Leakage Concrete Tunnels for Durability

Conventional concrete, which can be considered to be an artificial rock, is not intrinsically waterproof. It absorbs water. The rate of absorption depends upon many factors, including the porosity of concrete. In addition, excessive contents of water-soluble alkalis in cement (as in the case of Indian cements of recent periods) can make concrete highly absorbent1-4. Today’s concrete structures, compared to concrete structures of earlier decades, also suffer from higher thermal and shrinkage stresses and the resulting cracks1-5. The result: water retaining structures, e.g., tunnels, subways, basements, etc. are characterized by water seepage and leakage unless such structures will be effectively waterproofed.

In the case of tunnels and other water retaining structures, in addition to the seepage of water due to the development of thermal and shrinkage cracks or due to porosity in concrete, water leakages may also occur at expansion joints, construction joints, honeycomb areas, and isolated locations of defects and discontinuities as at locations of inserts and embedment’s. In addition to running water leakages through these locations of defects or planned structural separations, there can be minor sweating or dampness of the surface. This sweating or dampness is generally influenced by the general porosity or perviousness of concrete.

Besides general porousness or permeability, local defects or discontinuities, and besides the existence of planned structural gaps (expansion joints), the waterhead outside the structure greatly influences the rate of water leakage into underground structures. Thus, there will be many tunnels which may exhibit little or no water leakages during the dry season whereas the same tunnels may show profuse water leakages during the wet season of the year or when there will be flooding of grounds surrounding the tunnels.

water leakages Water leakages or seepages through a structure can go much beyond being an architectural nuisance or an operational inconvenience. The leakages may prevent the use of the facility. The leakages may also damage equipment and systems inside tunnels and other underground facilities.

In addition to ungainly sights, operational difficulties and damages to contents, etc., water leakages, seepages and dampness adversely affect the durability of tunnels and other concrete structures, as even minor dampness can lead to an early ruination of the structure by accelerating the process of corrosion in rebars and thereby inviting conditions of distress in the structure.

The Essence of Surface Protection

Kar1-14 highlighted the problem of early decay and distress in concrete structures that could be encouraged by the dampness in concrete or even by the exposure of the concrete structure to the general environment. In most cases of concrete structures, it is corrosion in reinforcing bars and prestressing elements, that leads to conditions of distress in such structures.

The Indian standard IS 456:200015 recognizes the problem of early distress in concrete structures as well as the causes for such early distress when it states in its Clause 8, Durability of Concrete: "One of the main characteristics influencing the durability of concrete is its permeability to the ingress of water, oxygen, carbon dioxide, chloride, sulphate and other potentially deleterious substances." The interior surfaces of tunnels are generally exposed to air and thus these surfaces, if not protected, will permit the ingress of oxygen and carbon dioxide. While the diffusion of carbon dioxide may lead to depassivation of reinforcing bars, making corrosion in such bars possible, the diffusion of oxygen into the structures will lead to corrosion in the rebars in the presence of the moist environment inside the structure.

As a solution to the problem of early distress, Kar1-14 recommended the prevention of the ingress of water into or flow of water through structural elements and the protection of all structural surfaces, exposed to either air or intermittently to water and air. The dual goal was to be achieved by the provision of waterproofing treatments on all exposed surfaces of structures. The Indian Standard15 followed suit when it wrote in its Clause 8: "The life of the structure can be lengthened by providing extra cover to steel, by chamfering the corners or by using circular cross-section or by using surface coatings which prevent or reduce the ingress of water, carbon dioxide or aggressive chemicals."

Kar12,13 explained why, among the four options recommended in IS 456:200015, the provision of surface coatings or other surface treatments to prevent the ingress or passage of water, and to prevent or reduce the diffusion of carbon dioxide and oxygen or permeation of aggressive chemicals, was the only viable option.

concrete structure Thus, a concrete structure needs to be waterproof and damp-proof. The mere arresting of running water leakages through isolated spots, like construction joints, as is the case when grouting methods are employed, may not make water retaining structures of concrete sufficiently durable. In other words, in order that a concrete structure may be durable, it, like a steel structure, needs to have surface protection. Dense concrete will help, but it may not be sufficient to make today’s concrete structures reasonably durable.

In order to make concrete structures durable, the exposed surfaces (i.e. all of inside surfaces of the tunnel) should be given a post-construction surface treatment to create an impermeable surface region that would, besides preventing water leakages and seepages, prevent the easy ingress of air (moisture, carbon dioxide, oxygen) into the structure. This is the essence of surface protection of concrete tunnels and other structures, and the surface protection is best achieved through the provision of waterproofing treatments on the surface of concrete structures. Though the most important objective of the provision of surface protection systems is to prevent or inhibit corrosion in rebars and prestressing elements of steel, it is not the recommendation of the writer to provide any surface treatment to such rebars or prestressing elements.

It need be emphasized in the context of durability of concrete structures that the mere absence of any visible sign of water leakage through a structure is not a proof of its being a waterproof structure. The development of conditions of distress in the columns in the middle of the tunnel (away from the leaking walls) of Metro Railway, Kolkata is an example.

A false impression of water tightness of a water retaining structure can also be created when the rate of evaporation of water from the structure is greater than the rate of ingress of water into the structure.

It must be recognized that most of the damages due to corrosion in rebars and prestressing elements inside concrete structures, adversely affecting the durability of such structures, will take place if there will be a moist environment inside the structure and air will enter into the structures15. in the absence of air even a lot of water inside a concrete structure may not cause much damage. This is exemplified in the virtual absence of any condition of distress in foundation structures of concrete, unless the ground water will be patently harmful for concrete.

Thus, whereas the absence of a surface protection system on the visible surfaces of a concrete tunnel is likely to invite conditions of distress early, the absence of any waterproofing system on the outer faces of the tunnel is not likely to cause any such problem in most of the cases.

It has thus become essential, specially in the case of today’s concrete structures, which are built with cement, with very high C3S/C2S ratios and excessive amounts of water soluble alkalis1,2, and with today’s high strength rebars with surface deformations1-5,11,16,17, that such structures be given surface protection in the form of waterproofing treatments1-14 . The provision of such a surface protection system will arrest the leakage of water on a long term basis. It will also help fulfill the second objective of the prevention of water seepage at the post-construction stage. Furthermore, it will prevent or inhibit the ingress of carbon dioxide and oxygen. It may also prevent the ingress of chlorides and other aggressive chemicals. The process of corrosion in rebars and prestressing elements will thus be arrested or slowed down, leading to a lengthening of the life span of the tunnel structure and prevention of damages to other systems housed inside the tunnel.

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