Shrinkage of Concrete: Types, Causes, Cracks and Control Measures
November 11, 2026
Shrinkage of concrete is a reduction in its volume or dimensions associated with moisture changes and cement hydration. Concrete also contracts when it cools, a related movement discussed in this guide. Shrinkage can begin before setting and continue after hardening. Shrinkage alone does not mean that concrete is defective. However, when its movement is restrained, tensile stresses can develop and cause cracking.
Understanding the types of shrinkage in concrete, the factors affecting it, and how cracks develop helps engineers and contractors plan the mix, curing, and jointing more effectively.
Key Takeaways
- Concrete can shrink before setting and after hardening.
- Shrinkage does not always cause cracks; restraint is a key factor.
- Plastic, drying, autogenous, chemical, and carbonation shrinkage have different mechanisms. Cooling also causes thermal contraction.
- Drying shrinkage slows with time but can continue for months or years.
- Mix proportions, moisture protection, curing, joints, and reinforcement influence shrinkage cracking.
What Is Shrinkage Of Concrete?
Shrinkage describes a reduction in concrete volume or dimensions without requiring an externally applied load. Different mechanisms cause this movement, including moisture loss, internal moisture changes during hydration, and reactions within the cement paste.
Thermal contraction is a related dimensional change caused by cooling. It is discussed alongside shrinkage because restrained cooling can also cause cracking.
Why Shrinkage Causes Cracks
Concrete can shrink without cracking if it is free to move. Restraint from the subgrade, foundations or adjoining elements creates tensile stress as concrete contracts. Cracks form when this stress exceeds the concrete’s tensile strength.
How Shrinkage Develops Over Time
Drying shrinkage develops as hardened concrete loses moisture. It is generally faster in the early months, then slows but may continue for years. The timeline depends on the mix, member size, curing and exposure conditions.
Types Of Shrinkage In Concrete
Concrete can shrink for different reasons. The type of shrinkage depends on what is happening inside the concrete and the stage at which it happens.
Plastic Shrinkage
Plastic shrinkage occurs before concrete has set, when surface moisture evaporates faster than bleed water can replace it. If the resulting contraction cannot be accommodated, cracks can develop.
Hot weather, low humidity, wind and direct sunlight increase evaporation risk. Large exposed surfaces, such as slabs, are particularly vulnerable. The cracks often appear within the first few hours, and their depth and extent can vary.
Drying Shrinkage
Drying shrinkage occurs after the concrete has hardened. It occurs as moisture slowly leaves the concrete.
Shrinkage is higher in the early stage and becomes less severe with time. However, concrete can continue to shrink for months or even years.
This can become a problem in slabs, walls, and other large concrete members. When the concrete is restrained and cannot shrink freely, cracks can develop. These cracks may appear around openings, at corners, between restraint points, or across large areas.
The amount of drying shrinkage depends on factors such as water content, cement paste, aggregate type, size of the concrete member, and humidity.
Autogenous Shrinkage
Autogenous shrinkage occurs when cement hydration consumes water within the concrete, causing self-desiccation and a reduction in internal relative humidity.
It is different from drying shrinkage because water does not have to leave the concrete surface for autogenous shrinkage to happen. As cement hydration continues, the amount of available water inside the concrete reduces. This can lower the internal humidity and cause the concrete to shrink.
It is more common in concrete with a low water cement ratio and in high-strength concrete because these mixes have less free water.
Thermal Contraction
Concrete becomes hot when cement reacts with water. As the temperature rises, the concrete expands. Later, when it cools down, the concrete contracts. If the concrete cannot move freely, this contraction can create tensile stress and lead to cracks.
Restrained thermal contraction can cause cracking in rafts, thick walls, large foundations, and other mass concrete works.
Carbonation Shrinkage
Carbonation shrinkage happens when the cement paste reacts with carbon dioxide from the air.
It is a slow process and mainly affects the surface exposed to air. It is generally more related to long-term durability and surface behaviour than cracks that occur soon after placing concrete.
Moisture also affects carbonation. Therefore, the amount of carbonation shrinkage can vary depending on the exposure and surrounding conditions.
Chemical Shrinkage
Chemical shrinkage happens during cement hydration. When cement reacts with water, the products formed take up less absolute volume than the original cement and water.
Chemical shrinkage refers to the reduction in internal volume caused by the hydration reaction. Autogenous shrinkage refers to the contraction seen in sealed concrete as hydration continues and the concrete undergoes self-desiccation.
| Type | When does it happen? | Why does it happen? | How can it be managed? |
|---|---|---|---|
| Plastic shrinkage | Before setting | Surface water evaporates too quickly | Reduce evaporation and begin curing promptly |
| Drying shrinkage | After hardening | Concrete loses moisture | Control mix water and paste content; use suitable curing and joints |
| Autogenous shrinkage | During hydration | Hydration uses up internal moisture | Evaluate the mix and consider internal curing where suitable |
| Chemical shrinkage | During hydration | The reaction products occupy less volume than the original materials | Account for this natural reaction during mix development |
| Carbonation shrinkage | During exposure to carbon dioxide | Carbon dioxide reacts with cement paste | Manage concrete quality, curing and exposure |
| Thermal contraction | During cooling | Concrete contracts as its temperature falls | Control temperatures and assess restrictions on movement |
Shrinkage Strain: How Much Does Concrete Actually Shrink?
Concrete shrinkage is normally expressed as strain. It means the change in length compared with the original length of the concrete member. Shrinkage strain is commonly expressed in microstrain. The practical dimensional movement can be calculated using:
Shrinkage movement = Shrinkage strain × Original member length
IS 456:2000, Clauses 6.2.4 and 6.2.4.1, state that in the absence of test data, the total shrinkage strain for design may be taken as 0.0003.
For a 10-metre member:
Theoretical free shortening = 10,000 mm × 0.0003 = 3 mm
This is shortening, not a predicted crack width.
The actual shrinkage can be different from the design assumption. It depends on factors such as water content, aggregate type and quantity, paste content, member thickness, ambient humidity, and curing conditions.
Therefore, code values should be treated as design assumptions and not as an exact prediction of how much ncob concrete mix will shrink.
Factors Affecting Shrinkage of Concrete
How much concrete shrinks depends on the mix, site conditions, and curing. The size and shape of the concrete section also affect how quickly it loses moisture.
1. Water Content
For otherwise comparable mixes, higher mixing-water content generally increases drying shrinkage.
Adding extra water at site may make concrete easier to place, but it can also increase shrinkage and the risk of cracks. The mix should provide the required workability without needing extra water during placement.
2. Aggregate Content and Stiffness
Aggregates help hold back the shrinkage of cement paste. Their quantity and stiffness affect how much the concrete can shrink.
A mix with more aggregate and less paste generally has lower shrinkage. However, the properties of the aggregate used also make a difference.
3. Cement Content and Paste Volume
Most drying shrinkage occurs in the cement paste. A higher paste volume can therefore increase shrinkage, depending on the materials and proportions used. However, cementitious content should not simply be reduced. The mix must still meet the specified strength, durability and workability requirements.
4. Member Size and Shape
The exposed surface area relative to the concrete volume affects how quickly moisture is lost. Thin sections generally dry faster than thick sections. Uneven drying through a slab can also contribute to curling.
5. Humidity and Exposure
Low surrounding humidity encourages drying. Hot, dry and windy conditions can also accelerate surface moisture loss, particularly while concrete is fresh. Exposure conditions therefore influence the rate of drying and the risk of early cracking.
6. Curing Conditions
Timely, consistent curing limits premature moisture loss and supports strength development. It helps manage early cracking risk but does not permanently prevent drying shrinkage after curing ends.
Creep and Shrinkage of Concrete: What's the Difference?
Shrinkage is a volume or dimensional change that mainly occurs due to moisture-related and hydration-related mechanisms. It can happen even when concrete is not carrying an applied load.
Creep is the gradual increase in deformation when concrete remains under a sustained load.
Both are time-dependent behaviours and can affect the long-term movement of concrete structures. Because of this, engineers consider creep and shrinkage of concrete together when calculating deflection, movement and prestress losses.
| Parameter | Shrinkage | Creep |
|---|---|---|
| Definition | Time-dependent volume or dimensional reduction | Time-dependent deformation under sustained stress |
| Requires an applied load? | No | Yes |
| Primary mechanism | Moisture change, hydration and related volume changes | Sustained stress acting over time |
| Reversibility | Some moisture-related movement may be partly reversible on rewetting | Some deformation may recover after unloading, while some may remain |
| Main design consequence | Cracking, dimensional movement and prestress loss | Increased deflection, deformation and prestress loss |
| Primary control | Mix design, curing, moisture management and joint detailing | Material design, loading control and structural design allowances |
Loss of Prestress Due to Shrinkage
As concrete shrinks, its shortening can reduce the stress in prestressing tendons and contribute to prestress loss. Designers account for this effect alongside creep and tendon relaxation when estimating the prestressing force available over time.
Shrinkage Cracks: Where They Appear and What They Mean
Concrete shrinkage cracks usually develop where the movement of concrete is restrained.
Shrinkage Cracks in Concrete Slabs
Shrinkage cracks in concrete slabs can occur because of restraint from the subgrade, walls, columns, and adjoining structural elements.
As the slab contracts, tensile stresses develop between the restraint points. Cracks can commonly appear near openings, re-entrant corners, and other changes in the slab geometry.
Different moisture loss between the top and bottom portions of a slab can also cause curling, where the edges or corners of the slab move vertically.
Shrinkage Cracks in Concrete Walls
Shrinkage cracks in concrete walls can develop because walls are often restrained by foundations, base slabs, or earlier concrete pours.
When a long wall contracts, the restraint at its base prevents the wall from shrinking freely. This can result in approximately vertical cracks at intervals along the wall.
The crack pattern and width depend on factors such as wall dimensions, reinforcement, concrete properties, environmental exposure, and the amount of restraint.
Shrinkage Cracks in Concrete Pavements
Shrinkage cracks in concrete pavement can occur because the pavement is restrained by the sub-base and friction with the supporting layers.
Contraction joints are provided so that shrinkage movement takes place at planned locations instead of causing random cracks across the pavement.
The purpose of the joint is not to stop the concrete from contracting. It provides a planned location where the movement can occur in a more controlled manner.
Shrinkage Cracks vs Structural Cracks
Shrinkage cracks are generally related to restrained concrete movement caused by moisture loss, temperature changes, or other volume changes. Their width and pattern depend on factors such as restraint, reinforcement, concrete properties, and exposure conditions.
Structural cracks can be associated with loads, structural movement, foundation movement, or other causes. Their significance depends on the crack location, pattern, width, movement, and structural behaviour.
If a crack is progressively widening, shows displacement, or is associated with visible structural movement, then it must be checked by a qualified structural professional.
Effects of Shrinkage on Concrete Structures
The main effects of shrinkage are cracking and dimensional movement, which can affect serviceability and durability.
Common consequences include:
- Cracking and water ingress: Shrinkage cracks can allow water and aggressive substances to enter the concrete and may affect reinforcement durability.
- Slab curling: Uneven moisture loss between the top and bottom of a slab can cause curling of edges and corners.
- Internal tensile stress: Restrained shrinkage can generate tensile stress and contribute to cracking.
- Prestress loss: Concrete shortening due to shrinkage contributes to loss of prestress in prestressed concrete members.
- Surface cracking: Fine shrinkage cracks can affect the appearance and serviceability of concrete surfaces.
How to Control and Prevent Shrinkage Cracks?
Shrinkage cannot be completely eliminated, but the risk of excessive movement and cracking can be reduced through proper control at every stage of construction.
Mix Design Stage
Proportion the mix to achieve the required strength, durability and workability without unnecessary water or paste. Suitable water-reducing admixtures can help achieve workability while controlling water demand.
Select appropriate aggregate types, quantities and grading. Aggregates generally restrain cement-paste shrinkage, but their own dimensional behaviour and stiffness influence the overall shrinkage of concrete.
Where dimensional movement is particularly important, specialist measures such as shrinkage-reducing admixtures or shrinkage-compensating systems require technical evaluation and trial mixes.
Placing Stage
Protect fresh concrete from conditions that cause rapid evaporation.
Windbreaks, temporary screens and suitable planning of placement times can reduce exposure to hot, dry or windy conditions.
Dry subgrades and absorbent formwork should be prepared appropriately so that they do not draw excessive moisture from fresh concrete.
Where evaporation risk is high, approved evaporation-control measures, fogging techniques or suitable fibres may be considered as part of the concreting plan.
Curing Stage
Curing should begin as soon as the finishing process and concrete condition permit.
The surface should be protected from premature moisture loss and maintained under the required curing regime.
IS 456:2000, Clause 13.5.1, specifies minimum moist-curing periods of 7 days for OPC and 10 days for blended cement or mineral-admixture concrete, with 14 days recommended for the latter. In hot, dry conditions, the minimum periods increase to 10 days for OPC and 14 days for blended cement or mineral-admixture concrete.
The concrete surface should be kept continuously damp using suitable methods such as ponding or wet coverings such as hessian, sacking or canvas, as applicable.
Curing does not permanently eliminate drying shrinkage. Its important role is to control early moisture loss and allow the concrete to develop sufficient strength before significant drying-related stresses develop.
Detailing Stage
Contraction joints create planned locations for cracking in suitable slab applications. Isolation joints allow movement between adjoining elements. Construction joints mark the meeting of separate pours and do not automatically function as movement joints; their detailing must follow the structural design.
Shrinkage strips may be considered in appropriate long structural layouts where staged closure can reduce restraint.
Distribution reinforcement helps control crack width by distributing shrinkage movement over a larger number of fine cracks. It does not stop concrete from shrinking.
Shrinkage-Control Checklist
| Stage | Control Measure | Purpose |
|---|---|---|
| Mix design | Avoid unnecessary water addition | Reduces evaporable water and shrinkage potential |
| Mix design | Optimise aggregate quantity and grading | Restrains shrinking cement paste |
| Mix design | Avoid excessive paste content | Reduces the volume of the component most prone to shrinkage |
| Placing | Protect concrete from sun and strong wind | Reduces rapid surface evaporation |
| Placing | Prepare dry subgrade and formwork appropriately | Prevents excessive moisture absorption from fresh concrete |
| Curing | Begin curing promptly and maintain it consistently | Limits premature moisture loss and supports strength development |
| Detailing | Provide properly designed joints | Directs cracking to planned locations |
| Detailing | Provide appropriate crack-control reinforcement | Controls crack distribution and width |
How Shrinkage of Concrete Is Tested?
Concrete shrinkage testing measures changes in the length of a concrete specimen over time.
IS 1199:1959, Clause 10, covers initial drying shrinkage, drying shrinkage and moisture movement. The maximum nominal aggregate size is 38 mm, with a typical 75 × 75 × 300 mm specimen.
ASTM C157/C157M, used with ASTM C490, measures length changes in hardened mortar and concrete under controlled conditions. A 25 × 25 × 285 mm prism is used for mixtures with aggregate smaller than 25 mm, with larger specimens used for larger aggregate.
Free-shrinkage tests measure contraction when concrete can shrink without significant restraint. ASTM C1581/C1581M uses restrained-shrinkage testing to assess cracking tendency, age at cracking and induced tensile stress, and covers mixtures with nominal maximum aggregate size up to 13 mm.
AASHTO T 334-08 is another restrained-shrinkage method and can accommodate nominal maximum aggregate sizes up to 19 mm.
These tests help engineers compare mixes and assess shrinkage-related cracking.
Controlling Shrinkage Starts at the Batching Plant
Several factors influencing shrinkage are established during mix design and batching. Site water addition, placing conditions, curing and structural restraint also affect the eventual cracking risk.
Water content, aggregate proportion and grading, cementitious material content, admixture dosage, and consistency between batches are all controlled through mix design and production.
Unplanned water addition at site can change the intended mix proportions and increase the quantity of evaporable water. This can increase the risk of shrinkage, segregation, and other performance problems.
Aparna RMC supplies ready mixed concrete using computerised batching of mix constituents and planned mix proportions. For slabs, rafts, pavements, or water-retaining structures where dimensional movement is an important design consideration, speak with the Aparna RMC team about the mix and placement requirements for the project.
Designing for Shrinkage, Not Against It
Concrete shrinkage cannot be eliminated completely, but it can be managed. A properly proportioned mix with controlled water content reduces unnecessary shrinkage potential. Good curing allows the concrete to gain strength before substantial drying occurs, while suitable joints and reinforcement control where cracking develops and how wide those cracks become.
Frequently Asked Questions On Shrinkage Of Concrete
Is shrinkage cracking a defect in concrete?
Not necessarily. Shrinkage is a normal material behaviour, but whether resulting cracks are acceptable depends on their cause, extent, effect on performance and the project requirements.
How wide can a shrinkage crack be before it becomes a concern?
IS 456:2000, Clause 35.3.2, specifies 0.3 mm for mild, 0.2 mm for moderate, and 0.1 mm for severe or more aggressive exposure. These are design limits, not a stand-alone assessment of existing cracks.
Can Shrinkage Cracks Be Repaired?
Yes, but the repair depends on the cause, crack movement and required performance. Stable cracks may be suitable for sealing or filling. Moving cracks need a repair that accommodates movement. Establish the cause before selecting the repair method.
When Do Shrinkage Cracks Appear?
Plastic shrinkage cracks can appear within a few hours after placing concrete. Drying shrinkage cracks can develop after hardening as the concrete loses moisture, and they may continue to develop over an extended period.
Does Concrete Shrinkage Ever Stop?
Drying shrinkage becomes slower with time. But it may continue for months or years. Concrete can also take in moisture and expand again. It may not come back fully to its original size.
Does Reinforcement Prevent Shrinkage Cracks?
No, reinforcement cannot stop concrete from shrinking. It helps keep the cracks smaller by spreading them over the concrete. So, reinforcement controls crack width rather than stopping shrinkage cracks completely.
What Is the Difference Between Shrinkage Cracks and Structural Cracks?
Shrinkage cracks result from restrained contraction. Cracks associated with loading, foundation movement or other structural actions can have different causes. However, appearance or width alone cannot reliably establish the cause or significance of a crack.
Does Shrinkage Reduce the Strength of Concrete?
Shrinkage is primarily a dimensional change, rather than a direct measure of compressive-strength loss. However, associated cracking can affect durability and performance. Uneven shrinkage through a slab can cause curling; curling is an effect, not a cause, of shrinkage.