Task Number: 2.2 Internal vs. External Tendons

Convention usually dictates the type of tendon used in segmental bridges. For example, experience has shown that top tendons in balanced cantilever bridges are most effective as internal tendons because of construction simplicity and structural advantage. There are however some instances where a designer may want to consider using external instead of internal tendons or vice versa.

Internal Tendons

Internal tendons are usually more structurally advantageous than external tendons. In most box girders, internal tendon sections have at least a 7" larger internal moment arm than external tendon sections. Internal tendons are also able to develop larger post-cracking stresses and therefore have a larger nominal moment resistance. Of course this is only an advantage where strength considerations control - an infrequent occurrence. Internal tendons have performed very well in accelerated corrosion testing of epoxy jointed precast segments [1]. The AASHTO Guide Specifications for Design and Construction of Segmental Bridges [2] prohibits the use of internal tendons in dry joints. Performance at cast-in-place joints has been good.

Internal tendons have some disadvantages, particularly regarding the fabrication segments with inclined ducts. Vertical tendon profiles in webs require shear key bulkheads to accommodate different duct locations at each segment face. On occasion, the ducts can dislodge and fill with concrete or, more frequently, cause honeycombing below. At the anchorage segment, web tendons must sweep inward for the required tendon anchor width. This sharp lateral movement increases frictional losses and cuts across the reinforcing grid. In small radius curves (horizontally curving webs, haunched bottom slabs etc.) out-of-plane tendon breakout must be checked. Field problems that can develop include increased friction losses due to kinks at joints, unraveling of the duct when threading tendons and grout crossover at joints. Lastly, the shear strength is decreased slightly as the effective web width is decreased by one-half of the duct width.

External Tendons

External tendons are appealing because the ducts can be installed easier than internal ones. Segments can be cast free of internal ducts and anchorage segment details are simpler. Friction losses are smaller as wobble becomes negligible. External tendons are easier to inspect and also simpler to replace if damaged.

External tendons can be extremely corrosion resistant. The duct is made of non-corrosive polyethylene, has fewer joints and is isolated within the cross-section. Problems can occur however if the box fills with water due to expansion joint leaks or drain overflows or in a high humidity environment. In these cases, a watertight duct jointing system is critical (see Item 1.1 "Grouting of Tendons"). Some draped external tendons have shown some significant corrosion but not because they were external, rather the duct/anchor system was breached at the anchor. In one instance, ducts were pierced to inspect for grout voids. This practice is not recommended; one corrosion barrier should not be sacrificed to inspect another.

External tendons have some inherent shortcomings. Since they are generally positioned further from the extreme cross-section fibers, external tendons are less structurally advantageous. They also develop less tendon force at ultimate. Additionally, deviation blocks typically need to be cast in two segments per span.

1. West, J.S., Vignos, R.P., Breen, J.E. and Kreger, M.E., "Corrosion Protection for Bonded Internal Tendons in Precast Segmental Construction, Report 1405," Center for Transportation Research, University of Texas at Austin, October, 1999.

2. AASHTO Guide Specifications for Design and Construction of Segmental Concrete Bridges, 2^nd Edition, 1999.