This article is about Types of Cracks in Fresh and Hardened Concrete, their Causes and Control. Concrete is a widely used construction material, and cracks are a common issue associated with it. There are different types of cracks in concrete structures, including plastic shrinkage cracks, settlement cracks, and various types of cracks in fresh and hardened concrete.
Plastic shrinkage cracks occur when the surface of freshly poured concrete dries too quickly, which causes it to shrink and crack. Settlement cracks, on the other hand, happen when the ground underneath the concrete shifts or settles unevenly, causing the concrete to crack.
Types of Cracks in Fresh and Hardened Concrete
Other types of cracks that can occur in concrete include thermal cracking, flexural cracking, and crazing. Thermal cracking is caused by temperature changes, while flexural cracking occurs when the concrete is subjected to bending stresses. Crazing, on the other hand, refers to a network of fine surface cracks that don’t penetrate deep into the concrete.
To prevent and control cracks in concrete structures, several measures can be taken. These include proper curing of the concrete, the use of reinforcement, and the addition of admixtures such as shrinkage-reducing agents or fibers. Additionally, proper design and construction practices can help prevent cracking, such as avoiding excessive water content, ensuring proper compaction, and providing adequate joint spacing.
Fresh and Hardened Concrete Cracks Types
Cracking in Fresh or Plastic Concrete
1. Plastic shrinkage Cracks
Plastic shrinkage cracking is a type of cracking that can occur in freshly placed concrete. This happens when the concrete experiences a rapid loss of moisture due to several factors, such as air and concrete temperatures, relative humidity, and wind velocity at the surface of the concrete. These factors can combine to cause high rates of surface evaporation, which can lead to the development of cracks in the plastic concrete.
The main cause of plastic shrinkage cracking is the differential volume change between the surface and other parts of the concrete. When moisture evaporates from the surface faster than it is replaced by bleed water, the surface concrete shrinks, which can cause cracking. Therefore, effective control measures for plastic shrinkage cracking involve reducing the relative volume change between the surface and other portions of the concrete.
There are several measures that can be taken to control plastic shrinkage cracking in freshly placed concrete. One approach is to use fog nozzles to saturate the air above the surface, which can help to reduce the rate of evaporation. Another approach is to use plastic sheeting to cover the surface between finishing operations. By covering the surface, the rate of moisture loss can be reduced, which can help to prevent the development of cracks. Overall, successful control measures for plastic shrinkage cracking require careful consideration of the factors that contribute to rapid moisture loss and the use of appropriate techniques to reduce the relative volume change between the surface and other parts of the concrete.
2. Settlement Cracks in Concrete
Concrete is a commonly used material in construction due to its durability and strength. However, even after the initial placement, vibration, and finishing, concrete continues to undergo a process of consolidation. This process can be affected by the presence of reinforcing steel, previously placed concrete, or formwork, which may locally restrain the plastic concrete.
Unfortunately, this local restraint can result in the formation of voids and cracks in the concrete adjacent to the restraining element. Settlement cracking, in particular, is more likely to occur when the reinforcing steel used is larger in size, the slump of the concrete is high, and the cover provided over the reinforcing steel is insufficient.
Studies have shown that by reducing the slump of the concrete and increasing the cover provided over the reinforcing steel, settlement cracking can be minimized. Therefore, it is recommended to use the lowest possible slump and provide adequate concrete cover over the reinforcing steel to reduce the likelihood of settlement cracking. By taking these measures, the durability and strength of the concrete can be maintained over time.
Cracks in Hardened Concrete
1. Drying Shrinkage Cracks
Concrete is a popular construction material, but one of its common issues is cracking, which can occur due to restrained drying shrinkage. This shrinkage happens when the cement paste loses moisture, leading to a shrinkage of up to 1 percent. However, the presence of aggregate in the concrete can provide internal restraint, reducing the shrinkage magnitude to about 0.06 percent. Nevertheless, when concrete is wet, it tends to expand, which is a natural characteristic of this material. If the shrinkage of concrete could take place without any restraint, cracking would not happen.
One of the key factors affecting the amount of drying shrinkage in concrete is the water content. Higher water content typically results in more significant shrinkage. To mitigate this issue, it is possible to increase the amount of aggregate in the concrete and decrease the water content. By doing so, it is possible to reduce the amount of drying shrinkage and minimize the likelihood of cracking.
In summary, restrained drying shrinkage is a common issue that can cause cracking in concrete. By understanding the factors that affect shrinkage, such as the water content and the amount of aggregate, it is possible to take steps to reduce the magnitude of shrinkage and prevent cracks from forming.
2. Cracks due to Thermal Stresses
Concrete structures can experience temperature differences due to various factors such as heat of hydration loss and weather conditions. These temperature differences can lead to changes in volume that create tensile stresses. When these stresses exceed the tensile stress capacity of the concrete, it can lead to cracking.
Mass concrete is particularly vulnerable to cracking caused by temperature differences, especially when the interior of the structure experiences higher temperatures than the exterior. To prevent such cracking, certain measures can be taken. These include reducing the maximum internal temperature of the structure, delaying the onset of cooling, controlling the rate at which the concrete cools, and increasing the tensile strength of the concrete.
By implementing these measures, the risk of thermally-induced cracking in concrete structures can be significantly reduced. It is important to address this issue, as cracks in concrete can compromise the structural integrity of the building and lead to costly repairs or even complete failure. Therefore, it is crucial to take preventive measures during the construction process to ensure the longevity and safety of concrete structures.
3. Cracks due to Chemical Reaction
Cracking of concrete can occur due to deleterious chemical reactions, which may be caused by the materials used in making the concrete or materials that come into contact with it after it has hardened. Although some general concepts for reducing these adverse chemical reactions are available, the effectiveness of specific measures can only be determined through pretesting of the mixture or extended field experience.
Over time, concrete may crack due to slowly developing expansive reactions between aggregate containing active silica and alkalies derived from cement hydration, admixtures, or external sources such as curing water, ground water, and alkaline solutions stored or used in the finished structure. The alkali-silica reaction leads to the formation of a swelling gel, which can draw water from other portions of the concrete, causing local expansion and accompanying tensile stresses. If not addressed, this may ultimately lead to complete deterioration of the structure.
4. Cracks due to Weathering
Concrete is a widely used building material that is subjected to various weathering processes. Natural weathering can cause visible cracks in concrete, giving the impression that the structure is about to disintegrate. However, this damage may not have progressed significantly below the surface. Freezing and thawing are the most common weather-related processes that cause physical deterioration of concrete.
To protect concrete against freezing and thawing, it is best to use the lowest possible water cement ratio and total water content. Additionally, durable aggregate and adequate air entrainment can be used to enhance its durability. It is also essential to cure the concrete adequately before exposing it to freezing conditions. Drying the structure after curing will further improve its resistance to freezing and thawing.
Other weathering processes that may cause cracking in concrete are alternate wetting and drying, and heating and cooling. These processes can lead to volume changes, which can result in cracks if they are excessive. Therefore, it is important to take measures to control these processes and prevent excessive volume changes in concrete.
5. Reinforcement Corrosion
Metal corrosion is an electrochemical process that involves an oxidizing agent, moisture, and electron flow within the metal. Chemical reactions occur on and near the metal surface, leading to corrosion. The key to protecting metal from corrosion is to halt or reverse the chemical reactions. This can be achieved by preventing the supply of oxygen or moisture or by providing excess electrons at the anodes, which prevents the formation of metal ions, a process known as cathodic protection.
Reinforcing steel typically does not corrode in concrete due to the formation of a tightly adhering oxide coating in the highly alkaline environment, known as passive protection. However, this steel may corrode if the alkalinity of the concrete is reduced through carbonation or if aggressive ions, typically chlorides, destroy the steel’s passivity. Corrosion of the steel leads to the formation of iron oxides and hydroxides, which have a much larger volume than the original metallic iron.
This increase in volume causes high radial bursting stresses around reinforcing bars, leading to local radial cracks that can propagate along the bar. This results in the formation of longitudinal cracks parallel to the bar or spalling of the concrete. Delamination, a well-known issue in bridge decks, occurs when a broad crack forms at a plane of bars parallel to a concrete surface. Cracks provide easy access for oxygen, moisture, and chlorides, which can accelerate corrosion and cracking.
Transverse cracks in reinforcement do not typically cause continued corrosion if the concrete has low permeability. This is because the exposed portion of the bar at the crack acts as an anode. Although wider cracks lead to greater corrosion at early ages, continued corrosion requires the supply of oxygen and moisture to other portions of the same bar or bars that are electrically connected.
If the combination of density and cover thickness is adequate, the corrosion process is self-sealing. However, if a longitudinal crack forms parallel to the reinforcement, corrosion can continue as passivity is lost at many locations, and oxygen and moisture are readily available along the full length of the crack.
To prevent corrosion-induced splitting in general concrete construction, low permeability concrete with adequate cover is the best protection. Increasing the concrete cover over the reinforcing delays the corrosion process and resists the splitting and spalling caused by corrosion or transverse tension. In severe exposure conditions, additional protective measures may be required, such as coated reinforcement, sealers or overlays on the concrete, corrosion-inhibiting admixtures, and cathodic protection.
Any procedure that prevents access of oxygen and moisture to the steel surface or reverses the electron flow at the anode will protect the steel. It is important to note that concrete must be allowed to breathe, meaning any concrete surface treatment must allow water to evaporate from the concrete.
6. Poor Construction Practices
Cracking in concrete structures can be caused by a range of poor construction practices, with one of the most common being the addition of water to improve workability. While this may seem like a simple solution, it can actually reduce the strength of the concrete, increase settlement and drying shrinkage, and lead to increased thermal stresses and potential cracking.
Even if the water-cement ratio remains constant, adding more cement to counteract the strength decrease caused by added water can still increase shrinkage, as the relative paste volume is increased. Furthermore, failing to properly cure the concrete can exacerbate the problem, as it allows for increased shrinkage when the concrete has low strength. This lack of hydration can lead to reduced long-term strength and durability of the structure.
In addition to water-related issues, other construction problems can also contribute to cracking. For example, inadequate formwork supports or insufficient consolidation can result in settlement and cracking before the concrete has had a chance to develop sufficient strength to support its own weight. Similarly, improperly located construction joints can result in joints opening at points of high stress, further contributing to cracking.
While there are known methods to prevent cracking caused by poor construction practices, such as those outlined in various ACI publications, it is crucial that these procedures are executed correctly during construction. Without proper attention and execution, even seemingly minor issues can lead to significant problems down the line.
Read Also: Grouting Procedure for Repair of Cracks in Concrete Structures
7. Construction Overloads
During construction, loads can be more severe than during normal use, and these conditions can occur when the concrete is most vulnerable, resulting in permanent cracks. Precast members are especially prone to damage if not properly supported during transport and erection. Lifting accessories should be approved by the designer, and caution must be exercised when operating lifting devices, as even with proper accessories, damage can still occur. The designer must also be aware of loads induced during transportation and consider support during shipment.
Pretensioned beams require special attention during stress release, as multiple strands must be detensioned in a specific pattern to avoid eccentric loads on the member, which can cause cracking. Similarly, tack welding embedded bearing plates to the casting bed to hold them in place during concrete placement can result in cracks near beam ends if the welds are too strong. Thermal shock can also cause cracking if not treated properly, and temperature restrictions should apply to the entire beam. Cast-in-place concrete can be unknowingly subjected to construction loads in cold climates, and heaters should be kept away from exterior walls to prevent unacceptably high thermal gradients.
Construction personnel must heed load limitations provided by designers to prevent unintentional construction overloads that can cause damage. Additional information on preventing and repairing cracking related to fabrication and shipment of precast or prestressed members can be found in the PCI Committee on Quality Control Performance Criteria.
8. Design and Detailing Errors
Structural design and detailing errors can have a range of negative consequences, from poor appearance to catastrophic failure. It is essential to have a thorough understanding of structural behavior to minimize these problems. Improperly designed or detailed structures may exhibit unacceptable cracking, which can be caused by several factors, such as the use of poorly detailed reentrant corners, inadequate reinforcement, restraint of members subjected to volume changes, lack of adequate contraction joints, and improper foundation design. Examples of these issues can be found in the works of Kaminetzky (1981) and Price (1982).
Reentrant corners, such as those found in window and door openings in concrete walls and dapped end beams, are susceptible to stress concentration and are, therefore, prime locations for crack initiation. High stresses can result from various factors, such as volume changes, in-plane loads, or bending, and designers must recognize that stresses are always high at reentrant corners. Additional diagonal reinforcement is required to keep inevitable cracks narrow and prevent them from propagating.
Inadequate reinforcement can also result in excessive cracking. A common mistake is to lightly reinforce a member because it is considered a “non-structural member.” However, this member may be tied to the rest of the structure in such a way that it is required to carry a significant portion of the load as the structure deforms. The “non-structural element” may then begin to carry loads in proportion to its stiffness, resulting in unsightly cracking.
Restraint of members subjected to volume changes can also cause cracking, as stresses resulting from restrained creep, temperature differential, and drying shrinkage can be significantly higher than those caused by loading. Restraining a slab, wall, or beam against shortening, even if prestressed, can easily generate tensile stresses sufficient to cause cracking. Properly designed walls should have contraction joints spaced from one to three times the wall height, while beams should be allowed to move.
Improper foundation design may result in excessive differential movement within a structure, leading to visual cracking or even catastrophic failure if the differential settlement is significant enough. Reinforced concrete’s advantage is that, given enough time, creep will allow for some load redistribution. However, proper design and detailing are critical for structures in which cracking may cause a significant serviceability issue, and continuous inspection during all construction phases is necessary.
Fig.4: Concrete Cracks at Restraint Corners
Fig.5: Crack Patterns at Dapped End Beams
9. Externally Applied Loads
Concrete design recognizes that load-induced tensile stresses can lead to cracks in concrete members. To mitigate this, current design procedures utilize reinforcing steel to not only carry the tensile forces but also ensure an adequate distribution of cracks and limit crack width. The American Concrete Institute (ACI) 318 and the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges are two commonly used standards in this regard.
Various factors influence cracking, including steel stress, cover thickness, and the area of concrete surrounding each reinforcing bar. Steel stress is the most critical variable. Surprisingly, bar diameter is not a significant consideration. The width of a bottom crack increases with a growing strain gradient between the steel and the tension face of the beam. The equation developed by Gergely and Lutz (1968) is considered the most accurate in predicting the maximum surface crack width in bending. ACI 318 uses a modified version of this equation, limiting crack widths to 0.016 in. (0.41 mm) for interior exposure and 0.013 in. (0.33 mm) for exterior exposure. However, these limits do not seem to be justified for corrosion control since there is little correlation between surface crack width and reinforcing corrosion.
There are several equations available for predicting the maximum crack width in prestressed concrete members (ACI 224R), but none have gained widespread acceptance. Tension members have a larger maximum crack width than flexural members due to the absence of a strain gradient and compression zone. An expression to estimate the maximum crack width in direct tension has been proposed (ACI 224R).
Crack width in both flexural and tensile members can increase over time, whether from sustained or repetitive loading. While there is considerable scatter in available data, it is reasonable to expect a doubling of crack width with time. To mitigate this, well-distributed reinforcing is the best option, with reduced steel stress also reducing cracking. However, designers should be aware that reducing cover to reduce surface crack width can be detrimental to reinforcing corrosion protection. Perpendicular cracks to the reinforcing steel do not have a significant effect on corrosion. In summary, while more research remains to be done, the fundamental principles of crack control for load-induced cracks are well understood.
Read Also: CRACKS TYPES IN REINFORCED CONCRETE SLABS & WALLS