3. Creep in Hardened Concrete

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 how concrete deforms over time when a constant load is applied. Unlike immediate deformation that happens when something is loaded, creep happens gradually. For example, if a heavy weight is placed on concrete, it will instantly compress slightly. However, over days or even months, the concrete may continue to deform slowly. This effect is particularly significant when the concrete is under compression, where it tends to sag or settle.

Think of a wet sponge being pressed down. Initially, when you press down, it squashes and absorbs the pressure. But if you keep pressing, it continues to deform gradually. Similarly, concrete under a constant load will keep deforming, leading to what we call creep.

Creep is caused by viscous flow and microcrack development in the cement paste. It is influenced by water content, temperature, humidity, and loading history.

The mechanism of creep involves two key processes: viscous flow and the development of microcracks in the cement paste. Viscous flow refers to the gradual movement of materials that can deform under sustained stress. In addition, as concrete ages and is subject to load, tiny cracks, known as microcracks, can form within the material. These are often exacerbated by environmental factors such as temperature and humidity, where higher temperatures or lower humidity levels will lead to more pronounced creep effects.

Imagine a plastic bag filled with honey. If you press down on it, at first, it won’t change much, but if you keep applying pressure, the honey will slowly ooze out. This is similar to how the internal structure of concrete can slightly deform over time when a load is continuously applied.

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).

The process of creep can be categorized into four distinct stages:

1. Instantaneous Strain is the immediate deformation that occurs as soon as the load is applied, such as when you place a book on a table.
2. Primary Creep refers to the quick increase in strain that occurs right after loading, where the material deforms significantly in a short period.
3. Secondary Creep happens over a longer duration, showing a slower, steady increase in strain that can persist for a long time under constant load.
4. Tertiary Creep is when the strain accelerates significantly and can lead to failure, although this is less common in practical structures.

You can think of these stages like a person settling into a couch. When they first sit down, the couch compresses immediately (instantaneous strain). As they continue sitting, they start to sink a bit more into the cushions (primary creep). After a while, they may gradually sink into the couch further (secondary creep). If they continue relaxing too long without adjusting, the cushions might get overly compressed and lose support (tertiary creep).

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

Several important factors can influence the amount of creep in concrete:

1. Stress Level: The greater the load on the concrete, the more it will creep.
2. Age of Concrete: Freshly poured concrete will exhibit more creep compared to older, more established concrete because it has not fully cured.
3. Humidity: When environmental conditions are drier, concrete tends to creep more because moisture loss increases the material’s susceptibility to deformation.
4. Temperature: Higher temperatures can speed up the processes that lead to creep, causing more deformation to occur.
5. Mix Design: Concrete with a higher paste content (more cement in relation to aggregates) tends to creep more due to less rigid material properties.

Consider baking bread. If you put it in a very warm oven (high temperature), it will rise (creep faster) compared to when it’s in a cooler spot. Similarly, if you use a lot of flour and less water in your dough mix (higher paste content), it becomes stiffer, just as a concrete mix with less water and more cement can creep more under pressure.

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

The effects of creep can lead to several structural issues:

1. Loss of Prestress: In prestressed concrete, creep can reduce the effectiveness of the prestressing, which can compromise its structural integrity.
2. Deflection Over Time: Beams and slabs can sag or deflect due to sustained loads, especially noticeable over long periods.
3. Redistribution of Internal Stresses: Creep can alter the internal stress distribution within the concrete, leading to unexpected stress concentrations and potential failure points.
4. Stress Relaxation: In structurally complex items, such as those that are statically indeterminate, creep can lead to a relaxation of stress, causing further misalignment or instability.

Imagine an elastic band stretched tightly. Over time, it may slacken, losing some tension (loss of prestress). Items hanging on it might droop lower as a result (deflection). If you pull that band too hard and for too long, the tension changes within the band, leading to points where it might unexpectedly snap (redistribution of internal stresses). This concept can be easily visualized in larger structures subjected to weight over extended periods.

--