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
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.