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1.
This investigation was carried out to study internal relative humidity (IRH) distribution of cement paste made with different water / cement ratios (w / c) and mineral admixtures in isothermal drying conditions. IRH changes in cement paste resulting from self-desiccation and moisture diffusion, respectively, at different ages were studied. The change laws of IRH in cement paste resulting from combining moisture diffusion with self-desiccation were discussed. The results indicate that IRH reduction of cement paste with w / c higher than 0.4 is mainly affected by moisture diffusion. However, IRH reduction of cement paste with w / c no higher than 0.4 is controlled by both moisture diffusion and self-desiccation. With the decrease of w / c, IRH reduction of cement paste resulting from self-desiccation increases, and IRH reduction resulting from moisture diffusion decreases at a given age. IRH decrement of cement paste incorporated with silica fume and ground blast-furnace slag is higher than that of control paste. w / c and the distance to the exposed surface play a significant role in IRH change resulting from moisture diffusion in isothermal drying condition. Change laws of IRH in cement paste with silica fume due to moisture diffusion considering self-desiccation are different from those in cement paste without silica fume.  相似文献   

2.
This paper presents an experimental study on the autogenous shrinkage of Portland cement concrete (OPC) and concrete incorporating silica fume (SF). The results were compared with that of the total shrinkage (including drying shrinkage and part of the autogenous shrinkage) of the concrete specimens dried in 65% relative humidity after an initial moist curing of 7 days. The water-to-cementitious materials (w/c) ratio of the concrete studied was in the range of 0.26 to 0.35 and the SF content was in the range of 0% to 10% by weight of cement.The results confirmed that the autogenous shrinkage increased with decreasing w/c ratio, and with increasing SF content. The results showed that the autogenous shrinkage strains of the concrete with low w/c ratio and SF developed rapidly even at early ages. At the w/c ratio of 0.26, the autogenous shrinkage strains of the SF concrete were more than 100 micro strains at 2 days. For all the concretes studied, 60% or more of the autogenous shrinkage strain up to 98 days occurred in the first 2 weeks after concrete casting. The results indicated that most of the total shrinkage of the concrete specimens with very low w/c ratio and SF exposed to 65% relative humidity after an initial moist curing of 7 days did not seem to be due to the drying shrinkage but due to the autogenous shrinkage.  相似文献   

3.
钟卓  黄乐鹏  张恒 《硅酸盐通报》2021,40(8):2609-2621
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。  相似文献   

4.
A dynamic method of determining sorption isotherm is proposed. Changes in humidity boundary conditions were applied to specimens of hardened cement paste having various water–cement ratios, as well as to autoclaved aerated concrete, and water adsorption rates were continuously recorded in a sufficiently short period. The diffusion equation was analytically solved and was fitted to the adsorption rates resulting in the rapid estimation of equilibrium moisture content. Results of equilibrium moisture content, the primary data of moisture capacity, showed good agreement with literature values. The adsorption rate measurement took 8 h for autoclaved aerated concrete and 12 h for hardened cement paste with a measurement interval of 60 s to predict an equilibrium moisture content at an ambient relative humidity.  相似文献   

5.
基于毛细管负压技术测试混凝土最早期的自干燥效应   总被引:1,自引:0,他引:1  
混凝土自干燥收缩与其内部相对湿度的下降密切相关.低水胶比混凝土在结构形成以后的初始阶段存在较大的自干燥收缩,但通常此时混凝土内部相对湿度较高,采用传统的湿度计难以测试出其自干燥效应.借鉴了土壤物理学中张力计的基本原理,研制开发了最早期毛细管负压自动测试系统,结合Laplace方程和Kelvin定律,实现了从浇筑成型开始至终凝后混凝土内部自干燥效应的自动化测试,提出了自干燥收缩零点判定的新方法.  相似文献   

6.
The coefficient of thermal expansion (CTE) of cement-based materials is strongly dependent upon their moisture content. In particular, with a drop of internal relative humidity (RH), an increase of the CTE is observed. This effect can be attributed to the internal RH change associated with a temperature change (ΔRH/ΔT coefficient). Published data show high scatter and moreover they mostly refer to drying and do not address the effect of self-desiccation in low water-to-cement ratio (w/c) materials. In this paper, the RH dependence upon temperature is quantified using the ΔRH/ΔT coefficient for cement pastes of different w/c. Next, the ΔRH/ΔT coefficient is coupled with the observed evolution of the CTE in cement pastes and mortars. The experimental investigation focuses explicitly on the early-age period up to about 7 days from water addition.  相似文献   

7.
The moisture content is of particular relevance in cement-based materials, as it has a strong impact on their fundamental material properties. For example, it directly affects their strength and elastic properties, which in turn are closely related to volumetric deformations and cracking susceptibility. This paper investigates the influence of the decreasing moisture content on the elastic properties at early-ages, when the material properties are still developing simultaneously to the drying process. Mortar mixtures containing either Portland cement or cement blended with slag were specifically designed to halt the hydration at predefined stages without altering the microstructure or promoting further hydration during drying (equivalent systems). The elastic modulus of the equivalent mortars as a function of the moisture content is measured through resonant ultrasound spectroscopy. At early age the elastic modulus remained constant during drying, while at later ages a steady reduction was observed as a function of the decreasing relative humidity.  相似文献   

8.
高性能混凝土自身相对湿度变化的研究   总被引:14,自引:0,他引:14  
蒋正武  孙振平  王培铭 《硅酸盐学报》2003,31(8):770-773,779
研究了不同水胶比m(w)/m(b)、浆体体积含量以及不同掺量与组合的矿物掺合料对混凝土自干燥引起自身相对湿度(autogenous relative humidity,ARH)变化的影响,并从热力学角度探讨了高性能混凝土自干燥引起ARH变化的机理。研究表明,不同龄期下混凝土的ARH下降均随着m(w)/m(b)的减小而增大。混凝土中浆体体积含量越高,ARH下降越大。不同矿物掺合料因对混凝土水化进程与速率的影响不同,ARH下降也不同。水泥矿物继续水化引起毛细孔中可蒸发水含量下降是产生自干燥效应的主要原因。  相似文献   

9.
In this paper, a three-dimensional finite element model for the analysis of non-structural cracks occurring in reinforced concrete (RC) walls is introduced. The numerical model could take into account both time-dependent temperature variations due to hydration heat and non-uniform moisture distribution during drying, and the coupling effect between the heat transfer and the moisture diffusion. Calculation of the temperature and internal relative humidity variations of RC walls is followed by determination of stresses due to thermal gradients, differential drying shrinkage, and average drying shrinkage. The mechanical properties of early age concrete, determined from numerous experimental studies, are taken into consideration to improve the accuracy of the numerical results, and a discrete steel element derived using the equivalent nodal force concept is also used to simulate reinforcing steels embedded in a concrete matrix. The validity of the proposed procedure is verified by comparing the measured experimental data with the analytical results for RC walls.  相似文献   

10.
The moisture diffusion coefficient of concrete is critical for moisture distribution calculation in concrete members. This article aims at building an experimental model to solve the moisture transfer coefficient of early age concrete and investigating its variation law with age as well. The model is based on experimental measurements on the interior humidity of concrete specimens under fully plastic sealing and environmental drying conditions. The model results show that the moisture diffusion coefficient of concrete at an early age is significantly dependent on age. The coefficient may vary from 10?8 to 10?10 from 3 to 28 days starting from concrete casting. The variation law of the coefficient with age can be described as a rapidly decreasing stage within 7 to 10 days from concrete casting followed by a slow decreasing stage. This obvious two-stage variation law may principally due to the change of the continuity of liquid and/or vapor phases during drying, which reflects the transformation of the moisture removal mechanism from liquid flow to vapor diffusion. High-strength concrete has a lower moisture diffusion coefficient than that of normal strength concrete under the same curing period.  相似文献   

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