3.2. Mechanism

Creep is the time-dependent increase in strain under sustained load. It occurs even when the stress level is constant, particularly in compression.

Creep refers to the gradual deformation of concrete over time when a constant load is applied. Unlike immediate deformation that happens at the moment a load is applied, creep happens over a longer duration, indicating a slow, progressive change in shape or size. This phenomenon is especially noticeable under compressive loads, which means when the concrete is being squeezed or compressed.

Imagine a rubber band that you stretch and hold in that position. Initially, it stays stable, but over time, the rubber band may become longer and thinner due to the extended tension. Similarly, concrete can gradually deform under a continuous load, which can impact the structural performance of a building over time.

• Caused by viscous flow and microcrack development in the cement paste. • Influenced by water content, temperature, humidity, and loading history.

The mechanism of creep involves two main processes: viscous flow, where the cement paste slowly deforms under stress, and the development of microcracks, which are tiny fissures that form within the concrete. These processes are affected by several factors, including the amount of water in the mix, the surrounding temperature, the humidity level, and how the concrete was loaded initially. All these variables can increase or decrease the amount of creep that occurs.

Think of a frozen pop. When you leave it out on a warm day, the ice begins to melt and changes shape. The rate at which it melts can be influenced by how warm it is outside and how much sunlight it’s receiving. In a similar way, the conditions in which concrete is set and loaded determine how much and how quickly it creeps over time.

1. Instantaneous Strain: Immediate deformation upon loading. 2. Primary Creep: Rapid strain increase immediately after loading. 3. Secondary Creep: Slower, steady strain development. 4. Tertiary Creep: Accelerated strain leading to failure (rare in structures).

Creep evolves through four distinct stages. The first stage, instantaneous strain, occurs the moment the concrete is loaded; this is where the concrete deforms immediately. Following this, primary creep occurs, where the deformation increases rapidly shortly after the load is applied. Then, secondary creep takes over, characterized by a much slower, steadier rate of deformation. Finally, in rare cases, the tertiary creep stage can lead to accelerated deformation and eventual failure. Each of these stages is important for understanding how concrete will behave over time under continued stress.

Consider a sponge that you press hard with your hand. When you first press down, it squishes immediately (instantaneous strain), then as you hold it there, it slowly compresses even more (primary creep). After a while, the sponge doesn't change as quickly but still is compressing at a steady rate (secondary creep). If you push too hard for too long, it might finally break or tear (tertiary creep). This progression shows how concrete also behaves under pressure over time.

• Stress level: Higher stress = higher creep. • Age of concrete: Younger concrete creeps more. • Humidity: Lower humidity = higher creep. • Temperature: Higher temperature accelerates creep. • Mix design: Higher paste content = more creep.

Several factors impact the amount of creep in concrete. The level of stress applied directly relates to the amount of creep; more stress results in more creep. Additionally, younger concrete tends to creep more as it hasn’t fully hardened yet. Humidity levels also play a role; lower humidity can increase creep because dry conditions tend to affect the hydration of the cement. Moreover, temperature is crucial; higher temperatures can accelerate the creep process. Lastly, the mix design affects creep, where a higher amount of cement paste in the mix leads to increased creep due to more viscous materials being present.

Think about how a wet sponge changes under different conditions. A freshly made sponge (young concrete) will absorb more water and deform easily compared to an old, dry sponge. If you stretch that fresh sponge in a warm room (high temperature), it will deform even more than if it were in a cool, humid environment. Similarly, concrete behaves differently based on its age, conditions, and how it was mixed.

• Loss of prestress in prestressed concrete. • Deflection in beams and slabs over time. • Redistribution of internal stresses. • Stress relaxation in statically indeterminate structures.

Creep has several consequences on concrete structures. It can lead to a loss of prestress in prestressed concrete, which is intended to counter act tensile forces. Over time, beams and slabs can also deflect under permanent loads, changing the shape of the structure. This can cause a redistribution of internal stresses, potentially leading to further structural issues. Additionally, structures that are statically indeterminate can experience stress relaxation, where the internal forces adjust due to prolonged loading and deformation.

Imagine a suspension bridge. Initially, cables holding the bridge might be tightly pulled (prestressed). Over time, as those cables experience creep, they may not be able to support the same weight as before, causing the bridge to sag (deflection). This change alters how the load is distributed across the bridge, potentially leading to weak points or unexpected stresses that could compromise safety.

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