Thermal Warping of Precast Segmental Bridges

thermal bowing of precast segments, quality defects of precast segments, precast segmental bridges

Thermal warping may be a serious quality defect of short-line match casting of precast segmental bridges. The segments are cast with high-strength concrete to achieve daily casting cycles. Concrete mixes designed for 75- or 100-year service life include sub-micronic pozzolanic materials such as slag, silica fume and fly-ash, and the hydration heat of these mixes can be particularly pronounced.

The heat of hydration in the new segment causes a thermal gradient in the match-cast segment. This gradient may cause a temporary backward bowing of the match-cast segment. The bowing that occurs before the concrete of the new segment has achieved initial set becomes a permanent curvature in the front joint of the new segment. Thus the new segment has one straight and one curved side, whereas the match-cast segment returns to its original shape after separation from the new segment and cooling to ambient temperature.

Warped segments may cause quality defects during span-by-span and balanced cantilever erection of precast segmental bridges. The size of the gap progressively increases as each joint is closed and may reach several millimeters in long assemblies comprising many segments. Self-weight bending and longitudinal post-tensioning cause irregular axial stress distributions in the top slab, with increased compressive stress in the side wings and reduced axial stress in the central top slab region between the webs.

Most design standards for prestressed-concrete bridges require zero tension in the epoxy joints between precast segments. When post-tensioning is designed for zero tension, compression deficiencies in top-slab regions directly exposed to traffic may cause shear and flexural failure of the epoxy joints and serviceability and durability issues. The time-dependent shortening of overloaded side wings will eventually decompress the midspan closure pours of balanced cantilever bridges and may actively pull match-cast joints apart. The photograph has been taken in a balanced cantilever bridge in the USA seven years after completion and shows pronounced deterioration that required structural retrofitting in every span.

Thermal warping of segments may also cause excessive thickness of the epoxy joints, which is frequently cause of voids, insufficient filling and stress concentration. Joint thickness should be kept as small as possible to minimize the influence of the physical parameters of epoxy (smaller elastic modulus than concrete, much larger creep coefficient, reoplastic time-dependent behavior instead of viscoelastic, etc.) on the behavior of precast segmental structures. Stress concentration and localized creep have often been associated with horizontal cracks in the webs radiating from thick epoxy joints.

Thermal warping may be noticeable in box girder segments with large width-to-length ratio. The courses of Mechanized Bridge Construction (2 days) and Precast Segmental Bridges (1 day) that Dr. Rosignoli teaches for the Continuing Education Program of the American Society of Civil Engineers and on-demand in the offices of bridge owners, designers and constructors provide exhaustive coverage of the topic and explain how to analyze and control the effects of thermal warping. With a production rate of one segment per day, a correct curing process for match-cast precast segments is essential to assure proper serviceability and durability of the bridge.