两类合成纤维在混凝土中的作用及使用效果(下)

按合成纤维在混凝土中所起的作用,可分为两大类别,即防裂纤维与増韧纤维。前者主要用于防止或减少混凝土的早期收缩裂缝并降低裂缝的尺度,有助于降低混凝土的渗透性与提高抗反复冻融性。后者主要用于提高混凝土的变形能力,有助于减少混凝土的干缩裂缝、增进混凝土的韧性、抗冲击性与抗疲劳性。二者均起着次要增强材的作用。作者分别论述了这两类纤维在混凝土中的作用机制,对混凝土物理、力学性能与耐久性的影响及使用效果。     

两类聚烯烃纤维在混凝土中的作用机制及应用

用于混凝土的聚烯烃纤维按原料与纤维尺度可分为聚丙烯细纤维与聚烯烃粗纤维两类.前者主要用于防止或减少混凝土的早期收缩裂缝,并可防止混凝土在火灾中发生爆裂.后者主要用于提高混凝土的变形能力,增进混凝土的韧性、抗冲击性与抗疲劳性.文章分别介绍了两类聚烯烃纤维的特性、主要品种以及在混凝土中的作用机制,并列举了在国内外混凝土工程中的应用实例.     

浅谈阻裂纤维混凝土在地下室防水工程中的应用

通过对掺加聚丙烯纤维配制的阻裂纤维混凝土性能的研究、讨论在"山水雅居"工程中的实际应用,表明阻裂纤维混凝土能够有效地减少混凝土的早期泌水,降低混凝土中的孔隙率,并且减少混凝土的早期干缩、塑性裂缝,阻止混凝土发生沉降裂缝,因而能较大幅度地提高混凝土的抗渗性、抗裂性,尤其是有效地抑制了连通裂缝的产生,提高结构的抗裂防水能力和耐久性.     

纤维和膨胀剂的混凝土路面开裂及力学性能研究

聚丙烯纤维和膨胀剂被用于混凝土以提高其收缩裂缝性和机械性能。混凝土试件用聚丙烯纤维或膨胀剂或两者准备。标本的形态是通过扫描电子显微镜来观测的,第一裂纹发生的时间、混凝土的机械性能如压缩强度和耐冲击能是通过拍击检验记录和测量的。结果表明,混凝土中的聚丙烯纤维可降低收缩率和延缓初裂,明显提高耐冲击性,并略微提高抗压强度。膨胀剂可以补偿收缩,明显减少混凝土路面裂缝,并略微提高混凝土路面的力学性能。这项研究为今后机场混凝土路面开裂的控制提供了建议。     

透水性聚丙烯纤维混凝土的耐久性研究

通过试验对比分析表明:掺人聚丙烯纤维后,试件的质量和强度损失率几乎为零;聚丙烯纤维有助于抑制和减少微裂缝的产生和发展,提高了透水性混凝土的抗冻融性能.同时借助有限元软件ANSYS分析了寒潮来袭时透水性聚丙烯纤维混凝土的温度场和温应力场变化情况,并得出可以通过改进材料,降低透水性混凝土的热交换系数,从而降低寒潮来袭时的温度应力,以提高透水性聚丙烯纤维混凝土的耐久性.     

聚丙烯纤维对混凝土早期开裂性能影响

纤维混凝土中加入的合成材料纤维丝,可增强塑性混凝土的抗拉能力,显著降低其塑性收缩微裂纹。这种减少或消除塑性裂纹可使混凝土获得最佳的长期整体性。本文经过试验研究与工程实践证明纤维可以防止收缩裂缝,亦可最大限度减小在同强度状态下混凝土可能出现裂缝的宽度和长度,可成倍增加抗塑性沉陷裂缝的作用。     

聚丙烯纤维快硬混凝土抗渗性能研究

为研究聚丙烯纤维对快硬混凝土抗渗性能的改善作用,将不同复合掺量以及不同纤维掺量的快硬混凝土进行抗渗对比试验。试验表明,10%硅灰、粉煤灰的复合掺合料快硬混凝土的抗渗性较空白组有所降低,抗渗系数提高了3倍,聚丙烯纤维的加入使得快硬混凝土的抗渗性能得到了提高,0.2%的纤维掺量能使抗渗系数降低至空白组的62.4%。说明适量聚丙烯纤维的掺入能够发挥抑制效果,减缓裂缝发展趋势,减少混凝土中的渗水通道,提高了混凝土的抗渗能力。最后通过建立抗渗模型,从机理上分析了纤维对混凝土抗渗性能的影响。     

玄武岩纤维混凝土抗盐冻性能试验研究

为了研究玄武岩纤维混凝土的抗盐冻性能,以纤维体积率、冻融循环次数为主要变化参数,在3.5%NaCl溶液中对玄武岩纤维混凝土进行了快速冻融试验。研究了不同纤维掺量和不同冻融循环次数下混凝土的质量损失率、相对动弹性模量、抗压强度和抗折强度的变化规律;采用扫描电镜对混凝土盐冻循环前后的微观形貌进行观察,分析玄武岩纤维对混凝土抗盐冻性能的影响机理。结果表明:在盐冻循环作用下,玄武岩纤维的掺入能够有效降低混凝土的质量损失率,减缓其相对动弹性模量的降低,而且能减弱冻融损伤对混凝土抗压、抗折强度的影响;适量玄武岩纤维的掺入能抑制混凝土中裂缝的扩展,减少基体内孔隙、坑洞的数量,延迟初始裂缝和相互贯通裂缝的出现,抗盐冻能力优于普通混凝土。     

现代混凝土增韧防裂原理及应用

从水泥基体和增强纤维两个方面综述混凝土增韧防裂的原理,分析增韧防裂技术的影响因素和作用效果,介绍了典型应用工程。从分子、微观和细观三个层次,阐述了纳米材料、有机高分子材料和聚合物对水泥基体增韧的作用机制。从纤维 基体界面作用力、裂缝形成和非稳态扩展抑制等方面,论述纤维种类和特性对混凝土硬化前后阶段增韧防裂的作用规律。基体增韧与纤维增韧相结合,能够提高混凝土的抗拉强度、拉伸应变和断裂能,并有效降低裂缝宽度,抑制裂缝扩展。     

纤维种类对水工混凝土抗裂性能的影响

本文采用混凝土平板开裂试验研究两种不同聚丙烯纤维和玻璃纤维对水工混凝土抗裂性能的影响。试验结果表明,纤维的掺入可延迟初始裂缝的出现时间,减小初始和最终裂缝长度、宽度以及裂缝数目,明显改变水工混凝土早期收缩抗裂性,提高水工混凝土抗裂等级。它们对混凝土开裂性提高幅度为:聚丙烯纤维Ⅰ>聚丙烯纤维Ⅱ>玻璃纤维。     

Influence of recycled polyethylene terephthalate fibres on plastic shrinkage and mechanical properties of concrete

Polyethylene terephthalate bottles production has drastically increased year after year due to high versatility of polyethylene terephthalate plastics and considerable consumption of beverages. In tandem with that increase, the major concern of society has been the improper disposal of this non-biodegradable material to the environment. To deal with this concern, recycled polyethylene terephthalate bottles were incorporated in concrete as fibre reinforcements in this study. The objective of this research is to evaluate the mechanical properties of recycled polyethylene terephthalate fibre reinforced concrete (RPFRC) in comparison with control concrete without fibres. polyethylene terephthalate fibres with three different diameters (0.45, 0.65, and 1.0 mm) and two lengths (20 and 30 mm) were added at various proportions (0.5%, 1.0%, 1.5% and 2.0%) by volume of concrete in order to determine the effect of fibres initially on compressive, flexural and splitting tensile strengths of concrete. The results revealed that none of the fibres have detrimental effects up to 1% volume fraction, however further addition caused slight reductions on mechanical properties in some conditions. Plastic shrinkage resistance and impact resistance tests were also performed according to related standards. Polyethylene terephthalate fibres were observed to have marked improvements on those properties. Such a good performance could be attributed primarily to the bridging effect of fibres.     

Effect of fibres on early age cracking of concrete tunnel lining. Part II: Numerical simulations

The early-age cracking of concrete structures increases permeability and diffusivity and moreover accelerates the penetration of liquid, gas and aggressive agents. Consequently, the serviceability of these structures could be reduced drastically. Early-age cracking might be due to external loading, but also to the internal or external restraint resulting from autogenous, drying and thermal shrinkage. This study focuses more specifically on these latter phenomena.In the first part of this study (see effect of fibres on early-age cracking of concrete tunnel lining - Part I: Laboratory testing), ring tests were performed to investigate the sensitivity of concrete to cracking due to both shrinkage strain and type of fibre (two organic fibres and one steel fibre were studied).Ring test results were then used to validate the capacity of a chemo-thermo-viscoelastic damage model aimed at reproducing the complex behaviour of fibre-reinforced concrete subjected to restrained shrinkage through identifying the material parameters with standardised tests. The numerical simulations conducted on a real tunnel lining show that for the studied geometries and concrete mixtures, thermal shrinkage constitutes the major phenomenon capable of causing early-age transverse cracks and moreover crack opening is highly dependent on the type of reinforcement. Modifications to both fibre type and lining thickness may serve to avoid the onset of transverse cracks.     

Efficiency of polypropylene and metallic fibres on control of shrinkage and cracking of recycled aggregate mortars

The use of recycled aggregates in mortar and concrete induces a large shrinkage due to the capacity of these aggregates to absorb a great quantity of water. The influence of polypropylene and metallic fibres (Fibraflex) has been studied in different proportioning to reduce shrinkage and cracking. The efficiency of polypropylene and metallic fibres to reduce restrained shrinkage cracking is evaluated by using a ring mortar cast around a stiff steel ring. By using fibres, the crack width decreases significantly. Free shrinkage results are equally investigated. Finally, crack width in reinforced fibre mortars is calculated analytically by using the model of Grzybowski and Shah. Results are compared with experimental values. The study demonstrates that theoretical values of crack widths correspond reasonably to experimental values.     

Effect of fibres on early age cracking of concrete tunnel lining. Part I: Laboratory ring test

The cracking of concrete structures can drastically reduce their serviceability, specifically by inducing a decrease in bearing capacity while their permeability and diffusivity are being increased. Cracking therefore leads to a faster penetration of liquid, gaseous and aggressive agents, which may reduce the durability and the tightness of the structures. It may be caused by an external loading or else by the self- or external restraint of autogenous, drying and thermal shrinkage. This paper will focus on the latter of these phenomena.In the first part of this study (part I: Laboratory test), ring tests are performed to investigate the sensitivity of concrete to cracking due to both shrinkage strain and fibre type (two organic fibres and one steel fibre have been studied). Results obtained show that the use of polypropylene microfibres does not delay the age at which the first crack appears but does slightly reduce crack opening. Polypropylene and steel macrofibres have a greater effect since cracking is delayed and the crack opening is significantly reduced. Tensile strength results indicate that a portion of the cracking delay may be attributed to the tensile strength increase resulting from fibre reinforcement.     

High Strength Polypropylene Fibre Reinforcement Concrete at High Temperature

Concrete is an inherently brittle material with a relatively low tensile strength compared to compressive strength. Reinforcement with randomly distributed short fibres presents an effective approach to the stabilization of the crack and improving the ductility and tensile strength of concrete. A variety of fibre types, including steel, synthetics, and natural fibres, have been applied to concrete. Polypropylene (PP) fibre reinforcement is considered to be an effective method for improving the shrinkage cracking characteristics, toughness, and impact resistance of concrete materials. Also, the use of PP fibre has been recommended by all of the researchers to reduce and eliminate the risk of the explosive spalling in high strength concrete at elevated temperatures. In this study, constitutive relationships are developed for normal and high-strength PP fibre reinforcement concrete (PPFRC) subjected to high temperatures to provide efficient modelling and specify the fire-performance criteria for concrete structures. They are developed for unconfined PPFRC specimens that include compressive and tensile strengths, elastic modulus, modulus of rupture, strain at peak stress as well as compressive stress–strain relationships at elevated temperatures. The proposed relationships at elevated temperature are compared with experimental results. These results are used to establish more accurate and general compressive stress–strain relationships prediction. Further experimental results for tension and the other main parameters at elevated temperature are needed in order to establish well-founded models and to improve the proposed constitutive relationships, which are general, rational, and fit well with the experimental results.     

合成纤维混凝土的性能及其工程应用

尼龙、维纶及聚丙烯纤维混凝土具有较好的抗裂性能，可提高混凝土工程的使用功能及耐久性，已成功担在８万人体育场、地铁工程及东方明珠电视塔等工程中推广应用。实践证明，它能控制裂缝的开展，避免面层起壳，是提高工程质量的有效措施。     

Post-cracking behaviour of steel and macro-synthetic fibre-reinforced concretes

The present paper describes the results of an experimental investigation on the performances of concrete specimens reinforced with either steel or macro-synthetic fibres under three-point bending. Steel fibres are often used to improve the flexural toughness of concrete and are used in various structural applications while synthetic fibres are more often used to reduce crack opening due to shrinkage. Macro-synthetic fibres have been proposed more recently with the aim of creating an alternative to steel fibres in structural applications but their use is still limited.In the tests performed, specimens cast with the same concrete mix, but containing different dosages of either steel or macro-synthetic fibres, were used and compared. In general steel fibres were more efficient in increasing the toughness of concrete than macro-synthetic fibres, even though their results were significantly more scattered.Test results were used to calculate the parameters of stress-crack opening relations via inverse analysis using a cracked hinge model. This numerical model provided results, in terms of force-Crack Mouth Opening Displacement (CMOD) curves, in very good agreement with the experimental data.     

高性能混凝土约束收缩对氯离子扩散的影响

对粉煤灰和矿渣粉双掺总量占胶凝材料总质量的百分比分别为30%,40%和50%及掺合比例分别为1∶2,2∶3和1∶1的高性能混凝土进行了平板收缩试验和快速氯离子扩散系数测定（RCM法）试验,分析了高性能混凝土有约束和无约束收缩开裂性能及对氯离子扩散系数的影响规律。研究结果表明:无约束的混凝土平板均未发生开裂,有约束的混凝土平板基本开裂,双掺矿物掺合料显著改善了混凝土的抗裂性能,当粉煤灰和矿渣粉的掺合比例为1∶1时,其抗裂性能最好,且随着掺量的增加,混凝土的抗裂性越好;矿物掺合料双掺总量和掺合比例一定时,各配比约束收缩混凝土的等效氯离子扩散系数均高于无约束收缩混凝土的氯离子扩散系数,即约束收缩增大了混凝土的氯离子扩散系数,收缩应变越大,混凝土的抗氯离子性能越差;当粉煤灰和矿渣粉掺合比例为1∶1时,约束收缩对混凝土内氯离子扩散的影响最小。     

混凝土抗裂材料应用研究

通过对聚丙烯腈纤维、聚丙烯酸酯乳液、膨胀剂、减缩剂几种混凝土抗裂材料进行抗裂及收缩性能研究,表明聚丙烯腈纤维、聚丙烯酸酯乳液在混凝土塑性阶段有显著阻裂及细化裂缝作用,而膨胀剂、减缩剂对混凝土干缩及自收缩有显著抑制作用,减小了混凝土后期收缩开裂的可能性.此实验结果可供设计和施工界参考.