1.5. Measurement of Creep

• A creep test is conducted using:
  • A creep frame with dead load.
  • Dial gauges or LVDTs to record strain.
  • Companion specimens without load to isolate shrinkage from total deformation.

To measure creep in concrete, a dedicated setup is needed. This includes a creep frame which holds a weight that applies stress to the concrete specimen. The strain, or deformation, that occurs over time is monitored using instruments such as dial gauges or Linear Variable Differential Transformers (LVDTs). These tools help in recording the exact amount of deformation. Additionally, companion specimens that are not subjected to any load are tested to separate the effects of shrinkage from those of creep, ensuring more accurate results.

Think of a sponge that swells when wet. If you apply a heavy object on it, it deforms over time, much like concrete under stress. However, to understand how much of its change is due to being squished (creep) versus just losing water (shrinkage), you can compare it with another sponge that doesn't have any weight on it. This setup in concrete testing helps achieve that clarity.

• The Creep Coefficient is a commonly used parameter:
  • Creep Strain
  • ϕ = Elastic Strain at loading
  • Typical values:
    • Normal concrete: φ = 1.5 to 2.5
    • High-strength concrete: φ = 0.8 to 1.5.

The Creep Coefficient (φ) is a critical parameter that quantifies creep. It is defined as the ratio of creep strain to the elastic strain that occurred when the load was first applied. For typical concrete, this coefficient ranges from 1.5 to 2.5, indicating how much deformation occurs in response to stress over time compared to the initial elastic deformation. In contrast, high-strength concrete, which is denser and often less susceptible to creep, shows a lower creep coefficient, ranging from 0.8 to 1.5.

Imagine stretching a rubber band. The immediate stretch can be compared to elastic strain, while the gradual extension over time, as it remains under tension, represents creep. If you were to measure how much longer the rubber band stretches after being held for a while versus how much it stretched instantly, you would get a coefficient akin to the creep coefficient in concrete testing. This helps engineers understand how structures might deform over the long term.

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