techniques
Many techniques are available to place the repair materials
and depend on the constraints and limitations of a project
(Emmons 1994). In addition, the past experience of the
contractor to successfully perform the placement technique
is also important. ICRI 03731 provides guidance for
selecting application techniques for repair.
2.6.1 Cast-in-place concrete, modified concrete, and
proprietary concrete or mortars—Repair by conventional
concrete placement is the replacement of defective concrete
with new concrete that is conventionally placed. This method is the most frequently used repair technique, and it is
usually the most economical.
Repair by conventional concrete placement is applicable to
a wide range of situations, including repair of deterioration
due to defects caused by poor construction practices.
Replacement with conventionally placed concrete should not be
used where aggressive exposure conditions caused deterioration
of the concrete, unless a protection system can mitigate the
factors that triggered the deterioration. For example, if the
deterioration was caused by acid attack, aggressive water
attack, or even abrasion-erosion, a repair made with
conventional concrete may deteriorate again for the same
reasons. Portland-cement concrete (PCC) modified with
silica fume, acrylics, styrene-butadiene latex, or epoxy,
however, have been successful in extending service life.
2.6.2 Shotcrete—Shotcrete is concrete or mortar that is
pneumatically conveyed at high velocity through a hose onto
a surface. The high velocity of the material striking the
surface provides the compactive effort necessary to consolidate
the material and develop a bond to the substrate surface.
The shotcrete process is capable of placing repair materials
in vertical and overhead applications without the use of
forms, and it can routinely place material several hundred
feet from the point of delivery.
There are two basic shotcrete processes. In wet-mix
shotcrete, cement, aggregate, and water are mixed and
pumped through a hose to a nozzle where air is added to
propel the material onto the surface. Dry-mix shotcreting
uses cement and aggregate that are premixed and pneumatically
pumped through a hose, then water is added at the
nozzle as the material is projected at high velocity onto a
surface.
Either method places suitable repair materials for normal
construction requirements. ACI 506R provides detailed
information on the two shotcrete processes and their proper
application.
In addition to placing conventional PCC and mortar, the
shotcrete process is also used for placing polymer-cement
concrete, fiber-reinforced concrete using both steel and
synthetic fibers, and concrete containing silica fume and
other pozzolans.
The application of repair materials by the shotcrete
process should be considered wherever access to the site is
difficult, the elimination of formwork provides economy,
and significant areas of overhead or vertical repairs exist.
Shotcrete is frequently used for repairing deteriorated
concrete or masonry on bridge substructures, piers, sewers,
dams, and building structures. It is also used for reinforcing
structures by encasing additional reinforcing steel added to
beams, placing bonded structural linings on masonry walls,
and placing additional concrete cover on existing concrete
structures (refer to Chapter 5).
The shotcrete nozzle operator’s skill determines the
in-place quality of the repair material. ACI 506.3R provides
a basis for determining the qualifications of a nozzle operator.
The nozzle operator should be certified by ACI. ACI 506.2
provides a basic specification for the application and
inspection of shotcrete. 2.6.3 Preplaced-aggregate concrete—Preplaced-aggregate
concrete is produced by filling the repair area with gap-graded
coarse aggregate, then filling the voids in the aggregate by
pumping in a cementitious or resinous grout. As the grout is
pumped into the forms, it fills the voids (displacing any
water that is present), and forms a concrete mass. The worker
should avoid the entrapment of air that result in voids. This
method is used for partial-depth repairs or for replacement of
whole members. This method reduces drying shrinkage
because the aggregate particles are in point-to-point contact
before and after grouting.
Generally, the same requirements for materials and
procedures that apply to preplaced-aggregate concrete in
new construction also apply to repair. Preplaced-aggregate
concrete is covered in detail in ACI 304R and ACI 304.1R.
2.6.4 Formed and pumped concrete and mortar—Formed
and pumped repair is a method of replacing damaged
deteriorated concrete by filling a formed cavity with a repair
mortar or concrete under pump pressure. This method can be
used for vertical and overhead repairs. Formwork should be
constructed to a strength sufficient to handle the pressure
induced by hydrostatic pressure and the additional pump
pressure required to consolidate the repair material. The
cavity and formwork design should provide for venting the
air. Venting can be accomplished by the removal profile of
the prepared concrete, vent tubes, or drilled holes in the
existing concrete. Pumping the cavity is started at the lowest
point in vertical repairs or at an extremity in overhead
repairs. Pumping continues until the material flows from an
adjacent port in the formwork. Pumping continues until the
cavity is completely full (Emmons 1994). During the final
pressurization, the repair material is consolidated around the
reinforcing steel and driven into the crevices of the prepared
substrate to improve bond.
2.6.5 Troweling and dry packing—
a) Troweling—Repairs applied by hand-troweling can be
used for shallow or limited areas of repair. These repairs can
be made using: portland-cement mortars; proprietary products
such as cementitious prepackaged materials; polymer-cement
grouts; polymer grouts; and mortars. Trowel-applied
systems are not recommended when reinforcing steel is
exposed and undercut due to the difficulty of consolidation
of repair material around and behind the reinforcing steel.
The paste of the repair material should be used as the
bonding medium. The repair material should be applied to
the grouted surface before the grout or paste sets. Where
multiple layers are needed to build up the total thickness of
the repair, the surfaces should be roughened to help bond
subsequent layers. For most applications, the surface of the
cavity should be saturated and surface dry at the time of
application of the material.
There are a wide variety of proprietary repair mortars and
concretes that have been modified with chemicals and
thixotropic agents. The placement techniques recommended
by the manufacturers do not always conform to accepted
placement techniques for portland-cement mortar and
concrete. This is particularly true for thin sections, such as
3 mm (0.1 in.), and vertical and overhead applications. The addition of chemicals and thixotropic agents that permit
deviations in placement techniques may compromise some
of the performance properties. Examples of some of the
properties that may be affected are shrinkage, bond strength,
and coefficient of thermal expansion. The specifier and
contractor should consult with the manufacturer and make
sure that the materials performance capability and limitations
meets the project criteria before using these materials. Refer to
Chapter 3.
Successful use of trowel-applied repairs is dependent on the
surface preparation and the skill of the mason. Masons should
be experienced, and close field observation of the work
should be made. Proper troweling technique should be used
to prevent the entrapment of air at the bonding surface, which
can cause reduced bond strength. Proper curing of portlandcement
mortar so that patch material does not dry before
hydration is complete is important. Special curing provisions
may be advisable for some proprietary repair materials or
where accessibility is difficult (ACI 308R and 308.1).
b) Dry packing—Dry packing is the hand placement of a
low-water portland-cement mortar and the subsequent
tamping or ramming of the mortar into place. Because of the
low water-cementitious material ratio (w/cm), these repairs,
when compacted properly, have good strength, durability,
and water tightness.
Dry packing can be used for repair of form-ties, cone-bolts,
and other holes and small areas with relatively high depth-tosurface-
areas ratios. Because of the labor-intensive nature of
this technique, it is not often used for large repairs.
2.6.6.1 Cement grouting—Cement grout is a mixture of
cementitious material, normally portland cement or microfine
cement, and water, with or without fine aggregate or
admixtures, proportioned to produce a pumpable consistency
without excessive segregation of constituents. Grout can be
injected into an opening from the surface of a structure or
through holes drilled to intersect the opening in the interior.
a) Grouting from the surface—When grout is to be
injected from the surface, short entry holes (ports), a
minimum of 25 mm (1 in.) in diameter and a minimum of
50 mm (2 in.) deep, are drilled into the opening. The surface
of the opening is sealed between ports with a portland
cement or resinous mortar. Whether or not short pipe nipples
are cemented into the holes for grout hose connections
depends largely on anticipated grouting pressures. If the
ports are drilled after sealing the openings, a hand-held,
cone-shaped fitting on the grout hose may be adequate for