service more quickly than conventional repair materials. This
advantage is important in repairing highways, bridges, airport
runways, and industrial plants because of the reduced protection
times, lower traffic-control costs, and improved safety.
b) Limitations—Although most rapid-setting materials are
as durable as concrete, some, due to their constituents, may
not perform well in a specific service environment.
Some rapid-setting materials obtain their strength
development and expansion from the formation of ettringite.
If the level of expansion is high and the time to attain the
maximum levels of expansion is long, strength retrogression
may occur. The potential for delayed expansion resulting
from insufficient initial curing followed by rewetting should
be recognized.
Because some of these materials may contain abnormally
high levels of alkali or aluminate to provide expansion, their
exposure to sulfates and reactive aggregates should be
limited. The specifying engineer should review appropriate
application and compatibility issues.
c) Applications—Rapid-setting cements are especially
useful in repair situations where an early return to traffic is
required, such as repair of pavements, bridge decks, airport
runways, and industrial plants.
d) Standards—ASTM C 928 covers packaged, dry,
cementitious mortar or concrete materials for rapid repairs
to hardened hydraulic-cement concrete pavements and
structures. ASTM C 928 does not provide bond strength or
freezing-and-thawing resistance requirements. The specification
permits up to 1% calcium chloride without acknowledgement
to the specifier or user. Also, a current footnote
cautions the user to check on exposure conditions (sulfate
exposure and alkali reactivity) that are not covered in the
specification. Therefore, additional testing should be
performed at anticipated application temperatures to verify if
properties not covered in the specification are important for
a given project. Substantial advances in the compounding of
rapid setting cements have taken place in recent years.
3.2.12 Shotcrete—Shotcrete is a mixture of portland
cement, sand, and water that is projected into place by
compressed air. In addition to these materials, shotcrete can
also contain coarse aggregate, fibers, and admixtures.
Properly applied shotcrete is a structurally adequate and
durable repair material that is capable of excellent bond with
existing concrete or other construction materials.
a) Advantages—In repair projects where thin sections less
than 150 mm (6 in.) in depth and large or small surface areas
with irregular contours or shapes are involved, shotcrete may
be more economical than conventional concrete because of
the savings in forming costs.
Shotcrete can be applied overhead in normal applications,
and materials can be mixed and transported several hundred
feet to the nozzle operator at project sites with restricted access.
Mechanical equipment is also available for remote placement
of shotcrete. For shotcrete dry-mix, refer to ACI 506R.
b) Limitations—The successful application of shotcrete is
dependent on the training, skill, and experience of the nozzle
operator. The nozzle operator should demonstrate his or her
skill by placing a test panel that reflects the site conditions.