What is a Waterproofing Membrane for Bridge Decks?
A bridge deck waterproofing membrane is a layer under an asphalt overlay that slows water and chlorides entering a concrete deck.
What it does
Chlorides and moisture can cause corrosion deterioration in a bridge deck. Protection systems are placed on bridge decks to slow that intrusion.
An asphalt overlay alone does not stop it. Conventional asphalt is permeable on a deck, because vibratory compaction is not allowed there. A membrane must be installed before the overlay is placed. The FHWA Bridge Preservation Guide lists asphalt with a waterproof membrane as one deck overlay treatment.
Sheet and liquid membranes
Membranes come as preformed sheets and as liquids. The deck decides which one fits. A grid deck may puncture a sheet. A liquid conforms to the surface contours, so its thickness varies on an uneven deck.
How a membrane fails
A membrane slows water. It does not stop it. Departments of transportation have reported that membranes often leak. On a Virginia bridge, a crack or open construction joint reflected through an epoxy membrane. Water reached it through the permeable asphalt. A repair would require removal of the asphalt and the membrane near the crack.
A weak bond is a second problem. On the same bridge, the asphalt debonded from the membrane when the crew took cores. That is typical for a bond strength below 50 psi.
A study of four decommissioned Utah decks on Interstate 15 found that a membrane and overlay, applied in a timely manner, can preserve a deck.
Airfield bridges
FAA AC 150/5300-13B, section 6.2, covers airfield bridges. Paragraph 6.2.3 sets dimensions, parapet height and loads. It has no requirement on deck waterproofing.
Frequently Asked Questions
- Does an asphalt overlay waterproof a bridge deck?
- No. Conventional asphalt is permeable when it is placed on a bridge deck, because vibratory compaction is not allowed on decks. A membrane must be installed before the overlay is placed.
- Do bridge deck membranes leak?
- Yes, they can. Departments of transportation have reported that membranes often leak. A crack or open construction joint in the concrete can reflect through the membrane, and the permeable asphalt then lets water reach it.
- How can you tell that a membrane leaks?
- Moisture on the underside of the deck is an indication that the asphalt and the membrane are permeable. A bond test on cores gives a second indication, because asphalt that debonds during coring typically has a bond strength of less than 50 psi.
References
- vtrc-13-r5
- Evaluation of the Installation and Initial Condition of Rosphalt Overlays on Bridge Decks (VCTIR 13-R5) Virginia Center for Transportation Innovation and Research, with FHWA
- fhwa-bridge-preservation-guide
- Bridge Preservation Guide: Maintaining a Resilient Infrastructure to Preserve Mobility (Spring 2018) FHWA
- byu-sumsion-2013
- Condition Assessment of Decommissioned Bridge Decks Treated with Waterproofing Membranes and Asphalt Overlays Brigham Young University (master's thesis, E. S. Sumsion, 2013)
- faa-ac-150-5300-13b
- AC 150/5300-13B, Airport Design FAA
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