3.5. Effects of Creep

Today, we'll dive into the concept of creep in hardened concrete. Creep is defined as the time-dependent increase in strain when load is sustained over time. It’s crucial to understand how this affects our structures.

What exactly causes this creep in concrete?

Great question! Creep occurs due to viscous flow within the cement paste and can be influenced by factors like humidity, water content, temperature, and loading history. This means that the conditions around your concrete can change how much it creeps!

So, is it that higher stress leads to higher creep? How does that work?

Exactly! Higher stress increases the rate of creep due to the material’s response to sustained load. Think of it like a sponge: the more you press down, the more it deforms!

What about the age of the concrete? Does it really matter?

Yes, the age does matter! Younger concrete tends to creep more because it has not achieved full strength. The cement particles are still reacting, and the structure is more malleable.

To summarize, creep is a gradual strain increase under constant load and is affected by stress level, age, temperature, and humidity.

Let’s explore the stages of creep. There are four stages: instantaneous strain, primary creep, secondary creep, and tertiary creep. Can anyone explain what happens in these stages?

I think the instantaneous strain is the immediate deformation when a load is applied?

Exactly! That’s the first stage. After that, we have primary creep where there's rapid strain increase.

And what follows that?

Next is secondary creep, which develops at a slower rate over time, leading to gradual and steady change. Tertiary creep is where the strain begins to accelerate, which can ultimately lead to failure, although we hope to design structures to avoid this stage.

Can you give an example of tertiary creep in real structures?

In well-designed structures, this stage is rare. However, it might occur in poorly maintained structures under constant heavy loads. It’s something we always try to mitigate!

To summarize, we discussed the stages of creep, highlighting instantaneous strain, primary creep, secondary creep, and the rarely occurring tertiary creep.

Now, let's focus on the effects of creep. Creep can have significant implications for prestressed concrete, especially. Can anyone tell me how?

I believe it can cause a loss of prestress, right?

Exactly! As creep progresses, it can lead to loss of prestress, causing potential performance issues in structures intended to be robust!

What about deflection? Does creep affect that?

Yes, indeed! It can lead to increased deflection over time, which is crucial in considering the serviceability and safety of beams and slabs.

So, how do engineers usually manage these issues?

Engineers factor in creep by designing with sufficient safety margins and considering long-term effects during analysis and design stages.

In conclusion, the effects of creep include loss of prestress, deflection in structural members over time, and a redistribution of internal stresses that must be managed in design.