Less is More: Structural Lightweight Concrete
Luke M. Snell, P.E.
When I was a young engineer, one of my first jobs was to provide quality control for a lightweight concrete project. Knowing very little about this type of material, I was on a steep learning curve. What is lightweight concrete, how does it differ from regular concrete, and what is needed to produce a quality lightweight concrete?
What is structural lightweight concrete?
There are many definitions, including the American Concrete Institute’s that defines structural lightweight concrete as having an air-dried density of not more than 115 pcf (pounds per cubic foot) and having a 28-day strength of at least 2,500 psi. Some specifications use the definition that the unit weight as measured in the field must be below a specified density (typically a value between 90-120 pcf). All concrete must exceed 2,500 psi to be considered for structural use. We needed to achieve a field unit weight below 120 pcf and a strength of 4,000 psi on my project.
Why use structural lightweight concrete?
Standard concrete weighs between 140-150 pcf. The use of lightweight concrete reduces the weight by 20-40 percent. A building must be designed to hold its own weight (dead load) plus the expected loads (live loads). Since the engineers have less of a total load to design for (the dead load from the concrete will be much less), they reduce the foundations' size or use a less expensive foundation system, smaller columns, and less reinforcement.
What are lightweight aggregates?
Most lightweight aggregates are clay, slate, or shale that have been expanded by heating in a rotary kiln. During this process, the material's internal gasses are released, creating voids or air pockets in the aggregates. This process is similar to how we make popcorn, where water becomes steam and causes the corn kernel to expand rapidly. Most lightweight aggregates will have about 10-30 percent voids, and often the aggregates would float in water. These voids will make the aggregates weaker; thus, a more complex mixture design is needed to achieve the required strength.
How is lightweight structural concrete batched?
There are two critical differences in batching lightweight concrete. The first is the aggregates must be saturated before or during batching. If the voids are not filled with water, the slump will decrease in the mixing and while pumping the concrete. This situation can cause placement problems and pump line blockages. The batch plant used rotating lawn sprinklers in several locations on my project's aggregate piles. This hydration was done continuously for at least five days before the aggregates were used.
The second difference is the amount of lightweight aggregates in the mixture has to be determined by volume. Since the batch plant weighs all of the mix materials, the aggregates' unit weight must be decided regularly. This calculation will allow them to adjust the mixture so that a constant volume of lightweight aggregates is added to each batch.
On my project, the batch plant had great difficulty getting consistent concrete because of their inexperience in working with lightweight concrete. I had to put a technician in the batch plant to run the aggregates' unit weight to make the appropriate adjustments to the mixture.
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This article originally appeared in the bimonthly Arizona Contractor & Community magazine, Jul/Aug 2021 issue, Vol. 10, No. 4.