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Permeability of Concrete | Factors Affecting Permeability

Permeability of Concrete

Permeability of Concrete means the ability of water (liquid) to pass out from the concrete. Best concrete is non permeable

About 38 percent water by weight of cement paste is required to fill the gel pores for complete hydration of cement

If water is used more than 38 percent, the excess water will cause undesirable capillary cavity and concrete convert into porous. Porous cement concrete has a higher permeability

The pores in cement paste/slurry consist of gel pores and capillary pores. The pores in cement concrete because of incomplete compaction of concrete so that voids of larger size which gives a honeycomb structure alternatively low strength of concrete, are not considered here. 

The gel is porous to the extent of 28 percent. But, the gel pores are so small that hardly any water can pass through under normal conditions.

The permeability of gel pores is about 7 x 10^(-16) m/s which is about 1/100 of that of paste. Hence the gel pores do not contribute to the permeability of cement paste

The capillary pores constitute about 0 to 40 percent of the paste volume. They are too large that the gel pores. It follows that the permeability of cement paste is controlled by its capillary porosity

Importance of Permeability

  • In reinforced cement concrete, the entrance of water and air will result in corrosion of steel reinforcement leading to expansion
  • If concrete becomes saturated with water due to permeability, it is more vulnerable to frost action
  • The penetration of deleterious materials in solution may adversely affect the durability of concrete, for example, Ca(OH2) leaches out and aggressive liquids attack the concrete
  • The permeability is very most important in any case of liquid retaining structures like water dams and tanks where water – tightness is essential

Factors Affecting Permeability

Various factors permeability of concrete but I list out main factors which are:
  1. Water/cement ratio
  2. Properties of cement
  3. Aggregate
  4. Absorption and homogeneity of the concrete
  5. Curing
  6. Use of admixture
  7. Age of concrete

Water/cement ratio

  • Permeability is proportional to the water/cement ratio
  • For the cement paste hydrated to the same degree, the permeability is higher with a higher w/c ratio or higher cement content

Properties of Cement

  • The properties of cement also affect the permeability of concrete
  • Coarse cement tends to produce a paste with higher porosity than a finer cement, for the same w/c rato
  • In general, if the strength of cement paste is higher than the permeability of concrete is lower

Aggregate

  • The permeability of aggregate also affects the behaviour of the concrete
  • If the aggregate has an extremely low permeability than it also helps to reduce the permeability because the presence of aggregate creates an obstacle to the flow of water
  • Further, since the flow path has to circumvent the aggregate particles, the effective path becomes significantly longer and the permeability decreases.
  • For the same water/cement ration, the large size of aggregate greater is permeability. This is because of the relatively large voids. Well graded aggregate decrease the permeability

Absorption and Homogeneity of The Concrete

  • Absorption is measure by the volume of pore space in concrete
  • Absorption is a physical process in which concrete draws water quantity into its pores voids and capillaries
  • Absorption depends on the structural composition of the concrete. Non-homogeneity affects permeability
  • The absorption increase in concrete due to cracks in the structure, voids available in concrete due to segregation or honeycombs
  • The permeability can be reduced by a workable mix so that segregation is eliminated

Curing

  • Continuative hydration of the cement paste results in a decrease in the size of the voids which help in decreases the permeability
  • Proper curing of concrete decreases also help to reduce the permeability of concrete
  • Permeability of steam cured concrete of normally  higher than that of wet-cured concrete structure element

Use of admixtures

  • The use of waterproofing admixture decreases the permeability of lean mixes
  • Normally the use of extra cement will be more effective in reducing the permeability because during the casing formwork is no total waterproof so that cement paste is gone outside. I provide so many tips for civil engineer in my every article so read all of them
  • In the case of porous concrete, you can use surface treatment to decrease permeability

Age of concrete

  • The age of concrete also affects the permeability of concrete
  • In the green concrete/fresh concrete paste the water flow is controlled by the size, shape and concentration of the original cement grains
  • With the process of hydration, the permeability decreases quickly because the gross volume of gel is approximately 2.1 times the volume of the unhydrated cement
  • Gel gradually fill the original water-filled space
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