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Flat suspended slabsreinforced only with
macro-synthetic fibres
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Citation: PUJADAS, P. ...et al., 2016. Flat suspended slabs reinforced onlywith macro-synthetic fibres. IN: Banthia, N., di Prisco, M and Soleimani-Dashtaki, S. (eds.) 9th RILEM International Symposium on Fiber ReinforcedConcrete (BEFIB 2016), Vancouver, Canada, Sept 19-21th, pp. 1300 - 1308
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• This is a conference paper.
Metadata Record: https://dspace.lboro.ac.uk/2134/32379
Version: Accepted for publication
Publisher: RILEM Publications S.A.R.L.
Rights: This work is made available according to the conditions of the Cre-ative Commons Attribution-NonCommercial-NoDerivatives 4.0 International(CC BY-NC-ND 4.0) licence. Full details of this licence are available at:https://creativecommons.org/licenses/by-nc-nd/4.0/
Please cite the published version.
Pablo Pujadas, PhD
Universitat Politècnica de Catalunya (UPC-Barcelona Tech)
Jordi Girona 1-3, Building C1-202
08034 Barcelona
Spain
Email: [email protected]
Tel: 0034 93 401 65 30
FLAT SUSPENDED SLABS REINFORCED ONLY
WITH MACRO-SYNTHETIC FIBRES
P. Pujadas 1, A. Blanco 2, S.H.P. Cavalaro 3, A. de la Fuente 4 and A. Aguado 5
1 PhD in Civil Engineering, Universitat Politècnica de Catalunya (UPC-Barcelona Tech), [email protected]. 2 PhD in Civil Engineering, Universitat Politècnica de Catalunya (UPC-Barcelona Tech), [email protected] 3 Associate Professor, Universitat Politècnica de Catalunya (UPC-Barcelona Tech), [email protected]. 4 Associate Professor, Universitat Politècnica de Catalunya (UPC-Barcelona Tech), [email protected]. 5 Full Professor, Universitat Politècnica de Catalunya (UPC-Barcelona Tech), [email protected].
ABSTRACT
Despite the remarkable advances in the field of macro-synthetic fibre reinforced concrete, the successful
use of plastic macro-fibres as the sole reinforcement in structural elements still requires thorough
research. Several questions hindering its wide-spread use have yet to be addressed. Do plastic fibres
guarantee a clearly defined structural reliability? Is there any influence of the formwork and casting
method on the orientation of this fibres? Does this orientation affects the structural behavior of the
slabs? To answer these questions this paper focuses on the study of the structural response of hyperstatic
concrete flat suspended slabs with different width/length ratio reinforced only with structural plastic
macro-fibres. First, the experimental program is described and then the structural results obtained are
presented. Then, a study of the influence of flowability and wall-effects of the formwork in the
orientation pattern of macro-plastic fibres is carried out in order to identify the preferential orientation
of fibres caused by the geometry of slabs.
Keywords: Fiber reinforced concrete, Macro-Synthetic Fibres, Slabs, Fiber orientation
1. INTRODUCTION
Plastic fibre-reinforced concretes (PFRC) attracted the attention of researchers during the first half of
the 1960s. Although they were not studied in the same depth as steel fibres, technical reports and studies
in the literature demonstrate significant improvements in the flexural strength, toughness and fatigue
behaviour with the addition of synthetic macro-fibres [1-3]. For this reason, synthetic macro-fibres are
used in the concrete industry with a purpose beyond the mere control of plastic shrinkage. Instead, these
fibres have been used in applications with a structural responsibility [4-8].
Despite the remarkable advances that have been attained in the field of PFRC, the successful use of
plastic macro-fibres as the sole alternative to conventional reinforcements in structural elements
requires thorough research on the material behaviour, analysis models, verification and structural
reliability. Thus, the main objective of this paper is to present the structural results of hyperstatic slabs
reinforced only with structural plastic macro-fibres [7, 8] and its fiber distribution/orientation [9].
2. EXPERIMENTAL CAMPAIGN
2.1. Elements tested
The elements tested in this experimental program were PFRC slabs without conventional reinforcement
of variable geometric shapes, simply supported on the four sides (see Fig 1). The length (blong) and the
height (h) of the element are fixed at 3.00 and 0.20 m, respectively. In order to evaluate the size effect
on the structural response, slabs with three different widths (bshort) were produced: 1.5 𝑚, 2.0 𝑚
and 3.0 𝑚. To distinguish between the different elements tested, the nomenclature L, M and S are used
for slabs with dimensions of 3.0 m x 3.0 m, 2.0 m x 3.0 m and 1.5 m x 3.0 m, respectively. These
designations are accompanied by the letters A or B, which identify the two slabs tested with each width
(i.e., L_A).
Figure 1. Dimensions of the supports for the slabs). (a) L_A or L_B; (b) M_A or M_B and (c) S_A or S_B.
2.2. Materials, dosages and production
Three batches (namely Series 1, Series 2 and Series 3) with the same mix proportion were required to
cast all slabs. The concrete mix was designed to obtain a fluid concrete with characteristics close to
self‐compactability. The concrete was produced with the following mixing process: initially the dried
components were mixed for one minute, subsequently the water was added and the mixing proceeded
for two minutes, afterwards the superplasticizer was added and finally the fibres were included. After
that, the concrete was mixed for two additional minutes. For this study, a reference concrete mix with
a) b) c)
a water/cement ratio of 0.50 was used (see Table 1) and a the plastic macrofibre content was 9 𝑘𝑔/𝑚3.
Both parameters were fixed in order to avoid introducing more variables to the study. The details of the
concrete mix are presented in Table 1.
Table 1. Composition of the FRC mixture (in kg/m3)
The plastic macro-fibres used in the tests were derived from polyolefin (specifically from
polypropylene) with a continuously embossed surface texture to improve adherence. The main
characteristics of the fibre used are presented in Table 2.
Table 2. Fibre characteristics (data provided by the manufacturer)
The casting followed a specified procedure for uniformity, due to its influence on the orientation of the
fibres. The concrete was poured through a cupola from the centre of the slab at an approximate height
of 1 m. During the manufacturing, a table vibrator was used to ensure proper compaction, although the
vibrations were not excessively energetic. The concrete did not show signs of segregation and the plastic
fibres were visible at the free-flowing edges of the poured mass. The average results at 28 days for the
elastic modulus (𝐸𝑐𝑚) and the compressive strength (𝑓𝑐𝑚) tests, and the coefficient of variation (CV)
for the three studied sets, are presented in Table 3. The proportionality limits (𝑓𝐿) and values of the
residual flexural tensile strenght 𝑓𝑅1, 𝑓𝑅2, 𝑓𝑅3 and 𝑓𝑅4 can also be found in this table, which correspond
to the different crack mouth opening displacement (𝐶𝑀𝑂𝐷) of 0.05 𝑚𝑚, 0.5 𝑚𝑚, 1.5 𝑚𝑚, 2.5 𝑚𝑚
and 3.5 𝑚𝑚, respectively, that were obtained with the flexural tension test as specified by the EN
14651:2005 standard.
Table 3. Characterisation of the PFRC at 28 days
Series 1 Series 2 Series 3
Average
[MPa]
CV
%
Average
[MPa]
CV
%
Average
[MPa]
CV
%
𝑬𝒄𝒎 32,000 0.74 33,200 1.49 29,900 5.65
𝒇𝒄𝒎 46.40 2.44 47.99 1.85 50.54 2.41
𝒇𝑳 4.82 7.71 4.83 7.07 4.34 10.92
𝒇𝑹,𝟏 3.56 8.47 3.04 11.95 2.96 31.01
𝒇𝑹,𝟐 4.70 6.05 3.94 11.62 3.72 34.67
𝒇𝑹,𝟑 5.19 4.81 4.36 11.38 4.54 36.0
𝒇𝑹,𝟒 5.23 5.17 4.42 10.60 4.61 34.56
SLABS L_A and M_A L_B and M_B S_A and S_B
Material Characteristics Dosage
Gravel (6/15 mm) Granite 520
Gravel (2,5/6 mm) Granite 400
Sand (0/3 mm) Granite 510
Concrete CEM I 52,5 R 350
Filler Marble powder 300
Water - 178
Superplasticizer Adva® Flow 400 12
Fibres 9
Material [-] Polyolefin
Length [mm] 48
Cross-section [mm] 0,8 x 1,1
Equivalent aspect ratio [-] 44
Tensile strength [MPa] 550
Elasticity modulus [GPa] 10
Number of fibres per kg [fibres] > 35,000
2.3. Test configuration
Tests performed in the laboratory at UPC consisted of the application of a point load; the slabs were
supported only along the central part of each edge (the central 2/4) simulating a hyperstatic support
configuration that should allow a redistribution of moments and the contribution of fibres in more than
one direction. The tests were performed with a MTS pseudo-dynamic servo-actuator with 1,000 𝑘𝑁 of
maximum compression capacity and a 500 𝑚𝑚 piston stroke that was end-coupled to a loading frame
structure by a three-dimensional ball-and-socket joint. During the course of the test, the load application
was controlled by displacement (see Fig. 2).
Figure 2. Test set-up [7, 8]
The loading process was divided in two stages, a proper loading speed for the set objective was defined
for each stage. In the first stage, a low displacement speed was used to permit the emergence and
propagation of cracks. Once the main cracks were established, a higher loading displacement speed was
imposed to evaluate the behaviour of slabs that had reached high displacement values. In addition to
the load and the piston displacement, the deflection was measured at different points.
3. ANALYSIS OF THE MECHANICAL BEHAVIOUR
3.1. Structural analysis
All slabs presented a ductile behaviour showing a great capacity to redistribute forces and keeping high
load levels after cracking. Figure 3, shows the load-displacement curves in the centre of the slab; and
the absorbed energy- equivalent rotation are presented for the six tested slabs. The energy absorption
capacity was analysed by calculating the area enclosed by the load‐deflection curves presented in this
work. As shown in Figure 3, the results corresponding to slabs of the same size vary only slightly, thus
showing low values of scatter and indicating that the test results are reproducible.
The smaller the 𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ ratio, the higher the 𝐹𝑚𝑎𝑥 obtained in the test, and the smaller the rotation
capacity. A mean 𝐹𝑚𝑎𝑥 of 171 𝑘𝑁 was obtained for the 3.0 𝑚 𝑥 3.0 𝑚 slabs (𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ =
1.00), 206 𝑘𝑁 for the 2.0 𝑚 𝑥 3.0 𝑚 slabs (𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ = 0.66) and 277 𝑘𝑁 for the 1.5 𝑚 𝑥 3.0 𝑚
slabs (𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ = 0.50).
The large slab (𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ = 1.00) was more ductile than the rest of the slabs. Once the main cracks
appeared, the fibre contribution permitted a redistribution of stresses and the emergence of new
secondary cracks. Consequently, the slab maintained a nearly constant load level until reaching
deflection levels of 70 𝑚𝑚 (𝐹𝛿=5𝑚𝑚 ≈ 𝐹𝛿=15𝑚𝑚 ≈ 𝐹𝛿=35𝑚𝑚 ≈ 𝐹𝛿=55𝑚𝑚) while maintaining
practically flat slopes (𝑚𝑓𝑖𝑛𝑎𝑙 ≈ 0𝑘𝑁/𝑚𝑚). However, the slabs became more rigid as the transversal
dimension (𝑏𝑠ℎ𝑜𝑟𝑡) decreased. Once the slabs reached the maximum load value, the fibres could not
a) b)
maintain the loads and losses in load-bearing capacity were accelerated, especially for slabs
with 𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ = 0.50.
Figure 3. Load-displacement curves at the centre of the slab (and equivalent rotation) and absorbed energy
displacement at the centre of the slab (and equivalent rotation) [7]
3.2. Assessment of fibre orientation
3.2.1. Core drilling strategy
In an effort to make a complete characterization of the fibre orientation in the slabs cylindrical 225x200
mm cores were drilled from the slabs: L; M and S (see Figure 4).
Figure 4. Slab M after drilling the cores
Given the importance of knowing the exact location and orientation of the cores with regards to the
slabs, a specific notation was assigned to each side of the slab: north (N), east (E), south (S) and west
(W). Before the drilling, the same notation was used to mark the cores that together with the location
map allows identifying accurately their location and orientation with regards to the sides of the slab.
Figure 4 shows a slab after drilling the cores. Each pair of drilled cores had to fulfil the following
criteria:
• being symmetric two to two (in order to test each core in each of the two directions N-S and E-
W),
• being representative of different boundary conditions
Thus, the amount of cores with a diameter of 225 mm drilled from slabs L, M and S were 16, 16 and
14, respectively. Each pair of cores was designated according to the notation PiA and PiB, in which i
represents a reference number (see Figure 5).
0
50
100
150
200
250
300
0 10 20 30 40 50 60 70 80
Lo
ad
[k
N]
Midspan displacement [mm]
L_A_3,0mx3,0m L_B_3,0mx3,0mM_A_2,0mx3,0m M_B_2,0mx3,0mS_A_1,5mx3,0m S_B_1,5mx3,0m
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
0 0.01 0.02 0.03 0.04 0.05 0.06
Ab
sorb
ed
en
erg
y [
kN
.mm
]
Equivalent rotación (c3m) [-]
L_A_3,0mx3,0mL_B_3,0mx3,0mM_A_2,0mx2,0mM_B_2,0mx3,0mS_A_1,5mx3,0mS_B_1,5mx3,0m
Figure 5. Location map of the cylindrical cores (dimensions in mm) for the slabs: a) L; b) M and c) S [9]
Then, the cores were cut into 150 mm cubic specimens required for the MDPT tests, as indicated in
Figure 6. The cuts were made so that the sides of the resultant cubes were parallel to the sides of the
slabs. The references regarding the orientation of the cubic specimen in the slab are marked in its sides
(W‐E and N‐S).
Figure 6. Details of the cutting procedure to obtain the cubic specimen [9]
3.2.2. Procedure of the test
The MDPT [10] is an indirect tensile test method that allows assessing the toughness and residual tensile
strength of FRC considering the distribution and orientation of fibres. The test setup is based on the
double punch test applied to FRC or Barcelona test [11,12]. According to that, two cylindrical steel
punches arranged concentrically above and below a FRC cubic specimen (with 150mm of edge)
transmit the load applied by the plates of the press that approach each other at a constant relative rate
of 0.5 mm/min. The punch is a metallic cylinder with 24mm of height and 35mm of diameter (1/4 of
the diameter of the largest circle inscribed in the face of the cube).
In the MDPT, the use of three cubic specimens allow three different loading directions, activating
different groups of fibres in each case, and thus assessing three different toughnesses and residual tensile
strengths. Unlike in the MDPT [10], in this research only two directions could be tested. Consequently,
based on the comparison of the post-cracking results obtained in two different specimens (drilled from
a) b) c)
N S
E
W
1st cut
different but analogous position in the slab), the orientation of fibres in the slabs was qualitatively
assessed.
3.2.3. Results of the fibre orientation in the slabs
The results show that plastic fibres also tend to be aligned parallel to the walls or surfaces of the
formwork. Nevertheless, based on the study conducted, the influence of flow in the orientation of
macro-plastic fibre also occurs in areas outside the influence of the formwork walls.
The results reveal that in the case of a free surface flow, the alignment of plastic fibres is mainly
perpendicular to the flow direction. For all the geometries of the slabs cast from the centre, the plastic
fibres are perimetraly aligned. Notice that external vibration facilitates the flow of the concrete, thus
leading to the rotation of the fibres and their alignment in the perpendicular direction to the flow.
As the cross-sectional dimension of the slabs is smaller, the orientation of the fibres is slightly affected
by the confinement effect of the side walls. The reorientation of the plastic fibres in the longitudinal
direction increases as the ratio𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ reduces.
It must be noted that, considering the radial distribution of the cracks, the perimetral distribution of the
fibres is more advantageous than the longitudinal orientation since the fibres are more likely to be
disposed perpendicular to the cracks (see Fig. 7). Therefore, a higher stress bearing capacity is observed
in the sectional level for the slabs a 𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ ratio close to 1.
Figure 7. Fibre orientation scheme depending on the 𝒃𝒔𝒉𝒐𝒓𝒕 𝒃𝒍𝒐𝒏𝒈⁄ ratio [9]
4. CONCLUSIONS
The most relevant conclusions regarding the experimental campaign analysis in this papers on slab-type
elements reinforced only with macro-plastic fibres are the following:
The slabs tested showed a ductile behaviour with great capacity to redistribute forces, keeping a high
load level after cracking. This shows the feasibility of PFRC to be used as a building material for
structures such as slab under hyperstatic configuration.
The orientation of macro-plastic fibres in slabs with different 𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ ratios was experimentally
assessed using the post-cracking results of the MDPT developed by Pujadas et al [10]. The results show
that plastic fibres tend to be oriented parallel to the walls or surfaces of the formwork. However, the
influence of the flow in the fibre orientation also takes place in areas far from the influence of the
formwork walls.
In this sense, the analysis conducted in this research shows that for free surface flow, the alignment of
the fibres is mainly perpendicular to the flow direction. The orientation of the fibres perpendicular to
the flow of concrete is slightly redistributed by the confinement effect of the side walls as the transverse
dimension of the slabs is reduced. In other words, the reorientation of the fibres in the longitudinal
direction increases as the width of the slab tends to lower 𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ ratios.
Finally, the perimetral orientation of the fibres together with the radial distribution of the cracks indicates that in elements with 𝑏𝑠ℎ𝑜𝑟𝑡 𝑏𝑙𝑜𝑛𝑔⁄ ≈ 1 (where the fibres are more prone to orient
themselves perpendicularly to the crack), the contribution of plastic fibers is more significant.
ACKNOWLEDGMENTS
The authors of this document wish to show their gratitude for the economic support received through
the Research Project BIA2010-17478: Construction Processes by means of Fibre Reinforced Concretes.
The authors thank the company ESCOFET S.A. for their collaboration in the development of the
experimental program. The first author acknowledges the grant PDJ provided by the Departament
d’Universitats, Recerca i Societat de la Informació de la Generalitat de Catalunya.
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