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Impact of Recovered and Embedded
Polypropylene Fibres on the Mechanical
Properties of Fibre-Reinforced Concrete
Guanzhi Liu1(B), Martin Hunger2, Nikola Toši´c1, Jürgen Bokern2,
and Albert de la Fuente Antequera1
1Civil and Environmental Engineering Department, Universitat Politècnica de Catalunya
(UPC), Jordi Girona 1–3, 08034 Barcelona, Spain
guanzhi.liu@upc.edu
2Master Builders Solutions Deutschland GmbH, Trostberg, Germany
Abstract. This study aims to investigate the effect of incorporating different quan-
tities of polypropylene fibres recovered during concrete recycling or embedded
in recycled aggregates on the mechanical properties of new polypropylene fibre-
reinforced concrete. Both recovered fibre (at replacement ratios of 0%, 10%, 30%
and 100%) and coarse recycled aggregate (0%, 100%) were used in new concrete.
A polypropylene fibre content of 9 kg/m3(1.0% by volume) was chosen for all
concretes.. The compressive strength, elastic modulus and stress-strain behaviour
of concrete in compression were tested. The residual tensile strength was also
tested for concretes with recovered fibres and fibres embedded in recycled aggre-
gate. The results show that mixing of recycled fibres recovered from concrete with
virgin fibres is feasible without significant effect on the mechanical properties of
concrete in compression.
Keywords: PPFRC ·recovered fibre ·replacement ratio ·recycled coarse
aggregate ·residual tensile strength
1 Introduction
Concrete is widely recognized as one of the most extensively utilized construction mate-
rials. The primary constituents of a concrete mixture include cement, aggregates (both
coarse and fine), water, and admixtures. Meanwhile, according to World Bank statistics
on solid waste management, most middle and low-income countries produce approxi-
mately 1.3 billion tons of solid waste annually. This amount is anticipated to rise to 2.2
billion tons by 2025 [1]. Consequently, the re-use of building elements and recycling of
building materials at the end of their service life avoid waste generation and preserve
natural resources at the same time [2,3]. Simultaneously, various innovative types of
concrete are being developed and employed for both structural and non-structural pur-
poses. Among these, there is a growing interest in fibre-reinforced concrete (FRC) due
to its ability to address challenges associated with the brittleness of conventional con-
crete. FRC offers advantages such as enhanced ductility, toughness, impact resistance,
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2024
V. Mechtcherine et al. (Eds.): BEFIB 2024, RILEM Bookseries 54, pp. 83–90, 2024.
https://doi.org/10.1007/978-3-031-70145-0_11
84 G. Liu et al.
and energy absorption capacity [4,5], properties that allow for more resilient structures.
As a result, the incorporation of recycled aggregate (RA) and recovered fibre (RF) from
FRC into new concrete is actively explored as a means to enable circular economy within
the field of research.
This study aims to investigate the recovery of fibres during polypropylene fibre-
reinforced concrete (PPFRC) recycling, the properties of the resulting coarse recycled
concrete aggregate (cRCA) and the effect of both recycled materials on the mechanical
properties of new FRC. The results of the paper are expected to contribute to facilitating
fully circular construction using macro-synthetic fibre-reinforced concrete.
2 Experimental Programme
2.1 Scope and Outline
In this study, the RF and cRCA were used to produce new PPFRC. Both RF and cRCA
were obtained from a laboratory-produced PPFRC whose compressive strength was
30 MPa and whose PP fibre content was 9 kg/m3(1.0% by volume). 0%, 10%, 30%,
and 100% of virgin PP fibres were replaced with RF. In terms of RCA, a new PPFRC
was produced with 100% of cRCA. To compare their mechanical properties, the control
group was made of virgin fibre (VF) and natural aggregate (NA). A fibre content of
9 kg/m3(1.0% by volume) was chosen. The mechanical properties of each group were
tested, which include the compressive strength ( fc), Elastic modulus (Ec) and residual
tensile strength.
2.2 Materials
The cement used in the study was CEM II/A-L 42.5N (Cementos Molins, Spain). The
NA was crushed limestone from a quarry in Villacarca (Barcelona province, Spain) used
in fractions 0/4, 4/12, and 12/20 mm. MasterPozzolith 7003 (Master Builder Solutions)
was used as a plasticizer. The cRCA was obtained by crushing concrete and sieving into
4/12- and 12/20-mm fractions, as shown in Fig. 1. The virgin PP fibre was an embossed
monofilament polypropylene fibre with a length of 54 mm, an aspect ratio of 67, a tensile
strength of 552 MPa, and a density is 910 kg/m3supplied by Master Builders Solutions.
The RF was obtained from tested PPFRC specimens, by crushing them and using a
washing and flotation method. The difference in appearance between the VF and RF is
given in Fig. 2. As illustrated in Fig. 3, the RF was manually categorized and enumerated
according to their shapes into three main groups: long, bent, and short. Long fibres are
defined as those whose lengths exceed half of the VF length, short fibres are those with
lengths shorter than half of the VF length, and the remaining fibres are designated as
curved fibres. The statistical results are presented in Table 1.
2.3 Mix Proportioning and Specimen Preparation
To compare concrete using cRCA and RF with concrete using NA and VF, the mix-
ture design used the same effective w/c ratio and cement dosage, 0.55 and 350 kg/m3
Impact of Recovered and Embedded Polypropylene Fibres 85
Fig. 1. Appearance of cRCA:4/12 mm (left);12/20 mm (right)
Fig. 2. Virgin PP fibres (left); recovered PP fibres (right).
Fig. 3. Shapes of RF.
Table 1. RF shape distribution
Type Category of fibres
short bent long
Percentage 19.0% 43.1% 37.9%
respectively. As the 28-day compressive strength of 30 MPa is widely used in building
construction, it was chosen for this study as the default mix design. For the substitution
rate of RF, four groups had been selected: 0%, 10%, 30% and 100% of RF with 100% of
NA (NA-0%, NA-10%, NA-30%, NA-100%,). For the cRCA, 100% of cRCA (4/12 mm
and 12/20 mm) was used with 100% of fine NA and 100% VF (RA-0%). Due to the dif-
ferences in density between cRCA and coarse NA, the amount of sand in RA-0% had to
be adjusted and decreased with respect to NA-0%. The quantity by mass is reduced due
to the lower density of cRCA compared to NA. Proportions of fine and coarse aggregate
86 G. Liu et al.
to meet the grading curve of the reference concrete. Due to the high water absorption
of cRCA, additional water had to be added keep the effective w/c ratio constant and to
meet the target workability (125 ±25 mm) of the concrete. A plasticizer was used in
all mixes to improve the workability. All groups and their mix designs are displayed in
Table 2.
Table 2. Mix design of the tested concretes
Concrete NA-0% NA-10% NA-30% NA-100% RA-0%
Cement (kg/m3)350 350 350 350 350
weff1(kg/m3)192.5 192.5 192.5 192.5 192.5
wadd2(kg/m3) 0 0 0 0 59
Plast. (wt%of cement) 1.50 1.23 1.23 1.23 1.50
NA (kg/m3)0/4 mm 770.6 771.7 771.7 771.7 659.5
4/12 mm 261.8 262.2 262.2 262.2 0
12/20 mm 695.5 696.5 696.5 696.5 0
cRCA (kg/m3)4/12 mm 0 0 0 0 457.2
12/20 mm 0 0 0 0 491.4
Fibre (kg/m3)Virgi n 9.0 8.1 6.3 0.0 9.0
Recovered 0.0 0.9 2.7 9.0 0.0
1 effective water; 2 water added for cRCA absorption.
For each concrete, three cylindrical specimens (Ø150 ×300 mm3) and three pris-
matic specimens (150 ×150 ×600 mm3) were cast in steel moulds for each concrete
mix. All batching was performed in a laboratory concrete pan mixer (Collomatic 65/2 K-
3) with a capacity of 35 l. For concrete with NA, fine and coarse aggregates and cement
were put together first and mixed for 60 s after which the water was added with the
plasticizer and mixed another 60 s, after which the fibre was added for another 60 s of
mixing. When fine NA and cRCA were placed together in the mixer, they were first
mixed for 30 s, then additional water to compensate for the water absorption was added
and mixed for a further 30 s. Further on the same procedure was followed as described
before. The specimens were unmolded 24 h after casting and kept in a curing chamber
(20 ±2°C,>95% relative humidity) until testing.
2.4 Test Procedures
All specimens were tested after 28 days. Each group of three cylindrical specimens
was used to test the stress-strain under compression and each group of three prismatic
specimens was used to test the residual tensile strength. For the stress-strain testing
according to EN 12390–3:2002 [6] and a compression testing machine (IBERTEST MEH
3000, Madrid, Spain) was used. As shown in Fig. 4, three linear variable displacement
transducers (LVDTs) were mounted at 120° relative to each other during the test. The
Impact of Recovered and Embedded Polypropylene Fibres 87
residual flexural tensile strength was tested according to EN 14651 [7]. The limit of
proportionality ( fLOP )and the residual flexural tensile strength ( fR,j)for CMODs of
0.5, 1.5, 2.5, and 3.5 mm. The Servohydraulic Test System (Instron 8505) was used;
Fig. 4shows the test layout.
Fig. 4. Experimental testing: compressive stress-strain relationship (left); residual tensile strength
(right) [8].
3 Results and Discussion
3.1 Stress-Strain Relationship
Figure 5shows, for the different replacement ratios of RF and 100% cRCA, the stress-
strain curves using a controlled load ratio for the fibre content of 9 kg/m3, where each
curve is an average of all tested cylindrical specimens in each group. About the shape
of the ascending branch of the stress-strain curve, it can be seen that the shape of all
the concretes’ stress-strain curves is very similar regardless of the RF replacement ratio
and cRCA. The relationship is linear up to approximately 0.4fc, after which it becomes
parabolic until the peak strain.
Fig. 5. Stress-strain relationship in compression for PPFRC
88 G. Liu et al.
From the stress-strain curve, the compressive strength ( fc) of concrete can be
obtained from the peak stress and the elastic modulus (Ec) as the secant between the ori-
gin and the point of 0.4fc[9]. Figure 6shows the fcand Ecfor different RF substitution
ratios and cRCA compared with the control group, where the error bar represents the
standard deviation. In terms of compressive strength, it can be observed that concrete
with 100% cRCA exhibited lower strength than that using 100% NA, indicating a 28.6%
reduction in compressive strength. Concretes with replacement ratios of RF compared
to using 100% VF showed minimal changes at 10% and 30% RF. However, at a substitu-
tion ratio of 100%, the compressive strength decreased significantly to only 28.15 MPa
(18.3%). Regarding Ec, the results for concretes using different replacement ratios of
RF mirrored those of compressive strength, except for a lower value for NA–100%, a
potential outlier. The fcfor the concrete with 100% of cRCA was lower than that of
NA-0%, as expected.
Fig. 6. The result of compressive strength and elastic modulus
3.2 Residual Flexural Tensile Strength
Figure 7shows the residual flexural tensile stress–CMOD curves with respect to RF
replacement ratios (0%, 10%, 30% and 100%) and 100% cRCA at fibre contents of
9 kg/m3, where each line represents an average of 3 specimens tested. The residual
strength of the control group was found to be the highest.
Table 3shows the comparison of cRCA and different replacement ratios on residual
flexural tensile strength. For concrete incorporating cRCA, the fLOP and residual flexural
tensile strength with NA were found to be higher than those with cRCA. Specifically,
cRCA exhibited a 22.2% decrease in fLOP and a significant 30% reduction in residual
flexural tensile strength. This decrease can be attributed to the fact that the fcof concrete
tested by cRCA was lower than NA, which was 28.5% lower.
Examining replacement ratios of RF, it was observed that all curves for concretes
using RF were lower than those using 100% VF. Interestingly, the residual tensile strength
did not consistently decrease with increasing RF content. Among different replacement
rates, 30% RF exhibited the highest residual tensile strength, followed by 10% RF, and
finally 100% RF. As for the residual strengths fR1 and fR3, the highest values were
obtained for 100% VF, reaching 2.89 MPa and 3.75 MPa, respectively.
Impact of Recovered and Embedded Polypropylene Fibres 89
Fig. 7. Residual flexural tensile stress-CMOD curves of three-point bending tests
A plausible explanation for the observed loss of residual strength when using RF
is the reduced average length (and consequently aspect ratio) combined with a certain
loss of mechanical properties for RF, as indicated in previous studies [10]. Inconsistent
trends may be a result of variations, e.g. with respect to the proportion of short fibres.
More data is needed to draw reliable conclusions.
Table 3. Effect of cRCA and different recovered fibre replacement ratios on residual tensile
strength
Specimen fLOP fR1 fR2 fR3 fR4
NA-0% 3.78 2.89 3.54 3.75 3.80
NA-10% 3.83 1.64 2.01 2.18 2.19
NA-30% 3.71 2.64 3.15 3.43 3.50
NA-100% 3.15 1.56 1.81 1.87 1.93
RA-0% 2.94 1.89 2.42 2.56 2.52
4 Conclusions
The main aim of this study was to explore the effect of incorporating different quantities
of free or embedded PP fibres recovered during PPFRC recycling on the mechani-
cal properties of new FRC. For this purpose, both the uniaxial stress–strain behaviour
in compression and the residual flexural tensile strength of PPFRC with different RF
replacement ratios and 100% cRCA were characterized. Based on the experimental
results, the following conclusions are drawn:
•The compressive strength and modulus of elasticity were almost unaffected at 10%
and 30% replacement of virgin fibres (VF) by recycled fibres (RF)
•The use of 100% of cRCA caused a reduction as expected in compressive strength
and modulus of elasticity, relative to a concrete with NA.
90 G. Liu et al.
•Replacing 30% of VFs by RFs led to slight decreases of the residual flexural strength
respect to the PPFRC produced with 100% of VFs. This reduction was of 8.6% for
fR1 and 8.5% in fR3. For higher ratios of replacement, the reduction of fRwas more
noticeable.
•The residual strength was influenced by the replacement ratio of RF, but more data
is needed to establish a clear relationship.
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