Civil Engineering Reference
In-Depth Information
tard the loss of water from newly placed concrete and it is
applied immediately after finishing. Surface sealing com-
pounds on the other hand retard the penetration of
harmful substances into hardened concrete and are typi-
cally not applied until the concrete is 28 days old. Surface
sealers are generally classified as either film-forming or
penetrating.
Sealing exterior concrete is an optional procedure
generally performed to help protect concrete from freeze-
thaw damage and chloride penetration from deicers.
Curing is not optional when using a sealer; curing is nec-
essary to produce properties needed for concrete to per-
form adequately for its intended purpose. Satisfactory
performance of exterior concrete still primarily depends
on an adequate air-void system, sufficient strength, and
the use of proper placing, finishing and curing tech-
niques. However, not all concrete placed meets those cri-
teria; surface sealers can help improve the durability of
these concretes.
Film-forming sealing compounds remain mostly on
the surface with only a slight amount of the material pen-
etrating the concrete. The relatively large molecular struc-
ture of these compounds limits their ability to penetrate
the surface. Thinning them with solvents will not improve
their penetrating capability. These materials not only
reduce the penetration of water, they also protect against
mild chemicals; furthermore, they prevent the absorption
of grease and oil as well as reduce dusting under pedes-
trian traffic.
Surface sealers consist of acrylic resins, chlorinated
rubber, urethanes, epoxies, and alpha methyl styrene. The
effectiveness of film-forming sealers depends on the con-
tinuity of the layer formed. Abrasive grit and heavy traffic
can damage the layer requiring the reapplication of the
material. Consult manufacturers' application recommen-
dations because some of these materials are intended for
interior use only and may yellow and deteriorate under
exposure to ultraviolet light.
The penetrating sealer that has been used most exten-
sively for many years is a mixture of 50 percent boiled lin-
seed oil and 50 percent mineral spirits (AASHTO M 233).
Although this mixture is an effective sealer, it has two
main disadvantages: it darkens the concrete, and periodic
reapplication is necessary for long-term protection.
A new generation of water-repellent penetrating
sealers have a very small molecular size that allows pen-
etration and saturation of the concrete as deep as 3 mm
(
1
⁄
8
in.). The two most common are silane and siloxane,
compounds which are derived from the silicone family.
These sealers allow the concrete to breath, thus pre-
venting a buildup of vapor pressure between the concrete
and sealer that can occur with some film-forming mate-
rials. Because the sealer is embedded within the concrete,
making it more durable to abrasive forces or ultraviolet
deterioration, it can provide longer lasting protection
than film-forming sealers. However, periodic retreatment
is recommended. In northern states and coastal areas
silanes and siloxanes are popular for protecting bridge
decks and other exterior structures from corrosion of rein-
forcing steel caused by chloride infiltration from deicing
chemicals or sea spray (Fig 12-12).
Application of any sealer should only be done on
concrete that is clean and allowed to dry for at least
24 hours at temperatures above 16°C (60°F). At least
28 days should be allowed to elapse before applying
sealers to new concrete. Penetrating sealers cannot fill
surface voids if they are filled with water. Some surface
preparation may be necessary if the concrete is old and
dirty. Concrete placed in the late fall should not be sealed
until spring because the sealer may cause the concrete to
retain water that may exacerbate freeze-thaw damage.
The precautions outlined earlier regarding volatile sol-
vents in curing compounds also apply to sealing com-
pounds. The effectiveness of water-based surface sealers is
still being determined. The scale resistance provided by
concrete sealers should be evaluated based on criteria
established in ASTM C 672. For more information on sur-
face sealing compounds, see AASHTO M 224,
ACI Com-
mittee 330
and
ACI Committee 362
.
Fig 12-12. Penetrating sealers help protect reinforcing steel
in bridge decks from corrosion due to chloride infiltration
without reducing surface friction. (69976)
