This item of work consists of
repairing concrete structures by spraying the area to be repaired with dry,
premixed sand and cement that is blended with water in a mixing nozzle. The pneumatically placed mortar is then
finished and cured. This type of
procedure is often referred to as “shotcrete” by the
industry.
All existing reinforcing
steel bars must have a minimum cover of 1 inch (25 mm). If the existing location of the reinforcing
bars would result in less than 1 inch (25 mm) of cover, they are to be driven
back into recesses cut into the existing concrete to achieve that coverage. If this is not practical due to the large
number of reinforcing bars, the coverage must be obtained by modifying the
finished surface. If the reinforcing
steel is epoxy coated, care should be taken to minimize the damage to the
existing coating.
Where the depth of the patch
exceeds 1-1/2 inch (38 mm), in addition to any existing reinforcing steel, wire
fabric is required. Where the depth of the patch exceeds 4 inches (100 mm), a
layer of fabric is to be placed for each 4 inch (100 mm) thickness of patch or
fraction thereof.
Prior to placement of pneumatically placed concrete,
the area to be repaired must be properly prepared. All soft, loose, and disintegrated concrete,
plus an additional depth of 1/4 inch (6 mm) of sound concrete, must be removed.
Failure to remove soft, loose, and disintegrated concrete will adversely affect
the bond of the mortar and shorten the life of the repair.
The edges or shoulders of the repair areas must be
square or slightly undercut. If this is
not accomplished, the mortar placed at the edges of the repaired area will be
feathered. These feathered areas will
not have adequate strength and will scale off.
After all concrete has been removed from the repair
area, all dowels and expansion hooks placed, all steel areas restored, and no
more than 24 hours prior to placement of mortar, the area to be repaired must
be abrasive blast cleaned. The abrasive blast cleaning must be done to remove
spalls, latence, and any other foreign material that
might be detrimental to achieving a bond with the pneumatically placed
mortar. The Contractor should select an
abrasive blast method that will control or minimize the amount of fugitive dust
escaping into the atmosphere. Suitable
blast methods may include high-pressure water blasting with abrasives in the
water, abrasive blasting with containment, or vacuum abrasive blasting. The
Contractor must ensure that all wastes generated by the surface preparation
operation are managed in accordance with 107.19.
Unless otherwise specified, the Contractor shall wet
the area to be repaired with water for at least 2 hours prior to placing the
mortar. The area must be kept wet until the mortar is placed. At the time of placement of the mortar, all
free water must be removed.
Due to past experiences with
pneumatically placed mortar that was improperly placed and prematurely failed,
each operator must demonstrate their ability to construct a sound, durable
repair prior to being allowed to place mortar on the structure. This is accomplished by gunning the mortar
onto a test panel. The mortar on this
test panel is then tested for strength and examined for hollow areas, sand
pockets, and bond to the reinforcement.
The cores taken for compressive strength samples cannot contain any
reinforcing steel. If the reinforcing
steel spacing is too tight to retrieve a non-reinforced core, it may be
necessary to construct a portion of the test panel without reinforcing. The easiest means to examine the test panel
for mixing and consolidation issues is to pull a core at the intersection of
the reinforcing steel or to simply saw the test panel in half. It is important to look at the cross-section
of the reinforcing steel, as the backside of the reinforcing steel is usually
the most suspect area. The test panel
should be water cured for 7 days and handled in the same manner as a cylinder. The sample should not be cored for at least 7
days and the cores should be handled in the same manner as a cylinder.
After the mortar is placed,
it must be cured. Curing shall consist of covering the patch with burlap or
cotton mats and keeping them wet for 7 days.
If it is not practicable to use mats, the surface of the patch must be
kept wet by sprinkling the surface with water for 7 days. If it is determined that the above methods
are impracticable due to isolated areas being inaccessible, they must be cured
according to the requirements of 511.14,
Method B.
After the curing of the
patched areas is completed and before they are accepted, they must be sounded,
and every 200 square feet (20 m˛) cored.
All unsound areas, or areas that exhibit cracking, must be removed and
replaced. The cores must be inspected
for hollow areas, sand pockets, voids around reinforcing steel, and lack of
bond to the underlying concrete. The
cores are to be tested for compressive strength. Any defective patches, as determined by the
cores, must be removed and replaced at the Contractor’s expense.
1. Unsound concrete removed plus 1/4 inch (1 cm).
2. 1 inch (2.5 cm) minimum clearance to reinforcing.
3. 1 layer wire fabric for each 4 inches (10 cm) of patch
depth.
a. Reinforcing fabric lapped 6 inches (15 cm) minimum.
4. Surface cleaned by water or sand blast.
5. Mortar composed of three parts sand to one part
cement.
6. Mortar placed as dry as possible.
a. No one coat greater than 1 inch (2.5 cm) in thickness.
7. Wet burlap cured 7 day minimum or membrane cured with
Engineer approval.
8. Inspect and test per 520.11.
a. Sounding.
b. Core taken for every 200 square feet of repair.
9. Measure patch area and pay by the square foot (square
meter).