Presented by: Amy L. Trahey, P.E. Great Lakes Engineering Group, LLC

Background and History

Typical Design Characteristics

Future

Repair Options

Types of Distress

First introduced to Michigan in 1954

Economical Solution

Statistics

These types of structures, until recently, were proven to have been virtually maintenance free

Many of the original pre-stressed concrete box beam bridges are nearing 60 years old.

Realizing this type of design lacks the ability to easily retrofit, repair, and rehabilitate.

Typical span lengths range from 20 feet to 100 feet Consist of either single-cell or double-cell units

Partial or Full-Depth shear keys are typically filled with non-shrink grout

Typical fit up beams in 1 to 3 inches

Transverse tie Post-tensioned strands common practice today

Beams built in the ‘50s and ‘60s used a threaded tie rod for the transverse tie.

Wearing surface is typically Concrete or HMA

Primary structural components are single or multiple rows of pre-stressing strands

Beams built before 1980 often did not have drain holes

Pre-stressed box beams built in the ‘50s and early ‘60s did not have shear stirrups that extended across the bottom flange

These are susceptible to longitudinal cracks along the bottom flange

Longitudinal cracking in the deck or wearing surface

Cracking leads to leakage, which allows chloride-laden water to saturate the sides and bottoms of the beams.

Leaching between box beams

Longitudinal cracks in the bottom of box beams

Delamination, incipient spalls

Spalls to exposed steel and exposed strands

Broken pre-stressing strands

Broken pre-stressing strands

Differential movement between adjacent box beams

Results in a loss of functionality of the load transfer mechanism

Shear or flexure cracks

Horizontal cracks along the fascia beams

Deterioration of the top flange of the box beam

Seal the longitudinal cracks in the surface

Replace the asphalt wearing surface with a concrete deck

Replace individual beams

Splice broken pre-stressing strands

Scarify the existing asphalt surface and install a waterproofing membrane and a new asphalt overlay

Repair grouted keyway and post tensioning

Typical course of “repair” utilized today …… let it rot, load rate it, post it, shut lanes down, inspect it more frequently, and ultimately close it. This is the potential fate for adjacent concrete box beam bridges. Be prepared.

Due to the limited visual inspection techniques, this type of structure can be a “Can of Worms”

Major concern is assessing corrosion of the pre-stressing strands – that you can’t see

Concern that corrosion is affecting load carrying capacity

It is difficult for inspectors to identify independent beam action

A call for specific rating guides for adjacent pre-stressed concrete box beams to be developed to help bridge inspectors

NDT – Thermal Infrared

Radar including Synthetic Aperture Radar (SAR)

Ground Penetrating Radar (GPR)

Infrared Thermography

More Research

Inspecting adjacent concrete box beams can be challenging

Difficult to define when corrosion will affect the load-carrying capacity the bridge

Difficult to determine independent beam action or if shear keys and transverse ties are still effective in sharing the load transfer from beam to beam

The quality and efficiency of each bridge inspection is influenced by the inspector’s knowledge of how the bridge works and what controls its strength and stability

An understanding of when to conduct a load rating is critical to ensure the safety of the public