Independent Seeks Freedom From Leaks
Midwest Pipe Producer Designs and Builds Watertight Conduit System
Independent Concrete Pipe Corp. (ICPC), a manufacturer of pipe and other precast structures, has developed an innovative bolted joint system for watertight conduit lines.
The system is called Prebolt, a proprietary rectangular concrete conduit that was developed at the request of consulting engineers who were seeking a watertight box design that would meet or exceed their sewer line leakage specifications.
Gasket is installed into groove before the box is lowered into the truck
The joint system uses steel tubes cast into each corner of the conduit. The tubes extend the full length of the conduit and are welded to the reinforcing cage structure.
The conduit is cast with flat face (blunt) ends. A machined piece of steel, shaped like a bullet nose, with a bolt passing through a hole in the center is threaded into the bolting end of each steel tube. A steel insert is threaded into the tubes' receiving ends.
Prior to installation, a U-shaped rubber gasket is installed in a groove (recess) in a blunt end of the conduit. A rubber spline (O-ring) is inserted between the legs of the U to hold the gasket in place. When two conduits are brought together and aligned, the bolts in the bullet noses are tightened into the inserts to bolt the conduits together.
Each joint is tightened within a 2 mm (l/16 inch) to 3 mm (l/8 inch) gap around its circumference. As the conduits are bolted together the gasket is compressed between the ends of the conduit.
The bullet noses project into the receiving ends of the steel tubes and support the joint load so that the rubber gasket cannot be subjected to shear loads. The jointing system also aligns the conduit walls for a smooth flowline at each joint.
The rectangular shaped Prebolt conduit offers certain hydraulic, structural, and installation advantages over circular pipe. Since the bolted joint is positively watertight, the rectangular shape can be expanded to projects where infiltration/exfiltration is an important design consideration.
The Prebolt rectangular conduit is the result of a completely different approach to building precast concrete structures relative to:
Independent Concrete Pipe Corp. |
- Forming
- Casting
- Handling
- Steel Placement
- Joint Assembly
- Tolerances
- Grading and Alignment
- Testing and Installation Procedures
The manufacturing procedures and equipment produce a conduit that is built to close tolerances and has an ideally smooth finish for greater hydraulic efficiency. The closely assembled flat joint faces eliminate energy losses due to offset and open joints. The close manufacturing tolerance, coupled with precise grade control, enables the construction of a conduit line with gun barrel-like properties.
"The flowline has been described as smoother than a bowling alley," says Eric Carleton, director of technical services for ICPC and the chairman of the ASTM C13.08 Committee on Concrete Pipe joints. Prebolt's compressed face seal gasket joint conforms to ASTM C443 and the performance limits of C36I, he adds.
Will Tolliver, ICPC's special projects consultant and Prebolt designer, notes that "The system's positive watertight gasket joint can be bolted together in less time than a round pipe gasket joint can be lubricated and assembled." Prebolt's flat base and rectangular shape simplifies bedding, fine grading and backfilling, he explains, since there are no haunches with which to contend.
Prebolt owes its watertight integrity to:
- Pump casting of the concrete
- Special forming and handling equipment
- Laser-guided alignment, grade and control of bedding
- Bolted joint connection
- A compressed face seal gasket
Prebolt Conduits Target Applications
- Combined Sewer Overflows
- Pedestrian and Utility Tunnels
- Storm Sanitary Sewers & Storage Lines
- Underwater Lines
- Storage Tanks
- Circulating Systems
- Siphons
- Treatment Plant Piping
- Minimum Cover/High Flow
- Jack and Tunnel Installations
Engineering refinements and formal installation and testing procedures were completed after pump casting a trial 2400 mm x 1800 mm x 1800 mm (8 ft x 6 ft x 6ft) box conduit. After selecting the bolt assembly system, other production equipment and components were selected as follows:
Pump casting
A high pressure concrete pump was deployed to handle 178 mm - 203 mm (7-8 inch) superplasticized mixes. Pump casting enables dense concrete to be wet cast with a minimum of vibration.
Conduit forming
In order to pump cast the conduits to closely held dimensions, the forms were built to controlled tolerances. The concrete is pumped into the forms under pressure. This pressure produces a marble-smooth surface texture that is free of voids on the conduit's blunt end without the rubber gasket recess. This provides a smooth surface for the gasket to seal against. The form design and pump casting allows conduits to be cast about every three hours.
Conduit handling
A custom built hydraulic tipping device is used to tip cured conduits from the cast vertical position to the horizontal position. This eliminates potential damage due to handling.
Reinforcing structure
A new type of bender was built to precisely form the ends of wire sheets into C-shaped cage reinforcing using high toleranced wire fabric.
These bent sheets are attached to flat fabric sheets on a unique cage assembly fixture resulting in inside and outside reinforcing cages.
When the cages are being assembled, A-shaped wire trusses are secured between the reinforcing cages to exactly space and add rigidity to the cages. The reinforcing cage structure is then welded to the full length steel tubes used to bolt the conduits together.
The cage assembly fixture closely controls the location of the steel tubes and reinforcing cages. The fixture assures that each reinforcing cage structure is uniformly built and precisely located in conduit walls.
Bends were pre-cut at ICPC plant and field positioned by contractor and then the open joints were bowled and poured in-place.
Compression joint
A U-shaped rubber gasket is inserted and securely locked in a recess formed in one end of the blunt end conduit. When the conduits are bolted together, the gasket is compressed between the ends of the conduit to provide a positive watertight seal.
The bolt assembly is designed to take any joint load condition. This provides a gasket joint where the gasket is free of joint centering and shear load.
Installing conduits
A laying device was designed to support the conduit being installed at an inclined angle. No lift holes are needed. The angle enables the top two bullet noses of the conduit being installed to be easily inserted into the top two steel tubes of the previously installed conduit. The laying device then hydraulically lowers the conduit caming the bottom two bullet noses in the bottom two steel tubes. The face ends of the conduits are now in alignment and the bottom of the conduit is at grade. The joint is ready to be quickly bolted together. The total time to bolt the conduits together is less than 10 minutes.
"Prebolt is the first new and revolutionary concrete pipe produd to enter the marketplace since the rubber gasket joint was introduced a half century ago."
-Will Tolliver
Joint testing
A mobile joint tester was designed to vacuum test the jointed conduits. The tester moves along inside the conduit to isolate and test the joints. A vacuum pump lowers the air pressure at the joint until the vacuum stabilizes at the inches of mercury specified for the joint test. The joint must hold the vacuum loss to less than 6.8 kPa (2 inches) of mercury over a specified time. The joints are tested in conjunction with the installation operations prior to backfilling.
Bedding
Grade of the conduits' bedding is precisely held by using a screeding device that uses the bottom two steel tubes of the previous installed conduit for grade control. The screed's steel guide tubes are attached to the installed conduits' bottom tubes and leveled with a laser tool. The screed is then pulled along the guide tubes to precisely level the bedding materials.
Gilding Eli Lilly
Prebolt's first installation was for the Harding Street stormwater sewer line near ICPC's plant in Indianapolis. The fall 1993 project required a stormwater design with sanitary watertightness, according to Carleton. Watertightness for the box conduit took on an added dimension, however, as the project abuts the site of pharmaceutical giant Eli Lilly & Co., whose recreational facilities revolve around a 12.15 ha 00 acre) lake. Lilly and city officials were concerned about the new line's potential for affecting the lake's water level and decreased the specification to a maximum allowable daily inflow of 29.8 Umm (200 gallons/ inch). Even though an allowable infiltration rate was specified, the intent was to have the line bottle tight.
The project was constructed along with widening and required 975 m 0,200 ft) of Prebolt 2400 mm x 1800 mm (8 ft x 6 ft) and 1128 m (3,700 ft) of 1050 mm, 1350 mm, 1950 mm (42, 54, and 78 inch) diameter concrete pipe. ICPC worked with American Consulting Engineers and Indianapolis Department of Public Works on the line, then engaged the low bid contractor, Bowen Engineering, in formal installation training. Using laser guided grading equipment, Bowen crews were able to conform to the installation procedures necessary for a watertight specification. The contractor's innovation and execution saw placement rates reach up to 36.6 m (120 ft) per day, with quick checking of each joint using the testing apparatus. The joints easily met the targeted integrity because they were found to be leak free. All Joints are free of patching materials.
The Harding Street job wrapped up late last year. After examining the results, Tolliver began looking at slight modifications, as well as additional applications for Prebolt. ICPC is presently developing a twin cell rectangular version for combined storm and sanitary sewer lines and other multiple use applications.
"Prebolt is the first new and revolutionary concrete pipe product to enter the marketplace since the rubber gasket joint was introduced a half century ago," Tolliver asserts.
He says that ICPC is marketing Prebolt through its Midwest plants and is considering joint ventures or license terms with other utility precast producers throughout North America. The system is being evaluated by a handful of major civil engineering firms and was exhibited at the American Society of Civil Engineers' Indiana Chapter Meeting and at the Water Environmental Federation, CSO Specialty Conference, Louisville, KY.