BY DARON BROWN
Precast concrete can contribute to more efficient and cost-effective construction. Because precast structures are poured and cured in a plant before being shipped to the jobsite, they can reduce the number of subcontractors working alongside each other as well as the amount of waste generated from having to place and cure concrete on site—both of which can streamline the process. While precast concrete has several benefits that help construction timelines proceed without delay, it can also contribute to better concrete in general. Further, when producers use structural lightweight concrete (SLC), they can maximize the value precast structures deliver to a project.
Uniformity and exactitude: The benefits of precast concrete structures. Often created in regulated factory conditions, precast structures are not beholden to sudden changes in weather that may slow the curing process or cause defects. This can lead to concrete that is more uniform and well-cured. It can also contribute to quicker construction since concrete contractors will not need to wait for the right conditions or cause delays if struck with inclement weather.
Additionally, due to the level of exactitude and standardization capable through precast concrete production, repair and replacement of concrete structures can be more cost-efficient. This helps reduce the total cost of investment for project owners. However, utilizing precast concrete is not without its challenges.
By truck and crane: The challenges of using precast concrete. The ability to transport precast concrete from the producer to the jobsite and then place oversized components are often the main challenges facing the precast industry. Precast structures can be made in sizes that outpace the load capacity of shipping trucks. Larger concrete components may also require heavier duty cranes, which may not be available for a project or unable to be used due to site conditions.
While these realities do not pose many problems for smaller projects, they can impact the efficiency and cost-effectiveness of projects that require larger concrete structures, such as bridge girders and tilt-up lightweight concrete wall panels. For these projects, lightweight concrete can simplify project logistics by allowing larger precast structures to be shipped and placed more easily. This can help solve the on-site challenges of using precast concrete.
Solving challenges with precast structural lightweight concrete. When produced with expanded shale, clay or slate (ESCS) lightweight aggregates, SLC is typically 25-35 percent lighter than traditional concrete. Despite its lower weight, the material retains comparable strengths to normal weight concretes as well as a higher resistance to microcracking and chloride penetration. When used in precast concrete, SLC’s main benefit is its ability to reduce weight to improve the transportability and handling of structures or elements.
For example, the Interstate 5 southbound lanes of the Puyallup River Bridge near Tacoma, Wash., used precast lightweight concrete to increase the length of the bridge girders to an unheard of 223 feet and 4.5 inches, which bests conventional girder lengths by more than 70 feet. The record-breaking size allowed the construction crews to work around tight schedules caused by a railway directly below a portion of the bridge, which would require crews to pause whenever a freight train passed underneath. Streamlining this aspect of construction helped ensure the project was completed with as few labor hours and profits lost to these interruptions as possible.
Likewise, tilt-up wall panels made with SLC supported the strength limitations of available cranes for a four-story office building in Anne Arundel County, Md. The 32 tilt-up lightweight concrete panels were 30 feet wide by 64 feet tall and, on average, 15 inches thick. The reduction in weight allowed the panels to be tilted by smaller cranes, easing construction logistics without compromising the performance capabilities of the walls.
Working as a team when using SLC. In addition to the general benefits of precast concrete, structures made with SLC can reduce logistic challenges and equipment costs to support more efficient construction. Creating SLC mix designs with ESCS poses some initial challenges as prepping the aggregates and batching the mix components can require additional steps. However, precast producers can contact lightweight aggregate suppliers to ensure these processes are easier to adapt to. When they do, they can help meet project owner expectations and simplify the job of a general contractor, both of which can mean more business in the long-term.
Daron Brown is regional sales manager for Arcosa Lightweight. He has over 20 years of experience working with concrete products and has spent the last 14 years working in the lightweight aggregate industry. He chairs the Expanded Shale, Clay and Slate Institute Concrete Pavement Committee and is a Structural Committee member.