Creep of concrete is the gradual increase in deformation that occurs when concrete remains under a sustained load for an extended period. Unlike immediate elastic deformation, creep develops progressively with time and can influence beams, columns, slabs and prestressed concrete structures. The extent of creep depends on factors such as concrete age at loading, moisture conditions, mix characteristics, member size and the duration and magnitude of sustained stress. Understanding concrete creep is important because excessive long-term deformation can affect serviceability, finishes and structural performance. This guide explains how creep occurs, its types, causes, influencing factors, effects and ways it is considered in structural design.
Quick Answer
Creep of concrete is the time-dependent deformation of concrete under a sustained load. It continues gradually after the initial elastic deformation and is affected by factors such as moisture, concrete age, strength, member size and duration of loading. Creep is especially important in long-span structures, columns and prestressed concrete because it can increase deflection and reduce prestress over time.
Creep of Concrete: Causes, Types, Effects and Factors
Concrete is generally considered a strong and durable construction material, but its behaviour changes over time.
When a concrete member is subjected to a sustained load, it initially undergoes an immediate deformation. If the load remains for a long period, the member can continue to deform gradually.
This time-dependent deformation is known as creep.
Creep is a normal characteristic of concrete rather than automatically indicating poor-quality construction. However, it needs to be considered during structural design because excessive creep can affect the long-term behaviour of a structure.
What Is Creep of Concrete?
Creep of concrete is the gradual, time-dependent increase in strain that occurs when concrete is subjected to a sustained stress.
For example, a reinforced concrete beam supporting a permanent load may initially deflect by a certain amount. Over time, continued deformation can increase the deflection even though the applied sustained load remains approximately unchanged.
Creep is different from instantaneous elastic deformation.
Immediate Deformation
When a load is first applied, concrete undergoes immediate elastic and other short-term deformation.
Creep Deformation
Under sustained stress, additional deformation develops gradually with time.
The total long-term deformation can therefore be greater than the deformation observed immediately after loading.
Why Does Concrete Creep Occur?
Concrete is not a completely rigid material. Its cement paste contains microscopic pores and moisture, and its internal structure changes gradually under sustained stress.
Creep is primarily associated with deformation of the cement paste.
Several mechanisms contribute to the phenomenon, including movement and redistribution of moisture within the cement paste and gradual internal adjustment of the hydrated cement structure.
The aggregate also influences creep because aggregates restrain deformation of the cement paste.
Therefore, creep depends on the combined characteristics of the concrete rather than on cement alone.
Creep of Concrete vs Shrinkage
Creep and shrinkage are both time-dependent phenomena, but they are not the same.
|
Feature |
Creep |
Shrinkage |
|
Main trigger |
Sustained stress or load |
Moisture loss and other volume changes |
|
Load required |
Yes, for conventional creep |
No external load required |
|
Main result |
Gradual increase in strain |
Reduction in volume |
|
Time dependence |
Yes |
Yes |
|
Structural effect |
Deflection, stress redistribution and prestress loss |
Cracking and dimensional changes |
Both need to be considered in appropriate structural designs because they can occur simultaneously.
Types of Creep in Concrete
Creep can be described in different ways depending on the conditions under which it occurs.
Basic Creep
Basic creep refers to creep occurring under sustained loading when the concrete is maintained under controlled moisture conditions, typically without moisture exchange with the surrounding environment.
Drying Creep
Drying creep occurs when concrete is subjected to sustained stress while also experiencing drying.
Drying can increase deformation compared with basic creep under otherwise comparable conditions.
Total Creep
For practical structural assessment, the combined long-term creep behaviour under the actual environmental and loading conditions may be considered as total creep.
The terminology and calculation approach can vary according to the design standard being used.
Creep Strain
Creep strain represents the additional strain that develops over time due to sustained stress.
A simplified relationship is often expressed using a creep coefficient:
Creep coefficient = Creep strain / Initial elastic strain
The actual calculation of creep coefficient depends on the adopted structural design standard and the concrete and environmental parameters.
It should not be treated as a universal constant because creep behaviour varies with the conditions of the concrete.
Factors Affecting Creep of Concrete
Many factors influence the amount of creep that develops.
1. Age of Concrete at Loading
Concrete loaded at a younger age generally experiences greater creep than concrete loaded after it has matured further.
As concrete gains strength and stiffness with age, its subsequent creep behaviour changes.
2. Magnitude of Sustained Stress
Higher sustained stress generally produces greater creep, provided the concrete remains within the stress range where the usual creep models apply.
Very high stresses can lead to more complex nonlinear behaviour.
3. Duration of Loading
Creep develops progressively while the sustained load remains.
The rate is generally higher during the earlier period and gradually reduces with time.
4. Moisture Conditions
Environmental humidity has a significant effect on creep.
Concrete exposed to drying conditions can exhibit greater deformation than comparable concrete protected from moisture exchange.
5. Member Size
Smaller structural members can generally exchange moisture more rapidly with their surroundings.
Member dimensions therefore influence the rate and magnitude of creep.
6. Concrete Strength
Concrete strength and stiffness affect creep behaviour.
Higher-strength concrete can generally exhibit lower creep under comparable conditions, although the actual behaviour depends on the complete concrete composition and curing history.
7. Aggregate Properties
Aggregates restrain deformation of the cement paste.
Aggregate type, stiffness, volume fraction and size can therefore influence concrete creep.
8. Water-Cement Ratio
Mix proportions influence the amount and characteristics of cement paste.
Concrete with a higher water-cement ratio may exhibit greater creep under comparable conditions.
9. Curing
Adequate curing allows cement hydration to progress and influences the resulting microstructure.
Poor curing can affect concrete properties and long-term deformation behaviour.
10. Temperature
Temperature affects moisture movement and cement hydration and can therefore influence creep behaviour.
The significance depends on the exposure and structural conditions.
Effects of Creep on Concrete Structures
Creep can have both manageable and significant structural effects.
Increased Deflection
Creep can increase the long-term deflection of beams and slabs.
This is particularly relevant for long-span members where serviceability limits are important.
Stress Redistribution
In reinforced concrete members, creep can redistribute stresses between concrete and reinforcement.
Prestress Loss
Creep of concrete contributes to the long-term loss of prestress in prestressed concrete members.
This can reduce the effective prestressing force over time.
Column Shortening
Creep can contribute to long-term shortening of concrete columns.
In tall buildings, differential shortening between structural elements can become an important design consideration.
Effects on Finishes
Long-term deformation can affect partitions, ceilings, cladding and finishes connected to structural members.
Excessive movement may contribute to cracking or serviceability problems.
Creep in Reinforced Concrete
Reinforcement influences the long-term behaviour of a concrete member.
When concrete creeps under sustained loading, the embedded reinforcement helps restrain some of the deformation.
This causes redistribution of stresses between the concrete and steel.
The effect depends on reinforcement ratio, member geometry, loading and other structural parameters.
For this reason, creep should be considered when evaluating long-term deflection and stress behaviour in reinforced concrete structures.
Creep in Prestressed Concrete
Creep is particularly significant in prestressed concrete.
Prestressed members are intentionally subjected to an initial compressive force. Over time, concrete creep causes shortening of the concrete.
Because the prestressing steel is bonded or otherwise connected to the member, this shortening can reduce the effective prestressing force.
Other sources of prestress loss can include:
- Shrinkage of concrete
- Relaxation of prestressing steel
- Anchorage-related effects
- Friction losses in applicable systems
The design must account for the relevant losses to achieve the required long-term performance.
Creep and Long-Term Deflection
One of the most important practical consequences of creep is increased deflection.
Consider a reinforced concrete beam supporting a permanent load. The beam experiences immediate deflection when the load is applied.
As creep develops, the concrete undergoes additional deformation, which can increase the beam’s long-term deflection.
This is why structural design considers both immediate and long-term serviceability.
How Can Creep Be Reduced?
Creep cannot be completely eliminated from normal concrete, but its magnitude can be controlled through appropriate design and construction practices.
Use Appropriate Concrete Strength
Concrete should be specified according to structural requirements. Higher stiffness can help reduce deformation under comparable conditions.
Control Water Content
Appropriate mix design can reduce excessive cement paste and water-related deformation.
Use Suitable Aggregates
Stiffer aggregates can provide greater restraint to cement-paste deformation.
Ensure Proper Curing
Good curing supports proper hydration and development of concrete properties.
Avoid Unnecessary Early Loading
Allowing concrete to develop adequate strength before significant sustained loading can reduce long-term creep compared with loading at a very early age.
Consider Member Dimensions
Structural member size affects moisture movement and creep behaviour and should be considered during design.
Creep Testing of Concrete
Laboratory creep tests can be used to evaluate time-dependent deformation under controlled conditions.
A typical test involves:
- Preparing concrete specimens.
- Allowing them to reach the required age.
- Applying a sustained load.
- Measuring deformation over time.
- Comparing the measured strain with reference or unloaded specimens.
The precise specimen dimensions, loading conditions, duration and measurement procedure depend on the applicable testing standard.
How Is Creep Considered in Structural Design?
Engineers use design standards and established creep models to estimate long-term deformation.
The calculation may consider:
- Concrete age at loading
- Relative humidity
- Member size
- Concrete strength
- Cement and aggregate characteristics
- Duration of loading
- Stress level
- Curing conditions
The appropriate method depends on the structural design standard being followed.
For important structures, long-term deflection and prestress effects should be assessed by a qualified structural engineer.
Creep of Concrete: Example
Suppose a concrete beam experiences an immediate elastic strain after a sustained load is applied.
If the concrete later develops additional deformation because the load remains in place, that additional time-dependent strain represents creep.
The process can be simplified as:
Sustained load → Initial strain → Continued time-dependent strain → Increased long-term deformation
The actual magnitude depends on the concrete, environment, loading conditions and duration.
Common Misconceptions About Concrete Creep
Creep Means the Concrete Is Failing
Not necessarily. Creep is a normal material behaviour under sustained stress.
Creep Happens Only in Poor-Quality Concrete
No. All ordinary concrete exhibits some degree of creep. Quality and mix characteristics influence its magnitude.
Creep Happens Immediately
Creep develops over time. The rate is generally higher during the earlier stages and decreases progressively.
Creep and Shrinkage Are the Same
They are different phenomena. Creep requires sustained stress, while shrinkage can occur without an external sustained load.
Creep Can Be Completely Prevented
Normal concrete cannot be made completely creep-free. Structural design instead accounts for expected long-term deformation.
Final Thoughts
Creep of concrete is a normal time-dependent response that occurs when concrete remains under sustained stress. Although it does not automatically indicate structural failure, it can influence long-term deflection, stress redistribution, column shortening and prestress losses. Its magnitude depends on factors such as concrete age, moisture, member size, strength, aggregate properties, sustained stress and loading duration. Good mix design, curing and construction practices can help control creep, but structural design remains essential for managing its effects. For significant structures, long-term deformation should be evaluated using the applicable design standards and engineering methods.
FAQs
- What is creep of concrete?
Creep of concrete is the gradual increase in deformation that occurs when concrete is subjected to a sustained load over time. The member initially undergoes immediate deformation and then continues to deform progressively. Creep is a normal property of concrete and is considered during structural design to control long-term deflection and other effects.
- What causes creep in concrete?
Creep is mainly associated with time-dependent deformation of the cement paste under sustained stress. Moisture movement, the internal structure of hydrated cement, concrete age, aggregate properties, stress level and environmental conditions all influence the amount of creep. The exact behaviour depends on the concrete composition and exposure conditions.
- What is the difference between creep and shrinkage?
Creep is time-dependent deformation caused by sustained stress, whereas shrinkage is a volume change that can occur without an external load. Both are affected by moisture, concrete composition and environmental conditions. They can occur simultaneously and should be considered separately when assessing long-term concrete behaviour.
- What factors affect the creep of concrete?
Important factors include concrete age at loading, sustained stress, loading duration, relative humidity, member size, concrete strength, aggregate properties, water-cement ratio, curing and temperature. These factors interact with one another, so creep cannot be accurately predicted from a single material property or construction parameter.
- How does creep affect reinforced concrete beams?
Creep can increase the long-term deflection of reinforced concrete beams and slabs. It can also redistribute stresses between concrete and reinforcement. The effect becomes particularly important for members carrying sustained loads over long periods, especially where serviceability requirements limit allowable long-term deformation.
- Why is creep important in prestressed concrete?
Creep causes gradual shortening of the concrete under sustained prestressing and other loads. This shortening contributes to a reduction in the effective prestressing force. Designers therefore consider concrete creep along with shrinkage, steel relaxation and other applicable losses when determining the long-term performance of prestressed members.
- Can creep of concrete be prevented?
No, creep cannot be completely eliminated from normal concrete. However, its magnitude can be controlled through suitable mix design, appropriate aggregate selection, proper curing, adequate concrete strength and sensible loading practices. Structural engineers also account for expected creep when designing members for long-term serviceability.
- Is creep of concrete the same as permanent deformation?
Creep is a form of time-dependent deformation under sustained stress, but the term should not simply be treated as synonymous with all permanent deformation. Some deformation may be recoverable after unloading, while another portion can remain. Structural design methods distinguish these behaviours when evaluating long-term concrete performance.
Sowjanya Siddanoor