Orientation factor and number of fibers at failure plane in ring-type steel fiber reinforced concrete

Considering the probabilistic distributions of fibers in ring-type steel fiber reinforced concrete, the orientation factor and the number of ring-type steel fibers crossing the failure plane were theoretically derived as a function of fiber geometry, specimen dimensions, and fiber volume fraction. A total number of 24 specimens were tested incorporating different fiber types, specimen geometry, and fiber volume fractions of 0.2% and 0.4%: 5 beams and 5 panels containing straight steel fibers; and 6 beams and 8 panels containing ring-type steel fibers. Measurements were made to assess the number of fibers at fractured surfaces of steel fiber reinforced concrete. The developed theoretical expressions reasonably predicted the orientation factor and the number of ring-type steel fibers at failure plane: the average and the standard deviation for the ratios of the test to theory were 1.03 and 0.26, respectively. Theoretical investigations and comparisons were made for the values of orientation factor and the number of fibers at failure plane for straight steel fibers and ring-type steel fibers.     

A micromechanical approach to elastic and viscoelastic properties of fiber reinforced concrete

Some aspects of the constitutive behavior of fiber reinforced concrete (FRC) are investigated within a micromechanical framework. Special emphasis is put on the prediction of creep of such materials. The linear elastic behavior is first examined by implementation of a Mori-Tanaka homogenization scheme. The micromechanical predictions for the overall stiffness prove to be very close to finite element solutions obtained from the numerical analysis of a representative elementary volume of FRC modeled as a randomly heterogeneous medium.The validation of the micromechanical concepts based on comparison with a set of experiments, shows remarkable predictive capabilities of the micromechanical representation.The second part of the paper is devoted to non-ageing viscoelasticity of FRC. Adopting a Zener model for the behavior of the concrete matrix and making use of the correspondence principle, the homogenized relaxation moduli are derived analytically. The validity of the model is established by mean of comparison with available experiment measurements of creep strain of steel fiber reinforced concrete under compressive load. Finally, the model predictions are compared to those derived from analytical models formulated within a one-dimensional setting.     

玄武岩纤维钢筋混凝土梁弯曲性能试验研究

通过玄武岩纤维钢筋混凝土梁四点弯曲试验,分析其破坏形态和破坏机理.通过四点对称加载方式研究不同玄武岩纤维掺量下梁的承载力、挠度、韧性、混凝土应变、钢筋应变等变化规律.试验结果表明:玄武岩纤维掺入对钢筋混凝土梁的开裂荷载和极限荷载都有一定的提高,开裂荷载最大提高幅度为32％,极限荷载最大提高幅度为6.5％.与普通钢筋混凝土梁相比玄武岩纤维钢筋混凝土梁的挠度、韧性均有所提高.玄武岩纤维对梁的受压区混凝土具有阻止裂缝扩展的能力,当梁上部受压区混凝土被压碎时,混凝土碎块会在纤维的桥接作用下不剥落,梁仍保持较好的整体性.     

Corrosion process and structural performance of a 17 year old reinforced concrete beam stored in chloride environment

The long-term corrosion process of reinforced concrete beams is studied in this paper. The reinforced concrete elements were stored in a chloride environment for 17years under service loading in order to be representative of real structural conditions. At different stages, cracking maps were drawn, total chloride contents were measured and mechanical tests were performed. Results show that the bending cracks and their width do not influence significantly the service life of the structure. The chloride threshold at the reinforcement depth, used by standards as a single parameter to predict the end of the initiation period, is a necessary but not a sufficient parameter to define service life. The steel-concrete interface condition is also a determinant parameter. The bleeding of concrete is an important cause of interface de-bonding which leads to an early corrosion propagation of the reinforcements. The structural performance under service load (i.e.: stiffness in flexure) is mostly affected by the corrosion of the tension reinforcement (steel cross-section and the steel-concrete bond reduction). Limit-state service life design based on structural performance reduction in terms of serviceability shows that the propagation period of the corrosion process is an important part of the reinforced concrete service life.     

Effect of Fiber Volume Fraction on the Off-Crack-Plane Fracture Energy in Strain -Hardening Engineered Cementitious Composites

In this paper, the results of an experimental study on the effect of fiber volume fraction on the off-crack-plane fracture energy in a strain-hardening engineered cementitious composite (ECC) are presented. Unlike the well-known quasi-brittle behavior of fiber reinforced concrete, ECC exhibits quasi-ductile response by developing a large damage zone prior to fracture localization. In the damage zone, the material is microcracked but continues to strain-harden locally. The areal dimension of the damage zone has been observed to be on the order of 1000 cm² in double cantilever beam specimens. The energy absorption of the off-crack-plane inelastic deformation process has been measured to be more than 50% of the total fracture energy of up to 34 kJ/m². This magnitude of fracture energy is the highest ever reported for a fiber cementitious composite.     

Influence of Fiber and Interfacial Properties on Fracture Behavior of Fiber-Reinforced Ceramic Composites

The fracture and fracture resistance behaviors of zirconmatrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide fibers are studied by performing experiments in three-point flexure and by analyzing results analytically using a cohesive crack model that incorporates crack bridging and fiber pullout mechanisms. A comparison of experimental results with the model predictions demonstrates good agreement. This analytical approach is then used in a parametrical study to demonstrate the role of fiber and fiber-matrix interfacial properties on the mechanical behavior of fiber-reinforced ceramic-matrix composites. Material parameters that enhance ultimate strength and ductility or toughness are elucidated.     

Prediction of load-deflection behavior of plain concrete from fracture energy

The paper examines two existing models for predicting the load-deflection behavior of plain concrete from fracture energy measured in three-point bending. These models are shown to overestimate the peak load attainable by concrete beams. This major drawback is overcome by proposing a new model that accounts for the strain hardening in the material prior to the attainment of peak load. It is also argued that fracture energy in its present form is dependent on the specimen size and that it is not therefore a reliable indicator of the fracture toughness of plain concrete.     

Impact resistance of poly(vinyl alcohol) fiber reinforced high-performance organic aggregate cementitious material

Poly(vinyl butyral) (PVB) which has many special engineering aggregate properties such as super lightweight, physical toughness, adhesion to a variety of surfaces and energy-absorbing characteristics is utilized as the sole aggregate in this study to develop a novel cementitious composite reinforced with Poly(vinyl alcohol) (PVA) fiber. Impact energy absorption capacity is evaluated based on the Charpy impact test. The results show that PVB composite material has lower density but higher impact energy absorption capability compared with conventional lightweight concrete and regular concrete. The addition of PVA fiber improves the impact resistance with fiber volume fractions. The remarkable change in the interfacial bond strength contributed by the non-covalent bond such as hydrogen bond and ether interactions at the interfaces between fiber, aggregate and matrix contributes to the improvement of the impact resistant capacity. A model based on fiber bridging mechanics and the rule of mixtures is developed to characterize the impact energy. A good correlation was obtained for the materials tested when experimental results are compared to those predicted by the developed model.     

Thermal and mechanical properties of fiber reinforced high performance self-consolidating concrete at elevated temperatures

This paper presents the effect of temperature on thermal and mechanical properties of self-consolidating concrete (SCC) and fiber reinforced SCC (FRSCC). For thermal properties specific heat, thermal conductivity, and thermal expansion were measured, whereas for mechanical properties compressive strength, tensile strength and elastic modulus were measured in the temperature range of 20–800 °C. Four SCC mixes, plain SCC, steel, polypropylene, and hybrid fiber reinforced SCC were considered in the test program. Data from mechanical property tests show that the presence of steel fibers enhances high temperature splitting tensile strength and elastic modulus of SCC. Also the thermal expansion of FRSCC is slightly higher than that of SCC in 20–1000 °C range. Data generated from these tests was utilized to develop simplified relations for expressing thermal and mechanical properties of SCC and FRSCC as a function of temperature.     

A model for predicting fracture resistance of fiber reinforced concrete

A theoretical model is presented to predict the crack propagation resistance of fiber reinforced cement based composites. A crack in the matrix is divided into a traction free zone, fiber bridging zone and the matrix process zone. The crack closing pressure due to fibers depends on the (Mode I) crack opening displacement (COD). A method is suggested to estimate this relationship from the pull-out tests. Although calculations of COD are based on linear elastic fracture mechanics concepts, the energy absorbed in the fiber bridging zone is included in the analysis. Theoretical results are compared with the experimental data of notched beam and double cantilever beam specimens.     

基于分形理论和扩展有限元法的钢纤维混凝土损伤破坏机理

为研究钢纤维混凝土损伤破坏过程和裂纹发展演化机理,基于分形理论和扩展有限元法,建立钢纤维混凝土立方体抗拉试验细观有限元模型和切口梁三点弯曲试验有限元模型,以相关试验测试结果为基础,比较验证了所建有限元分析模型的可靠性。以裂纹分形维数表征钢纤维混凝土损伤演化过程,考察不同钢纤维体积掺量和长度、粗骨料形状等重要因素对钢纤维混凝土损伤演化过程的影响。结果表明,基于裂纹分形维数的损伤值可以较好地反映钢纤维混凝土的损伤演化过程及特征,钢纤维体积掺量、长度的增加和骨料形状的不规则化会延缓钢纤维混凝土立方体试件的损伤演化过程,钢纤维体积掺量、初始裂纹距跨中距离的增加和初始裂纹缝高比的减小可在较小程度上延缓钢纤维混凝土切口梁的损伤演化过程。     

Flow of fresh concrete through steel bars: A porous medium analogy

Although being a very promising area of concrete technology, computational modeling of fresh concrete flow is a comprehensive and time consuming task. The complexity and required computation time are additionally increased when simulating casting of heavily reinforced sections, where each single reinforcement bar has to be modeled. In order to improve the computation speed and to get closer to a practical tool for simulation of casting processes, an innovative approach to model reinforced sections is proposed here. The basic idea of this approach is to treat the reinforcement zone as a porous medium in which a concrete is propagating. In the present paper, the numerical implementation of this concept is described. A methodology allowing for the computation of the equivalent permeability of the steel bars network is suggested. Finally, this numerical technique efficiency is evaluated by a comparison with experimental results of model fluids casting in model formworks.     

Concrete modelling for expertise of structures affected by alkali aggregate reaction

Alkali aggregate reaction (AAR) affects numerous civil engineering structures and causes irreversible expansion and cracking. In order to control the safety level and the maintenance cost of its hydraulic dams, Electricité de France (EDF) must reach better comprehension and better prediction of the expansion phenomena. For this purpose, EDF has developed a numerical model based on the finite element method in order to assess the mechanical behaviour of damaged structures. The model takes the following phenomena into account: concrete creep, the stress induced by the formation of AAR gel and the mechanical damage. A rheological model was developed to assess the coupling between the different phenomena (creep, AAR and anisotropic damage). Experimental results were used to test the model. The results show the capability of the model to predict the experimental behaviour of beams subjected to AAR. In order to obtain such prediction, it is necessary to take all the phenomena occurring in the concrete into consideration.     

Predicting the pullout response of inclined hooked steel fibers

Steel fiber reinforced concrete (SFRC) is symptomatically an anisotropic material due to the random orientation of fibers within the cement matrix. Fibers under different inclination angles provide different strength contributions at a given crack width. Therefore the pullout response of inclined fibers is a paramount subject to understand and quantify SFRC behavior, particularly in the case of fibers with hooked ends, which are currently the most widely used. Several experimental results were considered to validate the approach and to assure its suitability on distinct material properties and boundary conditions. The good agreement on predicting the pullout behavior of these fibers encourages its use towards a new concept of design and optimization of SFRC.     

玄武岩纤维筋全再生混凝土无腹筋梁受剪性能试验研究

通过4根玄武岩纤维筋与4根钢筋再生混凝土无腹筋梁的受剪试验,研究采用100%粗骨料取代率的再生混凝土梁的裂缝开展、破坏等情况;分析不同纵筋类型下,剪跨比、纵向配筋率和混凝土抗压强度对梁开裂荷载、极限承载力和跨中挠度变化的影响。比较中国规范(GB 50608—2010)、美国规范(ACI 440.1R-06)、加拿大规范(CSA.S 806-12)中规定的计算方法对玄武岩纤维筋再生混凝土梁受剪承载力的适用性。研究结果表明:钢筋再生混凝土梁的受力性能类似于传统的钢筋混凝土梁,而玄武岩纤维筋再生混凝土梁在荷载作用下,裂缝扩展较快且宽度更大;中国规范(GB 50608—2010)对试验梁抗剪承载力的计算值过于保守,美国规范(ACI 440.1R-06)最为接近,加拿大规范(CSA.S 806-12)次之。     

Probabilistic evaluation of initiation time in RC bridge beams with load-induced cracks exposed to de-icing salts

In this study, a reliability-based method for predicting the initiation time of reinforced concrete bridge beams with load-induced cracks exposed to de-icing salts is presented. A practical model for predicting the diffusion coefficient of chloride ingress into load-induced cracked concrete is proposed. Probabilistic information about uncertainties related to the surface chloride content and the threshold chloride concentration has been estimated from a wide review of previous experimental or statistical studies. Probabilistic analysis to estimate the time to corrosion initiation with/without considering the effect of the load-induced cracks on the chloride ingress into concrete has been carried out. Results of the analysis demonstrate the importance of considering the effect of the load-induced cracks for correct prediction of corrosion initiation in RC bridge beams exposed to chlorides.     

Experimental and theoretical investigation on the postcracking inelastic behavior of synthetic fiber reinforced concrete beams

A realistic method of analysis for the postcracking behavior of newly developed structural synthetic fiber reinforced concrete beams is proposed. In order to predict the postcracking behavior, pullout behavior of single fiber is identified by tests and employed in the model in addition to the realistic stress-strain behavior of concrete in compression and tension. A probabilistic approach is used to calculate the effective number of fibers across the crack faces and to calculate the probability of nonpullout failure of fibers. The proposed theory is compared with test data and shows good agreement. The proposed theory can be efficiently used to predict the load-deflection behavior, moment-curvature relation, load-crack mouth opening displacement (CMOD) relation of synthetic fiber reinforced concrete beams.     

On the mathematics of time-dependent apparent chloride diffusion coefficient in concrete

This paper provides an improved mathematical analysis of chloride penetration into concrete employing a time-dependent diffusion coefficient for the solution of Fick's second law of diffusion. In the paper the possible errors caused by the application of oversimplified mathematical expressions used in some models for the evaluation of service life of reinforced concrete structures are discussed. The results from this mathematical analysis demonstrate that some models based on the oversimplified error function complement (ERFC) solutions may easily overestimate the service life by orders of magnitude, especially when the age factor is high. Some chloride profiles after up to 10 years' field exposure were used to compare the oversimplified with the improved models. The results show that both the oversimplified and the improved models fairly well predict the 10 years' chloride ingress in Portland cement concrete, but the oversimplified ERFC model significantly underestimates the chloride ingress in concrete with fly ash.     

Influence of the meso-structure in dynamic fracture simulation of concrete under tensile loading

We investigate the dynamic behavior of concrete in relation to its composition within a computational framework (FEM). Concrete is modeled using a meso-mechanical approach in which aggregates and mortar are represented explicitly. Both continuum phases are considered to behave elastically, while nucleation, coalescence and propagation of cracks are modeled using the cohesive-element approach.In order to understand the loading-rate sensitivity of concrete, we simulate direct tensile-tests for strain rates ranging 1–1000 s⁻¹. We investigate the influence of aggregate properties (internal ordering, size distribution and toughness) on peak strength and dissipated fracture energy. We show that a rate independent constitutive law captures the general increase of peak strength with strain rate. However, a phenomenological rate-dependent cohesive law is needed to obtain a better agreement with experiments. Furthermore, at low rates, peak strength is sensitive to the inclusions' toughness, while the matrix dominates the mechanical behavior at high rates.     

Cover cracking of reinforced concrete due to rebar corrosion induced by chloride penetration

Cracking of concrete cover due to corrosion induced expansion of steel rebar is one of the major causes of the deterioration of reinforced concrete (RC) structures exposed to marine environments and de-icing salts.This paper presents two models that deal with the chloride-induced corrosion and subsequent cracking of concrete cover in RC structures. The former analyses the chloride diffusion within partially saturated concrete. A comprehensive model is developed through the governing equations of moisture, heat and chloride-ion flow. Nonlinearity of diffusion coefficients, chloride binding isotherms and convection phenomena are also highlighted. The latter describes the internal cracking around the bar due to expansive pressures as corrosion of the reinforcing bar progresses. Once a certain chloride concentration threshold is reached in the area surrounding the bar, oxidation of steel begins and oxide products are generated, which occupy much greater volume than the original steel consumed by corrosion. An embedded cohesive crack model is applied for cracking simulation.Both models are incorporated in the same finite element program. The models are chained, though not explicitly coupled, at first instance. Comparisons with experimental results are carried out, with reasonably good agreements being obtained. The work is a step forward for the integration of the two traditional phases (initiation and propagation) widely used in the literature and usually analysed separately. The estimation of the service life of the structure needs to evaluate the associated time for each one.