Page 176 - Trenchless Technology Piping Installation and Inspection

142 Cha pte r F o u r

compact a dry concrete mix [zero slump with low water-cement (w/c)
ratios] into a form. A conventional wet mix and casting method is
used in the fifth one. Each method is briefly discussed below:

1. Centrifugal casting: The form is rotated, acting as a centrifuge,
while the concrete mix is fed into it. Vibration and compac-
tion are also employed to consolidate the concrete mix. The
centrifuge promotes extraction of water from the mix.
2. Dry casting: Low-frequency, high-amplitude vibrations are
used to distribute and compact the dry mix in the form in this
process. The form is then removed and the pipe cured.
3. Packerhead process: In this process, a high-speed rotating
device, the packerhead, acts as the inner wall of the form and
compacts the concrete against the walls of the form. As the
concrete is fed from the top of the form, the packerhead grad-
ually moves up.
4. Tamping: The concrete is mechanically compacted with oscil-
latory tampers between the walls of the outer form and an
inner core. This method is declining in use.
5. Wet casting: This method is typically employed for the manu-
facture of large-diameter pipes, using a concrete mix with
higher water content than the other four methods. Again, the
pipe is made between the inner and outer walls of a form.

Types of Concrete Pipes There are six different types of concrete pipes
used for pressure and sewer applications:

1. Nonreinforced concrete pipe (CP): Used for nonpressure appli-
cations only, this pipe is manufactured in diameters of 4
through 36 in., in lengths up to 8 ft. Produced in three classes,
its minimum strength requirement is the three-edge bearing
per ASTM C14. Since it is nonreinforced, structurally, it can
only exhibit a brittle failure mode.
2. Reinforced concrete pipe (RCP): Typically produced to meet
ASTM C76, there are three types of reinforcements available
for RCP-welded wire reinforcing, hot-rolled rod made of
Grade 40 or 60 steel, and cold-drawn steel wire made from
hot-rolled rod. It is used in nonpressure and low-pressure
applications of up to 55 psi. There are five load-bearing
capacities, ranging from 1350 to 3000 in units of lb-force/ft of
diameter/ft of length known as “D-Load.” Higher load
capacities can be specially designed for unusual combina-
tions of fill height or live load as per ASTM C655.
3. Prestressed concrete cylinder pipe (PCCP): Often used in high-
pressure applications, PCCP is a composite pipe of concrete