PROVIDENCE — The state Department of Transportation said yesterday that it will have to replace the concrete deck on the entire Providence Viaduct, the elevated structure that carries Route 95 past the city’s downtown.

It is one of the most heavily traveled sections of highway in the state, and the DOT’s chief engineer, Kazem Farhoumand, said that the traffic disruptions that would result from removing even one lane could prove unacceptable. That could force the construction of a temporary bridge while the work is going on, he said.

However, he said, “It’s safe. I don’t want to give the wrong impression.”

The viaduct joins a list of urgent projects that the DOT says requires a massive increase in spending, in the range of $300 million per year, to catch up for years of lacking maintenance.

Also yesterday, TRIP, a Washington-based group affiliated with the road-building industry, issued a report repeating a number of DOT arguments that the state should spend more money on its roads and bridges. The report was timed to precede the election, where an $87-million transportation bond issue, $80 million of which would benefit the DOT, is on the ballot.

Farhoumand said replacing the viaduct deck is now the DOT’s third highest priority, after the replacement of two other critical, deteriorated bridges, the Route 95 bridge over the Pawtucket River, north of the viaduct, and the Sakonnet River Bridge, connecting Aquidneck Island with Tiverton and Southeastern Massachusetts.

Officials said that they have just begun design work for the viaduct project and that construction of the new deck is at least two years away. They said they don’t have a cost estimate yet. The cost is likely to be high. Asked if there’s any possibility that the deck can be replaced for less than $100 million, Michael P. Lewis, the director of the state transportation department, said, “It’s always possible.”

The quarter-mile-long viaduct carries the interstate highway across several roads and Amtrak’s railroad tracks and past the Providence Place mall. It is heavily traveled, carrying 170,000 to 200,000 vehicles per day, Farhoumand said.

“You really can’t shut down one lane,” he said, adding that the traffic problem could force the construction of a temporary structure while the work is going on.

Like most Rhode Island bridges, the viaduct’s deck is concrete with steel reinforcing rods inside. In many of the bridges, road salt has penetrated the concrete and caused the steel to rust and expand, forcing the surrounding concrete off and leaving the reinforcing exposed.

In the case of the viaduct, Farhoumand said, concrete is popping off both the top and bottom of the deck. On the bottom, wooden planks are visible in some places, held up by the flanges of the I-beams supporting the deck. The planks are there to keep chunks of concrete from falling onto traffic, he said.

The viaduct can be resurfaced, but that won’t cure the underlying problem. With new paving on top, Farhoumand said, the viaduct’s surface “looks very nice,” but the deterioration of the concrete underneath means the deck must be replaced. “It’s going to come to the point that you can’t keep paving on it,” he said.

If the concrete in one area crumbles completely, leaving the steel reinforcing bare, Farhoumand said, “You have the rebars there to stop you,” he said.

But he said that if holes appear, the DOT will patch the deck by cutting out the bad section and pouring new concrete. If a hole goes all the way through, he said, the DOT’s contractor would cover the bottom with a plywood form to hold up the new concrete until it could set.

blandis@projo.com