Permanent dimensional changes of concrete under sustained loading are known as:
Correct Answer: C. Creep
📚 Detailed Explanation: Creep — Time-Dependent Permanent Deformation Under Load
Why C (Creep) is correct: Creep is defined as the time-dependent, inelastic deformation of concrete under sustained (constant) load. It is permanent — only a fraction recovers after unloading (called creep recovery). Unlike shrinkage (which occurs without loading) and elastic deformation (which is immediate), creep is specifically the accumulation of strain over months and years under a constant applied stress.
Types of Deformation in Concrete
| Type | Definition | Cause | Recoverable? |
|---|---|---|---|
| Elastic deformation | Immediate strain on loading; follows Hooke's Law | Instantaneous application of load | Yes (fully, on unloading) |
| Creep | Time-dependent increase in strain under constant load | Sustained (constant) load over time | Partially (creep recovery ≈ 50% of total creep) |
| Drying shrinkage | Volume reduction as concrete dries | Evaporation of water; no load required | Partially reversible (swells when wetted) |
| Settlement | Vertical movement of structure into soil | Inadequate foundation; soil consolidation | No |
Creep Mechanism in Concrete
| Mechanism | Detail |
|---|---|
| Gel water movement | Adsorbed water in C-S-H gel migrates under sustained stress; creates irreversible flow |
| Microcracking | Sustained stress propagates microcracks slowly; adds to deformation |
| Viscous flow | Cement paste behaves as a viscoelastic material; deforms slowly under load |
Creep Coefficient (IS 456:2000):
φ = creep strain / elastic strain
φ = creep strain / elastic strain
Age at loading Creep Coefficient
7 days 2.2
28 days 1.6
1 year 1.1
Total strain = elastic + creep = εe (1 + φ)
Example: if elastic strain = 500 microstrain at 28-day loading:
Total strain = 500 x (1 + 1.6) = 500 x 2.6 = 1300 microstrain
- Creep = permanent time-dependent deformation under sustained load.
- Creep is highest in the paste phase; aggregate is essentially creep-inert.
- Creep is important in prestressed concrete design: it causes loss of prestress over time.