Steel Fiber Concrete Slabs on Ground: A Structural Matter is still a lack of design rules for steel fiber reinforced concrete slabs in building codes. Due to this lack, conventional

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  • ACI Structural Journal/July-August 2006 551

    ACI Structural Journal, V. 103, No. 4, July-August 2006.MS No. 05-026 received December 17, 2005, and reviewed under Institute publication

    policies. Copyright 2006, American Concrete Institute. All rights reserved, includingthe making of copies unless permission is obtained from the copyright proprietors. Pertinentdiscussion including authors closure, if any, will be published in the May-June 2007ACI Structural Journal if the discussion is received by January 1, 2007.

    ACI STRUCTURAL JOURNAL TECHNICAL PAPER

    An extensive experimental investigation with the aim of studyingthe structural behavior of slabs on ground made of steel fiber-reinforced concrete (SFRC) is presented in this paper. Several full-scale slabs reinforced with different volume fractions of steel fibershaving different geometries were tested under a point load in theslab center. A hybrid combination of short and long fibers was alsoconsidered to optimize structural behavior. Experimental resultsshow that steel fibers significantly enhance the bearing capacityand the ductility of slabs on ground.

    The nonlinear behavior of these SFRC structures is well capturedby performing nonlinear fracture mechanics analyses where theconstitutive relations of cracked concrete under tension wereexperimentally determined. Finally, from an extensive parametricstudy, design abaci and a simplified analytical equation for predictingthe minimum thickness of SFRC slabs on ground are proposed.

    Keywords: pavement; reinforced concrete; slabs on ground.

    INTRODUCTIONIn the last decades, the use of steel fiber-reinforced

    concrete (SFRC) has significantly increased in industrialpavements, roads, parking areas, and airport runways as aneffective alternative to conventional reinforcement (that is,reinforcing bars or welded mesh). Because heavy concentratedloads from industrial machinery and shelves may causeintensive cracking and excessive deformation of pavements,a diffused fiber reinforcement may help the structural behavior.

    Many of these pavements are slabs on ground that arestatically undetermined structures. For this reason, even atrelatively low volume fractions (

  • ACI Structural Journal/July-August 2006552

    toughness at small crack opening displacements17-19 (Fig. 1).Moreover, due to the better control of the cracking process,shorter fibers reduce the material permeability20 and HyFRCappears to be a promising application for pavementssubjected to aggressive environments.

    RESEARCH SIGNIFICANCEWhereas the structural behavior of plain concrete and

    conventionally reinforced slabs on ground is well known,

    there is still a lack of design rules for steel fiber reinforcedconcrete slabs in building codes. Due to this lack, conventionaldesign methods, based on the elastic theory, are used for fiberreinforced slabs whose behavior is significantly nonlinear.The behavior of slabs on ground with steel fibers was experi-mentally studied by performing full-scale tests; a designapproach based on nonlinear fracture mechanics is also proposed.

    To enhance the structural response, the use of HyFRCsystems, combining shorter and longer steel fibers, was alsoconsidered.

    EXPERIMENTAL PROGRAMFull-scale slabs on ground were tested under a point load

    in the center. The experimental model aimed to reproduce asquare portion of pavement, limited by joints, with a side (L)of 3 m (118.11 in.) and a thickness (s) of 0.15 m (5.91 in.).Additional tensile and bending tests were carried out to identifythe fracture behavior of SFRC. The slab tests presented inthis paper are part of an extensive research campaign whoseresults are published elsewhere.18,21

    MaterialsThe concrete matrix was made with cement CEM II/A-LL

    42.5R (UNI-ENV 197-1) and natural river gravel with arounded shape and a maximum diameter of 15 mm (0.59 in.);its composition is summarized in Table 1.

    Five different types of fibers were considered in thisresearch, as reported in Table 2 where geometrical andmechanical properties of fibers are shown; the fiber code isconventionally defined by the fiber length and the fiberdiameter (Lf /f , in millimeter unit). Two straight shorterfibers (12/0.18 and 20/0.4) and three longer fibers withhooked ends (30/0.6, 50/1.0(a), and 50/1.0(b)) were adopted.All the fibers have a rounded shaft, an aspect ratio rangingbetween 50 and 66, and a Young modulus of approximately210 GPa (30456.9 ksi).

    Seven SFRC slabs (S1, S3, S4, S5, S8, S11, and S14), witha volume fraction Vf of fiber smaller than 0.6% and a referenceslab made of plain concrete (S0) are reported in this paper(Table 3). Figure 2(a) shows the slab with a hydraulic jackplaced in its center.

    Luca G. Sorelli is investigating ultra-high-performance concrete (UHPC) structuralimplications by micromechanics and chemo-plasticity approaches at MassachusettsInstitute of Technology, Cambridge, Mass. He received his doctorate from the Universityof Brescia, Brescia, Italy.

    Alberto Meda is an Associate Professor of structural engineering, Department ofEngineering Design and Technology, University of Bergamo, Bergamo, Italy. Hereceived his degree in environmental engineering from the Milan University ofTechnology, Milan, Italy, in 1994. His research interests include concrete fracturemechanics, fiber-reinforced concrete, and fire design of reinforced concrete structures.

    ACI member Giovanni A. Plizzari is a Professor of structural engineering, Departmentof Civil Engineering, University of Brescia. His research interests include materialproperties and structural applications of high-performance concrete, fiber-reinforcedconcrete, concrete pavements, fatigue and fracture of concrete, and steel-to-concreteinteraction in reinforced concrete structures.

    Table 1Composition of concrete matrixMixture component Quantity

    Cement 42.5R (ENV 197-1) 345 kg/m3 (21.54) lb/ft3

    Water 190 kg/m3 (11.86) lb/ft3

    High-range water-reducing admixture (melamine-based) 0.38%vol

    Aggregate (0 to 4 mm) 621 kg/m3 (38.77) lb/ft3

    Aggregate (4 to 15 mm) 450 kg/m3 (28.09) lb/ft3

    Aggregate (8 to 15 mm) 450 kg/m3 (28.09) lb/ft3

    Table 2Geometrical and mechanical properties of steel fibers

    Fiber code

    Lf , mm (in.) f , mm (in.) Lf /f fft , MPa (ksi) Fiber shape

    50/1.0(a) 50 (1.97) 1.00 (0.0394) 50.0 1100 (159.5)

    50/1.0(b) 50 (1.97) 1.00 (0.0394) 50.0 1100 (159.5)

    30/0.6 30 (1.18) 0.60 (0.0236) 50.0 1100 (159.5)

    20/0.4 20 (0.79) 0.40 (0.0157) 50.0 1100 (159.5)

    12/0.18 12 (0.47) 0.18 (0.0071) 66.6 1800 (261.1)

    Table 3Volume fractions of steel reinforcement (steel fibers or welded mesh)

    Slab no.

    Steel fibersVf,tot,%vol

    50/1.0 %vol

    30/0.6 %vol

    20.04 %vol

    12/0.18 %vol

    S0 0.00

    S1 0.38 0.38

    S3 0.38(a) 0.19 0.57

    S4 0.38(a) 0.38

    S5 0.38 0.38

    S8 0.38(b) 0.38

    S11 0.57(a) 0.57

    S14 0.38(b) 0.19 0.57

    Fig. 2Test setup for: (a) slab on ground; (b) small beams;(c) deformed FE meshed for numerical simulation of slab;and (d) notched beams under bending.

  • ACI Structural Journal/July-August 2006 553

    Table 4 reports the mechanical properties of concrete ofthe different slabs, as determined on the day of the test; inparticular, Table 4 shows the tensile strength fct from cylinders(c = 80 mm [3.15 in.], L = 250 mm [9.84 in.]), the compressivestrength from cubes fc,cube of 150 mm side (5.91 in.); theYoungs modulus as determined from both compressiontests on cylinders Ec (c = 80 mm [3.15 in.]; L = 200 mm[7.87 in.]) and from core specimens Ec,core (c = 76 mm[2.99 in.]; L = 150 mm [5.91 in.]) drilled out from the slab(after the test). The slump of the fresh concrete was alwaysgreater than 150 mm (5.91 in.).

    Fracture properties were determined from six notched beams(150 x 150 x 600 mm [5.91 x 5.91 x 23.62 in.]) tested underfour-point bending according to the Italian Standard22

    (Fig. 2(b)). The notch was placed at midspan and had a depthof 45 mm (1.77 in.) (Fig. 2(b), (d)). These tests were carried outwith a closed loop hydraulic testing machine by using the crackmouth opening displacement (CMOD) as a control parameter,which was measured by means of a clip gauge positionedastride the notch. Additional linear variable differentialtransformers (LVDTs) were used to measure the crack tipopening displacement (CTOD) and the vertical displacement atthe beam midspan and under the load points (Fig. 2(b)).

    Test setup and instrumentationThe slabs were loaded by a hydraulic jack placed in the

    center by using the load frame shown in Fig. 2(a); theaverage loading rate was 2.5 kN/min (0.56 kips/min).

    To reproduce a Winkler soil, 64 steel supports were placedunder the slab at centers of 375 mm (14.76 in.) in both directions(Fig. 3(a)). These supports are steel plates on a square basehaving a side of 100 mm (3.94 in.; Fig. 3(b)). Previousnumerical simulations showed that the experimental subgradeprovides a good approximation of a continuous Winklersoil.16 Because of the curling of the concrete slabs due toshrinkage and the thermal effect, a layer of high-strengthmortar a few millimeters thick was placed on each spring toensure the contact with the bottom face of the slab. Theaverage spring stiffness was determined by compressiontests performed on each spring with results approximatelyequal to 11.0 kN/mm (2.47 kips/mm). By considering theinfluence area of each spring (375 x 375 mm [14.76 x14.76 in.]), the average Winkler constant kw was equal to0.0785 N/mm3 (289.2 lb/in.3), which corresponds to auniform graded sand soil according to ACI classification.23

    During the tests, the vertical displacements of 12 points onthe top surface of the slab were continuously monitored;furthermore, four in