Making it last

March 2, 2015

In Missouri, pavement preservation techniques innovate despite funding crunch

The largest asset for the Missouri Department of Transportation (MoDOT) is its pavement. MoDOT maintains more than 77,000 lane-miles of roads, making it the seventh-largest highway system in the U.S.

An agency’s ability to maintain pavement conditions is impacted by multiple factors, not least of which are reduced funding, harsh winter weather conditions, changes to the road surface at times needed for life-saving innovations, and dealing with sometimes unique road users and their impacts to the pavement. 

MoDOT operates its large highway system with the 46th lowest state revenue per paved mile, which is primarily funded by a 17-cent-per-gallon statewide fuel tax—a tax that has not been increased in nearly two decades. Fuel tax is an ever-diminishing funding source now that more fuel-efficient cars are being mandated and are occupying the roads; moreover, increased cost of materials to do pavement repairs can now eat ravenously through any modest budget. With voter-authorized bonding, MoDOT’s funding peaked at $1.3 billion in the past few years. However, by 2017 MoDOT’s funding will drop to $325 million per year. That is a 75% decline. 

MoDOT spends an average of $47 million per year clearing snow and ice off of its roads. In the winter of 2013-2014 alone, MoDOT spent $73 million dollars on its snow- and ice-removal efforts. Severe winters are a double-edged sword for MoDOT’s pavements. Removal of ice and snow uses funds that could be put toward pavement preservation, and as the cost of road salt continues to rise, winter operations usurp a larger chunk of the state’s maintenance budget. 

The increased efforts to remove snow and ice not only limit the funds available to preserve pavements, but they also increase the amount of damage sustained by the pavements. Treating roads to melt winter precipitation introduces freeze-thaw cycles that are tough on pavements, because as water enters cracks in the pavement, it freezes and expands and then thaws, leaving rifts and fissures in its wake. There is an insidious aspect to this process. This dynamic cycle expands the size of the crack incrementally; the more it happens, the faster the cracks become big. Road salt and deicing chemicals can chemically decrease the bond between the asphalt binder in asphalt pavements and its aggregates. Abrasives applied to the pavement surface to aid in vehicle traction can cause degrading physical damages to the pavement surface. The removal of snow and ice from the pavement using mechanical devices such as snow plows also causes physical damages to the pavement surface. All of these efforts can result in the stripping of an asphalt surface in which aggregates are lost from the surface in large quantities. 

Other damages are spalling in concrete surfaces where large portions of the surface of the concrete pavement separate from the concrete slab. Sometimes even a complete failure of the pavement occurs after a harsh winter on a tender, damage-prone stretch of pavement. The cost of winter weather to Missouri is twofold—the initial up-front cost of the snow- and ice-removal operations and the immediate and lasting impact to the pavements.

Thus, in anticipation of the aforementioned plummet in revenue, MoDOT staff has implemented several innovations to help meet the challenge of preserving Missouri’s pavements with reduced funding. These innovations include scratch and seal, next-generation microsurfacing and rumble stripe surface sealing.

Crumbling rumbles

Missouri crash statistics show 47.9% of the fatalities and 41.3% of the serious injuries over the past three years were attributed to vehicles leaving the roadway. This is the second-highest cause of traffic-related fatalities in Missouri. Consequently, MoDOT has focused on finding ways to reduce the number of crashes resulting from vehicles leaving the roadway, including the widespread use of edge-line rumble stripes. In addition, MoDOT has used multiple strategies including proper shielding of obstacles, adequate recovery areas, improved shoulders and public education on distracted driving. These strategies, along with the edge-line rumble stripes, have been very effective in reducing the fatalities resulting from these types of crashes by 33% from 2005 to 2011. Unfortunately, the construction of these rumble stripes is usually performed by grinding them into the pavement, exposing what is very often a porous aggregate to the elements. They also tend to hold water, which results in a higher deterioration rate due to freeze-thaw cycles. This very effective fatality- reducing strategy results in accelerated pavement edge deterioration. It is a situation of having to take the bad with the good.

Unique road users

MoDOT’s pavements endure heavy truck traffic and agricultural equipment on its large state highway system consisting of 34,000 miles and 10,000 bridges. While heavy trucks might not be unique to Missouri, their interactions with MoDOT’s roads are unique due to a couple of factors, primary of which is that MoDOT maintains a paved surface on a large portion of the collector road system in Missouri. This functional class of road is generally in rural areas and is designed for lighter vehicle traffic. Another factor is that Missouri has a large agricultural industry that uses very heavy trucks on its collector road system to deliver harvests and livestock. These two factors combine to create a significant challenge in maintaining this road system’s pavements. That alone is a lot to endure—but there’s yet another unique road user in Missouri. In several parts of the state, there are Amish communities. The Amish residents’ primary form of long-distance transportation is the horse and buggy. The metal horseshoes and very narrow buggy wheels are the culprits behind a great deal of surface damage. 

Double seal

The main combined impact of trucks, agricultural equipment and horse-and-buggy operators on Missouri roads is rutting and deformations on asphalt pavements. In response to the proliferation of these problems, MoDOT’s Northwest District Maintenance Engineer Martin Liles implemented in 2014 an innovative approach to provide minor corrections to asphalt pavement surfaces and sealed it with two different processes under the same construction project. 

The first process was the application of an asphalt scratch course in which the ruts and minor deformations in the pavement were paved over with a single and very thin lift of asphalt. The asphalt mix employed was a bituminous pavement PG 64-22 surface leveling mix with only a .25-in. thickness at the highest locations in the existing pavement cross-section. 

The second process, occurring 10 days later, applied a high-quality limestone chip seal over the scratch course. This double-process project replaced the 1-in. contract level course projects typically applied to these types of lower volume roads. It strategically addressed the surface deformations and rutting, and sealed the surface with a higher quality aggregate, thereby ensuring proper surface protection. This process provided little to no structural improvement of the overall pavement cross-section, but then neither do the typical 1-in. contract level course projects. 

To their benefit, however, these test projects came in at more than 10% under cost versus the typical 1-in. contract level course projects and performed essentially the same function with comparable results. Some other concerns with this type of project are due to the extremely thin asphalt application. With aggregate sizes in the mix larger than the minimum thickness, tears can form in the asphalt mat that may not be fully treated with chip seal. Also, the asphalt paver screed could incur damages from this tearing as well, which could result in higher contract prices if this is realized. In addition to monitoring the condition of the resurfacing project, MoDOT also engaged with its contracting partners to determine if there were screed damage issues and what could be done from a design perspective to mitigate this damage.

At the surface

What is next-generation microsurfacing? Micro-surfacing is usually performed on high-end hot-mix pavements that present with fewer distresses. Liles stated, “We need less expensive alternatives to treat as many miles of our roads as possible with a limited budget. If microsurfacing can accomplish this, then we have a solution that doesn’t require hot-mix asphalt plants in the proximity of many of our isolated rural roads.” For this reason, Liles looked at applying this treatment to lower-volume cold-mix-surfaced roads with higher amounts of surface distresses.  The contractor, Vance Brothers of Kansas City, employed various tactics such as adding fibers to the microsurfacing and adjusting microsurfacing application rates to assure its proper performance. This project is also an alternative to the typical 1-in. contract level course projects and came in at a 20% reduced cost while performing the same function. Applying a microsurface to a highly distressed, low-volume route is a new concept that has proven to be inexpensive up front. As more projects of this nature are developed in the future, the effective strategies incorporated into this project, both pre- and post-letting, will be used to ensure that these types of projects will save money over the life of the microsurfacing, as well. 

Ready to rumble

MoDOT’s innovative rumble stripes have been shown to save lives by alerting drivers when they are leaving the roadway. 

On a statewide level, several techniques were discussed on how to address the current rapidly deteriorating rumble stripes. The new technique ultimately chosen was to apply a surface or fog seal to the rumble stripes shortly after they were constructed—the idea being to seal the surface and prevent the water from entering the porous and exposed aggregate until it is removed from the rumble by traffic or evaporation. Application rates of the fog seal ranged from 0.05 to 0.15 gal per sq yd. The locations of these various applications were recorded and subsequently tracked for effectiveness. In addition to fog seals as a treatment, another method under consideration was applying a maltene-based asphalt rejuvenator, which would rejuvenate the asphalt binder that was oxidized and damaged from the paving and grinding process. The rejuvenated asphalt binder should provide a better bond with the exposed aggregate to help resist impacts from the elements.

It’s the good fight

All in all, the struggles of state DOTs to maintain an aging infrastructure with funding pools incapable of standing up to the backlogged demands is not something that is going to go away anytime soon. Finding innovative methods for addressing the most common problems in a cost-effective way is the most progressive means of, if not staying ahead of the problem, certainly not falling inexorably far behind. In Missouri, the hope is that these innovative methods will keep the roads functioning until such time as more invasive reconstruction methods can be realized. R&B

About The Author: R. Todd Miller, P.E., is contributing author for Roads & Bridges

Sponsored Recommendations

The Science Behind Sustainable Concrete Sealing Solutions

Extend the lifespan and durability of any concrete. PoreShield is a USDA BioPreferred product and is approved for residential, commercial, and industrial use. It works great above...

Powerful Concrete Protection For ANY Application

PoreShield protects concrete surfaces from water, deicing salts, oil and grease stains, and weather extremes. It's just as effective on major interstates as it is on backyard ...

Concrete Protection That’s Easy on the Environment and Tough to Beat

PoreShield's concrete penetration capabilities go just as deep as our American roots. PoreShield is a plant-based, eco-friendly alternative to solvent-based concrete sealers.

Proven Concrete Protection That’s Safe & Sustainable

Real-life DOT field tests and university researchers have found that PoreShieldTM lasts for 10+ years and extends the life of concrete.