Concrete provides considerable compressive strength and, if reinforced properly, can withstand flexure and shear caused by seismic activity. But, in seismic design, too much of a good thing can in some cases be detrimental. For concrete, a higher strength can contribute to brittleness. Brittle structures often perform less than optimally during seismic events. While there are several approaches to improve the seismic behavior of concrete structures, these solutions often address only one aspect of seismic design. Reducing the total mass of a structure, while still providing the same functionality, can also impact its seismic resilience.
Structural lightweight concrete (SLWC) has a lower density than normal weight concrete (NWC), which can significantly reduce the overall mass of a structure. Structural components can weigh less when SLWC is used because they are carrying less deadload from self-weight and the elements that they are supporting, such as SLWC floor slabs. Second, using SLWC may reduce the size of beams, columns, and decks. Finally, the reductions in size and weight of SLWC elements allow for smaller foundations.
Because lighter structures experience less seismic inertia during an earthquake, engineers and concrete producers can plan for enhanced seismic resilience by using SLWC to reduce the mass of a structure.
What is seismic inertia and what is its relationship to a structure’s mass?
Seismic events shake the ground. In response, a building is subjected to the ground motion at its base. Parts of a structure that rest on these base elements will experience seismic force indirectly via inertia.
The seismic force a structure experiences is a function of the ground acceleration during the seismic event and the overall mass of the structure. Therefore, since the ground motion will remain the same, reducing the mass of a structure will reduce the inertial forces for which the structure must be designed. SLWC concrete can have a density as low as 95 pounds per cubic foot (pcf)—compared to 145 pcf typical for NWC. The reduced weight can be expected to lower inertial effects a structure experiences.
Application of SLWC for bridge designs in seismic regions
Using SLWC can also improve bridge designs in seismic areas. For example, the Benicia-Martinez Bridge, part of a lifeline roadway three miles west of northern California’s Green Valley Fault, was designed to resist high seismic forces. After testing several SLWC mixtures, the engineering team specified a SLWC with a density range of 120-125 pcf and a minimum modulus of elasticity of 3.4 x106 psi at 28 days.
The choice of lightweight, high-performance concrete was key to mitigating the force a seismic event would exert on piles, footings, and piers—thereby improving the bridge’s ability to stay structurally intact during an earthquake. It is also likely that further reductions in mass of the bridge could have been realized if the piers and pier segments had been constructed using SLWC.
SLWC can provide seismic resilience for all types of structures
Reducing the density of concrete in a structure to improve seismic behavior is important for all types of major structures. A few examples include the Salt Lake City Airport expansion and the Allegiant Stadium in Las Vegas, Nev. The reduced mass of these projects reduced the seismic forces that the structures would experience should an earthquake occur. Importantly, the use of expanded shale, clay and slate (ESCS) lightweight aggregates that conform to ASTM C330 or AASHTO M 195 satisfied structural requirements as well as optimized economy in these projects.
Engineers are encouraged to consider the use of SLWC to reduce the seismic demand on structures, and then specify the properties of the concrete required to meet project performance criteria. It is also encouraged to avoid over specifying concrete properties as that may introduce challenges and unnecessarily increase project costs.
Michael Robinson earned his bachelors’ degree in business administration from Eckerd College and is a long-time associate of the concrete and lightweight aggregate industries. He is a past president of the Carolinas Chapter of the American Concrete Institute and remains an active member of the ACI Carolinas and National Capital Chapters. He is a voting member and past Committee Chair of ACI 213 (Lightweight Aggregate and Concrete) and is active on five other Technical Committees, including ACI 301. Robinson is an ACI Fellow and 2013 Cedric Wilson Award recipient. He is also 2017 recipient of the Expanded Clay Shale and Slate Institute’s Thomas A. Holm Award. He represents ESCSI in Masonry Alliance for Codes and Standards and Alliance for Concrete Codes and Standards activities and is an International Code Council member.