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Causes of cracks and Recommendations

Definition: A crack is a complete or incomplete separation of concrete or landcrete wall into two

or more parts, produced by breaking or fracturing.The nature of the material used in the construction of the building is an important factor in

determining the defects that it will be subjected to.

Buildings made of sandcrete blocks and concrete, landcrete, wattle and dub or bricks aresusceptible to shrinkage cracks. Buildings made of landcrete or wattle and dub have high clay

content. As such the clay shrinks and swells due to moisture changes introducing cracks in theprocess. Concrete being a material having very low tensile strength and readily cracks when

tensile stress beyond the tensile strength of concrete occur in structure.Cracks in buildings tend to follow lines of inherent weakness, for instance cracks in wall panels,

cracks between door and window openings which are the areas of weakness.

Cracking can be the result of one or a combination of factors, all of which involve some form ofrestraint. Some examples include:

1. Drying ShrinkageThis occurs as water used in the mix design evaporates.2. Thermal Contraction/ExpansionDue to temperature changes.3. Subgrade Settlement (or Expansion) - Resulting from poor soil conditions or changes in

soil moisture content.

4. Differential Bearing Capacity Harder soils under part of the foundation can causestresses as the building settles in.

5. Applied StressesForces such as building load, earth load, hydrostatic pressure, orheavy equipment operated too close to the wall.

6. Poor construction practice- Improper reinforcing steel placement, Improper bar detailing,Premature removal of forms, Improper column form placement

7. Ground water The presence of ground water can cause soil erosion and reduction of soilcompressive strength, reducing capacity of foundation, stressing and cracking building

materials.

Crack Pattern

Shrinkage and Temperature: Shrinkage and Temperature cracks are most often vertical to

diagonal. They usually emanate from a corner of a window, door, beam pocket, or other opening.Plastic shrinkage cracks include diagonal and random cracks. These occur on freshly rendered

surfaces. Cracks which are horizontal are most likely caused by an applied load.

Settlement cracks: A diagonally running stepped crack, with the appearance likened to that of a

stair when viewed edge on, can indicate that structural settlement is happening, possibly due to

upheaval at foundation level or some other form of slippage.

Vertical, or near vertical, cracks can also be a sign of serious trouble. If the cracks are wide at the

top and tight at the bottom this could mean that one or both ends of the buildings foundation are

dropping or that the middle of the foundations is rising. If the cracks are narrow at the top and

wide at the bottom, it could indicate that the middle portion of the building is dropping whereas

the extreme ends are still or heaving.

Alligator pattern: Alligator cracks on walls and slabs are indicative of drying shrinkage on a

small scale. The cracks usually occur when the surface layer of the concrete has higher watercontent than the interior concrete. The result is a series of shallow, closely spaced, fine cracks.

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Blasting: Blasting produces seismic waves similar to those produced by earthquakes, but with

relatively high frequency and low amplitude which can create damage to surrounding structures.

The degree of structural damage depends on the total energy of explosion, distance from the

source, and the characteristics of the medium.

The first damage generally occurs in the lower half of the wall nearest to the explosion. Thedegree of involvement of the rest of the structure depends on the size of the charge and its

distance from the structure.

Vibrations from blasting can affect buildings structurally. New cracks could be introduced,

existing cracks could be widened. Also buildings with reinforced concrete tend to resist ground

vibrations better than those made with bricks or landcrete. Buildings situated on soils with weak

or low bearing capacity would readily undergo differential settlement and thus introduce cracks

on the structure.

Cracks can also be categorized as:

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 andmay be difficult to be rectified. Extensive cracks of foundations walls, beams, columns or slabs.

Structural cracks could also be the result of ground vibrations from blasting.Non structural: 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.

Recommendation /Mitigation measures (For non structural cracks)

1. Construction and construction materialsThe first is the use of proper concrete mix designs. A mix with sufficient strength usingthe minimum amount of water necessary to distribute the concrete throughout the wall

without voids should be used. The type and amount of cement, as well as coarse and fineaggregates, can also have a large effect on the amount of shrinkage.

2. Rapid water loss and extreme temperature swings while the concrete is in the early stagesof curing should be avoided where possible.

3. Checking the proper water cement ratio.4. Proper support for formwork.5. Thin hairline cracks could be filled with groutRecommendation /Mitigation measures (Structural cracks)1. Affected walls could be pulled down depending on the extent of damage and rebuilt.2. The use of ground beams is also recommend .Ground beams would redistribute stresses

that are induced as a result of the ground vibrations from blasting.3. Reinforced concrete should be used in the construction of buildings.

Reference

1. Concrete Foundations Association, Canada2. Rajesh Zingade Gulbarga,Karnataka, India,cracks in buildings causes & preventions3. Stabilizing Heritage Buildings Founded on Reactive Clay, J W Jordan, B J Collins4. Charles C.Roberts ,Jr. Phd.,P.E.Evaluating cracks in buildings