shrinkage of concrete 4-04

Question 1

name the types of shrinkage:

Answer 1

-plastic shrinkage (capillary shrinkage)
-autogenous shrinkage
-drying shrinkage
-carbonation shrinkage
-chemical shrinkage
-total shrinkage

Question 2

what is plastic shrinkage ?

Answer 2

This shrinkage is manifested soon after the concrete is placed in the forms while the concrete is still in the plastic state. Plastic shrinkage is caused by loss of water by evaporation from the surface of concrete or by the absorption by aggregates or substrate.

Question 3

what is drying shrinkage?

Answer 3

Shrinkage due to the evaporation of water from hardened concrete. Typically observed when concrete is exposed to an environment with low humidity and high temperature. Caused by moisture movement that is manifested by moisture loss.

Question 4

what is autogenous shrinkage ?

Answer 4

The bulk deformation of a closed, isothermal cementitious material system not subjected to external forces.

Question 5

what is chemical shrinkage?

Answer 5

entire volume reduction due to hydration

Question 6

what is total shrinkage?

Answer 6

Shrinkage ε‘cs(t,t0) that typically incorporates both drying shrinkage component and autogenous shrinkage component εds(t,t0), i.e. εcs(t,t0)=εds(t,t0)+εas(t,t0)

Question 7

what is carbonation shrinkage ?

Answer 7

Shrinkage occurs as the various cement hydration products are carbonated in the presence of CO2. This shrinkage is probably caused by the dissolution of crystals of calcium hydroxide and its reaction to calcium carbonate. CaCO3 has a higher density
and subsequently a smaller volume than Ca(OH)2

Question 8

mechanisms of water absorption:

Answer 8

• Physical attachment of water molecules on the surface due to adsorption forces (surface tension)
• Water condensation: increasing for smaller pore diameters, increasing with rising ambient water vapor pressure
• In some cases: chemical attachment of water molecules due to attraction forces with the material of the pore walls (chemo sorption, e.g. dipole effect)

Question 9

mechanism of swelling and shrinkage:

Answer 9

1-. disjoining pressure
2-. tensile of compressive stresses in the pore walls
* decreasing relative humidity of the ambient
atmosphere
-smaller menisci radii in the capillary pores,
higher tensile stress in the water compressive
stress in the pore walls = shrinkage
* increasing RH of the ambient atmosphere
-selfsame mechanisms in reverse order = swelling

Question 10

crack formation due to plastic shrinkage

Answer 10

• Water evaporates.
• Formation of menisci between surface particles.
• Development of capillary pressure.
• Particle motion driven by various forces.
• Localization of stresses.
• „Crack formation“.

Question 11

autogenous shrinkage mechanisms

Answer 11

• shortage of free water
• decrease of the relative humidity in pores
• decrease in the menisci‘ radii of the pore water
• increasing tension within the pore water
• compressive stresses in the solid skeleton
• volume reduction of the entire system (autogenous shrinkage)

Question 12

potential materials used for water storage

Answer 12

Super Absorbent Polymers (SAP)
- most of SAP are cross-linked polyelectrolytes
- water absorption due to secondary chemical bonds
- in high-ionic solutions mostly < 20 g/g.