Structural Cracks in Concrete

• Cracks are defects commonly found in reinforced and plain concrete structures, especially in countries where most constructions are concrete.
• Studying cracks, their causes, and repair methods is crucial for ensuring building safety, longevity, and durability.
• Finding the suitable way to repair cracks and suggesting appropriate materials is essential.

Cracks Types

• Generally, there are two types of cracks based on the reason for their cause which are Structural and Non-structural.

Structural Cracks

• These occur in reinforced structural members like foundations, columns, or beams.
• They affect the integrity and load-bearing capacity of a structure.
• Structural Cracks extend through the entire thickness of a wall or slab.
• They are often dangerous, need rapid treatment and usually wider than 1/8 inch.
• Often appear in a straight line or follow a pattern.
• May be accompanied by signs of distress like bulging walls, sagging floors, or doors/windows that no longer close properly.
• Require professional assessment, involving structural engineers and specialized repair techniques.

Nonstructural Cracks

• These occur for reasons other than structural issues, such as shrinkage, temperature changes, or minor settling.
• Typically superficial and do not affect the structural integrity of the building.
• Often found in plaster, drywall, or other surface finishes.
• Are typically narrower than 1/8 inch, often appear as hairline cracks.
• May be random or follow the material's pattern, like mortar joints in brickwork.
• Can often be repaired by homeowners or general contractors by filling them with materials like caulk, epoxy, or mortar.
• These are not seen as dangerous at first but when ignored, they can pose harm to the structure over time.

Cracks Classification According to Their Nature

• Moving cracks increase in length, width, and/or depth and are the most dangerous, requiring expertise to repair.
• Constant cracks remain stable if the cause is removed, are less dangerous, and are easier to repair.
• Fresh concrete cracks often appear as hairline cracks on the concrete surface during the casting process, shortly after finishing.
• Hardened concrete cracks occur due to drying shrinkage, chemical factors, oxidation, sulfate attack, freezing, and differential settlement effects.

Cracks Diagnosis and Monitoring

• To determine the cause and best repair method of a crack, engineers should observe and study it, noting:
• Crack place and direction
• Crack path.
• Crack length, width, and depth
• If the crack is clear
• Seasonal extension of the crack over time
• Crack propagation degree and pattern (arbitrarily or systematically)
• Distance between two cracks

Causes of Nonstructural Cracks in Fresh Concrete

• Cracks due to construction movement can occur when the soil beneath the concrete subsides during the strengthening process, or from form movement due to design issues or concrete compaction.
• Cracks appear within hours or days after casting.
• Cracks due to settlement shrinkage are classified into two branches.
• Cracks around steel bars occur when concrete partially sets, causing water and volume reduction.
• Cracks weaken the bond between concrete and steel bars, which often occur in deep beams shortly after casting.
• Cracks around the aggregate and sedimentation and the mechanism is similar to steel bars
• Cracks due to settling shrinkage.
• Plastic shrinkage cracks occur when evaporation from the concrete surface happens faster than bleeding, leading to tensile stresses and small cracks within hours.
• Drying shrinkage cracks are due to water evaporation and concrete contraction, a process lasting up to two years.
• The structural part is restrained by surrounding parts and steel bars.
• Tensile stresses lead to surface cracks that may penetrate deeper over time, appearing from hours to weeks after.

Causes of Nonstructural Cracks in Hardened Concrete

• Late drying shrinkage cracks are similar to drying shrinkage but appear deeper after weeks or months.
• Cracks due to chemical reactions, like carbonation (reaction with carbon dioxide), result in thin cracks distributed irregularly on the concrete surface.
• Aggregate alkali reaction results from the reaction between active silica and cement alkalis, leading to internal stresses and cracks, similar in appearance to carbonation cracks.
• Oxidation cracks (corrosion) are caused by volume increase due to corrosion, resulting in longitudinal cracks parallel to steel reinforcement and appearing over months or years.
• Salt attack cracks are caused by sulfate from external soil that reacts with aqueous aluminate in concrete which causes swelling.
• Can also be caused by crystalization of salts in the pores between cement and aggregate, weakening the steel reinforcement.
• Can also be caused from internal attack of salts existed in the concrete during setting or in the succeeding stages.

Thermal Cracks

• Cracks due to cement hydration: Cement hydration is an exothermal reaction which emits heat, causes stresses during setting and solidification periods.
• The differentiation in temperature between concrete surface and inner concrete layers is the cracks cause. This differentiation increases when ambient temperature increases during casting days.
• These are self-closed when the temperature inside concrete equals concrete surface temperature.
• Cracks due to temperature fluctuation between day and night.
• In night time, concrete surface is colder than inside concrete, concrete surface shrinks more than the concrete beneath layers.
• These lead to stresses that try to bend up concrete mass, concrete weight resists these stresses.
• This causes tensile stresses in upper concrete surface.
• Freezing cracks; when water inside concrete freezes before initial setting, no setting will be.
• Can also happen after initial setting, the result is ice crystals and empty voids of equal volume.
• When water fills these voids and freezes, further volume increment takes place leading to tensile stresses and then concrete deterioration within time.
• Cracks due to differential consolidation: the large temperature difference from zone to another in a same section causes thermal stresses.

Causes of Structural Cracks

• Design Mistakes
• Reinforcement quantity calculations.
• Load calculations.
• Soil bearing capacity (no laboratory results).
• Overload the structure
• Structures used not for the purpose for which it is designed for
• Constructional mistakes
• Poor-quality control (QA).
• Wrong concrete mix proportions.
• Poor compaction
• Bad form work
• Cracks due to wrong use
• What the structure is designed for is changed (using a residential building as a warehouse)
• Making change in the the building without asking the designer (making openings for ventilation)
• Cracks from accidents
• Fire
• Earthquake
• Impact
• Dangerous ignorance to repair the non-structural cracks
• Deterioration due to the accumulation of steel reinforcement corrosion.

Structural Cracks Types

• Inclined shear cracks in beams and slabs.
• Horizontal structural elements deflection due to additional stresses associated with cracks perpendicular on the main reinforcement.
• Cracks parallel to the main reinforcement in columns due to extra loads.

Effect of Cracks on Structures

• Visual investigation in case of the crack is wide.
• Nondestructive tests.
• Core test.
• Load test. The structure is loaded by designed load and observing whether the cracks width is increased or structural failure takes place.