Civil Engineering Reference
In-Depth Information
dry mixture is consolidated by the impact force and can be
placed on vertical or horizontal surfaces without sagging.
Shotcrete is applied by a dry or wet process. In the
dry process, a premixed blend of cement and damp
aggregate is propelled through a hose by compressed air
to a nozzle. Water is added to the cement and aggregate
mixture at the nozzle and the intimately mixed ingredi-
ents are projected onto the surface. In the wet process, all
the ingredients are premixed. The wet mixture is pumped
through a hose to the nozzle, where compressed air is
added to increase the velocity and propel the mixture
onto the surface.
As the shotcrete mixture hits the surface, some coarser
aggregates ricochet off the surface until sufficient paste
builds up, providing a bed into which the aggregate can
stick. To reduce overspray (mortar that attaches to nearby
surfaces) and rebound (aggregate that ricochets off the
receiving surface) the nozzle should be held at a 90° angle
to the surface. The appropriate distance between nozzle
and surface is usually between 0.5 m and 1.5 m (1.5 ft and
5 ft), depending on the velocity.
Shotcrete is used for both new construction and repair
work. It is especially suited for curved or thin concrete
structures and shallow repairs, but can be used for thick
members. The hardened properties of shotcrete are very
operator dependent. Shotcrete has a density and compres-
sive strength similar to normal- and high-strength con-
crete. Aggregate sizes up to 19 mm (
3
⁄
4
in.) can be used,
however most mixtures contain aggregates only up to
9.5 mm (
3
⁄
8
in.); 25% to 30% pea gravel are commonly used
for wet mixes (
Austin and Robbins 1995
).
Supplementary cementitious materials, such as fly
ash and silica fume, can also be used in shotcrete. They
improve workability, chemical resistance, and durability.
The use of accelerating admixtures allows build-up of
thicker layers of shotcrete in a single pass. They also
reduce the time of initial set. However, using rapid-set
accelerators often increases drying shrinkage and reduces
later-age strength (
Gebler and others 1992
).
Steel fibers are used in shotcrete to improve flexural
strength, ductility, and toughness; they can be used as a
replacement for wire mesh reinforcement in applications
like rock slope stabilization and tunnel linings (
ACI
506.1R
). Steel fibers can be added up to 2 percent by
volume of the total mix. Polypropylene fibers if used are
normally added to shotcrete at a rate of 0.9 to 2.7 kg/m
3
(1.5 to 4.5 lb/yd
3
), but dosages up to 9 kg/m
3
(15 lb/yd
3
)
have also been used (
The Aberdeen Group 1996
).
Guidelines for the use of shotcrete are described in
ACI 506R
,
ASCE (1995)
, and
Balck and others (2000)
.
duces expansion during hardening and thereby offsets the
contraction occurring during drying (drying shrinkage).
Shrinkage-compensating concrete is used in concrete
slabs, pavements, structures, and repair work to minimize
drying shrinkage cracks. Expansion of concrete made with
shrinkage-compensating cement should be determined by
the method specified in ASTM C 878.
Reinforcing steel in the structure restrains the con-
crete and goes into tension as the shrinkage compensating
concrete expands. Upon shrinking due to drying contrac-
tion caused by moisture loss in hardened concrete, the ten-
sion in the steel is relieved; as long as the resulting tension
in the concrete does not exceed the tensile strength of the
concrete, no cracking should result. Shrinkage-compen-
sating concrete can be proportioned, batched, placed, and
cured similarly to normal concrete with some precautions;
for example, it is necessary to assure the expected expan-
sion by using additional curing. More information can be
found in
Chapter 2
and in
ACI 223-98
,
Standard Practice for
the Use of Shrinkage-Compensating Concrete.
PERVIOUS CONCRETE
Pervious (porous or no-fines) concrete contains a nar-
rowly graded coarse aggregate, little to no fine aggregate,
and insufficient cement paste to fill voids in the coarse
aggregate. This low water-cement ratio, low-slump con-
crete resembling popcorn is primarily held together by
cement paste at the contact points of the coarse aggregate
particles; this produces a concrete with a high volume of
voids (20% to 35%) and a high permeability that allows
water to flow through it easily.
Pervious concrete is used in hydraulic structures as
drainage media, and in parking lots, pavements, and airport
runways to reduce storm water run off. It also recharges the
local groundwater supply by allowing water to penetrate
the concrete to the ground below. Pervious concretes have
also been used in tennis courts and greenhouses.
As a paving material, porous concrete is raked or slip-
formed into place with conventional paving equipment
and then roller compacted. Vibratory screeds or hand
rollers can be used for smaller jobs. In order to maintain its
porous properties, the surfaces of pervious concrete
should not be closed up or sealed; therefore, troweling
and finishing are not desired. The compressive strength of
different mixes can range from 3.5 to 27.5 MPa (500 to 4000
psi). Drainage rates commonly range from 100 to 900 liters
per minute per square meter (2 to 18 gallons per minute
per square foot).
No-fines concrete is used in building construction
(particularly walls) for its thermal insulating properties.
For example, a 250-mm (10-in.) thick porous-concrete wall
can have an
R
value of 0.9 (5 using inch-pound units) com-
pared to 0.125 (0.75) for normal concrete. No-fines con-
crete is also lightweight, 1600 to 1900 kg/m
3
(100 to 120
pcf), and has low drying shrinkage properties (
Malhotra
1976
and
Concrete Construction 1983
).
SHRINKAGE-COMPENSATING
CONCRETE
Shrinkage-compensating concrete is concrete containing
expansive cement or an expansive admixture, which pro-
