碱对水泥基材料早期收缩性能的影响

用椭圆型开裂环自动测试装置和非接触式电阻率测定仪以及JAF量热计,分别测试不同水灰比、不同碱度水泥砂浆的初始开裂时间以及水泥浆体早期水化的电阻率与水化放热速率。结果表明：碱度增加虽然加速水泥的水化,但削弱了水泥基材料抵抗收缩变形的能力,在低水灰比下表现更为明显。水泥基材料的初始开裂时间与电阻率曲线上的特征点以及最大放热峰之间存在一定的联系。所提出的阻率步长与热率步长之比值K与水泥基材料的开裂敏感性及碱含量密切相关。     

Fracture of glass fiber reinforced cement

Fracture patterns produced when a crack advancing from a notch in cement paste intersected a Cem FIL-2 glass fiber strand placed perpendicular to it were studied. The specimens were small notched compact tension specimens that could be wedge loaded in the scanning electron microscope chamber using a wet cell. Four distinct cracking patterns were identified. In most cases the fiber caused a shift in the crack path and in some specimens, microcracking and separation of the crack into 2 to 4 branches were observed. The filaments maintained their continuity and bridged over the track. Similar tests with E-glass fibers after accelerated curing revealed brittle behavior in which the crack path was straight and many filaments were broken.     

Effect of superabsorbent polymer on the shrinkage and crack resistance of concrete at early age

The superabsorbent polymer (SAP) with different volumes and modes of entrained water is incorporated into concrete to study the effect of SAP on the compressive strength, flexural strength, shrinkage, plastic cracking and electrical resistivity of concrete at early age. The result indicated that the influence of SAP with different modes of entrained water on the early strength of concrete is relatively obvious. It is beneficial for the early strength of concrete to add pre-absorbed SAP whose internal curing (IC) is detracted from mixing water and unabsorbed SAP. But the early strength of concrete is decreased by adding pre-absorbed SAP. The effect of pre-absorbed SAP is limited on shrinkage of concrete when IC water is detracted from mixing water. The unabsorbed SAP gradually decreases the concrete shrinkage with its volume increase. The moderate volume of pre-absorbed SAP can decrease the early concrete shrinkage. In addition, SAP can effectively alleviate the early appearance and development of concrete cracks. Furthermore, the pre-absorbed SAP can efficiently enhance the crack resistance of concrete. When the unabsorbed SAP is added, the cracking of concrete is reduced with the increase of volume. The longer the curing age, the more obvious would be the influence of SAP on the internal humidity of concrete, which effectively delays the increase of concrete resistivity.     

Using acoustic emission to quantify damage in restrained fiber-reinforced cement mortars

This paper describes the use of acoustic emission (AE) for monitoring early-age cracking in restrained fiber-reinforced mortars. A steel-testing frame was used to prevent the length reduction associated with drying. AE sensors placed on both unrestrained and restrained specimens detected a high degree of activity that may be attributed to surface microcracking caused by moisture gradients that cause the surface to shrink more rapidly than the core. It was found that as the concrete neared the age of visible cracking, the acoustic waves generated in the restrained specimens had a greater amplitude and duration. For this reason, acoustic energy was utilized for these investigations. An increase in acoustic energy was detected before cracks were observed in the restrained specimens. It is believed that the role of fiber reinforcement is twofold. First, fibers arrest cracks thereby preventing unstable crack propagation, and second, they restrain the crack from opening preventing the cracking from becoming visible until a later age.     

Shrinkage cracking of lightweight concrete made with cold-bonded fly ash aggregates

Shrinkage cracking performance of lightweight concrete (LWC) has been investigated experimentally on ring-type specimens. LWCs with and without silica fume were produced at water-cementitious material ratios (w/cm) of 0.32 to 0.55 with cold-bonded fly ash coarse aggregates and natural sand. Coarse aggregate volume ratios were 30%, 45%, and 60% of the total aggregate volume in the mixtures. A total of 12 lightweight aggregate concrete mixtures was cast and tested for compressive strength, static elastic modulus, split-tensile strength, free shrinkage, weight loss, creep, and restrained shrinkage. It was found that the crack opening on ring specimens was wider than 2 mm for all concretes. Free shrinkage, weight loss, and maximum crack width increased, while compressive and split-tensile strengths, static elastic modulus, and specific creep decreased with increasing coarse aggregate content. The use of silica fume improved the mechanical properties but negatively affected the shrinkage performance of LWCs. Shrinkage cracking performance of LWCs was significantly poorer than normal weight concrete (NWC).     

Flaws Responsible for Slow Cracking in the Delayed Fracture of Alumina

The early stages of crack extension from inherent flaws were observed directly in order to identify flaws responsible for crack initiation. The specimen surface was immersed in fluorescent dye penetrant while cracks were forming; this procedure allowed the dye to penetrate into the fine cracks. Once the cracking sites were located, scanning electron microscopy was used to identify the flaws. An eccentrically loaded column testing system was used to produce a number of crack initiations on a surface of a specimen without causing catastrophic failure of the specimen. There were many inherent flaws which, either by themselves or as an assembly, became potential crack origins. The initial stage of delayed fracture was shown to involve the interaction and coalescence of nearby flaws with intergranular cracking. It was also found that the crack origins were multiple in delayed fracture.     

Humidity Drying of Plastic Clay

The humidity drying cycle gives rise to surface and body cracks in plastic clay. The cracks appear in the initial stages when the clay is being heated in a saturated atmosphere. Various factors affecting cracking in this initial heating stage were studied using a purified kaolin in the plastic state, and appropriate experimental models were used to isolate certain aspects for study. It is shown that the condensation of moisture on the clay surface is the major factor influencing the cracking of the plastic clay. The effect of thermal diffusion is insignificant and the loss of mechanical strength is of secondary importance. The balance af evidence from the literature and from this study indicates that capillarity plays a major part in the movement of moisture and the production of stresses during drying. When air spaces are present in the body, thermal capillarity may become important. Stresses caused by thermal expansion oppose those caused by drying shrinkage, while the thermal expansion of capillaries will have a negligible effect.     

黄麻与聚乙烯醇纤维增强水泥土的弯曲及早期抗裂性能

深层水泥土搅拌桩围护墙具有水泥基材料的特性,包括脆性破坏以及较低的拉伸强度和弯曲强度,且温度应力产生的干燥收缩可能会导致裂缝的产生并引起墙体渗漏和塌陷。本文研究了黄麻纤维和聚乙烯醇(PVA)纤维增强水泥土搅拌桩的弯曲性能和裂后性能,并对纤维改善水泥土早期干缩裂缝的效果进行了对比。结果表明,随着黄麻纤维和PVA纤维含量的增加,第一裂缝弯曲强度和峰值弯曲强度均逐渐增加。纤维对改善水泥土的裂后性能起着至关重要的作用,水泥土残余弯曲强度比、延性指数和韧性随纤维含量增加显著提高。黄麻纤维在韧性方面的表现略好于PVA纤维,在其他裂后指标上两种纤维差距较小。采用数字图像相关方法研究纤维对水泥土早期塑性收缩裂缝的影响,结果表明,水泥土中添加纤维可有效抑制干燥条件下收缩裂缝的形成和扩展,纤维的掺入有效减小了水泥土干缩裂缝的宽度和数量,且纤维含量越多效果越佳。     

Influence of silica fume on the desiccation cracks of compacted clayey soils

Clayey soils containing smectites are widely used for construction of liner and cover systems to reduce the hydraulic conductivity in geotechnical applications because of their low permeability and high cation exchange capacity. However, the compacted clayey soils crack on drying because of their high swelling potential, and their hydraulic conductivities increase. To solve this problem, it is essential to stabilize the clayey soils using additive materials. The aim of this study is to examine the suitability of silica fume as a stabilization material to reduce the development of desiccation cracks in compacted clayey liner and cover systems. Natural clayey soil and clayey soil–silica fume mixtures were compacted at the optimum moisture content and subjected to laboratory tests. The results show that silica fume decreases the development of desiccation cracks on the surface of compacted samples. We concluded that silica fume waste material can be successfully used to reduce the development of desiccation cracks in compacted clayey liner and cover systems.     

3D characterisation of indentation induced sub-surface cracking in silicon nitride using FIB tomography

In this study, a combination of 3D FIB tomography and incremental surface polishing has been used to characterize cracking beneath 0.5 kg and 1 kg Vickers indentations on silicon nitride. It is shown that a half-penny cracking regime exists even for low indentation loads with c/a ratios < 2 indicating that the c/a ratio cannot reliably be used to predict sub-surface crack morphology. For the first time, the presence of deep lateral cracks interconnected with radial cracks was also observed surrounding indentations of low loads on silicon nitride, and it is likely that these could contribute to material removal via spalling.     

Effect of cracking in drying and shrinkage specimens

The significance of cracking and microcracking caused by nonuniform drying shrinkage of test specimens is analyzed. To assure that no cracks are produced by drying in load-free specimens, one must lower the environmental humidity gradually and sufficiently slowly, and use very thin specimens (about 1 mm thick). Graphs for the maximum admissible rate of change of environmental humidity, calculated from both linear and nonlinear diffusion theories, are provided. The spacing and width of parallel cracks due to drying are estimated from fracture mechanics considerations. In normal size specimens the drying cracks are usually too narrow to be visible. Drying leads to discontinuous microcracking rather than continuous macrocracks and is represented better as strain softening than as an abrupt stress drop. Shrinkage cracking can increase drying diffusivity by several orders of magnitude.     

A preliminary SEM study of crack propagation in mortar

A device constructed to permit the testing of wedge-loaded compact tension specimens of mortar within the sample chamber of an SEM is described. Using this device, the process of cracking was observed in mortar specimens. It was found that the process of crack extension in mortars is very complicated: the crack is tortuous, there is some branch cracking, discontinuities in the cracks are observed, and there is some tearing away of small bits of material in some areas of cracking. The results suggest that the simple fracture mechanics models oversimplify the geometric features of the crack extension process.     

Influence of internal stress and extensibility on the exterior durability of wood coatings

Different types of coatings were characterised for: swelling by moisture, moisture permeability, elastic modulus and extensibility. These data were used to calculate maximum internal stress. Internal stress was also measured directly whilst changing temperature and humidity. The stress data were compared to cracking and flaking results in both natural and accelerated weathering tests. Results showed that high levels of stress occur if there is a difference in swelling between coating and wood. In most cases stress relaxation took place rapidly. Comparing maximum stress, extensibility and cracking, a good correlation between these data was found. The ratio between maximum stress and extensibility appeared to be a very good predictor for crack formation.     

Water desorption and shrinkage in mortars and cement pastes: Experimental study and poromechanical model

The aim of the present research was to study moisture changes and strains induced by smooth water desorption of several cement based materials. The main advantage of this small-steps drying is to dramatically limit the structural effect within tested samples, by lowering moisture gradients and therefore cracking due to differential shrinkage. Resulting data are of importance as they allow water retention curves, porosity distribution and desiccation shrinkage to be determined versus a large range of relative humidity. Experiments were conducted on ordinary mortars and cement pastes with water-to-cement ratio of 0.5 and 0.8. The role of the cementitious matrix and of aggregates over water-related behaviour of these materials can also be studied. Finally, a simple numerical model, based on experimental poromechanical results, was proposed to predict the shrinkage when the material is submitted to drying.     

The use of crack layer theory to predict the lifetime of the fatigue crack growth of high density polyethylene

In the case of high density polyethylene (HDPE), the fatigue crack propagates in a discontinuous manner, which can be observed by distinct striations. In this article, fatigue crack growth (FCG) experiments were conducted on two grades of HDPE pipe with compact‐tension (CT) and cracked round bar (CRB) specimens. The effects of the stress ratio (R‐ratio) which is defined as the ratio of minimum stress and maximum stress of fatigue loadings and the frequency on FCG behavior were experimentally studied. Although FCG rates showed a great dependence on the R‐ratio in terms of the range of the stress‐intensity factor, the effect of the frequency may be considered to be significant in the low crack growth region. In addition, these experimental data were employed for predicting the lifetime on the basis of the crack layer (CL) theory. Only a few steps of FCG are needed to determine all necessary parameters for CL theory, and the FCG behavior can be reconstructed based on a computer program that has been developed for the application of CL theory. The predictions from this program accord with experimental data. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Influence of water on stress corrosion cracking of epoxy bonds

Environment assisted fracturing, or stress corrosion cracking of adhesive joints in liquid water or a humid atmosphere was shown to occur at values of applied crack extension force ?_a considerably below those required for onset of opening-mode rapid fracture ?_c. The minimum value of ?_a below which no cracking was observed, ?_SSC, was dependent on the relative humidity and the particular adhesive used. For two adhesive systems, the one with the lower hardener content and post cure temperature showed both a lower absolute value of ?_SSC and a lower ratio of ?_SSC to ?_c. The value of ?_SSC for both adhesive systems decreased as the humidity was increased. As the relative humidity approached 100%, the value of ?_SSC approached its value for liquid water. Values of ?_c for the two adhesives differed by a ratio of approximately 2:1. When water was introduced to the crack tip, ?_c for the less tough material increased while it was not significantly changed for the tougher material.     

Modeling the effect of damage on electrical resistivity of melt-infiltrated SiC/SiC composites

Electrical resistivity (ER) measurements are a possible health monitoring technique for ceramic matrix composite components in future aerospace applications. In order to use ER measurements to detect and identify damage, it is necessary to understand how each specific damage state will affect the ER response. In this study, finite element models are developed and applied to quantify the effect of specific damage states on the ER response in a melt-infiltrated silicon carbide (SiC) fiber-reinforced SiC composite. The ER of several damage states are calculated by simulating the electric current flow through the damaged microstructure. This is achieved by performing the numerical solution of the steady-state conservation of charge density equation. Numerical results reveal the effect of various cracking features on the ER response such as type of cracking, extent of cracking, crack density and fiber/matrix debonding.     

Mechanosorptive Creep of Hemlock under Conventional Drying: I. The Determination of Free Shrinkage Strain

The dynamic free shrinkage behavior of Western hemlock (Tsuga heterophylla) was investigated in order to provide the theoretical basis for the determination of drying mechanosoptive creep mechanism. Small specimens taken from five distinct positions within a log were dried at three constant temperatures (40, 60, and 80°C) and stepwise decreasing relative humidity conditions and their free shrinkage strain S in the tangential and radial directions was determined. The anisotropic shrinkage ratios G were also calculated and analyzed as functions of moisture content and temperature. The influential mechanism of the drying temperature, moisture range, and specimen position on the free shrinkage behavior was revealed. Two different function types were chosen to simulate the relationship between the free shrinkage strain and its variables (temperature, moisture, wood positions) for the specified moisture ranges.