Application of neutron radiography in observing and quantifying the time-dependent moisture distributions in multi-cracked cement-based composites

Significant tensile strain capacity of SHCC under tensile stress can be reached by multi-crack formation, while the cracks remain bridged by fibres. Ductility of SHCC is due to this multi-crack formation. Cracks are preferential pathways for ingress of water and salt solutions into the material. In this contribution neutron radiography has been successfully applied to visualize the process of water penetration into cracked SHCC and to quantify the corresponding time-dependent moisture distributions in cracked SHCC. Results indicate that in uncracked SHCC, less water can be found. Once cracked, however, both the amount of water and the penetration depth increased with increasing of crack density and the wider crack pattern when higher tensile strain was applied. Even at comparatively modest imposed strain when micro-cracks were formed, water penetrated into the specimens along the cracks of 30 μm–50 μm immediately and then water migrated further into the surrounding matrix from water filled cracks. Water then moved into the matrix adjacent to the cracks which was mechanically damaged by direct tension. Therefore, if durability of SHCC is an issue for application, a maximum strain may not be exceeded. In order to prevent penetration of water or salt solutions into cracked SHCC, two approaches were used. Integral water repellent SHCC was prepared by adding silane emulsion to the fresh mortar. Compared with neat SHCC, the integral water repellent SHCC with multi-cracks absorbed much less water after imposed to the same tensile strain. Notice that there was still a small amount of moisture that could enter the matrix of integral water repellent SHCC via cracks when the tensile strain was over 1.5% in this study. As an alternative method, surface impregnation with silane gel was a more promising approach to protect cracked SHCC from water or salt solution penetration into the material when multi-cracks formed.     

Durability of strain-hardening cement-based composites (SHCC)

Strain-hardening cement-based composites were named after their ability to resist increased tensile force after crack formation, over a significant tensile deformation range. The increased resistance is achieved through effective crack bridging by fibres, across multiple cracks of widths in the micro-range. Whether these small crack widths are maintained under sustained, cyclic or other load paths, and whether the crack width limitation translates into durability through retardation of moisture, gas and other deleterious matter ingress, are scrutinised in this paper by evaluation of test results from several laboratories internationally. This contribution is a short version of the State-of-the-Art report by RILEM TC 208-HFC, Subcommittee 2: Durability, developed during the committee life 2005?C2009.     

Effects of crack properties and water-cement ratio on the chloride proofing performance of cracked SHCC suffering from chloride attack

This study aims at developing an SHCC mixture that offers improved workability and high rebar corrosion proofing performance while ensuring moderate tensile ductility. Specimens were made from several mixtures with part of the cement replaced with limestone powder, and with a reduced fiber content. Mechanical properties and corrosion proofing performance under tensile stress were examined to determine the effects of chloride in a simulated situation under uniaxial tensile strain. The results indicated that the fiber content in the mixture had no influence on the crack properties of steel reinforced SHCC under tensile load. It was found that the chloride proofing performance of SHCC with multiple cracks was affected by the crack properties such as the number of cracks and the accumulated crack width. Reducing the water-cement ratio was effective to enhance the chloride proofing performance of SHCC.     

Comparative testing of crack formation in strain-hardening cement-based composites (SHCC)

Two of the main activities of RILEM Technical Committee 208-HFC Subcommittee 2 were the preparation and publication of the state-of-the-art report on durability of strain hardening cement-based composites (SHCC), and the performance of comparative laboratory testing on SHCC. In this paper the comparative mechanical tests are reported, as performed in laboratories of five participating institutions. The purpose was to investigate and compare the crack patterns in terms of crack widths and spacing, and subsequently to make recommendations for a suitable test setup and procedure towards characterizing cracking in this class of materials. Such standardized procedures are required for future systematic and objective research towards durability of these materials in their in-service conditions, i.e. their resistance to deterioration processes in the cracked state. Standardized test procedures are also required for durability testing and guidelines for structural design with SHCC, which is the focus of follow-up committee activity in TC 240-FDS.     

Self-healing behavior of strain hardening cementitious composites incorporating local waste materials

The self-healing behavior of a series of pre-cracked fiber reinforced strain hardening cementitious composites incorporating blast furnace slag (BFS) and limestone powder (LP) with relatively high water/binder ratio is investigated in this paper, focusing on the recovery of its deflection capacity. Four-point bending tests are used to precrack the beam at 28 days. For specimens submerged in water the deflection capacity can recover about 65–105% from virgin specimens, which is significantly higher compared with specimens cured in air. Similar conclusion applies to the stiffness recovery in water cured specimens. The observations under ESEM and XEDS confirmed that the microcracks in the specimens submerged in water were healed with significant amount of calcium carbonate, very likely due to the continuous hydration of cementitious materials. The self-healing cementitious composites developed in this research can potentially reduce or even eliminate the maintenance needs of civil infrastructure, especially when repeatable high deformation capacity is desirable, e.g. bridge deck link slabs and jointless pavements.     

SHCC的配制与拉伸性能研究

采用聚乙烯醇纤维(PVA)纤维作为增强材料,选定不同的粉煤灰掺量、石英砂级配、纤维掺量和养护工艺配制应变硬化水泥基复合材料(SHCC),研究上述因素对SHCC力学性能的影响。研究表明,随着粉煤灰掺量的增加,SHCC极限拉伸强度有少许削弱,但极限拉伸应变不断增加,均高于3%。随着养护龄期增加,SHCC极限拉伸应变呈现先增加后减小的趋势,但拉伸强度随龄期增加而增大。自然养护有利于维持SHCC的高极限拉伸应变;蒸汽养护能提高SHCC早期的极限拉伸强度,但蒸汽养护使SHCC的极限拉伸应变随着龄期增加而明显降低。当m(FA)/m(C)=1.6,2.0和2.4,Vf=2.0%时,采用较细的石英砂和自然养护,28d龄期的SHCC极限拉伸强度在4 MPa以上,极限拉伸应变在3%以上。     

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.     

Water permeability of engineered cementitious composites

The water permeability of a unique class of high performance fiber reinforced cementitious composites (HPFRCC) called engineered cementitious composites (ECC) is investigated. These composites are deliberately tailored using microcmechanical design principles to exhibit pseudo-strain-hardening characteristics in uniaxial tension, up to greater than 4% strain. While undergoing tensile deformation, microcracks are designed to saturate the specimen rather than localize into large cracks. This tendency to form microcracks, which are experimentally shown to be approximately 60 μm in width, allows ECC material in the cracked state to maintain water permeability similar to that of uncracked concrete or mortar, and magnitudes lower than cracked reinforced mortar or concrete. It is also shown that the self-healing properties of cracks within ECC material significantly aids in reducing the coefficient of permeability of cracked ECC.     

Effects of inorganic surface treatment on water permeability of cement-based materials

The permeability of the cement-based materials can be used as an important indicator of their durability. Surface treatment is a simple way to reduce permeability and improve durability of cement-based materials. This paper studied the effects of fluosilicate and sodium silicate surface treatments on the permeability of cement-based materials using the Autoclam water permeability and water absorption testing method. The experimental results showed that both fluosilicate and sodium silicate surface treatments could effectively reduce the permeability of cement-based materials. However, fluosilicate worked within the first 28days after treatment, while sodium silicate showed more obvious effect at later ages. Autoclam water permeability index exhibited an exponential relationship with the water absorption of the cement-based materials. In addition, mercury intrusion porosimetry result suggested that these inorganic surface treatment agents could reduce the porosity of surface layer of cement-based materials.     

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。     

核壳粒子复合材料的等效磁导率

为了预测复合材料的等效磁导率,建立了填充核壳粒子复合材料等效磁导率的物理模型,应用电磁场理论推导了核壳粒子以单一介质球代替的等效方法并推广得到椭球核壳粒子情况.基于平均极化理论和Maxwell-Garnett理论给出了核壳粒子以特定浓度随机分布于复合材料时等效磁导率的预测公式,通过数值计算分析了核壳粒子浓度、结构参数以及磁导率对复合材料等效磁导率的影响.数值计算与实验结果吻合很好,验证了该等效方法用于复合材料电磁特性优化设计的有效性.     

Optimum strain gage location for evaluating stress intensity factors in single and double ended cracked configurations

A general methodology has been proposed for calculation of optimum radial strain gage locations for measurement of the stress intensity factors using strain gage technique. The upper bound (r_max) of strain gage locations for complex single ended and double ended cracked configurations has been determined using the proposed method. Further, dependency of the r_max on the crack length to width ratio and on the state of stress is investigated. Numerical results obtained from the present investigation are observed to be in accordance with the theoretical predictions. Using the proposed approach the correctness of strain gage locations used by the earlier researchers is also verified.     

Effect of fine crack width and water cement ratio of SHCC on chloride ingress and rebar corrosion

SHCC (Strain Hardening Cement-based Composite) is a material known for its strain-hardening behavior under tensile and bending stress and its characteristic numerous small cracks. SHCC is expected to show superior durability because of the fineness of the cracks. In this study, chloride ingress through cracks into SHCC and progress of rebar corrosion in three mixtures of SHCC with various water-cement ratios were investigated. Through a chloride solution immersion test, it was confirmed that chloride could penetrate through even very fine cracks. The resistivity of cracked SHCC against chloride ingress is mainly governed by the accumulated crack width and the water cement ratio. Chloride pre-mixed SHCC specimens were left in a high-temperature, high-humidity chamber for 11 months to promote rebar corrosion. While the accumulated crack width and the water cement ratio were both influential to an increase in corrosion area, only the water cement ratio had bearing on corrosion loss.     

Corrosion resistance of strain-hardening steel-fiber-reinforced cementitious composites

This study investigated the corrosion resistance of strain-hardening steel-fiber-reinforced cementitious composites (SH-SFRCs) in a chloride environment. Two types of steel fibers, hooked and twisted, were added (2% by volume) to a high-strength mortar matrix (90 MPa). All the specimens were exposed to cyclic wetting in a 3.5% chloride solution followed by drying. The corrosion resistance of SH-SFRCs was then evaluated by measuring the direct tensile resistance after the chloride cycles. The strain capacity and toughness of all the SH-SFRCs decreased significantly after 105 chloride cycles, whereas a slight reduction was observed in their post-cracking strength. The corrosion resistance of SH-SFRCs after the chloride cycles was strongly dependent on the width of multiple microcracks when the SH-SFRCs were pre-cracked by tensioning until 0.1% tensile strain. The addition of calcium nitrite (CNI) was successful in improving the corrosion resistance of the pre-cracked SH-SFRCs in the chloride environment.     

Simulation of water permeability and water vapor diffusion through hardened cement paste

The transport properties of cement-based materials significantly affect their durability. This results from the fact that most of the damaging reagents are transported, often solved in water, through the open pore space into the microstructure. This paper focuses on simulating water permeation (movement under a gradient of pressure) and water vapor diffusion (movement under a gradient of concentration) through hardened cement paste (hcp). The main goal is to derive the water permeability and the water vapor diffusion coefficient directly from the morphology of the 3D microstructure. For this purpose microtomographic images of a hcp made of ordinary Portland cement are used to represent the microstructure and especially the pore space through which the moisture transport will occur. With the use of a skeletonization algorithm, also known as “thinning algorithm”, the skeleton or centerline of the pore space is extracted. This skeleton is in a second step converted into a transportation network of cylindrical tubes. Bernoulli's law is applied to every tube for simulating water permeation. The permeability coefficient is then calculated by using Darcy's law. In the case of water vapor diffusion the diffusion coefficient is calculated using Fick's law. An erratum to this article is available at .     

MODELLING THE INFLUENCE OF STRAIN HARDENING AND PLASTIC CONSTRAINT ON CRACK CLOSURE OF ARBITRARILY THICK CCT-SPECIMENS

The triaxial stress constraint and the effective yield stress distribution in the plastic zone for strain hardening materials are studied, and then a modified strip-yield model is proposed to investigate the thickness effect of CCT specimens. Consequently, a plastic constraint factor α is defined and analysed in detail. The results show that the factor α can comprehensively account for the influence of thickness, crack length, loading level and hardening exponent. A simple expression for the plastic zone length and a fitting expression involving α are obtained. Application of the modified strip model to Newman’s crack closure model, and comparison with FEM results, show that the model can account for the influence of thickness on crack closure.     

Variation of the cyclic strain-hardening exponent in advanced aluminium alloys

The cyclic strain-hardening exponents for five fatigue-resistant aluminium alloys were determined throughout the fatigue life to study the degree of cyclic stability of these alloys. Data were compared with results for 2024-T4 aluminum and for two high-pressure steels. The strain-hardening exponent increased logarithmically in all cases except 2024-T4, although the increase was small and did not exceed 33% over the fatigue life. 7475-T351 aluminium alloy was found to be entirely stable, and 7075-T7351 almost so. These were followed in order of rising sensitivity by 2014-T6, 7050-T73651, and 2124-T851 aluminium alloys, and 28NiCrMo7.4 and 30CrNiMo8 steels. 2024-T4 aluminum alloy demonstrated a strong decrease in strain-hardening exponent with fatigue life.     

Impact response of a cracked orthotropic medium-revisited

Elastodynamics response of an infinite orthotropic medium containing a central crack under impact loading has been investigated. Laplace and Fourier transforms have been employed to reduce the transient problem to the solution of a pair of dual integral equations in the Laplace transform domain which has finally been solved by the method of iteration in the low frequency case. Analytic expressions for the stress intensity factors and crack opening displacement are also obtained for low frequency.     

Effect of loading rate on crack velocities in HSC

This paper presents the recent results of an experimental program aimed at disclosing the loading rate (loading-point-displacement rate) effect on the crack velocity in high-strength concrete (HSC). Eighteen three-point-bend tests were conducted using either a servo-hydraulic machine or a self-designed drop-weight impact device. Four strain gauges mounted along the ligament of the specimen were used to measure the crack velocity. Six different loading rates were applied, from 10⁻⁴ mm/s to 10³ mm/s (average strain rate from 10⁻⁶ to 10⁻¹ s⁻¹), i.e., a low loading-rate range (5.50 × 10⁻⁴ mm/s, 0.55 mm/s and 17.4 mm/s) and a high loading-rate range (8.81 × 10² mm/s, 1.76 × 10³ mm/s and 2.64 × 10³ mm/s). At low loading rates, the crack propagates with increasing velocity. Under high loading rates, the crack propagates with slightly decreasing velocity, though the maximum crack speed reached up to 20.6% of the Rayleigh wave speed of the tested HSC. In addition, the loading-rate effect on crack velocities is pronounced within the low loading-rate regime, whereas it is minor under the high loading-rate range.