In our October e-newsletter, we took a look at quality control and quality assurance (QA/QC) to understand the difference between the two, how they can overlap, and how each plays an important role to ensure durable roadway elements. Here, we
will look a little deeper into asphalt work to understand QA/QC’s role in constructing pavements that will be long lasting and provide ride characteristics the public expect.
Regardless of the project, durable asphalt pavements start with the proper mix design, maintaining target temperature ranges, and following good practices to avoid segregation and achieve good bonding and high compaction. Asphalt will degrade over time, but we should expect 7-10 years of good performance (even if we don’t practice pavement preservation, which we should) and well-constructed asphalt pavements have been known to perform reasonably well for as much as 20 years.
Some distresses, such as oxidation or environmental cracking, just come with age. Others, such as edge cracking and settlement, are likely due to poor subgrade. But some point directly at poor asphalt mix or poor laydown practices and both the contractor and owner share blame if a lot of them show up. If you have a lot of delamination between layers, potholes early in the pavement life, rutting, shoving, or early life raveling, it is likely you can trace those to poor construction, poor mix, or the wrong mix design. If both the owner and the contractor do their job, these should be rare. Part of how we improve the durability and performance of asphalt pavements is through good QA/QC.
The contractor’s quality assurance system is more than just testing that is carried out. It starts with best practices in the preparation of the mix at the batch plant, proper placement that avoids segregation, minimal hand working of the material, and good compaction. The contractor is responsible for ensuring that the asphalt batch plant providing the mix follows correct procedures to produce high quality mixes and you should consider only allowing sources that have a currently approved Quality Control Plan in accordance with the Delaware Department of Transportation’s (DelDOT) Standard Specifications for Road and Bridge Construction (see Sections 401.3 and 1014). The QC plan covers an array of practices as well as regular testing of aggregate, sand, asphalt binder, and other elements to ensure that the produced material will be consistent with the Job Mix Formula (JMF).
The agreed JMF should detail each component (and its source), amount of recycled asphalt pavement (RAP) or shingle (RAS), mixing temperatures, amount and type of asphalt cements, aggregate distributions, and target values for air void content. A well run batch plant should be regularly testing to ensure that the mix you require for your job is what arrives. But in the heat of the summer, there are a lot of asphalt trucks running around and there can be mix-ups, so have a look at each truck ticket to make sure that’s the asphalt your ordered.
Moving on from the contractor’s QC system to the owner’s QA role, compaction testing and core samples probably aren’t done enough by local agencies. They should be done more often so you can be sure that the unit weights are in range and that the compaction you sought is achieved. Compaction testing in the field is normally conducted with a nuclear test gauge and there is considerable cost, training, and medical surveillance needed to participate in that kind of testing. For smaller agencies, this may call for establishing an “on-call” contract with a testing consultant that can come out on selected projects to perform compaction testing on your behalf. You may also wish to have them cut cores on some projects to perform laboratory analysis.
As you can see from DelDOT’s Standard Specifications, there is a long list of checks and tests that the contractor and the batch plant perform on a regular basis (some of them daily) as part of the QC system. As the consumer, you are free to ask all the questions you like about the product and you should do so, just as you would do before you buy anything else. You also can ask to visit the production plant to hear first-hand how they produce high quality mix. This is part of your QA program as well. Chances are, you will leave with a greater confidence in the material that you are buying. If not, continue asking questions or change your source.
Long-life asphalt pavements are very achievable with good mix design and experienced contractors with the correct equipment, so as local agencies, you need to know a little about the process and set the expectations before the job starts or is bid out. If you are unsure of what good asphalt should look like and perform, be at our next offering of Asphalt Best Construction Practices, a half-day training workshop intended for those newer to asphalt pavement construction. The best way to hear about our training workshops is to be on our distribution list, so use this link to get on it.
The Delaware T2/LTAP Center’s Municipal Engineering Circuit Rider is intended to provide technical assistance and training to local agencies and so if you have construction management questions or other transportation issues, contact Matt Carter at matheu@udel.edu or (302) 831-7236.
You must be logged in to post a comment.