Shotcrete Additives used by Department of Transportation DOT

Strengthening Shotcrete for DOT Highway Construction

Departments of Transportation (DOTs) have increasingly used shotcrete in recent years since it’s usually less expensive to install. Shotcrete, composed of concrete fired through a hose at high velocity vertically or overhead, is ideal for complex shapes or forms. However, ensuring that shotcrete bonds rapidly and appropriately with the intended substrate often requires incorporating suitable cement additives into the cement manufacturing process and adding the right shotcrete admixtures during the concrete mixing process.

DOTs typically favor Portland cement, although an increasing number now use Portland-limestone cement (PLC). Cement manufacturing involves the combination of common materials, such as chalk, limestone, or shells, along with silica sand, iron ore, clay, shale, slate, and blast furnace slag, heated to high temperatures. During this process, supplementary cementitious materials (SCMs), such as slag cement, fly ash, or Pozzolan, and various chemical additives are often added, which can help enhance the core properties of the final product. After heating, the resulting rocky material, known as clinker, is ground into a fine powder. The resulting powder is ready to be transported to a concrete plant and mixed with water and aggregates to form shotcrete.

Common Shotcrete Applications

Given its affordability and versatility, shotcrete is frequently used by DOTs to enhance the structural support of bridges and roads, as well as to install sound walls, tunnel liners, and various roadside features. It’s also used effectively on projects involving water or soil retention. However, depending on the nature of the project, it’s essential to add the right shotcrete admixtures during the mixing process to ensure the shotcrete meets the performance requirements of the project at hand.

Common Admixtures for Strength

Multiple chemical admixtures may be added immediately before or during the mixing process, which include set retarding admixtures that delay how quickly the cement sets and increase its plasticity; air entrainment admixtures which protect the concrete from water freeze/thaw damage; water-reducing admixtures that enhance workability and strength by reducing the mixture’s water requirements; accelerators that can speed up the rate of concrete setting and early strength gain; superplasticizers which reduce the need for water by up to 30 percent and improve concrete strength; and corrosion-inhibitors that increase the durability of reinforced concrete by increasing the passivation of embedded steel.

SCMs include fly ash which can reduce concrete segregation, bleeding, and alkali-silica reaction while enhancing strength, workability, durability, and sulfate resistance. Ground granulated blast furnace slag (GGBS) is an SCM that is derived from extracting iron from iron ore. GGBS can increase strength and durability while reducing the impact of corrosion and chemical reaction from concrete placed in marine settings. Silica fume enhances water demand and is usually used with a superplasticizer. However, this SCM improves compressive strength and abrasion resistance. Using rice husk ash can also increase shotcrete strength and resistance to sulfate and chloride reactions. Finally, metakaolin (a mineral admixture made from adding regular clay to kaolin clay) can enhance shotcrete strength, durability, and resistance to chemical reactions, while reducing permeability and bleeding.

Fiber shotcrete admixtures can be categorized as macrofibers or microfibers. Microfibers are used to reduce the risk of shrinkage and cracking, as well as enhance durability, abrasion and impact resistance, and toughness. Macrofibers are typically used to reinforce welded wires and offer enhanced flexural strength and durability.

Admixtures for Durability

Some shotcrete admixtures are better suited for enhancing the base properties of shotcrete than others. For example, permeability-reducing admixtures (PRAs), such as crystalline waterproofers and hydrophobic pore blockers, reduce water egress while enhancing durability. Set retarding admixtures are excellent for crack reduction. To handle freeze/thaw cycles, air entrainment admixtures are frequently used. Accelerators and corrosion inhibitors can mitigate the risk of spalling, delaminating, and scaling.

Admixtures for Workability

For challenging structures and shapes, workability is paramount. Shotcrete admixtures that improve compressive strength, such as silica fumes, coupled with accelerators that help retain strength early, can optimize shotcrete pumping. Set retarding admixtures can help control hydration, allowing workers time to carve or finish the shotcrete as necessary.

Admixtures for Job Site Requirements

DOTs may also need to consider different shotcrete admixtures that will help the shotcrete meet specific project or job site requirements. In some cases, shotcrete may need to be delivered over a long distance, in which case, superplasticizers and retarders may be useful. In other cases, shotcrete must be applied in cold weather conditions. In these cases, the shotcrete must be prevented from freezing before reaching full strength. Accelerators, superplasticizers, and air-entraining agents are useful in this regard.

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