Quantcast Section IV- Maintenance and Repair of Duct Systems

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
5-9. Structure maintenance and repair.
will bond against sound concrete. The hole can then
be patched with calcium-chloride putty or quick-
Maintenance and repair of structures is a continu-
setting cement mortar.
ous procedure but is seldom extensive. Pump out
(3) Floor lea k age. If water is entering through
structures as necessary to allow complete inspec-
the floor of the structure, clean the floor and remove
tion. Major breaks or settlement of structures caus-
accumulated silt. If the floor is generally sound ex-
ing large cracks require investigation of the struc-
cept for the joints at the wall or for isolated cracks,
tural condition and rebuilding to eliminate the
repairs can usually be made as covered above. If the
concrete of the floor shows evidence of general po-
a. Duct Line entrances. Grout up chipped concrete
rosity or disintegration, it is better to pour a new
at the mouth of the duct line as necessary. Heavily
floor, as follows:
loaded cables will crawl because of expansion and
(a) Where a reduction in headroom of 4 to 6
contraction, which results from the alternate heat-
inches (100 to 150 millimeters) will not affect the
ing and cooling effects of changing loads. The mouth
utility of the structure, a new floor may be poured
of the duct line must be kept clean and free of burrs
directly over the old floor. Otherwise, the old floor
and small patches of concrete that will damage
should be broken out and a new floor poured.
(b) When necessary to break out the old floor,
b. Water leaks. Most structure repair require-
the first step is to excavate for a temporary pump
ments consist of stopping water leaks in the floor
sump about 12 inches (300 millimeters) below the
and walls of frequently entered structures. Depend-
old floor level if the structure has no existing sump.
ing on the terrain, the pumping of one structure
When the floor has been removed, continued pump-
may involve the removal of water from adjacent
ing may be necessary. Further excavation will be
structures. In applicable locations, all vacant ducts
necessary if added headroom is desirable. A perma-
should be plugged with standard duct plugs, and all
nent sump or a storm drain connection should be
occupied ducts should be sealed so as to prevent
considered when the new floor is poured.
water or gas from entering vaults or any users'
5-10. Duct line maintenance and repair.
(1) Occupied duct sealing. Use a nonhard-
ening sealing material that will not harm the ca-
Most damage to duct systems results from new un-
ble to seal occupied ducts. These nonhardening
related construction and settling of ducts. Too often,
compounds consist of emulsified vegetable oils con-
the new construction fails to locate an adjacent duct
taining fibers or asphalt compounds. Oakum is
line accurately and damages the line. Ducts some-
often packed around the cables as backing for
times settle where they cross older understructures,
the sealing compound. Use a ready-mixed com-
whose overlay was completed without adequate
mercial sealer and follow the manufacturer's direc-
backfilling and tamping. Duct settling is often not
apparent unless cable failure results or an empty
(2) Wall leakage. Water leakage through the
duct is rodded in preparation for pulling in new
walls of the structure will usually occur along joints
cable. In either event, the condition must be inves-
or void areas. Some leakage may be found where the
tigated and repaired. A new structure at the point of
ducts enter the structure. Using a cold chisel and
settlement may possibly be the quickest and cheap-
hammer, chip out the porous area so that the patch
est repair.
5-11. Cable faults.
intended to prevent extended outages due to tran-
sient disturbances on aerial lines. But repeated
Whenever cable insulation breaks down, resulting
reclosing on an underground cable fault tends to
in an underground cable fault, fuses should blow or
create unusually high fault resistances. Reclosing
circuit breakers should open to prevent further sys-
serves to aggravate an underground cable fault
tem damage. Faulted circuit indicators (FCIs),
which may then stress upstream circuitry.
where provided, may also provide an indication of a
cable fault.
b. Aerial-to-underground line connections. Fuse
a. Reclosing on a fault. The practice of applying
protection is required to be provided at or near riser
automatic reclosers on medium-voltage aerial dis-
poles where such connections are made. When any
tribution lines presents a problem when under-
aerial lines feeding underground cable systems are
ground distribution lines are supplied from aerial
provided with automatic reclosing, that feature
lines which have reclosing features. The recloser is
should be designed so that any permanent fault on


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