Pavia Systems: The Gravel Pit Blog

In our most recent RoadReady Newsletter, we discussed how agencies use Pavement Management Systems to make roadway planning decisions. These systems allow efficient use of resources not only to undertake new roadway projects, but to upkeep the condition of our pavement networks. But just how degraded are these networks? Let’s take a look at the condition of roadways across the U.S. to better understand the need for pavement management.

The American Society of Civil Engineers periodically publishes a “Report Card” for the Nation’s infrastructure. In the most recent report (2009), our roadways were given a D-. Our overall infrastructure was rated 23rd by the World Economic Forum. This means that the world’s largest roadway network is not performing up to task.

Pavement distress is a huge problem for our roadway network

Some of the most high-profile and high-volume roads in our country are interstate highways. In general, rural interstates are in better condition than urban interstates, where the majority of vehicle travel occurs. The FHWA reports that on rural sections of the Interstate highway system, 78% of roadway travel occurs on pavement in good condition, 20% on acceptable pavement, and 2% on poor pavement. For urban Interstates, these numbers drop to 62%, 33%, and 5% respectively. These ratings are based on the International Roughness Index (IRI), which is a measure of roadway smoothness.

When considering the nation’s overall roadway network, the picture gets worse. The FHWA collects data on the condition of Federal-Aid Highways, which includes all roads except for minor local streets and collectors. Although this represents a small portion of total roadway length, these roads account for around 85% of the nation’s vehicle travel. About 47% of the vehicle miles traveled on this system is on pavement in good condition, while about 39% is traveled on acceptable pavement. The remaining 14% of travel on Federal- Aid Highways is on pavement classified as being in poor condition. When considering total mileage across the U.S., travel on good roadways drops to 42% while acceptable travel increases to this same number, leaving 16% of U.S. roadway travel on poorly maintained roadways.

What are the consequences of a damaged roadway network? Here are some quick numbers:

• Poor roadway condition is cited as a cause of over 50% of annual roadway deaths.
• As a result, U.S. roadways have a 60% higher fatality rate than the OECD average.
• Condition related crashes cost the nation $217 billion
• Pavements in poor condition cost urban drivers $400 a year in vehicle maintenance on average.

Improving the state of our nation’s roadway network is an important goal. Well maintained roadways not only mean a more pleasant driving experience, but save money and lives.

In the latest edition of the RoadReady Newsletter, we explored how chip seals are used as a low-cost rehabilitation method for roadways. However, chip seals are not the only type of bituminous surface treatment (BST) in use today. In this blog, we’ll cover two additional types of BST, fog seals and slurry seals.

A fog seal application over an aging pavement

Fog Seals

A fog seal is a kind of bituminous surface treatment and consists of applying a slow-setting layer of emulsified asphalt to a pavement surface. Essentially, a fog seal is a chip seal without the aggregate. This treatment can create a seal, waterproof a road, and prevent raveling, or loss of particles from the roadway surface. This can delay the need for a chip seal, or a hot mix asphalt overlay in the short term. In addition, fog seals will leave a darker color on the roadway that improves the contrast of painted stripes, which is especially useful when applied over chip seals. They should only be applied where there is texture on the roadway surface, rather than a smooth pavement. This includes open graded pavements, aged dense graded pavements, and chip seals. Applying a fog seal on smooth pavements can result in a dangerous loss of friction, so safety impacts should be considered carefully before application.

Roads with alligator cracking indicate subgrade failure, and are not good candidates for slurry seals

Slurry Seals

A slurry seal combines the two steps of the chip seal into one by placing emulsified asphalt and aggregate in a single layer. This is basically the same as laying a chip seal in a single step. They can be used as a preventive maintenance measure early in the life of a pavement, and can also repair small defects. In addition, slurry seals will increase friction and like fog seals, provide a degree of waterproofing. However, the pavement should be free of any alligator cracking, and any major cracks should be sealed. For pavements that exhibit raveling, the slurry seal can be preceded by a tack coat. The asphalt and aggregate can also be combined with polymer additives before applying, a technique known as microsurfacing. These additives can help leave a higher residual asphalt content and can cause the asphalt to set faster. Slurry seals should be expected to last between three and five years before additional treatment.

Selection

Selecting the correct surface treatment for a rehabilitation project is critical. Without careful consideration of the deficiencies to be corrected, a project risks failure. In addition, an appraisal of available resources should be undertaken to formulate the most logical rehabilitation plan. If possible, these actions should be incorporated as part of a pavement management system. When the right one is chosen, bituminous surface treatments can stretch the life of a pavement for a minimal investment.

For more information on bituminous surface treatments, visit the following links:

Bituminous Surface Treatments

Fog Seals

Slurry Seals

Click the button below to learn about how the gas tax is structured, and how it contributes to roadway funding in the U.S.

Here is a list of some additional resources you can check out to learn more:

Current Status of the Highway Trust Fund – FHWA

Overview of the Highway Trust Fund – FHWA

The Highway Trust Fund and Paying for Highways – Congressional Budget Office

Spending and Funding for Highways – Congressional Budget Office

State Gasoline Tax Rates – Tax Foundation

State and Local Motor Fuel Tax Revenue – Tax Policy Center

In this week’s RoadReady Newsletter, we covered how a life-cycle cost analysis compares costs between two roadway projects. Below is an example of how a life-cycle cost is calculated for a basic asphalt roadway. This LCCA is deterministic, meaning that the output is a single estimated cost rather than a range of values and corresponding probabilities. For simplicity, we will only consider a few basic agency costs and leave out costs to the user of the road. The plot shown is known as an expenditure stream diagram, and is useful for mapping out the time relationships of spending and costs. Each arrow indicates an expenditure or value, and its height is proportional to the amount. Upward pointing arrows indicate expenditures, while downward pointing arrows indicate value or income. The bottom axis represents time, usually measured in years. Click on the arrows below to walk through the calculation.

For more information on Life-Cycle Cost Analysis, visit the following resources:
Federal Highway Administration’s LCCA Page

Pavement Interactive

Road and highway safety is a major concern for any transportation agency. One of the most important roadway characteristics affecting driver safety is the frictional properties of the pavement. Numerous studies have shown the relationship between roadway friction and driver safety, and as a result, considering friction is an important part of any pavement design.

Driving and Friction

Friction is the force that resists motion between two objects in contact. This force is proportional to the normal force, or the force pushing the object together. In the case of a vehicle, the normal force is caused by the weight of the vehicle pushing down on the tires. There are two types of friction we refer to when dealing with roadway safety. These are longitudinal frictional forces, which act parallel to the direction of travel, and lateral frictional forces, which act perpendicular to it. Longitudinal friction affects braking and accelerating, while lateral forces impact the ability to turn. Both of these forces have an important impact on the ability of a driver to safely control a vehicle.

The Economics of Friction… Supply and Demand

Turning and braking both have a frictional “demand” associated with them. This demand must be met by the available friction between the tires and the pavement surface. When demand exceeds the available frictional forces, skidding occurs. When turning and braking are done simultaneously, the available friction must exceed the demand for both.

Roadway friction is most important during wet weather conditions, as water on the roadway surface diminishes skid resistance. This is because water provides lubrication between the tire and roadway surface, and can partially submerge the texture of the aggregate.  As a result, when discussing pavement friction, wet conditions are usually considered as the critical scenario.

Surface Texture

Pavement friction is controlled by surface texture. Texture is defined by two main characteristics, which both affect friction and roadway safety. Which of these characteristics dictates overall friction and safety in wet weather depends on the speed of traffic.

• Microtexture refers to the surface characteristics of individual aggregate particles at the microscopic level, and dominates at low speeds.
• Macrotexture represents larger scale aggregate shape and gradation. Generally, macrotexture is more important at high speeds, and dictates drainage.

Macrotexture can be seen on the roadway surface, while microtexture cannot

Designing for Friction

Design decisions can affect both texture properties. For instance, microtexture will tend to degrade over time as traffic loads polish the surface aggregate. However, the use of hard aggregate will reduce this effect. Other mix design decisions will affect the macrotexture, such as concentrations of fine and coarse aggregate, the air voids in the mix, and the percent binder. These parameters will be selected such that the final frictional characteristics of the pavement meet a project specification. This will be based on factors such as the design speed of the road and the geometric layout.

Roadway friction is crucial for driver safety. Fortunately, pavement designers have a good understanding of how friction works, and how to create a safe roadway environment. Methods for measuring roadway friction are readily available, and agency specifications exist to ensure that safe levels are attained. As new mix design methods for improving roadway friction continue to be discovered, our pavement networks will become increasingly safe.

For additional information visit the following links:

Skid Resistance

Pavement Friction – FHWA Safety Program

If you’re reading this blog, you probably know that asphalt is the glue that holds roadways together. But how do we produce asphalt, and how do we make sure if holds up to our quality standards? In the most recent RoadReady newsletter, we explored how asphalt grades are used to describe different binders. In this edition of the blog, we’ll explore how asphalt is produced to meet specifications.

Asphalt is one of many constituents found in naturally occurring petroleum. Crude oils contain different concentrations of each of these constituents, and as a result oils exist that are composed almost entirely of asphalt. In the early days of asphalt road paving, Trinidad Lake, a naturally occurring asphalt seepage, provided a huge majority of the asphalt consumed worldwide. This lake contained primarily asphalt, and was the first source of asphalt for the U.S.

Components of Petroleum

Today, asphalt is usually obtained from the refinement of petroleum. Petroleum can be separated into paraffin, gasoline, naptha, kerosene, and diesel as well as asphalt. Separation of these constituents is performed by distilling the crude oil. This essentially means that the oil is heated and volatile compounds are allowed to evaporate at different rates. Asphalt is the heaviest constituent of crude oil and will be left as a residue following the distillation process. Modern sources of petroleum are characterized by their American Petroleum Institute (API) gravity value, a dimensionless indicator of density. Asphalt has a low API gravity relative to unrefined petroleum, and this fact can be used to estimate the asphalt content of petroleum. Oil with a high API gravity will contain a low percentage of asphalt, with oil with a low API will contain a high percentage of asphalt.

Distillation can separate the different components of crude oil

Chemical Composition

Apart from total asphalt content, the characteristics of the crude oil will impact the performance of the final asphalt binder. Asphalt is composed of three primary components, including:

• Asphaltenes – Heavy compounds that provide structure, strength, and stiffness to the asphalt
• Heavy Oils – Non-polar molecules that impart fluidity and viscosity to the asphalt
• Resins - semi-solid to solid compounds that cause ductility and malleability in the asphalt.

The relative proportion of each of these materials in the asphalt is dependent on the makeup of the crude oil, and will have important effects on the physical characteristics of the binder. For instance, the higher the asphaltene content, the lower the penetration value and higher the softening point, as asphaltenes form much of the structure of the asphalt. Asphalts with high resin contents will typically be very pliable. Each of these properties is important when determining the grade of an asphalt, and determining whether it is suitable for a given roadway project. Asphalts produced with a specific grade in mind are commonly created through blending of different asphalts, and understanding the effects of chemical composition on physical properties is important when selecting which asphalts to combine. Other characteristics of the crude oil can predict its suitability for paving asphalt, such as the initial concentration of wax and sulfur.

Polymer additives can reduce cracking in cold weather

Additives

When the desired properties are not provided by the asphalt residue, additives are used to bring the asphalt to grade. This is typically done through the addition of polymers, a class of compounds made up of small, repeating chemical units. When used in pavements, the addition of polymers is usually done to address thermal cracking at low temperatures or rutting at high temperatures. These polymers can be divided into plastomers and elastomers. The former creates a rigid lattice structure while the latter improves the elasticity of the binder.

The sophistication of asphalt grading has required the production of higher quality asphalt. Fortunately, the demand for ever tightening specifications has been accompanied by a greater understanding of refinement, crude oil, and additives. This combination of knowledge and expectations has led to a generation of better asphalt binders in our roadways.

For more information on asphalt binder production and grading, visit the following links:

Superpave Performance Grading

Asphalt Production and Oil Refining

In this week’s RoadReady newsletter, we looked at how warm-mix asphalt pavements have been performing in the field and in the lab. While warm-mix is certainly receiving a lot of attention, it doesn’t have a monopoly on innovation in asphalt technology. With the growing costs of fossil fuels, researchers are looking for ways that road builders can get more out of our asphalt binders. This has led to the investigation of new technologies, as well as a revisit of some old ones. Because of the lengthy testing and trial process for any paving technology, widespread application of these innovations is probably a ways away. However, in this edition of The Gravel Pit we’re going to explore three active concepts in asphalt binder technology.

Researchers are working on getting the most out of asphalt

Highly Modified Polymer Asphalt

Modification of asphalts is commonly done when a wide range of service temperatures are expected for a pavement. Combining additives with asphalt allows road builders to prevent rutting at high service temperatures and cracking at low service temperatures. However, use of additives, particularly polymers, can be taken to new extremes to produce high performance pavements. In a recent test on the test track at the NCAT, a pavement with a heavy concentration of the polymer styrene-butadiene-styrene (SBS) and a thickness of 5 ¾ inches was able to achieve similar structural support as standard asphalt with a depth of 7 inches due to the additional stiffness provided by the additive. Researchers are currently exploring the use of additives from complex synthesized polymers to common starch in asphalt pavements. As our understanding of how these polymers and other additives affect the properties of asphalt improves, we will increasingly be able to use them to reduce demand for virgin materials and improve our roadway networks.

Biologically Based Asphalt

Probably one of the more “out-there” new ideas in pavement is the use of plant based asphalts. These asphalts are made from renewable organic material such as corn, wood, and other plant products. They can potentially be used to replace a portion of the traditional binder or as the primary binder in a pavement. In 2007, two trial pavements were constructed in Norway using vegetable oil based binders. These asphalts met all relevant specifications for the region, and were more resistant to cracking at low temperatures than similar petroleum based binders. In the U.S., researchers at Iowa State University have developed a product known as Bioasphalt that is made non-food sources of organic material. This product was recently demonstrated on a paved bike trail in Des Moines, Iowa. Further investigation of these applications could eventually allow us to pave our roads without relying on fossil fuels.

Reductions in pavement cross section reduce the need for aggregate and other virgin materials

Sulfur Extended Asphalts

The idea of sulfur extended asphalt, or SEA, isn’t technically a new one, but it’s recently come back in style. Back in the 70’s and 80’s, a shortage of petroleum made road builders have to look outside the box to help pave their roads. This led to the addition of sulfur to asphalt binder. Addition of sulfur allows some of the liquid asphalt to be replaced while retaining the same ability to bind road materials. In a typical SEA pavement, up to 30% of the asphalt binder might be substituted with sulfur. However, this idea quickly became unattractive as the cost of sulfur rose relative to asphalt. Today, strict regulations on petroleum quality have led to surplus quantities of sulfur as a by-product of refinement, and petroleum prices are once again on the rise. This means that sulfur is again cost competitive with asphalt and that this technology is being given a second look.

How do sulfur additives affect the overall pavement properties? Studies by the FHWA and other researchers in the 1980’s concluded that pavements incorporating sulfur as a partial replacement of asphalt binder were similar in quality to traditional hot mix, with some increased vulnerability to freeze thaw cycles and enhanced stiffness. More recently, a sulfur additive produced by Shell has been tested in a number of settings, including at the test track at the National Center for Asphalt Technology (NCAT). The additive consists of small pellets which can be added during the mixing process without any plant modification. In this case, the increased stiffness and load bearing capacity provided by the sulfur additive allowed a significant reduction in pavement thickness when compared to typical asphalt concrete under the same loads. When this is considered along with the replaced asphalt in the mix, SEA’s look to be a promising technology moving forward.

For more information, visit the following resources:

NCAT Test Track - New Asphalt Technologies

Press Release - Bioasphalt Demonstration

Short video on Bioasphalt in Iowa

Shell Thiopave (SEA)

Federal Highway Administration - Advance High-Performance Materials for Highway Applications: A Report on the State of Technology

In our most recent RoadReady newsletter we discussed perpetual pavement, a concept that is relatively new on the scene. The idea of pavements that do not sustain significant structural damage under traffic loads has only been around for about 20 years, while a serious effort towards implementing this strategy has only gained ground over the last decade.   In 2000 the Asphalt Pavement Alliance (APA) formally defined perpetual pavements as “an asphalt pavement designed and built to last longer than 50 years without requiring major structural rehabilitation or reconstruction, and needing only periodic surface renewal in response to distresses confined to the top of the pavement”. This organization also established the “Perpetual Pavement Award” in 2001, awarded to pavements that have been in service for 35 years or longer without addition of four inches of thickness and overlays at an interval of 13 years or greater. Note that these requirements mean that projects receiving the award since its creation were built in the 1960’s or 70’s, before the concept of perpetual pavements was formalized. Since the award was created, 69 roads have been awarded in around 30 states. Tennessee and Minnesota have the most awards, with seven and eight respectively.

Evaluation of perpetual pavement designs is being conducted in different ways in different states

Because of the long expected lifetime of contemporary projects, there has been little opportunity to thoroughly examine how an intentionally designed perpetual pavement performs from the cradle to the grave. As a result, states are taking different approaches to incorporating perpetual pavements into their highway networks. An example of how some states are exploring this technology is given below:

Texas: TxDOT has recognized the perpetual pavement concept by revamping their design procedures for roadways expected to undergo 30 million ESALs, or equivalent single axle loads. The new method incorporates design concepts from perpetual pavements. In 2006, eight such perpetual pavements had been designed in this way. These projects are continually being monitored to evaluate the success of this design method.

Wisconsin: WisDOT has taken a unique approach to evaluating perpetual pavement designs. In addition to constructing highway test sections, perpetual pavement designs have been used to replace two different pavement sections used in truck weigh stations. This has allowed WisDOT to record the exact timing and magnitude of the loads that have been applied to each section. These areas were instrumented and monitored to determine their performance and suitability for highway application.

Kansas: KDOT decided to explore the potential of perpetual pavements by constructing four separate pavement sections on a portion of US-75. Each of these sections used a different perpetual pavement design, and was instrumented to examine their responses to traffic loads. The study paid particular attention to variables such as stresses which caused permanent structural damage, and the affects of temperature and vehicle speed.

Tools have also been developed to promote the implementation of perpetual pavement projects. The Asphalt Paving Association, the National Center for Asphalt Technology, and Auburn University have created a tool called PerRoad, a freely available piece of software which aids in perpetual pavement design. 

Planners and decision makers are finding ways to incorporate perpetual pavements into the nation’s roadways. Though it will be decades before we can see the final outcome of such a project, there is an important recognition of the savings in money and resources that can be made by building our pavements to last. As agencies continue to gather information and explore this concept, the upside of perpetual pavements will only grow.

In our most recent RoadReady newsletter we explored the exciting topic of recycling asphalt roofing shingles for use in asphalt pavement. In addition to being environmentally friendly and improving pavement quality, this practice can produce significant cost-savings on a paving project. An important part of deciding whether or not to incorporate recycled shingles in a project is quantifying what kind of savings or costs will be incurred.

Cost savings from asphalt shingles can put a smile on your face (image provided by Woodworth and Co.)

Estimating this amount requires knowledge of a number of factors. Probably the most influential parameter is the current price of liquid asphalt. Using recycled asphalt shingles (RAS) offsets part of the need for liquid asphalt in the mix, which is the primary source of cost savings. In addition to its importance, this is also a factor with a high level of volatility in recent decades.

Characteristics of the asphalt shingles are also important, such as the percentage of contained asphalt. This typically ranges from 19-36% based on the type of shingle used. Note that not all of the asphalt contained in the shingles is available to contribute to the mix. AASHTO requires additional testing of a mix if the expected amount of asphalt from recycled sources exceeds 30% of the total. Savings are also produced by the reduction of fine aggregate needed due to the granular material in the shingles. However, this amount will usually be negligible when compared to savings from other sources.  Of course, the amount of shingles in the mix must also be known. In most states that allow recycling of asphalt shingles, inclusion is limited to 5% in asphalt pavements.

Finally, a number of details must be determined regarding the handling and processing of the shingles. These include the tipping fees that are avoided by recycling the material, as well as the costs of transportation, processing, and the shingles themselves. In many cases the actual shingles are free, and processing costs are often overshadowed by tipping fees.

The easiest way to explore the economic effects of using shingles is to calculate how each of these factors influence the cost of a ton of asphalt. First, calculate the savings provided by offset asphalt and fine aggregate requirements. This is done by multiplying the cost of the given material by its representation in the asphalt shingles, and multiplying this product by the percentage of RAS in the mix.

Liquid Asphalt Savings = Price of asphalt ($/ton) × % Asphalt in shingles × % Shingles in mix

Fine Aggregate Savings = Price of F.A. ($/ton) × % F.A. in shingles × % Shingles in mix

The final source of savings is the waste tipping fees that are avoided by recycling. This value is computed by multiplying the tipping fee by the percentage of RAS in the mix.

To calculate the cost of using shingles, multiply the percentage of RAS to be used in the pavement by the cost of the shingles, their processing, and transportation.

Tipping Fee Savings = Tipping Fee ($/ton) × % Shingles in mix

Subtracting the costs from the sum of the savings above will provide a final net estimate for the savings per ton of asphalt pavement by use of RAS.

Cost = (Cost of shingles ($/ton) + Transportation costs ($/ton) + Processing costs ($/ton)) × % Shingles in mix

Paving site preparation was the topic of our most recent RoadReady newsletter. Site prep differs for new and existing pavements, as we will see in brief in the following.

For new pavements, make sure the paving site is cleared of structures and obstructions and cleaned of debris which will affect paving quality, check the site elevation and grade. Keep in mind the existing soil can serve as subgrade if it meets specs for density and moisture content.

• Subgrade – must be graded correctly to maintain proper grade and elevation, and needs to meet density according to specifications. Make sure construction activity doesn’t cause distortion or rutting
• Base course – unbound aggregate or asphalt mix, grade and compact correctly, may need to water the subgrade before adding asphalt mix base course, to deal with accumulated dust
• Subbase – use only if additional support is needed
• Prime coat – also optional, this is an asphalt emulsion that stabilizes fine material and prevents shoving, used to protect the subgrade when weather may dry it out, as in a long wait before paving
• Asphalt – final layer

For existing pavements, the process is similar. Start by examining the existing pavement for distress, and correct any deficiencies, so that they don’t propagate to the new pavement. Then, check for the existence of other problems and that elevation and grade are correct, and fix any bad spots.

• Level course – optional asphalt mix that corrects crowns or dips, by placing asphalt in the low spots. Watch out for differential compaction (more compaction in the thicker asphalt layer). Solve for this – an option is to create more than one leveling course and compact them separately.
• Mill – removes or “cold-planes” some of the existing pavement, but can level the surface as well. During milling, replace worn teeth and ensure all teeth are in good condition. Plan trucks for the rate of milling and volume of trucks, to remove the milled material.
• Sweep – clear debris and prepare surface for next stage of construction. Do this multiple times: after milling, after compaction, and prior to tack coat. Multiple sweeps are required after milling, to ensure that leftover grit does not interfere with the bond between layers.
• Tack coat – an asphalt emulsion spray, to help the layers of asphalt adhere