Designing with Lightweight Concrete By William H. Wolfe ach year, in the United States alone, designed utilizing SLC with strengths many years of process refinement, Hayde Eseveral hundred steel frame com- above 5,000 psi (34.5 MPa). developed the rotary kiln method for mercial office buildings are constructed expanding clays, shales and slates. The utilizing lightweight concrete and steel Early Beginnings aggregate that was manufactured with composite decking. This type of Lightweight concrete has Hayde’s process had a ceramic matrix® construction represents the ma- been utilized in construc- surrounding air voids contained within jority of mid-rise construction tion projects for over 2,000 the aggregate particle (Figure 1). Early today. During the design years. Some of the ear- development of lightweight concrete was process, many factors such ly structures from the accelerated when it was discovered that as initial costs, life cycle Roman Empire that lightweight concrete could be used as a costs, past product perfor- still survive today replacement for sheet steel, which was in mance, and availability are have elements that great demand during World War I and considered when choosing were constructed with World War II. Over one hundred ships appropriate building mate- lightweight concrete. were constructed from lightweight con- rials. Once all these factors Copyright A great example is the crete during this time period. have been evaluated, Structural Pantheon, which was Today’s Buildings Lightweight Concrete is often Figure 1: Lightweight aggregate completed in 27 BC. the material of choice. particle showing ceramic matrix The roman builders A great deal of the three story and and void structure. utilized natural light- higher commercial structures that domi- What is Structural weight volcanic materials found in the nate the suburban and inner-city Lightweight Concrete? region to modify the density of the con- landscape are constructed with structural The American Concrete Institute (ACI) crete used in the roof of the structure. lightweight concrete slabs on steel deck defines Structural Lightweight Concrete The 142-foot (43.3-meter) diameter (Figure 2). Annually, these suspended (SLC) as concrete with a minimum domed roof has concrete that varied in slabs account for over 200 million compressive strength of 2,500 psi (17 density based on changes in the amount square feet of flooring. One of the major MPa) and an equilibrium density of of lightweight aggregate placed in the advantages of lowering the density of between 70 and 120 pounds per cubic concrete. The structure is in excellent concrete on projects is the reduction foot (1,120-1,920 kg/m3). It consists shape and is still being utilized today. of dead loads. Typical normal weight of entirely lightweight aggregate, or a Lightweight concrete got its start in the concrete weighs approximately 145 combination of lightweight and normal- North America in the early 1900s. Steven pounds per cubic foot as compared to a density aggregates. Each structure is J. Hayde, a brickmaker from Kansas City, typically specified lightweight concrete design specific, with the majoritymagazine of the MO, discovered that bricks bloated after density of 115 pounds per cubic foot. concrete specified for suspended slabs they came up to temperature too quick- This reduction of density allows a con- Shaving anT equilibrium R density between U ly in kilns. TheseC bricks wereT distorted Ucrete weight savingsR of over 20 percent.E 105 and 120 pounds per cubic foot and out of the di- (1,620 – 1,920 kg/m3) and a com- mensional tolerances pressive strength of at least 3,500 psi needed for construc- (20.7 MPa). Other structures, such as tion. From this ini- bridges and precast elements, are often tial observation and LOCKS B UILDING discussion and updates on structural materials B Figure 2: Most of the structural lightweight concrete that is placed Figure 3: Fire endurance of concrete slabs as a function of thickness for in commercial structures is pumped. naturally dried specimens. STRUCTURE magazine 10 April 2008 Lawrence G. Griffis, P.E. NCSEA WEB SEMINAR Walter P Moore & Associates Houston, TX Wednesday April 16, 2008 Larry Griffis is President of the Structures 10:00 am Pacific 12:00 pm Central Division and Senior Principal with Walter 11:00 am Mountain 1:00 pm Eastern P Moore and Associates Inc., a national structural/civil engineering firm headquartered in ® Houston, Texas. Mr. Griffis is responsible for overseeing Designing Buildings for Wind the structural, diagnostic/forensic and parking services for nine offices around the country including Houston, Load By ASCE 7-05 Dallas, Atlanta, Tampa, Orlando, Kansas City, Austin, Las Vegas and Los Angeles. He is a registered professional On the surface, designing buildings for wind load according to engineer in numerous states including Texas, Florida ASCE 7-05 is a complex process. A complete evaluation of the and California. basic wind pressure equation in ASCE 7-05 for flexible buildings Mr. Griffis is very active in professional activities (buildings with the fundamental frequency n1 < 1 hertz) requires and is a frequent speaker at national and regional consideration of up to 48 different parameters. If a building is conferences. He serves as a member of the specifi- rigid (n1 1 hertz) the Gust Effect Factor may be taken equal cation committees for both AISC and ACI. He is a Copyright to 0.85 and the number of parameters to consider reduces by member of the ASCE 7 Committee responsible for the more than half, which is still a significant number. ASCE/SEI 7 load standard and is a member and past In practice, the process can be greatly simplified. This seminar Chairman of the Task Committee on Wind Loads for will explain how to design buildings for wind loads – the easy ASCE 7. He is past President and serves as a member of the board of directors for the Applied Technology way. The seminar will address the following topics: Conference (ATC). He currently is a member and past * Understanding the wind pressure equation Chairman of the ASCE Committee on Steel Buildings * Wind design flow charts and is a member of the Code of Standard Practice * The role of building frequency and damping Committee for AISC. * Simplifying the Gust Effect Factor Seminar Cost: $250 per connection * Parameters that most influence wind pressure * Dealing with non-typical building shapes Information: 312-649-4600 x202 * Direct determination of design wind pressures from tables Registration: www.ncsea.com * Controlling building torsion from wind and seismic loading ADVERTISEMENT – For Advertiser Information, visit www.STRUCTUREmag.org Additionally, thinner slab sections are allowed Fire Resistance ASTM E 119 tests have shown that slabs for lightweight concrete to achieve the same constructed with lightweight concrete have magazinedemonstrated greater fire endurance periods fire ratings of similar normal weight decks. A The fire resistivity of a structure is directly reduction in slab thickness from 42 inches related to the properties of the building ma- than slabs constructed with normal weight S T R U C T concreteU of an equivalentR thickness (FigureE 3). for normal weight concrete to a slab thickness terials that make up that structure. These of 33 inches for lightweight concrete, materials must be able to withstand high when combined with the concrete density temperatures and still maintain their structural Proportioning and Pumping savings, will reduce the total slab weight by stability. Lightweight concrete is more fire Like normal weight concrete, lightweight 38 percent. The load reduction often reduces resistant than normal weight concrete due to concrete should be designed in accordance the size of columns, beams, and girders, as its lower thermal conductivity and its lower with ACI 318. The proportioning of light- well as foundations and reinforcing steel. coefficient of thermal expansion. The expand- weight concrete mixtures can be done by An example of weight savings for a 2 hour ed aggregates used in lightweight concrete have volumetric or specific gravity methods. The fire rated deck is shown in Table 1. Seismic already been heated to temperatures in excess ready-mix supplier and the lightweight aggre- performance is also improved because the of 2000°F (1093°C). These characteristics gate manufacturer should be consulted when lateral and horizontal forces acting on a allow lightweight concrete to have thinner the concrete mixture is designed. The design structure during an earthquake are directly slab sections than comparable normal weight should have a cementitious content sufficient proportional to the inertia or the mass of a slabs that have identical fire ratings. Building to accommodate a 4- to 6-inch slump at the structure. The reduced slab weight also allows materials are tested for fire resistivity in point of placement, typically a minimum of for greater spans, thereby improving the accordance with ASTM E 119, Fire Tests of 611 pounds per cubic year (363 kg/m3). overall structural efficiency of the building. Building Construction and Materials. These Water reducing admixtures that aid pumping Table 1: Deck Comparison - UL Design No. D916 - 2 hour fire rating in 2 inch deck. Fire Pan Slab Total Slab Concrete Theoretical Concrete Volume Concrete Weight Concrete Type Rating Depth Depth Depth Density Slab Weight Reduction (hours) (inches) (inches) (inches) (lb/cu ft) (cu ft/sq ft) (cu yd/100 sq ft) (lb/sq ft) (%) Normal Weight 2 2 4.5 6.5 150 +/- 3 0.461 1.71 69.15 - Lightweight 2 2 3.25 5.25 110 +/- 3 0.357 1.32 39.27 43% STRUCTURE magazine 11 April 2008 line, and a rubber hose to allow movement at tolerances for this floor are F 25/FL20. On the end of the line. Use the largest line available some occasions, flatness/levelness tolerances with a minimum diameter of 5 inches (127 are higher to meet specific design requirements.