In fact, in many instances, cracks are merely cosmetic in nature, majority of which are caused by poor quality concrete, and/or inadequate cover to the reinforcement.
Cracks on plaster are due to silt content in sand. If sand is not washed before use then cracks occur in plaster.
They also occur due to poor curing which leads to drying shrinkage.
Curing is required to assist the chemical action of cement, to prevent rise in temperature due to the heat of hydration & to prevent drying of concrete resulting in contraction cracks. A crazy cracked surface is often indication of inadequate early curing.The early curing of concrete is very critical. Retaining moisture within the concrete may be sufficient for concretes of low to moderate cement content. Mixes rich in cement generate considerable amount of heat which may expell moisture from within the concrete. For such concrete, water curing should begin as soon as possible to replace any lost moisture and to dissipate heat. This is best done by ponding, continuous sprinkling, covering with wet cloth, cotton mats or similar materials on the surface of the concrete.

Typical causes of these non-structural cracks are :

• Poor workmanship
• Inappropriate joint detailing
• Higher shrinkage of concrete

Plastic Shrinkage

• Takes place in fresh ( green) concrete
• Takes place due to Water absorption by aggregates, sedimentation( bleeding), hydration of cement, thermal changes.
As a result of vibration during concreting excess water mixed in concrete floats on top (being lighter than sand, cement and stones). This water is called bleeding water.
• Affected by temperature, humidity, wind velocity.
• If rate of evaporation of water from concrete surface is faster than bleeding then cracking takes place.
• Plastic shrinkage cracks are usually straight line without any symmetry or pattern and are of considerable depth.
• Can be minimized by using saturated aggregates, low cement mixes, moist and cool casting conditions, tight and non absorbent forms, shallow lifts etc.
• Sometimes possible to eliminate cracks that have formed by revibration or retrowelling.
They are usually fine cracks which are fairly straight. They are often transverse in direction and in some cases several of them are parallel to each other with a spacing of around 50mm.

Drying Shrinkage

• Removal of water from the body of concrete due to thermal and hydral gradient between concrete and environment ( concrete moist- environment dry) causes drying shrinkage.
• Principally caused by contraction of cement paste when moisture content of the paste decreases.
• This shrinkage is about 0.4 %

Factors affecting drying shrinkage:

• Neat cement paste has more shrinkage than concrete
• Water content of mix, cement composition, quality and quantity of cement, admixtures, type and size of aggregates, amount and distribution of reinforcement, curing condition, length of drying period, humidity in air are some of the factors affecting drying shrinkage
• Advantage of prolonged curing is mainly that concrete can withstand greater shrinkage strains or stresses, without cracking.
• Differential volume changes take place in large concrete members with more shrinkage on exterior as compared to interior. This leads to surface cracks.
• It is observed that cumulative effect of individual factors increasing shrinkage is multiplication rather than addition.
Sharp 90 degree corners at windows, doors, and offsets in slabs and foundations are excellent locations for the cracks to begin.

Many of the cracks are typically seen between brickwork and concrete elements such as column or beam.

They occur due to the method of construction where beams and slabs are cast first followed by construction of walls. The top layer of cement mortar over the bricks is not packed properly. Moreover the thermal movement of brickwork and concrete is different.
This is a common problem in many buildings.
This happens because the original construction of beams and columns are not wide enough.
Hence brickwork is added to the narrow beams and columns for construction of doors and windows within the house.

Following figure shows the locations of such cracks.

As shown in the picture above, cracks usually develop between the brickwork and columns & beams. The additional brickwork which give extension to the columns and beams, are done for the construction of doors and windows.
Lintel, as shown in the figure is a horizontal member, usually of reinforced cement concrete, which is placed across the openings of doors or windows to support the wall over it.
Sometimes such nonstructural cracks as seen from inside the building become more than just an eyesore.

Thermally-induced cracking:

Results from stresses produced by temperature changes. The width of thermal contraction cracks is usually increased by normal drying shrinkage, this usually happens within 2-3 months after casting. Considerable heat is evolved by the chemical reaction between the water and the cement which results in a rise in temperature of the concrete causing expansion. When it cools down, it contracts.