From Alaska to the Bahamas, Minnesota to Texas, municipalities are taking a deeper look at the durability and life-cycle costs associated with manhole collars. Too often these systems for topping off, reinforcing and adjusting road elevations are a secondary line item in a larger storm sewer installation project, lumped into the overall task of final adjustments.
Conventional practice is to use concrete or brick and mortar to form a collar—which is often the first component to deterioration from the constant pounding and vibration of vehicles and/or weather, especially freeze/thaw conditions. Led by some proactive agencies in Minnesota, public works engineers around the country are rethinking concrete collars. They are switching to fully recycled high-density polyethylene (HDPE) manhole adjusting rings that are more economical, lighter and durable.
Inside the Ring
First manufactured in the 1990s, the LadTech HDPE manhole adjusting rings are made from 100% recycled plastic or high-density polyethylene (as defined by ASTM Standard D4976). The rings are designed to meet ASTM and AASHTO HS-25 specifications. The LadTech HDPE manhole adjusting ring was tested by American Engineering Testing, a third-party laboratory. The rings were loaded to 60,000 lbs and remained in serviceable condition. In fact, failure of the concrete catch basin and the manhole cover frame assembly halted further testing. Water penetration tests under ambient laboratory conditions were also conducted and showed no significant leakage.
Comparing the load/deflection performance of the ring stack exposed to asphaltic concrete with a similar unexposed ring stack, the deflection of the exposed ring stack was less than the unexposed stack (0.173 in. vs. 0.226 in.). In addition, HDPE materials will not corrode or deteriorate when exposed to the harsh hydrogen-sulfide environment found in most sewer systems.
The HDPE collars were first installed on sanitary manholes in 1996 in Apple Valley, a community outside Minneapolis-St. Paul, Minn. Jim Fruechtl, former city engineering construction coordinator with Apple Valley Public Works Dept., recalls, “We had such a problem with manhole collars in our harsh conditions that we decided to give HDPE rings a try to see if laboratory testing translated to real-life conditions.”
The engineering department regularly checked the collars over the following winter and next few years. “There was no deterioration, cracking or breaking,” Fruechtl confirms.
He performed his own test, setting a steel plate on one of the HDPE rings and driving a loaded cement truck over it. He explains, “I wanted to see if I could crush it—but it passed with flying colors.” By 1998, Apple Valley specified HDPE collars exclusively. Fruechtl notes, “Initially contractors resisted because it was something new. But they learned to like them mostly because installation is much faster and safer.”
On average, it takes 1–2 hours to install a concrete manhole cover, and then the crew has to return later in the day to backfill to match grade. With the HDPE, installation is 20–30 minutes, and crews can immediately begin backfilling and tamping. The system is ready for asphalt overlay within 10.5 hours. Crews like working with the lightweight (6-lb) HDPE rings that don’t crack or break, and there’s no leftover material.
To-date, the HDPE rings are now sole-sourced in more than 70 Minnesota cities as well as many cities in states that face freeze/thaw conditions and those with challenging soil conditions. In McAllen, Texas, HDPE rings are helping increase longevity of manhole covers in the region’s highly acidic soils since HDPE is highly resistant to stress cracking from exposure to chemicals and gases.
Just recently, the Washington Suburban Sanitary Commission (WSSC), among the largest water and wastewater utilities in the nation, approved HDPE collars for use by the maintenance division. In total, more than 4 million HDPE rings are in service nationwide with no signs of breakage or degradation.
Fruechtl confirms, “We installed LADTECH rings 20 years ago, and they look exactly the same today. These installs have eliminated inflow and virtually eliminated reconstruct.”