Influence of composite materials confinement on alkali-aggregate mechanical behaviour

In a previous study [6], we have presented an original experimental procedure for results expansion alcali aggregate reaction. Our main goal is to identify the influence of the confinement by composite materials on alkali-aggregate expansion. In this paper, we are going to present a serie of compressive tests that were carried out in order to investigate the mechanical behaviour during alkali aggregate attack at different ageing periods. We also evaluate the influence of composite material confinement on the mechanical characteristics of these concretes. This will be demonstrated in this second part. The specimens used for expansion were loaded in compression until they fractured at four successive stages in the ageing process. Our results show a significant increase in the mechanical characteristics of the confined alkali reactive specimens in comparison with the non confined specimens: the confined specimens not only withstand compressive stresses four to five times as high as non-confined ones, but their stress/strain curves follow a ‘linear evolution’ whereas non-confined specimens seem to behave like powdery materials. Our experiments show that the increase in strength is much more important with this kind of alkali aggregate concrete confined by composite materials.     

Effectiveness of LiOH in inhibiting alkali-aggregate reaction and its mechanism

A practical alkali-reactive aggreagate—Beijing Aggregate—was used to test the long-term effectiveness of LiOH in inhibiting alkali-aggregate reaction expansion. In this paper, not only the most rigorous conditions, such as the mortar bars being cured at 80°C, for 3 years after autoclaved 24 hours at 150°C, but also the 20°C long-term curing condition, were designed. At these conditions, LiOH was also able to inhibit ASR expansion long-term effectively. Not only the relationship between molar ratio of n(Li)/n(Na) and the alkali contents in systems was established, but also the governing mechanism of such effects was also studied by SEM and EDS.     

Utilization of quarry by-products for reduction of expansion due to alkali-aggregate reaction

The replacement of normal fine aggregate with high fines limestone screenings is a technique to reduce waste from crushed stone operations, while potentially improving the durability and performance of concrete. The main objective of this research is to evaluate varying percentages of high fines limestone screenings as a partial weight replacement of reactive fine aggregate (similar to limestone sweetening) in mortars to assess reductions in the expansion due to alkali-aggregate reaction (ASTM C 227, ASTM C 1260, and modified ASTM C 1105). Fresh and hardened properties have been evaluated to assess the effects of limestone screenings on performance criteria. Environmental scanning electron microscopy (ESEM) was performed to visually observe and confirm alkali-aggregate reaction products and associated damage. From the physical testing and microstructural analysis, it was found that 50% or more of limestone screenings significantly reduce expansions due to alkali-aggregate reaction, attributed to the preference for monocarbonate formation in the presence of limestone and/or reactive silica dilution. Replacements less than 50% did not significantly reduce expansion.     

Accelerated tests for assessing the potential exhibited by concrete aggregates for alkali-aggregate reaction

A number of tests are available for assessing the potential of aggregates for alkali-aggregate reaction. These are listed, for example, in ASTM C33-93. The majority of these test procedures have been widely used in researching aggregates, but there are serious reservations concerning the application of some of these tests to the selection of materials for structures. In recent years, many papers have been published detailing investigations of the tests. An examination of the literature shows that most of the tests are of little value for making engineering decisions. This paper reviews the literature relating to three commonly used tests, the ASTM C-289 Rapid Chemical Test, the ASTM C-227 Mortar Bar Test and what has become ASTM C-1260-94 Accelerated Mortar Bar Expansion Test. Attention has been given to a detailed practical study of the accelerated mortar bar tests since these are becoming very widely used. The experimental work, in essence, represents a determination of the errors to be expected from application of the test to a single aggregate and the factors that influence these errors. The repeatability of the tests suggests an upper bound standard deviation of 0.0365% expansion. This figure is used to establish discriminant criteria for the recognition of potentially damaging materials. It is evident that diagnosis of potentially highly reactive materials can be made with confidence, but that a large number of tests are required to assess marginal types. Petrographic evaluation of the mortar following the test is essential, particularly where the results are borderline, though the presence of traces of gel has to be interpreted with caution. Much of the uncertainty in the test results derives from inherent aggregate variability. A test schedule can be designed that has small contributions to the error from the experimental steps, but the error due to aggregate variability remains. This can lead to the requirement for the manufacture of multiple batches of mortar for establishing acceptance criteria for even a single aggregate batch, and further consideration is needed as to the procedures necessary for the assessment of aggregate sources.     

Influence of capillary forces on the regime of water evaporation in high-temperature rocks

A one-dimensional problem of the injection of water into a geothermal reservoir saturated with a superheated vapor in the presence of capillary forces has been investigated in an isothermal approximation. It is shown that in wettable rocks the capillary forces increase the velocity of motion of the front and decrease it in unwettable ones. If the capillary forces play a decisive role, then in wettable rocks an extended region of phase transitions is formed behind the evaporation front under the action of capillary forces. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 1, pp. 176–183, January–February, 2008.     

Influence of temperature on the sorption properties of rocks from the Nizhnekansky massif

The kinetics of radionuclide sorption onto finely divided samples of rocks from the Nizhnekansky massif, taken from the level of the planned arrangement of deep radioactive waste repository, was studied, and the times in which the sorption equilibrium was attained at 20 and 90°С were determined. The distribution coefficients of actinides (²³³U, ²³⁹Pu, ²⁴¹Am) and fission products (¹³⁷Cs, ⁹⁰Sr) were determined. Increasing the temperature from 20 to 90°С intensifies the sorption of the radionuclides studied, except cesium for which the sorption slightly decreases.     

Influence of surface microstructure on the reaction of the active ceramics in vivo

When porosity and macro-pore size differ in the same ceramic, the mode of bone regeneration and the degradation of ceramics in vivo is said to be different. However, the reactions in vivo of ceramics that have a different microstructure with the same porosity and the same macro-pore size, are not so far known. In this study, two kinds of -tricalcium phosphate (TCP) that had different microstructures but the same porosity and macro-pore size, were manufactured. These TCP were implanted in the distal femurs of 20 mature male rabbits, and their respective areas of ceramics and of regenerated bone were measured after 4, 12 and 24 wk. In both TCPs, the regenerated bone similarly decreased from 4–24 wk in a different way. The area of ceramics in one of these TCPs significantly decreased gradually throughout the observation period. On the other hand, the other TCP showed no marked decrease during the same period. This suggested a possibility that the difference in micro-structure has a large effect on the reaction of the ceramics in the bone.     

Influence of temperature on the speciation of radionuclides sorbed onto rocks from the Nizhnekansky massif

The influence of temperature on speciation of actinides (Pu, Am) and some fission products (Cs, Sr), sorbed from simulated groundwater onto rocks from the Nizhnekansky massif (gneiss, dolerite) under anaerobic conditions at 20 and 90°С, was studied by sequential extraction. An increase in the contact temperature leads to an increase in the radionuclide uptake percentage and, in some cases, in the content of tightly fixed forms. The radionuclide species undergo redistribution with time.     

Evaluating the reaction of actual compositions of concrete with respect to alkali-aggregate reactions preliminary testing at 110°C and 150°C

Different curing methods at 110°C and 150°C were tested on concrete prisms of various compositions in order to develop a rapid expansion test to evaluate the risl of AAR in actual concrete compositions. While no significant expansion occurred at 110°C (in water or water vapour), we obtained expansion values in the range 0–0.35% for concrete prisms cured at 150°C in water or in alkaline solution during 3–5 weeks. For both curing methods at 150°C, the highest expansion values are obtained for potentially reactive aggregates and high alkali levels in the concrete, whereas the lowest expansions correspond to the concrete mixes containing only non-reactive aggregates or having a total alkali content of less than 2.5 kg m⁻³. However, alkaline treatment accounts better for the effectiveness of mineral admixtures and, in general, for the reactivity of the concrete compositions, than curing in water. From the correlations obtained with data at 38°C or 60°C and 100% RH after 12 and 4 months, respectively, alkaline curing for 3 weeks at 150°C shows a reasonable potential for predicting accurately the influence of the total alkali content, nature of the aggregates, water/cement ratio and effectiveness of mineral admixtures on concrete expansion.

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Resume La réactivité potentielle des granulats vis à vis des alcalins du béton a fait l'objet de travaux nombreux portant sur des méthodes cccélérées de dépistage des granulats. Cependant, les essais visant à évaleur le risque associé à une composition réelle de béton sont appelés également à connaitre un développement rapide. Des travaux précurseurs ont été menés au Canada, au Royaume Uni, au Japon et en France. Nos travaux ont porté sur des méthodes d'expansion sur prismes de béton 7×7×28 cm de compositions variées. 12 ciments Portland, 19 sables, 20 graviers, ainsi que 11 ajouts pouzzolaniques et divers adjuvants ont été utilisés pour ces combinaisons. Les éprouvettes ont été soumises à des traitements tels que bain de vapeur à 110°C ou 150°C, immersion dans l'eau à 110°C ou 150°C, ou immersion en solution alcaline à 150°C, et cela pendant des durées allant jusqu'à 8 semaines. Parallèlement, des mesures d'expansion après conservation de prismes dans des conditions conventionnelles (38°C et 60°C en milieu saturé d'humidité) étaient réalisées. Les gonflements obtenus après plusieurs semaines de conservation à 110°C se sont révélés faibles quelle que soit la réactivité supposée des compositions de béton (présence de granulats réactifs, niveau élevé d'alcalins dans le béton). Les traitements à 150°C, que ce soit dans l'eau ou en solution alcaline, conduisent à des expansions de 0,00 à 0,35% en trois semaines. Les valeurs mesurées sont liées à la réactivité (ou la non réactivité) suspectée des bétons étudiés: présence de granulats très réactifs (respectivement non réactifs), teneur élevée (respectivement faible) en alcalins totaux dans le béton. Par contre, s'agissant de certains bétons, notamment à ajouts siliceux, le traitement dans l'eau à 150°C ne produit pas les résultats escomptés. En conséquence, les résultats d'expansion provoquée par ont été corrélés aux mesures de gonflement après conservation à 38°C pendant un an et plus d'une part, et à 60°C pendant 4 mois d'autre part. Les corrélations sont jugées satisfaisantes pour la plus grande partie des compositions testées, à, l'exception de certaines formules contenant des fumées de silice ou de la silice micronisée. Toutefois, la méthode proposée rend bien compte de l'influence vis à vis du gonflement de la teneur en alcalins totaux du béton, de la présence de granulats pouvant présenter un effet de pessimum, et des proportions des constituants tels que eau et ciment.

     

The effect of alkali-aggregate reactivity on the mechanical properties of high and normal strength concrete

In this investigation, a potentially highly reactive aggregate, and a potentially moderately reactive aggregate (identified by accelerated mortar bar testing and petrographic examination) were used in the preparation of concrete of normal and high strength concretes. After the initial 28 day curing period, the specimens were equally divided, and then submerged in a holding tank containing either a solution of a sodium hydroxide or de-ionised water at 80 °C for a period of 12 weeks. Normal strength concrete specimens containing the potentially highly reactive aggregate and exposed to the sodium hydroxide solution experienced more losses in mechanical properties than the concrete specimens prepared with potentially moderately reactive aggregates. However, in high strength concrete specimens exposed to the sodium hydroxide solution, there was a minimal loss in mechanical properties for both the specimens containing the highly reactive or moderately reactive aggregates. The superior performance of high strength concrete can be explained by the improved micro-structure and decreased permeability due to the formation of secondary calcium silicate hydrate formed as a result of the pozzolanic reaction.     

Influence of time factor on antimigration properties of a barrier based on sand-clay rocks

Variation of the speciation of Np, U, and Cs in a sand-clay rock from the Kornilovskoe deposit in relation to the time of radionuclide contact with the rock was studied, and mineralogical changes in the rock composition during 500-year period of functioning of a barrier based on this material were estimated. With an increase in the time of the barrier functioning, migration of radionuclides through sand-clay rocks will decrease. Neogenic montmorillonite will also be formed, which will enhance the sorption properties of the barrier under the conditions of a surface repository of solid radioactive wastes.     

不同反应气源对制备纳米金刚石膜的影响

为确定两种典型的反应气源对制备纳米金刚石膜的影响,分别以CH4/Ar/H2及CH4/N2混合气体作为反应源,用微波等离子体化学气相沉积(MWPCVD)法制备纳米金刚石薄膜.XRD和Raman分析表明两种气源条件下得到的膜材均为金刚石多晶膜,但用CH4/N2气为反应源沉积的膜材中非金刚石相成分明显更多;AFM和SEM对照分析证实所有膜层的平均晶粒尺寸及表面粗糙度均在几十纳米量级,但CH4/N2气源沉积的膜中容易形成异常长大的晶粒,不利于表面质量的提高.研究结果表明,以CH4/Ar/H2混合气体作为反应气源可制备物相组成纯度更高、表面形态更为优越的纳米金刚石膜.     

Influence of matrix properties on alkali silica reaction rates

Matrices with different cracking and confinement characteristics are utilized to isolate the important parameters that influence gel initiation and alkali silica reaction rate. The paper is divided into two parts. In Part I, the matrix exhibits crack growth resistance behavior which induces compressive stresses on the alkali silica gel and in addition prevents the alkali silica gel from leaving the reaction site, which slows down the alkali silica reaction rate and modifies the gel composition. In Part II the matrices are designed to provide little confinement and hence allow the alkali silica gel to expand almost freely. However, the matrices in Part II differ in their ability to allow the alkali silica gel to escape the reaction site. The results reveal that lack of confinement accelerates early gel formation whereas the lack of escape of alkali silica gel products away from the reaction site reduces the alkali silica reaction rate.     

反作用轮铰间隙对航天器动态特性影响分析

反作用轮是高精度航天器姿态控制系统的主要扰动源之一,其转动副间不可避免的含有间隙,研究铰间隙对航天器动态特性影响,对建立更精确的反作用轮扰动模型至关重要。基于虚拟样机技术,采用非线性弹簧阻尼模型建立间隙处的接触碰撞模型,同时采用修正的Coulomb摩擦模型考虑运动副间隙处的摩擦作用,进而建立考虑反作用轮铰间隙的卫星系统虚拟样机模型,对其进行动力学仿真分析,详细分析间隙对系统各动态变量的影响,为卫星系统的结构设计及控制策略的制定提供参考。     

Influence of the solid-phase reaction in fullerite-alkali-metal-halide systems on optically detectable vibrational states of C60

Spectral data on the solid-phase reaction of powdered C₆₀ with potassium halides, which is regarded as the main cause of the modification of the characteristic bands in the IR spectra, are presented.