Streets & Highways: New York DOT develops high performance concrete

What could be more tried-and-true than fly ash, something the Romans used for roads and a coliseum that are still intact today. The Romans discovered the durability of fly ash by accident after eruptions from the Mt. Etna volcano on Sicily.

Today, thousands of years later, the New York State Department of Transportation is using fly ash in a new high-performance concrete, designated Class HP.

Class HP concrete, consisting of 20 percent Class F fly ash and 6 percent microsilica (a byproduct of computer chip manufacturing), has better handling and workability characteristics than conventional concrete, reduced permeability and a greater resistance to cracking and scaling, says Don Streeter, field engineer for NYSDOT.

Reinforcing corrosion caused by concrete spalling has been the most common failure mechanism for bridge decks in New York state.

Based on computer modeling, with primary consideration given to concrete permeability, Class HP concrete is expected to increase the service life of a bridge deck to two to three times that of Class H concrete (the most commonly used of 10 different concrete classes in New York).

The model assumes 77 mm of concrete cover over black reinforcing steel, resulting in initiation of corrosion after 23 years in service for Class H and 62 years for Class HP. And because NYSDOT uses epoxy-coated reinforcing, the time before corrosion begins will be further increased.

While it would normally take about 35 years until significant maintenance was needed with Class H concrete, Class HP concrete should double that, says Streeter.

The original field test for Class HP concrete lasted just two months during 1994. “It was more verification than it was research,” says Streeter. The results showed a 12 percent cost increase for the concrete; other projects incorporating Class HP concrete averaged a 10 percent cost increase. Some increase is to be expected when using a new material by order-on-contract, Streeter says.

NYSDOT was concerned about possible higher costs, but found that bid prices for Class HP concrete on projects built in 1995 ranged from 25 percent less to 35 percent more than Class H prices.

Using a conservative life expectancy for Class HP that is double that of Class H and allowing for the higher material cost for Class HP, the expected life cycle cost would result in annual savings to NYSDOT of about $9 million.

NYSDOT has completed over 50 placements to date, has established Class HP concrete as the required class for all bridge deck construction, and also plans to use it in substructures. Several counties in New York, including Saratoga and Monroe, also are using Class HP concrete.