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.     

Multi-response optimization of post-fire performance of strain hardening cementitious composite

This paper presents results of an experimental program conducted to optimize the post-fire performance of Strain Hardening Cementititous Composites (SHCC) using Taguchi approach with utility concept. The experiments were first undertaken by determining nine SHCC mixes using a standard L9 (3⁴) orthogonal array of four parameters and each parameter with three levels. The four parameters of SHCC mixes included fly-ash/binder ratio, sand/binder ratio, water/binder ratio and fiber proportions. The responses of SHCC to be optimized were tensile strain capacity, compressive strength and post-fire compressive strength after subjected to 200 °C, 400 °C, 600 °C and 800 °C of isothermal heating. Utility concept was introduced to simplify the multi-response problem into mono-response question together with Taguchi method. The role of different parameters on the composite responses of SHCC was examined. Furthermore, an optimal SHCC mix to maximize multi-responses was determined based on statistical analysis and validated by additional confirmation tests.     

Characterising the time-dependant behaviour on the single fibre level of SHCC: Part 2: The rate effects on fibre pull-out tests

This paper is the second part of a two paper series about the time-dependant behaviour of Strain Hardening Cement-based Composite (SHCC) on the single fibre level. Having dealt with mechanisms of creep in SHCC in the first part, this paper reports single fibre pull-out tests that were done to investigate the effect of the pull-out rate on the mechanical response of the interface between the fibre and the matrix. It was found that not only the pull-out resistance increased with an increase of the pull-out rate but the probability of fibre rupture during pull-out as well. Another important finding was that the interfacial shear resistance and slip-hardening coefficient are not only dependant on the pull-out rate, but also the embedment length.     

PVA纤维增强应变硬化水泥基材料韧性性能研究

水泥基材料抗拉强度低、韧性差是其易于开裂、导致结构耐久性低劣的主要原因之一。高模量聚乙烯醇(PVA)纤维可增强水泥基材料韧性,使其呈现准应变硬化和多缝开裂特征,从而改善结构耐久性。本文通过四点弯曲试验得出了不同加载速率和不同配比应变硬化水泥基复合材料(PVA-SHCC)的力-变形曲线并用CONSOFT软件计算断裂能。结果表明,硅灰使材料的抗压强度有所提高,但最大抗弯承载力和变形下降,断裂能随之降低;甲基纤维素使PVA-SHCC脆性增大;随着加载速率的降低,材料表现出更好的应变硬化性能,微裂缝条数增多。     

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.     

混杂纤维增强应变硬化水泥基复合材料的弯曲性能预测

基于混凝土断裂力学与细观力学理论,同时考虑钢纤维(SF)/聚乙烯醇(PVA)混杂纤维对应变硬化水泥基复合材料(SHCC)弯曲性能的影响,提出了一种适用于SF/PVA纤维混杂SHCC(SF-PVA/SHCC)弯曲性能的预测方法.开展了SF-PVA/SHCC弯曲性能试验,分析了纤维种类和掺量对SHCC抗弯强度、极限弯曲挠度及弯曲荷载挠度曲线的影响.结果表明所提出的弯曲性能计算方法可以较好地预测SF-PVA/SHCC的抗弯强度和极限弯曲挠度.     

An improved image processing method for assessing multiple cracking development in Strain Hardening Cementitious Composites (SHCC)

Strain Hardening Cementitious Composites (SHCC) exhibit tension-hardening behavior accompanied by the formation of multiple cracks. To study the multiple cracking process, cracks are identified from digital images. As conventional image processing technique based on a single threshold of gray intensity cannot accurately determine the width of both fine and wide cracks, a new double-threshold algorithm is developed and its accuracy is verified by comparing with direct measurement under the microscope. Then, an additional algorithm for removing the noises and isolating individual crack regions is introduced. With the improved image processing method applied to a large number of sequential images, detailed information on the development of crack number and width is acquired. The average value and deviation of crack width at a given strain can be calculated to facilitate durability design. Also, with the stress-crack width relation obtained for various cracks, the fiber distribution among cracked sections can be estimated.     

Experiments and failure analysis of SHCC and reinforced concrete composite slabs

For all types of concrete structures, controlling of cracking, as well as the enhancement of serviceability and ultimate flexural capacity are important issues for deck slabs. This study presents an experimental campaign and accompanying nonlinear analysis of a series of Strain Hardening Cementitious Composite (SHCC) and reinforced concrete slab systems, simply-supported and subjected to four-point loading. In order to improve flexural performance both at the service and ultimate limit states, an SHCC layer with thickness of 150–400 mm was placed on the soffit of the composite slab; the SHCC was manufactured using two different processes, namely cast-in-situ SHCCs and extruded precast SHCC panel. Nonlinear analysis of SHCC and reinforced concrete slabs was also carried out to predict moment and curvature as well as deflections of the slab systems. The developed slab systems were found to have enhanced performance with regard to both at serviceability and flexural capacity, compared to the conventional reinforced concrete slab.     

应变硬化水泥基复合材料收缩性能的试验研究

湿度变化引起的收缩开裂在工程裂缝中占很大比重,而裂缝是混凝土结构性能退化和使用寿命缩短的主要原因之一。高模量聚乙烯醇(PVA)纤维的添加可以增强水泥基材料的韧性,使其呈现准应变硬化和多微缝开裂特性,从而显著改善结构的耐久性。自由收缩试验结果表明,应变硬化水泥基复合材料(SHCC)14d龄期能完成80%以上的总收缩,但纤维掺量对自由收缩的影响不显著。限制收缩试验表明,纤维掺量为1.5%的SHCC裂缝控制率可以达到90%以上,同时最大裂缝宽度控制在40μm左右,成为工程上认为的无害裂缝。因此,SHCC作为修复材料或新建结构的永久性模板,可带来巨大的经济效益和社会效益。圆环试验给出了基于耐久性能优化设计SHCC材料的一种实用方法。     

应变硬化水泥基复合材料(SHCC)弯曲韧性研究

砂浆、混凝土等水泥基复合材料易于开裂、耐久性低劣的主要原因是其抗拉强度低、韧性差。高模量聚乙烯醇（PVA）纤维的添加可以增强水泥基材料的韧性，使其呈现准应变硬化和多微缝开裂特性，从而显著改善结构的耐久性。通过四点弯曲试验研究了PVA纤维体积掺量分别为0、0.75%、1.5%的抗折强度，按照ASTM方法确定了SHCC的弯曲韧度指数，通过JCI方法得到了SHCC的弯曲韧性系数。结果表明，最大抗弯承载力和最大挠度均随纤维掺量的增加而增加。结果可由纤维增强材料的应变硬化特性来解释。同时，与数值模拟结果的比较也证实了上述结论。