With the strategy implementation of “One Belt and One Road”, “Western Development” and “Marine Potestatem”, a great deal of concrete structures are used in harsh environments, then the durability and safety of concrete structures meet great challenge. In actual project, the concrete structures serviced in tidal zone, splash zone and wetting-drying zone usually deteriorates at the earliest time and the damage degree is the most serious. The main reason is that the concrete structures in these zones are unsaturated and the harmful medium transport into concrete by the mechanisms of diffusion and convection, which seriously affect the service life of concrete structure and is the critical point to the durability design of concrete structures. Therefore, it is of great significance to investigate medium transport in unsaturated concrete, which not only improves the durability theory and design level but also provides theoretical support for the prediction of service life, repair and maintenance of concrete structures.
Based on the theories of porous media and composite material, moisture and chloride ion transport in unsaturated concrete were systematically investigated by experiment and simulation. Traditional test methods and modern analysis techniques were used together in this study. The isothermal adsorption-desorption behaviors of concrete were investigated firstly and water capillary suction process, feature and law of unsaturated concrete were studied quantitatively and visually. Meanwhile, the behavior of chloride transport in unsaturated concrete was investigated by experiment combined with modeling. In addition, water and chloride transport in cracked and damaged concrete were also investigated in this work. Several important innovative results are as follows:
(1) Isothermal adsorption-desorption behaviors of concrete
Special environment with different relative humidity was produced by various saturated salt solution and the isothermal adsorption-desorption curves of concrete were obtained from the experiment. In accordance with the curves, combined with Laplace equation and Kelvin equation, the moisture characteristic curves of concrete were obtained in further. Based on above results, the relationships between liquid/gas relative permeability, diffusion coefficient and saturation degree of concrete were established. Effects of water to cement ratio (W/C), mineral admixture type and content on the isothermal adsorption-desorption behaviors, permeability and diffusion coefficient of concrete were investigated systematically. The results show that hysteresis effect is obvious in the process of desorption and the area of hysteresis ring can be used for the characterization of pore structure. Gel pores in concrete can be saturated by adsorption when RH<70%, while capillary pores are gradually saturated when RH>70% and the saturation degree of concrete increases rapidly. The relative permeability of gas phase decreases with the saturation degree of concrete increasing while the permeability of liquid phase improves gradually. Water vapor diffusion is the main transport method when the saturation degree of concrete is lower (θ<0.5), however, pore water is continuous when θ>0.6~0.7 and the diffusion of liquid water is more and more dominant.
(2) Water transport in unsaturated concrete: quantitative and visualization
A technique of X-CT combined with Cs enhancing was created to continuously monitor water transport in cement pastes, mortars and concretes (patent No.201510617455.2). The contrast of CT images was greatly improved and the distance of water transport in materials could be determined exactly, which provided a powerful tool for the visualization of water transport in porous materials. With this technique, effects of W/C, mineral admixture and volume content of sand on water transport in cement-based materials were studied systematically. Based on the gravimetric method, water absorption of unsaturated concrete was characterized quantitatively and the relationship between sorptivity (S) and saturation degree (θ) was established, S=a(1-θ)b, which is the basis for the durability research of unsaturated concrete. In accordance with water transport theory of porous media and water absorption experiment, a model for calculating moisture distribution in the process of water absorption was proposed. Meanwhile, the theoretical relationship between capillary coefficient (k) and sorptivity (S) was established, S/k=φ, which was also verified by experiment results.
(3) Chloride transport in unsaturated concrete: experiment and modeling
Chloride transport in unsaturated concrete was studied in two conditions: 1) When the moisture in concrete does not exchange with environment and the saturation degree is constant, chloride transport in concrete is controlled by the mechanism of diffusion. The model of chloride diffuses in unsaturated concrete was modified and the chloride diffusion coefficients of concrete with different saturation degree were calculated by the modified model. 2) When water absorption takes place in concrete and the saturation degree changes with time, natural immersion method was used to investigate chloride transport in unsaturated concrete with the mechanism of diffusion-convection. The influences of W/C, fly ash content, slag content, concentration of salt solution and immersion time on chloride transport were investigated systematically. An empirical relationship between chloride diffusion coefficient and initial saturation degree of concrete was established, D(θ)/Ds=ea(1-θ). The relationship between sorptivity and chloride diffusion coefficient for unsaturated concrete was proposed, S2/D=100φ(1-θ)6.75(W/C)+0.42, which provided a new method for calculating chloride diffusion coefficient of unsaturated concrete.
(4) Water and chloride transport in cracked concrete (mortar)
Cracked mortars were prepared by inserting steel slices. The process of water transport in cracked mortar was in situ continuously tracked by the technique of X-CT combined with Cs enhancing. Effects of crack width (0.05mm, 0.1mm, 0.2mm, 0.4mm and 0.6mm) and direction (longitudinal and horizontal) on water transport in cracked mortar were investigated. With the consideration of absorption direction (upward and downward), the effect of crack width on the sorptivity is investigated by the gravimetric method. At the same time, the effect of crack width on chloride penetration depth and diffusion coefficient was studied by experiment and simulation, besides, the gravity effect was also considered in this work. In addition, effects of crack type, void and ITZ on chloride transport were investigated by finite element method. Results show that the height of water uptake in cracked mortar decreases with crack width increasing. The paths of water transport in unsaturated mortar can be broken by internal cracks, and then water bypasses the cracks and transport in matrix. At unsaturated state, the critical width of crack that affects the transport behavior of water and chloride is 0.1mm; however, at saturated state, the critical width of crack that affects chloride transport is 0.2mm. The existence of gravity effect accelerates the transport of water and chloride.
(5) Water and chloride transport in damaged concrete induced by freeze-thawing and loading
Damaged concretes were prepared by the methods of freeze-thawing and loading. The behaviors of water and chloride transport in damaged concrete were investigated by water absorption and electric flux method. The relationship between sorptivity, charge passed and damage degree of concrete was established, respectively. Results show that the sorptivity of concrete increases in linear or exponential with the damage degree increasing. With same damage degree, the sorptivity of concrete damaged by freeze-thawing is larger than that of concrete damaged by loading. Charge passed through damaged concrete induced by freeze-thawing and loading increases linearly with the damage degree increasing. At saturated state, for concrete suffered from same damage degree of freeze-thawing or loading, the resistance of chloride permeability is closed to each other; however, at unsaturated state, the concrete damaged by freeze-thawing is more vulnerable to harmful medium.