How to study drying shrinkage microcracking in cement-based materials using optical and scanning electron microscopy?

Three different ‘destructive’ microscopy methods were tested on their ability to show drying shrinkage microcracks on a specimen cross-section. The first two were methods in which the microcracks were impregnated with a fluorescent epoxy and examined with fluorescence microscopy. In one method, the impregnation was applied before making the cross-section and in the other after making the cross-section. In the third method, the sample was kept wet constantly and examined in an environmental scanning electron microscope (ESEM). It was concluded that the method in which the dried specimen was impregnated before making the cross-section was the most reliable method to record drying shrinkage microcracks. With this method, it was possible to impregnate the complete drying shrinkage microcrack pattern in the studied cement-based materials from the surface, and there was no risk of recording microcracks introduced by sample preparation.     

氧化锆增韧陶瓷的相变及相变增韧

本文阐述了氧化锆增韧陶瓷（ＺＴＣ）的相变增韧机理，并探讨了热处理工艺对ＺＴＣ的相变及显微组织的影响规律。     

Long-term performance of cement paste during combined calcium leaching-sulfate attack: kinetics and size effect

This paper presents experimental results obtained on cement paste samples (water/cement ratio of 0.4) subjected to a low-concentration (15 mmol/l) external sulfate attack during several weeks. Chemical and microstructural analyses include the continuous monitoring of calcium loss and sulfate consumption within the cement paste, periodic layer by layer X-ray diffraction (XRD)/energy-dispersive spectrometer (EDS) analyses of the solid constituents of the cement matrix (ettringite, portlandite, gypsum) within the calcium-depleted part of the samples. Scanning electron microscopy (SEM) and visual observations are used to follow the crack pattern evolution during the external sulfate attack. The relation between the size of the specimen and crack initiation/development is investigated experimentally by performing tests on samples with different thickness/diameter ratios.     

A comparison of bending strength between adhesive and steel reinforced concrete with steel only reinforced concrete

Cracking of brittle cementitious composites subjected to excessive loading causes a potential reduction in material performance. Steel bars or metal fibers typically act as tensile reinforcing in concrete composites to increase the material's structural capacity in bending and to delay or prevent matrix cracking.The goal of this research is to determine whether the performance in bending strength and material integrity of a typically reinforced cementitious composite may be improved through the release of “healing” chemicals, such as adhesives, from hollow fibers into cracks induced by loading in addition to the metal reinforcing. Adhesive-filled repair fibers are intended to break immediately upon cracking in the concrete thereby activating the healing process with the release of a sealing or adhering substance. This self-repair occurs whenever and wherever cracks are generated.     

Microcrack Formation During Gas Metal Arc Welding of High-Strength Fine-Grained Structural Steel

The recent development of high-performance-modified spray arc processes in gas metal arc welding due to modern digital control technology and inverter power sources enables a focused spray arc,which results in higher penetration depths and welding speed.However,microcracks occurred in the weld metal while approaching the process limits of the modified spray arc,represented by a 20-mm double layer DV-groove butt-weld.These cracks were detected in structural steel exhibiting a yield strength level of up to 960 MPa and are neither dependent on the used weld power source nor a consequence of the modified spray arc process itself.The metallographic and fractographic investigations of the rather exceptional fracture surface lead to the classification of the microcracks as hot cracks.The effects of certain welding parameters on the crack probability are clarified using a statistical design of experiment.However,these microcracks do not impact the design specification for toughness in the Charpy V-notch test(absorbed energy at —40 ℃ for the present material is 30 J).     

Damage monitoring of cement paste by electrical resistance measurement

Electrical resistance measurement is effective for monitoring damage (due to damage infliction and subsequent microcrack opening) and healing (due to microcrack closing) of cement pastes (plain, with silica-fume, and with latex) in real time during repeated compressive loading. Damage causes the resistance to increase; healing causes the resistance to decrease.     

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.     

Nonlinear ultrasonic evaluation of load effects on discontinuities in concrete

The presence of discontinuity surfaces in concrete structures, i.e. two or more layers in contact, may be an existing situation with evident relapses on damage formation and progression. Differences occur depending on the type of discontinuity, which could be a thin weaker layer or a pre-existing crack. The behavior of pre-existing interfaces is here studied by means of the Scaling Subtraction Method, a Nonlinear Ultrasonic Non-Destructive Technique, that revealed to be effective in describing the mechanical evolution of concrete samples with discontinuity surfaces under the effects of compressive loads.     

Crack effects on gas and water permeability of concretes

The relationship between load-induced cracking and concrete permeability is studied. Ordinary concrete (OC) and high-performance concrete (HPC), including steel fiber-reinforced concrete (HPFRC), are used. Two discs, 50 mm-thick slices, cut from 110-220 mm cylindrical specimens are diametrically loaded, as for a normal splitting test. The lateral displacement, also called the crack opening displacement (COD) is monitored for each loading cycle. After unloading, gas and finally water permeability tests are both performed, using constant head permeameter, to compare the influence of the percolating fluid and the COD. Due to the wide range of measured gas flow, Klinkenberg's and Dupuit-Forcheimer's laws are applied to compute the intrinsic gas permeability. Results suggest it increases proportionally to the cube of the COD and it matches water permeability, if only the first water percolating time is considered. The roughness parameter of the cracks induced in each concrete, is compared and discussed.     

Prevention of microcracking in dissimilar multipass welds of alloy 690 to type 316L stainless steel by Ce addition to filler metal

Abstract

The microcracking susceptibility in dissimilar multipass weld metals was investigated by a multipass weld test using different type 316L stainless steels with varying P and S contents and using different alloy 690 filler metals with varying Ce contents. The relation between microcracking susceptibility and (P+S) and Ce contents in every weld pass of the multipass weld was investigated. Ductility dip cracks occurred in the compositional range of Ce/(P+S)<0·22, and solidification/liquation cracks occurred in that of Ce/(P+S)>1·1, while no cracks occurred at Ce/(P+S) between 0·22 and 1·1. The ductility dip cracking susceptibility could be improved by adding Ce due to scavenging of impurity elements. Microcracking could be completely prevented in dissimilar multipass weld metals using two kinds of filler metals containing 0·077 wt-%Ce for the weld passes beside the stainless steel base metal (320 ppm P and 183 ppm S) and containing 0·032 wt-%Ce for the other weld passes.