A crack is a complete or incomplete separation of concrete into two or more parts, produced by breaking or fracturing. The crack in concrete is an inherent feature, which cannot be completely prevented but can only be controlled and minimized. Concrete being a material having very low tensile strength, readily cracks when such tensile stress beyond the tensil strength of concrete occur in structure.
An engineer should have a sound knowledge of all the facts of concrete technology i.e. of the behavior of construction materials, construction techniques, and types of crack likely to occur, their causes and respective remedial measure. In short treatment of cracks involves detection, diagnosis and remedy. Cracks also occur due to settlement, temperature, shrinkage effect, poor construction practice etc. In this seminar various causes for the above mentioned cracks is been discussed.
Types of cracks:
Cracks may be divided in two categories viz
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Structural cracks
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Non structural cracks
Structural cracks
Non structural cracks
i) Structural cracks:
Structural cracks may arise due to various reasons such as incorrect design, overloading of the structural components, overloading of the soil on which the building is constructed or other similar factors. Structural cracks endanger the stability of the building and may be difficult to be rectified. Extensive cracks of foundations walls, beams, columns or slabs etc, are examples of structural cracks.
ii) Non- structural cracks:
Non- structural cracks are generally due to internal forces developed in the buildings on account of change in the size of building components, due to moisture variation, temperature variations, the effect of gases, liquid and solids on the building components. The non-structural cracks can be repaired provided the reasons for cracks are identified and suitable remedial measures are taken to prevent their reoccurrence.
Investigation relating to cracks:
A careful study of the locations of cracks (starting and finishing points) their width and depth helps in dealing with the diagnosis of different types of cracks.
The following information helps in diagnosing the cracks:
- Whether the crack is old or new.
- Whether it appears on the opposite face of the member also.
- Pattern of the cracks.
- Soil condition, type of foundation used, and movement of ground if any.
- Observations on the similar structures in the same locality.
- Study of specification, method of construction, used and the test result at the site if any.
- Climatic condition during which the structure has been constructed.
Limitation of crack width (IS 456: 2000):
Depending on the exposure conditions limitations on crack width are imposed as follows
- For members in water storage units, sewage treatment plants, structures in chemically hazardous atmosphere, etc. Cracks are not permitted in R.C. members.
- In severe atmosphere up to 0.1mm crack width is permitted.
- Moderate atmosphere up to 0.2mm crack width is permitted
- In mild atmosphere the surface width of cracks should not, in general exceed 0.3mm in members where cracking does not have any serious adverse effects upon the preservation of reinforcing steel nor upon the durability of structure.
Permissible crack width in reinforced structure as per ACI
II) CAUSES FOR THE OCCURANCE OF CRACKS:
The importance causes responsible for occurrence of the cracks are
1.structural deficiency resulting from design deficiency or construction deficiency and overloading.
2. Settlement of ground
3.Temperature and Shrinkage effects.
4. Cracks due to faulty workmanship and poor construction practice
Cracks due to structural deficiency resulting from design deficiency or construction deficiency and overloads.
Concrete structure and individual members all carry loads. Some carry only the weight of the materials they are made of, while others carry loads applied to the structure. All material change volume when subjected to stress,
Concrete is no exception. When subjected to tensile stress, concrete stretches; when subjected to compressive stress it shortens. Concrete possesses high compressive strength but little tensile strength, and reinforcing steel provides the needed strength in tension. The loads induced during construction can be far more severe than they are experienced in service. Concrete problems, such as excessive deflection, cracking may be caused by volume changes associated with load effect.
Most concrete members are subjected to tensile forces. Slabs and beams are the most common members subjected to significant tension. Reinforcing bars are placed in the concrete to carry tension forces. When reinforced bar are subjected to tension they stretch. The concrete around the reinforcing bars is consequently subjected to tension and stretches. When tension in excess of tensile strength of concrete is reached, transverse crack may appear near reinforcing bars.
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Cracks occur due to shear, flexural and torsional steel deficiency.
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Cracks occur due to abrupt curtailment of reinforcing bars, construction joints etc.
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Improper anchorage.
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Cracks due to overloading of members
Cracks occur due to shear, flexural and torsional steel deficiency.
Cracks occur due to abrupt curtailment of reinforcing bars, construction joints etc.
Improper anchorage.
Cracks due to overloading of members
Preventive measures:
- Special care need to be taken in the design and detailing of structures in which cracking may cause a major serviceability problem. These structures also require continuous inspection during all phases of construction to supplement the careful design and detailing.
- Damages from unintentional construction overloads can be prevented only if designer provide information on load limitation for the structure and if the construction personnel heed to these limitations.
- Ensure proper anchorage to the reinforcing bars.
- Follow proper design specifications.
Cracks due to shrinkage and temperature effect:
Shrinkage crack:
Shrinkage cracks show up in two basic locations in most walls; the approximate mid-point of a long section of wall, and the narrowed section of the wall such as across a door or window head. Shrinkage cracks are virtually uniform in width from top to bottom and typically extend from the top of the wall to within a couple of feet of the foundation.
Common cause for shrinkage cracks in concrete walls would be excessive water content within the concrete. In general terms, higher water content within a concrete mix will result in a greater amount of shrinkage. This is quite evident in some concrete walls where there are an excessive number of cracks.
Shrinkage crack in wall Shrinkage crack in cantilever slab
masonry
On exposure to atmosphere, concrete loses some of its original water and shrink. Drying shrinkage, if unrestrained, results in shortening of the member without a build-up of shrinkage stress. If the member is restrained from moving, stress build-up may exceed the tensile strength of the concrete. This over-stressing results in dry shrinkage cracking.
Temperature effect:
The effect of temperature on concrete structure and member is one of volume change. The volume relationship to temperature is expressed by the coefficient of thermal expansion/contraction. Volume changes create stress when the concrete is restrained. The resulting stress can be of any type: tension, compression, shear, and etc. the stressed conditions may result in undesirable behavior such as cracking, spalling and excessive deflection.
A typical case of occurrence of cracks due to temperature variation is that of roof slab being exposed to the heat of sun, which is subjected to alternate expansion and contraction. This movement of slab may result in pushing out top course of masonry and develop horizontal cracks in the supporting walls.
Thermal crack in masonry
Preventive Measures:
- Adequate insulating or terracing treatment over roof slab and by introducing joint between the slab and the supporting wall.
- Painting top of roof with reflective finish such as white wash can also minimize cracks.
- Chances of cracking due to temperature variation can be minimized by introducing expansion, contraction joints at appropriate locations.
Cracks due to settlement:
Uneven (differential) settlement can be a major structural problem in small residential buildings, although serious settlement problems are relatively uncommon. Many signs of masonry distress are incorrectly diagnosed as settlement-related when in fact they are due to moisture and thermal movements.
Fig 14
Indications of differential settlement are vertical distortion or cracking of masonry walls, warped interior and exterior openings, sloped floors, and sticking doors and windows. Settlement most often occurs early in the life of a building or when there is a dramatic change in underground conditions. Often such settlement is associated with improper foundation design, particularly inadequate footers and foundation walls.
- Soil consolidation under the footings
- Soil shrinkage due to the loss of moisture to nearby trees or large plants
- Soil swelling due to inadequate or blocked surface or house drainage
- Soil heaving due to frost or excessive root growth
- Gradual downward drift of clay soils on slopes
- Changes in water table level
- Soil erosion from poor surface drainage, faulty drains, leaking water mains or other underground water movements (occasionally, underground water may scour away earth along only one side of a footer, causing its rotation and the subsequent buckling or displacement of the foundation wall above)
- Soil compaction or movement due to vibration from heavy equipment, vehicular traffic, or blasting, or from ground tremors (earthquakes).
Gradual differential settlement over a long period of time may produce no masonry cracking at all, particularly in walls with older and softer bricks and high lime mortars; the wall will elastically deform instead. More rapid settlements, however, produce cracks that taper, being largest at one end and diminishing to a hairline at the other, depending on the direction and location of settlement below the wall.
Cracking is most likely to occur at corners and adjacent to openings, and usually follows a rough diagonal along mortar joints (although individual masonry units may be split). Settlement cracks (as opposed to the similar-appearing shrinkage cracks that are especially prevalent in concrete block) may extend through contiguous building elements such as floor slabs, masonry walls above the foundation, and interior plaster work. Tapering cracks, or cracks that are nearly vertical and whose edges do not line up, may occur at the joints of projecting bay windows, porches, and additions. These cracks indicate differential settlement due to inadequate foundations or piers under the projecting element.
Often settlement slows a short time after construction and a point of equilibrium is reached in which movement no longer occurs. Minor settlement cracking is structurally harmful only if long-term moisture leakage through the cracks adversely affects building elements. Large differential settlements, particularly between foundation walls and interior columns or piers, are more serious because they will cause movements in contiguous structural elements such as beams, joists, floors, and roofs that must be evaluated for loss of bearing and, occasionally, fracture.
Buildings constructed on expansive soil are liable to cracks due to volumetric changes in the sub-soil conditions due to changes in moisture contents. Expansive soil is a kind of clayey soil, which exhibits swelling and shrinkage properties due to variation in seasonal moisture content. The structures built on such soils are subjected to severe stress due to alternate swelling and shrinkage and undergo distress. Light structures suffer more.
Preventive Measures:
- In case of shrinkable soils, adopt under reamed pile foundation.
- The structural design of the foundation should be carried out in such a manner as to achieve uniform distribution of pressure on the ground to avoid differential settlement.
- The foundation should be so proportioned that the safe bearing capacity of soil is not exceeded.
- The soil should be well compacted
Cracks due to faulty workman ship and poor construction practice:
Methods used to construct concrete structures are different from methods used in other type of construction. Concrete is one of the few materials in which raw ingredients are brought together at, or near, the construction site, where they are mixed, placed and molded into a final product. Every building process includes a sequence of necessary step-by-step operation-from conceptual plan to finished structure. There are so many variables affecting the production of concrete that there is always a potential for something to go wrong.
Causes:
- Improper reinforcing steel placement
- Improper bar detailing
- Premature removal of forms
- Improper column form placement
- Cold joints
- Segregation
- Plastic shrinkage cracking
Preventive measures:
- By providing proper water cement ratio.
- Proper curing.
- Proper support for forms.
- Following proper design codes and recommendations.
Case study:
The case study described in this paper is an example of a report on P.W.D office building Yelandur. The building was inspected on 21st May 1988 and the following observations where made.
The soil below the foundation and around the building is black cotton soil. Construction of the building is reported to be completed during 1981. It was learnt that that rainfall in the area where the building was situated was scanty. It was also learnt that no precautionary measures where taken during construction.
A team of experts from Karnataka Engineering Research station, Krishnarajasagar visited the spot and after investigation gave advise to implement certain remedies such as replacing the soil around the building by good gravelly soil for a depth of about 1foot, removing the vegetation and trees around the building, filling the pits around the building with gravelly soil, providing drainage facilities etc. But they where partially implemented, i.e., soil around the building was replaced by gravelly soil. It was observed that cracking continued further.
Possible causes of distress:
- When a building is founded on an expansive soil with normal footing, the swelling and shrinkage of the soil below the foundation due to variation in water content gives rise to moments. As the moisture content variation under the entire building will be uneven, this normally results in cracks in the buildings.
- Improper drainage around the building may lead to variation in water content of the formation soil.
- Unequal settlement of the structure may occur due to its construction on filled up soil.
- Growth of trees with dominant surface roots or fast growing trees, closely to the building may be the cause of distress.
- Due to increasing number of bore wells and drought situation, the water table may considerably go down. Due to this the water content of the soil may reduce causing considerable shrinkage. This may result in unequal settlement and finally leads to distress of the buildings.
Remedial measures:
Where the expansive soil is shallow, say about 2m, the most economical method will be to remove the soil & fill it with firm good soil and use sand or murram for the fill.
- Use under-reamed pile foundation.
- Damage due to uplift of expanding clay can be prevented by applying heavy superimposed loads.
- The building may be supported on footing at a depth below the surface and near enough to the water table, so that; the water content of the clay is not affected by climatic changes.
- Growth of trees near the foundation should be avoided.