Shrinkage of Concrete

Concrete is subjected to changes in volume either autogenous or induced. Volume change is one of the most detrimental properties of concrete, which affects the long-term strength and durability. It causes unsightly cracks in concrete and called concrete shrinkage.

Types of Shrinkage

• Plastic shrinkage in concrete
• Drying shrinkage in concrete
• Autogenous shrinkage in concrete
• Carbonation shrinkage of concrete

Plastic Shrinkage

• Cracks appear in the surface of fresh concrete soon after it is placed mostly or horizontal surfaces.
• Plastic shrinkage cracking is more likely to occur when high evaporation rates causes the concrete surface to dry out before it has set.

Drying Shrinkage

Shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water. This shrinkage causes an increases in tensile stress, which may lead to cracking, internal warping, and external deflection, before the concrete is subjected to any kind of loading.

Autogenous Shrinkage

Chemical shrinkage (lower volume of hydrates than cement and water) + self-desiccation (reduction in the pore water due to hydration).

Carbonation Shrinkage

Volume reduction due to the reaction of hydrated cement paste with CO₂ in the presence of moisture.

Factors affecting shrinkage are

Aggregate- Concrete with higher aggregate content exhibits smaller shrinkage. Concrete with aggregates of higher modulus of elasticity or of rougher surfaces is more resistant to the shrinkage process.

Water-Cement Ratio- The higher the W/C ratio is, the higher the shrinkage. As W/C increases, paste strength and stiffness decrease; and as water content increases, shrinkage potential increases.

Shrinkage Allowance

• Almost all cast metals shrink or contract volumetrically after solidification and therefore to obtain a particular sized casting, the pattern is made oversize by an amount equal to that of shrinkage or contraction.
• Different metals shrink at different rates because shrinkage is the property of the cast metal or alloy.
• Cast iron poured at higher temperatures will shrink more than that poured at lower temperature.
• Wood patterns used to make metallic patterns are given double allowance; one for the shrinkage of the metal of the pattern and the other for that of metal to be cast.

Factors that control shrinkage

• Construction: A tighter fabric construction reduces potential shrinkage.
• Yarn Twist: Optimum twist (based on yarn size) is very important for controlling shrinkage and torque.
• Type of Weave or Knit: Pain weave of jersey knit show more resistance to shrinkage than other types.
• Tension During Sewing of Garments: Uneven or too much sewing tension can lead to differential shrinkage causing puckering in the seam areas.
• Stability of Fiber and Yarns: Improper stabilization could lead to excessive shrinkage especially in blends where synthetic fiber shrinks differentially than the cellulosic fibers.