Common Challenges Inspecting Adjacent Prestressed Concrete Box ...

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

Background and
History
Typical Design
Characteristics
Types of Distress
Repair Options
Future

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.

Primary structural components are single or
multiple rows of pre-stressing strands
Wearing surface is typically Concrete or HMA

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
Scarify the existing asphalt surface and install
a waterproofing membrane and a new
asphalt overlay
Replace the asphalt wearing surface with a
concrete deck
Splice broken pre-stressing strands
Replace individual beams
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 prestressed
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