One of the critical elements in a successful paving project is the asphalt mix design. Another term for mix design is the “job mix formula” or “JMF”. The two main components in hot mix asphalt are mineral aggregate and the liquid asphalt binder. To develop a successful JMF, an aggregate type, typically determined by the area geology, along with an asphalt binder type are required. This is accomplished by reviewing the available aggregates and their gradations (the size, structure and distribution of the stone), selecting the applicable aggregate profile and combining the selected aggregate with the calculated ratio of asphalt binder.
It is well known in the asphalt-paving industry that the correct temperature of a finished asphalt mixture is vital to achieving a high-quality, long-lasting pavement. However, there are other environmental factors that must be taken into consideration, such as
Oftentimes when people think of road construction safety, they think of hard hats, reflective vests, cones, barrels and concrete barricades. While all of these things play an important role in keeping workers and the public safe, they do not account for some of the deadliest hazards faced in an active work zone. Being aware of our surroundings is something that many of us have been instructed to do by parents, coaches or teachers for much of our childhood and early adulthood. Whether it was on the field of play or in everyday life walking down the street, knowing who and sometimes more importantly what is around you can have a great impact on what happens to you.
Expansion joints are vital to the life of a bridge itself and must be correctly installed and maintained in order to function properly. The two major causes of joint failure are improper installation and insufficient maintenance of the joint. Oftentimes, when dirt, dust or other types of debris build-up in and around these joints (caused by traffic and environmental conditions), the joints lose their ability to expand and contract with the structure, rendering them less effective. When this happens, the reduced flexibility will result in cracking or distortion/crushing of the joint material, compromising the entire bridge’s structural integrity.
Often, preparing an asphalt mixture is compared to baking a cake. A contractor needs to make sure he/she has an accurate recipe and proper ingredients and that he/she blends everything at the correct ratios and “bakes” the mixture to produce a beautiful, homogeneous final product that everyone will be happy with – a blend that will perform as anticipated when properly applied on a roadway.
Tack Coat: The Bond That Promotes Optimal Performance
A vital but often overlooked part of an asphalt pavement is the bond strength between the layers that make up the finished structure. Tack coat is the main component responsible for ensuring a sufficiently strong bond.
What is a tack coat?
- It is an asphalt binder that is spray applied onto a substrate material -- typically cement, asphalt or steel (in the case of orthotropic bridges) -- prior to placing a new asphalt overlay.
- This thin application of bitumen is the glue that binds the two layers of pavement resulting in a uniform structure.
- This bonding of the layers creates a structure that behaves as a single unit as opposed to unbound, independent layers.
When looking at our nation’s infrastructure, it is not difficult to see there are many bridges in desperate need of rehabilitation. According to the latest Infrastructure Report Card, the U.S. has 614,387 bridges, almost four in ten of which are 50 years or older and nearing the end of their intended service life. In 2016, nearly 10% of these bridges (56,007) were considered to be structurally deficient.
When considering the durability of a bridge, there are two main areas thought to be the most vulnerable components:
- The concrete deck
- The untreated steel rebar used to provide structural reinforcement
When completed in 2018, the new twin span Tappan Zee (re-named the Governor Mario M. Cuomo) Bridge will have cost $3.9 billion dollars. The Bridge will be one of the widest cable-stay bridges in the world. It will have a combined deck width across both decks of almost 184 feet and it is approximately three miles in length. It is a critical transportation link in the New York Metropolitan area. The west bound span, connecting Rockland County, New York with Westchester County, New York, was opened in August of this year. The east bound span is expected to be completed Summer 2018. The Bridge will have total of eight travel lanes and a separate shared use path for pedestrian and bicycle use. The new Bridge was built by Tappan Zee Constructors (TZC), a design-build company composed of the Fluor Corporation, American Bridge Company, Granite Construction Northeast and Traylor Bros.
Asphalt has been used on bridge decks for over 100 years with the intent of providing an effective road surface area for traffic and limited water-proofing properties. Historically, standard materials have functioned well without the need for modification but as traffic volumes and loads have increased, demand has necessitated a shift to high- performance surfacing materials.