De-icing salt scaling resistance of mechanically loaded engineered cementitious composites

This paper reports the durability performance of non-air-entrained Engineered Cementitious Composites (ECC) with different fly ash content when subjected to mechanical loading and freezing and thawing cycles in the presence of de-icing salt. ECC is a newly developed high performance fiber reinforced cementitious composite with substantial benefit in both high ductility in excess of 3% under uniaxial tensile loading and improved durability due to intrinsically tight crack width. After 50 freezing and thawing cycles in the presence of de-icing salt, the surface condition visual rating and total mass of the scaling residue of ECC, even those with high volume fly ash content, remain within acceptable limits according to the ASTM C 672. This level of durability holds true even for specimens pre-loaded to cracking at high deformation level. Non-air-entrained mortar specimens with and without fly ash were also used as reference specimens. As expected, these mortar prisms under identical testing conditions deteriorated severely. Pre-loaded and virgin (no pre-loading) ECC coupon specimens were also exposed to freezing and thawing cycles in the presence of de-icing salts for 25 and 50 cycles to determine their residual tensile behavior. The reloaded specimens showed negligible loss of ductility, but retained the multiple micro-cracking behavior and tensile strain capacity of more than 3%. It is also discovered that multiple micro-cracks due to mechanical loading will heal sufficiently under freezing and thawing cycles in the presence of salt solutions to restore nearly the original stiffness. These results confirmed that ECC, both virgin and micro-cracked, remain durable despite exposure to freezing and thawing cycles in the presence of de-icing salts.     

Internal curing of engineered cementitious composites for prevention of early age autogenous shrinkage cracking

This investigation was carried out to study the effects of using a replacement percentage of saturated lightweight fine aggregate (LWA) as an internal curing agent on the shrinkage and mechanical behavior of Engineered Cementitious Composites (ECC). ECC is a micromechanically-based, designed high-performance, fiber-reinforced cementitious composite with high ductility and improved durability due to tight crack width. Standard ECC mixtures are typically produced with micro-silica sand (200 µm maximum aggregate size). Two replacement levels of silica sand with saturated LWA (fraction 0.59–4.76 mm) were adopted: the investigation used 10 and 20% by weight of total silica sand content, respectively. For each LWA replacement level, two different ECC mixtures with a fly ash-to-Portland cement ratio (FA/PC) of 1.2 and 2.2 were cast. In a control test series, two types of standard ECC mixtures with only silica sand were also studied. To investigate the effect of replacing a portion of the silica sand with saturated LWA on the mechanical properties of ECC, the study compared the results of uniaxial tensile, flexure and compressive strength tests, crack development, autogenous shrinkage and drying shrinkage. The test results showed that the autogenous shrinkage strains of the control ECCs with a low water-to-cementitious material ratio (W/CM) (0.27) and high volume FA developed rapidly, even at early ages. The results also showed that up to a 20% replacement of normal-weight silica sand with saturated LWA was very effective in reducing the autogenous shrinkage and drying shrinkage of ECC. On the other hand, the partial replacement of silica sand with saturated LWA with a nominal maximum aggregate size of 4.76 mm is shown to have a negative effect, especially on the ductility and strength properties of ECC. The test results also confirm that the autogenous shrinkage and drying shrinkage of ECC significantly decreases with increasing FA content. Moreover, increasing FA content is shown to have a positive effect on the ductility of ECC.     

高延性纤维增强水泥基复合材料的研究进展及应用

高延性水泥基复合材料(engineered cementitious composite,ECC)是经系统的微观力学设计,在拉伸和剪切荷载下呈现高延展性的一种纤维增强水泥基复合材料.综述了ECC的研究进展,介绍了配筋ECC结构的耐久性、安全性及可持续性等混凝土必须满足的关键性能.根据ECC近来的应用情况及在工程上推广应用的需要,总结了ECC长期性能方面的研究结果.     

Engineered cementitious composite with characteristic of low drying shrinkage

This paper reports a new class of engineered cementitious composite (ECC) with characteristics of low drying shrinkage, tight crack opening and high tensile strain capacity. Research emphasis is placed on the influence of different cementitious matrix on drying shrinkage, tensile property and early age cracking behavior of the composites. Experimental results show that drying shrinkage of the composite is greatly reduced as using the low shrinkage cementitious material in matrix, while the composite remains strain-hardening and multiple cracking characteristics. The measured drying shrinkage strain at 28 days is only 109 × 10^− 6 to 242 × 10^− 6 for low shrinkage ECCs. For traditional ECC, the shrinkage strain at 28 days is nearly 1200 × 10^− 6. The average tensile strain capacity after 28 days curing is 2.5% of the low shrinkage ECC with tensile strength of 4-5 MPa. Further, in the strain-hardening and multiple cracking stage, cracks with much smaller width compared to the traditional ECC are formed in the low shrinkage ECC.     

玄武岩复合材料筋增强ECC受拉性能及裂缝控制机理

工程水泥基复合材料(ECC)因其高韧性和多缝开裂特性成为研究热点,纤维复合材料(FRP)因具有抗拉强度高、密度小、耐腐蚀性好等优点而受到广泛关注。为研究玄武岩复合材料(BFRP)筋增强ECC(BFRP-ECC)的受拉性能以及筋材对基体的裂缝控制机理,考虑了基体类别和配筋率等因素,对ECC狗骨试件、BFRP-ECC和BFRP-砂浆薄板试件进行了单轴拉伸试验,同时借助数字图像相关法(DIC)技术获得了试件受拉过程中的全场应变和开裂状态,基于Richard的弹塑性应力应变公式提出了BFRP-ECC单轴受拉应力应变本构模型。结果表明：BFRP-ECC的极限拉应力随配筋率的增加而增大;ECC基体对复合材料的受拉性能增强效果优于砂浆基体,同时以ECC为基体的复合材料在裂缝间距和宽度控制上都明显优于以砂浆为基体的复合材料;BFRP筋能增加BFRP-ECC裂缝处的桥连应力,减小裂缝间距和宽度,增加裂缝数量。本文建立的BFRP-ECC单轴受拉应力应变本构模型与试验数据吻合良好,较好地反映了BFRP-ECC受拉应力应变关系。     

Autogenous healing of engineered cementitious composites under wet-dry cycles

Self-healing of Engineered Cementitious Composites (ECC) subjected to two different cyclic wetting and drying regimes was investigated in this paper. To quantify self-healing, resonant frequency measurements were conducted throughout wetting-drying cycles followed by uniaxial tensile testing of self-healing ECC specimens. Through self-healing, crack-damaged ECC recovered 76% to 100% of its initial resonant frequency value and attained a distinct rebound in stiffness. Even for specimens deliberately pre-damaged with microcracks by loading up to 3% tensile strain, the tensile strain capacity after self-healing recovered close to 100% that of virgin specimens without any preloading. Also, the effects of temperature during wetting-drying cycles led to an increase in the ultimate strength but a slight decrease in the tensile strain capacity of rehealed pre-damaged specimens. This paper describes the experimental investigations and presents the data that confirm reasonably robust autogenous healing of ECC in commonly encountered environments for many types of infrastructure.     

工程水泥基复合材料动态力学性能及抗爆抗冲击能力研究进展

工程水泥基复合材料(ECC)是一种基于微观力学设计的新型纤维增强水泥基材料,它通过连续稳态开裂过程表现出超高延性和韧性,从而克服了普通水泥基材料抗拉能力弱、易开裂的缺点。本文综述了ECC设计机理、动态力学性能及其在抗爆抗冲击方面的研究现状,分析了材料组分设计、高应变率和高温环境对ECC性能的影响,对ECC材料在抗爆抗冲击领域的进一步研究和发展提出了建议。     

基于ECC自愈能力的耐久性研究*

ECC是一种新型的高延性水泥基复合材料。与普通混凝土材料相比,ECC具有独特的微裂纹和应变硬化行为,具有较高的自愈合能力。因此,ECC在各种环境下比普通混凝土拥有更高的耐久性。为了研究ECC自愈能力与耐久性的关系,我们首先总结了ECC自愈能力的原因;然后,介绍了微生物法、玻璃胶囊填充愈合剂法、膨胀剂与其他外加剂等诱导ECC自愈的方法;最后,详细探讨了在冻融循环、酸腐蚀和疲劳条件下ECC自愈能力对其耐久性的影响。根据目前对于诱导自愈的研究现状,我们提出了要根据实际情况合理运用愈合剂来诱导ECC自愈的建议。     

基于ECC自愈能力的耐久性研究*

ECC是一种新型的高延性水泥基复合材料。与普通混凝土材料相比，ECC具有独特的微裂纹和应变硬化行为，具有较高的自愈合能力。因此，ECC在各种环境下比普通混凝土拥有更高的耐久性。为了研究ECC自愈能力与耐久性的关系，我们首先总结了ECC自愈能力的原因；然后，介绍了微生物法、玻璃胶囊填充愈合剂法、膨胀剂与其他外加剂等诱导ECC自愈的方法；最后，详细探讨了在冻融循环、酸腐蚀和疲劳条件下ECC自愈能力对其耐久性的影响。根据目前对于诱导自愈的研究现状，我们提出了要根据实际情况合理运用愈合剂来诱导ECC自愈的建议。     

Low-energy impact behavior of ambient cured engineered geopolymer composites

Engineered geopolymer composite (EGC) is a new kind of fiber reinforced geopolymer composite with tensile strain-hardening behaviors. This paper was intended to investigate the low-energy impact behaviors of EGC. To further reduce the carbon footprint and material cost of EGC, the feasibility of developing ambient cured EGC with cheap local PVA fiber was discussed according to the micromechanics-based analytical models. The compressive, tensile and impact tests of EGC, engineered cementitious composites (ECC), pure geopolymeric matrix and cementitious matrix were conducted and compared. It was found that the EGC specimens have similar tensile behaviors with ECC and the ultimate tensile strain of EGC can be as high as 7.5%. Under impact load, it was found that the PVA fibers could effectively restrict the crushing and spalling of geopolymeric matrix. Also, the dissipated energy of pure geopolymeric matrix is 3.8 times higher than that of cementitious matrix, indicating that it is recommendable to develop impact-resistant material based on geopolymeric matrix. The influences of NaOH molarity on the impact behaviors of EGC and geopolymeric matrix were discussed. It was found that the impact-resistance of EGC improved with the increase of NaOH molarity, while the threshold of NaOH molarity for geopolymeric matrix was recommended as 12 mol/L. Even though the compressive strength of EGC is lower than ECC, it can be concluded that EGC could have comparable or even higher impact-resistance than ECC under different low-velocity impact conditions.     

乌兰布和沙漠砂制备高延性水泥基复合材料的力学性能

使用内蒙古乌兰布和沙漠砂完全代替微石英砂配置了高延性水泥基复合材料(ECC),并以砂胶比为变量,对其抗压强度、抗拉强度、抗剪强度以及抗弯强度四个方面的力学性能展开了全面的研究。抗拉试验后进一步对纤维断面使用扫描电镜(SEM)进行了观测,并采用X射线衍射分析(XRD)方法研究了沙漠砂的物质组成。结果表明,以沙漠砂配置的ECC,在相同骨料含量的条件下,其抗压强度、抗拉强度、抗剪强度以及抗弯强度均与微石英砂配置的ECC接近,延性约为微石英砂ECC的一半。除抗剪强度外,沙漠砂ECC其他各项性能均随砂胶比增大而提高,优化配比设计的沙漠砂ECC延性能够达到微石英砂ECC的水平。     

Fatigue enhancement of concrete beam with ECC layer

The pseudo strain-hardening behavior of Engineered Cementitious Composites (ECC) is a desirable characteristic for it to replace concrete to suppress brittle failure. This widespread use of ECC in the industry is, however, limited by its high cost. To achieve higher performance/cost, ECC can be strategically applied in parts of a structure that is under relatively high stress and strain. In this paper, layered ECC-concrete beams subjected to static and fatigue flexural loads were investigated by experiments. The static test results showed that the application of a layer of ECC on the tensile side of a flexural beam increased its flexural strength and the degree of improvement increased with the thickness of ECC applied. In addition, the layer of ECC enhanced the ductility of the beam and the failure mode changed from brittle to ductile. Under four-point cyclic loading, the ECC layer significantly improved the fatigue life of the beam. Moreover, in comparison to plain concrete beams, layered ECC beams could sustain fatigue loading at a larger deflection without failure. The great improvement in fatigue performance was attributed to the effectiveness of ECC in controlling the growth of small cracks. The experimental findings reflect the feasibility of using ECC strategically in critical locations for the control of fatigue crack growth.     

矿物掺合料对混凝土中氯离子渗透性的影响

采用可蒸发水含量法、氯离子渗透快速实验法,研究了粉煤灰、硅灰、粉煤灰与硅灰复合掺入及不同龄期等条件制备的混凝土的孔结构、结合氯离子性能及渗透性的变化规律,探讨了掺粉煤灰、硅灰混凝土的孔结构、结合氯离子性能对其氯离子渗透性的影响.结果表明:粉煤灰、硅灰对混凝土的孔结构、结合氯离子性能及氯离子渗透性均存在不同程度的影响.对于掺粉煤灰、硅灰的混凝土,在胶凝材料水化前期,主要是混凝土的孔结构变化引起其6h库仑电量下降;而在胶凝材料水化中后期,主要是混凝土孔结构变化与混凝土对氯离子的结合共同作用导致其6h库仑电量降低.混凝土的孔结构改善及其对氯离子的结合是导致混凝土中氯离子渗透性降低的重要原因.     

The relation between fiber orientation and tensile behavior in an Ultra High Performance Fiber Reinforced Cementitious Composites (UHPFRCC)

In this study, the effect of the fiber orientation distribution on the tensile behavior of Ultra High Performance Fiber Reinforced Cementitious Composites (UHPFRCC) was investigated. The tensile behavior was explored separately in two stages; pre-cracking and post-cracking tensile behaviors. Pre-cracking tensile behavior is expressed using the mechanism of elastic shear transfer between the matrix and the fiber in the composites. Post-cracking tensile behavior was expressed as the combined behavior of the resistance by the fibers and the matrix, considering a probability density distribution for the fiber orientation distribution across crack surface and a pullout model of steel fiber. The effect of the fiber orientation distribution was found to be very small on pre-cracking behavior, but to be significant on post-cracking behavior of UHPFRCC. The predicted results were compared with the experimental results, and the comparison presented satisfactory agreement.     

高韧性混凝土复合材料研究与应用进展

聚乙烯醇(PVA)纤维具有良好的抗拉性能、化学稳定性能与热稳定性能。相对于普通混凝土,通过内掺PVA纤维制成的高韧性混凝土复合材料(ECC)的直接拉伸性能、抗压性能、抗拉性能和抗弯曲性能都得到明显改善,在工程应用中具有极好的社会效益与经济价值。总结了ECC研究与工程应用进展,为纤维在建筑用混凝土性能改性方面的研究提供一些参考。     

Autogenous healing of engineered cementitious composites at early age

Autogenous healing of early ages (3 days) ECC damaged by tensile preloading was investigated after exposure to different conditioning regimes: water/air cycles, water/high temperature air cycles, 90%RH/air cycles, and submersion in water. Resonant frequency measurements and uniaxial tensile tests were used to assess the rate and extent of self-healing. The test results show that ECC, tailored for high tensile ductility up to several percent and with self-controlled crack width below 60 μm, experiences autogenous healing under environmental exposures in the presence of water. However, the recovery for these early age specimens is not as efficient as the recovery for more mature specimen, for the same amount of pre-damage and exposure to the same environment. Even so, the self-healing for these early age specimens demonstrates high robustness when the preloading strain is limited to 0.3%. This conclusion is supported by the evidence of resonant frequency and stiffness recovery of the healed ECC materials.     

Nanoscale characterization of engineered cementitious composites (ECC)

Engineered cementitious composites (ECC) are ultra-ductile fiber-reinforced cementitious composites. The nanoscale chemical and mechanical properties of three ECC formulae (one standard formula, and two containing nanomaterial additives) were studied using nanoindentation, electron microscopy, and energy dispersive spectroscopy. Nanoindentation results highlight the difference in modulus between bulk matrix (~ 30 GPa) and matrix/fiber interfacial transition zones as well as between matrix and unreacted fly ash (~ 20 GPa). The addition of carbon black or carbon nanotubes produced little variation in moduli when compared to standard M45-ECC. The indents were observed by electron microscopy; no trace of the carbon black particles could be found, but nanotubes, including nanotubes bridging cracks, were easily located in ultrafine cracks near PVA fibers. Elemental analysis failed to show a correlation between modulus and chemical composition, implying that factors such as porosity have more of an effect on mechanical properties than elemental composition.     

蒸养条件下矿粉、粉煤灰对高铁相硅酸盐水泥基材料毛细孔和抗侵蚀性能的影响

为提高蒸养高铁相硅酸盐水泥(HFC)构件的抗侵蚀性能,推进HFC在海洋工程中的应用,本文通过力学性能测试、毛细孔测试、氯离子固化测试及X射线衍射等手段,研究了50 ℃蒸养条件下矿粉(SL)和粉煤灰(FA)对HFC水泥基材料毛细孔结构和抗侵蚀性能的影响。结果表明,由于SL具有较高的火山灰反应活性,掺入SL可以提高蒸养HFC砂浆的早期力学性能,同时降低HFC砂浆的28 d毛细孔率。掺入FA显著降低了HFC砂浆的早期力学性能,且后期强度增长缓慢,但FA的“微集料”效应导致HFC砂浆的毛细孔率降低。氯离子固化结果表明,SL促进了水泥对氯离子的物理吸附,而FA促进了水泥对氯离子的化学固化,SL和FA均提高了复合胶凝材料的氯离子结合能力。     

Tailoring engineered cementitious composites for impact resistance

This paper presents results of deliberate tailoring of engineered cementitious composites (ECC) for impact resistance. Microstructure control involving fiber, matrix and fiber/matrix interface was based on steady-state dynamic crack growth analyses accounting for rate dependence of composite phases. Uniaxial tensile stress–strain curves of the resulting impact resistant ECC were experimentally determined for strain rates ranging from 10^? 5 s^? 1 to 10^? 1 s^? 1. Low speed drop weight tower test on ECC panels and beams was also conducted. Damage characteristics, load and energy dissipation capacities, and response to repeated impacts, were studied.     

Tensile and fiber dispersion performance of ECC (engineered cementitious composites) produced with ground granulated blast furnace slag

An engineered cementitious composite (ECC) produced with ground granulated blast furnace slag was developed for the purpose of achieving moderately high composite strength while maintaining high ductility, represented by strain-hardening behavior in uniaxial tension. In the material development, single fiber pullout tests and matrix fracture tests were performed, followed by micromechanical analyses to properly select the range of mixture proportion. Subsequent direct tensile tests were employed to assess the strain-hardening behavior of the composite, which exhibited high ductility and strength with the addition of slag. High ductility is most likely due to enhanced workability and fiber dispersion performance which is attributed to the oxidized grain surface of slag, as verified by fiber dispersion tests. These results suggest that, within the limited slag dosage employed in the present study, the contribution of slag to fiber dispersion outweighs the side-effect of decreased potential for saturated multiple cracking, including a slight increase in matrix fracture toughness and fiber/matrix bond strength.