Testing - Concrete Mix Design, Quality Control and Specification - page 174

Chloride Penetration vs. Average Absorption during We tting Cycle

100
90
80
70
60
50

y = 33.3x - 6.9
R 2 = 0.82

40
30
20
10
0

0 0.5 1 1.5 2 2.5 3

Average Absorption per 3 Day We tting Cycle (L/m 2 )

Figure 7.11 Relationship between chloride penetration and absorption. (From Aldred, J.M.,
Chloride and Water Movement in Concrete with and without Admixtures,
M.Eng. thesis, National University of Singapore, Singapore, 1999.)

for 4 days and wetting with chloride solution for 3 days. Figure 7.11 shows
the correlation between chloride penetration and the average absorption
during the 3-day wetting cycle after 8 cycles (Aldred, 1999). Obviously,
under conditions of cyclic wetting and drying, sorptivity is a dominant
transport mechanism.
This is highlighted by cores taken from concrete with and without a
hydrophobic poreblocking ingredient after more than 15 years exposure
to daily wash-down with hypersaline bore water. The control concrete
exhibited aggregate exposure and decalcification due to magnesium sul-
fate attack as well as significant chloride and sulfate penetration. The
hydrophobic concrete exhibited no surface deterioration as well as lim-
ited chloride and sulfate penetration. The key difference between these
concretes was sorptivity. The 30-minute absorption of the concretes with
and without the hydrophobic admixture were 0.4% and 2.3%, respectively.
The sorptivity values according to ASTM C1585 were 9.7 and 15.8 × 10 -4
mm/s 0.5 . The chloride activation depth (0.06%) in the hydrophobic concrete
was approximately 15 mm compared to greater than 70 mm for the control.
Therefore the 30-minute absorption provided a better indication of relative
field performance in the field than the ASTM procedure where the speci-
mens were tested at much higher internal relative humidity. The difficulty is
determining an appropriate performance limit that would ensure acceptable
durability without reference to specific proprietary products.
Sorptivity also appears to be a good indicator of resistance to physi-
cal salt attack, which is not surprising considering the mechanism of salt