Cracks and Accelerated Migration

Author(s):
Paper category: Bibliography
Book title: Publications on Durability of Reinforced Concrete Structures under Combined Mechanical Loads and Environmental Actions: An Annotated Bibliography
Editor(s): Yao Yan, Wang Ling, Wittmann Folker
Print ISBN: 978-3-942052-03-0
Publisher: Aedificatio Publishers
Pages: 71 - 94
Total Pages: 24
Language: English

Abstract: 
One of the major factors affecting the durability and thereby the integrity of reinforced concrete structures is the migration of water and chemical compounds dissolved in water into the porous material as it is at the origin of various damage mechanisms. Cracking of reinforced concrete, which could be induced by structural loading, drying shrinkage, and thermal deformation, etc., is practically inevitable and often anticipated in restrained conditions. These cracks provide preferential paths for migration of water or aqueous solutions containing aggressive substances into cracked structural elements; and thus result in accelerated deterioration and decreased service life of reinforced concrete structures.
In relevant papers published so far, water permeation characteristics in mechanically crack-induced concrete were often evaluated and the relationship between crack width and accelerating permeability was studied. It appears that there is a threshold value for crack width or crack opening displacement in concrete, which is in the range of 50 μm to 200 μm according to different researches. That means there is normally little change in concrete permeability until this threshold crack width. Whereas beyond this value cracks increase the water permeability coefficient of concrete significantly from dozens of times to hundreds of times and, in such cases, water migration is controlled by the cracks. However, the migration rate of water flow reaches a constant as concrete approaches failure. It is often found in reinforced concrete that, when crack opening is less than 20 μm or so which is normally less than the critical value, about 80 % of crack opening is to be recovered after unloading. Thus it has very little effect on the permeability of water saturated concrete. Recent research indicates that, however, the presence of these fine cracks or micro-scale cracks will also increase the migration of water and aggressive agent into unsaturated concrete via powerful capillary suction, and thus accelerate its deterioration. As water penetration is closely related to the ion diffusion, cracks have similar effects on accelerating the diffusion of chloride in reinforced concrete. But it is usually reported to be less sensitive to crack opening than water permeability.
By means of the technique of neutron radiography, the process of water penetration into cracked reinforced concrete can be visualized and studied quantitatively. It has been concluded in recent studies that cracks can be instantaneously filled if the cracked surface is put in contact with water. The interfaces between steel and concrete are also filled with water from the crack as it is mechanically damaged. In this case, cracks serve as water duct for further migration into the material by capillary suction.
Due to the formation of multiple micro-cracks, wide cracks can be avoided in SHCC under an imposed strain. The high strain capacity, however, is beneficial with respect to durability only if the multi crack formation in SHCC does not lead to significantly increased water migration. Results published so far showed that once SHCC with multiple cracks is in contact with water, water will penetrate into fine cracks very quickly by capillary suction. In a second step, water migrates into the cement-based matrix adjacent to the cracks. This will also jeopardize durability of SHCC containing cracks in severe aggressive environment.
To set up an efficient barrier against water migration into cracked concrete, integral water repellent and surface impregnation are often utilized. The efficiency of water repellent treatments on cracked reinforced concrete is found to be highly dependent on the type and amount of agent, the crack width and whether impregnation is carried out before or after crack formation.
It should be pointed out that, due to the autogenous healing from the formation of calcite in the crack, the water migration through the cracks could be gradually reduced with time, and in extreme cases, the micro-cracks seal completely. In addition, autonomous sealing by the release of encapsulated agents in the matrix has also been proven to be a promising method to reduce the accelerating effect of cracks on water migration in cracked concrete.
The cumulative crack length appears to have no direct relationship with water migration in concrete. There is even a decrease in the permeability coefficient with time due to ongoing hydration and autogenous healing of cracks and is especially true for crack opening less than the critical value described above. It ought to be noticed that, however, water could migrate into the fracture process zone behind the crack tips where crack length ends. In this case, it will again lead to much more water ingress and consequently accelerate the deterioration of concrete.

Online publication: 2014
Publication Type: full_text
Public price (Euros): 0.00