DESIGN CONSIDERATIONS FOR UNBONDED CONCRETE

DESIGN CONSIDERATIONS FORUNBONDED CONCRETE TOPPINGSLABS IN PLAZA DECK SYSTEMSSUBJECTED TO VEHICULAR TRAFFICBY ENRIQUE VACA, NARENDRA K. GOSAIN, AND GABRIEL A. JIMENEZConcrete topping slabs are overlays applied ontop of structural floors to provide a finishedfloor surface for multiple purposes, such as providing a wearing course to support traffic loads inparking facilities and bus terminals, providing alevel surface for interior floors, and to resurfaceworn or damaged floors.The authors are not aware of existing designguidelines or recommendations for topping slabs,particularly for unbonded applications, and wherethe topping slab is subjected to vehicular trafficand exposure to the weather. Design of toppingslabs is not formally treated in engineering literature. The purpose of this article is to address theissues associated with the performance andbehavior of topping slabs in plaza deck systemssubjected to vehicular traffic.UNBONDED TOPPING SLABSIn unbonded systems, the topping slab is notmechanically adhered or otherwise bonded to theunderlying structural floor. Unbonded systems areprovided when it is desired that the two slab coursesmove independently or to permit easy replacementof the topping slab at a later period. To prevent thebond between the topping and base slabs, plasticsheeting, roofing felt, or other similar bondbreaking materials can be used. Very often, anintermediate layer of waterproofing membrane isprovided in applications where there is occupiedspace below. These systems are often referred to as“plaza deck systems” and will be the focus of thestudy presented in this article.ISSUES WITH TOPPING SLABSThe customary engineering practice has been totreat topping slabs as nonstructural without performing any rigorous analysis to determine reinforcement and thickness requirements. Toppingslabs are typically designed using the engineer’sexperience and reinforcement is provided for crackcontrol due to shrinkage and temperature effects. Acommon flaw or misconception is that the topping18CONCRETE REPAIR BULLETINJANUARY/FEBRUARY 2015slab is considered a “nonstructural” element bymost engineers and is merely seen as a medium totransfer the loads to the “real” structural deck orslab below the topping. This way of thinking istrue in the sense that failure of the topping will notjeopardize the structure or pose a life safety issue.However, extensive cracking and spalling of thetopping slab can lead to a significant reduction ofits service life and may compromise the waterproofing characteristics to occupied spaces below.For most owners, this condition will be unaccep table. Some cases of serviceability failures oftopping slabs have been observed because ofinadequate design and specifications for construction of these slabs.Topping slabs have many of the same issuesthat slabs-on-ground and structural concrete slabshave. Concrete practices related to crack control,shrinkage and temperature control, concretejointing, concrete mixture design, and concretecuring for elevated slabs and slabs-on-ground arealso applicable to topping slabs. Improper concretepractices can lead to poor performance of toppingslabs. Several American Concrete Institute (ACI)documents1-6 offer guidance for proper design considerations and construction practices for concreteslab construction.PLAZA DECK SYSTEMSPlaza deck systems typically consist of a structural slab, waterproofing membrane, protectionboard, insulation/drainage layer, and a topping slab(wearing surface). Figure 1 illustrates the typicalcomponents of a plaza deck system. These systemsare often used to provide waterproofing to occupied spaces located below driving surfaces orlandscaped areas. Plaza deck systems generallyoccur in parking garages, airport terminals, busterminals, and commercial properties such ashotels, condominiums, and office complexes.These systems are popular among owners becausethe topping slab provides protection to the waterproofing membrane with the anticipa et the required properties for design.When performing the recommended analysis, iftensile stresses in the topping slab are higher thanthe modulus of rupture of normalweight concrete(7.5 f 'c ), steel reinforcement should be designedto reinforce the topping slab for bending momentsproduced by traffic loads, considering the effectsof the waterproofing system and the topping slabbehavior. Providing reinforcement for shrinkageand temperature alone, as is normally done in top22CONCRETE REPAIR BULLETINJANUARY/FEBRUARY 2015ping slabs, will not be adequate, particularly forapplications with heavy traffic loads subjected totruck or bus loads (that is, a concentrated load of8000 to 13,200 lb [35.6 to 58.7 kN]).7CONCLUSIONSBased on this parametric study of various topping slab thicknesses, and a range of the stiffnessof the sandwiched waterproofing membrane thatcould be normally encountered in practice, the following conclusions can be drawn:1. An analysis of the structural slab plus toppingslab, taking into consideration the compressibility of the waterproofing membrane plusprotection board system and including the stiffness of the supporting structural frame, is recommended to evaluate the required thickness andreinforcement in the topping slab for the anticipated concentrated and uniform live loads thatwill be applied to the plaza deck system.2. Where a plaza deck system is to be used, consider the largest possible topping slab thicknessto minimize tensile stresses and cracking of thetopping slab. For applications with light traffic(concentrated load of 3000 lb [13.3 kN]), a topping slab with a minimum thickness of 3 in.(76 mm) is preferred. This is also the minimumthickness in which steel reinforcement can beplaced with adequate cover for exterior exposureas defined by ACI 318.11 For applications withheavy traffic (concentrated load of 8000 to13,200 lb [35.6 to 58.7 kN]), a minimumunbonded topping slab thickness of 5 in.(127 mm) will provide a better serviceable life.3. In certain cases, a waterproofing membranesystem (membrane plus protection board) withfoundation modulus higher than 300 lb/in.3(0.08 N/mm3) may help minimize the magnitudeof tensile stresses and, consequently, reduce theamount of cracking in the topping slab. Thefoundation modulus needs to be obtained fromthe membrane manufacturer. If this informationis not available, material tests to obtain therequired properties may be needed.4. It is important that, in addition to determining theappropriate topping slab thickness and reinforcement for the anticipated loads, proper constructionpractices for topping slabs should be followed.REFERENCES1. ACI Committee 302, “Guide for Concrete Floor and SlabConstruction (ACI 302.1R-04),” American Concrete Institute,Farmington Hills, MI, 2004, 76 pp.2. ACI Committee 224, “Joints in Concrete Construction(ACI 224.3R-95) (Reapproved 2008),” American ConcreteInstitute, Farmington Hills, MI, 1995, 41 pp.3. ACI Committee 325, “Concrete Overlays for PavementRehabilitation (ACI 325.13R-06),” American Concrete Institute,Farmington Hills, MI, 2006, 39 pp.WWW.ICRI.ORG

4. ACI Committee 308, “Guide to Curing Concrete (ACI308R-01) (Reapproved 2008),” American Concrete Institute,Farmington Hills, MI, 2001, 30 pp.5. ACI Committee 224, “Control of Cracking in ConcreteStructures (ACI 224R-01) (Reapproved 2008),” AmericanConcrete Institute, Farmington Hills, MI, 2001, 46 pp.6. ACI Committee 360, “Design of Slabs-on-Ground (ACI360R-10),” American Concrete Institute, Farmington Hills, MI,2010, 72 pp.7. ASCE/SEI 7-10, “Minimum Design Loads for Buildingsand Other Structures,” American Society of Civil Engineers,Reston, VA, 2013, 593 pp.8. Tarr, S. M., and Nussbaum, P. J., “Foundation Moduli forSub-Slab Insulation Board?” Concrete International, V. 24,No. 10, Oct. 2002, pp. 75-79.9. Dow Building Materials, “Guide to Plaza Constructionwith Styrofoam Brand Insulation,” Midland, MI, 8 pp.10. Owens Corning, “High Density Extruded PolystyreneInsulation.”11. ACI Committee 318, “Building Code Requirements forStructural Concrete (ACI 318-14) and Commentary,” AmericanConcrete Institute, Farmington Hills, MI, 2014, 520 pp.Enrique Vaca, PhD, PE, is aSenior Associate in the DiagnosticsGroup at Walter P Moore. Vacareceived his MS and PhD from TheUniversity of Texas at Austin,Austin, TX. He has 15 years ofdiversified experience in the analysis and design of new structuresand in the assessment and repair of existing facilities.Vaca is a member of PTI and is a licensed professional engineer in Texas and Georgia.Narendra K. Gosain, PhD, PE, isa Senior Consultant in the Diagnostics Group at Walter P Moore.Gosain received his PhD from RiceUniversity, Houston, TX. In his42-year career with Walter PMoore, he has been involved withthe design and restoration of several significant structures. Gosain is a member ofICRI, ACI, and ASCE, and is a licensed professionalengineer in Texas.Gabriel A. Jimenez, PhD, PE,SE, P.Eng, is a Senior Principal atWalter P Moore. He is the Executive Director of the DiagnosticsGroup. He received his MS andPhD in structural engineering fromthe University of Minnesota, Minneapolis, MN. Jimenez has performed numerous assessments and repair ofconcrete structures throughout the United Statesand Canada focusing on leakage, corrosion, andstructural-related problems. He is a member of ICRIand PTI and is a licensed professional engineer invarious states and provinces.WWW.ICRI.ORGJANUARY/FEBRUARY 2015CONCRETE REPAIR BULLETIN23

tion of these slabs. Topping slabs have many of the same issues that slabs-on-ground and structural concrete slabs have. Concrete practices related to crack control, shrinkage and temperature control, concrete jointing, concrete mixture design, and concrete curing for elevated slabs and slabs-on-grou