DYWIDAG Circular Structure Repair with Hoop Tendons
Prestressed Concrete Cylinder Pipe (PCCP) pipeline owners faced with potential pipeline ruptures, due to corrosion failures of circumferential prestress wires, can repair them, rather than replacing them, with the economical and efficient DYWIDAG external hoop tendon system. This method restores the prestress force and keeps PCCP pipelines in service. DYWIDAG Systems International specially developed this system, based upon its patent for extrusion coated strand for post-tensioned concrete and its strand coupler patent. With this system specifically designed for PCCP pipe, repairs can be carried out easily and quickly. It is of great benefit that the pipes do not have to be de-watered or removed, but they are strengthened in-situ once excavated. Only the pipes identified as having bad wires need to be repaired, there is no need to condemn the entire pipeline.
Our specialized strand is an extrusion coated, high tensile strength prestressing strand (ASTM A-416). Each hoop has an ultimate tensile strength of over 58.6 kips. Corrosion protection is assured because the steel is encased in a very high quality corrosion inhibitor, which is then encapsulated by extruding a molten seamless polypropylene (PP) polymer sheath over the strand. Due to our strand’s low coefficient of friction and high force transmission properties, the prestressing force is efficiently transferred along the entire 360-degree pipe circumference. The 80 mils thick, high strength, virgin polypropylene sheath provides toughness and durability during installation and under load.
A key component of the economical pipe repair system is the SURFACE LOKCOUPLERTM, designed to be used in conjunction with the high force trans - mission and corrosion resistive properties of our specialized strand tendons. This coupler system, with corrosive protection features, builds on DSI's experience of more than 25 years in the repair of circular tanks and silos by using its patented LOKCOUPLERTM. The SURFACE COUPLER is designed to bear on the surface of pipes as small as 48 inches in diameter.
Other couplers, or surface anchors, from the DSI array of post-tensioning accessories increase the range of application of hoop tendons on pipes 36" in diameter to 144" in diameter and larger. In particular, very large diameter structures, in the case of silo and tank repairs, can be done by engineering relay couplers around the circumference of the structure and reduce the overall post-tensioning losses.
The hoop tendon system has been proven since 1997 in a variety of pipeline applications from Arizona to Texas to Massachusetts. On one project alone, over 4,000 hoop tendons were installed by Arizona Public Service Co. on a water pipeline supplying the Palo Verde Nuclear Generating Station.
A recent addition to the range of application of hoop tendons is the use of Epoxy-Coated Strand instead of regular bare strand in the extruded sheath. The bare strand is subjected to a flow of epoxy, filling the gaps between the wires and adding on the exterior of the tendon a 30 mil epoxy coat. In itself, this strand can withstand the high strength, durability and coating resistance demands of hoop tendons.
Used in conjunction with the 80 mil coating and corrosion inhibiting grease, the system provides over 4,500 hours of resistance to the ASTM salt fog accelerated corrosion test. This is used in particularly corrosive soils, on pipes submitted externally to chemically aggressive runoffs or simply to reduce what additional cover would be required for impact and corrosion protection.