Concrete: Wonder Material in Aggregate

A miracle material invented by the Romans is still going strong. After more than two thousand years the material, cement-based concrete, is still durable, innovative, and cost competitive.

Consider the I-64 construction project. The Missouri Department of Transportation (MoDOT) picked concrete for the road material over asphalt. Dan Galvin, spokesman for Gateway Constructors, which is building the highway, said MoDOT looked at both the lifecycle costs and initial costs of using either material. Concrete has a lower lifecycle costs, because it requires less maintenance, he said. And due to the high price of oil at the time, the initial cost difference between asphalt and concrete “was minimal,” he said. So, MoDOT concluded taxpayers would get the most value from concrete.

Although the cost difference between asphalt and concrete for the reconstruction of I-64 may have been minimal, the amount of material used in the project is not. In the first half of the project, the portion west of I-170, Gateway Constructors poured 145,000 cubic yards of concrete in depths ranging from 9.5 to eleven inches, “and we should do something similar with the second half as well,” Galvin said. The eleven-inch depth comes from a full depth pour where Gateway Constructors completely removes and replaces the old concrete and roadbed. The 9.5-inch depth is when Gateway Constructors simply overlays new concrete on top of the existing road surface. In addition to the road surfaces, Gateway Constructors also poured 40,000 cubic yards of structural concrete in bridges and 85,000 linear feet of concrete in barrier walls. Gateway Constructors also set 264 precast concrete bridge beams, 149,000 square feet of precast concrete bridge deck and 290,000 square feet of precast concrete retaining wall.

“We are confident concrete is the most durable and cost effective pavement our tax dollars can buy,” said Patrick Juelich, marketing director for the St. Louis Concrete Council. “The benefits of concrete construction on the I-64 project and future roadways are more than getting a pavement that will last for 50 years. Another benefit is it supports job creation in Missouri through the use of rock, sand and cement all produced in and around the state,” he said.

Of course, concrete has been used in roads since the time of the Romans. Much more recently an innovative form of concrete has been used to reduce run-off from pavement and manage stormwater. Last year, the Metropolitan St. Louis Sewer District (MSD) approved something called pervious concrete for an exemption from its imperious user charge. “We have a charge based on the amount of impervious area – the area under surfaces that don’t suck up rainwater – a user has. If the paved area is pervious, where the water can percolate through, we do not assess that charge,” said Lance LeComb, a spokesman for MSD. “In the long run, we hope this impervious charge will spur the use of these greener, more environmentally friendly surfaces,” he added.

Pervious concrete works by letting water drain through to the gravel bed underneath it, explained Jerry Eidson, sales representative for Breckenridge Material Company. It can do that because it is full of interconnected spaces, “we’re talking 10-20 percent voids,” he said. That means pervious withstands repeated freeze-thaw cycles better than traditional concrete, he said. “Traditional concrete is very dense,” he said. Pervious concrete, however, has so many voids that water doesnt’t get trapped in cracks, but instead flows through to the gravel bed where the subgrade warmth keeps it from freezing, and all the voids in the concrete leave plenty of room for expansion and contraction.

“Pervious is new to the area. Five years ago you never saw it,” said Scott Inserra, president of Wilson Concrete, a concrete contractor, who has completed one pervious concrete job in the area.

Landvatter Ready Mix began making pervious concrete in 2007, although it has been used in other parts of the country, said Melissa Landvatter, quality control and sales manager for Landvatter Ready Mix. “It is a hugely different mix from traditional concrete,” she said. “It is a very dry, very thick mix, and the mix design is crucial to its success,” she said. Landvatter has made pervious concrete for three different jobs in the St. Louis area, but sees many more jobs requesting bids for it. “This potentially could be a significant market,” she said.

“What really kicked it off was MSD started raising stormwater rates. When big commercial parking lots and campuses started getting dinged on their MSD bills last year, they started looking at pervious concrete,” said Eidson.

Joseph Vitale, president of Concrete Strategies, which first installed pervious concrete at Lambert-St. Louis International Airport, agreed that storm water management is the biggest draw for pervious pavement. “Storm water infiltration using pervious pavement will become common practice as the engineering and construction community become more familiar with utilizing this green building tool as part of their Best Management Practice toolbox,” he said.

Cost

Pervious concrete costs 10-15 percent more from the supplier, “but you recoup that in your MSD bill,” Eidson said.

Inserra said the labor charge to install pervious concrete “is about the same” as the cost in installing traditional concrete, but the installation process is different.

“The installation process is unique and takes special tools and experience,” agreed Don Wade, program manager for Magruder Construction, a concrete contractor that has installed pervious concrete in St. Louis and elsewhere. “The cement/concrete industry requires that installers become certified through the National Ready-Mixed Concrete Association (NRMCA),” he said. There are three-levels of certification ranging from technician to crafstman. The first level, technician, requires attendance at a one-day training seminar and successfully completion of a two-hour test. The second level requires an additional one-two days experience in addition to the test. The third level, craftsman, requires at least 3,000 hours (almost two years) of verified experience. Magruder Construction has three employees that are NRMCA certified as craftsmen.

“I recommend that only those companies that have the craftsmen-level of experience be allowed to bid on any projects in the St. Louis area,” Wade said.

Savings

When properly designed as part of a new stormwater management system, pervious pavement can save an owner or developer money. “The cost advantages associated with using pervious concrete to manage stormwater go beyond eliminating MSD’s impervious charge of 14 cent per 100 square feet,” Vitale said. “Traditional pavements usually require traditional detention based stormwater management practices. With pervious pavement, stormwater can be detained under the pavement, where it infiltrates into the ground and reduces the need for storm sewers, detention basins and related infrastructure. Commercial users could realize a 45 percent savings in total ownership costs over the 20-year design period by using pervious concrete for the paving element in the design. Developers have been able to add more than 40 percent more retail rental space for projects that would have been required for detention,” he said.

“If your are able to eliminate storm sewers and detention structures from your project, the cost savings are overwhelming. If you can eliminate a half acre detention pond, that is 1/2 acre you don’t need to buy,” Inserra added.

Wade said pervious concrete is as durable as traditional concrete, which is more durable than asphalt. “Concrete gets stronger with age while asphalt deteriorates over time,” he said. Vitale agreed. “Parking areas properly designed and constructed will last 20-40 years with little or no maintenance,” he said. Pervious pavements, however, cannot be resurfaced as is often done with conventional asphalt pavements, Wade said, and routine maintenance practices have to change to avoid pushing dirt and debris down into voids to clog them.

Want to make sure the pervious mixture will work? A testing company such as Geotechnology, Inc., can test the permeability of the mix to measure water flow and can even test core samples for permeability after the concrete is poured.

With Clayton, Wildwood, Washington University, and the Missouri Botanical Gardens all planning to use pervious concrete, St. Louis will see more of this innovative form of an old miracle material.

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