Aggregates for Hot Mix Asphalt
(Second in a series about quality measures.)
By Timothy R. Murphy, P.E. and Amanda L. Murphy
As our roads have changed over the years, our road-building abilities have adapted. Contractor capabilities have improved over time to fit these needs, incorporating advanced technology, higher quality control standards and new specifications to improve safety, efficiency and value. In the process of road-building, the journey begins with aggregates.
When agencies engage contractors to build hot mix asphalt (HMA) projects, quite often they are being built as Percent Within Limits (PWL) projects. With this change to statistically-based payment schedules, the aggregate supplier has become even more actively engaged in the bidding and building processes.
Consistency of the aggregate materials, required to make the contract a success, is paramount. Material suppliers will be paid less for an inconsistent product because, as the FHWA describes a Quality Assurance Program, “confidence that a product or facility will perform satisfactorily in service” is the basis of payment. Material found to be non-compliant will not be allowed for use in the roadway; the agency will not pay the contractor; and the contractor will not pay the aggregate producer.
Before an aggregate is considered for use on an agency project, whether it is for aggregate base, HMA, or Portland cement concrete (PCC), the quality of the mined materials must go through rigorous agency evaluation to determine a variety of qualitative measures, including:
- Safety – Friction testing
- Angularity – Fractured-face testing
- Hardness – LA Abrasion test
- Durability – Soundness
- Absorption – Specific Gravity.
During the aggregate design phase, input from customers, marketing and those who manufacturer HMA is vital. In the first article of this series, we discussed the importance of product Total Quality Management (TQM) as it applies to Historical Analysis, Specification Integrity and People/Production Effectiveness. For TQM to work, it is important to realize that coarse and fine aggregates must be consistently well-graded and handled properly from the fire-in-the hole phase through stockpile load out.
Building a Team
When the asphalt industry set out to improve controls and automation in conjunction with the roll-out of Superpave, the initial response from the quarries was, “We’re not going to slow down our production or make any other expensive changes to our world.”
“As that attitude changed and quarries realized these specifications were here to stay, they got busy producing the materials the HMA facilities needed to make consistent hot mix asphalt,” says Matt Kinsella, Director of Operations at Wolf Paving of Oconomowoc, Wisconsin.
Stone for the Customer
Because we work in an industry that is aware of the need for continual improvement of quality, to simply “make stone” is no longer the quarry mantra. “Make stone for the customer” is a more accurate motto. Tell your customer what your needs are up front to ensure that your productivity and reputation are not compromised and that profits will remain stable for both parties.
Ask your customer to become your partner each new construction season. Identify your production goals and let your customer know you can deliver these goals throughout the construction season.
Recycle, Recycle, Recycle
Recycling is a bustling industry that has grown ever since the first HMA recycling project back in the 1970s. Today there is a renewed effort to grow the asphalt recycling program in the USA because it is economically practical and environmentally prudent. Agencies are committed to increasing the use of RAP successfully through proper engineering and production controls. Volumetric, aggregate and asphalt characteristics must meet or exceed the current specification requirements.
RAP consistency is important since it includes coarse, intermediate and fine particles throughout. Chris Holman, owner of Holman Consulting, describes the process as “crushing to fit the design instead of designing to fit the crush.” This is a critical approach to successful RAP use and HMA production.”
Recycled material must exhibit the desired engineering characteristics of consistently satisfying specification requirements. The material should provide an acceptable level of performance, be economically competitive with available materials, and be as consistent as virgin materials, aggregate from a quarry and asphalt from a refinery.
Consistency comes from knowledge of crushing and screening practices, building stockpiles and load-out procedures. For example, experienced contractors have realized that establishing a ½-inch top size produces a more consistent recycled product.
Reaching the Destination
As famed quality control expert Dr. W. Edwards Deming would say, “Quality is not so much an outcome as a never-ending process of continually improving the quality of what you produce.” This means that the work teams at the quarry need to inspect the yard daily, check the pile, and check primary, secondary and tertiary crushing and screening units often. Observing end-loader movements, as well as stockpiling and load-out techniques, is also necessary. It is particularly important to check screens for blinding, holes, slope and overall efficiency.
Inventory management is an essential part of the quarry’s success. Adequate inventory is as important as randomly grabbing aggregate samples and checking for consistent gradations. The entire process is part of the journey that improves the quality of pavements for agencies and our fellow travelers.
In the third installment in this series, we will discuss the HMA manufacturing facility and workmanship. You guessed it; we’ll be talking more about aggregates, asphalt and recycling throughout.
Tim Murphy is president of Murphy Pavement Technology, Inc. Amanda Murphy is a technical writer for Applied Pavement Technology, Inc.