Methods of repairing concrete structures

J Sudhakumar*, Regional Engineering College, India

26th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 27 - 28 August 2001, Singapore

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26th Conference on Our World in Concrete & Structures: 27 - 28 August 2001, Singapore

Methods of repairing concrete structures

J Sudhakumar*, Regional Engineering College, India

Abstract

There are three basiG symptoms of distress in a concrete structure: cracking, spalling and disintegration. Each of these basic symptoms in itself is fairly obvious and may be readily detected and differentiated from the others. Cracking is one of the most misunderstood problems of concrete. It is generally regarded as indicative of defective design or materials. Methods of repairing cracks include bonding with epoxies, routing and sealing, stitching, external stressing, grouting, blanketing, use of overlays, etc. Repairing cracks does not usually involve strengthening. In a structure showing spalling and disintegration, it is usual to find that there have been substantial losses of section and/or pronounced corrosion of reinforcement. Hence, the repair involves some requirement for restoration of lost strength. The principal methods used for repair of spalling and disintegration are jacketing, guniting, prepacked concrete, drypack, replacement of the concrete, and the application of overlays of several types. This paper gives a detailed description on all the above methods used for repairing cracks, spalling and disintegration in concrete structures.

Keywords: concrete, cracking, spalling, diSintegration, repair, stitching, grouting, jacketing

1. Introduction Concrete structures will show the degree of deterioration in the form of cracking, spalling and

diSintegration. Each one of these is clearly distinguishable. The reasons for their development may be poor materials, poor design poor construction practice, poor supervision or a combination. Crack formation in concrete is most interesting because sometimes the same causes produce a different cracking pattern, and sometimes the same cracking pattern is produced by different causes. Sometimes concrete cracks in a location where no cause can be found out, and in other places it does not crack where there is every reason for cracks to occur. However, fifty percent of the cases are straight forward.

Cracks in themselves are seldom indicative of structural danger; accordingly, repair usually does not involve strengthening. So their repairs are basically intended to seal the cracks against an objectionable flow of water or to improve the appearance of the construction. In the repair of a structure showing spalling and disintegration, it is usual to find that there have been substantial losses of section and/or pronounced corrosion of the reinforcement. Both are matters of concern from a structural viewpoint, and repair generally involves some urgency and some requirement for restoration of lost strength [1].

26th Conference on Our World in Concrete & Structures: 27 - 28 August 2001, Singapore

Methods of repairing concrete structures

J Sudhakumar*, Regional Engineering College, India

Abstract

There are three basiG symptoms of distress in a concrete structure: cracking, spalling and disintegration. Each of these basic symptoms in itself is fairly obvious and may be readily detected and differentiated from the others. Cracking is one of the most misunderstood problems of concrete. It is generally regarded as indicative of defective design or materials. Methods of repairing cracks include bonding with epoxies, routing and sealing, stitching, external stressing, grouting, blanketing, use of overlays, etc. Repairing cracks does not usually involve strengthening. In a structure showing spalling and disintegration, it is usual to find that there have been substantial losses of section and/or pronounced corrosion of reinforcement. Hence, the repair involves some requirement for restoration of lost strength. The principal methods used for repair of spalling and disintegration are jacketing, guniting, prepacked concrete, drypack, replacement of the concrete, and the application of overlays of several types. This paper gives a detailed description on all the above methods used for repairing cracks, spalling and disintegration in concrete structures.

Keywords: concrete, cracking, spalling, diSintegration, repair, stitching, grouting, jacketing

1. Introduction Concrete structures will show the degree of deterioration in the form of cracking, spalling and

diSintegration. Each one of these is clearly distinguishable. The reasons for their development may be poor materials, poor design poor construction practice, poor supervision or a combination. Crack formation in concrete is most interesting because sometimes the same causes produce a different cracking pattern, and sometimes the same cracking pattern is produced by different causes. Sometimes concrete cracks in a location where no cause can be found out, and in other places it does not crack where there is every reason for cracks to occur. However, fifty percent of the cases are straight forward.

Cracks in themselves are seldom indicative of structural danger; accordingly, repair usually does not involve strengthening. So their repairs are basically intended to seal the cracks against an objectionable flow of water or to improve the appearance of the construction. In the repair of a structure showing spalling and diSintegration, it is usual to find that there have been substantial losses of section and/or pronounced corrosion of the reinforcement. Both are matters of concern from a structural viewpoint, and repair generally involves some urgency and some requirement for restoration of lost strength [1].

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2. Repairing cracks In order to determine whether the cracks are active or dormant, periodic observations are done

utilizing various types of telltales [Fig.1]. Crack movement can be detected by placing a mark at the end of the crack. Subsequent extension of the crack beyond the mark indicates probable continuance of the activity that produced the defect originally. The deficiency of this technique is that it will not show any tendency for the crack to close or provide any quantitative data on the movement [2].

In another method, a pin or a toothpick is lightly wedged into the crack and it falls out if there is any extension of the defect. The deficiencies of this method, as before, are that there is no indication of closing movement or any quantitative measure of the changes, which occur [2].

A strip of notched tape works similarly. Movement is indicated by tearing of the tape. An advantage in this case is that some indication of closure can be realized by observing any wrinkling of the tape. However, this device is not reliable. The tape is not dimensionally stable under changing conditions of humidity, so that one can never be sure whether the movements are real or are due to shrinkage or swelling of the marker [2].

The device using a typical vernier caliper is the most satisfactory of all. Both extension and compression are indicated and movements of about one-hundredth of an inch can be measured using a vernier caliper of special type. If more accurate readings are desired, extensometers can be used. The reference points must be rigidly constructed and carefully glued to the surface of concrete, using a carborandum stone to prepare the bonding surface before attaching the reference marks.

Where extreme accuracy is required resistance strain gauges can be glued across the crack. They are, however, expensive, sensitive to changes in humidity, and easily damaged.

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2.1 Types of cracks Cracks can be divided into active cracks and dormant cracks. It should be considered that, in a

sense, every crack is an active one. For example, if there is any change in the load supported by a member, whether an increase or a decrease, the member must deform to accommodate the change. In this case, deformation is bound to occur and be concentrated at the pOints of weakness, ie. at the cracks. Thus, the proper differentiation between active and dormant cracks is one of magnitude of movement, and the telltales are a measure of the difference. If the magnitude of the movement, measured over a reasonable period of time (say 6 months or 1 year), is sufficient to displace or show significantly on the telltales, we can treat the crack as an active one. If the movements are smaller, the crack may be considered as dormant.

Cracks can also be divided into solitary or isolated cracks and pattern cracks. Generally, a solitary crack is due to a positive overstressing of the concrete either due to load or shrinkage. The cause for this will become apparent when the line of the crack is compared with the layout of the portion of the concrete, its reinforcement and the known stresses in it. Overload cracks are fairly easily identified because they follow the lines demonstrated in laboratory load tests. A crack due to setting and hardening shrinkage is formed in the first week of the life of the concrete. If the length of concrete under inspection is more than about 9 m, it is not likely that there is a solitary crack; usually there will be another of a similar type. The an