What is creep in concrete and how does it affect RCC structures?

Short Answer:

Creep in concrete is the slow and continuous deformation or strain that occurs in concrete when it is subjected to a constant load over a long period. Even if the load remains unchanged, concrete gradually shortens or compresses due to internal movement of moisture and microstructure changes.

In RCC (Reinforced Cement Concrete) structures, creep can lead to deflection of beams, sagging of slabs, or loss of pre-stress, especially in long-span structures. Though not sudden or dangerous, if not considered in design, creep may cause serviceability problems like cracks, tilting, or deformation over time.

Detailed Explanation:

Creep in concrete and how it affects RCC structures

Concrete is a time-dependent material, which means it behaves differently under long-term loading compared to short-term loads. One of the most important long-term behaviors is creep, which is the gradual and permanent deformation of concrete under sustained stress. Unlike immediate elastic deformation, creep progresses slowly and continues for months or even years after the load is applied.

This phenomenon must be understood and accounted for in RCC design, particularly in beams, slabs, columns, and prestressed members, as it can affect their shape, function, and long-term durability.

What is Creep in Concrete

Creep is defined as the increase in strain (deformation) in a material under constant stress over time. In concrete, this means a beam or column will gradually bend or shorten if it is subjected to a continuous load, even if that load is below the failure point.

Creep occurs due to:

  • Movement of water within the cement paste (gel pores).
  • Rearrangement of calcium silicate hydrate (C-S-H) particles.
  • Microcracking and internal slipping of particles.

It is more prominent when:

  • The concrete is young (not fully cured).
  • The environment is hot and humid.
  • The concrete has high water-cement ratio or poor quality.
  • The structure is under sustained load for long durations.

How Creep Affects RCC Structures

Creep can have the following effects on RCC structures:

  1. Deflection Increase in Beams and Slabs
  • Over time, beams and slabs under load will bend more than expected.
  • This can cause problems with appearance, ponding of water, and even stress in connected elements like walls.
  1. Column Shortening
  • In high-rise buildings, columns creep under sustained axial loads.
  • Uneven shortening leads to differential settlement, causing cracks in floors and walls.
  1. Loss of Prestress
  • In prestressed concrete, creep causes reduction in prestressing force, affecting the performance of the member.
  1. Crack Formation
  • Though creep itself doesn’t cause cracks, it adds to the stress and strain in concrete.
  • Combined with shrinkage and temperature changes, creep can lead to service cracks.
  1. Loss of Alignment and Level
  • In long-span bridges and cantilever structures, creep may lead to tilting or misalignment over years.

How to Minimize or Control Creep

  • Use low water-cement ratio concrete for better strength.
  • Proper curing to ensure hydration is complete before loading.
  • Avoid placing sustained loads early during construction.
  • Use admixtures and mineral additives like fly ash or silica fume to refine concrete microstructure.
  • Ensure balanced reinforcement to distribute stresses evenly.

Design codes like IS 456:2000 and other international standards provide formulas and factors to calculate creep strain and include it in design limits.

Conclusion:

Creep in concrete is the slow deformation that happens under constant load over time. In RCC structures, it can lead to increased deflections, uneven column shortening, and long-term service problems if not properly considered. By understanding its causes and including its effects in design, engineers can ensure safe, durable, and long-lasting RCC buildings and structures.