Skip to main content

Permeability as root cause of poor durability

Short answer

Permeability is the ease of fluid flow through concrete under pressure. It is the root cause of poor durability because most deterioration agents enter via connected pores. Materials with similar porosity can have low permeability with isolated pores or high permeability with connected pores.

Define permeability. "Permeability of concrete is often referred to as the root cause for lack of durability." Do you agree? Explain, and show with a schematic representation materials having similar porosity but different (low or high) permeability.

Definition

Permeability is the ease with which a fluid (water, gas, dissolved ions) can flow through concrete under a pressure difference. By Darcy's law for saturated flow,

qA=K ΔhL\frac{q}{A} = K\,\frac{\Delta h}{L}

where KK is the coefficient of permeability (m/s), Δh\Delta h is the head and LL the thickness. Dense concrete has KK of 10−1210^{-12} to 10−1010^{-10} m/s.

Agree: permeability is the root cause of lack of durability

Almost every cause of deterioration needs a fluid to move into the concrete:

  • Water carries sulphates, chlorides and acids inside (chemical attack, corrosion).
  • Oxygen and CO2_2 gas penetration cause corrosion and carbonation.
  • Water in pores freezes and causes freeze-thaw damage; it also supports alkali-silica reaction and leaching.

If the permeability is low, these agents enter very slowly, so the concrete lasts longer. Permeability depends on w/c, curing, compaction, cement and microcracks. Hence IS 456 limits the w/c ratio and requires a minimum cement content (Table 5) and proper curing (Cl. 13.5).

Porosity and permeability are different

Porosity is the total volume of voids; permeability depends on whether the pores are connected and how large they are.

 Low permeability           High permeability
 (isolated pores)           (connected pores)
 +----------------+         +----------------+
 | o   o    o   o |         | o==o==o   o=o  |
 |   o   o   o    |         |  \\  ||  //   |
 | o    o    o  o |         | o=o=o=o===o    |
 +----------------+         +----------------+
 same porosity (volume of pores), flow differs

For example, lightweight foamed concrete has high porosity but low permeability if its pores are discrete. A paste with continuous capillary pores (w/c above 0.7) has similar porosity but is highly permeable.

✓ Answer checked by Claude Sonnet 5.5

Related questions

All Concrete Technology and Masonry Structure chapter 6 questions →