Permeability is a measure of how easy it
is for water, air and other substances such
as chloride ions to enter concrete.
Concrete contains pores that allow these
substances to enter. Larger pores allow
easier entry, while smaller pores decrease
the rate at which these substances enter
A common way to measure permeability
of concrete is standard test method
ASTM C1202 "Electrical Indication of
Concrete's Ability to Resist Chloride Ion
Penetration," also known as the rapid
chloride permeability test. This method
is the most accepted test to determine the
relative permeability of concrete. A 60 V
electrical potential is established across
a sawed four inch diameter concrete
cylinder section. The total current passing
through the section over time and
measured in Coulombs is reported as
an indication of permeability. Lower
Coulomb values indicate lower permeability.
The permeability required is a
function of the concrete application.
Permeability and Chloride Induced Corrosion
Low permeability concrete can help
reduce the potential for reinforcing steel
to corrode when exposed to chlorides by
limiting the permeation of those chlorides
into concrete. When reinforcing
steel comes into contact with plastic concrete,
a chemical reaction occurs between
the steel and the concrete that causes a
protective layer (a passive layer) to
develop around the reinforcing steel.
This passive layer protects against corrosion
of the reinforcing steel. If the concrete
is exposed to de-icing salts, these
salts can migrate down to the reinforcing
steel through small pores in the concrete.
Over time, the chlorides in these salts can
react with the reinforcing steel, breaking
down the passive layer and causing the
steel to corrode. When reinforcing steel
in concrete corrodes, the product of that
corrosion (rust) takes up more volume
than the original steel, and causes the
concrete to crack. After cracks develop,
the deterioration accelerates as chlorides
are allowed easy ingress to the reinforcing
steel. The deterioration of both the
concrete and reinforcing steel compromises
the integrity of the structure.
Reducing Permeability with Slag Cement
When portland cement hydrates, it forms
calcium-silicate hydrate gel (CSH) and
calcium hydroxide (Ca(OH)2). CSH is
the "glue" that provides strength and
holds the concrete together. Permeability
is related to the proportion of CSH to
Ca(OH)2 in the cement paste. The higher
the proportion of CSH to Ca(OH)2, the
lower the permeability of the concrete.
When slag cement is used as part of the
cementitious material in a concrete
mixture, it reacts with Ca(OH)2 to form
additional CSH, which in turn lowers the
permeability of the concrete. Generally,
the higher the percentage of slag cement
in a concrete mixture, the lower the
permeability of the concrete. Concrete
with lower permeability can generally
be achieved by substituting between
25 to 65 percent slag cement for portland
cement. Figure 2 shows the ability of a
specific slag cement to reduce the permeability
of concrete as measured by the
rapid chloride permeability test.
- ASTM C1202-97 Standard Test Method for Electrical Indication of Concrete's Ability to Resist Chloride
Ion Penetration, American Society for Testing and Materials, West Conshohocken, PA 2001.
- Fernandez, J and Malhotra, V. M., "Mechanical Properties, Abrasion Resistance, and Chloride
Permeability of Concrete Incorporating Granulated Blast-Furnace Slag," Cement, Concrete and
Aggregates, CCAGDP, Vol. 12, No. 2, Winter 1990.