What's Behind the Black - Asphalt 101
Updated: Feb 9, 2022
If someone took a poll, “what is something society relies on every day, that the majority of people don’t fully understand”, asphalt would have to rank high up there. The whole goal of high quality asphalt, is to ensure you don’t even know notice that it is there on your daily commute.
Most people understand the basics, rocks mixed with tar equals asphalt, but there is a much more science and technical basis behind the stuff you drive your car over every day. The production of asphalt involves a large amount of knowledge and effort to ensure that the final product will last for years to come and to be smooth as butter so you don’t notice it while listening to your Murder Mystery podcast on Spotify. This article aims to give you a quick explanation on some of the basics and the testing procedures that are involved production of quality asphalt.
Asphalt mixtures have been commonly used in pavement construction since the beginning of the twentieth century. Asphalt mixtures primarily consist of mineral aggregate material that is bound together with liquid asphalt, which is then laid in layers and compacted to achieve optimum consistency and density. What most people do not know, is that the science behind the placement of asphaltic concrete involves many factors that require various testing procedures that need to follow applicable standards and specifications to ensure that it indeed achieves its ideal use. These specifications include the physical properties of the mineral aggregate being used, the physical properties and composition of the asphaltic concrete and the ability of the contractor to comply to an Asphalt Mix Design which is normally completed by a certified asphalt laboratory such as AllRock Consulting Limited.
Everything begins at the crushing phase. This is when a contractor mobilizes large amounts of heavy equipment to a pre-approved rock quarry source to start producing mineral aggregate to be used in asphaltic concrete. Normally, a set of specifications is provided to the contractor which highlights the various sizes of mineral aggregate and the composition that is required. Throughout mineral aggregate production, quality control testing is conducted on the newly crushed aggregate to ensure that the product meets the specifications for the project. This quality control testing generally consists of random sampling of the stockpiles during production where samples of the aggregate are taken by a certified laboratory for sieve analysis testing. These samples are tested according to the proper standard procedures and the results are then used to adjust the crushing operations to ensure compliance, but also to compile an average sieve analysis for the stockpile once crushing has completed. This average will be used in a certified asphalt laboratory later for the completion of an asphalt mix design.
What most people do not know, is that the science behind the placement of asphaltic concrete involves many factors that require various testing procedures that need to follow applicable standards and specifications to ensure that it indeed achieves its ideal use.
To ensure the best product possible, one of the most important aspects of asphaltic concrete is the physical properties of the mineral aggregate that was crushed and a naturally occurring blending sand used to make a smooth product. To do this, a laboratory that is certified in doing so, must complete a variety of property testing on the crushed aggregate that is to be used. This testing is comprised of subjecting the aggregate to various scenarios such as degradation, the effects of salt combined with the processes of freezing and thawing, as well as the way the aggregate was crushed and physical shape of the individual sized fractions. All these specified procedures are to test the aggregate to ensure that it is capable of withstanding the ever-expanding loading requirements, the interlocking of the liquid asphalt with the aggregate and to make sure that it is able to survive the environmental climate in which the asphaltic concrete will be placed, as well as other factors. Any laboratory which offers these aggregate testing services must be certified by the Canadian Council of Independent Laboratories (CCIL) and is subject to a biannual audit to prove that the laboratory is in good standing and capable of performing the required testing based on the applicable standards.
One might think that asphalt is asphalt, and that what you place in Northern Ontario is the same as what you place is a much warmer tropical climate, but that is not the case.
Whether you are completing an asphalt mix design in Newfoundland for a project that requires asphalt testing in St. Johns, or a mix design for asphalt testing in Toronto, Ontario the standards for each are different based on the highway classification and average traffic flow in the area. A rural road in Newfoundland is not equivalent to a major highway in Toronto, therefore the asphalt mix composition and physical requirements are different. A large highway is held to higher standards and specifications than a rural road, due to the amount of traffic present and the potential for more deterioration that may be caused. The role of the asphalt engineer is to highlight the requirements of which pavement structure that is to be produced, and to find the optimum composition and ratio of aggregates and liquid asphalt to produce the highest quality product possible. The way in which the engineer executes this is in the process of an asphalt mix design.
If an asphalt laboratory were a kitchen, then the asphalt engineer would be the chef and their goal is to create a recipe for their client for the best product possible. This recipe is the asphalt mix design. The engineer combines various proportions of the mineral aggregate that was produced earlier with the blending sand to create a composition that meets the gradation requirements of the final asphaltic concrete product that is required by the project specifications. The mix design procedure involves combining calculated amounts of the crushed mineral aggregate at its various size fractions with the blending sand and finally adding the liquid asphalt in appropriately sized testing batches and mixed to completely coat the aggregate with the asphalt. This combining and mixing aggregate and asphalt is completed at various intervals of liquid asphalt percent to provide a graphical analysis of how the mixture performs at different percentages of liquid asphalt. Each combination is then tested according to the applicable standards to determine the physical properties of the asphaltic concrete mixture such as density, air voids, voids in mineral aggregate, marshall stability and flow. Once the testing on all batches is complete, the engineer determines the optimum liquid asphalt content based on the laboratory results to produce the highest quality asphalt product. This combination of aggregate and liquid asphalt proportions is relayed onto the client, who then uses the recipe during asphaltic concrete production and placement. Now it’s time to finally place the asphaltic concrete on the pavement structure.
Figure 2: Asphalt Mix Design Marshall Specimens
The contractor takes the mix design provided by the engineer and uses it to produce the asphaltic concrete required for the project in which it is needed. During the placement phase, its not as simple as just placing it on the ground surface and compacting it; there are also compliance specifications that a contractor must uphold. During production and placement, random samples of hot mix asphaltic concrete are taken by a CCIL certified laboratory who can offer asphalt testing services and the samples are checked to ensure that the contractor is following the pre-approved mix design formula that was supplied by the asphalt engineer. If the asphaltic concrete meets the specifications and mix design then the contractor proceeds as normal, although if the specifications and mix design formula are not met then the contractor may need to adjust the asphalt plant to meet the criteria or face repercussions in the form of extended warranties, monetary penalties, or the form of rejection and removal of the already placed asphaltic concrete at the contractor’s expense. Also, if the asphalt does not meet the requirements, then there is a greater chance of degradation and failure of the pavement surface at a rate faster than its extended lifespan. Thus, showing the importance of quality control testing.
Not only are the samples important, but a vital component of asphaltic concrete placement procedures is the compacting of the hot surface with sufficient equipment to ensure it is compacted to project specifications.
Compaction testing is offered by CCIL certified asphalt laboratory using a nuclear density gauge during rolling, or by extracting asphalt cores from the roadway to determine the compaction percentage. As is with the physical asphalt mix, if the compaction of the asphaltic concrete doesn’t meet project specifications, then the contractor may face the same repercussions. It is important to note that the requirements for compaction testing in Toronto may be different than the requirements for compaction testing in Newfoundland as well as all other areas, so it is important to verify the project needs beforehand to establish a proper roller compacting pattern. Once the asphaltic concrete is compacted to the desired percentage, the contractor then moves on to the next area of the project knowing that in conjunction with the asphalt sample results, a proper job has been completed.
In summary, asphaltic concrete which is placed on our roads and highways across the country is more than a simple mixture of rocks and tar. There is a complicated and scientific process that goes on behind the scenes that most people do not see which is integral in supplying the best product possible and addressing the possible issues that can happen before they become a problem. This is only the tip of the iceberg of asphalt testing and monitoring and if one were interested in discovering more about asphalt mix designs or compliance then they can visit AllRock Consulting Limited’s website or reach out to any of their highly skilled staff located in Newfoundland & Labrador and Ontario.
Brett Evans, P. Eng
AllRock Consulting Limited
Gander, NL A1V2P2