Assessment of lightweight recycled crumb rubber composite

Advances in Concrete Construction, Vol. 11, No. 5 (2021) 409-417

DOI: https://doi.org/10.12989/acc.2021.11.5.409 409

Copyright © 2021 Techno-Press, Ltd. http://www.techno-press.org/?journal=acc&subpage=7 ISSN: 2287-5301 (Print), 2287-531X (Online)

1. Introduction

Tyre is a non-biodegradable material mostly made of

rubber (60-65wt. %) (Alsaleh and Sattler 2014). Every year,

1.5 billion tyres are manufactured over the world

(Mohajerani et al. 2020, Roychand et al. 2020, Thomas and

Gupta 2016) which generate about 4 billion tons of waste

(Sathiskumar and Karthikeyan 2019). Annually, 1 billion

tyres reached the end of their service life (Sofi 2018,

Thomas and Gupta 2016) and are stockpiled or landfilled.

These waste tyres present health, environmental and

economic risks. Landfilling of these waste tyres occupy a

large amount of land and causes air, water and soil pollution

(Mohammed et al. 2017). Besides that, stockpiling of waste

tyre provides breeding grounds for pests and insects that

may spread diseases (Roychand et al. 2020).

In order to overcome these problems, the waste tyre can

be utilised as partial or full aggregate replacement in

cement-based composite. This method will not only provide

a good avenue for sustainable utilisation for these wastes, it

is also possible to prevent natural aggregate from

exhaustion. As an alternative aggregate, waste tyres are

mostly used in the form of shredded or chipped rubber (13-

76 mm), crumb rubber (CR) (0.075-4.75 mm), and fine

Corresponding author, Ph.D.

E-mail: [email protected]

ground rubber (0.075-0.5 mm) (Roychand et al. 2020).

Literature has shown that various efforts were made to

incorporate CR in cement composite (Table 1). However,

the compressive strength, which is one of most important

properties in cement-based material, was found to be

significantly decreased in the presence of CR. The strength

reduction can be as high as 95% with the replacement of

sand by CR of just up to 60-70%. For instance, Ozturk et al.

(2020) and Angelin et al. (2019, 2017) reported about 65-

90% strength reduction in mortar with 30-45% CR as fine

aggregate substitute. Besides that, López-Zaldívar et al.

(2017) concluded that regardless of CR particle sizes, the

strength of mortar decreased up to 97% with CR

replacement level up to 70 vol.%. On the other hand,

Onuaguluchi and Banthia (2019) highlighted about 50%

reduction in strength of cement composite with the

inclusion of 15 vol.% CR as sand replacement. The

reduction of strength due to CR is also similarly evidenced

in alkali-activated mortar, where the strength dropped by

about 35% with the incorporation of 15 vol.% of CR

(Zhong et al. 2019). In cement concrete, researchers have

similarly reported strength loss by incorporating CR. Ren et

al. (2020) and Padhi and Panda (2016) investigated the

effect of CR as sand replacement in concrete and observed

about 30% reduction in strength due to 15 wt.% CR. In

another study, 20% strength reduction was reported when

24% of CR was employed to replace coarse aggregates in

concrete (Williams and Partheeban 2018). Furthermore,

Assessment of lightweight recycled crumb rubber-cement composite produced by preplaced method

Syed Nasir Shah1,2, Kim Hung Mo1, Soon Poh Yap1, Azma Putra3 and Muhammad Nur Othman3

1Centre for Innovative Construction Technology, Department of Civil Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

2Department of Civil Engineering, Faculty of Engineering, Balochistan University of Information Technology, Engineering, and Management Sciences, 87300 Quetta, Pakistan

3Centre for Advanced Research on Energy, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Malacca, Malaysia

(Received May 1, 2020, Revised March 7, 2021, Accepted April 21, 2021)

Abstract. The incorporation of non-biodegradable tyre waste in cement-based material has gained more interest towards

sustainable construction these days. Crumb rubber (CR) from waste tyre is an alternative for sand replacement in low strength

applications. Many researchers have studied CR cement-based materials produced by normal mixing (NM) method and reported

a significant decrease in compressive strength due to CR. To compensate this strength loss, this research aims to study the

innovative incorporation of CR in cement composite via the preplaced mixing (PM) method. In this investigation, cement

composite was produced with NM and PM methods by replacing sand with 0%, 50%, and 100% CR by volume. The test results

showed no significant difference in terms of densities of cement composite prepared with both mixing methods. However,

cement composite prepared with PM method had lower strength reduction (about 10%) and lowered drying shrinkage (about

20%). In addition, the sound absorption coefficient and noise reduction coefficient of CR cement composite prepared by PM

method were in similar range as those prepared with NM method. Overall, the results demonstrate that the PM method is

promising, and the maximum replacement level of 50% is recommended for CR in the cement composite.

Keywords: crumb rubber; sustainability; cement composite; preplaced aggregate; mechanical properties; sound

absorption

Syed Nasir Shah, Kim Hung Mo, Soon Poh Yap, Azma Putra and Muhammad Nur Othman

Solanki and Dash (2016) studied the effect of CR

replacement in concrete as both fine and coarse aggregate.

The reduction in strength due to 40 wt.% CR incorporation

was about 85% when CR was used as replacement of coarse

aggregate, whereas as fine aggregate the reduction was

about 40%. One of the major causes for strength reduction

due to CR inclusion was presence of entrapped air on CR

surfaces constituting irregularly shaped pores. The

entrapped air formed is due to low adhesion and poor bond

between CR and cement matrix Fig. 6 (Angelin et al. 2019,

2017, Padhi and Panda 2016).

Another reason for the reduction in strength is due to the

non-uniform dispersion of the CR in cement composite.

Besides that, CR decreases the flowability of cement

composite, which affects its uniformity (Dong et al. 2020,

Xue and Cao 2017). In very flowable mix, CR tends to float

upwards in the mix due to its lightweight nature (Fadiel et

al. 2014, Zhu et al. 2018). These drawbacks can be

addressed by using a more suitable mixing method which

facilitates uniform mixing of CR in cement composite.

Also, a uniformly mixed cement composite can exhibit

better load transfer mechanism and reduce the strength

reduction. This can be achieved by adopting the preplaced

aggregate mixing method commonly done in preplaced

aggregate concrete (also known as two-stage concrete, pre-

packed concrete and grouted aggregate concrete) (Najjar et

al. 2014). In preplaced aggregate concrete, the grout (or

cement mortar) is mixed separately and then injected into

the preplaced aggregate mass (Cheng et al. 2019).

Generally, grout is injected either by pouring on the top of

aggregate surface or pumping by network of pipes into the

bottom of aggregate. Preplaced concrete is basically a

skeleton of aggregate particles resting on each other with

grout filling the voids between them. Preplaced aggregate

concrete has a specific load distribution mechanism in

which the load is transferred through contact areas between

aggregate particles (Fig. 1).

Many researchers have investigated the characterisation

of mechanical strengths of preplaced concrete previously.

Li et al. (2019) produced preplaced ultra-high performance

concrete with excellent compressive strength (up to 151.8

MPa), low binder amount (down to 364 kg/m3) and high

Fig. 1 Load transmission in preplaced concrete (Najjar et al.

2014)

binder efficiency (up to 0.417 MPa·m3/kg). Cheng et al.

(2019) revealed that for the same w/c, the

tension/compression ratio of preplaced aggregate concrete

was higher than the reference concrete, suggesting a

beneficial effect on the tensile strength development of the

concrete. Alfayez et al. (2019) developed preplaced

recycled aggregate concrete from waste tyre CR as coarse

aggregates. Results revealed although decrease in

compressive strength was observed due to CR, the

preplaced method offered superior volume stability through

high resistance against shrinkage and thermal cracking.

Besides that, despite the reduction in compressive strength,

increase in impact energy absorption capacity was found

with the inclusion of up to 30% CR content as coarse

aggregate replacement in fibre-reinforced preplaced

rubberised concrete (Murali et al. 2019). Young and Hong

(2019) produced preplaced aggregate concrete using high

volume of other types of lightweight aggregates such as

expanded clay, bottom ash, and shale. The results showed

that the stiff aggregate network due to the preplaced

aggregates improved the compressive strength and lowered

the drying shrinkage. Du et al. (2017) conducted a

comparative study between preplaced and conventional

casting of lightweight aggregate (shale ceramsite) concrete.

It was concluded that the concrete prepared with preplaced

aggregate method exhibited higher compressive strength

and modulus of elasticity with much lower shrinkage. In

addition, it is a better solution for the segregation problem

of lightweight aggregate.

Table 1 Recent literature of five years on incorporating CR in cement composite

References Replacement

level

CR size

(mm)

Mechanism of incorporation Compressive

strength (MPa) Type of aggregate replaced Type of composite

Ozturk et al. (2020) 0, 15, 30, and 45 vol.% 0.1-4 Fine aggregate Mortar 8-28

Ren et al. (2020) 0, 5, 10, and 15 wt.% 0.1-0.8 Fine aggregate Concrete 26-38

Onuaguluchi and Banthia

(2019) 0, 10, and 15 vol.% 0.1-4.75 Fine aggregate

Cementitious

composite 20-46

Angelin et al. (2019) 0, 7.5, 15, and 30 vol.% 0.6 and 1.2 Fine aggregate Mortar 10-50

Zhong et al. (2019) 0, 5, 10, and 15 vol.% 0.5-6.0 Fine aggregate Alkali-activated

mortar 20-35

Williams and Partheeban

(2018)

0, 3, 6, 9, 12, 15, 18, 21,

and 24 vol.% 20 Coarse aggregate Concrete 30-50

Angelin et al. (2017) 0 and 30 vol.% 0.1-4.8 Fine aggregate Mortar 3.5-45

López-Zaldívar et al.

(2017)

0, 40.7, 51.6, 61.5, and

70.6 vol.% 0.5-4.0 Fine aggregate Mortar 0.5-23.5

Padhi and Panda (2016) 0, 5, 10, and 15 wt.% 0.15-4.75 Fine aggregate Concrete 25-55

Solanki and Dash (2016) 0, 10, 20, and 40 wt.% 0.1-4.75 Fine and coarse aggregate Concrete 16-48

410


Fig. 2 CR with size of 1-4 mm

Table 2 Major oxide composition of cement and fly ash

Oxides Cement Fly ash

SiO2 21.00 55.30

Al2O3 6.00 15.97

Fe2O3 3.50 4.87

CaO 65.00 2.70

MgO 0.70 1.13

SO3 1.50 0.37

Till date and to the best of the authors' knowledge, no

study has applied the preplaced method to incorporate high

volume (50% and 100%) CR in cement composite.

Therefore, this study intends to evaluate the effectiveness of

this method and to compare the performance of the

resulting cement composite with that of normal mixing

method. For this purpose, the mechanical properties,

acoustic properties, and drying shrinkage behaviour of

cement composite incorporating CR with conventional and

preplaced mixing methods are discussed in this paper.

2. Materials and methods

2.1 Materials and mix proportions

Ordinary Portland cement (OPC) type CEM I 42.5N/

52.5N (EN 197-1 2011) was used as the main binder of the

cement composite, and this cement type is similar as ASTM

type 1 (ASTM C150M 2012). The OPC fulfils the early

strength (≥20 MPa at 2 days) and standard strength (≥52.5

MPa at 28 days) requirements stipulated in EN 197-1. In

order to limit the maximum content of OPC to 500 kg/m3,

fly ash was introduced as 33% OPC replacement (by

weight). The major chemical compositions of FA and OPC

are given in Table 2. River sand (specific gravity: 2.59) was

used as fine aggregate in the study and sieved to obtain size

between 1-4 mm for the use in the preplaced method.

Recycled CR (specific gravity: 1.24) (Fig. 2) with particle

size of 1-4 mm was incorporated as sand replacement (by

volume) at different replacement levels. Tap water from the

laboratory was used as mixing water in this study.

Table 3 shows the mix design of cement composite

using different mixing methods, namely normal mixing

method (denoted by NM) and preplaced mixing method

Fig. 3 Sequence of preparing CR cement composite with

PM method

(denoted by PM). The sand/ binder ratio and water/ binder

ratio were fixed at 1.58 and 0.40, respectively. The low

water/ binder ratio was selected to prevent the floating of

CR. To maximise the usage of waste CR for producing a

more sustainable and lightweight cement composite, the CR

was utilised as sand replacement levels of 0%, 50%, and

100% (by volume). The selection of 50% replacement level

was based on trial mixes which showed that incorporating

50% CR gave a density of about 1850 kg/m3, which is

within the stipulated limit of lightweight concrete based on

ACI (ACI 213 2010). In addition, the cement composite

with 100% CR was chosen for investigation in this research

as other replacement levels between these CR replacement

levels would be expected to give linear changes in the

properties of the cement composite. Furthermore,

superplasticiser (SP) was added at 0.75% (by weight of

binder) for the PM mixes to ensure good flowing cement

slurry for the use in the PM mixing method.

411


(a)

(b)

Fig. 4 Mould with 100% sand (a) and 50% sand/rubber (b)

before placement of cement slurry in PM method

2.2 Mixing

For the NM method, it was carried out based on ASTM

C305-14 (2014). Firstly, mixing water and cement were

introduced in a bowl and mixed at a low speed of about 140

rpm for 30 sec. Secondly, the entire quantity of fine

aggregate (sand/CR) was added slowly over a 30-sec

period, while mixing at the same speed. Subsequently, the

SP was added, and the mixture was further mixed for 2-3

min at medium speed of about 285 rpm. Once the fresh

mixture was mixed properly, the flow table test was

conducted in accordance with method ASTM C1437-15

(2015). Then, the mixture was placed in a pre-oiled mould

and vibrated for 10-15 sec to eliminate air voids. The

moulds were then left in laboratory condition and de-

moulded after 24 hours and kept in water tank for curing up

to 28 days.

In the PM method (Fig. 3), the fine aggregates

(CR/sand) were placed in mould (Fig. 4). Separately,

cementitious materials (OPC and fly ash) were mixed with

water and SP in a similar manner as stated above for NM

method, to produce cement slurry with good flowability.

The cement slurry was then poured into the mould

containing fine aggregates, followed by gently tamping with

a steel rod. After tamping, the moulds were similarly de-

moulded after 24 hours and then water cured at temperature

of 25-30ºC until the age of 28 days.

2.3 Test methods

The compressive strength of the cement composite was

tested using 50 mm cube specimens at the age of 7 and

28 days based on ASTM C109M-16a (2016). The early 7-

day compressive strength of cement composite was

assessed due to partial replacement of OPC by FA adopted

in the study, as FA tends to delay the early strength of

cement composite (Radwan et al. 2020). Drying shrinkage

Table 3 Mix proportions for CR mortar composites

Mix ID NM0 NM50 NM100 PM0 PM50 PM100

Vol. of CR (%) 0 50 100 0 50 100

w/b ratio 0.4 0.4 0.4 0.4 0.4 0.4

OPC (kg/m3) 500 500 500 500 500 500

Water (kg/m3) 300 300 300 300 300 300

FA (kg/m3) 250 250 250 250 250 250

Sand (kg/m3) 1190 600 0 1190 600 0

CR (kg/m3) 0 290 570 0 290 570

SP (%) 0 0 0 0.75 0.75 0.75

test was conducted on 25×25×285 mm prisms using a

length comparator as per ASTM C596-18 (2018). The

shrinkage strains were continuously recorded up to

8 months. Additionally, cylindrical specimens measuring 35

mm diameter and 100 mm thickness were prepared with the

cement composite containing CR and were used for the

evaluation of sound absorption via the tube impedance

method (ISO 10534-2 1998). For all the evaluations, a total

of three specimens were tested for each mix and the average

value was reported.

3. Results and discussion

3.1 Consistency

Consistency is a key factor that affects the uniformity of

cement composite incorporating CR. Due to its lightweight

nature, CR particles tend to float upwards in the mix upon

vibrating and affect the uniformity of cement composite

(Fadiel et al. 2014, Zhu et al. 2018). Thus, to anticipate this

possibility, flow table test was carried out to evaluate the

consistency of the mixes with NM method only. On the

other hand, for mixes prepared using PM method, the flow

table test could not be conducted due to the nature of the

mixing method. The consistency of NM0, NM50, and

NM100 was 179, 183, and 170 mm, respectively, which

could be classified as plastic mortars (ASTM C270-19

2019) and could result in CR being susceptible to floating.

However, despite the constant w/b ratio in all mixes for

NM, a slight increase of about 2% in the flow results

(spread diameter) was observed for 50% replacement level.

This increase in flow might be due to less water-absorbing

nature of CR, which resulted in availability of more free

water in the fresh mix (Mohammed et al. 2017). Nora et al.

(2014) also reported about 4% increase in flow for CR

cement mortar of similar density. Yu and Zhu (2016)

observed very slight increase in flow due to 50% CR with

size of 1-3 mm, while no significant change in slump flow

was found for up to 50% CR replacement in self-

compacting mortars (Uygunoǧlu and Topçu 2010). On the

other hand, full CR replacement shows decrease in flow

(about 10%), which is attributed to angular particles of CR.

Even though with sufficient availability of free water, the

higher content of angular CR increased the friction between

CR particle in the fresh mix and hence confined its flow

(Mundo et al. 2020). Additionally, the surface roughness

and higher surface area of CR can cause reduction of flow

412


Fig. 5 Density of cement composite at 28 days with

different CR levels

Table 4 Compressive strength at 7 and 28 days

Mix CR

(vol. %)

Compressive strength (MPa)

7 day 28 day

NM0 0 40.92 63.42

NM50 50 8.29 12.11

NM100 100 2.49 2.95

PM0 0 28.01 41.73

PM50 50 7.12 12.87

PM100 100 2.43 3.91

(Li et al. 2020, Roychand et al. 2020).

3.2 Density

The 28-day densities of specimens prepared with

different mixing methods are presented in Fig. 5.

Irrespective of the mixing method, a clear reduction in

density of the cement composite was observed with the

incorporation of CR. The reduction in density is due to the

difference between the specific gravity of CR (1.24) and

sand (2.56) (Angelin et al. 2015, Boukour and Benmalek

2016, Nora et al. 2014). In addition, hydrophobicity of CR

caused air to be retained on the surface of CR, which

resulted in the density reduction (Angelin et al. 2019). It

was found that the decrease in density of the cement

composite with 50% and 100% CR replacement using

different mixing methods was similar, which was about

16% and 31%, respectively. The results are consistent with

those of Uygunoǧlu and Topçu (2010) and Yu and Zhu

(2016) whereby about 15% reduction in density was found

for 50% CR. Nora et al. (2014) also observed a 16%

reduction in oven-dry density of mortar containing 40%

CR. It should be noted that there was no significant

difference in densities when the PM method was adopted.

This clearly demonstrated that the CR cement composites

prepared with the PM method have no unfilled voids and

are as dense as NM mixes, indicating good filling ability of

the grout and feasible usage of the method.

3.3 Compressive strength

Table 4 shows the compressive strength of the cement

composite after 7 and 28 days of curing. It is worth noticing

that, regardless of curing days and mixing method, the

(a)

(b)

Fig. 6 SEM images of mixes (a) PM100 at magnification of

500× (b) NM100 at magnification of 370×

compressive strength of cement composite decreased with

the inclusion of CR. The reason for this loss of strength due

to CR can be attributed to hydrophobicity and rough surface

texture of the CR particles (Angelin et al. 2019, 2017, Di

Mundo et al. 2020, Moreno et al. 2020). This caused a low

adhesion and poor bond between CR and cement matrix,

which resulted in entrapped air on CR surfaces constituting

irregular shaped pores. (Fig. 6). This finding is consistent

with previous studies (Angelin et al. 2019, 2017, Padhi and

Panda 2016).

The reduction in the 7 days compressive strength for

50% and 100% CR with respect to the control mix was 80%

and 94%, respectively for the NM method. In contrast, these

reductions were lower 75% and 91%, respectively for the

PM method. Similarly, CR inclusion at both levels (50%

and 100%) in the NM series reduced the 28-day strength by

80% and 95%, while it was 69% and 90% for the PM

method. This reveals that PM method is more beneficial

than NM method in terms of reducing the strength decrease

due to CR, especially at 50% CR. In the NM method,

especially at high CR content (100%), the decrease in

flowability may affect the dispersion of CR and hence the

compressive strength of the cement composite (Dong et al.

2020, Xue and Cao 2017). On the other hand, in the PM

method, the aggregate particles make up a skeleton resting

on each other that has a specific stress distribution

mechanism (Najjar et al. 2014). The stresses were

transferred through contact areas between aggregate

CR

Cement matrix

Entrap air

Cement matrix

CR

Entrap air

413


Fig. 7 Drying shrinkage of cement composite with CR

particles that cause improvement of strength of the cement

composite prepared using PM method (Du et al. 2017).

Furthermore, due to high fluidity of cement grout in PM

method, there was less entrapped air into cement composite

matrix and on the surface of CR particles (Fig. 6(a))

compared to that prepared with NM method (Fig. 6(b)).

3.4 Drying shrinkage

Generally, initial shrinkage occurs when moisture dries

out in a cement composite. Nevertheless, the aggregate

type, size, quantity, and quality also have a significant

effect on drying shrinkage (Meshgin et al. 2012). Results of

drying shrinkage with different replacement levels by CR

for NM and PM methods are shown in Fig. 7. In terms of

CR content, both NM and PM mixes show similar trend

whereby the drying shrinkage increased with higher CR

replacement levels over time. This increase in shrinkage is

due to the flexible nature of CR (Boukour and Benmalek

2016), as replacing sand with the less stiffer CR led to a

lower internal restraint of the cement composite (Si et al.

2017). For example, the drying shrinkage for 50%

replacement was about 1.5 times of control mix in both NM

and PM mixes. This is consistent with previous work,

where the drying shrinkage for 50% CR (2-4 mm) was

reported to be about 1.4 times of the control specimen

(Fiore et al. 2014, Yu and Zhu 2016).

The drying shrinkage at 28 days for 50% and 100% CR

specimens with NM method was 1037 and 1998 micro-

strains, while for PM method these were 911 and 1819

micro-strains, respectively. The drying shrinkage of

lightweight aggregates concrete masonry unit is limited up

to 0.10% (1000 micro-strains) (ASTM C331/C331M-17

2017). Therefore, the CR replacement level of 50% can be

recommended for the cement composite. In addition, the

drying shrinkage strain of NM50 at 28 days is in similar

range as that reported by Boukour and Benmalek (2016) of

about 1000 micro-strains for CR mortar with similar

density. At the same time, the drying shrinkage of PM50 is

12% lower and in range with other lightweight aggregate

concrete. For example, Liu et al. (2019) reported drying

shrinkage strain up to 950 micro-strains for expanded shale

as lightweight aggregates in concrete. Additionally,

lightweight concrete incorporating cold-bonded fly ash as

Fig. 8 Sound absorption coefficients of cement composite

incorporating CR

Table 5 Sound absorption coefficient at selected frequencies for

NRC

Mix CR

(vol. %)

Frequency (Hz)

250 500 1000 2000

NM 50 0.089 0.103 0.104 0.088

100 0.106 0.110 0.099 0.094

PM 50 0.058 0.082 0.088 0.075

100 0.061 0.102 0.088 0.084

lightweight aggregate has a drying shrinkage strain in the

range of about 900-1000 micro-strains for mixes with w/c

ratio of 0.55 (Gesoglu et al. 2004, 2006).

The drying shrinkage at the age of 8 months for both

replacement levels (50% and 100%) in cement composite

prepared with NM method was about 57% and 229% higher

than NM0, respectively. In the case of the PM method, this

increment was lower, at 45% and 190% compared to PM0,

respectively. Hence, the drying shrinkage of all PM mixes

was less than that of the corresponding NM mixes. The

decrease in drying shrinkage due to the PM method can be

attributed to the connected and closely-packed aggregate

particles. Upon the loss of capillary water, the cement

composite tends to contract, where low compressibility of

PM method due to high one-to-one aggregate contact

skeleton provided movement resistance of cement paste,

thereby contributed to reduced shrinkage (Alfayez et al.

2019, Du et al. 2017, Najjar et al. 2014).

3.5 Sound absorption

Sound absorption coefficient (α) represents the ability of

material to absorb sound. In this study, the sound absorption

of the cement composite containing CR prepared by NM

and PM methods was measured for the valid frequency

range of 200-4500 Hz. Fig. 8 presents the results of average

1/3 octave band sound absorption coefficient measured by

impedance tube. Apparently, the results indicate that the

inclusion of CR marginally increased the sound absorption

with both mixing methods. For instance, in both mixing

methods, 50% CR yielded peak α values from 0.12-0.14. At

the same frequency range, 100% CR yielded only slightly

higher peak α values at 0.14-0.16. The increase of α due to

higher CR content can be attributed to the amount of air

entrapped on the CR surface (Corredor-Bedoya et al. 2017,

414


Table 6 Sound absorption coefficient at selected frequencies for

NRC

Mix CR

(vol. %)

Frequency (Hz)

250 500 1000 2000

NM 50 0.089 0.103 0.104 0.088

100 0.106 0.110 0.099 0.094

PM 50 0.058 0.082 0.088 0.075

100 0.061 0.102 0.088 0.084

Medina et al. 2016, Mohammed et al. 2012). Corredor-

Bedoya et al. (2017) also reported peak α value of about 0.2

(frequency range of 400-2500 Hz) for the CR cement

composite with density of about 1400 kg/m3. The peak α

value for 50% CR replacement in a hollow concrete block

(density about 1500 kg/m3) was observed to be 0.23

(frequency range of 100-5000 Hz) (Mohammed et al.

2012). However, the peak α value at both replacement

levels in cement composite prepared with PM method was

about 10% lower than that for NM method as there was less

entrapped air in the mixes prepared with the PM method

(Fig. 6).

It is difficult to compare the sound absorption properties

of materials using α values at different frequency ranges.

Thus, the noise reduction coefficient (NRC) is computed

using the average value of α at frequencies 250, 500, 1000,

and 2000Hz (Table 5) (Tie et al. 2020). Results of NRC

(Fig. 9) show slight increase in NRC of cement composite

with higher CR content by both mixing methods. Literature

also showed that there was an improvement in NRC with

increase in CR content (Rashad 2016). NRC is mainly

influenced by the amount of air void in cement composite.

Cement composite with low density due to the formation of

porosity has large value of NRC (Tie et al. 2020). In

addition, properties of material such as size, thickness,

density, and porosity can change the absorption behaviour

of material (Seddeq 2009). The tested specimen with

greater thickness will have more air volume, whereas higher

binder content tends to produce a denser structure. These

could be the reasons that the NRC varies with those

reported in the literature. Mohammed et al. (2012) found

about 12% NRC for dry-mix concrete block prepared with

50% CR, which was slightly higher compared to those of

cement composites from this study (NM50 and PM50).

Though the CR replacement level is similar, the higher

NRC of the former can be due to lower density and binder

content of the concrete block, as well as the dry-mix

production method. Similarly, Ling et al. (2010) observed

12% NRC for 30% CR concrete block having low binder

content of about 330 kg/m3. Whereas 18% NRC was

reported for concrete with 20% CR and binder content of

479 kg/m3 (Sukontasukkul 2009). Although the CR

replacement level was lower than that of NM50 and PM50,

the increase in NRC could be due to different binder content

and thickness of the specimen tested. CR as 100%

replacement of coarse aggregate in concrete with binder

content of 360 kg/m3 resulted NRC of about 8% (Medina et

al. 2016), which is lower than NM100 and PM100 cement

composites in this study, and this could be due to difference

in densities, thickness, and nature of material replacement.

Fig. 9 Noise reduction coefficients for cement composite

incorporating CR

It should be noted that the NRC of cement composite

containing CR prepared with PM method has inferior sound

absorption properties than that of NM method. The NRC of

PM50 and PM100 was about 20% less than the

corresponding NM50 and NM100 specimens. The slightly

higher value of NRC for specimens prepared with NM

method could again be due to more voids on CR surface in

this series (Fig. 6(b)), as the air entrapped in these voids can

easily absorb sound (Medina et al. 2016, Rashad 2016).

However, the improvement in α values and NRC for 100%

CR is very little compared to that of 50% CR. Thus, the

recommended maximum replacement level for CR is 50%

4. Conclusions

Based on the investigations carried out, the following

conclusions can be drawn from this study:

• The consistency of cement composite reduced (about

10%) with 100% CR in NM mix, whereas mixes

prepared with PM method do not have such concern due

to the nature of the mixing method.

• Inclusion of CR reduced the density of cement

composite by up to 31%; no significant difference was

found when PM method was adopted, demonstrating

that no additional void was introduced, and the PM

method is feasible.

• Although the compressive strength of cement

composite was reduced in the presence of CR, the

cement composite prepared with PM method exhibited

up to 10% lower strength reduction than that of NM

method.

• PM method was able to reduce the drying shrinkage of

the CR cement composite by 20% compared to NM

method, though the inclusion of CR increased the

shrinkage considerably. Thus, to limit the drying

shrinkage of cement composite, the maximum

replacement level for CR is up to 50%.

• The sound absorption and NRC were about 10%

higher for cement composite containing 100% CR

compared to that of 50% CR. However, the specimens

prepared with the PM method exhibited about 20%

lower NRC compared to those of NM method. Due to

the minor difference in α values and NRC, the

recommended replacement level for CR is 50%.

415


Findings from this paper show that PM method has

good potential to be adopted for CR cement composite as it

exhibited similar density, better compressive strength, lower

drying shrinkage and sound absorption properties within the

range as NM method. Furthermore, it is more economical

and easier to prepare the CR cement composite using the

PM method compared to the NM method, as it does not

require additional consolidation processes. Further works

dealing with the durability and thermal properties of the CR

cement composite can be explored to establish its feasibility

in usage for construction application.

Acknowledgement

The financial support provided by the University of

Malaya under the grant GPF034A-2018 is gratefully

acknowledged.

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Assessment of lightweight recycled crumb rubber composite