9.3. Creep of Concrete

Gradual, time-dependent increase in strain under sustained load.

Creep refers to the slow, continuous deformation of concrete when it is subjected to a constant load over time. Unlike immediate deformation which happens right after a load is applied, creep involves changes that occur gradually. This means that even if the load remains constant, the material may still deform slightly day after day or over months or years, leading to increased strain.

Imagine a sponge being pressed down with a weight. Initially, it squishes down quickly, but if you keep pressing down for a long time, the sponge will slowly compress even further over time. Similarly, concrete behaves like the sponge under sustained load.

Due to viscous flow in the cement gel and micro-cracking at the interfacial transition zone (ITZ).

Creep occurs primarily due to two main mechanisms. First, 'viscous flow' in the cement gel allows for gradual movement of the particles within the cement matrix, which contributes to the material stretching over time. Second, micro-cracking occurs at the interfacial transition zone (ITZ), which is the area where the cement paste meets the aggregates. These small cracks gradually propagate under sustained loads, leading to increased deformation.

Think of a rubber band that you pull and hold constantly. Over time, it stretches out slightly even more than right at the moment you first started pulling. The inner structure of the rubber band changes slightly, allowing more stretch – much like how the microstructure of concrete changes under constant stress.

• Water-cement ratio
• Aggregate type
• Age at loading
• Humidity and temperature
• Stress level

Several factors influence the amount of creep that occurs in concrete. The water-cement ratio is crucial; a higher ratio generally leads to more creep because it creates a more 'liquid' mix that can deform more easily. The type of aggregates used also plays a role; some aggregate types might be more resistant to creep than others. The age at which the concrete is loaded affects its creep behavior; younger concrete is typically more susceptible. Additionally, environmental conditions such as humidity and temperature can either increase or decrease creep tendencies. Finally, the level of stress applied – higher levels can lead to more rapid creep.

Consider playing with a silly putty that changes based on how you handle it. If you add more water to it, it becomes squishier (higher water-cement ratio), and if you leave it in a warm room, it might soften and stretch more. Likewise, the conditions and materials used in concrete affect how much it creeps over time.

• Loss of pre-stress in PSC structures
• Increase in long-term deflections
• Redistribution of internal stresses

The implications of creep can be significant for structural integrity. For pre-stressed concrete (PSC) structures, creep can lead to a loss of pre-stress, which is the tension applied to the concrete before service loads are applied. This may reduce the efficiency of the structure. Creep can also lead to increased deflections over time, which can affect the visibility and function of the structure. Additionally, internal stresses can get redistributed within the material, potentially causing issues like cracking if the stresses exceed the material's strength.

Imagine a bridge that was initially perfectly level and straight. Over time, as it creeps under its weight and traffic loads, it might start to sag or bend slightly. If significant enough, this could cause problems for vehicles crossing it or even impact structural safety, much like how the creep in concrete alters its performance over time.

--