This is the fourth in the Engineer’s Corner series of articles in ESCSI Lightweight Design eNews that address issues related to concrete made using
expanded shale, clay, or slate (ESCS) lightweight aggregate that has been manufactured using the rotary kiln process at temperatures around 2000 deg. F. When the particles reach these high temperatures, the material softens and bubbles are formed within the aggregate particles as gases are released from within. These bubbles become permanent pores in the aggregate when it cools and hardens. The pores in the vitrified ceramic aggregate particles give lightweight aggregate its reduced density and other unique properties.
The first three articles in the Engineer’s Corner series addressed common myths or misconceptions about lightweight aggregate or lightweight concrete. Future articles will continue this theme by addressing more misconceptions about lightweight aggregate and lightweight concrete. However, this article introduces a second direction for the series by presenting a topic for which structural lightweight concrete provides significant design advantages. Specifically, this article discusses design advantages offered by the common and efficient use of structural lightweight concrete on composite metal decks for floors in buildings.
The use of lightweight concrete on composite metal decks for floors and roof slabs in buildings is one of the most common uses of lightweight concrete. There are many benefits that can be obtained from the use of lightweight concrete for this system. The major advantages include:
• Longer spans
• Thinner concrete slabs for same fire rating
• Possible avoidance of fireproofing on underside of metal deck slabs
• Lighter loads for beams, columns, and foundations
• Lower mass for seismic design
• Reduced shrinkage cracking
Lightweight concrete slabs make sense simply because the unit weight of the concrete is reduced from about 145 pcf for normal weight concrete to about 110 pcf for the lightweight concrete (using equilibrium density with drying with time), which is a reduction of 24%. For buildings where an Underwriters Laboratory Inc. (UL) fire rated floor system must be used, an additional benefit for lightweight concrete is that a thinner lightweight concrete slab provides the same fire rating as a normal weight concrete slab. For example, if a 2 hour fire rating is required, 4.5 in. of concrete is needed to provide this rating (Underwriters Laboratories Design No. D916¹), so for a 2-in.-deep fluted metal deck, the total slab depth would be 6.5 in. for normal weight concrete. When using lightweight concrete, 3.25 in. of concrete is required for the 2 hour fire rating, for a total slab depth of 5.25 in. which is a reduction of 19%. When the effect of the reduced density of the concrete is combined with the reduced slab thickness, the benefits are compounded to reduce the total weight per square foot of floor slab from 68.1 psf to 40.6 psf for a typical metal deck², which is a 40% reduction in the weight of the floor slab system per unit area.
The 40% reduction in dead load from the floor slab reduces the loads for which beams, columns, and foundations must be designed. This results in significant savings in the design of a building, especially where seismic design must be considered. For example, consider a 55 story building constructed using a steel frame with concrete floor slabs on composite metal deck, with an average floor slab area of 390 ft². The building design criteria requires that the floor system provide a 2 hour fire rating; therefore, concrete deck slab thicknesses of 6.5 in. and 5.25 in. are required for normal weight concrete (NWC) and lightweight concrete (LWC) floor slabs, respectively, based on UL requirements for the D916 floor assembly¹. With normal weight concrete floors, the total concrete volume for the floor slabs in the building would be 33,000 CY; however, when lightweight concrete floors are used, a concrete volume of only about 27,900 CY would be required because the lightweight concrete floors would be thinner. The total weight of floor slab concrete for the 55 story building would then be 64,600 tons for normal weight concrete (145 pcf) and 41,400 tons for lightweight concrete (110 pcf), a reduction of over 23,000 tons or about 36%. Assuming costs of $100/CY and $115/CY for the normal weight and lightweight concrete, respectively, the normal weight concrete for the floor slabs would cost about $3,300,000 and the lightweight concrete would cost just over $3,200,000. Therefore, the use of lightweight concrete for the floors on metal deck provided savings in several areas: a reduction of about 5,100 CY in the concrete volume (and the associated trucks to deliver it); a reduction of over 23,100 tons for structure and foundation design loads; and a reduction of nearly $100,000 in material cost for the floor slab concrete (even though the cost of the lightweight concrete per CY was taken as 15% greater than for normal weight concrete). The quantities mentioned in this comparison are summarized in the table below. Additional savings would be expected in the structure and foundation because of the reduced dead load. This example is roughly based on the Bank of America Building in Atlanta that was completed in 1992 using lightweight concrete for the floor slabs³. The building was 55 stories tall and required approximately the volumes of concrete assumed.
Other notable examples of tall buildings have been constructed across the U.S. using lightweight concrete floors on composite metal deck. Examples from project reports on the ESCSI website³, www.escsi.org, and member company websites include: the Wilshire Grand Tower4 in Los Angeles, the tallest building on the West Coast, which has 73 stories and is 1100 ft tall, completed in 2017; the Salesforce Tower in San Francisco, the second tallest building on the West Coast, which is 61 stories and 1070 ft tall, completed in 2018; the Comcast Center in Philadelphia, which is 58 stories and 974 ft tall, completed in 2008; and the 150 North Riverside Building in Chicago, which is 54 stories and 742 ft tall, completed in 2017.
Use of this technology is not only limited to very tall buildings. It has also been widely used for mid-rise buildings and other types of structures in many parts of the country. Information about several such buildings, located in cities like Salt Lake City, Denver, Omaha, Houston, and New Orleans, are highlighted on the ESCSI and member company websites.
Lightweight concrete for these tall buildings has been delivered by pumping. Some engineers and contractors may be hesitant to use lightweight concrete in tall buildings because they think that it cannot be successfully pumped. However, the successful use of lightweight concrete for the buildings cited above, and many others not listed, clearly demonstrate that this is not true. I plan to address this topic in a future Engineer’s Corner article. The topic is also addressed in an ESCSI publication5.
Additional benefits can also be realized from the use of a thinner lightweight concrete deck: less concrete is used on the project, and that means that a smaller volume and therefore fewer trucks are required to deliver it, saving fuel and reducing traffic. In some locations, where concrete delivery routes may be load restricted, such as downtown New York City, concrete mixer trucks delivering lightweight concrete can be fully loaded, while trucks carrying normal weight concrete cannot deliver a full load. Also, with thinner floors, the floor height is reduced by 1.25 in. While this small reduction in floor height does not sound like it would be significant, a 12 story project in Denver saved $55,000 in the cost of the curtain wall alone, with additional savings realized in other parts of the building6. A study has also been prepared for ESCSI evaluating the embodied energy for a steel-framed building with lightweight concrete floor slabs7.
In recent years, some designers, owners, and contractors have moved away from using lightweight concrete on metal decks for buildings. This is based on the mistaken notion that lightweight concrete decks do not dry as quickly as normal weight concrete decks, which may delay installation of flooring and occupancy or may present problems with flooring performance after a building is in service. This notion has been dispelled based on test results8 and has been discussed in the second Engineer’s Corner article which appeared in the Summer 2020 issue of ESCSI Lightweight Design eNews9.
1. Underwriters Laboratories, Inc. (2009) Fire Resistance Directory, Volume 1. Underwriters Laboratories, Inc. Northbrook, IL, pp. 219-222.
2. ASC Steel Deck. 2018. Floor Deck Catalog. Section 3.3 2WH-36 Composite Deck. https://ascsd.com/wp-content/uploads/DL021_FloorDeck-Catalog.pdf. Accessed September 22, 2020.
3. ESCSI. n.d. Structural LWC/Featured Projects. https://www.escsi.org/structural-lightweight-concrete/featured-projects/. Accessed September 22, 2020.
4. Arcosa Lightweight. n.d. “HydroLite Lightweight Aggregate Plays Key Role in Record-Breaking Structure.” https://www.arcosalightweight.com/case-studies/structural-lightweight-concrete/wilshire-grand. Accessed September 22, 2020.
5. ESCSI. 2020. Go with the Flow – Pumping Structural Lightweight Concrete. Publication 4770.0. https://www.escsi.org/wp-content/uploads/2020/04/ESCSI_SLWC-Brochure-4770.0.pdf. Accessed September 22, 2020.
6. Arcosa Lightweight. n.d. “Lightweight Concrete Provides Economical Solution for 12-Story Civic Center Addition.” https://arcosalightweight.com/case-studies/structural-lightweight-concrete/denver-complex. Accessed September 22, 2020.
7. Walter P. Moore and Associates. 2012. Embodied Energy Study – Lightweight Concrete in Steel Framed Buildings. ESCSI. https://www.escsi.org/wp-content/uploads/2017/10/Embodied-Energy-Study-Report-FINAL-Letter-1.pdf. Accessed November 2, 2021.
8. Wolfe, W. H. and Ries, J. P. (2017) “Concrete on Metal Deck,” STRUCTURE, 24(9), 9-11.
9. Castrodale, R. W. “Engineer’s Corner: Myths and Misconceptions #2.” ESCSI Lightweight Design eNews. Summer 2020. https://www.escsi.org/e-newsletter/engineers-corner-myths-and-misconceptions-2/.