Drying shrinkage of portland cement pastes I. Microcracking during drying

The occurrence of microcracking of portland cement pastes during drying has been studied by comparing the effects of specimen thickness on shrinkage and cracking using light microscopy. Increases in specimen thickness tended to impede drying and wetting, but there were only slight changes (less than experimental errors) in total and reversible shrinkage once equilibrium was attained. Although microcracking occurred at the beginning of drying whenever the thin specimen (thickness <2mm) was suddenly exposed to low relative humidity (～50%), the cracks eventually closed up. It was concluded that no matter whether or not this microcracking happened, the shrinkage of the specimen after reaching equilibrium was unrestrained.     

Shrinkage cracking in Roman cement pastes and mortars

Roman cements were key materials used in the architecture of the nineteenth and early twentieth centuries. Fine cracks, caused by restrained shrinkage during drying, are a distinct characteristic of all Roman cement stuccoes. Today, cracking has become an important barrier preventing broader acceptance of Roman cement as a material by the restoration and construction sector. Drying shrinkage and tensile properties of Roman cement pastes and mortars submitted to various curing and drying regimes were determined as key parameters controlling cracking. A higher volume of aggregate in the mortar mix and a moderate curing time produce optimum Roman cement mortars from the standpoint of reducing the risk of cracking. Fast drying produced significant microcracking due to moisture gradients and differential shrinkage across the specimens. Stress relaxation observed during the long-time loading of the materials reduced their vulnerability to cracking.     

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.     

Temporal Characteristics of Desiccation Cracking and Resistivity of Lateritic Soil in Drying Process

To describe and measure the initiation and development of desiccation cracks in lateritic soil and to discuss the relationship between crack ratio and resistivity, three parallel specimens are prepared and dried under different temperatures and humidity. The geoelectrical resistivity technique combined with image processing method is applied. Water content, surface crack ratio, and electrical resistivity are monitored during the drying path. Results indicate that the calculated surface crack ratios can be identified on the basis of five distinct stages. The occurrence of the first crack in three specimens indicates that the first cracking time occurs earlier with increasing temperature and decreasing relative humidity. The time that crack ratios stabilize indicates that environmental parameters significantly influence crack evolution. The temporal characteristics of resistivity can also be identified on the basis of five distinct stages. The resistivity and crack ratio can be effectively described by a mathematical equation, which is considered as the basis for the use of the geoelectrical technique for the assessment of the temporal variability of soil desiccation cracking.     

In situ double torsion fracture studies of cement mortar and cement paste inside a scanning electron microscope

The characteristics of microcracking in cement and high strength mortar have been investigated by fracture experiments of small (33×11×1.1mm) double torsion (DT) type specimens, inside the specimen chamber of a conventional scanning electron microscope. The control of incremental crack growth, at less than a micron, is finer than has been reported before. The cracking path is tortuous and quite branched both around and through fine aggregate particles, thus absorbing significantly more energy than a corresponding straight crack. In the vicinity of the main crack tip a zone of diffuse microcracking was observed which may be regarded as a “process zone”. These two observations suggest that fracture mechanics models utilising discrete straight single cracks should only be used with circumspection. In cement paste there also appeared to be a transition from predominantly intergranular to transgranular fracture with increased age of hydration.     

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.     

Properties of concrete with and without calcium chloride during drying and wetting

The effect of calcium chloride on the direct tensile strength of hardened cement paste and concrete cylindrical and prismatic specimens are investigated. Two different techniques for the determination of the direct tensile strength i.e. cylinders with embedded bars and prisms with glued end blocks are used. The results indicate that the addition of 2% calcium chloride significantly increases the direct tensile strength and the dynamic modulus of hardened cement paste. It is concluded that the addition of calcium chloride can partially inhibit the cracking caused by drying and sorption induced microcracking in the concrete system.     

溶胶-凝胶制备二氧化硅薄膜的开裂问题研究

为制备微晶玻璃增强用二氧化硅薄膜,采用溶胶－凝胶工艺（ＳＯＬ－ＧＥＬ）在普通钠钙硅玻璃基片上做了预试,对过程中薄膜的开裂问题进行了详细的分析和研究。结果表明：溶胶的组成与凝胶干燥和热处理过程中的应力不均是开裂的主要原因。借助差热分析（ＤＴＡ）、红外吸收光谱分析（ＩＲ）、高倍光学显微镜等分析测试手段,通过调整原料配比、控制干燥过程的相对湿度和调节热处理过程中的升温试手段,通过调整原料配比、控制干燥过     

Drying-Induced Cracks in Thin Film Fabricated from Colloidal Dispersions

As a colloidal dispersion is coated and dried on a nonporous rigid substrate, the enormous stresses developing during the drying process can fracture the thin film. The drying-induced cracks can produce serous technological consequences and even destroy the efficacy of coatings, which is not desirable in most industrial cases. Therefore, as the first step of controlling cracks in thin film, understanding of crack properties and cracking mechanisms leading to fracture is of vital significance. Although numerous experiments and models have been proposed for cracking during drying of colloidal dispersions, there is little consensus on even the most basic mechanisms, and the effect of heat transfer on cracks as well as optimization of drying process are rarely taken into account. Additional, the broad employments of nanosuspensions bring both opportunities and challenges for this area. This review will give a comprehensive physical picture of thin film fabrication by drying of colloidal dispersions and cracking phenomenon, present current investigations for drying-induced cracks, and point out some prospects for cracking researches especially for industrial R&D, as well as propose combination of thin film preparation with drying technique for exploring crack-free thin film.     

Experimental characterization of the self-healing of cracks in an ultra high performance cementitious material: Mechanical tests and acoustic emission analysis

Self-healing of cracks in an ultra high performance concrete, considered as a model material, is investigated in this paper. An experimental program is carried out in order to quantify the phenomenon, which has been mainly highlighted by means of water permeability tests until now. Mechanical behaviour of self-healed concrete under three points bending, and acoustic emission analysis of the cracking mechanisms are reported. The mechanical tests demonstrate a recovery of the global stiffness, depending on the time of healing, for specimens initially cracked and then self-healed, and a slow improvement of structural strength. The acoustic emission (AE) analysis is performed in order to show that the mechanical response is due to new crystals precipitating in the crack. The microcracking of these products during three points bending tests is highlighted and an energy analysis provides insights about the cracking process of healed concrete, including damage of the newly formed crystals and continuation of the crack propagation.     

混凝土内部湿度场与自约束应力场的研究

为防治混凝土自约束作用导致的开裂现象,对混凝土内部湿度场和自约束应力场开展了研究。通过位移传感器、温湿度传感器对不同强度、养护条件下的混凝土收缩、内部温度和湿度进行测试。之后,通过理论推导,建立混凝土内部湿度、应变与内部湿度关系的理论计算模型。研究表明,混凝土内部湿度随着水泥水化作用和干燥作用的增强而降低。混凝土湿度扩散系数是表征其内部湿度的函数,随着与干燥面的间距增加,混凝土内部湿度扩散系数增大。混凝土的收缩变形与内部湿度之间存在显著的相关性,计算模型与试验结果拟合良好。混凝土内部湿度场的存在导致了应变梯度的存在,进而使混凝土内部产生自约束应力。相同环境条件下,高强混凝土内部自约束应力高于普通强度混凝土。     

Crack formation and self-healing behavior during the drying of alumina gels: Experimental studies

The high shrinkage of alumina gels during the drying process leads to cracks. In this work, the behavior of four alumina gel formulations was experimentally studied following two drying procedures: (i) at ambient temperature and ambient humidity, outside a convective dryer and (ii) inside the dryer, allowing the application of different conditions of temperature and humidity. During the experiments, the observations in real time showed shrinkage of gels and the formation of cracks on them due to drying. Two of the gels displayed a capacity for self-healing, requiring numerical treatment of pictures to distinguish the closure of cracks due to shrinkage from those due to self-healing. The results show the precise determination of shrinkage steps, the estimation of the “shrinkage diffusivity” and proved that the manifestation of the chemical reaction is responsible for self-healing, activated by both applied temperature conditions and residual water of gels. Additional in situ investigations at microscopic scale using an environmental scanning electronic microscope corroborate the self-healing phenomenon noticed at macroscopic scale.     

Measurements of strain and humidity within massive concrete cylinders related to the formation of ASR surface cracks

This study aims at proving the validity of a notion that the formation of non-expansive near-surface layer is responsible for surface cracking in ASR-affected concretes by a laboratory experiment. Relationship between the progress rate of the front of non-expansive layer toward inner portions and the formation of the first surface cracks was scrutinized by measuring relative humidity (R.H.) values and strains within a massive concrete cylinder (?450 mm × 900 mm) with reactive aggregates under a dry environment. It was presumed from the measurements that a non-expansive layer of about 40 mm had been formed at the first cracking. Thereafter, the environmental humidity was raised to > 95% R.H. Pursuit of the growth of surface cracks and subsequent measurements of strains and R.H. values within the concrete cylinder under the moist environment suggested that the re-saturation continuously gave rise to the generation of tensile stresses in near-surface regions leading to active extension of surface cracks.     

Inelastic Deformation Mechanisms and Damage in Structural Ceramics Subjected to High-Velocity Impact

The inelastic deformation mechanisms and damage features observed in structural ceramics subjected to nonpenetrating, high-velocity impacts are similar to those seen in quasistatic Hertzian indentation, albeit more severe. For impacts on large ceramic bodies (relative to impactor diameter), cone cracking is the primary mechanism in regions of high tensile stresses. In regions of nonhydrostatic compressive stresses, depending on the material characteristics, elasticity, grain-boundary microcracking, or plasticity are the primary mechanisms, and depending on their associated energetics, may be able to compete with the initiation and growth of cone cracks. In this regard, a new model is presented that examines the effect of grain-boundary microcracking on cone cracking through shear-induced dilatancy (i.e., bulking) within the quasiplastic zone that forms just underneath the impact site. Depending on the size of the quasiplastic zone and bulking pressure, it is shown that the bulking phenomenon has the potential to suppress cone cracking. Lastly, examples of other shear-driven inelastic deformation mechanisms are presented.     

HIGH HUMIDITY DRYING OF CORN: EFFECT ON DRYING RATE AND PRODUCT QUALITY

Drying of corn was studied at high and low humidity condiitions. Drying rate was measured, and test weight (bulk density), solid density, stress cracks, and breakage susceptibility of the dried grain at 15% (w.b.) were measured. Twenty four 350-g samples were dried under eight drying conditions in the 71° -104°C temperature range and 0.8%-80% RH relative humidity range. Each sample was exposed to the drying wnditions by using a semi-closed air flow cycle through a thin-layer of grain. High humidity drying reduced the drying rate by up to 44% and did not increase bulk density or solid density significantly as wmpared with low humidity drying. However, high humidity drying did reduce the multiple stress cracks in the corn kernels by up to 33.0 percentage points, increased the number of kernels with no stress cracks by up to 28.7 percentage points, and reduced Wisconsin and Stein breakage susceptibilities by up to 8.9 and 13.5 percentage poinu, respectively.     

Characteristics of microcracking in concrete under static and repeated tensile loading

The type and amount of microcracking in normal weight concrete were determined by the direct observation of the slices cut from the necked prismatic specimens subjected to static and repeated tensile loads. It was found from the results of observation under static tensile loading that bond cracks increased almost linearly with increasing load and that mortar cracks developed extensively at about 100% of the ultimate load. The results under repeated tensile loading indicated that the repetitive nature of loading merely conduced to a slight increase in the total amount of internal microcracking. The relations between these findings and two features of the tensile fatigue previously reported were discussed.     

Creep strain versus residual strain of a concrete loaded under various levels of compressive stress

An experimental study of the impact of microcracking of creep behaviour of concrete is presented in this article. Creep and residual strain results (after unloading) observed on specimens loaded in compression at various stress levels (30, 50, and 70% of the compression strength of the studied concrete) are presented for both sealed and unsealed (drying) specimens.The principal conclusion of this experimental study is the following: the residual strain is a linear function of the creep strain. This function is independent of hygral exchange conditions, sealed or unsealed specimens.This conclusion is explained by the assumption of a physical mechanism related to the presence of microcracking during creep.     

Surface modification of cellulose fibers III. Durability of cellulose–polyester composites under environmental aging

The effects of improved interfacial adhesion on the environmental aging behavior of cellulose–polyester composites were studied. The reduction in maximum water content of the composites upon immersion in water which occurs for surface-treated fibers is explained by the restrictions from fiber/matrix network. Whitening of the specimens based on the untreated fibers, or on fibers treated but not covalently bonded to the matrix, was found to be due to the formation of debonding cracks. Such cracks were formed on drying the materials. When the fiber and the matrix are covalently bonded, the matrix follows the fiber during shrinking, and thus no cracks are formed.     

Tensile and Fatigue Behavior of Silicon Carbide Fiber-Reinforced Aluminosilicate Glass

The onset of damage accumulation in ceramic-matrix composites occurs as matrix microcracking and fiber/matrix debonding. Tension tests were used to determine the stress and strain levels to first initiate microcracking in both unidirectional and cross-ply laminates of silicon carbide fiber-reinforced aluminosilicate glass. Tension–tension fatigue tests were then conducted at stress levels below and above the matrix cracking stress level. At stress levels below matrix microcracking, no loss in stiffness occurred. At stresses above matrix cracking, the elastic modulus of the unidirectional specimens exhibited a gradual decrease during the first 10 000 cycles, and then stabilized. However, the cross-ply material sustained most of the damage on the first loading cycle. It is shown that fatigue life can be related to nonlinear stress–strain behavior of the 0° plies, and that the cyclic strain limit was approximately 0.3%.