Concrete Additives used by DOT for highway construction

Reinforcing Concrete for DOT Construction Projects

Infrastructure projects, such as roads, bridges, highways, and water supply and resource management systems, have precise and exacting technical specifications. For example, roads must be appropriately abrasion, weather, and impact resistant, durable, and have sufficient compressive and flexural strength. However, even a low-trafficked road typically requires greater strength and durability than off-the-shelf Portland cement concrete provides. Department of Transportation (DOT) construction engineers and contractors consider using specific concrete admixtures during the mixing process to ensure the finished concrete can withstand the test of time.

Common Department of Transportation Concrete Applications

DOTs across the country rely on concrete for highways, roads, bridges, and bridge decking, among other applications. The concrete they use must have sufficient compressive and flexural strength to handle the continuous weight of vehicles and equipment and withstand varying weather conditions. Often, cement manufacturers tweak the composition of cement during the manufacturing process to increase or decrease its natural properties as desired. They also may include certain mineral additives, otherwise known as supplementary cementitious material or Pozzolana. Pozzolana may be natural, such as clay or shale, or artificial, such as silica fume, rice husk ash, or blast furnace slag. Additional cement additives can include grinding aids, activators, and air stabilizers. Construction managers and contractors may also add concrete admixtures during the concrete mixing process. These admixtures can help enhance concrete’s strength and durability, making them essential for infrastructure projects.

Strength-Enhancing Concrete Admixtures

Water reducers and accelerators are two of the most common chemical admixtures to increase concrete strength. As the name suggests, water reducers decrease the water-to-cement ratio anywhere between 10 and 40 percent. Water reducers on the higher end of that range are known as superplasticizers and are utilized to develop very strong and durable concrete that can withstand heavy loads. Water reducers and superplasticizers allow for the use of less water in the mix to maintain workability, while gaining the high strength that a lower water-to-cement ratio affords. Accelerators decrease the setting time and help concrete develop early strength. However, job-site conditions, such as high temperatures, may necessitate the conservative use of accelerators as rapid setting may reduce workability. Adding fiber to concrete may also enhance its strength and durability. Concrete is often reinforced with steel for DOT projects, making corrosion-inhibiting admixtures necessary.

Durability-Enhancing Concrete Admixtures

In addition to water reducers and accelerators, construction managers and contractors should consider the use of air entrainers to enhance concrete’s durability. These chemical admixtures make concrete more resistant to the freeze and thaw cycles that come with the seasons. Air entrainers generate a matrix of microscopic air bubbles in the concrete to relieve pressure created when water in the concrete matrix freezes and expands. Pozzolanic admixtures are essential for developing dense concrete with significantly reduced permeability. Using natural or artificial Pozzolanic materials can help when fabricating water-retaining infrastructure projects. Additionally, if the concrete is underwater, anti-washout admixtures from natural or synthetic rubbers can increase its cohesiveness and resistance to water pressure.

Using selected durability-enhancing admixtures can mitigate the risk of shrinkage, cracking, scaling, and delaminating. Corrosion and rust inhibitors can reduce the risk of spalling, particularly in marine environments or where heavy use of de-icing salts occurs during the winter. Using them to develop dense concrete can help ensure that the final construction project is appropriately weather- and abrasion-resistant as well.

Job-Site Considerations

Admixtures must also be selected depending on job-site conditions. For example, construction managers and contractors should consider accelerators for faster setting times if the project is proceeding during cold weather. Conversely, if the job site is far from the concrete plant, set retarders may be needed. These admixtures delay the setting time of concrete, allowing it to remain workable for longer periods during transportation from the plant to the job. Construction managers and contractors should also think through the job site terrain, other seasonal effects, and the weather on any given day, among other considerations, when determining the appropriate admixtures.