1.3. Types of Creep

1. Basic Creep: Occurs when concrete is loaded in a sealed environment without moisture exchange.

Basic creep refers to the type of creep that occurs when concrete is placed under a constant load without any exchange of moisture in a sealed environment. In this situation, as the stress remains constant over time, the concrete continues to deform, exhibiting a gradual increase in strain due to the viscoelastic nature of the material. This means that the concrete deforms slowly under a sustained load, even in the absence of moisture movement.

Imagine a sponge that has been squeezed firmly and then placed in a sealed bag. Even though you don't apply any more pressure, the sponge will continue to compress slightly over time because the moisture can't escape. Similarly, concrete under basic creep will deform gradually as time goes on, despite the load remaining unchanged.

2. Drying Creep (Pickett Effect): Occurs when concrete loses moisture under load. Drying enhances creep significantly.

Drying creep, also known as the Pickett effect, happens when concrete is subjected to load and simultaneously loses moisture. This type of creep is significantly enhanced due to the drying of the concrete, which affects its internal structure and increases the strain experienced. The loss of moisture can cause further deformation, as the material changes due to the changes in moisture content.

Think of a wet towel that you hang to dry. As the towel dries out, it shrinks. If you were to keep pulling on it while it's drying, it would stretch and might not return to its original size. In this analogy, the towel represents the concrete, and the moisture loss coupled with the load leads to increased deformation, just like drying creep increases strain in concrete.

3. Autogenous Creep: Happens even in the absence of external moisture movement, especially significant in high-strength concrete due to internal chemical shrinkage.

Autogenous creep occurs independently of external moisture conditions and happens due to chemical reactions within the concrete itself. Particularly in high-strength concrete, this type of creep is caused by internal changes, such as hydration reactions that lead to internal shrinkage. Unlike basic or drying creep, autogenous creep does not require moisture loss from the surface; instead, it is driven by the ongoing chemical processes within the concrete material.

Consider a balloon that continues to lose air slowly, leading it to shrink even when it is not exposed to outside air. In this case, the balloon represents high-strength concrete, where internal processes are causing it to shrink over time. Autogenous creep highlights how changes within the material itself can lead to deformation, even when external conditions appear stable.

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