Wondering why ITD seal coats highways? - Coeur d'Alene Press: Local News

Wondering why ITD seal coats highways?

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Posted: Thursday, July 12, 2012 11:20 am | Updated: 11:39 am, Thu Jul 12, 2012.

Every summer, particularly mid- to late-summer, seal coats and chip seals abound throughout the state. The Idaho Transportation Department reports that the practice always prompts many questions.

Here are a few of those questions, and answers, from ITD:

The short answer is that these applications protect the financial investment taxpayers have made in the road, and protect drivers.

Is it a seal coat or chip seal, and why are we doing it?

These two processes may differ ever-so-slightly in a few minor details, but it’s mostly a difference in semantics. The principle is the same – apply a protective layer of rock chips and liquid asphalt to the surface of the road to extend the life of the roadway surface and to provide better traction for drivers.

The ingredients in the two applications and the functions they serve have led many people to use the terms "seal coat" and "chip seal" almost interchangeably. In fact, a hybrid has recently evolved - the "chip seal coat."

It helps to minimize oxidation of the roadway surface. Oxidation causes the surface to dry out and the “pores” to open up to moisture, said Dave Kuisti, ITD southwest Idaho engineering manager.

“During the freeze/thaw cycles, any moisture allowed into the road will expand and contract, eventually causing the surface to crack and deteriorate, which in turn leads to potholes, broken pavement and repairs. The secondary reason for a seal coat is to create a rough, traction surface for winter driving,” Kuisti said.

Seal coating begins by applying a layer of liquid asphalt, then a cover coat of rock chips on the roadway surface. The highway is rolled and traffic allowed on to the roadway to help pack the surface. Like most ITD highway projects, the work is done by a general contractor with ITD inspecting the work to ensure quality standards are met. Seal coating helps to defer the cost of repairs that would be required much sooner without the protective application.

Why do we do it, especially over the top of a new, smooth roadway?

Many homeowners are dismayed when their new, smooth, blacktop road is seal coated or chip sealed. The smooth surface is covered with tiny rocks, eliminating many barefoot jaunts to the mailbox and potentially increasing the road-rash factor for bicyclists, skateboarders or other recreationists. However, a properly applied seal coat can add several years of service to a roadway by sealing the surface against weather and wear.

It is unfortunate timing that the best time to apply the seal coat is in the heat of the summer, but the primary benefit for drivers is greater traction when the snow flies several months later.

The work is done during the day to allow heat to help asphalt and rock bond more quickly to the roadway surface, providing a higher-quality finished product.

It may not be as appealing as it once was, but seal coating or chip sealing helps to protect the longevity of the road. Without the seal coat or chip seal, the asphalt is exposed and can begin to wear away prematurely.

How can I avoid rock chips in my windshield or paint job?

Before any seal coat is final, crews "broom" or sweep the roadway surface to remove excess gravel. This is usually done multiple times overnight. In addition, following posted speed limits and increasing following distance will reduce the risk of windshield damage caused by loose gravel. Windshield damage is often attributed to the seal coat but it is frequently caused by impatient drivers going too fast, whose tires fling the rocks indiscriminately at other drivers.

Drivers are also reminded to slow down and watch for equipment and workers. In a confined work area, safety is paramount.

  • Discuss

Welcome to the discussion.


  • concernedcitizen posted at 9:06 am on Sun, Jul 15, 2012.

    concernedcitizen Posts: 2530

    Wondering why ITD seal coats highways?

    Cuz someones well connected relative got the contract. LMBO

  • HK91 posted at 3:08 pm on Fri, Jul 13, 2012.

    HK91 Posts: 1

    MOST people do not care about road repairs. IF ONE DOES CARE ABOUT THE COST OF SEALING ONE'S DRIVEWAY THEN YOU SHOULD KNOW THE FOLLOWING. Chip seal is NOT SEALCOAT. It is illegal to use sealcoat on public highways and streets. The reason is that sealcoat is too slick when wet and lawsuits would abound if cities and towns used sealcoat on public right of ways.
    Chipseal, on the other hand, is clean chip size rock laid down on TOP of an asphalt tar for road covering. Chipseal is a rock additive and not slick as would be sealcoat if used for sealing roads. Chipseal also cracks as the tar coating to which it is applied eventually drys and cracks. RAVELING of the roadways occurs as this process continues. Raveling also occurs in driveways as the BINDER in the asphalt drys and starts the cracking process.
    THE PROBLEM WITH SEALCOAT is that it cracks and wears off in 1-3 years if not sooner. It is cheap and does not provide a good seal for asphalt. The fact that sealcoat cracks early in its life and when going through the drying process sealcoat accumulates billion and billions of tiny pinholes in its surface. These pinholes allow water to drain through the sealcoat and run into the asphalt road surface thus giving no protection to asphalt roadways. ASPHALT in itself is porous and asbords water like a sponge. When water runs through the asphalt roadways it eventually washes out the base rock material of which asphalt is formed for roadways. That is why we see "alligatoring" of Asphalt as the base underlying rock surface of the roads wash away.
    People sealcoat their driveways but are being shortchanged because sealcoat does not last more thant 1/12 to 2 years with starting the spider cracking process. That is the drying and cracking process. Ever think why sealcoat compnies want to re-surface your driveways after 2-3 years? When the sealcoat is new it works well but there is no longevity in the product. ANYONE INTERESTED IN THESE COMMENTS can call any asphalt sealcoat construction supply and speak with the manager about the life of sealcoat. The latex and silica in sealcoat help in the drying process and do not extend the life of the product. Currently Arrow constrution supply in Spokane, Washington produces sealcoat for its local suppliers.
    ASPHALT is nothing more than small size clean rock and coal tar binder mixed together and allowed to set-up to dry as asphalt pavement. The idea of sealing asphalt is to STOP THE WATER from penetrating the asphalt and thus causing sub structure damage. What is needed is a product that truely waterproofs the asphalt binder and reduces the drying process of that binder so that raveling and deteration of the asphalt is reduced. Unfortunately, sealcoat is cheap, looks good for a year or two, and then it is time to consider another application process. In many cases it would be just as productive to paint your driveway with spray paint as to use sealcoat if longevity is desired.

  • Tim Herzog posted at 7:50 pm on Thu, Jul 12, 2012.

    Tim Herzog Posts: 353

    OK..you caught me with a typo! I ride a '98 Ultra Classic 95th year Anniversary Edition. Maybe you have possible seen it when I was campaigning for commissioner. I pulled a matching cargo trail with my signs on it.

    I also have a collection of about 20 more, from 60's on up that will be restored, Triumphs, BSA's, a Norton, Honda's, Kawasaki's and a few I forgot about. So now you know!

  • mister d posted at 5:28 pm on Thu, Jul 12, 2012.

    mister d Posts: 1531

    Which motorcyclist have you ridden lately Tim (lol).

  • Tim Herzog posted at 2:56 pm on Thu, Jul 12, 2012.

    Tim Herzog Posts: 353

    MOTORISTS..PLEASE PAY ATTENTION TO THEN LAST PARAGRAPH. Those of us that ride motorcyclists really don't appreciate cars or trucks passing us on freshly seal coated roads. Those flying pebbles just don't feel good!

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