Creep

Creep resistance

Creep is a “time-dependent” deformation of a material under a constant applied load, below its yield strength. The rate of deformation depends on the material properties, exposure time, exposure temperature and the applied structural load. Creep is often responsible for excessive deflection at service loads which reduce the performance of the material and might result to cracking. The prediction of both the magnitude and rate of creep strain is an important requirement of the design process. Moderate creep can be sometimes welcome because it relieves tensile stresses that might otherwise lead to cracking.
Creep testing is run by loading and unloading the specimen under  constant load, below yield strength. The results are plotted as stress vs strain or strain vs. time charts.

Facade panels have long life span, so minimum creep rate is the most important material consideration. In facade panels we have 2 types of constant load, the wind load and the panels own weight. Deformation in the first 20 cycles is relatively big and panels will have creep deformation problems.

Conclutions-Results

  • One normal mix specimen broke during the test.
  • Normal mix appears to have the best results. Reinforcement increases the effect
  • New process doesn’t improve crepp reistance

We can’t prevent creep from happening, but we can slow the rate at which it occurs, this will increase durability and prolong the life of the structures.

  • Reduce the effect of grain boundaries (addition of fumes reduces the creep of concrete)
  • Change operating conditions
  • Accurately predict creep deformation and  integrated in the design (pre-curved or prestressed panels)
  • Change material

Stress-strain chart. Double cement specimens.

Specimen R0209. Time-Strain chart

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About Menelaos Kokkinos

Menelaos Kokkinos TU Delft mgkokkinos@gmail.com
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