Cut Sheet

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271
P&C
®
Duct and Telephone Duct – Installation
Typical Installation Practices for P&C
®
Duct Type EB and Telephone
Duct Type B
Trenching:
Whenever possible the walls of the trench for P&C Duct should
act as forms for concrete encasement. The trench should be
made no wider than necessary to provide the nominal size
concrete thickness.
Duct Spacing:
Duct spacing, both vertical and horizontal, is accomplished
with the use of Carlon PVC Snap-Loc
®
Spacers.
Recommended interval between spacer assemblies is 8 to 10
feet.
Terminating:
For smooth cable pulling and properly engineered terminations
into manholes, Carlon P&C Duct end bells should be used.
Concrete:
The concrete used with P&C Duct should be
3
/8" aggregate
with a nominal compressive strength of 2,500 lbs. per square
inch. The slump should be at the upper end of the range,
preferably 7 to 8 inches. It should have just enough slump to
flow to the bottom of the formation and yet not be so wet as
to cause the ducts to float. In placing concrete around P&C
Duct, adjust the delivery chute so the fall of the concrete into
the trench is minimal. Use a splash board to divert the flow of
concrete away from the trench sides and avoid dislodging soil
and stones.
Pressure Grouting:
This technique is used for ducts in a casing or bored
construction. Hydraulic pressure exceeding 25 psi is
common and thus dictates the use of a P&C Duct Type
DB-120 or a Schedule 40
®
product. (See collapse pressure
chart). Hydraulic pressure from grouting is a function of the
line pressure at the nozzle and back pressure created by
pumping. If the exhaust nozzle isn’t withdrawn properly, the
back pressure will rapidly build and equal the line pressure.
Depending on the type casing and pumping distance, line
pressures will go up to 90 psi.
Hydraulic Pressure:
The primary consideration for duct selection is the height
of the duct bank. Since concrete exerts a force of 1.03 psi
per foot of height, to determine the correct duct selection,
consider the following examples:
1. 16 way duct bank, 5" conduit, 4 x 4 configuration with
3" separation, and 3" concrete cover
Concrete Height – 4 x 5.563" + 4 x 3 =
22.5 + 12" = 34.2"
Hydraulic Pressure – (34.2"/12) x 1.03 = 2.9 psi
In this instance the maximum force on the bottom
ducts would be 2.9 psi, therefore, 5" EB-20 would be a
satisfactory choice.
2. 16 way duct bank, 5" conduit, 8 x 2 configuration with
3" separation, and 3" concrete cover.
Concrete Height – 8 x 5.563" + 8 x 3 =
44.5 + 24" = 68.5"
Hydraulic Pressure — (68.5"/12) x 1.03 = 5.9 psi
In this instance the hydraulic force is equal to the
theoretical collapse pressure of 5" EB-20, therefore,
the use of 5" EB-35 or DB-60 would be a satisfactory
choice.
Another alternative would be to use a sequential
pour technique. Pour approximately
1
/2 the height,
allow the concrete to set-up, and then pour the
remaining distance. Using this method, 5" EB-20
would be satisfactory, since the hydraulic pressure
has been reduced by 50%.
RUS Accepted