纤维混凝土的应用研究

通常队为,混凝士强度越高,其韧性越差,脆性越高,结构廷性和抗裂能力越不足,给结构抗震性能带来的安全隐患。在混凝土中掺入微细纤维的纤维混凝土不仅能够提高混凝土本身的延性和韧性,增强混凝土的抗裂性,而且对原材料加工几乎没有特别的要求,成本增加不大甚至还有可能降低。聚丙烯纤维具有耐化学腐蚀性强、强度高、加工性好、质轻、蠕变收缩小、价格低廉和在低掺量下对混凝土的抗裂、增韧效果显著等优良的技术经济性能,因而在建筑工程中得到越来越广泛的应用。聚丙烯纤维混凝土是一种掺入少量短切聚丙烯纤维来增强或改善混凝土性能的复合材料。聚丙烯纤维混凝土在国外应用较多,在我国的工程中应用也日渐增多。低掺量的聚丙烯纤维掺人混凝土后可显著控制混凝土早期收缩裂缝,提高混凝土的耐久性。     

水泥混凝土道面早期抗裂性能试验研究

针对传统的普通道面混凝土施工后经常出现裂缝的现象,分析了裂缝产生的原因,提出在普通道面混凝土中掺加粉煤灰、聚丙烯纤维的新型道面混凝土,采用平板法进行了抗裂性能对比试验。结果表明：在普通道面混凝土中单掺粉煤灰或聚丙烯纤维,都能提高混凝土的抗裂性,而粉煤灰和聚丙烯纤维双掺的新型道面混凝土更能有效防止裂缝的发生,显著延长道面使用寿命。     

高性能混凝土抗裂性能研究

该文对高性能混凝土的抗裂性能进行了系统的研究,主要包括:高性能混凝土收缩与抗裂性能的试验方法,研制出一种测试混凝土抗裂性能的诱导开裂试验方法,该方法避免了混凝土裂缝出现位置的随机性以及混凝土塑性沉降引起的材料不均匀等带来的测试结果不准确,同时研制出一种多点、实时、自动监测混凝土试件开裂时间的系统,可应用于测试混凝土早期的裂缝出现时间;进行了大量自生收缩试验和干燥收缩试验,对影响高性能混凝土自生收缩和干燥收缩的因素进行了深入的研究,提出了减少收缩变形的混凝土原材料选用和粉煤灰及矿渣单掺与多掺的最佳掺量,提出了高性能混凝土自生收缩和干燥收缩计算模型;采用平板诱导开裂法和圆环测试系统进行了大量的抗裂性能试验,深入研究了混凝土原材料特别是粉煤灰及矿渣影响高性能混凝土抗裂性能的规律,提出了实现高抗裂性能的混凝土原材料选用和粉煤灰及矿渣单掺与多掺的最佳掺量;研究成果成功地应用于实际工程中,取得了显著的技术和经济效益。     

聚丙烯纤维增强混凝土研究进展

聚丙烯纤维具有耐化学腐蚀、加工性好、质轻、蠕变收缩小、价格低廉和在低掺量时对混凝土的塑性干缩开裂改善效果显著、抗渗性好、防止火灾中混凝土爆裂等优良的技术经济性能。但同时也存在一些缺点,如随着纤维掺量的增加,混凝土工作性降低、弹性模量低、增韧效果差以及低掺量下对混凝土的强度(抗拉、抗压、抗折)无显著影响等等。综合论述了改善聚丙烯纤维增强混凝土目前的研究进展,展望了其发展趋势。     

纤维对薄层砂浆早期塑性收缩的影响

提出了一种利用视频显微镜技术来测量薄层砂浆早期塑性收缩的新方法,研究了纤维种类、聚丙烯纤维掺量及长度、纤维几何形态对薄层砂浆早期塑性收缩的影响。结果表明随着聚丙烯纤维掺量的增加,砂浆的塑性收缩值有所减小,说明聚丙烯纤维的掺入可以有效地抑制砂浆的塑性收缩;3mm、12mm长的聚丙烯纤维较6mm、9mm长的聚丙烯纤维对砂浆塑性收缩的抑制作用大;在抑制砂浆收缩性能上圆形截面纤维优于Y形截面纤维;掺聚丙烯纤维试样的塑性收缩较掺玄武岩纤维试样小,但两者的差距不大。     

玄武岩纤维对混凝土梁抗裂性能的影响

为研究玄武岩纤维对混凝土梁抗裂性能的影响,以纤维长度及纤维体积掺率为变化参数,对纤维长度分别为12mm和30mm,纤维体积掺率分别为0.1%和0.2%的4根纤维混凝土梁和1根对比梁进行静载试验,试验中对纤维混凝土梁开裂荷载、裂缝宽度以及挠度进行监测。结果表明:与普通钢筋混凝土梁对比,玄武岩纤维混凝土梁的开裂荷载显著增大,且裂缝宽度发展更为缓慢,相同荷载作用下的裂缝宽度和跨中挠度显著减小,其主要原因是玄武岩纤维改善了混凝土梁的抗裂和阻裂性能,提高了梁的整体刚度。     

复合型补偿收缩混凝土在高层建筑地下室外墙裂缝防治中的应用

混凝土裂缝问题是混凝土工程中一个相当普遍的技术难题,本文就复合型补偿收缩混凝土在高层建筑地下室外墙裂缝防治中的应用做了简要叙述。介绍了补偿收缩混凝土和聚丙烯改性纤维的应用特点,在高层地下室外墙混凝土使用时,配制掺加聚丙烯改性纤维的复合型补偿收缩混凝土,充分发挥了聚丙烯改性纤维抗裂防渗的物理作用和膨胀剂膨胀抗裂的化学作用。     

聚丙烯纤维在厦门某工程地下室混凝土中的应用及其作用分析

本文阐述了一种新型的合成纤维--聚丙烯纤维在混凝土中的应用效果。实践表明,聚丙烯纤维掺加在混凝土中可以起到阻裂和细化裂缝的作用,明显地改善了混凝土的微观结构,使混凝土中原生的微裂纹减少、裂缝宽度减小,这必然使混凝土硬化体的抗渗性和韧性得到相当程度的提高,使混凝土表面光滑,进而使混凝土的耐久性得到大幅度的提高。使得聚丙烯抗裂纤维混凝土被广泛应用在大面积地下筏板、侧墙、顶板及道路面层、水工混凝土之中。     

大掺量高吸附性石粉高强机制砂混凝土收缩开裂抑制试验

针对大掺量高吸附性石粉高强机制砂混凝土早期易发生收缩开裂的问题,开展了掺加膨胀剂、减缩剂、聚乙烯醇(PVA)纤维、高吸水树脂(SAP)混凝土的早期收缩、抗裂和力学试验,并利用核磁共振仪和扫描电子显微镜对混凝土孔结构和微观形貌进行了测试,揭示抑制收缩开裂的机制。试验结果表明:掺入膨胀剂、减缩剂、PVA纤维、预吸水SAP均能有效抑制混凝土早期收缩和开裂,其中掺预吸水SAP降低早期收缩的效果最好,收缩应变可降低93%,PVA纤维抑制早期开裂的作用最明显,总开裂面积可降低68.26%,同时,掺入两者会使混凝土力学性能提高,而掺入膨胀剂和减缩剂会使混凝土力学性能降低。SEM图像表明,SAP能将预吸收的水分释放到混凝土中,促进水泥水化反应,而PVA纤维的掺入改善了混凝土内部孔隙结构,有良好的填充作用和桥接作用。核磁共振试验表明,抑制收缩开裂的机制是通过改善混凝土孔隙结构,增强界面密实度,从而减小早期收缩,进而提高抗裂性能。      

纤维对开裂后混凝土渗透性及裂缝恢复的影响

通过劈裂试验和渗透试验,研究了结构型钢纤维、聚丙烯粗纤维和聚丙烯细纤维对开裂后混凝土的裂缝恢复率、劈裂韧性和渗透系数的影响。研究结果表明:钢纤维和聚丙烯粗纤维的掺入可限制裂缝扩展,使混凝土由脆性破坏转为韧性破坏,提高开裂混凝土在卸载后裂缝的恢复作用,显著减小开裂后混凝土的渗透系数。钢纤维掺量越高,裂缝恢复和渗透性降低越明显,钢纤维掺量由25kg/m~3增加至55kg/m3时,渗透系数减小了87%。钢纤维和聚丙烯粗纤维的掺入具有较好的正混杂效应,当裂缝宽度为150μm时卸载,单掺25kg/m~3钢纤维和4kg/m~3聚丙烯粗纤维与单掺35kg/m~3钢纤维相比,渗透系数减小了60%。而聚丙烯细纤维对开裂混凝土的裂缝恢复和渗透性影响较小。     

Mechanism for tensile strain hardening in high performance cement-based fiber reinforced composites

The mechanism responsible for the improvement in tensile strain capacity of FRC (fiber reinforced concrete) as a result of the addition of high volume fraction of discontinuous fibers was investigated, using energy changes associated with cracking. The energy terms considered include: matrix fracture energy, matrix strain energy. debonding energy, fiber strain energy and fiber frictional energy.

Assuming that the first observed crack is also the failure crack, it was found that multiple cracking occurs in high performance FRC. In such composites the energy needed to open the critical cracks exceeds the energy needed to form a new crack. The analysis predicts that the major energy term determining this behavior is the fiber debonding energy.

Multiple cracking was observed in fiber reinforced small densified DSP (particles) containing a high volume fraction (higher than 3%) of fine and short steel fibers. Because crack localization did not occur during multiple cracking, very large increases in total strain capacity were achieved with increasing fiber volume fraction. At 12% fiber volume fraction, a total strain capacity of about 0·2% was measured from flexures tests; an increase of about 15 to 20 times over that of the plain matrix.     

水工混凝土弥散型裂缝数值模型中开裂判据的研究

重大水利工程中,混凝土结构多处于高水压环境,坝体或多或少存在着裂缝,然而大部分开裂的混凝土坝仍能够安全运行,从而引申出是否可以允许混凝土结构出现裂缝,多大的裂缝才不至于引起破坏,以及用怎样一个合理的指标来对混凝土开裂程度进行描述评判的问题。该文以等效塑性应变表征混凝土弥散型裂缝模型数值模拟中的开裂破坏,通过混凝土单轴拉伸数值模拟,并根据试验资料及相关文献,建立了等效塑性应变与主拉应变、裂缝宽度之间的关系,得到对应于“有害裂缝”的等效塑性应变值。将方法和成果运用到混凝土重力坝的开裂研究中,具有较好的实际工程意义。     

改性聚丙烯纤维对发泡水泥性能的影响

采用丙烯酸化学接枝法对聚丙烯纤维进行表面改性, 研究了改性聚丙烯纤维对发泡水泥塑性收缩开裂、 力学性能及泡孔结构的影响。结果表明, 改性聚丙烯纤维可改善发泡水泥的泡孔结构, 并降低其塑性收缩开裂、 细化其塑性收缩裂缝, 同时可提高其抗折、 抗压强度及弯曲韧性。纤维与水泥的质量比为0.7%时, 试样的泡孔结构明显改善, 塑性收缩开裂值下降了85.4%, 且缝宽小于1 mm的塑性收缩裂缝比例高达73.1%, 同时试样抗折及抗压强度分别增加48.8%和30.3%, 弯曲韧性显著增加。利用傅里叶变换红外光谱仪、 SEM、 光学显微镜对改性前后聚丙烯纤维表面基团及发泡水泥试样的断面微观形貌、 泡孔结构进行了分析, 探讨了改性聚丙烯纤维的作用机制。     

Strain rate dependent properties of ultra high performance fiber reinforced concrete (UHP-FRC) under tension

The results of an experimental investigation of UHP-FRC tensile response under a range of low strain rates are presented. The strain rate dependent tests are conducted on dogbone specimens using a hydraulic servo-controlled testing machine. The experimental variables are strain rate, which ranges from 0.0001 1/s to 0.1 1/s, fiber type, and fiber volume fraction. Five different types of fibers are considered including straight and twisted fibers with different geometric properties. The rate sensitivity of the composite material in tension is evaluated in terms of its first cracking strength, post-cracking strength, energy absorption capacity, strain capacity, elastic modulus, fiber tensile stress and number of cracks. The test results show pronounced rate effects on post-cracking strength and energy absorption capacity. Further, post cracking strength varies linearly with the fiber reinforcing index and energy absorption capacity varies linearly with the product of the fiber length and the reinforcing index, as predicted from the theory for fiber reinforced concrete.     

Size and geometry dependent tensile behavior of ultra-high-performance fiber-reinforced concrete

This research investigated direct tensile stress versus strain response of ultra-high-performance fiber-reinforced concrete (UHPFRC) with various sizes and geometries. The UHPFRC in this research contained 1% macro twisted and 1% micro smooth steel fibers by volume. The effects of gauge length, section area, volume and thickness of the specimens on the measured tensile response of the UHPFRC were experimentally discovered. The different sizes and geometries of specimens did not generate significant influence on the post cracking strength of UHPFRC whereas they produced clear effects on the strain capacity, energy absorption capacity and multiple cracking behavior of UHPFRC. The strain capacity, energy absorption capacity and the number of multiple micro cracks within unit length obviously decreased as the gauge length, section area and volume of UHPFRC specimens increased. In contrast, as the thickness of the specimen increased, different tendency was observed.     

火灾下钢筋混凝土简支板力学响应机理

拉力膜理论针对的是板大变形下开裂状态的研究,对于火灾下板发生大变形尚未开裂的机理缺乏合理的解释。为此该文采用合理的材料热工性能和热-力耦合本构关系,应用ABAQUS有限元软件对火灾下钢筋混凝土简支单向板和双向板的温度场和热力耦合场进行了三维实体有限元分析,在实验验证的基础上深入探讨火灾下混凝土和钢筋的应力变化规律以及钢筋混凝土简支板的力学响应机理。分析结果表明：简支板发生了激烈的应力重分布现象,其变形过程经历弹性、弹塑性、塑性以及受拉开裂四个阶段,在弹塑性和塑性阶段板底混凝土双向受压的倒拱效应使得双向板抗火性能优秀;当火灾下钢筋混凝土简支板实验没有达到力学响应的受拉开裂阶段时,板底直接受火区域不会出现开裂现象;与单向板相比,双向板进入塑性与受拉开裂阶段的时间大幅度延长,且塑性与受拉开裂阶段变形速率减缓,因此其抗火性能更优。     

Development of sandwich panels combining Sisal Fiber-Cement Composites and Fiber-Reinforced Lightweight Concrete

This research proposes the development of an innovative structural panels based on the use of thin outer layers of Sisal Fiber-Cement Composites (SiFCC) together with a core layer of Polypropylene Fiber-Reinforced Lightweight Concrete (PFRLC).The influence of sisal fibers was studied in two different ways, short sisal fibers (50 mm) randomly distributed in the matrix, and long unidirectional aligned sisal fibers (700 mm) applied by a cast hand layup technique. Lightweight aggregates and polypropylene fibers were used in the concrete layer forming the panel's core in order to reduce its density and improve its post-cracking tensile strength and energy absorption capacity.The behavior of the sandwich panels in four-point bending test is described, and the various failure mechanisms are reported. Mechanical properties of both SiFCC and PFRLC were obtained, which were also used in the numerical simulations. Pull-off tests were performed to evaluate the bond strength between the outer SiFCC layers and the core PFRLC. The results revealed that the long sisal fibers were more effective in terms of providing to the panel higher flexural capacity than when using short sisal fibers, long fibers ensured the development of a deflection hardening behavior followed by the formation of multiple cracks, while short sisal fibers promoted a softening response after cracking.     

Reduction of fire spalling in high-performance concrete by means of superabsorbent polymers and polypropylene fibers: Small scale fire tests of carbon fiber reinforced plastic-prestressed self-compacting concrete

High-performance concrete (HPC) is prone to explosive spalling when exposed to fire, which may lead to failure of the concrete elements. Polypropylene fibers (PP) are often added to HPC, as upon their melting they create channels through which water vapor is evacuated, preventing the build-up of high vapor pressures. In self-compacting HPC (HPSCC), the amount of PP fibers needs to be limited in order to keep the self-compacting properties, which may reduce the fire resistance.In this paper, a novel strategy to reduce fire spalling in HPSCC is illustrated, based on adding small particles of superabsorbent polymers (SAP) during mixing. The SAP end up as empty macropores, similar to air voids, in the HPSCC matrix. The PP fibers-SAP voids system percolates at a lower fiber loading than the fibers alone, allowing maintenance of the self-compacting properties while reducing substantially the fire spalling. In particular, in this paper it is shown how addition of SAP is able to reduce fire spalling in thin-walled HPSCC slabs prestressed with carbon fibre reinforced plastic reinforcement.     

自密实钢纤维轻骨料混凝土的早期性能与损伤分析

在自密实轻骨料混凝土基础之上掺入钢纤维配制出自密实钢纤维轻骨料混凝土,分析了自密实钢纤维轻骨料混凝土的抗压强度、抗拉强度等主要力学性能以及收缩、抗碳化等耐久性能,并与普通骨料自密实混凝土进行对比分析。探讨了钢纤维对于改善自密实轻骨料混凝土损伤所起的作用及其机理。结果表明:掺入钢纤维后自密实轻骨料混凝土的抗压强度增大,劈拉强度明显提高,收缩及抗碳化能力也有明显改善。与普通骨料混凝土相比,自密实钢纤维轻骨料混凝土初始裂缝的产生与发展得到有效抑制。     

Experimental and analytical study of SIMCON tension members

The strength and ductility of slurry infiltrated mat concrete (SIMCON) tension members were investigated both experimentally and analytically to construct a mechanical model for simulating tensile force–displacement relationships. In addition to standard strength testing, special tests were conducted on tension specimens with preset cracks to determine the interaction between steel fibers and the cement matrix near an opening crack. These tests were conducted on two sets of preset-crack specimens: (i) with symmetrically inclined fibers and (ii) with aligned fibers having variable debonded lengths on each side of the crack. Using measured bridging forces of inclined fibers, an efficiency factor of plane random fibers, compared to aligned fibers, was determined to be approximately 0.58. It was found that the ductility of SIMCON mainly stems from plastic deformation of steel fibers rather than fiber pull-out. SIMCON tensile response was characterized by elastic, nonlinear hardening and softening regimes. The hardening response was notch insensitive without multiple crack formation. In the elastic regime, only minute stiffness reduction was observed. The nonlinear hardening regime was characterized by internal damage growth without visible crack formation and ended with the appearance of a co-linear set of partial cracks. The softening regime was described by a localized failure of fibers with variable failure strains at the co-linear cracks. Based upon the experimental observation that a co-linear set of partial cracks form at the ultimate composite stress, upper and lower bounds of the SIMCON stress–strain relation in the hardening regimes were obtained.