1.2. Mechanism of Creep

Creep primarily occurs due to the viscoelastic nature of the cement paste. The mechanism includes:

Creep happens because the cement paste in concrete behaves like a viscoelastic material. This means it has both elastic (which can return to its original shape) and viscous (which flows) properties. When a constant stress is applied over time, the material deforms and does not revert to its original shape. There are several processes contributing to this phenomenon.

Think of a sponge filled with water — if you press it down, it deforms and remains somewhat flattened. If you keep pressing it for a long time, it may never look completely round again, much like how concrete deforms under constant stress.

Moisture movement in the gel pores.

The movement of moisture within the gel pores of the concrete plays a crucial role in creep. As moisture migrates, it affects the internal structure of the cement paste. When moisture evaporates, it can lead to increased internal stresses and further deformation of the concrete over time.

Imagine a sponge again, but this time, think about how it loses water. As the water moves out, the sponge becomes less flexible, which might lead to permanent changes in its shape if it dries out too much.

Viscous flow of the hydrated cement paste.

Hydrated cement paste has a tendency to flow under long-term stress, similar to how honey flows slowly. This flow occurs within the microscopic structure of the hardened concrete, leading to gradual deformation known as creep. The ability of the paste to flow continues as long as the stress remains applied.

Picture syrup slowly pouring from a bottle. If you apply constant pressure on the bottle, the syrup would continue to flow and may not return to its original shape when the pressure is released. Concrete behaves in a somewhat similar fashion under load.

Microcracking within the transition zones.

As concrete cures and ages, tiny cracks, known as microcracks, may develop within the transition zones of the material. These microcracks contribute to the creep behavior, as they provide pathways for moisture and alter the mechanical properties of the concrete, leading to further deformation under constant stress.

Think of a road that has small cracks from the weight of vehicles; over time, these cracks can widen and lead to bigger structural issues. Similarly, microcracks in concrete can lead to more significant deformities when the concrete is under load for a long time.

Internal structural rearrangements in the calcium silicate hydrate (C-S-H) gel.

Calcium silicate hydrate (C-S-H) is the primary binding phase in cement. Over time, this phase rearranges itself under sustained loading conditions, contributing to creep. The rearrangement affects the overall stiffness and strength of the concrete, as the internal structure continuously adapts to the applied stress.

Imagine a Jenga tower that gets pushed slightly over time. The blocks might shift to maintain balance, but this new arrangement might not be as strong as the initial setup. In a way, the C-S-H structure in concrete adapts similarly under constant stress, leading to permanent changes.

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