The use of recycled materials in highway construction not only benefits the environment, but also results in durable, cost-effective roadways.
Consider this: the United States discards approximately 11 million tons of asphalt shingles each year. By using recycled asphalt shingles in highway construction, the country both prevents those materials from piling up in a landfill and decreases the cost of hot-mix materials. Blast furnace slag—a byproduct of iron produced in a blast furnace—provides another example of a material ripe for reuse. Rather than waste the silicates and aluminosilicates found in the slag, agencies and contractors can utilize the slag as an aggregate, a sub-base material, or an embankment material. Similarly, crumb rubber derived from scrap tires is being used to build retaining walls, as an additive for Hot-Mix Asphalt, and as a sealant.
The process of recycling either converts material into a reusable product or returns material to its original state. Because recycling provides ample material for highway construction, states do not need to tap into new resources. Materials such as blast furnace slag, carpet fiber, coal fly ash, glass, mill tailings, crumb rubber, recycled concrete, recycled asphalt, plastic, and shingles work as aggregates in the base and surface of a highway, and also serve as fill material, shoulder material, retaining walls, and pothole filler.
Pavement construction involves the use of either Hot-Mix Asphalt (HMA) or Portland Cement Concrete (PCC). The HMA process relies on aggregates, binders, and additives to produce materials that can withstand the stresses produced by daily high-speed traffic. PCC pavement types utilize aggregates, cement replacement/mineral additives, and reinforcing steel. Fly ash, reclaimed asphalt pavement, recycled concrete material, and boiler slag also work as subgrade, unpaved shoulders, or fill material. Other materials such as recycled crumb rubber, glass beads, and boiler slag can be employed as part of reflective crack control systems and as crack sealants.
Aggregates, or materials formed from a compacted mass of fragments, make up nearly 95 percent of the products used in HMA. While highway construction processes could use natural aggregates such as limestone or dolomite, those materials cannot be replicated and often exist far away from construction sites. Using recycled products enables construction companies to stretch nonrenewable resources and control transportation mixture costs. Additionally, waste products such as air-cooled blast furnace slag, reclaimed asphalt pavement, reclaimed asphalt shingles, recycled concrete material, steel slag, and boiler slag work as aggregates and provide better friction results than limestone or dolomite.
The hot-mix asphalt process often encounters problems with moisture susceptibility and mixture stability. Additives consisting of recycled materials, such as crumb rubber and reclaimed asphalt shingles, not only solve these issues, but also allow quicker compacting of the asphalt and increase its workable consistency.
Liquid asphalt binders account for the most expensive part of the HMA process; but using recycled materials, like reclaimed asphalt pavement and reclaimed asphalt shingles, reduces their cost. However, because the recycled materials produce a binder with a greater hardness than typical asphalt binders, construction projects must limit the amount of recycled materials used as binder.
Aggregates make up nearly 80 percent of the materials used in Portland Cement Concrete. In contrast to the aggregate used for Hot-Mix Asphalt, the aggregate for PCC must be higher quality to offset the potential for structural problems within the concrete mix. Aggregates that provide the best quality for PCC include granulated blast furnace slag and recycled concrete material.
PCC Cement Replacement/Mineral Additive
In the past, Portland cement, water, and other additives served as the best binding material for Portland Cement Concrete construction. Due to concerns about introducing possible environmental hazards, contractors have transitioned to the use of fly ash, ground blast furnace slag, and microsilica.
PCC Reinforcing Steel
Highway construction based on PCC requires the installation of reinforcing steel in the form of dowel bars, rebar, and welded wire. Dowel bars transfer the load from one pavement slab to another at uniformly spaced joints. Rebar both reinforces the concrete to prevent cracking and forms Continuously Reinforced Concrete Pavement, while welded wire reinforcement strengthens short joints. These reinforcement materials are composed of recycled steel from automobiles, appliances, and various structures.