Best Practices for Bridge Deck HMA Paving

Abstract

Newly constructed or rehabilitated bridges are frequently surfaced with hot mix asphalt (HMA); however, achieving high quality asphalt mats on bridges presents many challenges.  To prevent damage to the bridge, vibratory compaction is usually not permitted.  Additionally, coring to determine in-place asphalt mix properties and level of compaction is not desirable due to the need to protect the deck surface and any water-proofing membranes from damage during core extraction. 

Paving contractors often view and approach bridge paving in a similar way to roadway paving.  Bridges introduce a substantial level of complexity to paving operations that are often not adequately evaluated or understood.  Specialized compaction equipment is often required and attention to substrate surface preparation and tack coating may be lacking, resulting in a suboptimal final product. 

Bridge deck paving usually requires small quantities of HMA, often hauled to remote locations.  Hauling HMA for several hours can compromise available compaction temperature and result in under-compacted asphalt.  Although asphalt mixes may be available from smaller local asphalt producers where the haul distances would be shorter, this may mean using HMA that does not meet agency specifications. 

In order to ensure high quality, some agencies prescribe the method of compaction including size and type of rollers, specialized compaction equipment for compacting against bridge curbs/barriers/parapets, minimum compaction temperatures, and number of passes, in lieu of being able to measure the in-place density.  Some agencies require a premium-quality HMA for bridge decks, regardless of the site-specific demands such as traffic count, bridge size and bridge importance.  This can result in long haul of high-quality material from larger plants able to meet the specifications, usually arriving on site with less-than-optimal compaction temperatures.  This long haul can negate the quality of the HMA if it cannot be properly compacted due to heat loss.  Smaller local asphalt producers could be used to produce HMA for less important applications and reduce the difficulties related to the long haul.

Bridge decks with HMA should provide a smooth surface and adequate skid resistance, facilitate drainage, and protect underlying waterproofing membranes and reinforced concrete decks from deterioration due to penetration of water and deicing salts.  The surface should be durable and able to withstand traffic and environmental demands over the selected design life.  It must have a good bond to the substrate, and the bond must be durable over the design life.  Both construction cost and life cycle cost should be considered in HMA specification selection to balance cost and longevity.  The surfacing system should be one that can be installed and maintained by a trained local contractor.  Speed of construction should be considered to minimize lane closures.

In several respects, a bridge deck wear surface can be subjected to more severe conditions than highway pavements. Bridge deck surface temperatures can vary significantly and be different from neighboring highway pavements.  Cracks in the asphalt will cause accelerated deterioration of the HMA and underlying membrane.  Bridge deck pavement environmental conditions should therefore be treated separate from highway pavements, and the wear surface HMA should be specified considering these bridge-specific demands.

The project is intended to focus only on HMA paving of bridge decks.  Wear surface selection considering other material types is an important aspect of the design process, but is not intended to be part of the present scope.  Similarly, it is important that the owner satisfy themselves as to the condition of the underlying bridge deck structure before repaving.  It is assumed that these aspects are covered by the owner agencies as part of their project planning.

The project’s main objectives include:

• Development of a list of key input data for categorization of bridges with respect to HMA wear surface specification and construction considerations, for instance: traffic AADT and percent heavy trucks; bridge deck surface area; environmental demands (precipitation and temperature); bridge deck construction details; use of deicing chemicals; waterproofing system; distance to asphalt plant, minimum skid resistance, and design life.
• Suggested specifications and construction details for each category.
• A flowchart to determine the best HMA wear surface design/specification for any specific bridge HMA wear surface construction or renewal.
• Suggested QA/QC minimum requirements and test methods, preferably methods in keeping with an End Product type specification.

The final report for this project will be a Guide to Bridge Deck HMA Paving in Canada that includes findings from the literature review and survey of Canadian agencies; key performance indicators; selection criteria; a flowchart for individual projects; suggested specifications for various alternatives; and construction QA/QC minimum requirements and test procedures.