creep 4-05

Question 1

what are some negative and positive effects of creep?

Answer 1

Negative effects:
- increase of structure deformations
- reduction of pre-stressing level
Positive effects:
- reduction of eigenstresses, e.g. due to shrinkage
- reduction of stresses due to restraint

Question 2

what is creep?

Answer 2

the relation strain over time until irreversible damage

Question 3

creeps mechanisms:

Answer 3

• originates in hydrated cement paste (viscous flow of the cement paste caused by sliding or shear of the CSH phases)
• relates to the internal movement of absorbed or inter-crystalline water (consolidation due to seepage and redistribution of pore water under stress)
• partly results from delayed elastic deformation (restraint of elastic deformations of the skeleton
  formed by the aggregates and CSH phases by viscous cement paste)
• increases at higher stress (micro-cracking and failure of the paste-aggregate interface)

Question 4

parameters affecting creep:

Answer 4

-loading time
-age at loading
-stress
-temperature
-water content
-time
-Creep of concrete takes place in the cement paste; the aggregates creep very little, but they constrain the creep of the cement paste:
*creep ↑ with an ↑ in the cement paste content
*creep ↓ with an ↑ in the modulus of elasticity of the
aggregates
-Creep depends on the porosity of the cement paste:
*creep ↑ with an ↑ in the water-to-cement ratio
*creep ↓ with an ↑ in the degree of hydration or
concrete age
-Creep depends on the water content and the rate of the water loss:
*creep ↑ with an ↑ in the water content in concrete
*creep ↑ with an ↑ in the amount and the rate of
water loss
with ↓ of relative humidity of the environment
with ↓ in the size of the concrete member
-Creep depends on the temperature:
*creep ↑ with an ↑ in the temperature during loading
*creep ↓ with an ↑ in the temperature before loading

Question 5

Calculation of concrete strains under variable stresses from the creep behavior under constant load

Superposition principle by Boltzmann / McHenry

Answer 5

• perfectly valid for linear visco-elastic material
• fairly good accuracy as long as total strain of concrete
  does not decrease
• for a decrease in strain, an overestimation of delayed
  elastic strain is observe

Question 6

prediction and calculation methods

Answer 6

Product approach:
- easy application (exception: method by
Bazant/Panula)
- contradicting predictions for decreasing load/stress

Summation approach:
- consistent accurate prediction for load relief
- underestimation of creep deformation for loading at higher ages