Frequently Asked Questions (FAQ) About Flat Plate Voided Concrete Slab Systems
The CobiaxUSA voided slab system utilizes hollow, plastic voids made of recycled highdensity, polyethylene (HDPE) that are spaced regularly within the overall thickness of the concrete slab (See Figure 1). The latest innovation is the CLS System (See Figure 2). A voided slab with CobiaxUSA structural formers can be designed and planned by any structural engineer in compliance with ACI 318-19.
Introduction Flat plate voided concrete slab systems, which have been used for many years in Europe and other parts of the world, have become increasingly popular in the U.S. because of the many inherent benefits. Such benefits include reduced weight, which results in smaller seismic forces and larger allowable superimposed loads for given span lengths; economical longer spans without beams; reduced floor-to-floor heights; accelerated construction schedules; and inherent fire resistance that meets the fire-rating requirements in the International Building Code (IBC).
Figure 1 ― CobiaxUSA Slim Line Void formers
Figure 2 – CobiaxUSA CLS Void Formers
The buildings designed with voided slabs have lower floor-to-floor heights, resulting in reduction of building volume, HVAC requirements, façade area, and other vertical elements such as stairs, partitions, piping, and elevator runs. In addition, the thermal mass of concrete buildings helps the cooling and heating cycles as concrete stores and releases the energy to balance the temperature. The energy use of buildings once in service should be considered in the life cycle analysis.
CobiaxUSA routinely receives inquiries concerning various aspects on the design and construction of flat plate voided concrete slab systems. This Technical Note addresses a collection of typical questions, which came from design professionals, including structural engineers, architects, field inspectors, code enforcement officials, and contractors.
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Analysis Methods
with the same loading in mind. The resulting deflections will be less if the same slab thickness is used due to the reduced selfweight.
How are flat plate voided slab systems analyzed?
From laymen’s point of view, how can one tell whether a floor is of Cobiax floor design?
Just like other two-way slab systems, flat plate voided slab systems are permitted to be analyzed and designed by any procedure satisfying equilibrium and geometric compatibility in accordance with ACI 318
Once the concrete has been cast, there is no visible difference between that of a Cobiax floor slab and a conventional floor slab.
How do the void formers impact the stiffness of the overall system?
Will the Cobiax void former weaken over time? No. Once the concrete has achieved its strength, the void former does not serve any function; hence it will not result in the weakening of the floor capacity.
A stiffness correction factor, which can be obtained from CobiaxUSA literature for the various void formers, must also be applied when using software programs, mainly for calculation of deflections. Depending on the software, either the modulus of elasticity of the concrete 𝐸𝐸𝑐𝑐 is multiplied by this factor and input accordingly or the global stiffness factor of the slab is multiplied by this factor. Uncracked flat plate voided slab systems typically have a slightly less (about 87 – 93%) flexural stiffness than solid slabs with the same thickness.
Fire Resistance Regarding fire rating, have there been any issues or concerns getting permits/approvals? As a follow-up, have voided slabs reached 1hour or 2-hour ratings? Voided slabs are explicitly covered in IBC section 722.2.2.1.4 with a fire rating of up to two-hours. This provision is based on a review of the tests sponsored by CRSI and conducted at NGC Testing in Buffalo NY
Are there vibration criteria available for flat plate voided systems? Please visit www.CRSI.org for technical resources regarding vibrations of voided concrete floor systems.
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Will the floor capacity / loading capacity be different when comparing with floor of conventional design? There is no difference for the floor loading capacity between the Cobiax Voided slab and a conventional RC slab, if they are designed
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In the cast-in-place system, can the concrete be placed in one lift?
Sound Control Is there information available for the sound transmission class and impact insulation class for voided systems?
The entire depth of the slab can be placed in one lift in a cast-in-place system. However, the void system must be anchored directly to the formwork to prevent buoyancy forces from lifting them upward due to the fluid concrete. The more common approach is to use two lifts as described above in the construction sequence.
Studies have shown that it is justified to assume the same acoustic insulation properties for a Cobiax slab as that for a solid slab having the same weight.
What kind of concrete should be used for casting?
Construction What is the typical site installation of a castin-place voided concrete slab? 1. 2.
3. 4.
5.
Just like other typical two-way slab systems, normal-weight concrete with a specified compressive strength that proves most efficient for flat plate voided concrete slab systems with conventional reinforcement.
Formwork is erected and bottom reinforcing bars are placed. Void systems are placed and tied perpendicular to the top layer of bottom reinforcing bars. Top reinforcing bars are placed directly on and tied to the void system. First layer of concrete is cast, which covers the bottom reinforcing bars and the lower portion of the cage modules. This layer of concrete essentially anchors the cage modules and prevents possible upward movement due to buoyancy when the second layer of concrete is placed. Second layer of concrete is cast to the required overall thickness of the slab system.
Should self-consolidating concrete be used in order to get concrete under the voids? A properly vibrated normal-weight concrete mix is typically adequate and is generally the most cost effective If self-consolidating concrete is required for other reasons then the void former system must be anchored to the formwork.
Is there a cold joint concern when using the cast-in-place system where two concrete placements is used? CONTINUES ON PAGE 4
In addition to around the columns, void formers are typically omitted around the perimeter of the slab for a width not less than 1 ft. Reinforcement details similar to those used in other two-way slab systems can be provided along the edges.
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When casting concrete for the cast-in-place systems, a horizontal cold joint is created whenever the bottom layer of concrete dries and hardens before the top layer is cast.
Larger post-installed anchors can be used provided the void formers are located by any rational method, including visual observation of the soffit, placement/shop drawings, nondestructive testing, or any combination thereof. Holes drilled in the slab for temporary use during the construction phase (i.e. dowels fixing braces) are to be sealed after use to avoid the unintentional filling of the void former with water which might freeze and cause damage.
Are there any special considerations for embedded items? Small items, such as electrical juncture boxes and pipes less than 3 in. in diameter, can be installed within the depth of a flat plate voided slab with essentially minor or no interference with the void formers. Larger items, like MEP conduits, usually require modification of the regular pattern of the void formers; some of the void formers are either omitted or shifted to accommodate larger embedded items. The structural engineer and architect of record should be notified whenever changes are proposed to the layout of the void formers.
Does the installation of Cobiax void formers require specialized trades?
No. Typically for the first slab installation, Cobiax will assign one of our technical advisors to be on site to guide the contractors in the installation. The installation process is simple and straightforward.
Are there any special considerations for coring through voided slabs?
Availability What is the availability of the voids in different parts of the country?
Holes can easily be diamond core drilled through the cast-in-place as long as it meets ACI 318 criteria. As with all cores the prevention of creating a space or draft for fire to spread is accomplished by appropriately sealing the space between the cored opening and the pipe/chase installed within.
CobiaxUSA has supplied voided slab products throughout the US.
What is the cost of a voided slab system? It is recommended to contact Cobiax USA to obtain the latest information on cost. As with all structural systems; the costs vary between regions, project size, length of spans, etc.
Can post-installed anchors be used with voided slabs? Typically, 2-1/2” post installed anchors will not hit the void. Post installed anchor manufacturers provide anchors that can be installed with void former systems.
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References
ASTM E492 – 16, West Conshohocken,
ACI (American Concrete Institute). 1995. “Control of Deflections in Concrete Structures.” ACI 435R-95, Farmington Hills, Michigan.
CRSI (Concrete Reinforcing Steel Institute). 2014a. Design Guide for Voided Concrete Slabs, Schaumburg, Illinois.
ACI (American Concrete Institute). 2019. “Building Code Requirements for Structural Concrete and Commentary.” ACI 318-14, Farmington Hills, Michigan.
CRSI (Concrete Reinforcing Steel Institute). 2014b. Design Guide for Vibrations of Reinforced Concrete Floor Systems, Schaumburg, IL.
ASTM (American Society for Testing and Materials). 2009. “Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.” ASTM E90 – 09, West Conshohocken, Pennsylvania.
DIN (Deutsches Institut für Normung). 1977. “Fire Behavior of Building Materials and Building Components; Building Components; Definitions, Requirements and Tests.” DIN 4102-2, German Institute for Standardization, Berlin, Germany.
ASTM (American Society for Testing and Materials). 2012. “Standard Classification for Determination of Impact Insulation Class (IIC).” ASTM E989 – 12, West Conshohocken, Pennsylvania.
Harmathy, T.Z., Sultan, M.A., and MacLaurin, J.W. 1987. “Comparison of Severity of Exposure in ASTM E119 and ISO 834 Fire Resistance Test,” Journal of Testing and Evaluation, ASTM, 15(6), 371-375.
ASTM (American Society for Testing and Materials). 2016a. “Standard Test Method for Fire Tests of Building Construction and Materials.” ASTM E119 – 16, West Conshohocken, Pennsylvania.
ICC (International Code Council). 2015. International Building Code, Washington, D.C.
Pennsylvania.
ISO (International Organization for Standardization). 1978. Acoustics –
Measurement of Sound Insulation in Buildings and of Building Elements – Part 4: Field Measurements of Airborne Sound Insulation Between Rooms, ISO 140-4.
ASTM (American Society for Testing and Materials). 2016b. “Classification for Rating Sound Insulation.” ASTM E413 – 16, West Conshohocken, Pennsylvania.
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ASTM (American Society for Testing and Materials). 2016c. “Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine.” CobiaxUSA.com 5
ISO (International Organization for Standardization). 1982. Acoustics – Rating of
ISO (International Organization for Standardization). 1999 (Amended 2012). Fire-
resistance Tests - Elements of Building Construction - Part 1: General Requirements,
Sound Insulation in Buildings and of Building Elements – Part 1: Airborne Sound Insulation in Buildings and of Interior Building Elements,
ISO 834-1.
ISO 717-1.
Mota, M. 2010. “Voided Slabs – Then and Now,” Concrete International, ACI, Farmington Hills, Michigan.
ISO (International Organization for Standardization). 1989. Evaluation of Human
Exposure to Whole-Body Vibration – Part 2: Human Exposure to Continuous and ShockInduced Vibrations in Buildings (1 to 80 Hz),
Scanlon, A. and Bischoff, P.H. 2008. “Shrinkage Restraint and Load History Effects on Deflections of Flexural Members,” ACI Structural Journal, ACI, 105(4), 498-506.
ISO 2631-2.
ISO (International Organization for Standardization). 1995. Acoustics –
UL (Underwriters Laboratories). 2015. “Standard for Fire Tests of Building Construction and Materials.” UL 263, Northbrook, Illinois.
Measurement of Sound Insulation in Buildings and of Building Elements – Part 3: Laboratory Measurements of Airborne Sound Insulation of Building Elements, ISO 140-3.
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