High-strength concrete has many benefits, but it also can present some challenges to concrete contractors. One aggregate provider shared his insights working with the material during a recent American Concrete Institute educational session.
High-strength concrete is defined as concrete with a specified compressive strength of 55 MPa (8,000 psi) or higher, according to the ACI, but concrete with a specified compressive strength of more than 69 MPa (10,000 psi) is routinely produced. It provides a higher compressive strength, a higher modulus of elasticity, a higher tensile strength, reduced creep and greater durability than normal-strength concrete.
One recent construction project, The Grand LA, utilized high-strength concrete with high-quality aggregates and sheer walls that achieved a concrete strength of 13,000 psi after 120 days, according to Ken Sears, a technical innovation director for Orca, an aggregates company that is part of Polaris Materials Corp.
The complex, part of a revitalization effort in downtown Los Angeles, includes two buildings: a 46-story residential tower and a 29-story hotel. Sears said during the ACI session it used more than 130,000 cubic yards of concrete for the project.
High-strength concrete helps support The Grand LA’s unique concrete sections, which Sears said look like misaligned building blocks stacked atop each other.
The project, a public-private partnership with the Los Angeles Grand Avenue Authority, includes more than 176,000 square feet of retail space anchored by restaurants, shops, a movie theater complex, a hotel, and luxury and affordable-rate residences, according to the Downtown LA website. Supplier Catalina Pacific Concrete, structural engineering firm DCI Engineers and concrete contractor Conco worked on the mixed-use development, which was designed by architects from Gehry Partners.
Opened in 2022, The Grand LA complex features a modulus elasticity of 6,500 ksi and a steel-buckling restraint-bracing frame system tied into concrete columns and beams, Sears said. He said modulus elasticity in concrete is similar to what happens with marshmallows when they bulge out on the sides after being compressed.
“That bulging, or its ability to withstand that bulging, that’s your modulus elasticity,” he said.
Modulus of elasticity is an important mechanical property of concrete, according to technical information compiled by Pennsylvania State University College of Engineering. The higher the value of the modulus, the stiffer the material is. When compared with normal-strength concrete, the elastic modulus for high-performance concrete will be higher, making it a stiffer type of concrete. Stiffness is a desirable property for concrete because the deflection a structure may experience will be decreased.
Sears said modulus of elasticity is “the next hot thing everybody’s asking for,” and can help deflect seismic loads, which is a pressing concern for Southern California and other earthquake-prone areas on or near fault lines.
Find a lab with experience
Sears said experience matters when it comes to testing high-strength concrete.
“Your standard deviation is going to be all over the place,” he said. “So, this takes a heightened sense of knowing what you’re doing.”
Sears said labs testing high-strength concrete, which isn’t synonymous with high-performance concrete, should have a demonstrated track record of accuracy.
“The guy that is out there making cylinders on 3,000-psi concrete is not the guy you want testing 12,000-psi concrete and modulus of elasticity,” he said. “He doesn’t get it and he’s going to test it like it’s 3,000 psi.”
Use high-quality aggregates
Sears said for high-strength concrete to be effective, high-quality aggregates are required to reach high-strength levels because the material has a very low water-to-cement ratio.
But high-quality aggregates are hard to come by in Southern California, where The Grand LA project is located.
“Aggregate quality, density and strength has a tendency to improve as we go north,” he said.
For the Los Angeles area, Sears said Orca utilizes aggregates for high-strength concrete from its Vancouver Island, Canada, quarry.
“The aggregate quarry was formed by the melting of a glacier,” he said. “So, when the glacier melted it basically did all the gradation for us.”
Because of that, Sears said Orca doesn’t have to do much crushing. Instead, it mostly has to size, screen and wash the aggregate.
When selecting aggregates for high-strength concrete, the Portland Cement Association recommends considering the strength and optimum size of the aggregate, the bond between the cement paste and the aggregate, and the aggregate’s surface characteristics. Any of these properties could limit the ultimate strength of high-strength concrete, the association says.
There are other challenges to consider when using high-strength concrete, the Penn State engineers noted. Click here for more information.