These type of cracks occur, when the structure shows signs that it cannot safely support the loads to which it was being subjected. A majority of structural cracks occur as a result of the following conditions:
• Design deficiency
• Construction deficiency
• Settlement or swelling of soil
• Reinforcement corrosion

Sometimes structural cracks manifest themselves with some side effects. Doors and windows do not open and close easily. Floors feel uneven. Vinyl flooring tears as a result of crack movement. Plaster begins to show new cracks and even interior corners may develop new cracks. Cracks can develop along the length of the foundation as a result of corrosion of reinforcement.

Types of Structural Cracks

Rust cracking:
Rust cracking is usually a result of corrosion of the reinforcing steel or structural over stressing. The corrosion products are more in volume than steel and these push the concrete in cover region and cause cracks. Such cracks lead to further corrosion. Typically these are seen running vertical in columns and along bottom of the beams.
Inadequate reinforcement cover causes the most common and most serious cause of structural cracking. It gradually develops at varying rates over time depending upon the degree of protection offered by the concrete cover.
Various anti-corrosive coatings are available in the market to prevent the rusting and deterioration of concrete.

Some tolerable crack widths along with their exposure conditions are given in the table.

Observed crack widths, however, are not necessarily very good indicators of durability (the structure’s resistance to corrosion and deterioration). It may be (and ACI 224 states that is should be expected) that some of the cracks in a structure will be significantly wider than those in the table. The amount of concrete cover over the reinforcement may be a better indicator of corrosion resistance than crack width.

Another point to consider when using this table is that it applies only to structural reinforced concrete. These values should not be used to judge crack widths in unreinforced or lightly reinforced slabs on grade or in unreinforced basement walls. Also take into account the high variability in crack widths. Isolated locations where cracks are wider than those in this table 224 should not be grounds for rejection or reduced pay.

Settlement Cracking :
Settlement happens when parts of a house drop below the elevation or height where they were placed during the original construction. Settlement cracking in houses can be minimized if precautions are taken. If building is on filled land, make sure it is properly compacted. Settlement cracking can also be caused due to overload on the foundation.
Geotechnical and soils engineers can perform soil tests to make sure if the soil can support the building.

As you can see from the picture below, which is a typical case for a settlement crack to occur.

Removal of a big tree next to a house can cause sudden changes in the soil moisture level and cause soil movement.

Case 1 shows cracks due to tree removal.

Case 2 shows cracks due to plantation of a new tree.

Tension cracking:
Such type of cracking occurs only in reinforced concrete and is caused by elongation of the reinforcement (steel) in tension zones.
It is sometimes seen around columns in flat slabs (slabs resting directly on columns, without beams) and on beam soffits (bottom side of beams) near the middle of a span.

Concrete has good compressive strength, while steel has good tensile strength.
Steel bars are cast in positions where maximum tensile stresses occur, which the concrete alone cannot withstand. Such a combination of concrete and steel is called reinforced cement concrete.

Tensile Strength : Ability to resist forces that cause stretching.
Compressive Strength : Ability of a material to withstand compression.

As seen in the picture above, when there is a load on the slabs, pressure is created and the steel at the lower portion of the slab takes all the tension due to the property of the steel. When the steel tends to stretch from the middle (where tension zone is created), a reverse pressure is created at the junctions.
Thus additional steel is inserted at the junctions at the top of the slab.

When in doubt, consult a professional concrete engineer to investigate the cause of distress and evaluate the condition of the concrete.
One could assist such a professional by keeping a periodic record of :
• Crack progression
• Crack widths
• Landscape irrigation schedule, especially big tree removal dates