Adam Matteo, Virginia Department of Transportation
Expansion joints in bridge decks have long been recognized as the one of the leading factors in bridge deterioration. Expansion joints, placed at the ends of bridge decks to allow for thermal expansion and contraction and live-load deflection, inevitably leak, allowing water to seep below the bridge deck. The water often carries chloride-laden road salts, which rapidly accelerate the corrosion rate of superstructure and substructure elements. A typical expansion joint is shown in Figure 1. Figures 2 and 3 show areas under leaking joints that exhibit accelerated corrosion due to the intrusion of water, salt, grit and oil.
Difficulty in Maintaining Expansion Joints
Ideally a bridge owner should be able to maintain joints in a satisfactory condition to prohibit the inflow of water through the seal. However, the durability of commonly used seals has proven to be highly problematic. In high traffic environments seals may last less than 1 year, and the cost of traffic control to re-seal joints continues to increase. Figure 1 shows three common problems affecting the durability of joint seals:
- Localized failure of the concrete adjacent to the joint
- Accumulation of grit and debris above the joint seal, causing failure of the seal
- Debonding of the seal from the adjacent concrete
VDOT collects data on the performance of its bridge joints during each regular bridge inspection. VDOT has approximately 1 million linear feet of expansion joints in it inventory, 900,000 linear feet of which are pourable seals or compression seals. Of these joints, the following percentages are in Condition State 1 (no leakage).
- Pourable Seals: 68%
- Compression Seals: 61%
Taken together, only 2/3 of the total length of pourable and compression seals are functional. However, because of the way that joint condition is measured, the problem is actually more severe. If a particular joint is 50’ long and has 2’ of unsatisfactory seal, the joint is reported as having 48’ in Condition State 1 and 2’ in Condition State 2 or 3 (deteriorated or failed). However, that 2’ of compromised joint seal is enough to allow the flow of chloride-laden water below the deck surface. An analogy may be made to the gutters on a home. If a house has 50’ of rain gutter, 48’ of which is functional and 2’ of which is not, the entire gutter is non-functional.
Addressing the Problem
In the early 1990s VDOT acknowledged the difficulty of maintaining expansion joints and began to experiment with the concept of eliminating the joints entirely. VDOT began to place closure pours in the locations of existing joints, essentially eliminating the joint entirely. These closure pours were initially placed over pier supports and called “link slabs”. Later, VDOT began to install closure pours called “deck extensions” at the ends of bridges over the abutments. Since that time we have successfully eliminated over 700 joints on more than 300 bridges. Figures 4 and 5 provide typical engineering details for link slabs and deck extensions. Figures 6 – 10 show photographs of the joint elimination process.
The performance of the link slabs has been excellent. While small cracks often appear at the interface between the closure pour and the existing deck, these cracks don’t allow significant moisture to reach the bearings. Link slabs have been used on highly skewed and heavily travelled (interstate) bridges with good results.
Considerations Affecting Decision to Eliminate Joints
VDOT has established that joint elimination is the most cost-effective maintenance option when viewed from the perspective of life-cycle investment. Typical costs to eliminate joints are about $700 per linear foot, which is an investment that pays strong dividends in the overall durability of bridges. VDOT’s current guidance requires that joint elimination be evaluated as the primary joint treatment on all rehabilitation projects. Each of VDOT’s 9 districts has a target to eliminate 1% of its joints per year. While our standard 4,000 psi low-permeability concrete has provided good results, VDOT has also begun to deploy flexible concrete mixes utilizing fiber reinforcement.
The two greatest impediments to joint elimination are traffic control and apprehension about the effects on the piers. In high traffic environments it is difficult to obtain approval for lane closures of adequate duration for joint eliminations. In order to address this concern, VDOT is developing generic details for temporary driving surfaces that will span the opening required for joint elimination. This will allow contractors to perform the work over multiple shifts.
To address concern about pier loads we are developing standard calculation templates to allow for quick analysis of pier loads after joint elimination. Because joint elimination changes the articulation of a bridge, certain piers will receive additional lateral loads after project completion. These lateral loads are rarely enough of a concern to preclude the elimination of a joint, but example calculations currently under development will provide an additional level of security for designers in the future.
For more information, please contact Adam Matteo at Adam.Matteo@vdot.virginia.gov