ISBN: 378-26-138420-01
INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN ENGINEERING RESEARCH, ICCTER - 2014
REPAIRING CRACKS IN CONCRETE STRUCTURES Anilkumar P M and Dr. J Sudhakumar B.Tech Student, Professor Department of Civil Engineering, National Institute of Technology, Calicut anilkumar_bce11@nitc.ac.in, skumar@nitc.ac.in Abstract— Studies related to crack formation in concrete is most interesting because sometimes the same causes produce a different cracking pattern and sometimes the same cracking pattern produced by different causes. Sometimes concrete cracks in allocation where no cause can be found and in other places it does not crack where there is every reason why it should. Eighty percent of these cases, however, are straight forward. In order to determine whether the cracks are active or dormant, periodic observations are to be done. Various factors have impeded improvements in the durability of concrete repairs, including: inadequate condition evaluation and design; lack of quality construction practices and quality control; and the choice of repair materials. It is necessary to reconsider some recent developments in structural repairs from the view point of extending the service lives of structures under repairs [1]. Index Terms—Concrete, cracks, reinforcement, repair
I. INTRODUCTION Cracks can be broadly divided into two categories namely solitary cracks and pattern cracks. Generally, solitary crack is due to a positive overstressing of concrete either due to load or shrinkage, the cause of which 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 test. A crack due to setting and hardening shrinkage is formed in the first week of life of the concrete. If length of concrete under inspection is more than about 9m, it is not likely that there is a solitary crack, usually there will be another one of a similar type and the analysis of the second confirms the finding from the first. Regular pattern of cracks may occur in the surfacing of concrete and in thin slabs. The term pattern cracking is used to indicate that all the cracks visible have occurred more or less at the same time. II. TYPES OF CRACKS Cracks can be divided into two types:-Solitary cracks and pattern cracks A. Solitary cracks ( Isolated cracks) Generally, a solitary crack is due to a positive over stressing of the concrete either due to load or shrinkage, the cause of which will become apparent when the line of
the crack is compared with the layout of the portion of the concrete, is 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 life time of concrete. If the length of concrete under inspection is more than 9m, it is not likely that there is a solitary crack, usually there will be another one of a similar type and the analysis of the second confirms the finding from first. In a long retaining wall or long channel, the regular formation of cracks indicates faults in the design rather than the construction, but an irregular distribution of solitary cracks may indicate poor construction as well as poor design. B. Pattern cracking Regular pattern of cracks may occur in the surfacing of concrete and in thin slabs. The term pattern cracking is used to indicate that all the cracks visible have occurred more or less at the same time. III. IDENTIFICATION OF CRACKS 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 originally producing the defect. The deficiency of this defect is that it will not show any tendency for the crack to close or provide any quantitative data on the movement. In another method, a pair of tooth pick is lightly wedged into the crack and 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 qualitative measure of the changes which occur. A strip of notched tape works similarly. Movement is indicated by tearing of the tape. An advantage is that same indication of closure can be realized by observing any writing on the tape. This device is not reliable; however, the tape is not dimensionally stable under changing conditions of humidity, so that one can never be sure whether the movements are due to shrinkage or swelling of the marker. A device using a typical vernier caliper is the most satisfactory of all. Both extension and compression are indicated and movements of about (1/100) inch can be measured using a vernier caliper. If more accurate readings are desired, extensometers can be used. The reference point must be rigidly constructed and carefully glued to the surface of concrete, using a carborundum stone to prepare the bonding surface before attaching the reference mark.
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ISBN: 378-26-138420-01
INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN ENGINEERING RESEARCH, ICCTER - 2014 Where the extreme accuracy is required, resistancestrain gauges can be glued across the crack. They are however, expensive, sensitive to changes in humidity, and easily damaged.
IV. MEHODS OF REPAIR A. Bonding with epoxies Cracks as narrow as 0.05mm can be bonded by the injection of epoxy. The technique generally consist of establishing entry and venting ports at close intervals along the crack, sealing the crack on exposed surfaces, and injecting the epoxy under pressure [2]. Usual practice is to drill into the crack from the face of the concrete at several locations; inject water or a solvent to flush out the defect; allow surface to dry (using hot air jet, if needed); surface-seal the cracks between the injection points; and inject the epoxy until it flows out of the adjacent sections of the crack or begins to bulge out the surface seals, just as in pressure grouting. Usually the epoxy is injected through holes of about 0.75 inch diameter and 0.75 inch deep at about 6 to 12 inch centers (smaller spacing are used for finer cracks). This method of bonding represents an application for which there is no real substitute procedure. However, unless the crack is dormant (or the cause of cracking is removed, thereby making the crack dormant), it will probably recur, possibly somewhere else in the structure. Also, this technique is not applicable if the defects are actively leaking to the extent that they cannot be dried out, or where the cracks are numerous. B. Routing and sealing: This method involves enlarging the crack along its exposed face and filling and sealing it with a suitable material. This is a simplest and most common technique for sealing cracks and is applicable for sealing both fine pattern cracks and longer isolated cracks. The cracks should be dormant. This technique is not applicable for sealing cracks subjected to a pronounced hydrostatic pressure. The routing operation consists of following along the cracks with a concrete saw or with hand or pneumatic tools, opening the crack sufficiently to receive the sealant. A minimum surface width of 0.25 inch is desirable. Smaller openings are difficult to work on. The surface of the routed joints should be rinsed clan and permitted to dry before placing the sealant. The method used for placing the sealant depends on the material to be used and follows standard technique. Routing and sealing of leaking cracks should preferably be done on pressure face so that the water or other aggressive agents cannot penetrate the interior of the concrete and cause side effects such as swelling, chemical attack or corrosion of the bars. The sealant may be any of several materials, depending on how tight or permanent a seal is desired. On roadway pavements it is common to see cracks which have been sealed by pouring hot tar over them. This is a simple and in expensive method where thorough water tightness of the
joint is not required and where appearance is not important. C. Stitching: The tensile strength of a cracked concrete section can be restored by stitching in a manner analogous to sewing cloth. Concrete can be stitched by iron or steel dogs in the same way as timber. The best method of stitching is to bend bars into the shape of a broad flat bottomed letter U between 1 feet and 3 feet long and with ends about 6 inches long (or less if the thickness of the concrete is less), and to insert them in holes drilled to match in the concrete on either side of the crack. The bars are then grouted up- some grout being placed in the holes in advance of the bars. In case of stitching of concrete pavements, after stitching a longitudinal crack it may be necessary to treat a nearby longitudinal joint. A primary concern is whether a crack has formed below the saw cut for longitudinal joints. If a crack has occurred and the joint functions properly, then no treatment other than joint sealing is warranted. However, if there is no crack extending below the saw cut joint, then it is advantageous to fill the saw cut with epoxy to strengthen the slab at this location. If the joint is not functioning, but a joint sealant has already installed, then no further action is recommended [3]. Usually cracks start at one end and run away from the staring place quicker on the side of the concrete than on the other. The stitching should be on the side which is opening up first. The following points should be observed, in general i. Any desired degree of strengthening can be accomplished, but it must be considered that the strengthening also tends to stiffer the structure locally. This may accelerate the restraints causing the cracking and reactivate the condition. ii. Stitching the cracks will tend to cause the problem to migrate elsewhere in the structure. If it is decided to stitch, investigate and if necessary, strengthen the adjacent areas of the construction to take the additional stress. In particular, the stitching dogs should be of variable length and /or orientation and so located that the tension transmitted across the crack does not devolve on a single plane of the section, but is spread out over an area. iii. Where there is a water problem, the crack should be sealed as well as stitched so that the stitches are not corroded, and because the stitching itself will not seal the crack. Sealing should be completed before stitching is commenced both to avoid the aforementioned corrosion and because the presence of dogs tends to make it difficult to apply the sealant. iv. Stress concentrations occur at the ends of cracks, and the spacing of the stitching dogs should be closed up at such locations. v. Where possible, stitch both sides of the concrete section so that further movement of the structure will not exert any bending action on the dogs. In bending members it is possible to stitch one side of the crack only, but this should be the tension side of the section, where movement is originating. If the members are in
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INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN ENGINEERING RESEARCH, ICCTER - 2014 a state of axial tension, then a symmetrical placement of the dogs is a must, even if excavation or demolition is required to gain access to opposing sides of the section. vi. As an alternative to stitching cracks at right angles which method does not resist shear along the crack, sometimes it is necessary use diagonal stitching. One set of dogs can be placed on each side of the concrete if necessary. The length of the dogs is random so that the anchor points do not form a plane of weakness. vii. The dogs must be grouted with a non-shrink or expanding mortar, so that they have a tight fit, and movement of the cracks will cause the simultaneous stressing of both old and new sections. If this is not possible, proportion the stitching to take the entire load without precipitation. viii. The dogs are relatively thin and long and so cannot take much in the way of compressive force. Accordingly, if there is a tendency for the crack to close as well as to open, the dogs must be stiffened and strengthened by encasement in an overlay or by some similar means. D. External stressing Development of cracking in concrete is due to tensile stresses and can be arrested by removing these stresses. Further, the cracks can be closed by inducing a compression force sufficient to overcome the tension and to provide a residual compression. The compressive force is applied by use of the usual prestressing wires or rods. The principle is very similar to stitching, except that the stitches are tensioned; rather than plain bar dogs which apply no closing force to the crack and which may in fact have to permit the crack to open up a bit before they begin to take the load. All the points noted regarding stitching must be considered. An additional problem is that of providing an anchorage for the prestressing wires or rods. Some form of abutment is needed for this purpose. The effect of the tensioned force on the stress conditions in the structure should be analyzed. E. Grouting Grouting of the cracks can be performed in the same manner as the injection of an epoxy, and the technique has the same areas of application and limitations. However, the use of an epoxy is the better solution except where considerations of fire resistance or cold weather prevent such use, in which case grouting is the comparable alternative. The procedure is similar to other grouting methods and consist of cleaning the concrete along the crack; installing built-up seats at intervals along the crack, sealing the crack between the seats with a cement paint or grout, flushing the crack to clean it and test the seal; and then grouting the whole. The grout itself is high-early-strength Portland cement. An alternative and better method, where it can be performed, is to drill down the length of the crack and grout it so as to form a key. The grout key functions to prevent related transverse movements of the sections of concrete adjacent to the crack. However, this technique is applicable only where the cracks run in a reasonably
straight line and are accessible at one end. The drilled hole should preferably be 2 or 3 inches in diameter and flushed to clean out the crack and permit better penetration of the grout. F. Blanketing Blanketing is similar to routing and sealing, but is used on a larger scale and is applicable for sealing active as well as dormant cracks. The following are the principal types of blanket joints i. Type 1 – Where an elastic sealant is used: The sealant material is the one which returns to its original shape when not under an externally induced stress, i.e., acts elastically. The recessed configuration is used where the joint is subjected to traffic or a pressure head. The strip sealant is applicable where there are no traffic or pressure problems and is somewhat less costly. The first consideration in the selection of sealant materials is the amount of movement anticipated and the extremes of temperature at which such movement occur. It should be capable of deforming the required amount under applicable conditions of temperature. The material should be able to take traffic, be resistant to chemical spillage, be capable of being pigmented, if desired. ii. Type 2 – Mastic filled joint: The sealant is a mastic rather than a compound having elastic properties. This type of joint is for use where the anticipated movements are small and where trafficabilty or appearances are not considerations. The advantage is that the mastic is less costly than the elastic type of sealant material. iii. Type 3 – A mortar plugged joint: Proper sealing to the crack is provide against leakage using a temporary mortar plug. The mortar plug provides the strength for the joints. The plug resists the pressure on the joint by arching the load to the sides of the chase. Where the pressure acts on the face of the joint, static balance requires the development of the tensile and shear stresses between the plug and the sides of the slot. Compression and shear are developed when the pressure head is internal. Theoretically, the edges of the chase should be undercut so that the mortar plug, which dries more quickly and more thoroughly at the surface, does not shrink differentially and so pull away from the sides and bottom of the chase. iv. Type 4 – A crimped water bar: Mortar is required where the junctions which bear traffic. A crimped water stop joint sealant is not applicable for the use where the crack is subject to a heavy pressure head from inside the joint, or where movement occurs as a shear along the length of the crack. In the first case, the pressure would tend to bulge out the crimp, and in the second case, the longitudinal movement would tend to tear the stop. Accordingly, this type of joint is primarily for use where the anticipated movements are limited to a simple extension or contraction and where the pressure head is either small or acts from the face of the joint. However, while not specifically intended for such applications, a rubber type or similar crimped joint
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INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN ENGINEERING RESEARCH, ICCTER - 2014 material will take a little longitudinal movement, and the crimped stop will function under limited amount of relative transverse displacement. v. Dealing with the reinforcement: What ever be the detail used, when cutting the chase, it is possible that some of the reinforcement will be exposed. If this is the case, cut deep is enough so that the sealant will be behind the reinforcing, clean the bars, and paint them with bitumen as a protection against moisture penetration and corrosion. If the crack in an active one with substantial movements, cut the bars so that they do not impede the movement of the joint. G. Overlays: Overlays may be used to seal cracks and are very useful and desirable where there are large numbers of cracks and treatment of each individual defect would be too expensive.
The mechanism whereby healing occurs is the carbonation of calcium oxide and the calcium hydroxide in the cement paste by carbon dioxide in the surrounding air and water. The resulting calcium carbonate and calcium hydroxide crystals precipitate, accumulate and grow out from cracks. The crystals interlace and twine producing a mechanical bonding effect, which is supplemented by a chemical bonding between adjacent crystals and the surfaces of the paste and aggregate. As a result, some of the tensile strength of the concrete is restored across the cracked section, and the crack may become sealed. Saturation of the cracks and the adjacent concrete with water during the healing process is essential for the development of any substantial strength. Submergence of the cracked section is desirable. Alternatively, water may be ponded so that the crack is saturated. Upto 25% of the normal tensile strength may be restored by healing under conditions of submergence in water. The saturation must be continuous for the entire period of healing. A single cycle of drying and re-immersion will produce a drastic reduction in the amount of healing strength. Healing should be commenced as soon as after the crack appears as possible. Delayed healing results in less restoration of strength. Healing will not occur if the crack is active. Healing also will not occur if there is a positive flow of water through the crack, which dissolves and washes away the lime deposit, unless the flow is slow so that complete evaporation occurs at the exposed face causing re-deposition of the dissolved salts. Concrete cracks both dormant and active subjected to water pressure are able to themselves with time. The greatest autogenous healing effect occurs between the first 3 to 5 days of water exposure. In additional skin reinforcement proves to be highly effective in supporting the autogenous healing effect. Growth rate of calcium carbonate crystals depends on crack width and water pressure, whereas concrete composition and type of water has no influence on the autogenous healing rate [4].
i. Active cracks: Sealing of active cracks by use of an overlay requires that the overlay be extensible. The occurrence or prolongation of crack automatically means that there has been an elongation of the surface fibers of the concrete. Accordingly, an overlay which is flexible but not extensible, i.e., can be bent but cannot be stretched, will not seal a crack that is active. A two or three-ply membrane of roofing felt laid in a mop coat of tar, with tar between the plies; the whole covered with a protective course of gravel, concrete, or brick, functions very well for this purpose. The type of protective course depends on the use to which it will be subject. Gravel is typically used for applications such as roofs, and concrete or brick is applicable where fills is to be placed against the overlay. An asphalt block pavement also works well and may be used where the area is subjected to heavy traffic. If the cracks are subjected to longitudinal movements parallel to their axis, the overlay will wrinkle or tear. Be very careful in repairing such joints. Blanketing may be a better V. CONCLUSION solution. ii. Dormant cracks: If the cracks are dormant, almost any General precautions to be followed for the repair of type of overlay may be used, provided that it will take cracks in concrete as listed below. the traffic to which it is subject and that it is either i. Do not fill the cracks with new concrete or mortar adequately bonded or thick enough so that curling due ii. Try to avoid the use of brittle overlay to seal an active to differential deformations is not a problem. Epoxy crack. compounds are coming into increasingly frequent use iii. Do not fail to remove the restraints causing the crack. for this purpose. iv. Do not surface –seal cracks over corroded reinforcement without encasing the bar. H. Autogenous healing v. Do not bury or hide a joint so that it is in accessible. The inherent ability of concrete to heal, cracks within itself is termed as autogenous healing, and is a phenomenon which has been known for some time. It has REFERENCES practical application for sealing dormant cracks, such as in [1] C.S. Suryawanshi, “Structural concrete repair – A durability the repair of precast units cracked in handling, reknitting based revised approach is needed”, The Indian Concrete of cracks developed driving the driving of precast piling, Journal, 2012. sealing cracks in water tanks, and sealing of cracks which [2] “Causes, Evaluation and Repair of Cracks in Concrete are result of temporary conditions or inadvertencies. The Structures”, Reported by ACI Committee 224. effect also provides some increase in strength of concrete [3] “Stitching concrete Pavement”, International Grooving and damaged by vibration during set and of concrete disrupted Grinding Association, June 2010. by the effects of freezing and thawing. INTERNATIONAL ASSOCIATION OF ENGINEERING & TECHNOLOGY FOR SKILL DEVELOPMENT
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INTERNATIONAL CONFERENCE ON CURRENT TRENDS IN ENGINEERING RESEARCH, ICCTER - 2014 [4] Carola Edvardsen, “Water permeability and Autogenous healing of cracks in concrete”, ACI Material journal, Title no 96-M 56, July/August 1999.
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