What Causes Concrete Shrinkage ? . Shrinkage is caused by decrease in either concrete length or volume resulting from changes in moisture content or chemical changes. The need to facilitate placement and consolidation of concrete often results in the decision to use a greater amount of mixing water than is needed for the hydration process. The used water content creates movement and generates tensile stresses that pull the cement paste closer causing shrinkage. Concrete shrinkage can lead to cracking, curling of slabs, and dimensional instability of concrete members.
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What is Shrinkage?
Shrinkage measures the change in concrete dimensions and volume after casting. The change is measured by noting the difference in the dimensions between two points in time.
What Causes Shrinkage?
Shrinkage primarily takes place because of the porous inter-molecular space that’s present in concrete, grout and mortar. Thus, when these materials are mixed with water and an exothermic reaction takes place, they expand. Similarly, when they begin to dry and cool, they contract or shrink.
Shrinkage of cement products can be generally classified into four categories:
| TYPE OF SHRINKAGE | CAUSE | TIME OF CRACKS APPEARING |
| Plastic shrinkage | Moisture loss prior to set | First 24 hours |
| Drying shrinkage | Evaporation after set | 1 to 56 days |
| Thermal shrinkage | Cooling after set | 1 to 7 days |
| Autogeneous | Surface tension capillaries | Typically negligible |
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The Role of Water in Concrete Shrinkage
Water plays a dual role in concrete shrinkage. While necessary for hydration and workability, excess water contributes to drying shrinkage. Striking a balance in the water-cement ratio is vital for minimizing volume changes.
Why Measure Shrinkage?
Measuring shrinkage allows us to understand how a particular cement product will perform onsite under different conditions. Determining the temperature, available moisture, humidity and restraints are all important factors that we can understand before placing large quantities of material.
The Mechanisms For Dealing With Shrinkage
Our mechanisms for preventing cracks from occurring are:
- Reduce shrinkage (more on this in the next blog)
- Compensate against shrinkage
- Release movement in the structure to allow for shrinkage.
Decisions around all of these options relies upon understanding the magnitude of shrinkage we’re expecting.
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Most importantly, the magnitude of that shrinkage relevant to the actual environment, not a lab. In addition, we must measure shrinkage at various times ie. Not just in the drying phase. Therefore, selecting the right type of test is critical.
Why Does Concrete Shrinkage Cause Cracks?
There are many reasons that may cause concrete to crack:
- On a very hot and windy day, the surface of the concrete may lose moisture faster than the concrete can tolerate and cause surface shrinkage, known as plastic shrinkage cracking. This can occur throughout the initial setting to final setting phases of the concrete-curing process. The cracking is only on the surface and is not full depth.
- As the concrete goes past the final setting time (up to 7 days) the volume change of the concrete occurs as the excess water not used in hydration leaves the concrete. This type of cracking is typically full depth and can be controlled by placing joints within 8 foot to 12-foot square layouts to force this cracking to occur with aesthetics instead of random placement.
- When the top of the concrete is drying faster than the bottom of the concrete, this can cause what is known as thermal differential cracking. This type of cracking is very distinctive as the cracking appears to tear with wide gaps and has a very jagged edge in appearance.
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Different types of concrete shrinkage and their preventions
Plastic shrinkage
Plastic shrinkage develops on the surface of the fresh concrete. It is a consequence of water evaporation from the surface of fresh concrete or absorption in the concrete form or bedding. It is a contraction in volume due to water movement from the concrete while still in the plastic state, or before it sets. This movement of water can be during the hydration process or from the environmental conditions leading to evaporation of water that resides on the surface on the wet concrete. So, the more the concrete bleeds, the greater the plastic shrinkage should be. It is proportional to cement content and, therefore, inversely proportional to the w/c ratio [weights of water and cement used in the concrete mix.
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Prevention of plastic shrinkage
- Preventing the rapid loss of water from the surface.
- The surface covering with the help of polyethylene sheeting
- Proper vibration of the concrete
- Use of expansive cement or shrinkage compensating cement
- Use of aluminum powder
- Reducing the temperature of a concrete mixture
- The use of micro-synthetic fibers
- The use of an evaporation reducer
Drying shrinkage
Most of the building materials having pores in their structure in the form of intermolecular space, expand on absorbing moisture from the atmosphere and shrink on drying. These movements are reversible, causing cracks in buildings in the form of Drying shrinkage. Drying shrinkage occurs when water starts evaporating from the exposed surface and the moisture differential along the depth of the slab causes strain which induces tensile stresses.
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Prevention of drying shrinkage
- Using a concrete with a lower water content
- Selection quality ingredients for the concrete mix
- Maintaining the water /cement ratio of the concrete mix
- Reduction of cement paste volume
- Good curing
- Providing movement joints and sufficient crack control reinforcement.
Autogenous shrinkage
Autogenous shrinkage occurs after the initial setting of concrete due to hydration, since this process requires water and therefore reduces the internal free water. It is caused because of a self-desiccation process in the pores of cement stone, it is the use of water in the cement hydration processes. The process of self-desiccation begins in these pores because of the process of hydration i.e. inside the cement stone structure all water is used for hydration and humidity cannot reach fast enough from the outside due to density structure.
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Prevention of autogenous shrinkage
- Leveling the cement content in the mix
- The use of low-heat cement
- Use of shrinkage-reducing admixture
- Application steel fibers and polypropylene fibers
- Addition of pre-soaked lightweight aggregate
Carbonation shrinkage
Carbonation shrinkage occurs in hardened concrete. CO2 (carbon dioxide) from the environment on cement stone causes chemical reactions. This leads to the formation of calcium carbonate and water and, consequently, a reduction in volume. It is observed in areas that have intermediate humidity conditions.
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Prevention of carbonation shrinkage
- Suitable joints can be provided in the structure during its casting
- Providing regular contraction joints in the concrete
- Use of higher the modulus of the aggregate
- Use of lower cement content
Chemical and Thermal shrinkage
Other than four pivotal shrinkage types, two other types of shrinkage occur viz. chemical and thermal shrinkage.
Chemical shrinkage occurs due to a decrease of volume cement paste occurring due to the chemical binding of water in the process of cement hydration. Thermal shrinkage is caused by the contraction of the movement of concrete. Seasonal variation of temperature could lead to cracking of concrete that can cause as a result of the thermal contraction in the concrete. These types of shrinkages can be prevented by controlled by the use of shrinkage compensating admixture and reducing agents.
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FAQs
1. What is the primary cause of concrete shrinkage?
Excessive water loss from the concrete mix, whether due to evaporation or chemical reactions, is the primary cause of shrinkage.
2. How does curing help reduce shrinkage?
Curing maintains moisture levels in the concrete, ensuring gradual drying and preventing rapid volume changes.
3. Can admixtures completely eliminate shrinkage?
While admixtures significantly reduce shrinkage, complete elimination is not feasible. They should be combined with other preventive measures.
4. Does aggregate quality affect shrinkage?
Yes, high-quality, well-graded aggregates reduce the total cement paste volume, minimizing shrinkage.
