced polymer cement composite that enhances the fire-resistance of underground structures. A mix ratio was determined through preliminary tests to satisfy the target compressive strength and maintain the minimum thermal conductivity. A mix with 10% silica fume, 30% perlite replacement ratios and 0.2% of polypropylene fiber satisfied the desired standards. The polymer cement composite had high compressive, flexural, and bond strengths of approximately 40, 5, and 1.6 MPa, which are sufficient for the surface pressures encountered in road and railway tunnels.     

芳纶纤维和超高分子量聚乙烯纤维制备工程用纤维/水泥复合材料的适用性

研究了两种高强合成纤维在工程用纤维/水泥复合材料制备过程中的适用性,其中,芳纶纤维的表面为亲水性,超高分子量聚乙烯（UHMWPE）纤维的表面为憎水性。研究结果表明:工程用芳纶纤维/水泥复合材料拉伸破坏过程中无应变硬化能力且表现为单裂纹破坏现象;工程用UHMWPE纤维/水泥复合材料拉伸破坏过程中表现出良好的应变硬化能力和多裂纹开裂特性。因此,两种纤维相比,UHMWPE纤维适宜于工程用纤维/水泥复合材料的制备。随着水胶比的降低,工程用UHMWPE纤维/水泥复合材料抗拉强度增大,但应变硬化能力降低,因此,在制备工程用UHMWPE纤维/水泥复合材料的过程中,应协调纤维抗拉强度和基体与纤维之间界面过渡区的品质。      

混杂纤维增强水泥基复合材料的力学性能

研究了化学改性聚丙烯（PP）纤维以及掺加聚丙烯纤维和芳纶纤维混杂比例和混杂效应对水泥基复合材料力学性能的影响,并构建了纤维增强水泥砂浆界面层的物理模型,描述了纤维对水泥砂浆的增强机制。实验表明,聚丙烯纤维经改性后使水泥砂浆前期抗折强度明显提高,聚丙烯纤维和芳纶纤维的混杂使水泥砂浆的后期抗折强度显著提高。改性聚丙烯纤维掺加体积分数为0.56%,芳纶纤维的体积分数为0.24%时,混杂纤维增强水泥砂浆试样较空白试样,3天、28天抗折强度分别提高了18.48%、31.17%,3天、28天抗压强度分别提高了7.16%、5.19%。     

Electrochemical corrosion monitoring of steel fiber embedded in cement based composites

Steel fibers are commonly used in cement based materials for many applications such as floors, structural elements, repairing works, etc. The chloride-induced corrosion via ingress of seawater may become a risk for performance of the steel fiber reinforced cement based composites. Despite of few studies that have dealt with the corrosion behavior of steel fibers embedded in cement based composites, there are lack of information about the chloride-induced corrosion of steel fibers embedded with sufficient cover in non-cracked matrixes, reactive powder concrete, and especially polymer-modified cement based mortars. The open-circuit potential and corrosion current density of single steel fibers embedded in various cement based matrices were monitored after 200, 400, 600, 1200 wetting-drying cycles in 3.5% NaCl solution. The corrosion and microstructure analyses revealed that the steel fibers can be protected by a well-designed mixture, non-cracked matrix and sufficient cover. However, it could be corroded in long terms depending on the type and the dosage of the polymer latex used. In addition, the residual stresses in the deformed regions of the hooked-end steel fibers is critical in terms of the protection against to chloride-induced corrosion.     

表面氧化处理对碳纤维及其水泥石性能的影响

碳纤维与水泥石界面的粘接状况是影响碳纤维水泥石材料性能的关键因素之一，为此，通过测试碳纤维的表面浸润性以及碳纤维水泥石的力学性能，研究了经过NaClO氧化处理后，碳纤维表面性能以及碳纤维与水泥石界面粘结性能的变化．研究结果表明：碳纤维的氧化处理可以提高其表面对水的浸润性，改善碳纤维与水泥石的粘结界面，提高碳纤维水泥石的压敏特性。     

碳纤维增强水泥基复合材料屏蔽性能的研究

现代电子技术的迅速发展不可避免地带来了电磁辐射所造成的环境污染.选择合适的屏蔽材料,利用其反射、吸收、多层反射等功能是消除电磁污染的有效手段.碳纤维除具有高弹性、高模量、低密度、耐腐蚀等优良性能被当作许多复合材料的增强体外,还具有良好的导电性,将其加入到水泥基体中制成碳纤维增强水泥基复合材料(CFRC),不仅使水泥自身力学性能得到改善,而且可作为防止电磁辐射或核辐射的优良屏蔽体.     

碳纳米管-碳纤维/水泥基材料微观结构和热电性能

研究了碳纳米管-碳纤维/水泥基材料的微观结构以及碳纳米管-碳纤维/水泥基材料升温与降温过程中的热电性能。实验结果表明,当水泥基材料中碳纳米管掺量较低时(碳纳米管掺量占水泥质量百分比不高于0.5%),碳纳米管能有效改善水泥基体性能,密实基体结构。在碳纤维水泥基材料中掺入碳纳米管能有效提高和改善复合材料热电性能;当碳纳米管掺量为水泥质量0.5%,水泥基材料热电势率最多能提高260%,达到22.6μV/℃。与此同时,碳纳米管掺入能增强热电效应中温差电动势与温差关系的线性规律和可逆性规律。     

环境湿度对玻璃纤维增强水泥力学性能的影响

通过开展在不同龄期、不同环境湿度下玻璃纤维增强水泥(GRC)试件的抗折强度、抗压强度试验和基体pH值测定,研究了环境湿度对掺加粉煤灰和硅灰等活性矿物掺合料的GRC试件力学性能的影响。结果表明:环境湿度对GRC试件的抗折强度有重要影响,相对湿度越大,随着龄期增加, GRC试件抗折强度降低越严重;在温度60℃、相对湿度95%条件下,经过56 d龄期后,掺有40%粉煤灰和10%硅灰的GRC试件抗折强度比未掺加粉煤灰和硅灰的GRC试件的抗折强度提高48.5%、抗压强度提高23.6%, GRC基体pH值降低6%。在相同的湿度条件下,掺有粉煤灰和硅灰试件的pH值在各个龄期都低于普通硅酸盐水泥试件,说明粉煤灰和硅灰的掺入能降低水泥水化液相的碱度,进而延缓了纤维受侵蚀的速度,显著改善了GRC试件的力学及耐久性能。通过对试验结果进行分析,利用MATLAB软件建立了GRC试件抗折强度和抗压强度与水泥砂浆基体pH值及时间的关系式。      

Properties of strain hardening ultra high performance fiber reinforced concrete (UHP-FRC) under direct tensile loading

Enhanced matrix packing density and tailored fiber-to-matrix interface bond properties have led to the recent development of ultra-high performance fiber reinforced concrete (UHP-FRC) with improved material tensile performance in terms of strength, ductility and energy absorption capacity. The objective of this research is to experimentally investigate and analyze the uniaxial tensile behavior of the new material. The paper reviews and categorizes a variety of tensile test setups used by other researchers and presents a revised tensile set up tailored to obtain reliable results with minimal preparation effort. The experimental investigation considers three types of steel fibers, each in three different volume fractions. Elastic, strain hardening and softening tensile parameters, such as first cracking stress and strain, elastic and strain hardening modulus, composite strength and energy dissipation capacity, of the UHP-FRCs are characterized, analyzed and linked to the crack pattern observed by microscopic analysis. Models are proposed for representing the tensile stress–strain response of the material.     

紫外辐照对玻纤增强树脂基复合材料冲蚀性能的影响

为了探讨紫外老化对玻璃纤维增强环氧乙烯基酯树脂基复合材料冲蚀性能的影响,对玻璃纤维增强环氧乙烯基酯树脂基复合材料进行人工紫外加速老化试验,并对不同老化时间下试样进行冲蚀;采用扫描电镜、红外光谱仪及差式扫描仪研究了老化前后材料表面形貌、化学结构以及玻璃化转变温度的变化,并分析了不同老化时间对试样巴氏硬度、质量损失率及冲蚀失重率的影响.结果表明：随着老化时间的增加试样表面发黄现象逐渐加剧,并出现树脂基体的开裂、剥离及纤维暴露;紫外老化7 d后试样的抗冲蚀性能提升了5.3%、巴氏硬度上升3%,但老化56 d后试样抗冲蚀性能下降了15.1%、巴氏硬度下降14.6%;老化56 d后试样玻璃化转变温度上升9.3 ℃.紫外老化是一个由表及里的过程,首先造成表层树脂性能的下降;紫外老化引起树脂基体的分解、玻璃纤维与树脂基体界面的脱黏、树脂基体的脆化,进而导致材料抗冲蚀性能的下降.     

Influence of reinforcing bar type on autogenous shrinkage stress and bond behavior of ultra high performance fiber reinforced concrete

This study investigated the effects of reinforcing bar type and reinforcement ratio on the restrained shrinkage behaviors of ultra high performance fiber reinforced concrete (UHPFRC), including autogenous shrinkage stress, degree of restraint, and cracking potential. In addition, the influence of the type and embedment length of reinforcing bars on the bond behavior of UHPFRC was evaluated by performing pullout test. Three different reinforcing bars (deformed steel bar, round steel bar, and GFRP bar) were investigated in the restrained shrinkage and pullout tests. The GFRP bar exhibited the best performance in relation to the autogenous shrinkage stress, degree of restraint, and cracking potential because of its low stiffness. The highest bond strength was obtained for the deformed steel bar, and the bar yielding was observed when the bar embedment length of l_b = 2d_b was used. The round steel bar exhibited the poorest behaviors for both of the restrained shrinkage and pullout.