Energy dissipation capacity of concretes reinforced with R-PET fibers

Abstract

We investigate on the strength and ductility properties of recycled PET fiber-reinforced concretes
(RPETFRCs) showing different mix-designs and PET filaments with variable mechanical and geometric properties.
Available literature results of compression tests and four-point bending tests on such materials are reviewed comparing laboratory results in terms of compressive strength, first crack strength and ductility indices. The examined tests highlight that most relevant effects of the recycled PET fiber reinforcement are concerned with material toughness and ductility. In the case of low-strength concretes, significant compression and flexural strength enhancements due to the addition of recycled PET fibers are also observed.

Full text

61
Anno XXXI – N. 2 – aprile-giugno 2014
Energy Dissipation Capacity of Concretes Reinforced with
Recycled PET Fibers
Saverio Spadea*, Ilenia Farina*, Valentino P. Berardi*, Fabio Dentale*, Fernando Fraternali*
SUMMARY– We investigate on the strength and ductility properties of recycled PET fiber-reinforced concretes
(RPETFRCs) showing different mix-designs and PET filaments with variable mechanical and geometric properties.
Available literature results of compression tests and four-point bending tests on such materials are reviewed compar-
ing laboratory results in terms of compressive strength, first crack strength and ductility indices. The examined tests
highlight that most relevant effects of the recycled PET fiber reinforcement are concerned with material toughness
and ductility. In the case of low-strength concretes, significant compression and flexural strength enhancements due
to the addition of recycled PET fibers are also observed.
Keywords: PET, FRC, recycling, compressive strength, first crack strength, structural ductility
1. Introduction
Fiber-reinforced concretes (FRCs) are being used
increasingly for the construction of new structures
with energy dissipation capabilities, and the retrofit-
ting of existing constructions. It has been recogni-
zed that plastic filaments extruded from polyethylene
terephthalate flakes recovered from post-consumer
bottles (R-PET) are well suited to manufacture eco-
friendly and crack-resistant mortars and concretes,
which can be profitably used to manufacture indu-
strial floors, tunnel coatings, and for structural re-
trofitting [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]. The
effects of the addition of R-PET fibers to cementitious
materials with different mix-designs are not fully un-
derstood at present, but there is some evidence that
highest strength enhancements are produced by adding
R-PET fibers to low-strength concretes, while ductility
improvements are more uniform over concretes with
different water/cement ratios and particle sizes [1] [2]
[3] [4]. We deal in the present work with a review
of available literature results on the concrete rein-
forcement with R-PET fibers. We examine different
mix-designs and fiber properties, focusing our atten-
tion on the compressive strength, first crack strength,
energy absorption capacity and ductility indices of
RPETFRCs. Our final goal is to investigate on how
the effects of the R-PET reinforcements vary with
the nature of the binder, the water/cement ratio and
the geometrical and mechanical fiber properties. We
carry out such a comparative analysis by examining
experimental results recently presented in [1] [2], and
previous results given in [3] [4]. We begin in Sect.
2 by diffusely describing the examined RPETFRCs.
Next, we review the experimental setup and the out-
comes of the examined laboratory tests (Sect. 3). We
end in Sect. 4 with a summary of the present study
and guidelines for future research.
2. Materials
We examine concretes reinforced with plastic fi-
bers produced through industrial extrusion lines in the
plants of Techno Plastic (TP) S.r.l. (Castelfranco Emi-
lia, Modena, Italy) and FHP S.a.s. – Plastic Division
(Roncello, Milan, Italy). The examined fibers include
several plastic filaments obtained through R-PET fla-
kes extrusion lines, which show different mechanical
properties, cutting and aspect, as well as polypropylene
(PP) filaments extruded from virgin material (cf. Tab.
1 and Fig. 1).
Concrete specimens were prepared using the mix-
designs shown in Tab. 2 and Tab. 3, which respec-
tively make use of the Portland limestone cement
CEM II/A-LL 32.5 R and the pozzolanic cement
CEM IV/B 32.5 R (European standard UNI EN 197-1
[11]). Hereafter, we name UNRC1 and UNRC2 the
unreinforced concretes realized according to the mix-
designs given in Tabs. 2 and 3, respectively. Simi-
* University of Salerno, Department of Civil Engineering, 84084 Fi-
sciano (SA), Italy
Corresponding author Fernando Fraternali

62 Anno XXXI – N. 2 – aprile-giugno 2014
Tab. 1. Properties of PET and PP fibers examined in the present study.
Proprietà delle fibre di PET e di PP prese in considerazione in questo studio.
Property PET/a PET/b PET/c PP
Specific gravity 1.34 1.34 1.34 0.90
Cross section Circular Circular Circular Oval
Aspect Straight Straight Crimped Embossed
Diameter (mm) 1.10 0.70 0.70 0.80 x 1.30
Length (mm) 40 52 52 47
Tensile strength (MPa) 550.00 263.72 274.29 250.00
Ultimate strain (%) 27 26 19 29
Fig. 1. Illustrations of the examined PET and PP fibers [2].
Illustrazione delle fibre di PET e di PP.
Tab. 2. Mix design of UNRC1 and RPETFRC1 [1].
Composizione del conglomerato UNRC1 e RPETFRC1.
Concrete
Type
Coarse
Aggregate
(10-20 mm)
Medium
Aggregate
(4-10 mm)
Sand
(0-4 mm)
CEM II
A-LL 32.5
Fluidizing
Additive
SKY 624
Water
Water/
Cement
Ratio
Fibers
Portland cement CEM II/A-LL 32.5 R Kg/m3Kg/m3Kg/m3Kg/m3Kg/m3Lt./m3Kg/m3
UNRC1 605.0 170.0 944.1 496.0 4,35 187.9 0.38 –
RPETFRC1/a 605.0 170.0 944.1 496.0 4,35 187.9 0.38 13.4
RPETFRC1/c 605.0 170.0 944.1 496.0 4,35 187.9 0.38 13.4

63
Anno XXXI – N. 2 – aprile-giugno 2014
reinforcement at 1% fiber volume fraction of ordinary
Portland cement-based concretes with water/cement
ratio equal to 0.41 (always comparing to UNRC). It
is possible to recognize a general trend indicating a
decrease of the RPETFRC compressive strength with
the water/cement ratio, at constant fiber volume ratio
(1%). This might imply that the beneficial effects of the
R-PET reinforcement in terms of compressive strength
are more pronounced in presence of low strength class
concretes.
Let us now pass to examine the results of the four
point bending tests presented in [1], which were
conducted on three prismatic 150 mm x 150 mm x
600 mm specimens of UNRC1, RPETFRC1/a and
RPETFRC1/c after 28 curing days, according to the
Italian standards UNI 11039-1 [13], and UNI 11039-2
[14] (cf. Tabs. 6 and 7). Each specimen was prelimi-
nary notched at the midspan (notch width 2 mm at the
mouth; notch depth: a0 = 45 mm [14]). The crack tip
opening displacement (CTOD) was measured through
two displacement transducers placed on the opposite
faces of the specimen (denoted as CTOD1 and CTOD2
in Fig. 2), in correspondence with the crack tip. He-
reafter, we denote the mean value the displacements
measured by such transducers by CTODm (mean crack
tip opening displacement). The midspan deflection d
was measured through a vertical displacement transdu-
cer (denoted as DT in Fig. 2). A 50 kN load cell
was used to measure the total load P applied to the
top surface of the specimen. The test setup and the
employed instrumentation are shown in Fig. 2. The
results of similar four point bending tests performed
on UNRC2, RPETFRC2 and PPFRC specimens are
given in Tab. 8 [2].
Tab. 6 provides the individual values of the peak
load Pmax; the CTODm corresponding to Pmax; the
first-crack load PIf; and the first-crack strength fIf,
which were observed for each different specimen of
UNRC1, RPETFRC1/a and RPETFRC1/c, together
with the mean values of Pmax and fIf. According to
UNI 11039-2 [14], we let CTOD0 denote the mean
value of CTODm
Pmax for the UNRC (plain concrete). In
addition, we define PIf as the maximum load obser-
ved in the CTODm range [0, CTOD0]. The first crack
strength was computed according to the following
formula (UNI 11039-2 [14])
()
fbh a
Pl
0
2
lf
lf
=- (1)
where l is the clear span of the specimen, while b and
h are the width and the height of the specimen cross-
section, respectively (cf. Fig. 2).
By analyzing the results presented in Tab. 6, we no-
tice a slight decrease of Pmax(–4.34%) and fIf(–2.65%)
in RPETFRC1/a, as compared to UNRC1. Differently,
we record remarkable increases of Pmax approximately
show twice greater tensile strength than PET/c fibers
(550 MPa vs 274 MPa). The results in Tab. 6 suggest
that the waviness of the fiber profile might play a gre-
ater role than the tensile strength, for what concerns
larly, we name RPETFRC1 and RPETFRC2 the fiber
reinforced concretes based on such mix designs and
the PET/a,b,c fibers shown in Tab. 1, at 1% volume
content. Finally, we name PPFRC the fiber reinfor-
ced concrete using the mix design in Tab. 3 and the
PP fibers described in Tab. 1 (1% volume content).
It is worth noting that UNRC1 has a considerably
smaller water/cement ratio (0.38 vs 0.53), and grea-
ter compressive strength class (C30/37), as compared
to UNRC2 (C25/30, cf. Tabs. 2 and 3). Reinforcing
fibers and concrete were mixed through a concrete
mixer. The specimens were unmolded within 3 days
after casting and cured for a period of 28 days. Ma-
terials and mixing devices were kindly provided by
Calcestruzzi Irpini S.p.A. (Avellino, Italy).
3. Review of Available Experimental Results on
RPETFRCS
Table 4 shows the results of compression strength
tests presented in [1], which were conducted on cu-
bic UNRC1 and RPETFRC1 specimens (three cu-
bes with 150mm side for each examined material),
in accordance with the European standard UNI EN
12390-1 [12]. The results in Tab. 4 provide the spe-
cific gravity and cube compression strength (fc, cube) of
each tested specimen. The same table also provides
the mean value ( f,ccube
r
), the standard deviation and
the coefficient of variation of (fc, cube) for UNRC1,
RPETFRC1/a and RPETFRC1/b. Analogous results
on UNRC2, RPETFRC2 and PPFRC specimens are
presented in Tab. 5, which provides 95% Confidence
Intervals (CI) and percentage variations of (fc, cube) over
UNRC2 (FRR, cf. [2]).
The results in Tab. 4 highlight that the UNRC1,
RPETFRC1/a and RPETFRC1/c exhibit f,ccube
r
, equal
to 42.4 MPa, 40.0 MPa and 38.9 MPa, respectively,
after 28 days of curing. We observe a reduction of
f,ccube
r
, equal to 5.66% in RPETFRC1/a and 8.25%
in RPETFRC1/c, as compared to UNRC1. Such re-
sults significantly deviate from those competing to
RPETFRC2, which instead shows 35.14% and 0.03%
increments of f,ccube
r
in RPETFRC2/a and RPETFRC2/c,
respectively, against UNRC2. Ochi et al. [3] recorded
8.41%, 13.79% and 5.99% increases of f,ccube
r
, in the
case of R-PET reinforcements at 1% fiber volume
fraction of concretes with water/cement ratios equal
to 65%, 60% and 55%, respectively; while Kim et al.
[4] observed a 7% decrease of f,ccube
r
, or the R-PET
Tab. 3. Mix design of UNRC2 and RPETFRC2 [2].
Composizione del conglomerato UNRC2 e RPETFRC2 [2].
Component Dosage (kg/m3)
Pozzolanic Cement CEM IV/B 32.5 R 340
Sand (0-4mm) 923
Medium aggregate (4-10 mm) 185
Coarse aggregate (10-20 mm) 743
Water 181
Water/cement ratio (%) 53
Fluidifying agent 2.4

64 Anno XXXI – N. 2 – aprile-giugno 2014
Tab. 4. Results of compression strength tests on RPETFRC1 and UNRC1 [1].
Risultati dei test di resistenza a compressione per RPETFRC1 e UNRC1.
Specimen ID Specic Gravity Cube compression strength fc, cube
Specimen strength Mean Value Standard Deviation Coefcient of Variation
[Kg/m3] [MPa] [MPa] [MPa] [%]
UNRC1 -1 2222 40.6
42.4 1.6 3.85
UNRC1-2 2311 42.6
UNRC1-3 2207 43.9
RPETFRC1/a-1 2299 40.6
40.0 0.8 1.94RPETFRC1/a-2 2258 39.1
RPETFRC1/a-3 2214 40.3
RPETFRC1/b-1 2228 37.6
38.9 1.6 4.02
RPETFRC1/b-2 2252 38.5
RPETFRC1/b-3 2240 40.6
Tab. 5. Results of compression strength tests on UNRC2, RPETFRC2 and PPFRC [2].
Risultati dei test di resistenza a compressione per RPETFRC2 e UNRC2 [2].
Material # Specimens Specic gravity Compressive strength MPa
fc, cube 95%Cl FRR(%)
UNRC2 8 2.27 31.50 4.85 0.00
RPETFRC2/a 6 2.32 42.57 2.72 +35.14
RPETFRC2/b 6 2.31 38.44 3.16 +22.03
RPETFRC2/c 6 2.28 31.51 1.69 +0.03
PPFRC 6 2.30 36.80 4.91 +16.83
Fig. 2. Four point bending test setup and instrumentation.
Configurazione e strumentazione di prova a flessione.
Tab. 6. Results of four point bending tests on UNRC1 and RPETFRC1 [1].
Risultati di prove a essione su UNRC1 e RPETFRC1 [1].
Specimen ID Pmax CTODm at Pmax Mean Value of Pmax CTOD0PIf fIf Mean Value of fIf
[N] [mm] [N] [mm] [N] [MPa] [MPa]
UNRC1-1 13366 0.084
13910 0.077
13289 3.6
3.77
UNRC1-2 15241 0.079 15144 4.1
UNRC1-3 13124 0.069 13124 3.6
RPETFRC1/a-1 13813 0.076
13331 0.077
13813 3.8
3.67
RPETFRC1/a-2 12268 0.078 12218 3.3
RPETFRC1/a-3 13913 0.063 13919 3.8
RPETFRC1/c-1 13591 0.079
15181 0.077
13520 3.7
4.07
RPETFRC1/c-2 16740 0.086 16680 4.5
RPETFRC1/c-3 15212 0.084 14720 4.0

65
Anno XXXI – N. 2 – aprile-giugno 2014
CTOD0 + 3mm] of the P – CTODm curves, respectively
(UNI 11039-2 [14])
()
()
UPCTODdCTOD
UPCTODdCTOD
.
2
0.6
3
mm
CTOD
CTOD mm
mm
CTOD mm
CTOD mm
1
06
0
0
0
0
=
=
+
+
+
#
#
(2)
According to UNI 11039-2 [14], we characterize the
energy absorption capacity of the examined materials
through the following “ductility indices” D0 and D1
Df
fDf
f
(.)
(.
)
(. )
lf
eq
lf
eq
0
006
1
006
06 3
==
-
-
- (3)
where feq(0 – 0.6) and feq(0.6 – 3) are the following
equivalent stresses associated with the CTODm ran-
ges [CTOD0, CTOD0 + 0.6mm] and [CTOD0 + 0.6mm,
CTOD0 + 3mm], respectively
()
.
()
.
fbh a
lU
fbh a
lU
06
24
(.)
(. )
eq
eq
006
0
2
1
06 3
0
2
2
=-
=-
-
-
(4)
It is worth noting that the above ductility indices
measure the ratios between the equivalent strengths
of the material in two different post-crack regimes
([CTOD0, CTOD0 + 0.6mm] and [CTOD0 + 0.6mm,
CTOD0 + 3mm], respectively) and the first-crack
strength, which can be understood as dimensionless
indicators of the nature of the post-crack response
(or “ductility class”) of the material (“hardening”
the bending response of RPETFRC1 (we remind that
PET/c fibers have crimped profile, while PET/a fibers
have straight profile, cf. Tab. 1). An opposite trend
is instead observed in Tab. 8, for RPETFRC2. As a
matter of fact, the flexural strength of RPETFRC2/a
is about 40% greater than that of UNRC2, while the
flexural strength of RPETFRC2/c is about 8% greater
than the fIf of UNRC2 [2]. Interestingly, the increase
in fIf of RPETFRC1/c (over UNRC1) remains appro-
ximately the same in RPETFRC2/c. Ochi et al. [3]
recorded 7.85%, 2.18% and 15.20% increases of the
bending strength of fiber reinforced concretes with
1% R-PET fiber volume fraction and water/cement
ratios equal to 65%, 60% and 55%, respectively (over
UNRC). Kim et al. [4] observed a 32% increase in the
value of Pmax of 200mm x 300mm x 2000mm concrete
specimens reinforced by R-PET fibers at 1% volume
fraction plus steel rebars, as compared to concrete
specimens reinforced by rebars only (mix-design ba-
sed on an ordinary Portland cement and 0.41 water/
cement ratio). Interestingly, the representative load-
deflection curve of concrete specimens reinforced by
rebars only shows slightly greater turning point (Pcr),
as compared to the representative load-displacement
curve of specimens reinforced by 1% R-PET fiber
volume fraction plus steel rebars (cf. Tab. 4 and Fig.
11 of [4]).
Fig. 3 shows the P – CTODm curves that were obtai-
ned for RPETFRC1/a and RPETFRC1/c specimens,
respectively [1]. Fig. 4 illustrates the analogous cur-
ves obtained for RPETFRC2/a,b,c and PPFRC [2].
The results presented in [1][2] point out that both
UNRC1 and UNRC2 exhibit brittle failure with prac-
tically zero energy absorption capacity after crack
onset (first-crack load). Regarding the post-crack be-
havior, we introduce the following fracture energies,
U1 and U2, which are associated with the CTODm ran-
ges [CTOD0, CTOD0 + 0.6mm] and [CTOD0 + 0.6mm,
Tab. 7. Energy absorption capacities and ductility indices of RPETFRC1 specimens [1].
Capacità di assorbimento di energia e indici di duttilità per provini RPETFRC1 [1].
Specimen ID [CTOD0, CTOD0 + 0.6mm] [CTOD0 + 0.6mm, CTOD0 + 3mm] Total Post-Crack Energy
U1[Nmm] feq(0 – 0.6)[MPa] D0U2[Nmm] feq(0.6 – 3) [MPa] D1U1 + U2[Nmm]
RPETFRC1/a-1 3217 1.5 0.4 11608 1.3 0.4 14825
RPETFRC1/a-2 3338 1.5 0.5 10517 1.4 0.4 13855
RPETFRC1/a-3 4530 2.1 0.5 17049 2.5 0.5 21579
RPETFRC1/c-1 3443 1.6 0.4 12530 1.8 0.4 15973
RPETFRC1/c-2 2273 1.0 0.2 11334 1.6 0.3 13607
RPETFRC1/c-3 2805 1.3 0.3 12521 1.8 0.4 15326
Tab. 8. Results of four point bending tests on UNRC2, RPETFRC2 and PPFRC [2].
Risultati di prove a essione su UNRC2, RPETFRC2 e PPFRC [2].
Material First crack strength D0D1Class
fIf (MPa) fIf-FRR(%)
UNRC2 3.39 0 0.71 0.09 DS0
RPETFRC2/a 4.78 +41.00 0.82 0.68 DS1
RPETFRC2/b 3.46 +2.06 0.77 0.45 DS0
RPETFRC2/c 3.65 +7.67 0.95 0.58 DS1
PPFRC 3.73 +10.03 0.92 0.73 DS2

66 Anno XXXI – N. 2 – aprile-giugno 2014
of D0 and D1 equal to 0.30 and 0.37, respectively
(cf. Fig. 3). RPETFRC2/a instead shows D0 = 0.82
and D1 = 0.68, while RPETFRC1/c features D0 = 0.95
and D1 = 0.58 (Tab. 8). It is evident that RPETFRC2
exhibits much lower ductility indices (and, conse-
quently, much lower fracture toughness), as compared
to RPETFRC1. We also notice that the RPEFTRC/a
exhibits higher ultimate ductility than RPETFRC/c,
both in correspondence with the mix-design of Tab.
2, and in correspondence with the mix-design of Tab.
3 (cf. Figs. 3 and 4). This can be explained by a
fiber-debonding-type failure mode of RPETFRC/a
(due to the high strength and straight aspect of PET/a
fibers, which are expected to not break up to mate-
rial failure), and a fiber-rupture-type failure mode of
RPETFRC/c (due to the relatively low tensile strength
of PET/c fibers, and the enhanced bonding strength
induced by the crimped aspect of such fibers). Ochi
et al. [3] measured relative energy capacities (aver-
age energies absorbed by the analyzed RPETFRCs
divided by the average energy absorbed by the unre-
inforced concrete) equal to 5.14, 5.70 and 5.95 for
RPETFRCs with 1% R-PET fiber volume fraction and
water/cement ratios equal to 65%, 60% and 55%, re-
spectively. Kim et al. [4] observed relative energy
capacity equal to 4.34 for concrete specimens rein-
forced by 1% R-PET fiber volume fraction and steel
rebars (ordinary Portland cement-based concrete with
0.41 water/cement ratio).
4. Concluding remarks
We have reviewed the results of recent experimental
studies on the mechanical properties of different con-
behavior corresponds to ductility indices greater
than one; while “softening” behavior corresponds to
ductility indices smaller than one). In particular, D0
characterizes the “ductility” of the material (defined
as above) in the CTODm range that immediately fol-
lows the crack onset (“first crack ductility”), while
D1 characterizes the “ductility” in a heavily cracked
regime (“ultimate ductility”). Tab. 7 summarizes
the results obtained in terms of energy absorption
capacities and ductility indices, for each of the ex-
amined RPETFRC1 specimens [1]. It is easy to rec-
ognize that RPETFRC1/a exhibits average values of
D0 and D1 that are equal to 0.47 and 0.43, respec-
tively, while RPETFRC1/c exhibits average values
Fig. 3. Total applied load vs CTDOm curves for RPETFRC1/a and RPETFRC1/c specimens [1].
Carico totale applicato in funzione di CTDOm per provini RPETFRC1/a e RPETFRC1/c [1].
Fig. 4. Total applied load vs CTDOm curves for UNRC2, RPETFRC2
and PPFRC specimens [2].
Carico totale applicato in funzione di CTDOm per provini UNRC2,
RPETFRC2 e PPFRC [2].

67
Anno XXXI – N. 2 – aprile-giugno 2014
of RPETFRC structural members, under service and
ultimate loads.
Acknowledgments
The authors gratefully acknowledge the great techni-
cal support received by Mauro Mele and Calcestruzzi
Irpini S.p.A. during the course of the present work.
FF also acknowledges financial support from the Ital-
ian Network of Seismic Engineering Laboratories (Re-
LUIS-DPC grant 2014-2016).
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3: Compressive strength of test specimens; 2009.
cretes reinforced with recycled PET fibers at 1% fiber
volume content, examining the compressive strength,
first-crack strength, energy absorption capacity and
ductility indices of the examined materials. An exten-
ded comparative study based on the results given in
[1] [2][3][4] has allowed us to draw some conclusions
about to-date available results on the mechanical per-
formance of concretes reinforced by R-PET fibers.
All the results taken into account in the present
work point out that the effects of the R-PET reinfor-
cement are highly beneficial in terms of energy ab-
sorption capacity of the material. As a matter of fact,
the outcomes of the laboratory tests given in [1][2][3]
[4] indicate that the addition of R-PET fibers to the
mix design dramatically increases the relative energy
capacity of concrete (from 400% up to 700% increases
in the energy absorption capacity). We are therefore led
to conclude that RPETFRCs might be a good match for
earthquake resistant structures.
More complex is the analysis of the material re-
sponse in terms of compressive strength and first-crack
strength. The results presented in Sect. 3 allow us to
recognize a general trend indicating the reduction of
the compressive strength of RPETFRC with the wa-
ter/cement ratio for fixed fiber volume content (1%).
We also record reductions of the first-crack strength
with the water/cement ratio in RPETFRCs showing
the same reinforcing fibers and two different mix-de-
signs (Tabs. 2 and 3). In particular, for one particular
material (RPETFRC1/a), we have observed a slightly
negative effect of R-PET fibers in terms of first-crack
strength, in presence of Portland limestone cement
and 0.38 water/cement ratio. A comparative analysis
between the results given in [1] and [2] also indicates
that high tensile strength R-PET fibers (PET/a) ap-
pear to be the most beneficial in terms of compressive
and tensile properties of the final material in the case
of pozzolana cement-based concretes with high wa-
ter/cement ratio (0.53), while, on the contrary, fibers
with crimped aspect and relatively low tensile strength
(PET/c) appear to be the most beneficial in terms of
compressive strength and first-crack strength of Port-
land limestone cement-based concretes with low water/
cement ratio (0.38). Overall, we are led to conclude
that the addition of R-PET fibers to cementitious ma-
terials needs to be accurately tailored to the employed
mixed-design, both for what concerns the choice of the
fiber properties, and in relation to the desired strength
and ductility properties of the final material (cf. also
[10] for what concerns the R-PET reinforcement of ce-
ment mortars). An experimental study on the durability
and seawater curing of RPETFRC has been recently
presented in [1].
We address to future work the mechanical mo-
deling of elastic stability and crack propagation
in RPETFRC structural elements, to be conducted
through free-discontinuity models, the quasiconti-
nuum method and mesh-free approaches [15-30]; as
well as the study of multiaxial prestess [31], and the
formulation of tensegrity models of RPETFRC struc-
tures [32-34]. Such studies are expected to provide
useful information about the load carrying capacity

68 Anno XXXI – N. 2 – aprile-giugno 2014
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ent elastic response in tension and compression”.
Compos Struct, 2013, 100: 280-289.
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“Highly Nonlinear Pulse Splitting and recombi-
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M., Computational Assessment of ballistic impact
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posite plate. Comput. Mech., 43(4), 525-534, 2009.
/29/ Fraternali F., Lorenz C.D., Marcelli G., “On the
estimation of the curvatures and bending rigid-
ity of membrane networks via a local maximum-
entropy approach.”, J. Comput. Phys. 2012, 231:
528-540.
/30/ Ascione L., Mancusi G., Spadea S., “Flexural be-
haviour of concrete beams reinforced with GFRP
bars” Strain 2010, 46 (5): 460-469.
/31/ Fraternali F., Carpentieri G., Palazzo B., “Multia-
xial Prestress of Reinforced Concrete I-Beams”,
Ingegneria Sismica, 2014, 31 (1): 17-30.
/32/ Fraternali F., Senatore L., Daraio C., “Solitary
waves on tensegrity lattices”. J. Mech. Phys. Sol-
ids., 2012, 60: 1137-1144.
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cheletti A., “Minimum mass design of tensegrity
bridges with parametric architecture and multiscale
complexity” Mech. Res. Commun., 2014, 58: 124-
132.
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Skelton R.E., Fraternali F., “Experimental inve-
stigation of the softening-stiffening response of
tensegrity prisms under compressive loading”,
Compos. Struct., 2014, 117: 234-243.
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definitions, classification and designation”, Milan
(Italy), UNI Editions, 2003.
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method for determination of first crack strength
and ductility indexes. Milan (Italy), UNI Editions,
2003.
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els in two-dimensions for in-plane crack problems”.
Theor. Appl. Fract. Mec., 2007, 47: 274-282.
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ture: an eigendeformation approach to variational
fracture”. Multisc. Model. Sim, 2009, 7 (3): 1237-
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vergence of 3d free discontinuity models in vari-
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lumped stress method for plane elastic problems

69
Anno XXXI – N. 2 – aprile-giugno 2014
Capacità di dissipazione energetica di calcestruzzi
rinforzati con fibre PET da riciclo
S. Spadea, I. Farina, V.P. Berardi, F. Dentale, F. Fraternali
Parole chiave: calcestruzzo bro-rinforzato, PET da riciclo, tenacità, resistenza a compressione, resistenza di prima
fessurazione, indici di duttilità.
L’industria edilizia contribuisce notevolmente al
degrado ambientale, anche più del traffico automo-
bilistico e di altre rinomate attività inquinanti, ma gli
imprenditori edili negli ultimi anni hanno fatto passi da
gigante verso una significativa riduzione dell’impatto
ambientale del processo di costruzione. Nel contesto
di un crescente interesse verso il riciclo di materiali
derivanti da rifiuti solidi urbani ed industriali e la co-
siddetta architettura “sostenibile” o “verde”, una sem-
pre maggiore attenzione viene rivolta ai giorni nostri
alla sperimentazione ed allo studio del rinforzo del
cemento armato con aggregati e/o fibre ottenuti dal
riciclo di materiali recuperati dai rifiuti solidi urbani e/o
industriali. Diversi materiali recuperabili dai rifiuti solidi,
quali, ad esempio, le materie plastiche, il vetro, la cel-
lulosa, il legno, ecc., sono dotati di estrema versatilità
e/o peso contenuto, durabilità, resistenza agli attacchi
chimici, eccellenti proprietà di isolamento termico ed
elettrico. Tali proprietà possono essere utilmente sfrut-
tate per costruire nuovi materiali compositi innovativi
ed eco-sostenibili. Particolarmente interessante è il
caso del rinforzo dei conglomerati cementizi (armati
e non) con fibre ricavate da materiali da riciclo, che si
configura come una tecnica di rinforzo a basso costo
in grado di migliorare significativamente la resistenza
meccanica, la duttilità strutturale e l’isolamento termico
della matrice cementizia. Il miglioramento della dutti-
lità è particolarmente significativo nelle zone sismiche,
dove gli edifici e le infrastrutture necessitano di avere
a disposizione una notevole capacità di dissipazione
di energia e di deformazione plastica, soprattutto in
presenza di importanti eventi sismici. D’altra parte, la
ridotta conduttività termica del calcestruzzo rinforzato
con fibre plastiche da riciclo, rispetto ad un calce-
struzzo non rinforzato, consente di produrre compo-
nenti strutturali in grado di ridurre l’impatto ambientale
e migliorare il rendimento energetico degli edifici.
Fibre di rinforzo in materiale riciclato possono es-
sere estratte, ad esempio, da scarti di polietilene te-
reftalato (PET), polipropilene (PP), polietilene, nylon,
aramide, poliestere, vetro, gomma e cellulosa. Il cre-
scente interesse della comunità scientifica internazio-
nale verso il rinforzo del calcestruzzo con fibre di ma-
terie plastiche riciclate è illustrato nel recente articolo
di Siddique et al. [5] e nei riferimenti ivi citati, che
analizzano gli effetti di tale rinforzo in termini di un
gran numero di proprietà del materiale, quali la den-
sità, il contenuto d’aria, la lavorabilità, la resistenza a
compressione, la resistenza a trazione, il modulo di
elasticità, la resistenza all’urto, la permeabilità e la
resistenza all’abrasione.
Per quanto riguarda specicamente il rinforzo del
calcestruzzo mediante bre di PET da riciclo, si distin-
guono, tra gli altri, lavori [3-7]. Ochi et al. descrivono
in [3] la tecnologia di produzione di bre di rinforzo
del calcestruzzo a partire da bottiglie PET da riciclo.
Gli stessi autori analizzano anche gli effetti beneci
derivanti dall’aggiunta di tali bre al mix-design del
conglomerato cementizio, in termini di duttilità, di resi-
stenza a essione e di resistenza a compressione del
materiale. Nello studio di Kim S.B. et al. [4] bre di PET
di vario aspetto (lisce, ondulate e goffrate) sono impie-
gate per indagare sulla fessurazione da ritiro plastico
in materiali compositi a base cementizia. Kim J.J. et
al. esaminano in [6] il rinforzo del calcestruzzo con di
bre di PET riciclato, esaminando diverse percentuali
volumetriche delle bre (0,5%, 0,75% e 1%). Le bre
di PET esaminate da questi ultimi autori sono realiz-
zate attraverso macchine di taglio e di deformazione,
partendo da bobine ricavate dalla lavorazione di bot-
tiglie di PET da riciclo. Kim J.J. e coautori misurano
la resistenza a compressione ed il modulo elastico di
provini di RPETFRC, nonché la resistenza a essione
di provini di calcestruzzo rinforzato con bre di PET
da riciclo e barre di acciaio. I risultati presentati in [6]
mettono in luce aumenti signicativi della resistenza a
essione e della duttilità del RPETFRC e, talvolta, a
lievi diminuzioni della resistenza a compressione e del
modulo elastico, rispetto al calcestruzzo non rinforzato
(“Unreinforced Concrete” o UNRC). Foti esamina in [7]
un processo economico di produzione di scaglie di PET
riciclato per il rinforzo del calcestruzzo, che si basa

70 Anno XXXI – N. 2 – aprile-giugno 2014
lanico (CEM IV/B 32.5 R) e diversi tipi di fibre R-PET.
L’attenzione è concentrata sulla resistenza a compres-
sione, sulla resistenza di prima fessurazione, sulla
capacità di assorbimento di energia e sugli indici di
duttilità degli RPETFRC esaminati, con l’obiettivo finale
di indagare su come gli effetti dei rinforzi R-PET varino
con la natura del legante, il rapporto acqua/cemento
e le proprietà geometriche e meccaniche delle fibre
di rinforzo.
sul semplice taglio di bottiglie recuperate dai riuti. Il
rinforzo di malte cementizie con bre PET da riciclo è
trattato in [8,9].
Il presente lavoro mette a confronto i risultati dei
recenti studi sperimentali sul rinforzo del calcestruzzo
con fibre di R-PET [1-2]. Si prendono in esame due
diverse miscele di calcestruzzo, che utilizzano rispet-
tivamente un cemento Portland calcareo (classe di
cemento CEM II/A-LL 32.5 R) ed un cemento pozzo-