Researches On The Moisture Transport Of Cement-Based Materials

The reaction of corrosive media with concrete or steel in concrete is one of the most vital factors, which result in the durability problem of concrete structures. A lot of researches indicate that water are necessarity for almost all corrosive reactions, and almost all of the corrosive agents are transferred into concrete through concrete pore solution. Generally, the water saturated condition is not met in real field structural materials. So, it is very important for the unsaturated water (or water solution) transfer, although so many research works are focused on the water saturated permeability and chloride diffusivity. Based on the summarizing of the internal and international researches, a systematical research on unsaturated water transport through cement-based materials were carried out in this paper, including unsaturated vapor transmission, unsaturated liquid transport, moisture diffusion and water adsorption et al, and some test methods applied in water transport were also concerned in the paper, an application of water transfer and in site monitoring of water content in real field were introduced in the 6^th chapter.In general, the main content and some primary results are concluded as following:(1) A review on the researches of water transport and the corresponding measurements is given, the research object of unsaturated water transport is proposed based on the review and the real conditions of cement-based material structures.(2) An improved method based on ASTM E 96 is suitable to the water vapor transmission (WVT) measurement of cement-based materials. The results of WVT test of material with different constitutes under different conditions show that there are great effects of water-cement ratio, addition of fly-ash and environment conditions of materials on the WVT results. The lower the water-cement ratio, the lower the WVT, 20 percent addition of fly-ash decreases the WVT greatly, the liquid contact on the vapor inlet face has greater WVT than the water vapor contact, and the increase of the concentration of chloride or sulphate also lowers the WVT.(3) The water vapor adsorption method is introduced to the investigation of moisture diffusion. The moisture diffusion law, water content retention of materials and porosity distribution of materials are investigated by the combination of thermodynamics and diffusion and adsorption theories. The results show that the change of moisture diffusivity with moisture content is not the same during adsorption and desorption, the highest diffusivity at the initial low moisture content during adsorption, and the highest at somewhat high moisture content during desorption. With other conditions the same, moisture diffusivity and adsorption energy increases with the increasing of water-cement ratio, and diffusivity varies linearly with water-cement ratio. The adsorption isotherms of cement-based materials mainly depend on hydrated products, but have nothing to do with the non-hygroscopic aggregates, including fine and coarse aggregates, and porosity distribution, ranging from gel pore to capillary pore sizes, can be calculated from sorption isotherms.(4) The unsaturated water transport refers to the capillary wicking action of water in materials, mainly the water imbibition and capillary water diffusivity. A moisture diffusion term is introduced into the conventional imbibition equation to improve the simulated transport process, and an improved slicing-weighing method based on the traditional one is used to measure the capillary water diffusivity and to get a continuous water content distribution, without the dependence on some expensive equipment.The results of water imbibition show that the capillary imbibition coefficients of cement-based materials decrease with the decrease of water-cement ratio, the increase of aging and initial moisture content of materials and with the addition of fly-ash, and the coefficient varies linearly with strength for a given mixed cement-based material, the coefficient of penetration depth is determined by the critical pore diameter and pore tortuosity, but the mass coefficient is determined by the effective porosity of material. The theoretic calculation and the experimental results indicate that the contribution of moisture diffusion to the whole capillary transport content can not be neglected, especially for the materials which hold small pores and low porosity.Capillary water diffusivity of cement-based materials is greatly influenced by the microstructure and moisture content of the materials. Diffusivity increases sharply when moisture content approximates to the level of total capillary porosity and has little change prior to this level, the wider the pore range of materials, the lower value of moisture content to total capillary porosity ratio when the sharp change of diffusivity happens. For the same moisture content of materials, the larger the pores and the better connectivity of pores result in the higher of diffusivity.(5) In this paper, the AC electrical property is established for the measurement of water content distribution or diffusivity. The water transport processing under load, and the evidence of heterogeneous distribution of microstructure and hydrated products under bending force during hydrating period are found by the analysis of electrical properties, mineralogy and chemical composition, and the phenomenon of transport processing and the resulting heterogeneous distribution by external forces is named force-transport effect by Professor Shui Zhonghe firstly and formally.(6) Carbonation under certain condition, interface bonding of new-old concrete and healthy inspecting of concrete pavement by in site electrical measurement are investigated in this paper. From the CO₂ adsorption/desorption experiment of cement paste under very low moisture condition, a new possible mechanism of carbonation by the direct gas-solid reaction is proposed. From the experiment of concrete repairing by pretreatment of concrete under different moisture conditions, it shows that lowering the moisture content of old concrete increases the interface bonding strength between new and old concrete. The reason of this is that the lower moisture content of old concrete will accelerate the mass transport from new concrete to old concrete, which results in a high bonding strength. After paving asphalt concrete on the rebuilt cement concrete subbase, the in-site electrical measurement shows that the moisture content of the subbase can be well reflected by electrical property, a reasonable health evaluation of the whole pavement can be given by the combination of electrical measurement, inner temperature surveying and stress-strain measurement.