FAQs

Talking Asphalt: Asphalt’s Advantages
by Dwight Walker
(Courtesy of the Asphalt Institute and the Asphalt Pavement Alliance)

Ninety-four percent of U.S. roads (and roads in Hawaii) are surfaced with asphalt, and there are about 18 billion tons of asphalt in place on our roads. There must be a reason for such wide spread usage. Actually, there are many reasons. It is my opinion that asphalt is truly the most adaptable and versatile paving material. Here are some of the reasons for using asphalt.

Versatility/adaptability
Asphalt is a marvelous engineering material. It lends itself to working in a great number of applications. It can be a load-carrying or structural component; it can be a wearing course or a waterproofing layer. It can be extremely dense or free-draining. It can be placed in thick layers to last indefinitely or it can be placed in ultra-thin overlays to preserve an existing road.

Recyclable/reusable/rejuvenation
Asphalt can be and is re-used. RAP or recycled asphalt pavement has incredible value. Nearly 100 million tons of asphalt is re-used each year. The aggregate component of RAP saves resources and the asphalt can be restored to approximately its net original properties by using a compensating binder with the recovered materials. Other pavement types can only be re-used as an aggregate.

In addition to re-using RAP, asphalt mixtures routinely incorporate other scrap or waste materials, such as tire rubber, roofing shingles, glass, and other materials. An asphalt pavement can, effectively, recycle concrete through the rubblization process. Failed concrete pavements are broken down in-place and used as an aggregate base for a new asphalt pavement. This technique offers the environmental advantages of conserving aggregate and saving landfill space.

Speed of construction
Asphalt is quick to place and requires less traffic delays due to congestion or lanes being out of service. Much of the work can be done during off-peak hours, and this saves the traveling public time and money. Asphalt pavements can be opened to traffic as soon as they are compacted and cooled, while other pavement types can take days or even weeks to cure.

Quiet
Asphalt pavements are quieter than other pavement types. Some asphalt mix types (OGFCs and SMAs) typically reduce noise levels by three to five decibels, which is like doubling the distance from the source of the noise. Noise barrier walls are incredibly expensive and paving with asphalt is the better choice.

Smooth
Asphalt pavements are undeniably smoother. Because asphalt is smoother, some agencies hold asphalt to higher smoothness specs. The public appreciates a smooth ride, and as an added benefit, vehicles get better fuel mileage on smooth pavements.

Perpetual Pavements
These pavements are designed so that only the top, surface layer is renewed on a 15-year or longer cycle. The base, structural layers last indefinitely (40 years or more); hence, the name perpetual pavements. Over their lifetime, these pavements use less materials and have a lower overall life cycle cost.

Readily repaired
If needed, asphalt can be readily repaired. These repairs can be done during off-peak travel times, often over-night, while other pavement types may require weeks for repairs or replacement.

Warm mix asphalt
New technologies have allowed asphalt mixtures to be mixed and placed at significantly lower temperatures. A 50-degree F reduction is typical and 100-degree is possible. The warm mix processes (there are numerous methodologies) save burner fuel and reduce emissions. Other observed benefits may include easier compaction, extended haul distances and/or paving seasons and higher RAP contents.

Porous pavements
Asphalt mixtures can be designed to be free-draining. Such engineered installations can be used in storm-water management. The filtering process of these installations can improve the water quality by removing contaminants during the retention time.

Safety
Specially designed asphalt mixtures, such as Open Graded Friction Courses (OGFCs), can allow water to travel through the pavement surface layer and away from the actual riding surface. This reduces spray, decreases hydroplaning and increases skidding resistance.

Life cycle cost
At one time, asphalt had a significantly lower initial cost than other pavement types. This may not always be the case today, but asphalt stills remains the lowest overall cost paving material. When alternate bids are received, asphalt routinely wins.

Familiarity
I started off saying that 94 percent of our roads have asphalt surfaces. With that much volume, it is not a problem to find a qualified, competent contractor. They are out there working every day to build and preserve our roads.

With all these advantages, it is easy to see that asphalt is the obvious choice.

ASTM versus AASHTO Standards
by Brian J. Johnson, AASHTO Accreditation Program Director, AASHTO re:source

ASTM and AASHTO are two separate entities that serve slightly different purposes with different members making decisions even though they both end up at about the same place

ASTM is primarily a standards development organization that operates in just about every industry. Construction materials is just one of them. ASTM membership is voluntary with members from all over the world and with varying levels of expertise and roles in their industries, which contributes to a broad collective perspective. Anyone can vote on the electronic ballots, but most of the effort maintaining the standards comes from those who attend the meetings in person (at least in construction materials).

Those members are mainly materials producers, testing firms, equipment manufacturers, consultants, researchers, governmental agencies, accreditation bodies, certification programs, calibration providers, and other interested parties. These members drive forward changes in practices, test methods, and specifications to keep up with improvements in equipment, quality concepts, and techniques.

AASHTO serves many roles in the transportation industry, and one is as a standards development organization for the state departments of transportation (DOTs). State DOTs work together on maintaining this set of standards through AASHTO’s Committee on Materials and Pavements (COMP) that all states can use instead of maintaining their own set of standards in their states.

The members of COMP are usually state materials engineers. Anyone can comment or suggest changes to AASHTO standards, but it takes a member to drive it forward, and only members have voting rights. However, some of the same members from corresponding ASTM committees contribute to ballots and discussions in COMP so the perspective is not as narrow as it sounds based on the limitation of membership, but ultimately only the DOTs are deciding on whether a change is going to be made in an AASHTO standard. COMP also serves a unique role in the standards development arena because it evaluates research proposals that leads to the advancement of technologies that often are the basis for new or revised standards.

I mentioned that they end up at about the same place, but there are significant efforts going on to harmonize the AASHTO and ASTM standards so that the most common standards are identical. Also, new standards can now only be published by either ASTM or AASHTO according to an agreement between the two organizations. That helps reduce the proliferation of more standards that are not quite the same covering the same type of test.

What I would really like to see is one super-committee in which the interests of the DOTs, industry, and other interested parties are working on the same exact standard together, and then everyone could just use that standard…but like I said, ASTM covers just about every industry, and this would be a rather significant change to their operations to accommodate one industry. It would also cause a change in the policies, balloting system, and membership management for COMP – so even though it sounds like it might be easy to do, it would be quite a massive undertaking for both organizations.

If you would like to read more about this topic, you can check out these newsletter articles: The Birth of a Standard – Part I: Who Are AASHTO and ASTM? and The Birth of a Standard – Part II: The Harmonization Effort.

For private roads, several HAPI members do pothole repairs.  Please go to our members page and review the services of each company.  Those that do will have pothole repairs listed as a service.

For public roads, the pothole would be repaired by the agency that owns the road:

The City has a Pothole Hotline: https://www.honolulu.gov/dfm/pothole.html.  Phone numbers for the State and HART pothole hotline can be found at that site, as well as a listing of roads under State or HART jurisdiction.

Potholes within HART work areas can be reported directly to HART at info@honolulutransit.org or the City’s pothole reporting . HART’s contractors routinely repair portholes during construction.

The County of Hawaii has a website  for reporting problems such as potholes.
https://www.dpw.hawaiicounty.gov/divisions/highway-maintenance/report-problems

Is RAP just a black rock?
by Dr. Grover Allen, Ph.D., P.E.
(Courtesy of the Asphalt Institute)

If you’re reading this article, you probably already know that RAP stands for “reclaimed asphalt pavement” and is valued by users and producers alike because it can be recycled as a component of a new asphalt mixture. In addition to being resource-responsible, the use of RAP in an asphalt mixture means that at least some portion of the reclaimed asphalt binder can be used to offset the amount of new asphalt binder used in the mix. But how much of that reclaimed asphalt binder is actually available to act as a binder and how much is too hard to serve that role – acting more like a black rock?

Before we discuss reclaimed asphalt pavement, let’s first discuss reclaimed asphalt shingles (RAS).

Just a few decades ago, binder from RAS was being ushered in as a cost-cutting and environmentally friendly alternative to virgin binder. Yet, over time, the material has become intrinsically linked with dry mixtures (low active/effective binder content) and premature pavement failures leading to its steady decline as an asphalt mixture component since its peak in 2014.(1)

Per “AASHTO PP78 – Standard Practice for Design Considerations When Using RAS in Asphalt Mixtures”, additional binder must be included at about 0.2 percent by weight of mix for every five percent RAS used “to account for RAS asphalt binder that does not become available and effective during the asphalt mixing process.”

This means AASHTO PP78 assumes approximately 20 percent of RAS binder is inactive, but this also means it is implicit in AASHTO PP78 that 80 percent of the RAS binder is active. How could this much RAS binder be assumed to be active if nearly all RAS binder is demonstrably inactive?

Consider this basic experiment as evidence: When RAS and virgin aggregates are heated to a typical production temperature of 350°F and mixed, it is readily observed that RAS binder doesn’t actually soften, nor does it separate from RAS aggregates or coat virgin aggregates to any measurable degree, as shown below.

The marginal amount of RAS binder that should be assumed as a mix contributor is the portion filling RAS aggregate pores and coating RAS aggregates. I would not even advise a Department of Transportation (DOT) agency to allow RAP (yes, RAP) binder to replace virgin binder at a 1:1 ratio; or in other words, I wouldn’t advise assuming the RAP binder is 100 percent active, let alone RAS, which is observed to have only an insignificant amount of active binder.

However, according to a recent survey, there are still a few DOT agencies assuming RAS binder is 100 percent active and allowing RAS binder to replace virgin binder at a 1:1 ratio of up to 20 percent of the total required binder content.(2) There are obvious environmental benefits in using RAS as an aggregate and as a RAS aggregate pore filler/coating, but without substantially improving the RAS binder quality and availability, that’s about where the benefits end. So why is the RAS black rock theory taking so long to become widely accepted and properly addressed despite the clear evidence for it? If nothing else, the RAS situation demonstrates that we sometimes get it wrong and are very slow to course-correct.

RAP binder availability

This leads us to our next important question: Is RAP just a black rock? Cue the skeptics: Is he serious?

Asking this just a few years ago would have certainly been considered blasphemy, and the person asking it would have been ridiculed. I mean, let’s get real! RAP is a very different material than RAS. RAP binder isn’t nearly as stiff as RAS binder and examples of its successful use are evidence RAP binder is clearly not a black rock, right? Not to mention, RAP is the most recycled material in the world, so why would anyone question its utility? If you’ve been following along lately, you already know the question of whether RAP is a black rock is no longer controversial, and in fact, it has become the primary focal point of a major research project, “NCHRP 9-68 – Recycled Asphalt Materials: Binder Availability and Its Impact on Mix Performance” (3), on which I serve as a technical advisor.

There’s certainly been a major shift in thinking around RAP in recent years. Despite cases of successful use of RAP, there are also many instances in which the use of RAP has resulted in premature failures and overall poor pavement performance. Not unlike RAS, investigations into performance-related issues linked to RAP have led agencies, asphalt technologists and practitioners to pursue ways to improve RAP mixture performance.

The presumption of RAP binder as a viable replacement for virgin binder has hit more than a few major roadblocks. Let’s start with an area of widespread agreement: RAP binder, despite being less brittle than RAS binder, is still generally much more brittle than virgin binder, and therefore, it should either be used in mixtures at relatively low percentages, blended with a softer virgin binder (grade dropping), rejuvenated or some combination of these strategies. Each of these approaches highlights an attempt to minimize the recognized damaging effects of using highly oxidized and brittle RAP binder as a direct replacement for higher quality and ductile virgin binder. However, even these strategies fail to address the issue of “Is RAP just a black rock?”

You see, if RAP binder is inactive (black rock) or only partially active (partial black rock), it dramatically changes the strategies that we’d use to overcome just the inferior properties of the binder. We’d not only need to compensate for these highly aged and oxidized binder that lacks ductility but also the portion of the binder that is completely inactive – in other words, the portion that is just a black rock.

For example, would it make sense to “grade drop” by using a softer binder in conjunction with RAP if the RAP binder isn’t capable of completely liquifying, separating from the RAP aggregate, blending with virgin binder, coating virgin aggregates and gluing aggregates together? Assuming that it is doing all of this at 100 percent efficiency when it is not will inevitably lead to an active binder portion that is too soft and a mixture that doesn’t have enough total effective binder. A better strategy to use in this case would be to specify a stiffer grade of virgin binder and more of it.

How did we arrive at this point?

So how did we transition so quickly from RAP binder being an equivalent replacement for virgin binder to casually questioning “Is RAP just a black rock?”

The roots of “RAP binder availability” probably began the instant “RAS binder availability” began due to obvious similarities in use of the two materials, but the recent and ongoing cascading effect of DOT agencies investigating and adopting partial RAP binder credit policies (assuming partial black rock) has ignited great interest in the topic. In fact, the Southeastern Asphalt User-Producer Group (SEAUPG) annual meeting held in November 2023 featured multiple sessions on RAP binder availability, including a panel discussion highlighting two DOT agencies and two contractor representatives from each of those states purporting the benefits of implementing partial RAP binder credit policies, which leads to more virgin binder.

Feedback from the panel members was practically all in favor of a partial RAP binder credit policy. Cited benefits include better performance and greater ease in meeting quality assurance (QA) requirements. These benefits do come at an added initial cost, but according to recently completed research by NCAT (“Determining the Effect on Asphalt Mixture Performance by Increasing New Asphalt Binder Content Due to Inactive RAP Binder in the Mixture”), it will only take about two months of additional service life to justify the cost of the additional virgin binder if we assume 0.2 percent is added (reflecting the assumption that only 80 percent of the RAP binder is active.)

All agencies with current partial RAP and RAS binder credit policies (as of 2021) are shown in the following map. In addition to these state agencies, Florida DOT and Texas DOT are deep into the process of implementing their own policies with others exploring policies and expected to soon follow suit. The assumed active RAP binder among these states ranges between 60 and 80 percent. All other states currently assume 100 percent active RAP binder as does AASHTO M 323 – Standard Specification for Superpave Volumetric Mix Design (2022).

A major influencer and early adopter of partial RAP binder credit policy was the Georgia DOT (GDOT). A bit of this history was highlighted in Asphalt magazine Vol. 35, No. 3 (2020). I won’t rehash that article, but I do recommend reading it if you’d like to learn more about GDOT’s procedure for calculating and accounting for the partial RAP binder credit via a process GDOT calls “corrected optimum asphalt content (COAC).”

I will briefly explain a few key points from GDOT’s research into RAP binder availability, which I believe opened a lot of eyes and contributed to the momentum we’re now seeing on this topic. Around 2012, after noticing premature deterioration of high RAP pavements (≥ 25% RAP), GDOT performed a lab study similar to the one mentioned earlier with RAS – virgin aggregates heated to a 350°F production temperature and mixed together in a pugmill with non-heated RAP (since RAP is not typically pre-heated in plant-produced mixtures). Interestingly, the experiment revealed no visible transfer of binder from RAP to virgin aggregates (similar to RAS). This then led GDOT to partner with Reeves Construction to conduct a follow-up plant study of a similar design – 30 percent stockpiled RAP plus virgin aggregates were mixed in a typical plant production process, and once again, there was no observable transfer of RAP binder to the virgin aggregates.

This is quite a surprising outcome considering that RAP binder is assumed by most DOT agencies to be 100 percent available. With this new information, GDOT (and many others, I’m sure) became believers in the RAP black rock theory, or at a minimum, very skeptical that 100 percent of RAP binder is available. Despite zero binder transfer observed in GDOT’s research, GDOT does presently allow 60 percent credit (assumed availability) of the RAP binder, which is significantly above the percent consumed in filling pores and coating RAP aggregates. This is also notably less credit than is given for RAS binder according to AASHTO PP78, which is something I can’t quite grasp.

It is important to highlight that virgin binder was intentionally omitted from these GDOT experiments to prevent virgin binder from painting all virgin aggregates black and making it impossible to determine how much RAP binder transfers to virgin aggregates during the experiment. Some contend that omitting virgin binder from such experiments means eliminating an important thermodynamic component (hot liquid virgin binder) that may have led to additional RAP binder activation during a normal mixture production process. This is a valid consideration and was likely a factor in the reconciliation of GDOT’s policy as they assume more RAP binder availability (60 percent) than indicated in their experiments (0 percent).

NCHRP 9-58 (“The Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios”) researchers performed experiments that did include the virgin binder component in addition to the RAP and virgin aggregate components and arrived at the following linear correlation between RAP binder stiffness and availability for a variety of RAP sources. So basically, the stiffer the RAP, the less available the RAP binder, i.e. the lower the RAP binder availability factor (BAF). That actually makes a lot of sense.

My personal favorite method for quantifying RAP binder availability was employed originally by Mohajeri et al. (2015)(4) and recently adopted by the NCHRP 9-68 research team – it involves using borosilicate glass beads as tracers (representing virgin aggregates), shown below. All components of the mixture, including virgin binder and a portion of the virgin aggregates replaced by glass beads, are added and mixed. Afterward, the glass beads (easily identified as “virgin aggregates”) along with a new binder coating on the beads are removed and the binder extracted/recovered/tested to determine how these properties compare to force-blended properties of the RAP and virgin binder at 100 percent assumed availability. The ratio of actual properties, e.g. PGH (high-temperature performance grade), on the glass beads to assumed 100 percent availability properties will reveal the RAP binder availability of that particular RAP source as shown in the basic expression that follows.

Where,

RBA = RAP Binder Availability
PGB = Property extracted and recovered from glass bead
Pv = Property of virgin binder
PRAP-blend = Property of extracted and recovered RAP binder force-blended with virgin binder at 100% assumed availability

In closing

Successful use of RAP requires nuance and consideration of variables that are known to change from one RAP stockpile/project to another, including fractionation, moisture control, stiffness of RAP binder, target RAP percentage used/allowed, asphalt layer containing the RAP, rejuvenation and blending methods. Most asphalt practitioners understand these variables well.

Over just the last few years, we’ve gone from not even questioning the presumed 100 percent contribution of RAP binder to basically agreeing that it’s unlikely to ever be 100 percent and could be as low as 40 percent or less in certain cases. Now we must add RAP binder availability to the list of key considerations for its success. DOT agencies are quickly implementing partial RAP binder credit policies in an industry that is not typically known for fast course correction. That says a lot!

If the common goal is to maximize the use of RAP, it will not be achieved by using RAP the way it has been used in the past, which includes incorporating RAP at higher and higher percentages until performance issues are revealed and then backing down to a lower percentage until the performance issues go away. A better approach is to acknowledge the weaknesses of RAP materials in the beginning and address them. If a RAP source is only 75 percent active, or 25 percent black rock, let’s go ahead and say that up front. We can then account for it by adding additional virgin binder or implementing other methods known to increase active binder percentage, including additional heat or rejuvenation – and then reap the rewards of higher performance at higher RAP percentages.

Allen is an Asphalt Institute Regional Engineer based in Florida.

Resources

(1.) FHWA-HIF-22-001 Tech Brief – Practices and Lessons Learned When Using Reclaimed Asphalt Shingles in Asphalt Mixtures. (2021)
(2.) NCHRP 9-58 – The Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios (2014-2018).
(3.) NCHRP 9-68 – Recycled Asphalt Materials: Binder Availability and Its Impact on Mix Performance (2022-2025).
(4.) Mohajeri et al., 2015. Blending of Virgin bitumen and RA Binder in Mixtures with High Amounts of RA.

Click here for a PDF file of the magazine article.

What is the Cure Time for Asphalt?
by Dr. Grover Allen, Ph.D., P.E.
(Courtesy of the Asphalt Institute)

An engineer colleague recently reached out to me and asked, “what is the cure time for asphalt?” My initial response to his question was “are you referring to cold-mix asphalt?”

Cold-mix asphalt is a mixture of asphalt and a solvent and/or water, aggregates, reclaimed asphalt pavement (RAP) and other minerals typically used for patching and for repair of small sections of pavement. The asphalt and solvent portion, excluding the other components of the cold mixture, is often referred to as “cutback asphalt.”

Evaporation, or curing, of these components – solvents and/or water – in a cold-mix asphalt leads to the hardening of the asphalt mixture. I assumed
this was likely what my colleague was referencing with his question. Another thought occurred to me that he may be referring to an asphalt emulsion, which is an aqueous solution containing tiny droplets of suspended asphalt meant to be applied in ultra-thin layers on or within a pavement structure.

Asphalt emulsions also require the evaporation of water and a relatively short “cure time.” However, it is not common or appropriate to refer to these products as “asphalt” without including some type of additional descriptors, such as “cold-mix”, “cutback”, or “emulsion.” When the word “asphalt” is used without a descriptor, it is most commonly referring to an “asphalt mixture”, which includes the heated liquid asphalt and aggregates that are compacted to build a typical roadway; otherwise, the term “asphalt” may be referring solely to the hot liquid asphalt portion of the mixture. “Hot-mix asphalt” is a term often used to refer to an asphalt mixture, and “warm-mix asphalt” may also be used to describe an asphalt mixture that is either processed with an additional additive or using foaming techniques that allow the temperature of the mixture to be reduced slightly, although the mixture is still very hot.

Simple answer

Asphalt is liquified by applying external heat, which allows it to be mixed, transported and compacted and the mixture rapidly hardens as heat dissipates and achieves full strength upon cooling to ambient temperature. Therefore, I want to make this as clear as possible – asphalt, which is the material used to build 94 percent of our roadway infrastructure in the U.S., does NOT have a cure time.

Asphalt begins to cool immediately after mixing and cooling continues during transport, construction and post-construction. It will typically gain full strength and can support traffic within minutes after construction but can take hours in rare cases, depending on lift-thickness, ambient conditions and other variables.

Complex answer

For the more technical readers, there are two key temperature ranges that should be considered to ensure that asphalt construction operations cease and traffic operations commence at the proper times:

1. Cessation temperature (internal asphalt temperature in which all construction operations must cease to prevent damage caused by construction equipment)

2. Maximum ambient temperature (internal asphalt temperature below which a newly constructed asphalt pavement has cooled sufficiently to be trafficked without causing premature damage)

The internal portion of an asphalt pavement during construction and immediately post-construction may be slower to cool than the surface. This effect can be much more pronounced for pavement lifts thicker than two inches, so internal temperature may be temporarily higher than surface temperature until equilibrium is reached. For pavement lifts two inches and thinner, the surface temperature is a more reasonable approximation of internal temperature during the cooling phase.

Asphalt Institute’s MS-22 “Construction of Quality Asphalt Pavements” says the following about cessation temperature:

“Compaction should be completed before the internal mix temperature falls below what is referred to cessation temperature, typically around 175-180°F (80-82°C). The cessation temperature will vary from project to project. Therefore, it is important to establish a target cessation temperature at the beginning of each project.”

Once an asphalt pavement has cooled to ambient temperature, surface temperature will closely trend with ambient air temperature, but due to the effects of surface heat radiation, pavement temperature is typically greater than the ambient air temperature. For example, if it is 90°F outside, the pavement surface may be up to 120°F or greater, depending on UV intensity and other factors. The precise post-construction maximum pavement temperature permitted for traffic opening will differ based on the climate and materials selected for construction.

Choosing a binder

Performance-graded asphalt binders are selected based on a design maximum pavement temperature 20 mm below the pavement surface. This design value is dependent on a maximum air temperature and the geographical latitude for a given region. For example, there is more than 99 percent reliability that a pavement temperature will not exceed 136.4°F (58°C) in Cleveland, Ohio. Therefore, a pavement constructed with a PG 58-28 grade binder in Cleveland, Ohio would be ready to open to traffic once the internal pavement temperature post-construction has cooled below the maximum design temperature of 136.4°F (58°C). In a hotter climate,
such as my home state of Florida, a stiffer grade of asphalt, such as PG 76-22, is commonly specified.

An asphalt grade that can support traffic at a temperature up to 76°C (168.8°F) means that a newly built roadway should be able to open to traffic very shortly after final roller pass occurs between 175-180°F. This matches closely with the language shown in the Florida Department of Transportation’s (FDOT’s) Specification Section 330-10: “To prevent rutting or other distortion, protect sections of newly finished dense-graded friction course and the last structural layer before friction course from traffic until the surface temperature has cooled below 160°F.”

The specification then goes on to say the contractor may use artificial methods to cool the pavement and may be directed to do so when it is desirable to open the pavement at the earliest possible time. Therefore, the required time to open a newly constructed two-inch asphalt surface to traffic would generally be just a few minutes. Thicker lifts will take longer to cool internally, and in rarer cases of asphalt construction, it could take hours. DOT and airfield project engineers or other representatives present at the construction site may give the order when an asphalt pavement is ready to be opened to traffic.

The lack of a cure time for asphalt, or its speed of construction, is one of the unique advantages of using asphalt as the primary roadway and airfield building material and an attribute which makes it uniquely different than portland cement concrete (PCC). The load-bearing capacity of PCC is normally measured and estimated according to its compressive strength over a slow curing time.

A typical strength-gain curve for PCC is:

• 1-day: 16%
• 3-day: 40%
• 7-day: 65%
• 14-day: 90%
• 28-day: 99%

Honest question

Back to my engineering colleague who asked, “what is the cure time for asphalt?” to which I responded, “are you referring to cold-mix asphalt?” He responded: “no; I’m referring to hot-mix asphalt.”

Now, let me provide a little more background on my engineer colleague. He’s no rookie. He’s a very accomplished engineering professional. His engineering career has mostly encompassed working with structures, including concrete mix design, concrete construction, blast testing, blast mitigation, etc., and he’s been practicing engineering for over 30 years. He will soon retire.

When he clarified his question, one thing occurred to me. If a structural engineer with decades of experience does not understand that asphalt does NOT “cure”, there must be an abundance of other individuals involved in public service, infrastructure project planning, construction, design, life cycle analysis, infrastructure safety planning, user-delay analysis, etc. who also do not understand this.

At this point in the article, you already have more background about asphalt strength-gain than my engineer colleague with over three decades of engineering experience. That is the primary reason I felt compelled to write this one.

Misinformation

I wanted to better understand where he and others might be getting their information about asphalt, so I went to the Google search bar and typed in “What is the cure time for asphalt?” The results, which you can verify for yourself, were very disappointing. Nearly every search result, including the following examples, falsely indicates there is a cure time associated with asphalt construction:

• “6-12 months”
• “It can take up to a full year for asphalt
to cure, but within 30 days asphalt is
cured to a point it can be driven on”
• “48-72 hours to dry completely”
• “Asphalt takes six to twelve months to fully cure and remains a little more susceptible to damage for that time”
• “The drying process happens in phases. Your asphalt may look and feel dry, but a complete curing typically takes much longer than three days.”

Almost none of the information returned in the Google search accurately answers the precise question my colleague asked, which I admit, was quite surprising to me. At best, the information is extremely misleading, and at worst, it results in the implementation of poor infrastructure policy based on a misunderstanding of how asphalt gains strength after placement.

Now I understand why my colleague was confused about how asphalt hardens and why just about anyone else outside of the asphalt industry might be also. I learned a couple of lessons in this case –don’t assume what others might know and understand, and always verify the information found on the internet.

If you have any asphalt-related questions, please do like my engineer colleague, and come to a trusted source of information, such as the Asphalt Institute. We can’t eliminate misleading information on the internet, but we can provide accurate answers to questions like this, and even much tougher questions presented by practitioners, engineers, the general public, those interested in transportation and particularly those involved in making important transportation investment decisions impacting billions of dollars in infrastructure funding.

Allen is an Asphalt Institute Regional Engineer based in Florida.

Click here for a PDF file of the magazine article.

Or as typically asked, does porous asphalt cost more than dense graded asphalt?

The answer to question is not a straightforward material cost comparison between porous asphalt and dense graded asphalt. To compare the two asphalt types, a cost study should be done to include the entire pavement section. The materials used beneath a porous asphalt and a dense graded asphalt are quite different.

The other major cost item to consider is that the use of a porous asphalt pavement with a stone reservoir eliminates the need for a drainage system, other than provisions for an overflow system.

Therefore, the costs to compare are a porous asphalt pavement section with an overflow system versus and dense graded pavement section with an underground drainage system.

Lastly, it is also important to consider:

• The life of the two asphalt systems – dense graded pavement will typically have a longer life that porous asphalt
• The maintenance costs associated with each pavement system.
  

One question that seems to be on everyone’s mind is how does the cost of warm mix asphalt (WMA) compare to the cost of hot mix asphalt (HMA)? The answer is that depending on the technology used, the cost may either be more or less. For example, once the cost of the equipment needed for the foaming process is recovered, there is a minimal cost associated with this technology as it involves the addition of a small amount of water to the asphalt binder; the cost would be about the same or less as HMA. For other technologies, the savings in fuel costs from producing warm mix at lower temperatures may not offset the material cost of the warm mix technology. In addition, the cost savings from reduced emissions and improvements in working conditions are in the process of being quantified.

Asphalt is flammable if overheated when oxygen is present. Some asphalt cements and air-blown asphalts are not combustible until heated above 450˚F. The ability of asphalt to catch fire varies with the type and amount of solvent. TD Tip – Most PG 64-22 (neat binder grade used in Hawaii) binders flash around 525-575°F degrees F. Beware of using transport companies or hauling trailers that may contain trace amounts of other materials that usually have a lower flash point.