properties of crumb rubber hollow concrete block

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Journal of Cleaner Production 23 (2012) 57e67

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Journal of Cleaner Production

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Properties of crumb rubber hollow concrete block

Bashar S. Mohammed a,*, Khandaker M. Anwar Hossain a, Jackson Ting Eng Swee b, Grace Wong b,M. Abdullahi c

aDepartment of Civil Engineering, Ryerson University, 350 Victoria Street, Toronto, ON M5B 2K3, CanadabDepartment of Civil Engineering, Universiti Tenaga Nasional, Selangore, MalaysiacDepartment of Civil Engineering, Federal University of Technology, Minna, Nigeria

a r t i c l e i n f o

Article history:Received 22 December 2010Received in revised form22 October 2011Accepted 23 October 2011Available online 2 November 2011

Keywords:Crumb rubberConcreteAcousticThermalElectricalAbsorptionHollowBlock

* Corresponding author. Tel.: þ1 416 770 8755.E-mail address: [email protected] (B

0959-6526/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jclepro.2011.10.035

a b s t r a c t

Several researches have been conducted to determine the fresh and hardened properties of crumbrubber concrete (CRC), concrete containing crumb rubber as partial replacement to fine aggregate. Thebenefits of outlined from these works include low density, good thermal resistivity, better soundabsorption, increase slump values and toughness, and better impact resistivity of the resulting concrete.Reduction in strength, increase in water absorption and are some of the adverse effect of utilizing crumbrubber in concrete. The study reported in this paper is a development of crumb rubber hollow concreteblock (CRHCB). For the purpose of this work, sixty-four trial mixes were prepared to produce hollowconcrete blocks of dimension 390 mm � 190 mm � 190 mm using 0%, 10%, 25% and 50% crumb rubber(CR) as replacement of fine aggregate. Tests conducted on the hardened concrete include compressivestrength, thermal conductivity, electrical resistivity, acoustic absorption and transmission loss, andelectrical resistivity. It has been found that CRHCB can be produced as load-bearing hollow blocks as wellas lightweight hollow blocks. The CRHCB also has better thermal, acoustic and electrical properties incomparison with conventional hollow block.

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1. Introduction

Accumulations of discarded scrap tires are non-biodegradableand have been a major concern. Even after long-period of landfilltreatment, unmanaged waste tire poses environmental and healthrisk through fire hazard and as a breeding ground for disease-carrying mosquitoes (Milanez and Buhrs, 2009; Pelisser et al.,2011). Therefore, utilization of crumb rubber (CR) from this scraptires for the production building materials in the constructionindustry would help to preserve the natural resources and alsomaintain the ecological balance (Mohammed, 2010).

Many research works have been carried out to investigate theproperties of fresh and hardened crumb rubber concrete (CRC).Previous works show that the unit weight of CRC decreases as thepercentage of the CR replacement increases due to the low specificgravity of CR particles (Sukontasukkul and Chaikaew, 2004). Also,air content increases as CR content increases due to the non-polarity of CR causing water to be repelled and air trapped on thesurface (Turatsinze and Garros, 2008). Partial replacement of the

.S. Mohammed).

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fine aggregate by CR will improve the workability of CRC(Mohammed and Azmi, 2011). The strengths (compressive, flexural,splitting and modulus) of CRC decrease as the partial replacementof fine aggregate with CR increases (Siddique and Naik, 2004;Khaloo et al., 2008; Ganjian et al., 2008; Zheng et al., 2007) CRCexhibits high capacity for absorbing plastic energy under bothcompression and tension loading which also possesses highertoughness (Mohammed et al., 2011). It has also been asserted thatinclusion of CR into concrete decreases the thermal transmittanceor improves thermal insulation performance (Yilmaz andDegirmenci, 2008). In addition, CRC also exhibits better soundabsorption and noise reduction coefficient compared to conven-tional concrete (Sukontasukkul, 2008). All the above studiesconcentrated on investigating wet-mix CRC properties instead ofdry-mix, which is the subjected matter of this study.

Conventional hollow concrete block is a large rectangular brickused in construction. Concrete blocks are made from cast concrete,i.e. Portland cement and aggregate, usually and small size gravelwith sufficient water content to produce dry-mixed mixtures. SuchHollow concrete block are known for their high-density block aswell as known low to moderate thermal, acoustic and electricalresistivity properties. Due to these drawbacks, hollow concreteblock is an unpopular construction materials among developers.

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Table 2Properties of fine aggregate, coarse aggregate and crumb rubber.

Properties Fine aggregate Coarse aggregate Crumb rubber

Water absorption, % 4.48 1.13 e

Specific gravity 2.71 2.65 0.95Moisture content, % 16.7 0.94 1.15Fineness modulus 2.32 e 0.92

B.S. Mohammed et al. / Journal of Cleaner Production 23 (2012) 57e6758

Therefore, an improvement on these drawbacks is essential toencourage government agencies, developers and contractors onutilization of hollow concrete block. This would also be a novelsolution for industrialized building system.

Therefore, the main objective of this research work reported inthis paper is to improve acoustic, thermal and electrical propertiesof the hollow concrete block by partially replacing the fine aggre-gate by CR. Inclusion of fly ash and silica fume for strengthenhancement purposes to achieve the product requirements is alsoanother aspect of the study. The outcome of this research isa lightweight high performance hollow concrete block known asCrumb Rubber Hollow Concrete Block (CRHCB) that can be usefulfor practical construction.

2. Material

2.1. Cementitious and pozzolanic materials

Ordinary Portland Cement (OPC) Type I conform to therequirements of ASTM C150 (ASTM, 2005f) was used in producingthe concrete mixtures. Fly ash obtained from Kapar Energy PowerStation (Malaysia) was used in this study. The fly ash was classifiedas class F and has pozzolanic properties due to its chemicalcomposition according to the specification of ASTM C618 (ASTM,2005j). It has a total amount of silicon dioxide (SiO2), aluminumoxide (Al2O3) and iron oxide (Fe2O3) of 89.4% and loss on ignition ofless than 6%.

Silica fume used in this study was classified as pozzolans withstrength activity index (SAI) greater than 105%. The main purposeof using silica fume was to enhance the bonding between cementpaste and the CR particles of the CRC dry-mix. The density, specificgravity, chemical composition and SAI of the silica fume were ob-tained according to the requirements of the ASTM C1240 (ASTM,2005a). Similar properties for fly ash were obtained according tothe requirements of ASTM C311 (ASTM, 2005h). Specific gravity ofOPC was determined accordance with the requirements of theASTM C188 (ASTM, 2005g). Table 1 shows the chemical composi-tion and properties for the cement, fly ash and silica fume.

2.2. Coarse aggregate, fine aggregate and crumb rubber

The processed CR mesh size 30 or No.30 (600 mm) from Ara JayaEnterprise (Malaysia), river sand and 10 mm nominal size gravelwere used as aggregate in the CRC mixtures. Tests conducted oncoarse aggregate, fine aggregate and CR include oven-dry density,

Table 1Chemical composition and properties of cementitious/pozzolanic materials.

Chemical composition/properties

Cement (%) Fly ash (%) Silica fume (%)

SiO2 21.54 62.5 89.22Fe2O3 3.63 3.5 0.421Al2O3 5.32 23.4 0.065CaO 63.33 1.8 0.073MgO 1.08 0.34 0.061Na2O e 0.24 0.029K2O e 0.95 0.268SO3 2.18 1.2 e

C3S 46.96 e e

C2S 26.33 e e

C3A 7.96 e e

C4AF 11.05 e e

Loss on ignition, % 2.5 5.61 e

Specific gravity 3.1 2.04 2.27Strength activity index (SAI) e 76 106Specific surface area, cm2/g 3091 2262 e

specific gravity, water absorption and fineness modulus accordingto the requirements of the ASTM C127 (ASTM, 2005b), ASTM C128(ASTM, 2005c), and FM 5-559 (FM, 2004), respectively. Test resultsare shown in Table 2. The grading of the river sand, coarse aggre-gate and CR are shown in Fig. 1. The grading limits (upper andlower) are only applicable to coarse aggregate and sand stated inthe requirement of the ASTM C33 (ASTM, 2005i). The sieve analysisfor river sand and coarse aggregate was performed in accordancewith the requirements of ASTM C136 (ASTM, 2005d) and FM 5-559(FM, 2004) for CR.

2.3. Crumb rubber manufacture processes

Preparation of crumb rubber started with shredding processthat reduced the scrap tire into 100mme50mm. This was followedby granulation process in two stages where primary and secondarygranulation further reduced the size from 50 mm to 10 mm.Separation of steel wire from the tire chips occurred after primarygranulation before fed into secondary granulation. Tire chips werethen grinded into smaller mesh sizes to produce crumb rubber ofrequired gradation by cracking or grinding in rolling mills. Screens/gravity separators and aspiration equipment were used to removemetal and fibers, respectively in the production process.

3. Mixture proportions

3.1. Mixtures investigation

This work involved fifteen trial mixes at three levels of crumbrubber replacement (10%, 25%, and 50%) by volume and a controlmix. Replacement of fly ash (5%, 15%, and 30%) and silica fume (5%,10%, and 20%) by volume to cement content was made. Optimumreplacement for crumb rubber is 50% in order to achieve theconcrete block requirement according to ASTM C90 and BS EN 771.Thus, a total of sixty-four different mixtures were prepared, cast

Fig. 1. Fine aggregate, coarse aggregate and crumb rubber particles size distribution.

Table 3Ratio of mixture proportioning by volume for dry-mix concrete.

Mix Cementitious material CR 0% CR 10% CR 25% CR 50%

C F S CA FA CR FA CR FA CR FA CR

Control 1 0 0 1 2 0 1.8 0.2 1.5 0.5 1 1S5F0 0.95 0 0.05 1 2 0 1.8 0.2 1.5 0.5 1 1S10F0 0.9 0 0.1 1 2 0 1.8 0.2 1.5 0.5 1 1S20F0 0.8 0 0.2 1 2 0 1.8 0.2 1.5 0.5 1 1S0F5 0.95 0.05 0 1 2 0 1.8 0.2 1.5 0.5 1 1S0F15 0.85 0.15 0 1 2 0 1.8 0.2 1.5 0.5 1 1S0F30 0.7 0.3 0 1 2 0 1.8 0.2 1.5 0.5 1 1S5F5 0.9 0.05 0.05 1 2 0 1.8 0.2 1.5 0.5 1 1S10F5 0.85 0.05 0.1 1 2 0 1.8 0.2 1.5 0.5 1 1S20F5 0.75 0.05 0.2 1 2 0 1.8 0.2 1.5 0.5 1 1S5F15 0.8 0.15 0.05 1 2 0 1.8 0.2 1.5 0.5 1 1S10F15 0.75 0.15 0.1 1 2 0 1.8 0.2 1.5 0.5 1 1S20F15 0.65 0.15 0.2 1 2 0 1.8 0.2 1.5 0.5 1 1S5F30 0.65 0.3 0.05 1 2 0 1.8 0.2 1.5 0.5 1 1S10F30 0.6 0.3 0.1 1 2 0 1.8 0.2 1.5 0.5 1 1S20F30 0.5 0.3 0.2 1 2 0 1.8 0.2 1.5 0.5 1 1

Fig. 3. Electrical resistivity test specimens.

B.S. Mohammed et al. / Journal of Cleaner Production 23 (2012) 57e67 59

and tested for compressive strength with different percentage of CRas a partial replacement to the fine aggregate by volume. Table 3shows the ratio for mixture proportioning. The conventional mixratio of 1:1:2 by volume [ratio of Cement (C): Fine aggregate (FA):Coarse aggregate (CA)] was adopted in this study. In Table 3; themix S10F15 for example means the cement has been partiallyreplaced (by volume) by 10% of silica fume and 15% of fly ash. It hasbeen reported that the total water content required for theproduction of zero slump concrete mixture for dry-mix is about5.5% of the total batch weight (Jablonski, 1996). In this study, thewater content required for all themixtures was 8% of the total batchweight as practiced by hollow concrete block manufactures inMalaysia. The slightly higher w/c ratio has also been recommendeddue to the higher water absorption of the fine aggregate (4.48).Compressive strength test has been carried out on the samples ofthe 64 mixtures and only the best three series with total twelvemixtures (based on the compressive strength values) have beenselected for further investigations.

3.2. Preparation of test specimens

The compressive strength test specimens were prepared inaccordance with the requirements of the BS EN 12390-2 (BS,2000a). Modification was carried out on the compaction method

Fig. 2. Thermal conductivity test specimens.

(rod tamping) to comply with the requirements of the concretehollow blocks’ manufactures. Inside the factory, concrete hollowblock was produced by subjecting the mold to vibration and pres-sure to reduce the percentage of void ratio as much as possible. Thematerials for the block making were initially compacted threetimes manually using a hammer and of approximately equal depth.Then compaction was carried out by using compression machine;a compression force at a rate of 600 kNmin�1 is applied for 1min tocompact the material in the mold. Test specimens were removedfrom the mold after 24 h and subjected to water curing. Testspecimens for thermal conductivity (Fig. 2), electrical resistivity(Fig. 3) and acoustic absorption and transmission loss (Fig. 4)underwent the same compaction method. Table 4 shows the totalnumber and size of the test specimens as well as adopted Standardtest methods.

3.3. Production of CRHCB in the laboratory

The CRHCBs shown in Fig. 5 were fabricated in steel molds withthe internal dimension of 390 mm � 190 mm � 190 mm(length � width � depth), two hollow cavities and 75% of totalvolume. After the compaction is done, CRHCBs were removed

Fig. 4. Acoustic absorption and transmission loss test specimens.

Table 4Number and size of test specimens.

Test Dimension Number Standard

Compressivestrength e cube

100 mm � 100 mm �100 mm

192 BS EN 12390-3(BS, 2001)

Thermal conductivity 24 mm dia. � 35 mm 36 ASTM C1045(ASTM, 2004a)

Acoustic absorption andtransmission loss

100 mm dia. � 10 mm(Low freq.)

36 ASTM E1050(ASTM, 2005k)

28 mm dia. � 10 mm(High freq.)

36

Electric resistivity 100 mm � 100 mm �100 mm

36 e

Hollow concrete block 190 mm � 190 mm �390 mm

Compressive strength 36 ASTM C140(ASTM, 2005e)Density and water

absorption12

Fig. 6. Compressive strength test for crumb rubber hollow concrete block.


immediately from the steel mold (to simulate the normal proce-dure in the factory) and left at room temperature for 24 h. Watercuring was then carried out for 28-days before tests wereconducted.

4. Test procedures

4.1. Cube compressive strength

The compressive strength for the 64 mixtures was determinedusing cube specimens in accordance with the requirements of BSEN 12390-3 (BS, 2001). The test was carried out at 28-days usingGOTECH compression-testing machine in accordance with therequirements of the BS EN 12390-4 (BS, 2000b).

4.2. Compressive strength, density and water absorption tests forCRHCB

The compressive strength, density and absorption tests for theCRHCB were performed in accordance with the requirements ofASTM C140 (ASTM, 2005e). The compressive strength of CRHCBwas obtained (Fig. 6) at 28-days using GOTECH compression-testing machine equipped with rigid steel plates having a dimen-sion of 200 mm� 400 mm. The length and width of the steel platesare at least 6.3 mm greater than the length and width of the CRHCBunits according to ASTM C140 (ASTM, 2005e). The tested surfaces ofthe CRHCB were prepared using saw-cut to produce flat surface forall specimens.

Fig. 5. Crumb rubber hollow concrete block unit.

4.3. Acoustic absorption and transmission loss test

Sound absorption test was performed in accordance with therequirements of ASTM E1050 (ASTM, 2005k) using impedance tubemethod. Sound transmission loss was obtained using the samecomputerized apparatus for sound absorption having transmissionloss tube kit, sample holder and additional extended tube. Fig. 7shows the apparatus used to conduct the testing. Sound trans-mission class (STC) is an integer rating of how well a building wallattenuates airborne sound. STC calculation was performedaccording to the guideline of the ASTM E413 (ASTM, 2005l). Theability of material to absorb sound can be calculated using onesingle value known as the noise reduction coefficient (NRC) asgiven in Eq. (1) (Sukontasukkul, 2008).

NRC ¼ a250 þ a500 þ a1000 þ a20004

(1)

where a is the sound absorption coefficient at different frequency

4.4. Thermal conductivity test

Linear heat conduction is concerned with the transmission ofheat in material. This method, also known as steady-state method,was introduced by Tritt (Terry, 2004). Fig. 8 shows the linear heat

Fig. 7. Acoustic sound absorption and transmission loss testing apparatus.

Fig. 8. Thermal conductivity testing apparatus.

0

10

20

30

40

50

0 10 20 30 40 50 60

Co

mp

res

sive S

tre

ng

th

, N/mm²

Crumb Rubber Replacement, %

S00F00S05F00S10F00S20F00

50

a

b


conductivity test equipment. The rate of heat conductivity wasobtained by using Eq. (2) according to the guideline of the ASTMC1045 (ASTM, 2004a):

l ¼ QLAðTh � TcÞ (2)

where Q is the time rate of one-dimensional heat flow through themetering area; A is the surface area for heat transfer; L is the

Fig. 9. Quadtech 1920 LCR meter.

thickness of the matter through which the heat is passing; l isa thermal conductivity, which is dependent on the nature of thematerial and its temperature; and Th and Tc are the hot and coldsurface temperature of the specimen, respectively.

4.5. Electrical resistivity test

The Quadtech 1920 LCR meter (Fig. 9) using two electrodesmethod was preferred in this experiment as it offers flexibility and

0

10

20

30

40

0 10 20 30 40 50 60

Co

mp

res

sive S

tre

ng

th

, N/mm²


0 10 20 30 40 50 60


S00F00S00F05S00F15

0

10

20

30

40

50

Co

mp

res

sive S

tre

ng

th

, N/mm²

c

S05F05S05F15S05F30S10F05S10F15S10F30S20F05S20F15S20F30

Fig. 10. a. Compressive strength versus crumb rubber replacement for silica fumereplacement at 28-day of curing. b. Compressive strength versus crumb rubberreplacement for fly ash replacement at 28-day of curing. c. Compressive strengthversus crumb rubber replacement for combination of both silica fume and fly ashreplacement at 28-day of curing.

Fig. 11. Adhesion of cement paste (CRC).


simplicity in measuring inductance, capacitance and resistance. Inthis study, two copper electrodes were used to measure resistance.It acts as a probe, attached and clamped to the concrete sample.Connectors were fabricated to minimize movement of the cableswhich could cause the result to be inconsistent. It is important toobtain the resistivity rather than the resistance, as it is related tothe specimen shape and size. The resistivity of the specimen wascalculated by using Eq. (3) (Chen and Lui, 2005; Koleva et al., 2008):

r ¼ RSL

(3)

Where r is the electrical resistivity, R is the electrical resistance, S isthe cross-sectional area of the specimen, and L is the length ofspecimen.

BANon-loadbearing(ASTM C129)> 3.45N/mm2

Loadbearing (BS EN 771-3)

> 7N/mm2

Loadbearing(ASTM C90)> 11.7N/mm2

Non-loadbearing(BS EN 771-3)

> 3N/mm2

fcu = -0.216CR + 11.76R² = 0.986

fcu = -0.241CR + 13.26R² = 0.995

fcu = -0.209CR + 11.13R² = 0.971

0

2

4

6

8

10

12

14

16

0 10 20 30 40 50 60

Com

pres

sive

Str

engt

h, f c

u,N

/mm

2


S0F15 S5F15 S10F15

Linear (S0F15) Linear (S5F15) Linear (S10F15)

Fig. 12. Compressive strength of CRHCB versus crumb rubber replacement at 28-day ofcuring.

5. Result and discussion

5.1. Cube compressive strength

Fig. 10aec show the overall compressive strength results for thesixty-four trial mixtures at 28-day of curing. Three series ofmixtures which are S0F15, S5F15 and S10F15with three levels of CR(10%, 25% and 50%) replacement and the control concrete mixtureswere selected. These mixtures were selected as the reductions intheir compressive strength were less than others.

Partially replacing the cement with a combination of silica fumeand fly ash has further enhanced the compressive strength. Thiswas expected because of the filling effect of silica fume due to itsfiner particle size, thus providing a good adherence between theaggregate and cement matrix (Erhan et al., 2004). The properamount of fly ash replacement to cement which was 15% may havepositive effects on the interfacial bond between the cement paste

CCA

CAFA

CR

Cement pasteCement paste

TrappedTrappedAirAir

Fig. 13. Microstructure of crumb rubber concrete.

Light Weight(ASTM C90 &

C129)< 1680kg/m3

Medium Weight(ASTM C90 &

C129)1680kg/m3 -2000kg/m3

Normal Weight(ASTM C90 &

C129)> 2000kg/m3

1400

1500

1600

1700

1800

1900

2000

2100

0 10 20 30 40 50 60

Den

sity

, kg/

m3


S0F15 S5F15 S10F15

Fig. 14. Gross dry density of CRHCB versus crumb rubber replacement at 28-day curingage.

1.13

0.97

0.79

0.67

1.06

0.95

0.78

0.64

1.01

0.94

0.74

0.61

Ordinary Concrete

0.50

0.60

0.70

0.80

0.90

1.00

1.10

1.20

1.30

0 5 10 15 20 25 30 35 40 45 50 55

The

rmal

Con

duct

ivit

y, W

/m.K


S0F15

S5F15

S10F15

Fig. 16. Thermal conductivities versus crumb rubber replacement at 28-day of curingin air.


and the aggregate. In addition, the fly ash in concrete not onlyreduces the amount of calcium hydroxide (lime), but in the processconverts it into calcium silicate hydrate providing more cementingmaterial and thereby enhancing the strength of the concretemixture. Lam et al. (1998) have reported that the compressivestrength of a concretemixture would be increased when the properamount of silica fume and fly ash replacement to the cement isused.

Also, it can be seen that as the percentage of CR replacementincreases, the compressive strength decreases (Fig. 10aec). This isdue to the physical properties of the CR which repels water duringthemixing process and entrapping air on its surface. The entrappedair on the surface of CR particles causes an increase in air content inthe CRC mixture, producing a resultant reduction in compressivestrength.

The reduction in the compressive strength as the amount of CRincreases can also be explained from matrix of the fresh dry-mixedCRC as shown in Fig. 11. Basically, this figure shows the cementpaste adhesion of fresh dry-mixed CRC.

As the CR content in the mixture increased the adhesionbetween the cement paste and coarse aggregate decreased. The

Light weight(ASTM C90)< 288kg/m3

Medium weight(ASTM C90)< 240kg/m3

Normal weight(ASTM C90)< 208kg/m3

150

200

250

300

0 10 20 30 40 50 60

Abs

orpt

ion,

kg/

m3


S0F15S5F15S10F15

Fig. 15. Crumb rubber concrete hollow block absorption versus crumb rubberreplacement at 28-day curing age.

coarse aggregate surface shown in Fig. 11A is fully coated by cementpaste whereas the replacement of the CR to fine aggregate was 10%.As the replacements of CR increased to 25% (Fig. 11B) and 50%(Fig. 11C), the cement paste adhesion decreased as the coarseaggregate surface was exposed with less cement paste coating. Thespecific surface area increased as the crumb rubber replacementincreased since crumb rubber is finer than sand particles. Due tothe less adhesion of the concrete matrix, weak bonding betweencoarse aggregate and cement paste formed in hardened concrete.

5.2. Compressive strength, density and water absorption of CRHCB

The compressive strengths of the CRHCBwhich have been madefrom the selected three series containing 12 mixtures are shown inFig. 12. It can be seen from Fig. 12 that load-bearing CRHCB can beproduced, from mixture S5F15 with 6.5% CR replacement, inaccordance with the requirements of the ASTM C90 (ASTM, 2004c).Non load-bearing CRHCB can be produced, with 40.7% maximumCR replacement, in accordance with the requirements of the ASTMC129 (ASTM, 2004b). However, according to the requirements ofthe BS EN 771-3 (BS, 2003), load-bearing and non load-bearingCRHCB can be produced by maximum replacement of CR in therange 19.8%e38.9%, respectively.

Generally it can be noticed that the compressive strength ofCRHCB decreased as the percentage of CR increased. The reductionin strength is owing to the weak bonding between cement pasteand CR particles. The weak bonding formed by the entrapped air onthe CR particles’ surface is due to the non-polarity of CR causingwater to repel.

As show in Fig. 13, the entrapped air on the surface of CRparticles leads to reduction in the bonding areawithin the concrete

0

0.05

0.1

0.15

0.2

0.25

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Soun

d A

bsor

ptio

n C

oeff

icie

nt

Frequency, HzS0F15CR0 S0F15CR10 S0F15CR25 S0F15CR50

Fig. 17. Sound absorption coefficients versus frequency for silica fume 0% replacement.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Soun

d A

bsor

ptio

n C

oeff

icie

nt


Fig. 18. Sound absorption coefficients versus frequency for 5% silica fume replacement.

5.576.03

8.44

11.77

5.455.84

7.77

11.34

5.365.74

7.30

11.24

0

2

4

6

8

10

12

Noi

se R

educ

tion

Coe

ffic

ient

, %

Crumb Rubber Replacement, %S0F15 S5F15 S10F15

Fig. 20. Noise reduction coefficients for CRHCB.


matrix. The CR particles were not fully bonded to the cementmatrix but rather more bonded to fine aggregate (FA) and coarseaggregate (CA) surface. As the load is applied to CRHCB during test,micro cracks start to form at weak interface between cement pasteand CR due to stress concentration and eventually leads to failurewith continuous application of load.

The optimum replacement percentage of the silica fume is foundto be 5% based on the results obtained in Fig. 12. As the replacementincreases to 10%, the strength decreases. The increase in strengthwas due to the reaction between silica fume and calcium hydroxidein the interface zone. The higher the silica fume concentration atthe interfacial zone, the narrower the thickness of interfacialtransition zone and the higher the interfacial bond strength. Thecalcium hydroxide in crystalline form, is the end product ofhydration and has no strength value within the concrete matrix butonly serve as a filler. Knowing that, the amount of calciumhydroxide in the interfacial zone is limited and related to thehydration of cement. Therefore, the consumption of silica fume inthe interfacial zone is proportional to the amount of availablecalcium hydroxide around aggregates. In other words, there shouldbe a critical silica fume dosage to react with calcium hydroxide inorder to improve on the interfacial bond strength (Shannag, 2000).

The gross density of the CRHCB is shown in Fig.14. In accordancewith the requirements of ASTM C90 (ASTM, 2004c) and C129(ASTM, 2004b), test results revealed that the density of CRHCBranges from lightweight to medium weight hollow blocksdepending on the percentage of CR replacement.

The density of CRHCB decreases as the percentage of CRreplacement increases. This is due to the low specific gravity of CR

0

0.05

0.1

0.15

0.2

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Soun

d A

bsor

ptio

n C

oeff

icie

nt

Frequency, Hz

S10F15CR0 S10F15CR10 S10F15CR25 S10F15CR50

Fig. 19. Sound absorption coefficients versus frequency for 10% silica fumereplacement.

(0.95) compared to fine aggregate (2.57). In addition, the silica fumecontributes to the slight reduction in density as silica fumereplacement to the cement content increases. This is because thespecific gravity of silica fume (2.27) is less than that of cement(3.10). Also, entrapped air on the CR particles’ surface in the CRCmicrostructure may contribute to the reduction in density of theCRHCB.

The water absorption of the CRHCB is shown in Fig. 15. For allmixes, water absorption of CRHCB increases as the percentage of CRreplacement to the fine aggregate increases. However, in accor-dance with the requirements of ASTM C90 (ASTM, 2004c), thewater absorption of the CRHCB is within the range of light tonormal weight hollow blocks.

The main reason for the increase in water absorption of CRHCBis the presence of air in the CRC microstructures. The air contentincreases as the percentage of CR replacement to fine aggregateincreases. This is because CR repels water during mixing andthereby allows entrapped air on the surface of CR particles. Uponhardening, voids are formed inside the CRC mixture.

As for the effect of silica fume on thewater absorption of CRHCB,the optimum amount of silica fume replacement to cement is 5%.This is the optimum amount required to react with calciumhydroxide to create densified interfacial transition zone andreduces the thickness of the zone as well. This produced concretematrix with the highest compressive strength compared with othertwo series containing 0% and 10% silica fume as a replacement tocement contain. However, 10% replacement of silica fumes tocement leave excess amount of un-react silica fume inside the CRCmatrix. This fills the air void inside the microstructure of the CRCdue to its micro filling ability, resulting in a decrease in waterabsorption.

15

20

25

30

35

40

45

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Tra

nsm

issi

on lo

ss c

oeff

icie

nt, d

b


Fig. 21. Sound transmission loss versus frequency for silica fume at 0% replacement.


15

20

25

30

35

40

45

Tra

nsm

issi

on lo

ss c

oeff

icie

nt, d

b

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000


28.5

29

29.5

30

30.5

31

31.5

32

32.5

33

33.5

34

0 10 25 50

Soun

d tr

ansm

issi

on c

lass

, db

Crumb rubber, %

S0F15

S5F15

S10F15

Fig. 24. Sound transmission classes, STC versus crumb rubber replacement.

400

450

500

.m

Ω


5.3. Thermal conductivity

The thermal conductivities of the CRC mixtures are shown inFig. 16. Test result show that the thermal conductivity deceases asthe percentage of CR replacement increases. In addition, inclusionof fly ash and silica fume produces a reduction in the thermalconductivity.

This can be explained by the microstructure of CRC. Air istrapped on the surface of CR leading to an increase in the amount ofair content. The thermal conductivity of the air, 0.025 W m�1 K isless than that of the concrete, 1.7 W m�1 K. Therefore, the air voidsinside the CRC mixture opposes the thermal transfer through theCRC. In addition, CR particles also restrain thermal flow because thethermal conductivity of the CR (0.16 W m�1 K) is less than that offine aggregate (1.5 W m�1 K).

The combination of silica fume and fly ash as a replacement tothe cement content leads to further reduction in the thermalconductivity. This is due to the lower thermal conductivity of silicafume and fly ash compared to cement. Similar observation on theeffect of silica fume and fly ash on the thermal conductivity of theconcrete mixture was reported earlier (Demirbo�ga, 2003).

5.4. Sound absorption and transmission loss properties

CRC has better sound absorption compared to conventionalconcrete as shown in Figs. 17e19. The noise reduction coefficient(NRC) is given in Fig. 20. Test result show that NRC increases as thepercentage of CR replacement increases. Sound absorption isdefined, as the incident sound that strikes a material and is notreflected back. Hence, the sound was easily absorbed through theentrapped air on CR surface inside the microstructure of CRHCB.The same observation was reported by Sukontasukkul (2008).However, it is noteworthy that for the same CR content; NRCdecreases as the silica fume content increases. This is due to the

15

20

25

30

35

40

45

Tra

nsm

issi

on lo

ss c

oeff

icie

nt, d

b

Frequency, Hz

S10F15CR0 S10F15CR10 S10F15CR25 S10F15CR50

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000


silica fume effects by narrowing the thickness of the interfacialtransition zone at optimum replacement of 5% silica fume tocement content and hence, reduce the amount of air content in CRCmixture. As the replacement of silica fume at 10%, the un-reactedsilica fume left inside the microstructure of CRC acted like filler(micro filler) which reduces the air void further. Therefore, thesound is not easily absorbed into the CRHCB.

Transmission loss coefficients are shown in Figs. 21e23. As thepercentage of CR replacement increases in the CRC; the air contentincreases due to the entrapped air on the CR surface, therefore, thesound is easily absorbed into the CRHCB. Hence, the reminder of thesound that passed through the concrete was measured as trans-mission loss. As the porosity of concrete increases due to anincrease in air content, the transmission loss coefficient decrease asthe sound is easily passed through the air void present inside theCRC. Transmission loss coefficient can be summarized into STC asshown in Fig. 24. The STC decreases as the percentage of CRreplacement increases due to an increase in air voids in CRC.However, the reduction in STC can be partially restored by inclusionof higher amount of silica fume due to its effects as micro filler. It isnoteworthy that the reduction in STC as the CR content increases, isinsignificant and the CRC mixtures remain in the same class.

5.5. Electrical resistivity

As can be seen from Fig. 25, the electrical resistivity of CRCmixture increases as the percentage of CR replacement increases.

0

50

100

150

200

250

300

350

0 5 10 15 20 25 30

Ele

ctri

cal r

esis

tivi

ty,

dayS0F15CR0 S0F15CR10 S0F15C25 S0F15CR50S5F15CR0 S5F15CR10 S5F15CR25 S5F15CR50S10F15CR0 S10F15CR10 S10F15CR25 S10F15CR50

Fig. 25. Electrical resistivity for CRHCB.


This is due to the nature of CR itself, being used as an insulator inelectrical industry and function as dielectric material (Malik et al.,1997). The CR exists inside the microstructure of the CRHCBacting as a blocking pathway for electrical transfer between the twomeasure electrodes.

In addition, the electrical resistivity also increased as the dosageof silica fume increased. This is due to the modification of theinterfacial transition zone (higher bonding) as a result of addition ofsilica fume dosage into CRHCB. On the other hand, the pore in theCRHCB is filled by excessive silica fume and that will form a blockpathway for electrical transfer through the CRHCB (Dotto et al.,2004). It can also be seen from Fig. 25 that as the test ageincreases, the electrical resistivity increases. This is because at anearly age of hydration of CRHCB, semi-liquid form containing highion concentration are involved in the hydration reaction such asOH�, Naþ, Kþ, SO2�

4 , CL� and Ca2þ (Shi, 2004). All these ionconcentrations will conduct the electrical charges from one end toanother end of electrodes during measurement. As the ageincreases CRHCB become hardened as more and more end productof hydration (calcium silicate hydrate) are produced and acting likea barrier against the follow of electrical charges through CRHCB.

6. Conclusion

The following conclusions can be drawn from this research:

1. Due to decreasing in adhesion between the cement paste andcoarse aggregate; both compressive and splitting tensilestrength decreases as the CR percentage increases.

2. CRHCB can be produced as load-bearing hollow block withmaximum 6.5% CR replacement and non load-bearing hollowblock with maximum 40.7% CR replacement.

3. CRHCB can be produced as lightweight to normal weighthollow block, in accordance with the requirements of ASTMC90 and C129, depending on the percentage of CR replacement.

4. CRHCB has lower thermal conductivity compared to conven-tional hollow concrete block.

5. CRHCB has better sound absorption with higher noise reduc-tion factor than conventional hollow concrete block. Although,STC for CRHCB decreases as percentage of CR replacementincreases, the reduction in STC is marginal and can be partiallyrestored by inclusion of higher percentage of silica fume.

6. CRHCB has higher electrical resistivity than conventionalhollow concrete block.

Acknowledgement

The authors would like to thank the Ministry of Science, Tech-nology and Innovation (MOSTI) of Malaysia for Granting the Projectunder code 03-02-03-SF0091.

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properties of crumb rubber hollow concrete block

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