【作者】 李美利；

【导师】 钱觉时；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2011， 博士

【摘要】 要使混凝土获得充分的潜在性能,合理充分的养护是必须的,尤其是早期的充分潮湿养护。然而,混凝土施工过程中并未给予足够的重视,甚至经常被完全忽视。其主要因为是目前尚没有一种有效的方法对混凝土的养护效果进行定量评价,对早期不良养护造成的后期混凝土质量的下降无法进行后评估,也无法在合同中对养护达到的效果进行约束。因此,准确评价混凝土的养护效率,有益于混凝土的质量控制。混凝土的电阻率对其内部含水率的影响非常敏感,混凝土含水率越高,电阻率越低,良好养护的混凝土在早期通常含有比较高的含水率,因此其电阻率比较低,而且由于不同养护情况下混凝土的含水率变化只在距离表层较小区域内,本文通过研究不同养护条件下表层混凝土电阻率的变化规律,来建立混凝土潮湿养护效率的电阻率评价方法。为了保证电阻率测量值稳定,首先研究了采用恒电位仪测量混凝土电阻率的稳定性和复现性,通过恒电位仪和数字万用表对阻值为0.33～100 k?十二个标准电阻进行了测试,发现恒电位仪与数字万用表测试电阻具有良好的相关性,而恒电位仪测试高电阻更为稳定,因此采用恒电位仪测试具有高电阻特性的混凝土电阻率更为合适。进一步采用恒电位仪研究了电极间距、离子浓度、电极与混凝土接触面积等因素对混凝土电阻率测试结果的影响,表明恒电位仪可以用于混凝土电阻率的测量,特别是比较容易获得稳定测试结果。设计了埋入深度10、20、30、40 mm和50 mm的多对电极可测量混凝土试件距表面不同位置的电阻率,混凝土的水灰比从0.25到0.45范围内变化,考虑到粉煤灰在混凝土中普遍采用,粉煤灰掺量在0%～50%范围内变化,采用水中养护6、13 d和27 d,以及自然干燥(不养护)、湿布覆盖、薄膜覆盖和涂刷养护剂等多种养护方式;同时还研究环境湿度变化对混凝土表层电阻率的影响。结果显示,混凝土表面层0～2 cm的电阻率对养护条件非常敏感,而混凝土表层3 cm以下的电阻率对养护条件的敏感性较小;掺加粉煤灰的混凝土对养护条件变化更为敏感;水养时间越长,混凝土表层电阻率随深度的变化越小,较短时间水中养护的粉煤灰混凝土的表层电阻率明显高于普通混凝土的电阻率;混凝土表层电阻率对环境相对湿度非常敏感,环境相对湿度降低,电阻率升高,环境相对湿度升高,电阻率降低。根据良好养护的混凝土表层与内部的电阻率差通常保持一个较小值的特点,通过对比分析不同养护条件下、混凝土水灰比以及粉煤灰掺量变化条件下混凝土表层电阻率变化规律,以不同位置电阻率之间的关系来建立混凝土的潮湿养护效率的评价方法。结果显示,以距离表面1 cm、2 cm的混凝土电阻率平均值和距离表面4 cm、5 cm的混凝土电阻率平均值之差ρd,能比较好地反映养护条件变化;混凝土28 d的ρd最为适合评价混凝土的养护效果;以ρd值评价混凝土养护效果,可排除混凝土本身性质的影响,而只与养护条件有关。为了使电阻率法能有效应用于混凝土养护效果的现场评定,对预埋电极与钻孔后埋置电极的电阻率测试结果相关性,并采用1000 mm×1000 mm×100 mm的大尺寸试件进行试验,还研究了电极形式以及钢筋的影响。结果显示,对于接近实际混凝土结构尺寸的试件,以28 d的ρd值来评价混凝土的养护效果仍然有效;预埋电极与钻孔后埋置电极的电阻率测试结果有非常好的相关性,棒状电极可以取代片状电极更有利于钻孔后埋置电极,表明实际工程应用无需预埋电极,可在测试前钻孔后埋置电极进行电阻的测量,这不仅有利于自由选取测试点,也更便于现场对混凝土养护效果的评价。采用电阻率法可实现对混凝土潮湿养护效果的评价,有望能发展成为混凝土养护效果进行现场检测的方法,进一步的研究可根据混凝土浸水后不同深度电极对的电阻率变化特征,再现混凝土的养护历程,实现对其养护效果的后评估。更多还原

【Abstract】 Proper and adequate curing, especially adequate curing during early stage affect significantly the desired properties of concrete. However, in practice in China, it has not been paid an attention to that the curing of concrete structure and member in-site for a long time. Curing has always either been ignored or performed perfunctorily by specifiers or contractors. Lack of necessary care needed to insure good curing can be attributed to lack of a means by which a resulting poor end product can be identified at early age. Part of this problem may be due to at present lack of a effective method for making a quantitative assessment of how well concrete is cured and to contract documentation that makes no provision for penalties in the event of curing not being carried out. Therefore, it is beneficial for quality control of concrete through assessing its curing history accurately.The electrical resistivity of concrete is very sensitive to internal moisture content, the higher moisture content of concrete means the lower resistivity. The idea was that if a specimen was adequately cured, it would have a higher humidity within the near-surface layer concrete and smaller resistivity difference between the internal and near-surface layer during early stage, and in a case of poor curing, the opposite would be true. In this research, an improved method using potentiostat for measuring the electrical resistance of near-surface layer concrete at different depths is presented. The effects of different curing conditions on change in the electrical resistance of near-surface concrete with depths were investigated by the method. Moreover, the electrical resistivity method for assessing curing efficiency of concrete in-site was explored.To ensure the preciseness of measured values of resistivity, the stability and reproducibility of the electrical resistance test method performed on PS-6 potentiostat were investigated. Twelve standard resistances, resistance values varying from 0.33 k? to100 k?, were tested by potentiostat and digital multimeters, respectively. The test results were shown that the resistance values obtained by two test methods had a good correlation. And the high resistance values obtained by potentiostat were more stable. Thus, using potentiostat to measure the resistance of concrete is more appropriate for typically the resistance of concrete is higher。An investigation of the influence of the distance between two electrodes, ionic concentrations and electrode contact areas on the measuring results was carried out. The results indicate that the potentiostat is suitable for measuring the resistance of concrete with preciseness and stability.The small copper electrodes were embedded horizontally 50 mm within the surface 10, 20, 30, 40 mm and 50 mm of the specimens at the time of casting, the test of the electrical resistance of concrete was performed on PS-6 potentiostat. In this research, we made normal concrete and concrete containing fly ash. Fly ash concrete was prepared with the cement partially replaced by 15%, 30% and 50% of equal mass of fly ash, respectively. Three water binder ratios were 0.25、0.35 and 0.45, respectively. The curing conditions included 6、13 d or 27 d water curing, natural curing, plastic sheet or wet burlap curing and curing agent curing, etc. Simultaneously, the effect of different curing conditions on the electrical resistance change of near-surface concrete was examined. It is found that curing conditions have pronounced influence on the electrical resistivity of near-surface layer of concrete. Thus it affect mainly the resistivity of the concrete within about 0-2 cm from the surface, and the contribution of curing condition to electrical resistivity of the concrete beneath some 3 cm from the surface is negligible. Fly ash concrete was more sensitivity to curing condition. The longer the water curing duration, the smaller the resistivity changes with depths. The resistivity of the fly ash concrete specimens exposed to shorter duration of water curing was much higher than that of the plain concrete specimens. The results indicate that the resistivity of near-surface layer concrete is sensitive to relative humidity; when relative humidity decreases, the resistivity of near-surface layer concrete increases, and when relative humidity increases, the resistivity of near-surface layer concrete decreases.Based on the fact, i.e., the resistivity difference between the internal and near-surface layer is normally small under adequate curing condition. The change rule of resistivity under different curing conditions, water binder ratios and fly ash contents was investigated. The electrical resistivity method to assess curing efficiency of concrete was established based on the relationship between resistivity values at different positions. It is indicated that the difference (ρd) between the average value of the resistivity values of 1 cm (ρ1) and 2 cm (ρ2) and the average value of the resistivity values of 4 cm (ρ4) and 5 cm (ρ5) is most suitable for evaluating curing efficiency of concrete. Moreover, it can eliminate the effect of mix proportion itself, only related to curing conditions.To make the method more effectively applied to field test, the test of the electrodes embedded in advance and embedded afterwards electrodes by drilling hole was performed. It is found that the testing results of the electrodes embedded in advance can be well correlated with those of the embedded afterwards electrodes by drilling hole. For large-scale specimens measuring 1000 mm×1000 mm×100 mm under natural ambient condition, the electrical resistivity of concrete within about 0-2 cm from the surface is very sensitive, and the electrical resistivity of the concrete beneath some 3 cm from the surface is not sensitive regardless of by the electrodes embedded in advance or the embedded afterwards electrodes by drilling hole. The difference between internal and surface resistivity (ρd) was smaller under continuous plastic sheet curing than natural ambient condition. Test results for large-scale specimens were consistent with that for small-scale specimens. In practical field testing, the electrical resistance is measured by drilling hole, then embedding electrodes, it is not only beneficial for selecting freely test location, but also being used in-site. The electrical resistance test method that can be used to assess the curing history of concrete was validated using large-scale specimens. The resistivity difference between internal and near-surface (ρd) at 28 days obtained regardless of by the electrodes embedded in advance or the embedded afterwards electrodes by drilling hole is shown to be very sensitive to curing condition. The resistivity difference between internal and near-surface (ρd) at 28 days obtained by this testing method can reflect accurately the curing history of concrete.The test procedure is quick and uncomplicated, and found to be extremely sensitive to reflect changes in concrete curing condition. It will can be developed a new approach for making a quantitative assessment of how well concrete is cured in-site. By studying resistance profiles at different depths of water immerged specimens, concrete curing history and efficiency can accurately be evaluated.更多还原