Steel Fiber Concrete Slabs on Ground: A Structural Matter the structural behavior of slabs on ground

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Text of Steel Fiber Concrete Slabs on Ground: A Structural Matter the structural behavior of slabs on ground

  • 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, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including author’s closure, if any, will be published in the May-June 2007 ACI Structural Journal if the discussion is received by January 1, 2007.

    ACI STRUCTURAL JOURNAL TECHNICAL PAPER

    An extensive experimental investigation with the aim of studying the 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 fibers having different geometries were tested under a point load in the slab center. A hybrid combination of short and long fibers was also considered to optimize structural behavior. Experimental results show that steel fibers significantly enhance the bearing capacity and the ductility of slabs on ground.

    The nonlinear behavior of these SFRC structures is well captured by performing nonlinear fracture mechanics analyses where the constitutive relations of cracked concrete under tension were experimentally determined. Finally, from an extensive parametric study, design abaci and a simplified analytical equation for predicting the minimum thickness of SFRC slabs on ground are proposed.

    Keywords: pavement; reinforced concrete; slabs on ground.

    INTRODUCTION In the last decades, the use of steel fiber-reinforced

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

    Many of these pavements are slabs on ground that are statically undetermined structures. For this reason, even at relatively 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 HyFRC appears to be a promising application for pavements subjected to aggressive environments.

    RESEARCH SIGNIFICANCE Whereas 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 reinforced concrete slabs in building codes. Due to this lack, conventional design methods, based on the elastic theory, are used for fiber reinforced 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 design approach based on nonlinear fracture mechanics is also proposed.

    To enhance the structural response, the use of HyFRC systems, combining shorter and longer steel fibers, was also considered.

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

    in the center. The experimental model aimed to reproduce a square 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 identify the fracture behavior of SFRC. The slab tests presented in this paper are part of an extensive research campaign whose results are published elsewhere.18,21

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

    42.5R (UNI-ENV 197-1) and natural river gravel with a rounded 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 this research, as reported in Table 2 where geometrical and mechanical properties of fibers are shown; the fiber code is conventionally defined by the fiber length and the fiber diameter (Lf /φf , in millimeter unit). Two straight shorter fibers (12/0.18 and 20/0.4) and three longer fibers with hooked 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 ranging between 50 and 66, and a Young modulus of approximately 210 GPa (30456.9 ksi).

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

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

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

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

    Table 1—Composition of concrete matrix Mixture 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 2—Geometrical and mechanical properties of steel fibers

    Fiber code

    Lf , mm (in.) φf , mm (in.) Lf /φf 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 3—Volume fractions of steel reinforcement (steel fibers or welded mesh)

    Slab no.

    Steel fibers Vf,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. 2—Test 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 of the different slabs, as determined on the day of the test; in particular, Table 4 shows the tensile strength fct from cylinders (φc = 80 mm [3.15 in.], L = 250 mm [9.84 in.]), the compressive strength from cubes fc,cube of 150 mm side (5.91 in.); the Young’s modulus as determined from both compression tests on cylinders Ec (φc = 80 mm [3.15 in.]; L = 200 mm [7.87 in.]) and from core specimens Ec,core (φc = 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 always greater 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 under four-point bending according to the Italian Standard22

    (Fig. 2(b)). The notch was placed at midspan and had a depth of 45 mm (1.77 in.) (Fig. 2(b), (d)). These tests were carried out with a closed loop hydraulic testing machine by using the crack mouth opening displacement (CMOD) as a control parameter, which was measured by means of a clip gauge positioned astride the notch. Additional linear variable differential transformers (LVDTs) were used to measure the crack tip opening displacement (CTOD) and the vertical displacement at the beam midspan and under the load points (Fig. 2(b)).

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

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

    To reproduce a Winkler soil, 64 steel supports were placed under the slab at centers of 375 mm (14.76 in.) in both directions (Fig. 3(a)). These supports are steel plates on a square base having a side of 100 mm (3.94 in.; Fig. 3(b)). Previous numerical simulations showed that the experimental subgrade provides a good approximation of a continuous Winkler soil.16 Because of the curling of the concrete slabs due to shrinkage and the thermal effect, a layer of high-strength mortar a few millimeters thick was placed on each spring to ensure the contact with the bottom face of the slab. The average spring stiffness was determined by compression tests performed on each spring with results approximately equal to 11.0 kN/mm (2.47 kips/mm). By considering the infl