corrosion bridge component steel frame failure

Bridge Expansion Joint Designs: Corrosion of Steel Framing Below Deck and Pier Cap Reinforcement

Image 1 – Typical Beam End Corrosion at Abutment Backwall

My wife thinks that my driving skills are poor.  She is not entirely wrong in this opinion.  Whenever I am on a long drive or I am driving in an unfamiliar area, my eyes tend to stray from the road and examine any bridge or sign structures I come upon.  If you do the same (hopefully only when you are a passenger), you may have noticed a few recurring characteristics on many bridges.  Oftentimes there appears to be a higher degree of corrosion on steel girders at the abutments and sometimes at the piers.  Yet other bridges will have newer looking paint on the girders for some distance at the abutments and at some piers.  Less common is the abutment wall or pier concrete having patches of newer looking concrete in odd patterns.  All these examples are the result of corrosion; specifically accelerated corrosion of the steel bridge members or concrete reinforcing steel at bridge expansion joints.

Image 2 – Pier Cap Beneath Neglected Expansion Joint

I once heard it stated, “bearings and bridge deck joints are approximately 5% of total bridge costs, but when improperly designed, poorly installed, and maintained, are 95% of the headaches of bridge operations.”  The simple fact is that bridge expansion devices are very often needed to accommodate bridge movement due to thermal expansion, live loads, shrinkage, and possibly even creep.  Note that efforts are being made to eliminate traditional bridge deck expansion joints using link-slabs and approach slabs (these will not be discussed in this article).  The secondary function of bridge expansion joints is to protect bridge components below the wearing surface from corrosion.  What you really need to protect the bridge components against is moisture, de-icing salts, dirt, and debris, which can all cause corrosion at faster rates than on the parts of the bridge away from the expansion joints.  While this is most obvious on steel girders or truss members (Image 1), concrete beams and substructure components deteriorate from reinforcing bar corrosion in the form of cracking and spalling (Images 2, 3).

Image 3 – Beam End and Pier Concrete Damage from Corrosion

I once heard it stated, “bearings and bridge deck joints are approximately 5% of total bridge costs, but when improperly designed, poorly installed, and maintained, are 95% of the headaches of bridge operations.”  The simple fact is that bridge expansion devices are very often needed to accommodate bridge movement due to thermal expansion, live loads, shrinkage, and possibly even creep.  Note that efforts are being made to eliminate traditional bridge deck expansion joints using link-slabs and approach slabs (these will not be discussed in this article).  The secondary function of bridge expansion joints is to protect bridge components below the wearing surface from corrosion.  What you really need to protect the bridge components against is moisture, de-icing salts, dirt, and debris, which can all cause corrosion at faster rates than on the parts of the bridge away from the expansion joints.  While this is most obvious on steel girders or truss members (Image 1), concrete beams and substructure components deteriorate from reinforcing bar corrosion in the form of cracking and spalling (Images 2, 3).

Bridge expansion joints are designed to accommodate thermal expansion (and sometimes rotation) of the bridge from temperature changes.  A 100-foot span steel structure will expand approximately 0.8-inches in a temperature change of 100oF (0.7-inches for a concrete structure in same conditions).  Several bridge expansion joint types exist; some of the more commonly used ones are shown and briefly described in the Table 1.

In my experience examining highway bridge structures for corrosion, I frequently encounter strip seal joints.  The second most common is finger joints (I routinely and incorrectly called these “tooth dams” in the past).  Reportedly compression seal joints are being replaced with strip seals in many bridge rehabilitation projects.  Unfortunately, I haven’t seen many new construction projects firsthand so my experience with the as-installed condition or newest technology is somewhat limited.  What I typically see to a greater or lesser degree is a failure in the secondary function of the expansion joints (i.e., keeping moisture and de-icing salts from leaking through the joint and accumulating on the bridge components beneath).  While occasionally there are installation issues like the strip seal not fully seated in the extrusion (Image 4), the majority of inadequate expansion joint performance is due to a lack of routine maintenance.  Dirt and debris accumulate in the seal (for strip seals and compression seal joints), or the troughs (neoprene or metal) of finger joints are damaged or filled with debris so runoff water does not drain as intended (Images 5, 6, 7). 

Image 4 – Strip Seal not Fully Seated in Steel Extrusion
Image 5 – Strip Seal Joint Full of Dirt and Debris
Image 6 – Somewhat Damaged Finger Joint Trough
Image 7 – Finger Joint with Trough Missing

Cleaning and maintaining the joints is a difficult and expensive endeavor for any bridge owner.  Add to that the traveling public despises traffic restrictions and lane closures and news media reporting on traffic delays can exacerbate the situation, so it is understandable why the joints become “the headaches of bridge operations.”

One key to improving expansion joint design may be to limit the debris that can accumulate in the joint opening(s).  Several products and designs have this goal in mind, and include improvements in sealing elements, alternate materials, and simple improvements to standard designs (Images 8, 9).

Image 8 – Inverted Seal with Epoxy Adhesive Limits Debris Accumulation
Image 9 – Silicone Treated Top with Foam Seal Could Improve Water tightness

One interesting simple change was to move the strip seal location to the underside of the bridge deck and add a sliding plate at the deck wearing surface to reduce or eliminate debris accumulating in the joint (Figure 1).

Figure 1 – Strip Seal Location Moved to Below Deck

Another change that has been in use for some time is a foam seal directly below the fingers of a finger joint (Figure 2).  This foam seal is supported from below by an additional, much thinner finger plate.  This allows water to drain into a trough while road debris and dirt remain on the bridge deck surface for easier removal.  I am not aware of the performance history of this system; and the level of adoption of this (or similar design) by bridge owners is unknown.

Figure 2 – Finger Joint with 4” Thick Foam Seal and Trough

As often occurs, planning for future maintenance and longevity are neglected due to funding.  If more consideration for future corrosion is given during the planning of new structures or bridge rehabilitation projects, the less likely we will be to see the conditions in images 10 and 11 as you are driving by.

Image 10 – Spalled Concrete and Cracks Pier Cap Under Expansion Joint
Image 11 – Corrosion on Bearing and End Diaphragm

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