Asphalt Paving and Sealcoat  Blog

The Performance of Asphalt - Pt.2

Noise reduction

Asphalt is the quiet pavement. Quiet pavement technologies include open-graded surfaces, fine-graded surfaces, and two-layer open-graded pavements. Studies show that the noise-reducing properties of asphalt last for many years.

Noise reductions of 3 to 10 dB(a) are common. Reducing noise by 3 dB(a) is about the same as doubling the distance from the road to the listener, or reducing traffic volume by 50 percent.

Congestion reduction

Asphalt pavements are faster to construct and rehabilitate. In crowded urban areas, where closing a road for rehabilitation or reconstruction would dump increased traffic on neighboring routes-adding to congestion-asphalt is the answer. Highways and roads can be milled for recycling, then overlaid, during off-peak hours, so that most motorists are never inconvenienced.

Smoothness and conservation

Studies at a pavement test track in Nevada have shown that driving on smoother surfaces can reduce fuel consumption in the neighborhood of 4.5 to 5 percent.4

When trucks are driven on rough surfaces, the tires bounce and deliver heavy, punishing impacts to the pavement. Some experts estimate that a 25 percent increase in smoothness can result in a 9 to 10 percent increase in the life of pavements.

Building smooth asphalt roads is simple and cost-effective. Keeping them that way is fast, easy and inexpensive.

Taking the punishment

Any doubts about whether asphalt is durable can be answered at racetracks and airports. These pavements are punished by heavy loads, but asphalt stands up to them.

When vehicles reach speeds of 200 mph and more, and a car's undercarriage clears the pavement by about one inch, the pavement had better be smooth. That's why nearly all racetracks-for both Formula One and NASCAR racing-use asphalt.

The Performance of Asphalt - Pt.1

Roads that don't wear out

One of the keys to sustainability is long life. With Perpetual Pavements, asphalt pavements have an extremely long lifespan.

A Perpetual Pavement is constructed so that distress occurs in the top layer only. The only rehabilitation required is removal of the surface and resurfacing with an asphalt overlay. Using current pavement technologies, this can be done on an infrequent basis-every 15 to 20 years. The reclaimed material is then recycled. Perpetual Pavement is the ultimate in sustainable design and construction.

While the Perpetual Pavement name is relatively new, the concept is not. In fact, more than 35 pavements have received the Perpetual Pavement Award since 2001. These award-winning roads, streets, highways, and airport runways have been in place for at least 35 years, with a minimum of maintenance and no full-depth reconstruction.

Rubblization

When concrete pavements reach the end of their useful life, they must undergo expensive, time-consuming rehabilitation or be completely removed and replaced. This process squanders precious natural resources, in addition to inconveniencing the traveling public. Prolonged road closures also lead to congestion, which consumes energy and produces excess emissions. Asphalt's answer is a sustainable process called rubblization, in which the concrete pavement is left in place, "rubblized" (fractured), and used as the base for a new Perpetual Pavement.

In addition to the environmental advantages, cost savings can be significant. The rubblization process is much faster than the remove-and-replace option. It can also be accomplished through temporary lane closures, without the necessity for traffic to be detoured onto parallel routes.

Public safety

Smooth asphalt roads give vehicle tires superior contact with the road.

One type of asphalt surface, known as open-graded friction course, allows rainwater to drain through the surface layer and off to the sides, reducing the amount of splash and spray kicked up by vehicles.

Porous asphalt provides better water quality - Pt3

Image: Hydrograph comparison showing how porous pavement reduces peak flow and total volume of runoff
Source: Cahill Associates

Impact on groundwater
Asphalt pavements are compatible with clean water. Studies show that asphalt pavements and stockpiles of reclaimed asphalt pavement do not leach.

Contaminants on the surface of pavements tend to become part of runoff, but with a porous pavement, they are washed into the stone bed. From there they flow down into the soil, where beneficial bacteria and other natural processes cleanse them. Data are limited, but indicate a very high removal rate for total suspended solids, metals, and oil and grease.

Figure 1 shows the effect of a porous asphalt pavement on the hydrology of a developed site.

Cool cities
Porous asphalt pavements have been shown to mitigate the urban heat island effect. Open-graded asphalt roads and highways-which use the same surface material as porous parking lots-have been shown to lower nighttime surface temperatures as compared to impervious pavements. In at least one city, the hottest heat signature is at the airport, with its thick, dense, impervious runways.

Comparisons to other asphalt pavements
The surface of a porous asphalt pavement wears well. While slightly coarser than some pavements, it is attractive and acceptable. Most people driving or walking on the pavement will not notice (or believe) that it is porous.

Like all asphalt pavements, porous pavements are ADA-friendly.

Environmental applications
Asphalt pavements have been used for many years to enhance water quality. At landfills, asphalt liners and caps keep contaminants from leaking into groundwater. Drinking water reservoirs lined with asphalt pavement have been used in California since the 1950s. Salmon hatcheries and fish rearing ponds in the Pacific Northwest use asphalt liners.

Variations on the theme
Porous asphalt can be used successfully in parking lots, walkways, and playgrounds. Several current suburban projects are exploring its use in subdivision roads. A few porous highways and city streets have been constructed, both in the U.S. and in Europe, and have performed well.

The open-graded asphalt surface used for porous pavements has been used extensively to surface high-volume highways that carry heavy trucks. Its benefits include noise reduction, a decrease in splash and spray kicked up by vehicles in heavy downpours, and mitigation of the urban heat island effect.

Porous asphalt provides better water quality - Pt2

Construction and performance

Porous asphalt pavements are fast and easy to construct. With the proper information, most asphalt plants can easily prepare the mix and general paving contractors can install it.

The stone bed, often eighteen to thirtysix inches in depth, provides a tremendous subbase for the pavement. As a result, porous asphalt pavements tend not to exhibit cracking and pothole formation problems. The surface wears well. Under the stone bed is a geotextile which keeps fine particles from moving into the stone bed from below and filling in the spaces.

Porous asphalt has been proven to last for decades, even in extreme climates, and even in areas with many freeze-thaw cycles. The underlying stone bed can also provide stormwater management for adjacent impervious areas such as roofs and roads. To achieve this, stormwater is conveyed directly into the stone bed, where perforated pipes distribute the water evenly.

Economics

Porous pavement is a sound choice on economics alone. A porous asphalt pavement surface costs approximately the same as conventional asphalt. Because porous pavement is designed to "fit into" the topography of a site, there is generally less earthwork. The underlying stone bed is usually more expensive than a conventional compacted sub-base, but this cost difference is offset by eliminating the detention basin and other components of stormwater management systems. On projects where unit costs have been compared, the porous pavement has been the less expensive option. Porous pavements are therefore attractive on both environmental and economic grounds.

An installation at the University of North Carolina in Chapel Hill included parking lots where some sections were constructed from porous asphalt and others used porous concrete. The cost differential was approximately 4:1 - that is, the porous concrete pavement cost four times as much as the porous asphalt pavement.

Porous asphalt provides better water quality - Pt1

Porous asphalt...
• Conserves water
• Allows for better use of land
• Reduces runoff
• Promotes infiltration
• Cleans stormwater
• Replenishes aquifers
• Protects streams
Porous asphalt offers a powerful tool in the toolbox for storm-water management.
In the natural environment, rainfall sinks into soil, filters through it, and eventually finds its way to streams, ponds, lakes, and underground aquifers. The built environment, by way of contrast, seals the surface. Rainwater and snowmelt become unoff which may contribute to flooding. Contaminants are washed from surfaces directly into waterways without undergoing the filtration that nature intended.

For these reasons, managing stormwater is a significant issue in land use planning and development. Stormwater management tools can serve to mitigate the impact of the built environment on natural hydrology. Unfortunately, however,they also can lead to unsound solutions such as cutting down stands of trees in
order to build detention ponds.

Porous asphalt pavements allow for land development plans that are more thoughtful, harmonious with natural processes, and sustainable. They conserve water, reduce runoff, promote infiltration which cleanses stormwater, replenish aquifers,
and protect streams.

A typical porous pavement has an opengraded surface over an underlying stone recharge bed. The water drains through
the porous asphalt and into the stone bed, then, slowly, infiltrates into the soil. If contaminants were on the surface at the time of the storm, they are swept along with the rainfall through the stone bed. From there they infiltrate into the subbase so that they are subjected to the natural processes that cleanse water.

Asphalt

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Seal Coats as a Preventive Maintenance Technique

General
Many companies have had a formal preventive maintenance program for years. Some folks, however, are behind in adopting a regular seal coat application schedule for every few years. Seal coats are used extensively as a preventive maintenance technique for asphalt pavements throughout the San Francisco Bay Area. While routine maintenance can be characterized as a reactive process in which immediate repairs are made to existing distress, preventive maintenance treatments help to preserve a pavement and extend its useful performance life. This concept is illustrated in Figure 2-1. It is desirable in California to place seal coats on a 2 to 5 year cycle, but this is not always possible due to funding constraints. In rough economic times, it is a great alternative to complete repair.

The process for selecting pavements to receive seal coats will vary among the locations and many circumstances. In
general, the process starts with maintenance grounds supervisors because they are most familiar with the roads in the section and inspect them often. They are also aware of the resurfacing and maintenance history of a particular pavement area. The maintenance grounds supervisor generally prepares a list of candidate projects, and the list is reviewed and modified as appropriate by the facility manager or property manager before being submitted for approval.

Seal coats provide no additional structure to an existing pavement, so pavements that are structurally deficient are not candidates for seal coats. In addition, because wide cracks or cracks experiencing large movements are expected to reflect through the seal coat, pavements with extensive amounts of distress are generally not good candidates for a preventive maintenance treatment. There is, however, still a considerable amount of seal coat applied as a Band Aid to hold the pavement for a few years until reconstruction or rehabilitation can be funded.

In many cases, spot base repair, edge repair, crack sealing, strip/spot seal coat, or level-up work may be required to make an existing pavement ready to receive a seal coat and to maximize the life of the pavement.
Factors that can affect the decision to use a seal coat as a maintenance treatment include the following:
♦ condition of the existing pavement
♦ effectiveness of a seal coat in addressing the existing pavement deficiencies
♦ cost of the seal coat compared to other treatments
♦ traffic volume
♦ percent truck traffic
♦ repairs needed prior to the seal coat.

Pavement Preservation Plan

What is Pavement Preservation?
Pavement Preservation is a planned system of treating pavements to maximize their useful life.

All pavements require some form of maintenance due to the effects of traffic and the environment on the exposed pavement materials. Many times,the only treatment a pavement receives is crack filling or patching, while surface distresses are ignored. Applying a surface treatment to a pavement with light to moderate
distress can greatly increase the life of a pavement.

Pavement Preservation takes the maintenance process one step further by carefully choosing and timing pavement maintenance applications to extend the life of a pavement.

WhatAre the Benefits Of Pavement Preservation?
The most obvious benefit of Pavement Preservation is the extension of the life of the pavement. Other benefits of a Pavement Preservation program are:
• Lower costs over time - Studies show every additional dollar spent on preventive maintenance treatments saves up to $10 in future rehabilitation costs.
• More predictable costs - If you schedule your treatments and keep your pavements maintained, you should be better able to predict and plan future costs.
• Fewer premature pavement failures - Many premature pavement failures are caused by pavement damage that goes untreated, such as water seeping into cracks.
• Better condition pavements - Scheduled monitoring and pavement treatments keep pavements in better overall condition
than random or insufficient maintenance.
• Reduced user delays and user costs - The more extensive damage a pavement has, the longer drivers will be delayed
due to construction. Pavements that are in good condition are also easier on a vehicle's daily wear and tear.
• Better utilization of resources - Regularly scheduling treatments allows better use of available resources, and planning for those you may need (such as contractors, equipment, etc.).
• A happier driving public - Drivers will get to their destinations on time over safe,well-maintained roads and parking lots.

HowDo I Start A Pavement Preservation Program?
1 INVENTORY your road system and components - This will give you a starting point.You might also want to consider gathering Average Daily Traffic (ADT) or ESAL counts on those roads that seem to have heavier traffic, have a lot of growth nearby, or that have not been surveyed in a while.
2 FIELD SURVEYS determine your pavements' conditions - We've provided a Pavement Rating Form for you to use.The field surveys will give you information on how to prioritize your treatment strategy.
3 ANALYZE field surveys and reports to determine maintenance strategy - Pavements that are severely distressed aren't the best candidates for a surface treatment because you may have to reconstruct them fairly soon. Instead, it may be better to apply a surface treatment to a pavement with light or moderate distress.
4 PLAN STRATEGY using analysis from PMS as a tool (if available) - A Pavement Management System (PMS) makes your decisions easier, especially if you have many pavements that need attention in the near future.
5 IMPLEMENT, execute, and document costs and work performed for future use - Keeping track of the work performed and related costs can help determine the life of a treatment and the true costs over time.

Concrete Pavement Restoration and Preservation

Faced with limited funding for both capital improvements and
maintenance, many cities and counties are facing serious
challenges to repairing streets and roads.

In some cases, the service life can be extended using techniques
known as pavement restoration methods. These include minor
concrete slab repair, diamond grinding the surface, and resealing
joints. When done at the proper time, these repairs can bring new
life to streets and roads ... and are less expensive than asphalt
overlays.

Portfolio of Solutions
The concrete pavement industry
offers a portfolio of restoration
methods known as Concrete
Pavement Restoration (CPR).

CPR is a is a viable alternative to
thin asphalt overlays, which have
been proven time and again to be
nothing more than short-term
solutions.

CPR techniques have been shown
to add 12 to 15 years of service life
to a roadway, compared with
asphalt overlays, which provide
between 2 and 5 years. Equally
important, the costs are about the
same for either method.

Why worry? Routine pavement evaluations and other "CPR"
strategies can help prolong the life of a pavement and help
agencies set the stage for funding, plans, and specs.

The key to any preservation strategy is timing.
Planning CPR typically needs to begin at approximately
2/3 to 3/4 of the pavements original design life, so for a
20-year pavement, agencies can expect to do start
thinking about pavement preservation at about 12 to 15
years.

This helps ensure that maintenance funding, project
plans, and specifications are ready before extensive
deterioration has occurred.

CPR techniques typically required for city streets and
other local roads include: full- or partial-depth
pavement patching, diamond grinding, and joint
sealing.

These and other techniques are outlined in the
American Concrete Pavement Association's "Concrete
Pavement Repair Manual."

Here's a quick guide to these common CPR techniques, all of which can be done inexpensively and quickly by local contractors.

Diamond Grinding
> Contractors remove top 1/8 in. to 1/4 in. of
pavement using special grinding equipment.

Advantages
• Removes bumps, quiets noisy pavements
• Does not affect ground-level or overhead structures
• Does not reduce gutter capacity
• Does not require expense driveway tie-ins
• Requires only minimal traffic control requirements
• Cost less than a typical asphalt overlay

Partial Depth Replacement
> Mark outer perimeter of patch
> Remove affected area
> Clean allow to dry patch area
> Place, finish, and cure new patch

Advantages
• Fast, easy, and inexpensive
• Can be done under traffic
• Pre-cast concrete panels are an option

Full Depth Replacement
> Mark outer perimeter of patch
> Saw-cut the full depth of pavement
> Remove deteriorated concrete
> Replace and compact subgrade/subbase
> Install load transfer dowels
> Place, finish, and cure new concrete

Advantages
• Fast, easy, and inexpensive
• Can be done under traffic
• Pre-cast concrete panels are an option

All Concrete Isn't Created Equal

What You See is What You Hear...
Each concrete pavement surface texture shown above produces a different noise level, and at different frequency spectrums (tones). Historically, the highway community has focused only on overall sound levels. In the 1990s, however, it was demonstrated that the frequency of the sound produced by the surface texturing effects what you hear as much as the overall sound level does, particularly with uniform transverse tined pavements, which were recommended by the FHWA until recently. Transverse tine textures produce tonal spikes that we recognize as the "tire whine" heard as you travel down the road. Today, these issues are understood and the FHWA, in 2005, modified their requirements to allow other textures for concrete pavements that could potentially eliminate the tire whine.

Sound level is primarily a function of the surface finish, not the material it is onstructed of... The plot above shows typical ranges in over all sound levels for different concrete surface textures and types and typical hot mix asphalt. There is variability in the sound produced from all textures (even porous asphalt)...  The range of sound levels produced by concrete pavements is on the order of 16 dBA.  Then a sound increases by 10 dBA it seems twice as loud, so such variability can be quite significant.
You get what you spec... Simply by specifying a turf drag, diamond ground or longitudinal tined texture in lieu of a random transverse tined texture, it is possible to cut as much as half the noise on a concrete pavement.

Guess That Frequency?
The figure on the left shows the sound spectrum produced by two concrete surface textures. The tonal spikes that occur in transverse tining at about 600 and 1200 hz are what we hear as tire whine. Unfortunately, transverse tining is the texture FHWA has required for the last 25 years. Today, states may use textures that remain durable and safe over many years of service, but without the tire whine.