By Eric Steinberg, PhD, PE, Ali Semendary, and Kenneth Walsh, PhD
For years, adjacent prestressed concrete box-beam bridges have been employed for short and medium spans—two-thirds of all state Department of Transportations (DOTs) use them. While they are popular due to ease of construction and low profile, cracks in the longitudinal grouted joints between adjacent beams have often been observed. This can lead to leakage through the joint, accelerating corrosion of reinforcement and diminishing load transfer. New use of ultra-high-performance concrete (UHPC) grout may provide a solution to this problem.
The poor transverse connection of the shear keys is often the main cause of these longitudinal cracks. Different solutions have been suggested to reduce this cracking, including the study of shear key configurations and grouting materials in a full-scale box beams. In that research, a typical small-depth shear key was tested either with non-shrinkage grout or with epoxy, and a mid-depth shear key was tested with non-shrinkage grout. The small-depth shear key with non-shrinkage grout was cast in the November of 1998, with only minor shrinkage cracks observed after inspection.
Prior to load testing in January 1999, cracks were observed in the key that were attributed to the cold winter climate. The strain gages embedded in the beams and shear keys recorded a large jump in strain when the temperature reached −18 C (0 F). When the beams were tested under cyclic load, the temperature induced cracks propagated. However, it was difficult to determine whether the cracks were caused by the loading or cold temperature as the test was performed outside.
The mid-depth shear key was tested in May, and the shear key cracked after just one week. The temperature was also determined to be the main cause for the cracking.
The author concluded the vertical temperature gradient in the beams, rather than the ambient temperature, was the main cause for the cracking. Daily temperature variation caused deflections that led some joints to open and others to close. As the beams did not sit perfectly on the abutments, the movement created sufficient strains to cause cracking in the shear key.
Epoxy was also used as a grouting material in the shear key, and no cracks were observed due to temperature or loading. However, epoxy is undesirable to use because of the large difference in the thermal expansion coefficients between epoxy and concrete. The mid-depth shear key was constructed with non-shrinkage grout. The cracking was reduced because the throat was not grouted, which helped to reduce the temperature stresses. The recommendations were to use transverse post-tensioning to limit joint movement, neutral axes shear keys with non-grouted throats, or full-depth shear keys.
In-service adjacent box beam bridges in Michigan were inspected to evaluate changes in the design of side-by-side box beam bridges. The design changes were full-depth grouted shear keys, a high level of transverse post-tensioning, and a cast-in-place deck with seven-day moist curing.
It was determined the Michigan box beam bridges still experienced longitudinal reflective cracks irrespective of age. One of the bridges inspected while still under construction showed reflected cracks developed within three days after grouting before post-tensioning was applied, and continued to crack even after post-tensioning. When the bridge was inspected after three weeks, cracks were observed in every shear key of the bridge. It was determined the cracking was a result of de-bonding between the shear key and adjacent box beams. The authors concluded a redesign should be employed.