A basic building material used in many construction projects worldwide is concrete. It is perfect for constructions like roads, bridges, and buildings because of its strength and durability. However, concrete shrinkage is a frequent problem that develops during the hardening process.
When the concrete mixture shrinks due to water loss, it happens. If left unchecked, this can cause cracking and other structural issues. For any concrete structure to last a long time and remain intact, it is essential to understand why concrete shrinks.
Concrete shrinkage is caused by a number of variables, such as the type of cement used, the water-to-cement ratio, and ambient conditions. The water in the mix evaporates as the concrete dries, resulting in a volume reduction in the concrete. The curing procedures and mix design can have an impact on this organic process.
Shrinkage can be reduced and associated problems can be avoided with the right methods and supplies. Building professionals and engineers can ensure stronger and more durable constructions by taking measures to mitigate the effects of concrete shrinkage by understanding its causes and effects.
Reason | Description |
Water Evaporation | As concrete hardens, water evaporates, causing it to shrink. |
Chemical Reactions | When cement reacts with water, it forms compounds that take up less space, leading to shrinkage. |
Because of the chemical reactions that take place during the curing process and the loss of moisture from the mixture as it dries, concrete shrinks during the hardening process. Concrete loses volume as the particles get closer to one another as hydration and evaporation persist. In order to guarantee the longevity and integrity of concrete structures, it is essential to comprehend and regulate the drying process because improper management of this shrinkage can result in cracks.
- What is shrinkage
- The shrinkage coefficient is insignificant
- Types
- Deformation
- Primary and secondary stages
- Main types
- Reasons for formation
- Shrinkage during dehydration of the solution: losses during operation
- Existing types
- Shrinkage nuances in different conditions
- Conclusion
- Working seams
- Expansion
- Shrinkage
- Technology
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What is shrinkage
Concrete shrinks as a result of various factors, including compaction, moisture loss from the material, and hardening from chemical, physical, and physicochemical processes. If the pertinent data are taken into consideration during the design and construction process, the volume decrease is almost imperceptible, rarely surpassing 1%.
The service life of the structure will be greatly shortened if the foundation or monolith has poor strength characteristics because even a small amount of shrinkage will cause cracks to form, the surface to peel off, and other deformations.
The shrinkage coefficient is insignificant
Various plasticizers and additives are added to cement by modern manufacturers in an attempt to find the best possible component ratio that will allow the building to remain true to its original specifications for hundreds of years. The settling coefficient at the level of 1.5%–1.7% won’t be noticeable if the foundation is small.
Furthermore, the maximum allowable shrinkage coefficient of 3% is specified by GOST. However, it is preferable to consider this concrete indicator if the structure is large.
Types
Concrete shrinkage can occur before the solution hardens, concurrently with the mixture setting, or after the concrete has hardened, depending on the time of day. There are two possible causes of shrinkage: either hydration, or the chemical reaction between the components of the cement, caused it, or physical impact, or physicochemical reactions (like when concrete dries out and loses moisture).
Types of shrinkage that occur when concrete strengthens and hardens:
1) Plastic: This material emerges right away after pouring, persists for eight hours, and is then ignored. happens as a result of the water in the solution evaporating; the maximum indication is equal to 4 millimeters per meter. Water is continuously applied to the concrete during the whole drying process, particularly in the initial hours, to prevent any unfavorable effects.
2) Autogenous: This happens when relatively "young" concrete gains strength and hardens. Equal to one millimeter per meter, this factor is frequently ignored completely but is crucial when designing large objects because even tiny changes in geometry can result in microcracks.
GOST permits minimum values for shrinkage that occurs during the drying process because a well-prepared and laid solution exhibits strength and durability. However, after a few years, concrete shrinkage can occasionally be as little as 5 millimeters per meter. This is the reason why the concrete foundation was previously left in place for a year before construction started. These days, the issue is resolved by reinforcement and accurately determining the concrete mixture’s composition.
Deformation
By adding mineral additives to concrete, deformation and shrinkage can be minimized in modern construction. The application of these additives causes the cement crystals’ linear dimensions to grow as they harden. High water resistance and strength are given to concrete, guaranteeing the structure’s longevity.
Deformations can be eliminated, permeability can be decreased, and tensile strength can be reduced with the use of additives. Each additive’s particular type and volume are decided upon separately.
Primary and secondary stages
There are two phases to concrete shrinkage. Primary: Evaporation occurs when moisture is lost through the formwork or by absorption into the base of the road while the solution is still liquid or plastic. Secondary shrinkage occurs as the composition dries and solidifies.
By using the proper concrete care system, selecting the base, and correctly installing the formwork, the primary type of shrinkage can be minimized. Once all the required steps have been taken, reducing the primary shrinkage will be simple. Since secondary deformation is irreversible, the original parameters will not be restored even if the concrete’s humidity is changed.
It’s crucial to keep in mind that different concrete structures can experience different types of shrinkage in their individual components. As a result, drying and shrinkage will happen far more slowly after pouring the foundation of large structures if a concrete panel quickly loses moisture due to internal heating caused by the atmosphere. Internal stresses brought on by this may result in fractures.
Further deformation between the air’s carbon dioxide and lime may be seen as a result of chemical reactions occurring in the upper layers. When the cement mortar hydrates, air is released. This reaction between the elements is known as carbonation, and it also causes an increase in the surface’s overall shrinkage.
Main types
The types of shrinkage that occur in concrete are directly related to the solution’s composition and work stage. Since moisture is the primary cause of cracks, the types of shrinkage depend on the moisture’s appearance and departure points as well as how it interacts with the composition’s materials. Plastic, autogenous shrinkage can happen to young concrete or mature concrete during the drying process.
Reasons for formation
Both the evaporation of moisture from the solution and specific capillary forces acting within the cement structure itself can cause shrinkage. Water causes capillaries with a diameter of less than 200 nanometers to narrow, compacting and distorting the material.
Moisture is crucial during the shrinkage processes, in one way or another. Furthermore, it is significant as a component of the cement itself as well as an outside factor. It is important to keep in mind that shrinkage occurs most during the hardening process when there is a high percentage of aluminates in the composition. Cements of the alite type, which form calcium hydroxide and exhibit negligible shrinkage, are used to lower the indicator.
The chemical reaction between cement and water is the primary cause of a concrete monolith’s shrinkage in size. It makes sense that the shrinkage value will decrease with the amount of these substances in the composition. Therefore, the factor must be considered when working with high-strength grades.
The first three to four months are typically when all deformations are noticed; the size decreases linearly at a rate of two millimeters per meter on average. Then there are multiple slowdowns in the process. The modulus of elasticity, which is influenced by a number of variables including the kind of binder, the kind of filler, and their ratio in the solution, is largely what determines how much concrete shrinks.
Because they are porous, heavy types shrink less than light ones by up to one centimeter per meter. In this case, the rule of thumb is that the solution made using the filler elements will shrink less the smaller they are.
Shrinkage during dehydration of the solution: losses during operation
One of the primary issues with concrete shrinkage is shrinkage during the hardening process; initially, water loss initiates the process, and subsequently, physical processes and a chemical reaction in the concrete’s surface layers cause the composition to shrink.
Existing types
It is important to first discuss the contraction compression of concrete that takes place when the solution is being prepared. Hydrates that are created when cement and water interact have a lower volume than the original substance. The porosity of the concrete is impacted by this shrinkage, although there is little volume loss.
The dehydration of the solution—which can be plastic or hydraulic—is more dangerous. Because the composition loses water quickly after pouring, plastic shrinkage happens in the first three to six hours. The solution deforms less the less water there is in it. Structures that are properly reinforced also lose less volume.
Following solution solidification, settling progressively transitions into the hydraulic phase, which is slower, less violent, and less hazardous to the structure. Uneven drying and shrinking of concrete can cause internal stresses to show. It is important to keep in mind that when concrete is poured in hot weather, the lack of water will cause cracks to appear even at the initial stage and will intensify the second.
Even after the concrete has strengthened and hardened completely, settling processes still take place in it. Even though different additives are added to modern building materials, the foundation is frequently left in place for a while. Concrete can settle for up to two years, depending on the quality of the cement. In addition, carbonation can cause a 5 millimeter shrinkage under seasonal temperature fluctuations.
Considering all the aforementioned considerations, it is advisable to pour the foundation in the spring or early summer to allow the concrete to strengthen, dry, and avoid being torn by wintertime deformations.
Shrinkage nuances in different conditions
- The use of special vibration equipment allows you to immediately remove air from the still uncured solution, reducing future shrinkage. The process is carried out using special equipment or independently (tamping, punching, trampling, piercing the mixture with a bayonet).
- Many modern compositions involve the use of technologies that are designed to reduce shrinkage. And although it will not be possible to completely eliminate the phenomenon, its impact on the structure can be neutralized. Thus, foam concrete, aerated concrete give minimal shrinkage values, especially if reinforcement is performed.
- It is believed that floating compositions are less susceptible to shrinkage, but more liquid mixtures often demonstrate deterioration of other properties.
- Shrinkage increases due to the addition of special mixtures to concrete that allow it to work in freezing temperatures and change some other characteristics.
- Much depends on the surface humidity – the optimal indicator is considered to be 55-70%: if the humidity is lower, the shrinkage coefficient will increase, the material will begin to press.
- The smaller the mass of the solution, the lower the shrinkage coefficient.
Conclusion
If shrinkage is not taken into consideration, there may be major issues when the building is being operated. The larger the object, the lack of reinforcement, the addition of plasticizers, working in harsh environments, and noncompliance with solution preparation and use protocols, the more likely an object is to shrink.
Working seams
The joints that form between freshly poured concrete and concrete that has already set within a single monolith are known as working seams. Since cold joints weaken the monolith, they are typically avoided in construction and formwork is filled all at once. If this isn’t feasible, layers of formwork are applied, with the next layer being applied before the preceding layer solidifies.
If there are noticeable breaks in the concrete, adhere to these guidelines: the previous layer should only be covered with new layers once the set concrete has reached a strength of 150 kgf/cm. The old surface should also be sandblasted and cleaned of cement films using a brush or other tool. To guarantee improved adhesion, bitumen, various glues, and primer are also used.
When discussing columns, the shrinkage joint should be positioned at the level of the top of the foundation, the bottom of the purlin, and the beams. The beam joints are made a few centimeters below the floor slab’s surface during the pouring process; in standard slabs, the joints should run parallel to the smaller slab sections.
Expansion
The purpose of expansion joints is to counteract the effects of crack-causing factors such as thermal expansion. These joints split the structure into multiple distinct monoliths.
These situations call for expansion joints:
- The length of the monolith is more than 50 meters (but in harsh climates it is recommended to take 25 meters instead of 50)
- Around the perimeter of a large monolithic floor (to avoid stress and cracks due to shrinkage of the foundation and walls)
- Around columns that stand on compacted soil or backfill
Joint requirements include a minimum thickness of 6 millimeters, the ability to make round or square (45 degree) joints around columns, and the filling of cavities with sealant or insulating material to keep out moisture, dirt, and microorganisms.
Shrinkage
A shrinkage joint is used to make up for the uneven drying of the screed following pouring. Concrete with thick layers typically dries unevenly, with more shrinkage occurring on top and less on the inside, which can distort the base. However, forcing the surface to be divided into sections will prevent cracks from forming.
Fundamental guidelines for implementation:
- The screed fragment inside the seams is made rectangular or square, the aspect ratio should be a maximum of 1:1.5
- Only straight lines, no bends
- Indoors, the maximum size of the fragment between the seams is 6×6 meters, outdoors – 3×3. Concrete paths are divided by a longitudinal seam if their width exceeds 3.6 meters
- L-shaped sections are divided into rectangles or squares
- The seams are made with a depth of a quarter/third of the screed thickness
Technology
The following is how expansion and shrinkage joints are made: the monolith is divided into sectors during the pouring process using slats, boards, plastic lining, or pieces of glass; the elements are then removed, and the concrete is ground with a diamond-wheel grinder after it has set.
The natural process of concrete shrinking during hardening is impacted by a number of variables. The concrete mix shrinks as a result of volume changes brought on by water loss. Anyone working with concrete needs to be aware of this phenomenon in order to guarantee the longevity and structural integrity of their creations.
Concrete shrinkage is caused by a number of variables, including the type of cement used, the water content of the mix, and external factors. When the extra water evaporates, a high water content can cause shrinkage that is more noticeable. Humidity and temperature also matter; the process is sped up by hotter, drier weather.
Taken in advance, preventive actions can reduce shrinkage. To preserve moisture in the concrete, these methods include using low-shrinkage concrete mixes, regulating the amount of water in the mix, and using the right curing procedures. These procedures can lessen the possibility of structural problems and cracks.
Builders and engineers can lessen the effects of concrete shrinkage by knowing why it happens during the hardening process. Having this knowledge is crucial to building durable, superior concrete structures. To guarantee the best outcomes for concrete projects, it is important to pay attention to mix composition, environmental factors, and curing procedures.