Theoretical And Experimental Study On The Capillary Absorption Within Concrete

In many cases, concrete structures are exposed to some mechanical and environmental loadings, such as the compression and tension strain, drying shrinkage, freezing and thawing, and so on. These factors can always induce micro-cracks the internal damage of the concrete structure. When the damage exists in the materials, it will be not conservative to predict the durability of structure based on the results obtained from the laboratory tests for undamaged specimens. The micro-cracks or the connected cracks caused by mechanical loading are the main factors for the deterioration of concrete. The capillary absorption of water by unsaturated concrete is the critical reason of degradation of structures subjected to an aggressive environment since water often acts as the transporting medium for damage contaminants and the prior condition for chemical reaction. The gradient of the total potential is the motivated force for capillary suction. The main objectives of this paper are to build up a theoretical model and solving method to describe the capillarity process with gravity effection, and analyze the loading effection on the capillary water sorptivity. The main contents and conclusions can be summarized as follows:(1) The theoretical models and the study progress are listed here; it also includes the application of the capillary theory in soil and porous building materials without taking the gravitation into consideration. Herein, two parameters used to describe the capillarity process are introduced. They are capillary coefficient and sorptivity, the first represent the height of the wetting front position, and the second one stands for the velocity of water absorption. Meanwhile, the importance of the study on the capillary water absorption related to the durability of concrete was improved.(2) Based on the theoretical model with gravity effect, the half-analytical solution and numerical solution are calculated. By comparing these results with the experimental data obtained from absorption testing of the mortars, it is obvious that, the gravity effect is going up with the time increasing.(3) As to the capillarity of the damaged concrete, static loading, repeated loading and the composition of these two paths are used on the specimens to obtain the damage condition of materials. They are loaded to the designated level, i.e.70%,80%,90% under uniaxial tensile and compressive loading respectively, then unloaded to 0. After this, they are cut to designated geometry. The capillary absorption of concrete is conducted in the improved experiment apparatus used to measure the cumulative water absorption. Through this equipment, the cumulative water content versus the square root of time can be obtained shown as bilinear. As a result, the slope of the curve, initial sorptivity and final sorptivity, can be calculated through the linear fitting method. The parameters sorptivity and porosity were used to predict the water distribution. Comparing these results obtained from damaged and undamaged concrete, it shows that the sorptivity, total water content, and the wetting front position are increased when the load level is over 70%. However, it doesn't show a monotonically increasing trend between these three items and the load level.