Cracks are common to most of the concrete structures. Concrete cracks easily due to its low tensile strength. When a crack appears in concrete, our first assumption is that something wrong with the concrete. But this is not true for every crack that appears in the concrete structure. In fact, the American concrete institute has also stated that “even with best floor designs and proper construction, it is unrealistic to expect crack-free and curl-free floor”.
Concrete can crack due to many reasons. Some are avoidable and some are not. So it is important to investigate the severity of the crack at first and decide on treatment methods. Concrete mostly subjected to cracks due to the following major reasons
- Joint arrangement
- Construction speed and sequence
- The heat of hydration and temperature
- Change of sections
- Possible settlements, movements
- Composition of concrete
- Type, maximum size, and grading of aggregate
- Admixtures using
- Environmental temperature (cooling and heating rate)
- Amount of compaction and re-vibration
- Lift height and volume
- The time between concrete placing
- Curing condition and insulation
- Evaporation rate
- Quality of the formwork and supports
- Chemicals present in water
- Alkali aggregate effect
- Freezing and thawing
- Seismic action
The crack investigation is the first phase after the appearance of the crack. Making necessary observations and taking records is the immediate step. Soon after the appearance of the cracks, investigating the following parameters are mandatory.
- Time of cracking
- Crack depth
Concrete crack types
The best parameter to categorize the cracks is the cause of cracking. Identifying what are the reasons behind cracking is also helpful when treating.
Plastic settlement cracks
Plastic settlement cracks are categorized as very early age cracks. These cracks can appear in 15 minutes to 3 hours after placement. Reasons for plastic settlement cracks are bleeding and compaction of air pores. Plastic settlements cracks are possible in concrete members with different thicknesses, arches, and over reinforcement.
When concrete members change their thickness, different plastic settlements take place. This generates a separation between members and ultimately turns in to a plastic settlement crack.
Concretes having a high amount of reinforcement as top layer are most likely to have plastic settlement cracks. Heavy particles tend to settle between rebars during compaction time as well as setting time. This leaves weak concrete composition on top of rebars and ultimately cause a crack.
Concrete can settle down due to the low quality of the formwork material and supports. Weak formwork material may sag as a result of concrete weight itself. Note that formwork bears the highest concrete load immediately after the pour. When the concrete hardens, the lateral load on the formwork reduces. This way we can reduce the risk of formwork failure.
Plastic shrinkage cracks
Cement has a shrinking property during hardening. Cement rich concrete mixes shrink more than the leaner mixes. Shrinkage at the plastic stage of concrete can result in random cracking on the surface.
Concrete subjected to self-drying because of hydration consumes internal water. The process has a considerable impact on the concretes with w/c is below 0.45. Generally, self-desiccation cracks do not affect the structure directly. However, it can speed up deterioration and reduce the lifetime.
Crazing is known as pattern cracking or map cracking. Cracks due to crazing can appear at 1 to 7 days. This closely spaced shallow crack appears on the surface due to the rapid hardening of the top concrete layer. The reasons for these cracks are high temperature and surface drying due to wind.
Thermal cracks belong to the early age category cracks. These can appear at 1 day to a few weeks. The main reason for thermal cracks is the hydration heat of cement. Concrete increases its internal temperature during hydration and starts to cool down after a few days. During this heated time concrete undergoes thermal stresses that are large enough to crack the concrete. Usually, thermal cracks are straight and develop through the whole concrete lift.
Drying shrinkage cracks
Drying shrinkage cracks can occur due to different drying rates on different sides. One side drying, different external restraints, and differential final shrinkage make the situation severe. Cracks can appear even after a few months.
When the concrete dries it shrinks slowly. A different side of the concrete may open to different exposure conditions. This makes the concrete dries different rates through those sides. Ultimately the drying shrinkage rate is going to be different on those faces of concrete, making drying shrinkage cracks.
Cracks due to pre-stressing
These are loading cracks that appear in pre-stressed concrete elements while pre-stressing. Most cracks appear due to excessive loading rate or construction errors.
During the service life of the structure, loading cracks appear. Typical loading cracks are,
Tensile cracks: Cracks due to tensile forces, the crack propagates through the weakest section.
Flexural cracks: These cracks appears due to bending. In most cases, the mid-span of beams undergo flexural cracks. Usually, cracks propagate only half of the section
Shear cracks: Appears near high shear zones. A commonplace for these cracks is beam-column joints.
Torsional cracks: Beams having voids on one side and cantilever slabs on other sides are the most common example of torsional members. These cracks propagate around the beam
Foundation movements: Differential ground settlements can cause cracks in the structure.
Chemicals can create unfavorable conditions for concrete as well as reinforcements. Both deteriorations lead to cracks.
Cracks due to corrosion of reinforcements
These cracks appear after years of time. Reinforcements swell with the corrosion reaction and cracks appear due to this. When chlorides are available cracks appear after 1 to 2 years and carbonation can leads to corrosion in more than 5 years.
Cracks due to chemical attack
Common chemical attacks that lead to cracks are,
- Sulfate attack
- Alkali – silica reaction: silica in the aggregates reacts with the alkaline ingredients in the cement and forms a gel that expands with external water. This movement in the hardened concrete creates cracks. Normally these cracks appear more than 5 years.
- Alkali – carbonation reaction: This occurs due to impure dolomitic limestone or marble.
Cracking due to heat
Generally concrete is better against heat compared to steel. Both the aggregates and cement expand slowly with the heat and comes back to the original size when cooling. However, at higher temperatures, cement paste starts to shrink because of internal water evaporation. This will develop the micro crack around the aggregates.
|Time of cracking||Crack type||Possible reasons|
|10 minutes to 3 hours||Plastic settlement cracks||Thin cover Change of concrete depths|
|1 hour to 5 hours||Cracks due to formwork settlement||Insufficient supports Low-quality form boards|
|Mostly within the first 5 hours||Plastic shrinkage cracks||High evaporation rate|
|During hardening||Self – desiccation cracks||Low w/c|
|Generally within the first week||Cracks due to crazing||High environment temperature|
|First few weeks||Thermal cracks||High contrast between internal and external temperature|
|Within several months||Drying shrinkage cracks||Different drying rates in the concrete External restraints|
|After pre-stressing||Pre-stressing cracks||Too much pre-stressing rate|
|While loading the structure||Loading cracks||Defects in the construction Foundation movements|
|At least 1 to 5 years||Cracks due to rebar corrosion||Accelerated corrosion due to chemicals|
|More than 5 years||Cracks due to deterioration||Alkali – silica reaction Alkali – carbonation reaction Sulfate attack|