Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 98

on the surface. The process can happen very gently without having a great
deal of effect on the concrete properties. If bleeding is severe the rising
water tends to leave well-defined capillary passages and it is then known as
channel bleeding. Water penetration of the hardened concrete is obviously
greatly facilitated by both the vertical channels and the voids formed under
the coarse aggregate and even fine aggregate particles. It is important to
note that concrete does not become “impermeable”, as commonly thought,
when the capillaries become discontinuous. Discontinuity is a change in
the rate of increase in permeability with an increase in capillary porosity as
shown in Figure 5.2 (Nokken, 2004).
Reduction of permeability can be effected either by avoiding bleeding in
the first place or by blocking the channels after formation. Pore blocking
after they have formed takes place as cement continues to hydrate and
extends gel formation into the pores. This requires the concrete to be well
cured and is greatly affected by w/c ratio. Curing is much more critical for
permeability than it is for strength. Another means is to line the pores in the
concrete with hydrophobic or pore-blocking material. Such materials are
marketed as water resisting admixtures. Hydrophobic materials generally
reduce sorptivity more than permeability. Some hydrophobic material may
provide an initial benefit but lose its effectiveness in the longer term and so
proven materials must be used.
Studies on the effect of drying on permeability by Powers et al. (1954),
Vuorinen (1985), and Hearn (1998) showed an increase in the permeability

1.E-05

1.E-07

1.00

Continuous
Capillary Pores

1.E-09

0.90

1.E-11 0.80

0.64

0.60

0.55

0.50

1.E-13

0.71

Discontinuous
Capillary Pores

1.E-15

1.E-17
0.00 0.05 0.10 0.15 0.20 0.25

Capillary Porosity (as fraction of total volume)

0.30 0.35 0.40 0.45

Equation 2-14
Relationship at 18c

Powers et al. (1959a) data
0.64 with time

0.71 with time

Figure 5.2 Change in permeability due to discontinuity of capillary pores. (After
Nokken, M. R., Development of Discontinuous Capillary Porosity in
Concrete and Its Influence on Durability, PhD thesis, University of
Toronto, Ontario, Canada, 2004.)