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.
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.
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
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
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
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
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)
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