Creep of Concrete – Definition, Mechanism, Factors and Effects

Imagine a concrete beam spanning between two columns. The day after loading, it deflects by 10 mm. A year later, without any change in load, the deflection has grown to 25 mm. No one added more load. The concrete itself continued to deform slowly under the sustained stress — this is creep. It’s one of those concrete behaviours that catches inexperienced engineers by surprise, and it has caused real structural problems in long-span beams and prestressed concrete structures.

1. What is Creep of Concrete?

Creep is defined as the time-dependent (long-term), inelastic deformation of concrete under a sustained constant stress, at constant temperature and moisture conditions.

Let’s break down the key parts of this definition:

• Time-dependent: Creep doesn’t happen all at once. It begins the moment load is applied and continues for months and years, gradually tapering off (approaching a final value after 3–5 years).
• Under sustained constant stress: Creep only occurs under load. Remove the load and creep stops. Part of the creep strain also recovers over time after load removal (recoverable creep).
• At constant temperature and moisture: This distinguishes creep from thermal deformation and drying shrinkage (which also cause time-dependent strains but are driven by temperature and moisture changes respectively).

Creep strain is typically 1 to 3 times the initial elastic strain for concrete loaded at 28 days. For young concrete loaded early (at 7 days), creep can be up to 5 times the elastic strain.

2. Mechanism of Creep

The mechanism of creep in concrete is complex and not fully understood even today. Several mechanisms operate simultaneously:

• Viscous flow of C-S-H gel: The C-S-H gel, which constitutes 50–60% of hardened cement paste, can slowly flow and rearrange its layer structure under sustained stress. This is the dominant mechanism at low stresses (well below 50% of strength).
• Moisture migration: Sustained stress increases the chemical potential of water in loaded zones, causing pore water to migrate from high-stress regions to lower-stress regions or to the surface. This drying under stress (called drying creep or Pickett effect) contributes significantly to creep at normal relative humidity.
• Microcrack development: At higher stress levels (above 40% of strength), slow growth of micro-cracks contributes to creep. This component is often irreversible.
• Crystal dissolution: Under high sustained stress, cement crystals slowly dissolve at contact points and re-precipitate in void spaces — a process akin to pressure solution in rocks.

3. Strain Components Under Sustained Load

When a sustained load is applied to a concrete member, the total strain has several components:

• Elastic strain (ε_e): Instantaneous deformation on load application. ε_e = σ ÷ E_c. This is fully recoverable on load removal.
• Creep strain (ε_cr): Additional time-dependent strain under sustained load. Partly recoverable (reversible creep) and partly permanent (irreversible creep).
• Drying shrinkage strain (ε_sh): Occurs simultaneously with creep — they interact (creep is greater in drying conditions — “drying creep” or Pickett effect).

4. Creep Coefficient — IS 456:2000

The Creep Coefficient (C_c) is defined as the ratio of creep strain to elastic strain:

IS 456:2000 Table 2 specifies the following creep coefficient values:

The trend is clear: the older the concrete at the time of loading, the less creep it undergoes. This is because older concrete has a more fully developed, less viscous C-S-H gel structure.

Using Creep Coefficient in Design

IS 456:2000 Clause 6.2.5 uses the concept of Effective (Long-term) Modulus of Elasticity:

This reduced effective modulus is used to calculate long-term (creep) deflection of beams and slabs.

5. Factors Affecting Creep

• Stress level: For stresses below ~40% of f_ck, creep is approximately proportional to stress (linear creep). Above 50%, creep accelerates non-linearly. Above 80%, sustained loading leads to creep failure (tertiary creep).
• Age at loading: Younger concrete creeps more. Delaying loading by a few weeks significantly reduces creep (as shown by IS 456 Table 2: C_c drops from 2.2 at 7 days to 1.6 at 28 days to 1.1 at 1 year).
• W/C ratio: Lower W/C → denser paste → less viscous C-S-H gel → less creep.
• Relative humidity: Lower RH → more drying → more drying creep. Sealed (no drying) specimens show much less creep than drying specimens.
• Temperature: Higher temperature accelerates creep (thermal activation of C-S-H gel viscous flow).
• Cement content: Higher cement content → more paste → more creep potential. Well-graded aggregates provide internal restraint of paste creep.
• Type of aggregate: Stiffer aggregates (granite, quartzite) restrain paste creep better than soft aggregates (limestone, sandstone).
• Member size (volume/surface ratio): Larger members dry more slowly, so drying creep is lower in mass elements.

6. Effects of Creep on Structures

Creep causes several important structural effects that must be accounted for in design:

• Increased deflection: Long-term deflection can be 2–4 times the initial elastic deflection. Beams and slabs sag more over time. IS 456:2000 requires that total long-term deflection (including creep and shrinkage) not exceed span/250 (for simply supported beams).
• Prestress losses in PSC: In prestressed concrete, creep causes the concrete to shorten progressively under the compressive prestress force. This shortening reduces the tension in the prestressing tendons — a loss of prestress. This is one of the major long-term losses calculated in IS 1343 design.
• Redistribution of stresses in indeterminate structures: In frames and continuous beams, creep causes gradual redistribution of moments and forces from highly stressed to lower-stressed regions — generally beneficial as it reduces peak stresses.
• Lateral deformation of columns: Sustained eccentric loads cause columns to creep sideways progressively, increasing eccentricity and P-delta effects. Critical in slender columns under sustained load.
• Reduction in concrete stiffness: The effective modulus E_ce = E_c/(1+C_c) is significantly lower than the short-term modulus, affecting frame stiffness and natural frequencies.

7. Beneficial Effects of Creep

Not all creep effects are harmful. Some are actually beneficial:

• Relief of stress concentrations: In areas of high local stress (near supports, around openings, at construction joints), creep redistributes stresses to adjacent lower-stressed zones, reducing the risk of cracking.
• Beneficial redistribution in frames: In indeterminate structures, creep-induced moment redistribution generally moves stresses away from peak locations, improving overall structural safety.
• Compensation for differential settlement: In statically indeterminate structures on differential settlements, creep gradually reduces the secondary stresses caused by settlement.

8. Minimising Creep

• Delay loading: Load concrete as late as possible after casting — creep coefficient drops from 2.2 (7 days) to 1.6 (28 days) to 1.1 (1 year).
• Low W/C ratio: Denser, less viscous C-S-H gel structure with lower creep potential.
• High aggregate content: Aggregates don’t creep (rigid minerals). More aggregate → less paste volume → less total creep.
• Stiff aggregates: Hard rocks (granite, quartzite, basalt) restrain paste creep better than soft rocks.
• Steam curing: Accelerates C-S-H gel maturation → concrete loaded after steam curing shows significantly less creep.
• Lower stress levels: Stress-to-strength ratio below 0.4 keeps creep in the linear range and minimises long-term deformation.

9. Creep vs Shrinkage — Key Differences

10. Diagram — Creep Strain Components and IS 456 Coefficients

11. Exam Tips (RTMNU)

• ✅ Definition: creep = time-dependent deformation under sustained constant stress — state all three conditions (time-dependent, constant stress, constant temperature).
• ✅ IS 456:2000 Table 2 creep coefficients: 2.2 (7 days), 1.6 (28 days), 1.1 (1 year) — memorise all three.
• ✅ Effective modulus: E_ce = E_c ÷ (1 + C_c) — for M25 at 28 days: E_ce = 25,000/2.6 = 9,615 MPa.
• ✅ Creep vs shrinkage comparison table — excellent format for 5-mark questions.
• ✅ Effects of creep: increased deflection, prestress losses (PSC), stress redistribution — list all three with brief explanations.
• ✅ “Why is loading concrete early (at 7 days) worse than loading at 28 days?” — C_c = 2.2 vs 1.6 answer.

12. Key Takeaways

• Creep = time-dependent deformation under sustained constant stress. Continuous for 3–5 years then plateaus.
• Creep coefficient C_c = creep strain/elastic strain. IS 456 Table 2: 2.2 (7d), 1.6 (28d), 1.1 (1 year).
• Effective modulus: E_ce = E_c/(1+C_c). For M25 at 28 days: E_ce = 9,615 MPa.
• Key effects: increased deflection, prestress losses, column lateral deformation.
• Minimise by: delaying loading, low W/C, stiff aggregates, steam curing.
• Creep differs from shrinkage: creep needs sustained load; shrinkage occurs even without load.

13. FAQs

Q1. What is creep of concrete?

Creep is the time-dependent, inelastic deformation of concrete under a sustained constant stress, occurring at constant temperature and moisture conditions. It begins at the instant of loading and continues for months to years, typically reaching 70–80% of its final value within 5 years.

Q2. What are the creep coefficient values per IS 456:2000?

IS 456:2000 Table 2 specifies: C_c = 2.2 (concrete loaded at 7 days), C_c = 1.6 (loaded at 28 days), C_c = 1.1 (loaded at 1 year). These values assume average humidity and typical concrete. Older concrete creeps less because the C-S-H gel is more fully developed and less viscous.

Q3. How does creep affect prestressed concrete?

In prestressed concrete, the high compressive prestress causes the concrete to shorten progressively over time (creep shortening). This shortening reduces the extension (and hence tension) in the prestressing tendons — a long-term prestress loss called creep loss. IS 1343:2012 requires designers to calculate this loss explicitly and account for it in the design prestress.

Q4. What is the effective modulus of elasticity for long-term loading?

IS 456:2000 Clause 6.2.5 defines the effective modulus of elasticity for long-term loading as E_ce = E_c/(1+C_c), where C_c is the creep coefficient. For M25 concrete loaded at 28 days: E_ce = 25,000/(1+1.6) = 25,000/2.6 = 9,615 MPa. This is used to calculate long-term deflections.

Q5. What is the difference between creep and shrinkage?

Creep is time-dependent deformation that occurs only under sustained load (it requires stress to exist). Shrinkage occurs due to moisture loss and chemical reactions even without any applied load. Both cause long-term deformation, but their causes, controlling factors, and design treatment are different. IS 456 accounts for them separately: creep via C_c and E_ce; shrinkage via design strain of 0.0003.

🔗 Related: Shrinkage of Concrete – Types, Causes and Control

🔗 Related: Modulus of Elasticity of Concrete – IS 456 Formula

📚 Reference: IS 456:2000 Clause 6.2.5 and Table 2 – Creep of Concrete, BIS