Plastic/Rubber Waste in Concrete: A Review

Review
Functions and impacts of plastic/rubber wastes as eco-friendly aggregate
in concrete – A review
Xuemiao Li
a
, Tung-Chai Ling
a,
⇑
, Kim Hung Mo
b
a
Key Laboratory for Green & Advanced Civil Engineering Materials and Application Technology of Hunan Province, College of Civil Engineering, Hunan University, Changsha
410082, Hunan, China
b
Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
highlights
Recycling and reusing of rubber and plastic waste as aggregate in cement-based composites.
Effect of rubber and plastic aggregate on the fresh properties and hardened properties.
Effect of rubber and plastic aggregate on the durability.
Functional application of concrete/mortars containing rubber and plastic aggregate.
Recommendations for the future studies of rubber and plastic waste.
article info
Article history:
Received 13 April 2019
Received in revised form 26 November 2019
Accepted 13 December 2019
Available online 28 December 2019
Keywords:
Functional properties
Plastic aggregate
Rubber aggregate
Waste recycling
Eco-friendly aggregate
abstract
About 6.5 billion tons of discarded plastic and rubber wastes are generated every year globally, and the
disposal of these wastes poses a great threat to the environment due to their long period for degradation.
Therefore, a direct recycling and reusing of these wastes as green construction material can potentially
reduce the environmental burden. Considering plastic and rubber are both synthesized polymer materi-
als containing similar basic elements with distinct structures, this paper aims to comparatively review
the functions and impacts of plastic and rubber wastes used as eco-friendly aggregates on the fresh
and hardened properties as well as the durability performance in mortar and concrete. The type and
source of plastic and rubber wastes used as aggregate, as well as their effects in terms of aggregate size,
replacement content, shape and treatment method are highlighted. In general, the presence of plastic
aggregate decreases the workability of concrete, while the effect of rubber aggregate is mainly governed
by the size and replacement content. Also, the smooth surface and low specific gravity of plastic/rubber
aggregate compared to natural aggregate increased the pore structure which reduced the matrix density
and mechanical strength. Nevertheless, chemical treatment of rubber aggregate was found to be an effi-
cient and feasible method to compensate the mechanical strength loss of rubber incorporated concrete.
Apart from improving ductility of concrete, one of the most promising characteristics of the plastic/rub-
ber aggregate is the low thermal, acoustical and electrical conductivity and therefore these aggregates are
excellent options for producing thermal and sound insulating concrete.
Ó2019 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . ........................................................................................................ 2
2. Turning plastic and rubber wastes into aggregates . . . . . . . . . . .................................................................. 3
2.1. Plastic aggregate . . . . . . . ........................................................................................... 3
2.2. Rubber aggregate. . . . . . . ........................................................................................... 3
3. Fresh properties ........................................................................................................ 5
3.1. Workability . . . . . . . . . . . ........................................................................................... 5
https://doi.org/10.1016/j.conbuildmat.2019.117869
0950-0618/Ó2019 Elsevier Ltd. All rights reserved.
⇑
Corresponding author.
E-mail addresses: [email protected],[email protected] (T.-C. Ling).
Construction and Building Materials 240 (2020) 117869
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat

3.1.1. Effect of plastic aggregate ................................................................................... 5
3.1.2. Effect of rubber aggregate ................................................................................... 6
4. Hardened properties . . . . . . . . . ........................................................................................... 6
4.1. Dry density . . . . . ................................................................................................. 6
4.1.1. Effect of plastic aggregate ................................................................................... 6
4.1.2. Effect of rubber aggregate ................................................................................... 7
4.2. Compressive strength . . . . . . . . . . . . . . . . .............................................................................. 7
4.2.1. Effect of plastic aggregate ................................................................................... 7
4.2.2. Effect of rubber aggregate ................................................................................... 7
4.3. Tensile and flexural strengths . . . . . . . . . .............................................................................. 8
4.3.1. Effect of plastic aggregate ................................................................................... 8
4.3.2. Effect of rubber aggregate ................................................................................... 8
5. Durability performance . . . . . . . ........................................................................................... 8
5.1. Water absorption. ................................................................................................. 8
5.1.1. Effect of plastic aggregate ................................................................................... 8
5.1.2. Effect of rubber aggregate ................................................................................... 8
5.2. Drying shrinkage . ................................................................................................. 9
5.2.1. Effect of plastic aggregate ................................................................................... 9
5.2.2. Effect of rubber aggregate ................................................................................... 9
5.3. Freezing and thawing resistance . . . . . . . .............................................................................. 9
5.3.1. Effect of plastic aggregate ................................................................................... 9
5.3.2. Effect of rubber aggregate ................................................................................... 9
5.4. Chloride ion penetration . . . . . . . . . . . . . . .............................................................................. 9
5.4.1. Effect of plastic aggregate ................................................................................... 9
5.4.2. Effect of rubber aggregate ................................................................................... 9
5.5. Electrical resistivity . . . . . . . . . . . . . . . . . . ............................................................................. 10
5.5.1. Effect of plastic aggregate .................................................................................. 10
5.5.2. Effect of rubber aggregate .................................................................................. 10
6. Functional properties . . . . . . . . . .......................................................................................... 10
6.1. Thermal conductivity . . . . . . . . . . . . . . . . ............................................................................. 10
6.1.1. Effect of plastic aggregate .................................................................................. 10
6.1.2. Effect of rubber aggregate .................................................................................. 10
6.2. Sound absorption ................................................................................................ 10
6.2.1. Effect of plastic aggregate .................................................................................. 10
6.2.2. Effect of rubber aggregate .................................................................................. 10
7. Discussion and conclusion . . . . . .......................................................................................... 10
8. Recommendations for the future studies . . . . . . . . . . . . ....................................................................... 11
Declaration of Competing Interest . . . . . . . . . . . . . . . . . ....................................................................... 11
Acknowledgements . . . . . . . . . . .......................................................................................... 11
References . .......................................................................................................... 11
1. Introduction
Polymeric wastes such as post-consumer plastic and rubber
represents a major component of solid waste which forms a huge
environmental burden due to their non-degradability. The genera-
tion of plastic waste which mainly comes from disposable packag-
ing and abandoned building materials has reached about 12
million tons [1]. Polyethylene terephthalate (PET), polypropylene
(PP), polystyrene (PS), polyethylene (PE) and polyolefins (PO) are
the main sources of plastic waste. In Korea, about 2.2 billion PET
bottles are produced each year, which is equivalent to about
87,000 tons [2]. The production of polyvinyl waste has reached
about 12 million tons [3]. Whereas the majority of rubber waste
originate from discarded tires. In Taiwan, more than 100,000 tons
of tires waste are generated yearly [4], while the UK alone used
about 37 million tons of tires in 2002 [5]. In Europe, 355 million
tyres are produced in 90 plants yearly, representing 24% of the
world production [6]. It is projected that about 1200 million of tires
will reach the end of their useful life worldwide [7].
At present, the general treatments of these wastes are landfill,
incineration and recycling. However, the current recycling practice
is found to be not sustainable and landfill is still the most com-
monly adopted method. Statistics showed that about 51% are bur-
ied, 27% are incinerated, and only 22% are recycled for plastic waste
[8]. Data collected from Mohammadi et al. [9] had showed that the
trend for accumulated waste tires was rising at a rate of 2% and it
was estimated more than 20 million tires were deposited in landfill
and stockpiles in Australia. The disposal of the wastes at landfill
causes environmental issues like pollution and contamination,
which could subsequently lead to health-related problems. These
wastes have been explored for use as composite material, such as
wood-plastic composite and rubber asphalt. Realizing the distinct
features (lightweight, flexibility, chemically inert, etc.) of plastic
and rubber, the post-consumer plastic and rubber have also been
explored for use in construction application as a type of recycled
aggregate for producing a more environmental-friendly cement
mortar and concrete.
Plastic waste is usually used as fine/coarse aggregate and fiber,
while rubber waste is more commonly utilized in the form of fine/-
coarse aggregate; though there are also some usages as fiber [10]
and binder [11]. The use of recycled plastic and rubber has been
explored as an efficient way of improving the properties of con-
crete and mortar for specific applications [8]. For instance, concrete
with recycled polyolefins (PO) waste as aggregate showed a better
post-fire mechanical performance [12,13]. When mixed with plas-
tic fiber, concrete showed excellent ductility in the post-crack
region and flexural toughness of concrete [14]. The addition of rub-
ber particles can be used in non-primary structures irrespective of
the decrease in workability and compressive strength such as (i)
road safety islands and roadblocks (ii) shock absorber, in sound
barriers; (iii) sound buffer (which controls the sound effectively)
and (iv) in buildings as an earthquake shock-wave absorber [15].
2X. Li et al. / Construction and Building Materials 240 (2020) 117869

Furthermore, plastic and rubber can lower the density and reduce
the brittleness of concrete and mortar effectively, as well as
demonstrating good performance in thermal insulation, water-
proof and noise reduction features [16–18].
Both rubber and plastic are lightweight, have low water absorp-
tion, strength and stiffness, as well as exhibiting hydrophobic char-
acteristic which repels water and entraps air onto its surface.
Despite the similarity in the inherent properties, the comparison
of post-consumer rubber and plastic as synthesized polymeric
solid waste aggregate in mortar and concrete are not discussed
in previous papers. Therefore, the main aim of the paper is to com-
pare and provide an overview of turning plastics and rubber wastes
as potential eco-friendly aggregate for producing mortar and con-
crete. For effective usage, plastic/rubber solid waste can primarily
be treated by re-extrusion, mechanical, chemical and energy recov-
ery schemes and technologies [19]. Based on the type of plastic/
rubber waste and their processing method, different shapes of
plastic/rubber can be obtained. The shape, size and content of plas-
tic/rubber aggregate are important factors that influence the per-
formance of mortar/concrete. These factors can be studied by
comparing previous works which utilize various forms of the
aggregates as listed in Tables 1 and 2. The impacts of incorporating
plastic and rubber wastes as aggregate substitution on the fresh
properties, hardened properties and durability performance as well
as its functional properties are discussed in the following sections.
2. Turning plastic and rubber wastes into aggregates
2.1. Plastic aggregate
Polyethylene terephthalate (PET) and virgin polypropylene (PP)
is the main source of manufacturing plastic aggregate [32]. Usually,
these materials need to be processed through three different stages
before it can be further used as aggregate in cement concrete [67].
Firstly, impurities such as labels and adhesives should be
removed by washing with disinfectants and detergents. This is
important for ensuring quality of the finished product. The second
stage is the shredding stage, whereby it involves tearing up the
plastic into small fragments or flakes. Finally, the shredded plastic
is melted and then extruded into pellets. Extrusion is the oldest
and simplest way of processing PET. The most common extrusion
processes are extrusion moulding and extrusion to produce foam
[68].
Plastic waste inherently has low density and water absorption.
Examples of the physical properties of plastic waste and natural
aggregate are compared in Table 3. As can be seen in Fig. 1, the
PET and PP particles have smoother surface than natural river sand,
causing weak bond between plastic aggregate and cement matrix.
Pozzolanic materials and plasticizer are commonly adopted to
compensate the loss of mechanical strength caused by this weak
interface bond [36]. In addition, modifying the surface layer of
plastic aggregate by granulation technology or foaming technology
can considerably improve the overall performance. For example,
the modified expanded polystyrenes (EPS) [69] and plastic gran-
ules [70] (Fig. 2) demonstrated better bonding due to the obtained
rougher and porous surface.
2.2. Rubber aggregate
Vehicle tires are one of the main products of rubber industry. It
is necessary to recycle the discarded waste tires by separating the
metal, textile and rubber that may be further used as raw materials
in manufacturing of other products [48]. Normally, rubber can be
ground into small particles/powder form by mechanical methods.
Table 1
Plastic aggregate used in literatures.
Author Type Shape Size (mm) Replacement level Aggregate replacement type
[19] PET Flaky (a) 2.6
(b) 11.4
10 and 20 vol% Fine
[20] PET (a) Strip
(b) Square
(c) irregular
Thickness: 1 Kept at 40 cm
3
by volumetric amount Fine
[21] EPS Spherical (a) < 4
(b) < 16
25, 50, 75, and 100 vol% Fine/coarse
[3] PS + PE Fabriform Length: 0.15–12
width: 0.15–4
10, 15, and 20 wt% Fine
[22] PS Beads/ granulous 4 20, 40, 60 and 80 vol% Coarse
[23] PET Irregular 0.1–5 5 wt% Fine
[24] PET Irregular <4 25.64 and 16.95 wt% Fine
[25] HDPE Flaky 4–12 10, 15, 20 and 25 vol% Coarse
[8] PET (a) Flaky
(b) Cylindrical
2–16 5, 10 and 15 vol% Coarse
[26] PS Irregular <4 10, 20 and 50 vol% Fine
[27] PET (a) Lamellar and irregular
(b) Cylindrical granulate
<20 7.5 and 15 vol% Fine/coarse
[28] PO + PET Irregular <2 10–50 wt% Fine
[29] Plastic bags Annular cylindrical <0.01 10, 20, 30 and 50 wt% Fine
[13] PET (a) Flaky
(b) Regular pellet
(a) 2–11.2
(b) 1–4
7.5 and 15 wt% Fine/coarse
[18] EPS Irregular <1 10, 30, 50 and 70 vol% Fine
[30] PET (a) Flaky
(b) Regular pellet
(a) 2–11.2
(b) 1–4
7.5 and 15 wt% Fine/coarse
[31] PET Flaky <4.75 5, 10 and 15 vol% Fine
[32] Modified PP Short column Length: 1.5–4 10, 15, 20 and 30 vol% Fine
[33] PET (a) Flaky
(b) Regular pellet
1–4 5, 10 and 15 vol% Fine
[34] PET (a) Flaky
(b) Spherical
(a) 2–11.2
(b) 1–4
5, 10 and 15 vol% Fine/coarse
[35] PP + PE Spherical 0.18–2 10 and 25 vol% Fine
[12] PO + PET Irregular 4.75–6 10, 20 and 30 vol% Coarse
[36] PET Flaky <4.75 5, 10 and 15 wt% Fine
[37] PET Flaky greater than 4.75 20, 30, 40 and 50 vol% Coarse
X. Li et al. / Construction and Building Materials 240 (2020) 117869 3

The recycling process mainly consists of three stages [75,76].
Firstly, waste tires are crushed and shredded followed by the
grinding process. In this process, grinding/rolling mill were often
used to produce rubber particles larger than 4.75 mm. Secondly,
these coarse rubbers particles are converted to granular rubber
material or fine crumb rubber particles with size between 75 mm
and 4.75 mm achieved by rolling/rotary crushing mill. If ultrafine
rubber powder is needed (<75 mm), it can be ground further by
rotary colloid mill.
In general, cryogenic grinding is the main process in production
of crumb rubber. Grains of rubber granulates obtained through
cryogenic grinding have a smoother surface as compared to the
grains of rubber granulates obtained through the ambient grinding
method. Fig. 3 showed the microstructure of rubber particles
obtained by two grinding methods. Ground rubber granulates
obtained from ambient grinding are spongy in nature [77]. The
detailed process of the two grinding methods are given in Fig. 4
and Table 4.
Table 2
Rubber aggregate used in literatures.
Author Types Shapes Size (mm) Replacement level Aggregate replacement type
[38] Tires (a) Chipped
(b) Crumbed
Chipped: 5–20
Crumbed: 1–5
25, 50, 75, and 100 vol% Fine/coarse
[39] Tires (a) Spheroidal-like
(b) Fiber-like
Spheroid-like: 0.02–0.06/ 1.7–2.1
Fiber-like: 0.06–0.1/ 2.5–2.9
30 wt% Fine
[40] Tires Angular 10–20 5, 10, 15, and 20 vol% Coarse
[41] Tires Crumbed (a) 1.18
(b) 2.36
10, 20, 30, 40 and 50 vol% Fine
[42] Tires Crumbed <4.75 15, 25, 35 and 50 vol% Fine
[43] Tires Crumbed 0.6 4, 4.5, 5 and 5.5 wt% Fine
[44] Tires Crumbed 1.44–2.83 15, 25, 35 and 50 vol% Fine
[45] Tires Crumbed (a) #30 mesh
(b) 1–3
(c) 2–4
5.33–21.11 vol% Fine
[46] Tires Angular/ flaky/elongated (a) 0.5–1.5
(b) 2–8
(c) 5–25
2, 4, 6 vol% Fine/coarse
[47] Tires Powder <4.75 10, 20, 30 and 40 vol% Fine
[48] Tires Chips (a) 2–4
(b) 0.8–2
(c) powder
0 – 20 vol% Fine
[49] Tires Angular (a) 0–5
(b) 5–10
(c) 10–20
10, 20, 40, 60, 80 and 100 vol% Fine/coarse
[50] Tires Ground (a) 3
(b) 0.5
(c) 0.3
20 vol% Fine
[51] Tires Crumbed (a) 0–0.6
(b) 0.84–2
10, 20, 30 and 40 wt% Fine
[52] Tires Crumbed <4.75 40, 50, and 60 vol% Fine
[53] Tires Granulated 0.5–4 10, 20 and 30 vol% Fine
[54] Tires Irregular and jagged 0, 0.063–8 10, 20, 30 and 40 wt% Fine/coarse
[55] Tires Crumbed 0.075–4.75 15, 25, 50 and 100 vol% Fine
[56] Tires Angular 4 10, 20, 30, 40, 50, 60 and 70 wt% Fine
[4] Tires Powdered (a) 0.6
(b) 0.3
5, 10, 15 and 20 vol% Fine
[57] Tires Granulated 0.15–4.75 5, 10 and 15 vol% Fine
[58] Tires Granulated <11.2 5, 10 and 15 vol% Fine/coarse
[59] Rubber-shoe outsole Granulated <8 10 and 30 vol% Fine
[60] Tires Crumbed 0.7–5 20 and 30 vol% Fine
[61] Tires Angular <10 15 wt% Coarse
[62] Tires Elongated <20 10, 20, 30, 40 and 50 vol% Fine/coarse
[63] Tires Powder 1–4 10, 20, 30, 40 and 50 wt% Fine
[64] Tires Chipped greater than 4.75 5, 7.5, and 10 wt% Coarse
[65] Tires (a) Chipped
(b) Crumbed
(a) Chipped: 4.75–15
(b) Crumbed: <4.75
12.5, 25, 37.5 and 50 vol% Fine/coarse
[66] Tires Irregular <0.045 2.5, 5, 7.5 and 10 wt% Fine
Table 3
Example of the physical properties of plastic/rubber aggregate and natural aggregate.
Type Bulk density (kg/m
3
) Water absorption 24 h (%) Specific gravity (g/cm
3
) Reference
Plastic waste PET 438.2 0.09 1.34 [27]
PP 515 0 0.90 [32]
EPS 30 0 0.34 [72]
PE 24 0.10 1.34 [3]
Rubber waste Mechanically ground rubber 44 0.80 1.01 [73]
Cryogenic rubber 46 1.30 1.07
Natural aggregate Gravel 1624 1.32 2.79 [74]
Sand 1656 1.80 2.65
4X. Li et al. / Construction and Building Materials 240 (2020) 117869

Similar as plastic waste, rubber waste can be used as light-
weight aggregate due to its low density. The hydrophobic nature,
though limits its use. The physical properties of rubber aggregate
are listed in Table 3. Similar as plastic aggregate, rubber particles
have a relatively smooth and elastic surface (Fig. 5) which is
expected to weaken the interface bonding between the rubber
particles and the cement paste. To compensate the drawback,
pre-treatment of rubber aggregate by chemical method has been
proposed. NaOH solution treatment is the most often used since
it could provide a weak alkali condition around the rubber parti-
cles, which is beneficial for the hydraulic conductivity, rubber/
cement water transfer rate, and hydration at the interface, thus
improving the rubber/cement adhesion [78]. Moreover, the treat-
ment can improve the hydrophilicity of rubber aggregate, resulting
in thinner water film and porosity of ITZ between rubber particles
and cement paste [42]. Similar findings were also reported for
using hydrochloric acid solution treatment method [79]. However,
some other researches showed that although NaOH treatment
resulted in notable enhancement in compressive strength, it did
not improve the hydrophobic nature of rubber, with water contact
angle of the rubber surface still higher than 90°[9,80]. Thus, He
et al. [81] proposed introducing strong polarity groups to the rub-
ber surface by a more advanced pretreatment method shown in
Fig. 6, so that a stronger chemical bond between rubber and
cement could be formed. FTIR results showed that the pretreat-
ment greatly reduced the thickness of water film as well as the
contact angle (Fig. 7), resulting in better adhesion strength.
3. Fresh properties
3.1. Workability
3.1.1. Effect of plastic aggregate
The main findings of studies indicated that workability of plas-
tic aggregate concrete mainly depends on the friction between
irregular shape particles which was influenced greatly by the plas-
tic shape and content. Rahmani et al. [31] found that the workabil-
ity of concrete with PET waste decreased due to their flaky shape
which impaired the fluidity of fresh concrete. The decrease of
workability was more pronounced as the plastic waste content
Fig. 1. ITZ between cement paste and natural aggregate/waste PET lightweight aggregates (WPLA) in mortar (a) natural aggregate (28 days, magnification: 700) and (b) WPLA
(28 days, magnification: 700) [2].
Fig. 2. SEM of EPS [71].
Fig. 3. SEM analysis at 200 l m and 400 magnification of (a) ambient grinding and (b) cryogenic grinding [6].
X. Li et al. / Construction and Building Materials 240 (2020) 117869 5