In this article we will discuss about:- 1. Assessment of Cracks in Concrete Structures 2. Evaluation of Cracks 3. Identification of Weak Spots 4. Selection of Repair Procedure.
Cracking is the most common indication of the defects in a concrete structure. It may affect appearance only or may indicate significant defect or lack of durability. Cracks may represent the extent of the damage or they may point to problems of greater magnitude. Cracks may cause corrosion of reinforcement due to entry of moisture and oxygen.
Cracks in a structure broadly may be classified into two groups as superficial and structural cracks. The structural cracks may further be divided into two categories as (i) Active and, (ii) Dormant. An active crack is that in which a movement is observed to continue, where as in a dormant crack no movement occurs.
Assessment of Cracks in Concrete Structures:
For the assessment of the cracks following information will be useful:
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1. Pattern of cracks
2. Type of crack, whether it is active or dormant.
3. Whether the crack is old or new
4. Whether it appears on the opposite face of the member
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5. Type of foundation used, soil condition, sign of movement of ground, if any
6. Observations on the similar structures in the vicinity
7. Study of method of construction, test results of the site, specifications etc.
8. Weather in which the structure has been constructed.
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Thus the cracking is a complex phenomenon and before any repair work is taken in hand the cause of damage must be identified clearly after careful investigations.
Evaluation of Cracks in Concrete Structures:
Before starting repair of cracked structure, the knowledge of location and extent of cracking is essential for the effective and proper repair. The objectives of repair are restoration and enhancement of durability of structural strength, functional requirements and aesthetics.
Following are the purposes of the crack evaluation:
1. To identify the cause of cracking
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2. To assess the safety and serviceability of the structure
3. To establish the extent of cracking
4. To establish the likely extent of future deterioration
5. To study the feasibility of various remedial measures, and
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6. To make the final assessment of serviceability after repairs.
Identification of Weak Spots in Concrete Structures:
Apart from visual inspection, hammering the surface with a light hammer and listening the sound from the area is one of the simplest methods of identifying the weak spot as dull sound indicates hollowness of the surface. The suspected areas are then opened up by chipping the weak concrete for further assessment.
The comparative strength of concrete in the structure can be assessed by nondestructive tests and tests on cores taken from the concrete of the structure to a reasonable accuracy. Commonly used nondestructive tests are rebound hammer test and ultrasonic pulse velocity test.
Visual Examination:
The possible causes and nature of cracks can be ascertained by visual examination. The appearance of the concrete surface may suggest the possibility of chemical attack by the general softening and leaching matrix. In case of sulphate attack, the surface of the concrete becomes white. The rust stains arc the indication of corrosion of reinforcement or contamination of aggregate with iron pyrites. The discolouration or presence of dirt on the broken surface of the cracked concrete gives an indication that the crack is quite old.
The general flaking of the exposed surface of the concrete suggests frost damage. The colour of concrete damaged in fire hazard indicates the maximum temperature reached.
The crack pattern is also useful in evaluating the cracks. A mesh pattern suggests the drying shrinkage cracking and surfacing crazing may indicates either frost attack or alkali-aggregate reaction cracking. Generally it is frost attack. The cracks caused by unidirectional bending will be the widest in the zone of maximum tensile stress and will tapper along their length, while cracks caused by direct tension will be roughly of uniform width.
The width and location of cracks should be noted on the sketch of the structure. Crack width may be measured to an accuracy of 0.025 mm with the help of a small hand held microscope. It has a scale on the lens which is held close to the surface to be viewed. Location of spalling, exposed reinforcement, rust staining and surface deterioration etc. should be noted on the sketch.
Non-Destructive Testing:
Nondestructive test may be used for determining accurate and reliable information about the presence of internal cracks and voids, depth of penetration of cracks which are visible on the surface. By tapping the surface with an ordinary light hammer, often useful information may be obtained. A dull sound indicates the separation of concrete due to fire or otherwise, it also indicates the presence of cracks below the surface.
The most common technique to detect the crack using ultrasonic nondestructive test equipment is through transmission testing using. Soniscope. The method consists of transmitting a mechanical pulse to one face of the concrete member and receiving it at the opposite face, and calculating the pulse velocity from the time taken by the pulse to pass through the member and the distance between the transmitting and receiving transducers.
If there is internal crack, the pulse will travel at the corner or edge of the crack, increasing the path length of the pulse. The increase in path length of the signal as it passes around the end of the crack there will be a significant change in the pulse velocity, generally higher the pulse velocity, better the quality and durability of the concrete.
If the signal is displaced on the oscilloscope, the internal discontinuities can also be detected by weakening or reducing the strength of the signal.
If no signal arrives at the receiving transducers, it indicates a significant internal discontinuity, such as void or crack. The extent of the discontinuity can be assessed by taking readings at a series of positions on the member. The result of ultrasonic testing should be interpreted continuously. Generally with fully saturated cracks the ultra-sonic testing will be ineffective. The arrangement of the test is shown in Fig.26.7.
Selection of Repair Procedure for Concrete Structures:
The repair of concrete structures may vary to a great extent, giving it just a superficial treatment to the total replacement. By proper investigations using the well-designed equipment, tools and materials a number of structures have been re-innovated economically, which were though damaged beyond repairs. An appropriate repair method can be selected depending upon the cause, extent of damage, importance of the structural member and its location. The success of the repair will depend upon the proper choice of the method.
The procedure should be selected to accomplish one or more of the following objectives:
1. To improve its durability
2. To improve the appearance of concrete surface
3. To increase its functional performance
4. To increase the stiffness of the member
5. To increase the load carrying capacity of the structure
6. To increase the water tightness of the structure, and
7. To protect the reinforcement from corrosive material etc.
Depending upon the extent and nature of the damage, one or more methods may be adopted for example to increase the tensile strength of the cracked member epoxy grout can be used along with additional reinforcement. If further cracking is not anticipated, then epoxy injection can be used to restore flexural stiffness of the member. In water retaining structures cracks likely to cause leakage should be repaired unless the leakage is insignificant. When the appearance of the cracks becomes un-acceptable, they should be repaired.
The long term success of the repair procedure mainly depends upon the nature of cracks as well as their cause. If the cracks are developed due to the continuing settlement of the foundation, repair will not be effective till the settlement problem is tackled.