About Flat Plate Voided Concrete Slab Systems

ENGINEERING Frequently Asked Questions ETN-B-4-17 (FAQ) About Flat Plate Voided Concrete Slab Systems

Introduction a Flat plate voided concrete slab systems, which have been used for many years in Europe and other parts of the world, are becoming increasingly popular in the U.S. because of 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- b to-floor heights; accelerated construction schedules; and inherent fire resistance that meets the fire-rating requirements in the International Building Code (IBC).

Various types of flat plate voided concrete slabs have been used throughout the years, including one-way Sonotube systems and two-way joist (waffle slab) systems that are constructed using dome forms (Mota c 2010). Contemporary proprietary systems pioneered by BubbleDeck® and Cobiax® utilize hollow, plastic balls made of high- density, recycled polyethylene (HDPE) that are spaced regularly within the overall thickness of the concrete slab. These are commonly referred to as void formers.

Technical Note Technical The BubbleDeck® system is usually con- Figure 1 — Void formers (a) BubbleDeck® – Semi- structed as a semi-precast system (Figure precast system (b) BubbleDeck® – Cast-in-place 1a); however, a cast-in-place system is also system (c) Cobiax USA® – Cast-in-place system (Photos courtesy of BubbleDeck and Cobiax USA). available (Figure 1b). The Cobiax® system is typically employed as a cast-in-place system (Figure 1c). More information on this publication can be obtained by visiting www.crsi.org. CRSI routinely receives inquiries concern- ing various aspects on the design and con- Analysis Methods struction of flat plate voided concrete slab systems. This Technical Note contains a col- How are flat plate voided slab lection of typical questions, which mainly systems analyzed? How do the void came from design professionals, including formers impact the stiffness of the structural engineers, architects, field in- overall system? spectors, code enforcement officials, and Just like other two-way slab systems, flat contractors. plate voided slab systems are permitted to be analyzed and designed by any proce- Specific questions and responses are pro- dure satisfying equilibrium and geometric vided below to frequently asked questions compatibility in accordance with ACI 8.2.1.1 (FAQ). Additional information on these and The direct design method of ACI 8.10 or other topics can be found in Design Guide the equivalent frame method of ACI 8.11 is for Voided Concrete Slabs (CRSI 2014a). permitted where applicable; finite element analysis can must be satisfied, namely, the design strength ΦMn be used in any case and is recommended. must be greater than or equal to the required strength Mu at the critical sections. The strength reduction factor Because the voids reduce the self-weight of the sys- Φ is determined in accordance with ACI 21.2 and is equal tem, the application of self-weight reduction for flat plate to 0.90 for tension-controlled sections, that is, sections voided slabs needs to be applied properly in commercial where the net tensile strain εt in the extreme layer of the slab design software programs, which automatically cal- longitudinal tension reinforcement at nominal strength culate self-weight based on full slab thickness. For flat is greater than or equal to 0.005. Like any two-way rein- plate voided slabs, an additional load case must be con- forced concrete slab system, flat plate voided concrete sidered: a uniform load of 70 percent of the self-weight slabs should be designed as tension-controlled sections reduction must be applied over the entire slab surface for overall efficiency. in a direction opposite the gravity loads to account for 2 the reduction in self-weight. Assuming 70 percent of The nominal strength Mn at a section is determined the self-weight reduction is a good, conservative start- based on the equilibrium and strain compatibility as- ing point in the design process because 70 to 80 percent sumptions in ACI 22.2.1 through 22.2.3. Illustrated of the of the total slab area is typically “covered” with in Figure 2 are the strain and stress distributions at a voids. A more accurate calculation can always be per- section in a flat plate voided concrete slab at nominal formed once the actual net void former area has been strength where positive reinforcement is required. The determined. discussion that follows is also applicable to sections where negative reinforcement is required. A stiffness correction factor, which can be obtained from manufacturers’ 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 Ec 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.

Strength Requirements Figure 2 — Strain and stress distributions in a flat plate voided concrete slab. What are the design requirements for flat plate voided slab systems? Does ACI 318 contain Under design loads that are typical for usual building flexure and shear design provisions for such occupancies, the depth of the equivalent stress block systems? Do you check shear the same way as a will be within the zone of solid concrete between the a solid flat plate? compression edge of the slab and the void formers (the concrete cover to the edge of the void former depends Flexural Strength Requirements. Flat plate voided on the size of the void former and can be obtained from concrete slabs systems are designed and detailed in ac- the manufacturers’ literature). This has been confirmed cordance with requirements of ACI 318 (ACI 2014) just by numerous laboratory flexural tests using various over- like any other two-way slab system. It is important to all slab depths and void former sizes. Thus, the equation note that the void formers do not contribute to the nomi- for nominal strength is the same as that for any rein- nal flexural and shear strengths of the slab system; their Mn forced concrete flexural member: only role is to provide voids in the slab.

The slab is divided into column strips and middle strips and the flexural design strength requirements in ACI 8.5 where and s is the center-to-center spacing of the void formers. 1 All references to ACI 318 (ACI 2014) are given as “ACI” followed by the appropriate section number. Required strength Mu is determined using the load 2 Self-weight reduction is equal to the weight of the solid slab assum- combinations in ACI Table 5.3.1. The required amount of ing no void formers minus the weight of the slab with void formers. flexural reinforcementA s at a section can be obtained The weight of the slab with void formers is equal to the volume of by setting ΦM = M and solving for A . Minimum flex- concrete per square foot times the unit weight of the concrete. The n u s volume of concrete is obtained by subtracting the volume of concrete ural reinforcement requirements are given in ACI Table displaced per square foot (which is the volume of one void former 8.6.1.1. For Grade 60 reinforcement, As,min = 0.0018hs. times the number of void formers per square foot) from the overall slab thickness.

2 Frequently Asked Questions (FAQ) about Flat Plate Voided Concrete Slab Systems [ETN B-4-17] Shear Strength Requirements. Both one-way and must be checked at critical sections located a distance two-way shear requirements must be satisfied for flat d/ 2 from the face of the column and d/ 2 from the edge plate voided concrete slabs. For one-way shear, the fol- of the solid section of concrete around the column. lowing equation must be satisfied at the critical section, which is located a distance d from the face of the sup- port (see Figure 3 and ACI 22.5):

In this equation, strength reduction factor Φ = 0.75 and ℓ2 is the width of the critical section (which can be taken as the center-to-center spacing of the columns). The factor fsr, which is not in ACI 318, is the shear reduction factor that is obtained from the manufacturers’ literature; this factor modifies the design shear strength due to the presence of the void formers in the slab. Depending on the void former, fsr is in the range of 0.50 to 0.60. A solid section of concrete is usually provided around the columns (see Figure 3). It is evident that the critical section for one-way shear typically traverses through a portion Figure 4 — Critical section for two-way shear. of the slab that is solid and mostly through portions that contain void formers. The factor f is applied to the de- sr At the critical section located a distance d/ 2 from the sign shear strength assuming that the solid slab section face of the column, a solid section of concrete is pres- around the column is not present, which is conserva- ent and the following equation must be satisfied (see tive. However, the weight of the solid section should be ACI Table 22.6.5.2): included when calculating the required shear force Vu. Except for long, narrow slabs, one-way shear is seldom critical, but it must be checked to ensure that one-way shear provisions are satisfied.

In these equations, β = ratio of long-to-short dimen- sion of the column; αs = 40 for four-sided critical sections, 30 for three-sided critical sections, and 20 for two-sided critical sections; and, bo = perimeter of the critical section.

At the critical section located a distance of d/ 2 from the outer edge of the solid section of concrete around the column, the design shear strength Φvc must be multiplied by the shear reduction factor fsr, which was discussed previously for one-way shear. The area of the solid section of slab must be made large enough so that two-way shear strength requirements that account for the presence of the void formers are satisfied at that critical section.

The use of headed shear stud reinforcement has prov- Figure 3 — Critical section for one-way shear. en to be a cost-effective way of increasing the design Two-way or punching shear is usually the critical of shear strength of two-way slabs without increasing the the two types of shear. In slabs without shear reinforce- overall slab thickness and/or the plan dimensions of the ment, the shear stresses due to direct shear forces and supporting columns (see Figure 5). ACI 22.6.6.1 contains bending are resisted by the concrete slab section around provisions for two-way slabs with shear reinforcement, a column. The critical sections for flat plate voided con- including headed shear stud reinforcement, which can crete slabs are depicted in Figure 4. Shear requirements be used in flat plate voided concrete slabs.

CRSI Technical Note 3 in the orthogonal direction. The average Ιe for a column strip or a middle strip is used to account for positive and negative regions in the strips.

Calculated deflections must be less than or equal to the maximum permissible deflections in ACI Table 24.2.2. A minimum overall thickness is established based on immediate deflection calculations assuming a void former thickness and corresponding top and bottom cover to the void formers, which can be obtained from manufacturers’ literature. Two-way shear stresses should also be checked at critical locations at this stage; if it has been decided that shear reinforcement will not be used, the overall slab thickness or the supporting column sizes must be increased accordingly if it is found that factored Figure 5 — Headed shear stud reinforcement. (photo courtesy of shear stresses are greater than allowable values. Cobiax Technologies AG). Time-dependent deflections can be calculated in ac- Serviceability Requirements cordance with ACI 24.2.4.1 based on the overall slab thickness determined for immediate deflections. What is the required overall slab thickness to satisfy deflection requirements? Are there vibration criteria available for flat Unlike other two-way reinforced concrete slab sys- plate voided systems? tems, empirical serviceability requirements for flat plate Acceptance criteria are available for human comfort voided concrete slabs are not specifically provided in ACI and sensitive equipment. Recommended acceleration 8.3.1. Because this system is similar to two-way joist and limits for human comfort (walking and rhythmic excita- two-way slab systems, the overall thickness can be ini- tions) for vibrations due to specific activities were devel- tially estimated by ℓ /36 where ℓ is the clear span length n n oped by the International Organization of Standardization in the long direction of a panel measured face-to-face of (ISO 1989) and have been successfully implemented in supports (see ACI Table 8.3.1.1 for minimum thickness a wide variety of situations. Limits for different occupan- of nonprestressed two-way slabs without interior beams cies are provided in terms of root-mean-square (rms) ac- and with drop panels). celerations as multiples of a baseline curve. Suggested In lieu of using an estimated thickness, it is recom- peak accelerations based on these multipliers are given mended that deflection calculations be performed in in Figure 6. accordance with ACI 24.2 to determine an appropriate 100 overall slab thickness. Immediate deflections must in- Unacceptable Zone: clude effects of cracking and reinforcement on member Areas above the curves stiffness, where stiffness is a function of the modulus of for each structure type elasticity of the concrete Ec (determined in accordance with ACI 19.2.2) and the effective moment of inertia Ιe, 10 Rhythmic which may be calculated by ACI Equation (24.2.3.5a) Activities [ACI 24.2.3.1].3,4 Shopping Malls,Dining For flat plate voided concrete slab systems,Ι e can be and Dancing approximated by adding the average Ιe of the column 1 Of ces/ strip in one direction to the average Ιe of the middle strip Residential Peak Acceleration Acceleration Peak g 3 For members with low reinforcement ratios, such as two-way slabs, it has been suggested that ACI Eq. (24.2.3.5a) overestimates member stiffness and that the equation for Ιe in Scanlon and Bischoff (2008) 0.1 ISO Baseline provides more realistic results. Curve Acceptable Zone: 4 The effective moment of inertia Ιe is a function of the cracking Areas below the curves moment Mcr, which in turn is a function of the modulus of rupture where λ is the modification factor that accounts for the reduced mechanical properties of lightweight concrete relative to ’ 0.01 normalweight concrete of the same compressive strength fc. Scanlon 1 10 100 and Bischoff (2008) recommends using and ACI Commit- Frequency cs tee 435 (ACI 1995) recommends using in deflection calculations for two-way slabs to account for the contribution of shrinkage Figure 6 — Recommended peak acceleration for human comfort (ISO restraint to cracking. 1989).

4 Frequently Asked Questions (FAQ) about Flat Plate Voided Concrete Slab Systems [ETN B-4-17] Flat plate voided concrete slab systems designed in accordance with the minimum serviceability criteria discussed above have been found to readily satisfy vibration acceptance criteria for human comfort under typical service conditions and occupancies. The acceptance criteria for walking excitations are easily satisfied by providing an overall slab thickness based on deflection requirements only. Minimum overall slab thicknesses that satisfy acceptance criteria for walking excitations as a function of span length are given in Figure 7. This figure is based on normalweight concrete with a compressive strength of 4,000 psi and Figure 7 — Minimum slab thickness / maximum span lengths for walking excitations. mild reinforcement with a yield strength of 60,000 psi. Superimposed loads custom- arily specified in commercial and residential occupancies were used in the analysis. As an example, if 30-ft spans are required, a flat plate voided concrete slab with a minimum thickness of 10 in. must be provided to satisfy the acceptance criteria for walking (maximum span length is equal to 30.5 ft from the Figure).

Acceptance criteria for rhythmic excitations are directly related to the fundamental frequency of a floor system. Minimum overall slab thicknesses that satisfy the acceptance criteria for various types of rhythmic excitations as a function of span Figure 8 — Minimum slab thickness / maximum span lengths for walking excitations. length are given in Figure 8 based on the same material proportional to the stiffness of a floor system; thus, to and load assumptions used for walking excitations. A satisfy the acceptance criteria in such cases, floor sys- 13.5-in.-thick flat plate voided concrete slab with a maxi- tems must be provided that are very stiff. Minimum mum span length of 30.8 ft must be used if 30-ft spans overall slab thicknesses that are to be used to satisfy are required for jumping exercises and aerobics (which acceptance criteria as a function of span length and have the most stringent acceptance criteria of all the walking pace based on the same material and load considered rhythmic excitations). assumptions noted above are given in Figure 9. Results are presented for a maximum velocity V of 8,000, For sensitive equipment, the expected maximum 1,000, and 130 µin./sec. These velocities correspond to, velocity due to walking-induced vibrations is inversely for example, residences and computer systems; micro,

CRSI Technical Note 5 Figure 9 — Minimum slab thickness / maximum span lengths for sensitive equipment.

6 Frequently Asked Questions (FAQ) about Flat Plate Voided Concrete Slab Systems [ETN B-4-17] eye, and neuro surgery; and, manufacturing highly-sen- structural floor systems, including flat plate voided con- sitive microelectronic equipment, respectively. A thicker crete slabs. The user enters material properties, span (stiffer) slab is required to satisfy the acceptance crite- and column dimensions, and superimposed loads, and ria for faster walking paces. Slab thicknesses that are the program provides estimates of concrete and rein- not provided for certain walking paces and span lengths forcement quantities for flat plate, flat slab, wide-module means that a flat plate voided concrete slab system is joist, two-way joist, one-way beam supported slab, and unable to satisfy the acceptance criteria in those cases. flat plate voided slab systems. This free tool can be used Other stiff reinforced concrete floor systems, such as to determine required slab thickness for any span and two-way joist or grillage systems, should be considered loading condition, and can be accessed at http://www. instead. crsi.org/index.cfm/rcconcept.

Additional information on vibration of flat plate voided Are there any limitations when voided slabs concrete slab systems (and other reinforced concrete are used as cantilevers? floor systems), including approximate methods to determine natural frequency and worked-out design ex- Reducing the dead load and corresponding bending amples, can be found in Design Guide for Vibrations moments and shear forces on cantilevers using flat of Reinforced Concrete Floor Systems (CRSI 2014b). plate voided slab systems has been successfully em- Included in the Design Guide is a report that summa- ployed in projects. Cantilevered flat plate voided slabs rizes the findings of the experimental vibration study are attainable provided all the applicable requirements performed at the University of Wisconsin – Madison. for strength and serviceability are satisfied. It is recommended to check with the manufacturer in cases where What is a typical column spacing and slab cantilevered slabs are proposed. depth for a 40 psf live load? Fire Resistance Flexural strength, shear strength, and serviceability requirements all play a role in determining the overall Regarding fire rating, have there been any slab depth required for a given span length and superim- issues or concerns getting permits/approvals? posed loads. Because the voids reduce the self-weight As a follow-up, have voided slabs reached of the system, a flat plate voided concrete slab system 1-hour or 2-hour ratings? can typically span longer distances than conventionally reinforced flat plate systems of equal overall depth for Fire-resistance rating (or, fire rating), is the period of given superimposed loads. Similarly, flat plate voided time (usually expressed in hours) a building element, concrete systems can carry larger superimposed loads component, or assembly maintains the ability to contain for a given span length. Another way to compare spans a fire, continues to perform a given structural function, achievable with flat plate voided slabs versus conven- or both. Fire ratings are determined by tests or by the tional flat plate systems is to consider that for the same methods prescribed in Section 703.3 of the IBC (ICC volume of concrete per square foot, flat plate voided 2015). slabs facilitate spans that are 35 to 40 percent greater Required fire-resistance ratings for elements in build- than flat plates, mainly because of the greater section ings are given in Table 601 of the IBC based on the properties of the flat plate voided slab. construction type (I though V). Types I and II are types Many solutions are available for required span and of construction where the building elements are of loading criteria. As an example, an overall slab thickness noncombustible materials, which includes reinforced of 9 in. is required for 26-ft spans based on the follow- concrete. The minimum fire-resistance rating for floor ing assumptions: 24-in.-square columns, a live load of 40 elements in Type I construction is 2 hours. psf, a superimposed dead load of 10 psf, 4,000-psi nor- IBC Section 703.2 permits the test procedures in malweight concrete, Grade 60 reinforcement, no shear ASTM E119 (ASTM 2016a) and UL 263 (UL 2015) for de- reinforcement, and walking excitations only. Longer termining fire-resistance ratings of building elements, spans and/or smaller columns are possible if shear re- components, and assemblies. Standard fire tests are inforcement is provided in the 9-in. slab (two-way shear conducted by placing an assembly in a furnace and sub- stresses govern). In the case of 35-ft spans, an overall jecting it to a fire that follows a standard time-tempera- slab thickness of 12.5 in. is required; 24-in. columns with ture curve. shear reinforcement can be used in this case. A 23-in.- thick slab with shear reinforcement can span 50 ft with Numerous fire tests have been performed on 36-in. columns. BubbleDeck® systems and Cobiax® systems in accordance with the provisions in DIN 4102-02 (DIN 1977). CRSI’s Reinforced Concrete Concept is an online tool The time-temperature curve used to test specimens in for preliminary slab design that provides design and the DIN requirements is essentially the same as that construction teams with comparative evaluations and prescribed in ISO 834 (ISO 1999). ISO 834 and ASTM estimates for the most popular reinforced concrete

CRSI Technical Note 7 E119 time-temperature curves are also essentially the machine is used to generate impact noise: the machine same, and it has been shown the differences in severity drops five hammers in rapid succession at equal inter- between the two tests are negligible (Harmathy 1987). vals on the test specimen. Impact sound pressure lev- Therefore, it follows that the results obtained from spec- els are measured in the receiving room below the test imens tested in accordance with the DIN requirements specimen. The information from these tests is then used would essentially be the same as those that would be to determine the IIC, which is calculated in accordance obtained if the specimens were tested in accordance with ASTM E989 (ASTM 2012). with ASTM E119 requirements. Section 1207 of the IBC contains sound transmission Fire tests have revealed that the concrete cover to the requirements that apply to common interior walls, parti- reinforcing bars on the side of the fire is the controlling tions, and floor/ceiling assemblies between (1) adjacent parameter in the determination of the fire resistance for dwelling units and sleeping units or (2) dwelling units flat plate voided concrete slab systems. It was found and sleeping units and adjacent public areas such as that the voids act as thermal isolators, that is, the heat halls, corridors, stairways, or service areas. For airborne from the fire is dammed below the voids. This leads to sound, a minimum value of 50 is required for the STC if slightly higher temperatures in the reinforcing bars that the measurements are performed in a laboratory or 45 are positioned below the voids. A cover of ¾ in. to the if field tested. For impact sound, a minimum value of main flexural reinforcing bars resulted in a fire-resistance 50 is required for the IIC if the measurements are per- rating of at least 2 hours. Void formers were found to formed in a laboratory or 45 if field tested. It is important be intact after the fire tests; the internal temperature to check sound transmission requirements with the local remained below the melting temperature of the HDPE, building authority that has jurisdiction over the project. which is approximately between 200 and 300°F. While STC and IIC ratings of 50 are the minimum pre- There have been numerous projects in the U.S. that scribed by the IBC, it is not uncommon to stipulate STC have utilized flat plate voided concrete slab systems and and IIC ratings of 60 or higher, especially in high-end CRSI is not aware of any projects where fire resistance properties. While an STC of 60 is readily achievable, IIC has been an issue regarding issuance of permits. As values of 60 generally require the use of resilient floor- the tests revealed, a minimum 2-hour fire rating can be ing (vinyl) or carpet and are difficult to attain using hard achieved with a concrete cover of ¾ in., which is the surfacing such as ceramic tile. smallest cover based on void former sizes. Additional fire tests are currently being planned in the U.S. Sound insulation tests have been performed on the BubbleDeck® and Cobiax® systems in the laboratory Sound Control and in completed buildings. For the thinnest concrete slabs (8 and 9 in.) without any floor coverings or ceil- Is there information available for the sound ings, airborne sound reduction indices (similar to STC) transmission class and impact insulation class of 55 and 56 dB were obtained, respectively. These in- for voided systems? dices are calculated in accordance with ISO standards (ISO 1978, ISO 1982, and ISO 1995). Impact sound pres- Sound Transmission Class (STC) is the measure, in sure levels (similar to IIC) of 76 dB were also obtained. decibels (dB), of how much airborne sound a floor or The fundamental methods for the actual measurements wall assembly blocks. STC ratings only measure sound and the mathematical calculations behind the ASTM and blocked in the 125 to 4,000 Hz range, which correspond ISO acoustic standards are similar; however, they are to normal and amplified speech. An assembly with an significantly different in the details of tests and how the STC rating of 50 can reduce 110 dB of airborne sound numerical results are processed. It is recommended to generated on one side of the assembly to 60 dB of air- contact the manufacturers for the latest information on borne sound on the other side. This is the equivalent of sound control for the various systems. reducing the noise level of a rock concert to the level of normal speech. To determine STC ratings of assemblies, Void Formers measurements are taken in a laboratory in accordance with the requirements in ASTM E90 (ASTM 2009). The Are the plastic balls a proprietary product? methodology for calculating the STC is given in ASTM The plastic balls in flat plate voided concrete slab sys- E413 (ASTM 2016b). tems are commonly referred to as void formers and are Impact Insulation Class (IIC) ratings measure how proprietary products. much structure-borne sound is blocked in floor/ceiling assemblies. The higher the IIC, the better the floor/ceil- What are the sizes of the voids in voided ing assembly is at blocking the impact noise. Impact systems? What is the spacing between the sound transmission loss of floor/ceiling assemblies voids and what is the thickness of the concrete is measured in a laboratory and must conform to the above and below the voids? requirements in ASTM E492 (ASTM 2016c). A tapping

8 Frequently Asked Questions (FAQ) about Flat Plate Voided Concrete Slab Systems [ETN B-4-17] BubbleDeck® produces spherical void formers that range in 1 3 diameter from 7 /8 in. to 17 /4 in. Recommended minimum slab thickness for these void formers 3 are 9 in. to 22 /4 in.

Cobiax® produces both ellipsoi- dal (Slim-Line) and spherical (Eco- Line) void formers. The heights of the Slim-Line void formers range 5 from 4 in. to 8 /8 in. (corresponding 1 to slab depths of 8 in. to 13 /2 in.). Slim-Line void formers have been stacked to accommodate slabs as thick as 30 in. Eco-Line void form- 7 ers range in diameter from 8 /8 in. 3 to 17 /4 in. (corresponding to slab 1 depths of 13 /2 in. to 23 in.). Figure 10 — Section of BubbleCage mat being lifted into place (Photo courtesy of BubbleDeck).

The minimum clear spacing be- 1. Cage modules with top and all the bottom structural tween the void formers is 1 in. and the thickness of the reinforcement are precast into 3-in.-thick panels. concrete above and below the void formers depend on the cover required for the reinforcing bars, the type and 2. The panels are set on shoring that is spaced 7 ft on size of the void former, and other geometrical require- center. ments. Specific information on these dimensions can 3. On site, a layer of concrete is cast to the required be obtained from product catalogues obtained from the overall thickness. manufacturers. In addition to around the columns, void formers are typi- Construction Considerations cally omitted around the perimeter of the slab for a width of at least 2 ft (but not less than 1 ft). Reinforcement de- What are the construction sequences for the tails similar to those used in other two-way slab systems cast-in-place and semi-precast systems? can be provided along the edges. Typical site installation of a cast-in-place voided concrete slab is as follows: An emerging trend in the construction of flat plate voided slab systems is tying together assembled cages 1. Formwork is erected and bottom reinforcing bars are as one mat and lifting them into place. This helps in re- placed. ducing overall construction time. Depicted in Figure 10 is a 10 ft by 20 ft section of a BubbleCage mat being lifted 2. Cage modules that contain the void formers are by a crane for final placement. placed and tied perpendicular to the top layer of bottom reinforcing bars. In the cast-in-place system, can the concrete 3. Top reinforcing bars are placed directly on and tied to be placed in one lift? the cage modules. The entire depth of the slab can be placed in one lift 4. First layer of concrete is cast, which covers the bot- in a cast-in-place system. However, the cage modules tom reinforcing bars and the lower portion of the cage must be anchored directly to the formwork to prevent modules. This layer of concrete essentially anchors buoyancy forces from lifting them upward due to the flu- the cage modules and prevents possible upward id concrete. The more common approach is to use two movement due to buoyancy when the second layer of lifts as described above in the construction sequence. concrete is placed. What kind of concrete should be used for 5. Second layer of concrete is cast to the required overall casting? Should self-consolidating concrete be thickness of the slab system. used in order to get concrete under the voids? In the semi-precast system, a precast concrete layer Just like other typical two-way slab systems, normal- essentially replaces the horizontal formwork of the cast- weight concrete with a specified compressive strength in-place system described above. Installation of this sys- of 4,000 psi is usually the most efficient for flat plate tem is as follows: voided concrete slab systems with conventional reinforcement.

CRSI Technical Note 9 In the cast-in-place system, the flowability of the con- layer that is cast first, act as shear dowels and provide crete mixture forming the bottom layer is particularly sufficient interlocking between the two concrete layers. important. The concrete must flow in between and be- Manufacturers’ literature contains the size of the vertical neath the void formers and must be consolidated (see bars in the cage modules for the various void formers. Figure 11). In addition, concrete must flow between This information can be used to verify by calculation that the cage modules, the flexural reinforcing bars, and any the provided vertical bars are adequate to transfer the conduits or other embedded items in the slab. A fully horizontal shear between the concrete layers. consolidated bottom concrete layer is also important for architectural reasons where the underside of the slab is For the semi-precast system, what are the to be left exposed. The concrete must be vibrated and typical details between the precast units at the admixtures can be added to the concrete mixture that bottom of the assembly? help facilitate concrete placement. Precast panels as large as 11 ft by 40 ft have been used The grid of the void formers provides a convenient vi- in projects in North America. These panels are located sual indication for the operator of a needle vibrator to adjacent to each other to form the floor and roof systems follow the concrete flowing beneath the void formers; of the structure. In order to provide an aesthetic finish, this helps in obtaining a consistently consolidated slab. a ¼- to ½-in. bottom bevel/fillet is usually formed along Concrete flows readily around the rounded shapes of the edges of adjacent panels. Similar to other types of the void formers. Like any concrete mixture, prolonged precast units, the resulting ½- to 1-in.-wide grooves can vibration may result in segregation of the aggregates be taped, filled with silicone caulk, or left intact in cases and should be avoided. where the precast components are made with geomet- rically precise fit. This joint is generally not conducive to Self-consolidating concrete is not required in order to wet cement paste leakage. For wider gaps, backing rod get concrete under the void formers and around rein- and silicone caulk may be considered to close the open- forcing bars and embedded items. A properly vibrated ing and to allow cement paste to fill the gap from above. normalweight concrete mix is typically adequate and is generally the most cost effective. To facilitate lowering the precast panels into place, chamfers that are 1-in. wide and at a 45-degree angle Is there a cold joint concern when using the are often provided along the upper edges of the panels. cast-in-place system? Are there any special considerations for When casting concrete for the cast-in-place systems, embedded items? a horizontal cold joint is created whenever the bottom layer of concrete dries and hardens before the top layer Small items, such as electrical juncture boxes and is cast. In cast-in-place systems, the friction between pipes less than 3 in. in diameter, can be installed with- the two layers (weight of top layer of concrete on the in the depth of a flat plate voided slab with essentially unfinished surface of the first layer of concrete) helps minor or no interference with the void formers. Larger in transferring horizontal shear between the two layers. items, like MEP conduits, usually require modification In addition, for both cast-in-place and semi-precast sys- of the regular pattern of the void formers; some of the tems, the vertical bars of the cage modules that contain plastic balls are either omitted or shifted to accommo- the void formers, which are embedded in the concrete date larger embedded items. The structural engineer and

Figure 11 — Typical cross-section of a cast-in-place flat plate voided concrete slab.

10 Frequently Asked Questions (FAQ) about Flat Plate Voided Concrete Slab Systems [ETN B-4-17] architect of record should be notified whenever changes Numerous types of anchoring systems are available are proposed to the layout of the void formers. for a variety of situations. The type and size of anchor depends on the magnitude of load that needs to be sup- It is always good practice to coordinate all the trade ported. For relatively small loads, screw-type anchors drawings with the structural drawings prior to construc- installed in concrete between the void formers may be tion. In this way, the placement/shop drawings for the sufficient. For larger loads, one technique that has been void formers can include any modifications required for used is to install anchors into the voids, which are then embedded items or large openings. partially filled with adhesive or mortar. Through-bolting the slab is an option for heavy loads. In addition to check- Are there any special considerations for coring ing global strength and serviceability requirements, local through voided slabs? shear stresses must be checked at the location of each Holes can easily be diamond-core-drilled through the through-bolt; the load must be adequately spread over cast-in-place and semi-precast voided systems. There is the slab surfaces to avoid any local punch-through fail- essentially no limit on the size and location of holes as ures. This usually entails providing plates at the location long as it can be shown by analysis that all the pertinent of the through-bolts, which may not be architecturally strength and serviceability requirements are satisfied. feasible. Cutting through one or more void formers is not an issue because they are not required for strength. In or- If an anchor unintentionally penetrates or comes near der to prevent the creation of a space or draft for fire to a void former, the anchor should be relocated. Knowing spread, the space between the cored opening and the the spacing and the size of the void formers enables the pipe/chase should be appropriately sealed to meet the anchor to be located where there is sufficient concrete required fire rating. between the voids.

The provisions in ACI 8.5.4 can be used as a guide for Holes drilled into the slab for temporary use during the openings in slab systems without beams. Openings ad- construction phase (for example, dowels fixing braces jacent to columns must be analyzed carefully; a portion attached to the top side of the slab) should be sealed of the critical section resisting two-way shear stresses after use to avoid void formers from unintentionally be- may need to be considered ineffective depending on the ing filled with water, which may later freeze and cause size and location of the opening (ACI 22.6.4.3). damage.

Just like any other cast-in-place or precast concrete It is recommended to consult with anchoring manufac- floor system, it is important to have structural drawings turers where it is anticipated that post-installed anchors and placement/shop drawings of the flat plate voided will have an important impact on the project. concrete slab system when determining locations for Availability and Cost cores and other types of penetrations through the slab. The information on the drawings should be verified in What is the availability of the voids in different the field. parts of the country? How many plants are making the voids? Can post-installed anchors be used with voided slabs? What if an anchor hits a void? It is recommended to contact BubbleDeck® and Cobiax USA® to obtain the latest information on avail- Post-installed anchors can be used provided the void ability. Representatives are available that can assist formers are located by any rational method, including vi- through every phase of a project. sual observation of the soffit, placement/shop drawings, nondestructive testing, or any combination thereof. The What is the cost of a voided slab system? minimum thickness of concrete cover at the underside of the plastic balls should be established using nonde- It is recommended to contact BubbleDeck® and structive testing techniques. It is important to note that Cobiax USA® to obtain the latest information on cost. for spherical void formers, minimum concrete thickness When comparing costs of different types of structural is applicable only at the center point of each sphere (the systems, it is important that the costs are calculated minimum concrete thickness is 2 in. for a ¾-in. cover using the same span and loading conditions for each and 2 layers of #5 flexural reinforcement); a substantial system. The following cost-savings items should also increase in concrete thickness occurs away from the be considered, to name a few: (1) reduction of concrete center point. In contrast, minimum concrete thickness displaced by the voids, (2) the design of the columns and extends over a larger horizontal region for elliptical void foundations due to the reduced dead load, and (3) the formers. reduced structural depth and its impact on all the vertical runs in the building.

CRSI Technical Note 11 References ACI (American Concrete Institute). 1995. “Control of Deflections in Harmathy, T.Z., Sultan, M.A., and MacLaurin, J.W. 1987. “Compari- Concrete Structures.” ACI 435R-95, Farmington Hills, Michigan. son of Severity of Exposure in ASTM E119 and ISO 834 Fire Resis- tance Test,” Journal of Testing and Evaluation, ASTM, 15(6), 371-375. ACI (American Concrete Institute). 2014. “Building Code Require- ments for Structural Concrete and Commentary.” ACI 318-14, Farm- ICC (International Code Council). 2015. International Building Code, ington Hills, Michigan. Washington, D.C. ASTM (American Society for Testing and Materials). 2009. “Stan- ISO (International Organization for Standardization). 1978. dard Test Method for Laboratory Measurement of Airborne Sound Acoustics – Measurement of Sound Insulation in Buildings and of Transmission Loss of Building Partitions and Elements.” ASTM E90 Building Elements – Part 4: Field Measurements of Airborne Sound – 09, West Conshohocken, Pennsylvania. Insulation Between Rooms, ISO 140-4. ASTM (American Society for Testing and Materials). 2012. “Stan- ISO (International Organization for Standardization). 1982. dard Classification for Determination of Impact Insulation Class (IIC).” Acoustics – Rating of Sound Insulation in Buildings and of Building El- ASTM E989 – 12, West Conshohocken, Pennsylvania. ements – Part 1: Airborne Sound Insulation in Buildings and of Interior Building Elements, ISO 717-1. ASTM (American Society for Testing and Materials). 2016a. “Stan- dard Test Method for Fire Tests of Building Construction and Materi- ISO (International Organization for Standardization). 1989. Evalu- als.” ASTM E119 – 16, West Conshohocken, Pennsylvania. ation of Human Exposure to Whole-Body Vibration – Part 2: Human Exposure to Continuous and Shock-Induced Vibrations in Buildings (1 ASTM (American Society for Testing and Materials). 2016b. “Clas- to 80 Hz), ISO 2631-2. sification for Rating Sound Insulation.” ASTM E413 – 16, West Con- shohocken, Pennsylvania. ISO (International Organization for Standardization). 1995. Acoustics – Measurement of Sound Insulation in Buildings and of ASTM (American Society for Testing and Materials). 2016c. “Stan- Building Elements – Part 3: Laboratory Measurements of Airborne dard Test Method for Laboratory Measurement of Impact Sound Sound Insulation of Building Elements, ISO 140-3. Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine.” ASTM E492 – 16, West Conshohocken, Pennsylvania. ISO (International Organization for Standardization). 1999 (Amended 2012). Fire-resistance Tests - Elements of Building Con- CRSI (Concrete Reinforcing Steel Institute). 2014a. Design Guide struction - Part 1: General Requirements, ISO 834-1. for Voided Concrete Slabs, Schaumburg, Illinois. Mota, M. 2010. “Voided Slabs – Then and Now,” Concrete Interna- CRSI (Concrete Reinforcing Steel Institute). 2014b. Design Guide tional, ACI, Farmington Hills, Michigan. for Vibrations of Reinforced Concrete Floor Systems, Schaumburg, Illinois. Scanlon, A. and Bischoff, P.H. 2008. “Shrinkage Restraint and Load History Effects on Deflections of Flexural Members,” ACI Structural DIN (Deutsches Institut für Normung). 1977. “Fire Behavior of Journal, ACI, 105(4), 498-506. Building Materials and Building Components; Building Components; Definitions, Requirements and Tests.” DIN 4102-2, German Institute UL (Underwriters Laboratories). 2015. “Standard for Fire Tests of for Standardization, Berlin, Germany. Building Construction and Materials.” UL 263, Northbrook, Illinois.

Contributors: The principal author on this publication is David A. Fanella, Ph.D., S.E., P.E., Senior Director of Engineering, CRSI, with review by CRSI members and staff.

Keywords: flat plate voided concrete slab systems, reinforced concrete floor systems, voided slabs

Reference: Concrete Reinforcing Steel Institute - CRSI (2017), “Frequently Asked Questions (FAQ) About Flat Plate Voided Concrete Slab Systems”, CRSI Technical Note ETN-B-4-17, Schaumburg, IL, 933 North Plum Grove Rd. 12 pp. Schaumburg, IL 60173-4758 p. 847-517-1200 • f. 847-517-1206 Historical: None. New Technical Note. www.crsi.org

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