再生钢纤维增韧超高性能混凝土的力学性能

采用来自于废旧轮胎的两种再生钢纤维制备含粗骨料的超高性能混凝土,并测定其抗压强度、劈裂抗拉强度、断裂能和静弹性模量等力学性能,空白组及普通钢纤维增韧超高性能混凝土作对比性能试验。结果显示,未附着橡胶颗粒的再生钢纤维使超高性能混凝土的抗压强度略微下降,降低幅度为3.91%,其余各类型钢纤维均有利于提高超高性能混凝土的力学性能;而附着橡胶颗粒的再生钢纤维显著提高了超高性能混凝土的断裂能,约为普通钢纤维增韧超高性能混凝土的4倍。此外,再生钢纤维对超高性能混凝土的劈裂抗拉强度和静弹性模量的提高效果均优于普通钢纤维。再生钢纤维,尤其是附着橡胶颗粒的再生钢纤维,可以作为一种增韧材料替代普通钢纤维应用到超高性能混凝土工程结构中。      

Material and bond properties of ultra high performance fiber reinforced concrete with micro steel fibers

For investigating the effect of fiber content on the material and interfacial bond properties of ultra high performance fiber reinforced concrete (UHPFRC), four different volume ratios of micro steel fibers (V_f = 1%, 2%, 3%, and 4%) were used within an identical mortar matrix. Test results showed that 3% steel fiber by volume yielded the best performance in terms of compressive strength, elastic modulus, shrinkage behavior, and interfacial bond strength. These parameters improved as the fiber content was increased up to 3 vol.%. Flexural behaviors such as flexural strength, deflection, and crack mouth opening displacement at peak load had pseudo-linear relationships with the fiber content. Through inverse analysis, it was shown that fracture parameters including cohesive stress and fracture energy are significantly influenced by the fiber content: higher cohesive stress and fracture energy were achieved with higher fiber content. The analytical models for the ascending branch of bond stress-slip response suggested in the literature were considered for UHPFRC, and appropriate parameters were derived from the present test data.     

Experimental evaluation of fiber reinforced concrete fracture properties

Concrete is now universally recognized a construction material vital and essential for the regeneration and rehabilitation of the infrastructure of a country. The last few decades have now shown that high strength concrete, with a compressive strength of 100–120 MPa can be readily designed and manufactured. There have also been several advances made in the development of fiber reinforced concrete to control cracking and crack propagation in plain concrete, and to increase the overall ductility of the material. However, there are now many types of fibers with different material and geometric properties, and the exact fracture behavior of fiber reinforced concrete materials is not clearly understood. The overall aim of this paper is to establish the fracture properties and fracture behavior of concrete containing two widely used types of fibers, namely, steel (high modulus) and polypropylene (low modulus). The experimental investigation consisted of tests on cubes and notched prismatic specimens made from plain concrete and fiber concrete with 1% and 2% of steel or polypropylene fibers. The cube tests and the three point bending tests on notched specimens were carried out according to RILEM specifications, and extensive data on their compressive and flexural tensile behavior and fracture energy were recorded and analyzed. The results obtained from the tests are critically assessed, and it is shown that fibers contribute immensely to the structural integrity and structural stability of concrete elements and thereby improve their durable service life.     

纳米碳纤维增强混凝土抗冻性能试验

对六种不同纳米碳纤维掺量的72个纳米碳纤维/混凝土试件进行了慢冻融循环试验,通过测量纳米碳纤维/混凝土经不同冻融循环次数作用后的抗剥落能力、质量损失率、相对动弹性模量和抗压强度损失率,研究了纳米碳纤维掺量对纳米碳纤维/混凝土抗冻性能的影响。另外进行了纳米碳纤维/混凝土的FE-SEM试验和压汞试验,分析了纳米碳纤维对纳米碳纤维/混凝土抗冻性能的微观改性机制。结果表明:纳米碳纤维通过改善混凝土的微观形貌,细化其孔隙结构,提高其整体性和密实度,显著改善了混凝土的抗冻性能;纳米碳纤维掺量为3vol%时,纳米碳纤维/混凝土的抗冻性能最佳。同普通混凝土相比,300次冻融循环后,纳米碳纤维/混凝土的相对动弹性模量提高了33.2%,抗剥落能力显著增强;相同冻融次数下,随着纳米碳纤维掺量的增加,纳米碳纤维/混凝土相对动弹性模量和抗压强度损失率均先增大后减小,质量损失率先减小后增大。但纳米碳纤维掺量最大为5vol%时,纳米碳纤维/混凝土的抗冻性能仍优于普通混凝土;冻融循环次数越多,纳米碳纤维对混凝土抗冻性能的改善作用越显著。      

Performance evaluation of structural concrete using controlled quality coarse and fine recycled concrete aggregate

This paper presents the fresh, mechanical, and durability performance, of a structural concrete mix classified as C-1, by the Canadian Standards Association (CSA) made with controlled quality Recycled Concrete Aggregate (RCA). Five mixes with water-to-cementing material (w/cm) ratio of 0.40 were produced with various RCA contents and tested against two 0% RCA control mixes made with General Use (GU) cement, and General Use Limestone cement (GUL). The RCA contents in the mixes were 10%, 20%, and 30% by coarse aggregate volume replacement, as well as 10% and 20% fine and coarse (granular) aggregate volume replacement. All evaluated mixes met the specifications from the CSA for fresh, mechanical, and durability properties. The coarse RCA mixes performed better than the granular RCA mixes in terms of flexural and splitting tensile strengths, linear drying shrinkage, water sorptivity, and rapid chloride-ion permeability, where the test results were significantly affected by the ultra fines present in the granular RCA.     

玄武岩纤维对再生混凝土抗碳化性能的影响

采用快速碳化的方法研究了玄武岩纤维(BF)掺量和再生粗骨料(RCA)取代率对再生混凝土(RAC)抗碳化性能的影响,实测了3天、7天、14天、28天的碳化深度,对随碳化天数的增加,碳化深度与BF掺量、RCA取代率之间的关系进行了分析.结果表明,RAC的碳化与天然混凝土(NAC)类似,其碳化深度均随碳化天数的增加而增加,随...     

混杂纤维增强再生砖骨料混凝土增强机制与抗压性能计算方法

通过钢-聚烯烃混杂纤维增强再生砖骨料混凝土（HF/RBAC）的抗压与弹性模量试验,研究了再生砖骨料（RBA）取代率、混杂纤维掺量、纤维种类对混凝土抗压强度和弹性模量的影响。根据RBA的XRD图谱、X-CT图像、RBA火山灰活性成分与水泥水化产物反应原理及能量平衡原理,分析了HF/RBAC的破坏机制和纤维增强机制。研究表明,当RBA全取代天然骨料（NA）时,HF/RBAC立方体抗压强度、轴心抗压强度和弹性模量分别降低了36.72%、24.95%和43.53%。当钢-聚烯烃混杂纤维体积掺量为1.5%时,HF/RBAC立方体抗压强度、轴心抗压强度和弹性模量分别增加了20.51%、30.33%和35.84%。最后,提出了考虑RBA压碎指标和取代率、纤维种类和掺量等因素影响的HF/RBAC抗压强度和弹性模量的计算方法。     

Development of recycled PET fiber and its application as concrete-reinforcing fiber

We describe a method that can be used to produce concrete-reinforcing PET fiber from used PET bottles. Using this method, the concrete and PET fibers are easily mixed at a fiber contents as high as 3%. The primary characteristic of the PET fiber is that it is easy to handle. The issue of concern in the development of PET fiber is its alkali resistance; however, we encountered no problems when using the fiber in normal concrete. The wetting tension of PET was found to be lower than that of PVA but higher than that of PP. No toxic gas was generated during a combustion test of the PET fiber. We describe two example applications: a gateway support at Hishikari Mine, Japan, and the pavement of bush roads.     

The fracture and fatigue performance in flexure of carbon fiber reinforced concrete

The fracture parameters and fatigue performances of carbon fiber reinforced concrete is investigated by three point bending tests. In comparison with the results of quasi-static tests where no pre-cyclic loading is applied, the influence of pre-cyclic loading history on fracture parameters was researched by using compliance calibration. The test results show that the fracture parameters of carbon fiber reinforced concrete and plain concrete will be reduced if the pre-cyclic loading stress levels are higher than a certain threshold, and this threshold value for carbon fiber reinforced concrete is higher than that of plain concrete. The critical effective crack length for carbon fiber reinforced concrete is significantly larger than that of plain concrete and independent of the pre-cyclic loading history and fatigue life. Carbon fiber reinforced concrete has a considerable beneficial effect on the behaviour of concrete subjected to flexure fatigue loading.     

Ultra-high performance concrete and fiber reinforced concrete: achieving strength and ductility without heat curing

Ultra-high performance concrete (UHPC) and ultra-high performance fiber reinforced concrete (UHP-FRC) were introduced in the mid 1990s. Special treatment, such as heat curing, pressure and/or extensive vibration, is often required in order to achieve compressive strengths in excess of 150 MPa (22 ksi). This study focuses on the development of UHP-FRCs without any special treatment and utilizing materials that are commercially available on the US market. Enhanced performance was accomplished by optimizing the packing density of the cementitious matrix, using very high strength steel fibers, tailoring the geometry of the fibers and optimizing the matrix-fiber interface properties. It is shown that addition of 1.5% deformed fibers by volume results in a direct tensile strength of 13 MPa, which is 60% higher than comparable UHP-FRC with smooth steel fibers, and a tensile strain at peak stress of 0.6%, which is about three times that for UHP-FRC with smooth fibers. Compressive strength up to 292 MPa (42 ksi), tensile strength up to 37 MPa (5.4 ksi) and strain at peak stress up to 1.1% were also attained 28 days after casting by using up to 8% volume fraction of high strength steel fibers and infiltrating them with the UHPC matrix.     

Comparative evaluation of early age toughness parameters in fiber reinforced concrete

Early age strength development is a major consideration for design and construction processes such as the shotcrete mixtures used for tunneling applications. Adding the fibers to high strength concrete helps in resisting potential early age thermal and shrinkage cracking in addition to maintaining long-term strength. The post cracking tensile strength is one of the critical safety parameters to insure a safe level of ground support. Results of several bending tests on early-age fiber reinforced concrete are presented as load–deflection responses. A strain softening response is used to model the behavior of different types of fiber reinforced concrete and simulate the experimental flexural response. Closed form equations for moment–curvature response of a rectangular beam in conjunction with crack localization rules are utilized. As a result, the stress distribution that considers a shifting neutral axis can be simulated which provides a more accurate representation of the residual strength of the fiber cement composites. The analysis is performed to evaluate effects of age and fiber type on back calculated tensile stress strain response, along with experimental and simulated flexural load–deflection curves. The back-calculated tensile post cracking strengths are compared and correlated with the corresponding parameters used by ASTM, JCI, and RILEM methods and scale factors for the elastic methods are proposed which are in-line with the current fib Model Code. Caution must be exercised in application of results from the standard test methods due to the overestimation of the residual strength parameters that are based on elastic approaches.     

High performance fiber reinforced concrete: progress in knowledge and design codes

High performance fiber reinforced concrete is developing quickly to a modern structural material with a high potential. As for instance testified by the recent symposium on HPFRC in Kassel, Germany (April 2008) the number of structural applications increases. At this moment studies are carried out with the aim to come to an international recommendation for the design of structures with HPFRC. Research projects are being carried out in order to supply missing information in relevant areas. Some examples of recent research at TU Delft are given. For the preparation of an internationally acceptable design recommendation for HPFRC a number of principles should be respected. The code should as much as possible be in harmony with the code for conventional fiber concrete. Moreover it should be consistent with existing design recommendations for structural concrete. Second thoughts on the introduction of such a new code are given.     

Short and long-term behavior of structural concrete with recycled concrete aggregate

Recycling concrete construction waste is a promising way towards sustainable construction. Coarse recycled concrete aggregates have been widely studied in recent years, however only few data have been reported on the use of fine recycled aggregates. Moreover, a lack of reliable data on long-term properties of recycled aggregate concrete has to be pointed out.In this paper the effects of both fine and coarse recycled concrete aggregates on short and long-term mechanical and physical properties of new structural concrete are investigated. The studied concrete mixes have been designed by adjusting and selecting the content and grain size distribution of concrete waste with the goal to obtain medium–high compressive strength with high content of recycled aggregates (ranging from 27% to 63.5% of total amount of aggregates).Time-dependent properties, such as shrinkage and creep, combined with porosity measurements and mechanical investigations are reported as fundamental features to assess structural concrete behavior.     

圆钢管玄武岩纤维再生混凝土短柱轴压力学性能

为研究圆钢管玄武岩纤维再生混凝土(BFRRC)短柱的轴压力学性能,以再生粗骨料取代率和玄武岩纤维掺量为变化参数,设计并完成了15根圆钢管BFRRC短柱试件的轴压试验。观察了试件的受力全过程及破坏形态,获取了试件的荷载-位移曲线及荷载-应变曲线,分析了变化参数对圆钢管BFRRC短柱轴压性能的影响,建立了可行的组合截面应力-应变全过程曲线方程。研究表明:试件均发生鼓曲破坏,但核心混凝土在钢管约束下处于碎而不散状态;随着再生粗骨料取代率的增大,试件的耗能性能、延性系数逐渐增大,耗能因子、延性系数提升幅度最高可达1.84%和10.36%,承载力逐渐降低,降低幅度最大达5.03%;随着玄武岩纤维掺量的增大,试件的耗能性能、延性系数逐渐增大,增加幅度最高可达2.97%和4.93%,承载力提高幅度不大;不同的玄武岩纤维掺量下,试件实测的荷载-位移曲线饱满,且具有较长的变形流幅,延性较好。      

纤维增强树脂复合材料约束超高性能混凝土轴压性能的细观数值模拟

利用LS-DYNA有限元分析软件建立纤维增强树脂(FRP)复合材料约束超高性能混凝土(UHPC)圆柱细观有限元模型,以研究其单轴受压性能。通过已有试验数据验证了模型的有效性,并建立了能准确反映FRP复合材料约束作用的K&C模型的剪切膨胀参数预测公式。在此基础上进行参数分析,研究FRP复合材料厚度、纤维缠绕角度和钢纤维掺量的影响。结果表明,本文模型不仅能模拟随机分布钢纤维对试件应力分布的影响,且能较准确反映FRP复合材料约束作用对核心UHPC强度和延性的提高效果。模型在轴压作用下的破坏模式和应力-应变曲线与试验结果基本一致。参数分析表明,随FRP复合材料厚度或纤维缠绕角度的增大,试件极限承载力和延性均增大,而增大钢纤维掺量虽可限制核心UHPC斜裂缝的开展,但对试件强度和延性影响较小。      

Time dependent behavior of elements combining ultra-high performance fiber reinforced concretes (UHPFRC) and reinforced concrete

Structural elements combining Ultra-High Performance Fiber Reinforced Concretes (UHPFRC) and concrete offer a high potential in view of rehabilitation and modification of existing structures. The investigation of the time-dependent behavior of composite “UHPFRC-concrete” elements is a fundamental step in the determination of durability and serviceability. For this, an experimental program was conducted on large composite “UHPFRC-concrete” beams and a numerical model was validated with the test results. The experimental results and a parametric study performed with the numerical model showed that UHPFRC and normal strength reinforced concrete are compatible in the long-term and that the critical period of composite “UHPFRC-concrete” elements are the first 90 days after the casting of the UHPFRC layer. Thus, the high potential of such composite elements can be exploited also in the long term.