causes and prevention of cracks in building

What is a crack?

A crack is a complete or incomplete seperation of concrete into two or more parts, produced by breaking or fracturing.

CLASSIFICATION OF CRACKS

1.Structural Cracks

2.Non Structural Cracks

INVESTIGATION RELATING TO CRACKS • Whether the crack is old or new. • Pattern of the crack. • Soil condition, type of foundation used, movement of ground if any. • Observation on the similar structure in the same locality.

• Study of specification, construction method and climatic conditions.

LIMITATION OF CRACK WIDTH 1. For members in water storage units, sewage units, chemically hazardous atmosphere cracks are not permitted. 2. In severe atmosphere up to 0.1mm crack width is permitted. 3. Moderate atmosphere upto 0.2mm crack width is permitted. 4. In mild atmosphere width of crack is permitted upto 0.3mm .

PERMISSIBLE CRACK WIDTH AS PER ACI EXPOSURE CONDITION MAXIMUM ALLOWABLE CRACK WIDTH IN mm Dry air, protective membrane

0.41

Humidity, moist air

0.30

Sea water and seawater spray, wetting and drying

0.15

Water retaining structure

0.10

CAUSES FOR THE OCCURANCE OF CRACK Crack may develop due to :-

1. Structural deficiency resulting from design deficiency or construction deficiency and overloading. 2. Temperature and shrinkage effects.

3.Settlement of ground. 4. Faulty workman ship and poor construction practice .

1.

STRUCTURAL DEFICIENCY RESULTING FROM DESIGN DEFICIENCY OR CONSTRUCTION DEFICIENCY AND OVERLOADING

CRACKS OCCUR DUE TO

• Shear, flexural and torsional steel deficiency.

• Abrupt curtailment of reinforcement bars. • Overloading of member. • Improper anchorage.

FLEXURAL CRACKS IN BEAMS

• Occurs due to flexural steel deficiency. • Occurs at maximum bending moment region

FLEXURAL CRACKS IN CANTILEVER BEAMS

• Occurs due to shear steel deficiency. •Occurs in maximum shear region.

PREVENTIVE MEASURES • Special care need to be taken while designing and detailing.

• Requires continuous investigation.

• Damages from unintentional construction overloads can be prevented only if designer provide information on load limitations and the construction personnel heed to these limitations.

• Follow proper design specification.

2 SHRINKAGE AND TEMPERATURE EFFECT CRACKS DUE TO SHRINKAGE • Show up in two basic location in most walls, approximately mid point of long section wall , across door or window head. • Uniform in width. • Excessive water content within the concrete. • Higher water content results in greater shrinkage . • On exposure, concrete loses some of its original water and shrink.

PREVENTIVE MEASURES • Minimise the use of rich concrete mix.

• Use lean cement mortar in masonry works.

• Allow adequate time for curing . SHRINKAGE CRACK IN WALL MASONRY

TEMPERATURE EFFECT • Volume changes .

• Volume relation to temperature is expressed by coefficient of thermal expansion/contraction.

• Volume change induces stress.

PREVENTIVE MEASURES • Adequate insulating or terracing treatmemt.

• Painting top roof with reflective finish such as white wash. THERMAL CRACKS IN WALL MASONRY

• Introducing of expansion and contraction joint at apropriate locations.

3 CRACK S DUE TO SETTLEMENT

• Uneven settlement can be a major structural problem in small residential building.

• Vertical distortion or cracking of masonry walls, wrapped interiors and exterior opening. • Occurs early in life of building.

SETTLEMENTS ARE CAUSED DUE TO

• Soil consolidation under footing. • Loss of moisture. • Water table level. • Faulty drains, leaking water mains. • Soil compaction or movement due to vibration .

Building settlement due to cut and fill

Differential settlement caused due to variable soil type

• Cracking likely to occur at corners and adjacent to opening. follows a rough diagonal along morter joints.

• May extend through continuous building element such as floor slabs, masonary walls above the foundation and interiors.

• Building constructed on expansive soil are liable to cracks.

• The structure is subjected to severe stress due to alternate swelling and shrinkage.

PREVENTIVE MEASURES

• Under reamed pile foundation. • Foundation design for uniform distribution of pressure. • SBC is not exceeded.

• Soil should be well compacted.

FAULTY WORKMANSHIP AND POOR CONSTRUCTION PRACTICE CAUSES • Improper reinforcing steel placement. • Improper bar detailing. • Premature removal of forms.

• Cold joints. • Segregation. • Plastic shrinkage cracking.

PREVENTIVE MEASURES • Water cement ratio.

•Proper curing. •Support for forms. •Design recommendations.

CASE STUDY OF P.W.D OFFICE BUILDING YALENDUR • Inspected on 21s t may 1988.

• Foundation soil is a black cotton soil.

•Construction completed In the year 1981.

CAUSES FOR DISTRESS

• Swelling and shrinkage of soil.

• Improper drainage.

• Filled up soil.

• Growth of trees .

REMEDIAL MEASURES •Remove and fill with firm good soil.

•Under reamed pile foundation.

•Apply heavy super imposed load.

•Avoid growth of trees in the vicinity.

CONCLUSION

• The individual cracks in members are unique by themselves.

• They are versatile diagnostic devices, clearly indicating the specific problem in concerned members. In addition they also indicate the degree of seriousness of the problem.. • The crack throw light on the discipline in the associated structural designs and quality . • The corrective measures to be implemented are clearly indicated by these cracks.

REFERENCES • Peter H Emmons- “Concrete repair and maintainence” `

•P.C Varghese- “Limit state design of Reinforced Concrete”

•H.P. Rossmanith- “Fracture and Damage of Concrete”

• M.S.Shetty- “Concrete Technology”