A large concrete water main that exploded this spring along busy Connecticut Avenue in Chevy Chase has brought to light a little known local distinction: The Maryland suburbs have more of a notoriously problematic stock of pipe than almost any major U.S. water utility.

The Washington Suburban Sanitary Commission has 350 miles of concrete mains that have been prone to exploding without warning. The particularly large mains are designed to carry high volumes of pressurized water. Utilities around the world have struggled with this type of pipe since the 1980s, when they began bursting decades before their 100-year life expectancy was up.

The WSSC’s inventory of large concrete pipe is second only to Detroit’s and two to three times that of many other U.S. cities and suburbs, according to a Washington Post survey of 21 large utilities.

Many of the WSSC’s large concrete mains were installed during the 1970s, when industry standards in place at the time permitted design and manufacturing changes that were later found to make the pipes significantly more prone to breaking. The WSSC also has significant amounts of pipe from a defunct New Jersey company blamed for making some of the most vulnerable mains during the 1970s and 1980s, The Post found.

“Washington Suburban stands out as having the biggest problem of failed [concrete] pipes, especially in the East,” said Wayne Brunzell, an engineering consultant and chairman of the American Water Works Association’s committee that sets industry standards for concrete mains.

“Their problems are much more severe than the average utility,” Brunzell said.

The scope of the problem has been a key question among local officials wrestling with how to best protect people who live, work, attend school and drive within what the utility has declared an 80-foot danger zone of the massive mains’ potentially explosive forces.

The pipes span up to eight feet in diameter, big enough to hold a minivan. Because they carry so much pressurized water, they can blow like a bomb, leaving 50-foot craters in roads and hurling rocks and other debris “like shrapnel,” said Gary Gumm, the WSSC’s chief engineer.

The 350 miles of large concrete pipe — technically called prestressed concrete cylinder pipe — form the backbone of the 5,600-mile water distribution system for 1.8 million people in Montgomery’s and Prince George’s counties. The large transmission mains carry water from the treatment plants to the smaller pipes that reach into neighborhoods. After decades of development, some of the mains, buried in what was once the countryside, now sit just beyond back fences and alongside or beneath major roads.

In the densely populated Washington suburbs, they’re too big to move, WSSC officials say, and replacing all of them would cost a prohibitive $2.9 billion. Doing so also wouldn’t be cost-effective because inspections have shown that only 1.5 percent of concrete pipe sections need to be repaired or replaced, WSSC officials said.

Nine of the WSSC’s concrete mains have blown apart since 1996. The most dramatic incidents occurred in 2008, when motorists had to be rescued by boat and helicopter from a torrent of water on River Road in Bethesda, and in 2011, when a broken main blew out doors and walls in a Capitol Heights office park.

The Chevy Chase pipe happened to rip open on the side farthest from Connecticut Avenue, sparing motorists from a 40-foot geyser.

The WSSC has denied requests for maps of the mains’ locations, citing an exemption in Maryland’s public records law that allows a utility to do so out of homeland security concerns. However, WSSC officials have said 1,768 homes, schools and other buildings are within the 80-foot danger zone.

“It’s everywhere,” Gumm, the WSSC’s chief engineer, recently told a panel of local officials studying the issue.

In addition to a potential public safety hazard, the WSSC’s decaying pipes pose a financial burden for residents and businesses. WSSC officials cited the utility’s aging infrastructure as the basis for a 50 percent rate increase over the past six years, and utility experts have said they expect those rate increases to continue.

Like much of the country’s infrastructure, water systems are deteriorating faster than they can be replaced. According to a recent Environmental Protection Agency estimate, the nation’s water distribution systems will require $384 billion over the next 20 years to keep drinking water safe.

But the breaks in the WSSC’s concrete pipes have little to do with age. The 60-inch main that burst in Chevy Chase was 33 years old, relatively young in an industry where pipes can date to the Civil War.

Instead, the WSSC’s concrete transmission mains owe their troubles more to bad timing and challenging geography.

The Maryland suburbs grew far more quickly than anticipated in the 1960s and 1970s. The WSSC had to rapidly expand its water distribution system and installed 1,685 miles of new pipe of all sizes and materials between 1959 and 1976, according to a utility history.

During those years, concrete pipes reinforced with tightly wrapped steel wire were the least expensive for contractors seeking the lowest bid and the only option for the largest pipes, which were then unavailable in steel.

Most of the WSSC’s concrete pipe came from Interpace, the defunct company, which had a manufacturing plant near Baltimore, a longtime WSSC engineer said. That allowed Interpace to keep its trucking costs low and, in turn, clinch more low bids, the engineer said.

Interpace and other pipe manufacturers at the time took advantage of changes in industry standards that allowed them to produce cheaper — but potentially weaker — pipe. Interpace, which was later successfully sued by the WSSC and other utilities, made pipes with reinforcing wire that was particularly prone to corrosion.

Other suburbs also boomed during the era of poorly made pipes. But in Northern Virginia, delivery to homes is carried out by a number of water utilities, with Fairfax Water serving as a sort of wholesaler. As such, it required smaller distribution pipes and far fewer of them, utility officials say.

Jamie Hedges, director of Fairfax Water’s planning and engineering division, said the utility replaced about five miles of concrete Interpace pipe a decade ago after it had some breaks. She said the system’s 109 miles of concrete pipe have had three breaks — one of them occurred when a contractor accidentally hit a pipe — in the past 17 years. Moreover, she said, most of Fairfax Water’s concrete mains are three feet in diameter or less — significantly smaller than many WSSC pipes — so they probably wouldn’t cause as much damage if they broke.

“We don’t have any concerns about the prestressed [concrete] pipes that remain in our system,” Hedges said. “We just haven’t had those catastrophic failures.”

The WSSC, on the other hand, must move water across 1,000 square miles. Most of the utility’s water comes from the Potomac River, on the western border of its service area. The rest comes from the Patuxent River, at its eastern edge.

Moving 170 million gallons of water daily around the Capital Beltway, up toward the Frederick and Howard county lines, and down into southern Prince George’s with enough pressure to run showers and flush toilets required hundreds of miles of large pipe, utility experts said. Most of those mains were too big — and too expensive — to be anything but prestressed concrete, utility experts said.

The District has 37 miles of concrete pipe. The city’s older water system was mostly built out by the time concrete pipes became widely used, said George S. Hawkins, D.C. Water’s general manager.

Hawkins said the utility stopped installing new concrete pipes in 1985 and replaces deteriorated sections with ductile iron. D.C. Water has had one break in a concrete main since 1996, he said.

“We watch them pretty carefully,” Hawkins said. “We know exactly where they are.”

The scope of the WSSC’s problems has forced it to become a leader in managing its larger pipes to detect breaks before they occur, industry experts said.

By late summer, the utility expects to have inspected about 18 miles of concrete pipe in a 14-month period, up from 8.2 miles in fiscal 2008.

The WSSC also has spent $21.2 million since 2007 on a break-detection system designed to record the sounds of the reinforcing wire snapping as the pipe weakens. It is supposed to provide enough warning of an impending break to allow the utility to shut down a pipe before it explodes — something WSSC officials say they have done twice. The WSSC also accelerated repairs on 12 other sections of pipe after they had a high number of wire breaks, Gumm said.

The WSSC now has 77 miles of concrete pipe monitored by the acoustic equipment — the most of any U.S. water utility, according to Pure Technologies, the Columbia-based firm that patented the technology. The system provides alerts of a wire break almost daily, Gumm said.

The WSSC is about to begin using robots to inspect 68 miles of smaller concrete mains. Those pipes also will have acoustic equipment installed for monitoring.

But the Chevy Chase pipe break exposed a gap in that break-detection system. The main’s acoustic cables never gave an alert about an impending break because it blew apart near a joint — a part that has no reinforcing wire to monitor. About 6 percent of breaks in large concrete pipes occur near joints, according to an industry study.

That leaves the utility wrestling with what to do next, with no easy — or inexpensive — answers. The WSSC has proposed requiring that new buildings be set back at least 80 feet from its largest mains or be reinforced to withstand the blast of a break. But local officials and developers have balked, saying it wouldn’t protect people already within the 80-foot zone and would deter the kind of dense development needed to spur and focus economic growth.

As the WSSC struggles with problems from its past, it must continue to plan for the future. An 84-inch pipe is being installed in a new tunnel beneath the northern side of the Beltway between Rockville and Kensington. It is intended to be the last major link in the WSSC’s water transmission system.

That pipe is made of steel.