UPDATE 3-Some Republican U.S. senators snipe at $1 trillion bipartisan infrastructure bill

(Adds details on cryptocurrency provision, background)

By Makini Brice and Richard Cowan

WASHINGTON, Aug 2 (Reuters) - The U.S. Senate will try to complete work this week on a $1 trillion infrastructure investment bill that would bring long-awaited improvements to roads, bridges and mass-transit systems, even as some Republicans began complaining about the details.

Following long weekend sessions, senators on Monday began voting on amendments to the 2,702-page bill.

But battles already were shaping up.

Republican Senator Pat Toomey said in a statement that the bill's proposed tax reporting regime for cryptocurrencies was overly broad and unworkable. He said he plans to offer an amendment to change it.

Congress' Joint Committee on Taxation estimates the provision would raise $28 billion in new revenue over 10 years, which is important to lawmakers' goals of paying for the costs of the infrastructure bill.

"Congress should not rush forward with this hastily-designed tax reporting regime for cryptocurrency, especially without a full understanding of the consequences," Toomey said in an emailed statement.

Senate leaders were trying to reach a deal on the number of amendments that would be put to votes, as the chamber's 100 senators were eager to begin an August recess.

The legislation, if enacted, would be the largest U.S. infrastructure investment in decades. Its passage would mark a major win for President Joe Biden, a Democrat, and the deeply divided Congress. It would come on the heels of a $1.9 trillion economic stimulus and coronavirus aid bill that was enacted earlier this year without Republican support.

The measure would include $550 billion in new spending over five years for items such as roads, rail, electric vehicle charging stations and replacing lead water pipes on top of $450 billion in previously approved funds.

Its wide-ranging provisions include $343 billion for improvements on highways, bridges and related projects, $48.4 billion to make drinking water and water infrastructure safer, $7.5 billion to help construct electric vehicle charging stations - far below what Biden had sought - and $350 million over five years for reducing vehicle collisions with wildlife. The bill would provide grants for "wildlife crossing structures."

The Joint Committee on Taxation, the nonpartisan research arm of the U.S. Congress, projected tax provisions of the bipartisan Senate infrastructure bill would increase federal revenue by $51 billion over 10 years.

While the $1 trillion bill represents a large investment, the American Society of Civil Engineers estimates that over the next two decades the United States needs to spend around $13 trillion to upgrade aging public works projects that they have graded at a "C-" level currently.

The group said in its 2021 assessment that there is a $5.6 trillion gap between infrastructure funding that is needed and what is being planned across the country.

The $1 trillion bill also would clear the way for Democrats, acting without Republican support, to begin work on a budget framework that would sketch out plans for a $3.5 trillion "human infrastructure" bill.

That bill would shovel federal dollars toward fighting climate change, help millions of immigrants gain legal protections and fund an expansion of healthcare, including for senior citizens needing assistance at home.

The Senate first needs to put the finishing touches on the infrastructure bill, which also would expand high-speed internet access to rural areas where economies have been hobbled by old technology.

Barring surprise developments, the bill could be ready for a final vote as early as this week, according to some senators.

Nevertheless, on Monday, three Republican senators - John Cornyn, Rick Scott and Martha Blackburn - criticized the infrastructure bill, saying they had inadequate time to consider it and taking issue with one of the ways it would be financed. (Reporting by David Morgan, Makini Brice, Richard Cowan, Susan Cornwell and David Lawder in Washington Editing by Alistair Bell and Matthew Lewis)