Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 96

CSH needle

Meso gel pore
Micro gel pore

Meso gel pore

CSH needle

Capillary pore Capillary

pore

Cement paste
particle

(a) (b)

Figure 5.1 (a) C 3 S after 21 days curing time, CSH-needles with diameter approximately
5 nm. (From Espinosa, R. M., and Franke, L., Cement Concrete Res ., 36, 1956-
1970, 2006b.) (b) Scheme of the pore structure of hardened cement paste.
(From Jennings, H.M., Cement Concrete Res ., 36, 101-116, 2000.)

5.3.1 Porosity
Porosity is an important characteristic of concrete that influences many
aspects of its behaviour: mechanical properties, transport properties, and
durability. For cementitious materials, Espinosa and Franke (2006a) define
pores with hydraulic radii as follows (Figure 5.1):

• Micro-gel pores, <1.0 nm
• Meso-gel pores, 1.0-25 nm
• Micro-capillary pores/meso-capillary pores, 25-50 nm
• Macro-capillary pores, 50 nm-1 μm

The simplest measure of porosity is the total voids content as estimated
by the ASTM C642 Volume of Permeable Voids test. More sophisticated
techniques measure pore size distribution. Mercury intrusion porosimetry
(MIP) has an advantage over other testing techniques, such as a capillary
condensation in being able to measure pore radii ranging from a few
nanometers to several hundred micrometers (Diamond, 1971). Mercury is
a nonwetting liquid for cementitious materials and consequently it has to
be forced into the pores. Pore size and volume quantification are calculated
from the pressure required to force the mercury into the sample. Although a
valuable tool, Diamond (2000) suggested that it was inappropriate method
for the absolute measurement of pore size distributions in “cement-based
materials primarily because of the so-called” ink-bottle effect. This refers
to a larger void, which is accessed through a smaller pore.

5.3.2 Permeability
The flow of water (with or without chloride ions) caused by a pressure
head is water permeability. In this transport mechanism flow through the
capillary system is assumed to be laminar (i.e., a steady-state condition