Underground Construction - January 2022 - 37

agents is by constructing a barrier on the concrete surface using a
suitable coating or sealer. A coating forms a fi lm on the surface and
blocks the pores and cracks on the concrete surface, whereas sealers
penetrate the concrete and make its surface hydrophobic
However, most sealers are not permanent, requiring periodic
re-applications. To provide long-term durability, another CIGMAT
study investigated the eff ectiveness of combining sealers with
coating to protect reinforced concrete from chloride and moisture
intrusion in salt water.
Materials/Methods
Diff erent combinations of three coating materials were applied to
the concrete substrate.
Coating-1 was a water-based latex, with rapid drying, good
adhesion, and high fl exibility.
Two commercially available silane coatings were:
* Silane-1, a micro-emulsifi able concentrated alkoxysilane, with
typical dilution of 9 parts water to 1 part silane (volume based).
* Silane-2, a clear, penetrating, breathable and water-repellent
version designed to protect the exterior above-grade concrete.
Some of the properties of these materials are summarized in
TABLE 5.
Materials/Methods
Obtained from an onsite concrete batch plant used for constructing
a bridge across a causeway near the coastline, 6-inch diameter by
6-inch height concrete specimen cylinders had an average unit
weight (based on 20 specimens) of 140.2 pcf, standard deviation of
0.9 pcf, COV (coeffi cient of variation) of 0.64 percent, and average
compressive pulse velocity of 13,353 ft/sec (4070 m/s).
Procedures were developed for preparing various combinations
of specimens and evaluating their protection of reinforced concrete
during immersion (wetting phase) for 21 days and exfi ltration/
breathing (drying phase) for 21 days.
Measured the changes in weights under the wetting and drying
testing conditions, to assess degree of deterioration, and modeled
performance of the various composite coatings.
In the wetting phase, specimens were totally immersed in one
of two test solutions: tap water (pH = 7 to 9) and 15 percent NaCl
solution, which represents the accelerated test condition in marine
system, as recommended by NCHRP 244 (National Cooperative
Highway Research Program Report).
Th e 21-day drying phase was under room condition of 23 degrees
C and 55 percent relative humidity (FIGURE 8).
FIGURE 9: Change in weight for concrete in water coated with Silane-1
and Coating-1
Results, Discussion
Th e relationships between the percent weight change and time
for coated specimens in water are shown in FIGURES 9 and 10
for silane-1 and silane-2 respectively.
In the fi rst four days, based on the observed weight changes
with time, there was infi ltration, followed by diff usion of the
water into the coated specimens.
Test results are also summarized in TABLE 6:
* Th e average weight increase in water after 21 days, for
specimens coated with Silane-1 was 1.1 percent as compared to
0.7 percent for specimens coated with Silane-2.
* Weight increase for specimens coated with Silane-1/Coating-1
FIGURE 10: Change in weight for concrete in water coated with Silane-2
and Coating-1
TABLE 6. Weight changes in coated concrete specimens during water wetting cycle
Percentage Weight Gain in Wetting Time (Days)
Coatings
Coating-1
Silane-1
Silane-1 and Coating-1
Silane-2
Silane-2 and Coating-1
Remarks
1
0.7%
0.3%
0.5%
0.1%
0.6%
Silane-2 had the lowest
weight change
4
2.3%
0.6%
1.8%
0.4%
1.1%
Coating-1 had the highest
weight change
10
2.4%
0.75%
2.0%
0.5%
1.2%
Silane-1 had the second
lowest weight change
21
2.6%
1.1%
2.3%
0.7%
1.2%
Weight changes were sensitive
to the type of coating
Remarks
Highest change in weight.
Second lowest change in weight.
Second highest change in weight.
Lowest change in weight.
Slightly higher than Silane-1.
Of all the coatings investigated,
Silane-2 had the lowest weight change
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