Creep of Concrete: Causes, Factors, Creep Coefficient, Effects, IS 456 & Practical Guide

Introduction

Creep of concrete is an important long-term deformation phenomenon that occurs when concrete remains subjected to sustained stress or load. Unlike the immediate deformation that occurs when a load is first applied, creep develops gradually with time while the sustained stress continues.

Creep is particularly important in RCC beams, slabs, columns, prestressed concrete members and high-rise structures because it can influence long-term deflection, column shortening, differential movement, prestress losses and overall serviceability.

Understanding creep is therefore important for Civil Engineers, Structural Engineers, Site Engineers, QA/QC Engineers and students studying concrete technology and RCC design.

Quick Answer

Creep of concrete is the gradual increase in strain or deformation with time when concrete is subjected to sustained stress.

In simple terms:

Sustained Stress + Time → Additional Deformation = Creep

Creep should not be confused with the immediate elastic deformation that occurs when a load is first applied.

What Is Creep of Concrete?

When a concrete member is loaded, it initially undergoes an immediate deformation.

If the stress remains substantially sustained for a long period, the concrete continues to deform gradually. This additional time-dependent deformation is called creep.

Consider an RCC beam carrying a sustained load.

Immediately after loading:

Load → Immediate Deflection

With time:

Sustained Load → Additional Time-Dependent Deflection

The additional deformation that develops with time includes the effect of creep.

A simple conceptual representation is:

Total Long-Term Deformation ≈ Immediate Deformation + Time-Dependent Deformation

The actual structural response depends on the concrete properties, age at loading, loading history, environmental conditions, reinforcement, cracking, restraint and structural system.

Why Does Concrete Creep?

Concrete is not a perfectly elastic material. Its hardened structure consists of aggregate embedded in hydrated cement paste containing pores and moisture at different scales.

When sustained stress acts on concrete, time-dependent processes within the cement paste and its moisture system allow additional strain to develop gradually.

Aggregate restrains the deformation of the cement paste. Consequently, creep depends on the properties and proportions of both the cementitious matrix and aggregate.

The magnitude of creep is therefore influenced by the combined effects of:

• Sustained stress
• Age at loading
• Duration of loading
• Concrete strength and stiffness
• Water-cement ratio
• Cement paste content
• Aggregate properties
• Relative humidity
• Member size
• Curing
• Environmental conditions

Basic Mechanism of Creep

A simplified sequence is:

Sustained Load

↓

Immediate Deformation

↓

Load Remains Applied

↓

Time-Dependent Internal Response

↓

Additional Strain

↓

Creep

Creep is therefore a progressive time-dependent response, not an instantaneous deformation.

Immediate Deformation vs Creep

Concrete response under sustained loading can be understood in two broad stages.

Stage 1 — Immediate Deformation

When the load is applied, concrete experiences an immediate strain or deformation associated with its short-term response.

Stage 2 — Creep Deformation

If the stress remains sustained, additional strain develops progressively with time.

This additional time-dependent strain is known as creep strain.

Creep Strain

Creep strain is the additional time-dependent strain that develops in concrete because of sustained stress.

Conceptually:

Total Strain under Sustained Stress = Initial Strain + Creep Strain

Other strain components such as shrinkage and temperature-related strain may also need to be considered separately in an actual structural assessment.

Creep Coefficient

The creep coefficient expresses creep strain relative to the initial elastic strain at the age of loading.

Conceptually:

Creep Coefficient, φ = Creep Strain / Initial Elastic Strain

Therefore:

Creep Strain = φ × Initial Elastic Strain

The creep coefficient is not one fixed value for all concrete structures. It depends on factors including age at loading, duration, concrete properties, environmental conditions and member characteristics.

Creep of Concrete as per IS 456:2000

IS 456:2000 addresses creep of concrete in Clause 6.2.5.

The code states that creep depends on factors including the stress in concrete, age at loading and duration of loading. For stress not exceeding one-third of the characteristic compressive strength, creep may be assumed to be proportional to stress.

In the absence of experimental data and detailed information on the relevant variables, Clause 6.2.5.1 gives the following creep coefficients for estimating ultimate creep strain.

For the simplified IS 456 values, earlier age at loading corresponds to a higher creep coefficient.

For long-span structures, IS 456 advises determining the actual creep strain likely to occur rather than relying only on the simplified values.

Engineering Note: These values are design provisions from IS 456:2000 and should not be treated as universal material constants. Project-specific structural design should follow the applicable standard, design basis and approved engineering methodology.

Factors Affecting Creep of Concrete

The magnitude of creep is controlled by several interacting material, loading, environmental and structural factors.

1. Age of Concrete at Loading

Age at loading is one of the most important factors.

Concrete loaded at an early age generally develops greater creep than concrete first loaded at a more mature age, all other relevant conditions being comparable.

This trend is also reflected in the IS 456 creep coefficients.

2. Duration of Sustained Loading

Creep develops progressively while sustained stress continues.

In general:

Longer Sustained Loading → Greater Accumulated Creep

However, the rate of creep is not constant. It is generally greater during the earlier period and reduces progressively with time.

3. Magnitude of Sustained Stress

Higher sustained stress generally produces greater creep strain within the range where the applicable creep relationship remains valid.

For the simplified IS 456 treatment, creep may be assumed proportional to stress when the concrete stress does not exceed one-third of its characteristic compressive strength.

4. Concrete Strength and Stiffness

Concrete strength, stiffness and maturity influence its time-dependent deformation.

Higher-strength concrete can still experience creep; therefore, it is incorrect to assume that increasing concrete grade eliminates creep.

5. Water-Cement Ratio

The water-cement ratio influences the structure and properties of hardened cement paste.

Uncontrolled addition of water at site can change the concrete properties assumed in the approved mix design and should therefore be avoided.

6. Cement Paste Content

Creep is strongly associated with the cementitious matrix, while aggregate provides restraint.

Consequently, the paste content and overall mix proportions can influence creep behaviour.

7. Aggregate Properties

Aggregate restrains deformation of the cement paste.

Important aggregate characteristics include:

• Stiffness
• Volume fraction
• Type
• Size and grading
• Quality

A stiffer aggregate skeleton can generally provide greater restraint against paste deformation.

8. Relative Humidity

Environmental humidity affects moisture movement within concrete.

Drying conditions can influence time-dependent deformation, particularly when drying occurs while concrete is under sustained stress.

9. Member Size

Member dimensions affect moisture movement between the interior of the concrete and the surrounding environment.

Smaller or thinner members generally respond more rapidly to environmental moisture changes than massive members.

10. Curing

Proper curing supports hydration and development of the intended concrete properties.

Poor curing can adversely affect concrete quality and its subsequent structural performance.

11. Environmental Conditions

Temperature, humidity, exposure and moisture conditions can influence long-term behaviour and should be considered together with the material and loading conditions.

Types of Creep

Creep can be discussed in different ways depending on the moisture and loading conditions.

1. Basic Creep

Basic creep refers to creep under sustained stress when moisture exchange with the surrounding environment is substantially prevented.

It represents the intrinsic time-dependent response of the concrete under the specified conditions.

2. Drying Creep

Drying creep is the additional time-dependent deformation associated with drying while concrete is simultaneously subjected to sustained stress.

It should not be confused with drying shrinkage.

Drying creep requires sustained stress.

Drying shrinkage can occur without sustained structural stress.

Creep vs Shrinkage

Creep and shrinkage are both time-dependent phenomena, but they are fundamentally different.

An easy way to remember the difference is:

Creep → Stress/Load related

Shrinkage → Volume-change related

Creep vs Elastic Deformation

Creep vs Plastic Deformation

Creep should also be distinguished from ordinary plastic deformation.

Creep is a time-dependent deformation that develops under sustained stress.

Plastic deformation refers to permanent deformation associated with material response beyond the elastic range.

Therefore, creep should not simply be described as plastic deformation.

Why Is Creep Important in Civil Engineering?

Creep can influence:

• Long-term beam and slab deflection
• Column shortening
• Differential shortening
• Prestress losses
• Redistribution of stresses and internal forces
• Serviceability
• Structural alignment
• Interaction with non-structural elements
• Long-term structural performance

Its importance depends on the structural system, loading, material properties and environmental conditions.

Creep in RCC Beams

An RCC beam undergoes an initial deflection when a load is applied.

If a significant portion of the load remains sustained:

Initial Deflection

↓

Sustained Loading

↓

Creep Develops

↓

Additional Long-Term Deflection

A beam that satisfies an immediate deflection requirement can therefore still require assessment for long-term deflection.

Creep in RCC Slabs

Creep can increase long-term slab deformation, particularly in slender slabs and slabs carrying significant sustained loads.

Excessive long-term deformation may affect:

• Floor finishes
• Partition walls
• Drainage slopes
• Doors
• Facades
• Building services
• Architectural appearance

Creep in RCC Columns

Columns are commonly subjected to sustained compressive stress.

Creep can therefore contribute to gradual column shortening.

In multi-storey structures, differences in:

• Column load
• Column dimensions
• Concrete properties
• Age at loading
• Construction sequence
• Environmental conditions

can result in different amounts of long-term shortening.

Differential Shortening in High-Rise Buildings

Differential shortening occurs when adjacent vertical structural elements undergo different amounts of long-term vertical deformation.

A simplified mechanism is:

Different Loads/Properties

↓

Different Time-Dependent Strains

↓

Different Long-Term Shortening

↓

Differential Movement

This movement can affect:

• Floor levels
• Cladding and curtain walls
• Partitions
• Finishes
• Connections
• Pipes and ducts
• Mechanical and electrical services

Creep and shrinkage are therefore important considerations in high-rise design and construction planning.

Creep in Prestressed Concrete

Creep is particularly important in prestressed concrete because it contributes to long-term loss of prestress.

A simplified mechanism is:

Prestressing Force

↓

Compression in Concrete

↓

Concrete Creep

↓

Additional Concrete Strain

↓

Change in Prestressing Steel Stress

↓

Prestress Loss

Creep must therefore be considered together with other relevant prestress-loss mechanisms according to the applicable design standard.

Effects of Creep on Concrete Structures

The principal structural effects of creep include:

Increased Long-Term Deflection

Creep can increase beam and slab deformation under sustained loading.

Column Shortening

Sustained compression can result in progressive shortening of columns and other vertical concrete elements.

Differential Shortening

Different members may undergo different amounts of time-dependent shortening.

Prestress Loss

Creep contributes to long-term loss of prestressing force.

Stress Redistribution

Time-dependent deformation can cause redistribution of stresses in statically indeterminate, composite and other restrained structural systems.

Serviceability Problems

Excessive long-term deformation may affect finishes, partitions, facades, doors, windows, drainage and services.

Creep Calculation — Basic Concept

For a simplified educational explanation:

Initial Elastic Strain = Sustained Stress / Modulus of Elasticity

or:

εₑ = σ / E

Creep strain can then be represented as:

εcr = φ × εₑ

Therefore:

εcr = φ × (σ / E)

where:

• εcr = creep strain
• φ = creep coefficient
• εₑ = initial elastic strain
• σ = sustained concrete stress
• E = relevant modulus of elasticity

These equations explain the basic concept. Actual structural calculations should follow the applicable design provisions.

Worked Example — Creep Strain

Suppose a concrete member is subjected to:

• Sustained stress = 5 N/mm²
• Modulus of elasticity = 25,000 N/mm²
• Assumed creep coefficient for this educational example = 2.0

Step 1 — Calculate Initial Elastic Strain

Initial elastic strain = 5 / 25,000

= 0.0002

or:

= 200 microstrain

Step 2 — Calculate Creep Strain

Creep strain = 2.0 × 0.0002

= 0.0004

or:

= 400 microstrain

This example is provided only to explain the relationship between elastic strain, creep coefficient and creep strain. The assumed creep coefficient of 2.0 is illustrative and should not be treated as a universal design value.

Worked Example — IS 456 Creep Coefficient

Consider the same simplified conditions:

• Sustained stress = 5 N/mm²
• Modulus of elasticity = 25,000 N/mm²
• Age at loading = 28 days

For the simplified IS 456 provision:

Creep coefficient = 1.6

Initial elastic strain:

εₑ = 5 / 25,000

= 0.0002

Ultimate creep strain:

εcr = 1.6 × 0.0002

= 0.00032

Therefore:

Ultimate creep strain = 320 microstrain

The creep strain calculated here is additional to the initial elastic strain.

Hence, considering only these two components for this simplified illustration:

Initial Elastic Strain + Ultimate Creep Strain

= 0.0002 + 0.00032

= 0.00052

or:

520 microstrain

This is an educational demonstration of the IS 456 creep-coefficient concept, not a complete member-level deflection calculation.

Simplified Creep Deformation Example

If a uniform creep strain of 0.00032 were used only for a simplified axial illustration over a member length of 3,000 mm:

Change in length = Strain × Original Length

= 0.00032 × 3,000

= 0.96 mm

This relationship is appropriate as a simple uniform axial-strain illustration.

It should not be used directly as a formula for beam or slab deflection. Actual structural deformation depends on stress distribution, cracking, reinforcement, curvature, restraint, geometry and the structural analysis method.

Creep and Long-Term Deflection

Creep is an important consideration in long-term deflection calculations.

Conceptually:

Immediate Deflection + Time-Dependent Effects → Long-Term Deflection

However, long-term beam and slab deflection should not be calculated simply by multiplying an initial deflection by a creep coefficient without following the applicable design method.

The structural calculation may need to account for:

• Sustained and transient loading
• Cracking
• Reinforcement
• Effective stiffness
• Shrinkage
• Creep
• Geometry
• Boundary conditions
• Construction sequence
• Serviceability requirements

Creep and Serviceability

Creep is often particularly important from the standpoint of serviceability.

Excessive long-term deformation may cause:

• Visible sagging
• Damage to finishes
• Partition distress
• Door and window problems
• Drainage problems
• Facade movement
• Misalignment of services

A structure must therefore satisfy not only strength requirements but also the applicable long-term serviceability requirements.

Practical Example — RCC Beam

Suppose an RCC beam supports permanent loads for many years.

Immediately after loading, it develops an initial deflection.

As the sustained loading continues, creep can cause additional deformation.

Therefore:

Initial Beam Deflection + Time-Dependent Effects → Greater Long-Term Deflection

If excessive deflection is observed, however, creep should not automatically be assumed to be the sole cause.

Practical Example — RCC Column

Consider two columns carrying different sustained loads.

Column A may have a higher sustained stress than Column B.

If their material properties, dimensions, loading ages and environmental conditions also differ, their long-term strains may differ.

Therefore:

Different Time-Dependent Strain → Different Shortening → Differential Movement

This effect can become important in tall buildings.

Practical Example — Prestressed Beam

A prestressed beam is subjected to sustained concrete compression resulting from prestressing.

With time:

Concrete Creep

↓

Additional Concrete Strain

↓

Change in Prestressing Steel Stress

↓

Prestress Loss

Creep is therefore one component of the total long-term prestress-loss assessment.

How Can the Effects of Creep Be Controlled?

Creep cannot normally be eliminated completely. Its magnitude and structural consequences are managed through appropriate design, materials and construction practices.

1. Follow the Approved Mix Design

Concrete should be produced according to the approved mix proportions and project specifications.

2. Control Water Addition

Unauthorised water addition at site should be avoided because it changes the concrete properties assumed during mix design and structural design.

3. Use Suitable Aggregates

Aggregate quality, volume and stiffness influence concrete deformation behaviour.

4. Provide Proper Curing

Concrete should be cured according to the project requirements to support proper hydration and development of the required properties.

5. Avoid Unauthorised Premature Loading

Significant construction or permanent loads should not be applied earlier than permitted by the approved construction sequence and project requirements.

6. Control Permanent Loads

Permanent loads should remain consistent with the assumptions of the approved structural design.

7. Consider Long-Term Behaviour During Design

Where creep is significant, structural design should account for its effects on deflection, shortening, stress redistribution and prestress losses as applicable.

8. Monitor Critical Structures

Long-term deformation may need to be monitored in structures where differential shortening or deflection is particularly important.

Site Engineer’s Creep Inspection Checklist

When long-term deformation is suspected, record the relevant information systematically.

Concrete

☐ Concrete grade

☐ Approved mix details

☐ Date of casting

☐ Age at loading

☐ Curing records

☐ Available material test records

Loading

☐ Date significant loading started

☐ Permanent loads

☐ Changes in loading

☐ Construction sequence

Structural Behaviour

☐ Beam/slab deflection

☐ Column or wall shortening

☐ Differential movement

☐ Crack locations and development

☐ Floor-level variation

☐ Change with time

Environment

☐ Temperature where relevant

☐ Relative humidity where available

☐ Exposure conditions

☐ Moisture/drying conditions

Documentation

☐ Structural drawings

☐ Approved design information

☐ Previous measurements

☐ Current measurements

☐ Dates of observations

☐ Relevant construction and inspection records

What Should a Site Engineer Do if Excessive Long-Term Deflection Is Observed?

Do not immediately conclude that creep is the cause.

Follow a systematic investigation.

Step 1

Measure and record the location and magnitude of deformation.

Step 2

Check the age of the structure and loading history.

Step 3

Review the concrete grade, available test results, mix information and curing records.

Step 4

Check whether permanent or other sustained loads differ from the approved design assumptions.

Step 5

Review cracks, support conditions and other visible signs.

Step 6

Measure the deformation at suitable intervals where monitoring is required.

Step 7

Consider alternative or contributing causes such as:

• Excessive loading
• Cracking
• Structural design or detailing issues
• Construction tolerances
• Material properties
• Support movement
• Shrinkage
• Temperature effects
• Foundation or structural movement

Step 8

Where necessary, obtain assessment from the responsible structural engineer.

The practical principle is:

Observe → Measure → Record → Investigate → Assess → Act

rather than assuming:

Deflection = Creep

Common Misconceptions About Creep

Misconception 1 — Creep means concrete is failing

Incorrect.

Creep is a normal time-dependent characteristic of concrete. The important engineering question is whether the resulting deformation and associated effects remain within acceptable design and serviceability requirements.

Misconception 2 — Creep occurs only in old buildings

Incorrect.

Creep begins after sustained stress is applied and develops progressively with time.

Misconception 3 — Creep and shrinkage are the same

Incorrect.

Creep requires sustained stress, whereas shrinkage does not require sustained structural stress.

Misconception 4 — High-strength concrete has no creep

Incorrect.

Higher-strength concrete can still experience creep.

Misconception 5 — All long-term deflection is caused by creep

Incorrect.

Creep may contribute to long-term deflection, but cracking, shrinkage, loading, stiffness, construction sequence, support movement and other factors may also be involved.

Important Practical Points

1. Creep is time-dependent deformation under sustained stress.
2. It develops progressively after sustained loading begins.
3. Age at loading strongly influences creep.
4. Earlier loading generally results in greater creep potential.
5. Creep can increase long-term beam and slab deflection.
6. Creep contributes to column and wall shortening.
7. Differential shortening can be important in high-rise structures.
8. Creep contributes to prestress losses.
9. Aggregate properties influence creep.
10. Relative humidity and member size influence time-dependent behaviour.
11. Proper mix control and curing are important.
12. Unauthorised water addition should be avoided.
13. Significant premature loading should be avoided where it conflicts with approved project requirements.
14. Critical long-term deformation should be monitored where required.
15. Never diagnose creep from deflection or cracking alone.

Frequently Asked Questions on Creep of Concrete

What is creep of concrete?

Creep is the gradual increase in strain or deformation of concrete with time when it is subjected to sustained stress.

What causes creep in concrete?

Creep results from the time-dependent response of concrete under sustained stress. Its magnitude is influenced by age at loading, duration and magnitude of stress, concrete properties, aggregate characteristics, humidity, member size, curing and other factors.

What is the creep coefficient?

The creep coefficient is the ratio of creep strain to the initial elastic strain at the age of loading:

φ = Creep Strain / Initial Elastic Strain

What is the creep coefficient as per IS 456?

For the simplified estimation of ultimate creep strain in the absence of detailed information, IS 456:2000 gives creep coefficients of 2.2 for loading at 7 days, 1.6 at 28 days and 1.1 at 1 year.

Does creep occur without sustained stress?

Creep is associated with sustained stress. Time-dependent volume changes occurring without sustained structural stress are generally considered separately, such as shrinkage.

Does creep increase with time?

Creep accumulates progressively with time under sustained loading, although its rate generally decreases as time progresses.

What is basic creep?

Basic creep is the creep occurring under sustained stress when moisture exchange with the surrounding environment is substantially prevented.

What is drying creep?

Drying creep is the additional time-dependent deformation associated with simultaneous sustained stress and drying.

What is the difference between creep and shrinkage?

Creep is time-dependent deformation associated with sustained stress. Shrinkage is a volume-change phenomenon that does not require sustained structural stress.

Does creep cause cracks?

Creep is primarily a deformation phenomenon. However, creep-induced deformation and stress redistribution can influence cracking in some restrained or structurally complex situations.

Why is creep important in RCC beams and slabs?

Creep can increase long-term deflection and therefore affect serviceability.

Why is creep important in columns?

Creep under sustained compression contributes to long-term shortening. Different amounts of shortening between vertical elements can result in differential movement.

Why is creep important in prestressed concrete?

Creep contributes to changes in concrete strain and therefore to long-term loss of prestress.

Can creep be completely eliminated?

No. Creep is an inherent time-dependent characteristic of concrete. Its effects are considered and controlled through appropriate design, materials and construction practices.

How can the effects of creep be reduced or controlled?

Important measures include appropriate mix proportioning, suitable aggregate, controlled water content, proper curing, avoiding unauthorised premature loading and accounting for long-term deformation during structural design.

Is creep the same for every grade of concrete?

No. Creep depends on several interacting parameters and should not be represented by one universal value for every concrete grade or structure.

Civil Engineering Interview Questions on Creep of Concrete

What is the simplest definition of creep?

Creep is the gradual additional deformation of concrete with time under sustained stress.

What are the major factors affecting creep?

Important factors include age at loading, stress magnitude, loading duration, concrete properties, aggregate properties, water-cement ratio, paste content, relative humidity, member size, curing and environmental conditions.

Why does younger concrete generally experience greater creep?

Concrete loaded at an earlier age is less mature and generally has greater potential for time-dependent deformation.

What is the effect of aggregate on creep?

Aggregate restrains deformation of the cement paste. Its stiffness, quantity and characteristics therefore influence the overall creep of concrete.

Why is creep important in high-rise buildings?

Creep can contribute to long-term shortening of columns, walls and other vertical members. Differences in shortening can produce differential movement affecting floors, facades, partitions, services and connections.

Can a Site Engineer identify creep only by looking at a deflected beam?

No. Creep may contribute to long-term deflection, but visual observation alone cannot establish the cause. Loading, cracking, material properties, support conditions, construction history and other possible causes should also be investigated.

Conclusion

Creep of concrete is the gradual time-dependent increase in deformation under sustained stress. It is an important long-term behaviour of concrete and can influence beam and slab deflection, column shortening, differential movement, stress redistribution and prestress losses.

The magnitude of creep depends on several interacting factors, particularly the age of concrete at loading, magnitude and duration of sustained stress, concrete properties, aggregate characteristics, relative humidity, member size and curing conditions.

IS 456:2000 provides simplified creep coefficients of 2.2, 1.6 and 1.1 for loading at 7 days, 28 days and 1 year respectively when detailed information is unavailable. These values should be applied within the context and limitations of the applicable design provisions.

From a construction perspective, the effects of creep are managed through proper mix control, suitable materials, adequate curing, controlled loading and appropriate structural design. When unexpected long-term deformation occurs, engineers should measure and investigate the behaviour systematically rather than attributing every crack or deflection to creep.

Creep cannot normally be eliminated, but its effects can be understood, predicted and appropriately considered in structural design and construction practice.