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Construction and Building Materials 16 (2002) 281289

0950-0618/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S0950-0618 02 .00019-3

Use of recycled aggregates in molded concrete bricks and blocksC.S. Poon*, S.C. Kou, L. Lam

Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China

Received 5 January 2002; received in revised form 25 March 2002; accepted 17 April 2002

Abstract

This study aimed to develop a technique for producing concrete bricks and paving blocks using recycled aggregates obtainedfrom construction and demolition waste. Laboratory trials were conducted to investigate the possibility of using recycledaggregates from different sources in Hong Kong, as the replacement of both coarse and fine natural aggregates in molded bricksand blocks. A series of tests were carried out to determine the properties of the bricks and blocks prepared with and withoutrecycled aggregates. The test results showed that the replacement of coarse and fine natural aggregates by recycled aggregates atthe levels of 25 and 50% had little effect on the compressive strength of the brick and block specimens, but higher levels ofreplacement reduced the compressive strength. However, the transverse strength of the specimens increased as the percentage ofreplacement increased. Using recycled aggregates as the replacement of natural aggregates at the level of up to 100%, concretepaving blocks with a 28-day compressive strength of not less than 49 MPa can be produced without the incorporation of fly ash,while paving blocks for footway uses with a lower compressive strength of 30 MPa and masonry bricks can be produced withthe incorporation of fly ash. 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Concrete bricks and blocks; Recycled aggregates; Construction and demolition waste; Compressive strength

1. Introduction

In Hong Kong, a huge quantity of construction anddemolition (C&D) waste is produced every year w1x.The disposal of waste has become a severe social andenvironmental problem in the territory. The possibilityof recycling of waste from the construction industry isthus of increasing importance w2,3x. In addition to theenvironmental benefits in reducing the demand on landfor disposing the waste, the recycling of C&D wastescan also help to conserve natural materials and to reducethe cost of waste treatment prior to disposal.C&D wastes are normally composed of concrete

rubble, bricks and tiles, sand and dust, timber, plastics,cardboard and paper, and metals. Concrete rubble usuallyconstitutes the largest proportion of C&D waste. It hasbeen shown that crushed concrete rubble, after separa-tion from other C&D waste and sieved, can be used asa substitute for natural coarse aggregates in concrete or

*Corresponding author. Tel.: q852-2766-6024; fax: q852-2334-6389.

E-mail address: [email protected] (C.S. Poon).

as a sub-base or a base layer in pavements w47x. Thistype of recycled material is called recycled aggregate.Successful application of recycled aggregate in con-

struction projects has been reported in some Europeanand American countries, as reviewed by Desmyster andVyncke w8x. While this type of material has been usedin large amounts in non-structural concretes or used asroad bases, its use in structural concrete is limited. Onlya few cases have been reported on the use of recycledaggregates in structural concrete, and the amount ofrecycled aggregate used has generally been limited to alow level of replacement of the total weight of coarseaggregate. An example is a viaduct and a marine lockproject in the Netherlands in 1988, and an office buildingin the UK in 1999 w8x. In the first case, a total of 11 000m of concrete in which 20% of the coarse aggregates3were replaced by recycled aggregates were used in allparts of the structures. Another reported case involvedthe use of 4000 m of ready mixed concrete, which was3prepared with recycled aggregates obtained from crushedconcrete railway sleepers to replace 40% of the coarseaggregates w8x. It should be noted that in these cases

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Table 1Properties of cement and fly ash

Properties Cement Fine fly ash Coarse fly ash

SiO (%)2 19.61 56.79 47.23Fe O (%)2 3 3.32 5.31 8.42Al O (%)2 3 7.33 28.21 24.54CaO (%) 63.15 -3 8.28MgO (%) 2.54 5.21 1.62SO (%)3 2.13 0.68 0.39Loss on ignition (%) 2.97 3.90 8.06Density 3.16 2.31 2.19Specific surface area (cm yg)2 3520 3960 2338

Table 2Properties of natural aggregates

Properties River sand Coarse BS limitaggregate w1217x

Dry surface density (gm )3 2.52 2.57Water absorption (%) 1.01 1.25Ten percent fine value (kN) 159.7 )100Impact value (%) 10.10 -30Crushed value (%) 23.34 -30Acid soluble materials (%)0.65 mm 0.70-0.6 mm 2.80

Table 4Constitutions of recycled aggregates

Material Constitution (% by weight)

TKOF KTF

Old concrete 77.7 98.8Natural stones 21.8 0Clay bricks 0.2 0Other impurity 0.3 0.2

Table 3Sieve analysis of combined natural aggregates containing 35% riversand and 65% crushed granite

BS test sieve Percentage passingby weight

10 mm 99.875 mm 63.522.36 mm 44.491.18 mm 30.37600 mm 20.35300 mm 12.26150 mm 3.14

recycled aggregates were used only to replace the coarsenatural aggregates.The limited use of recycled aggregate in structural

concrete is due to the inherent deficiency of this typeof material. In comparison with natural normal weightaggregates, recycled aggregates are weaker, more porousand have higher values of water absorption. The resultsof research studies show that when recycled aggregatesobtained from crushed concrete are used to replace upto 20% by weight of the coarse natural aggregate inconcrete, little effect on the properties of concrete isnoticed w9x. However, when used at a higher level ofreplacement, the high water absorption ability of recy-cled aggregate results in a higher total water demand.This renders the control of the free water-to-cement ratio(wyc) and the workability of fresh concrete difficultand, results in a higher shrinkage and creep of thehardened concrete when compared with the concreteprepared with natural aggregates. The extent to whichthe properties of concrete are affected by the use ofrecycled aggregate depends on the water absorption,crushing value and soundness of the recycled aggregatew10x.However, the disadvantages of using recycled aggre-

gates in structural concrete can be avoided in concretemixtures for mechanized molded concrete bricks andblocks. This is because in manufacturing concrete bricksand blocks using a mechanized molding machine, themixed materials are molded under a combined vibratingand compacting action. The requirement for maintaining

a workable mix is not so important. Only a minimalamount of water is needed to make the mixture fluidenough to be fed into the molding machine w11x. Thisreduces the difficulties of controlling the wyc ratio andworkability. Also, the low water content of the concretemixtures for the molded bricks and blocks significantlyreduces the creep and shrinkage of the hardened prod-ucts. An attempt has been made by Collins et al. w12xto use recycled aggregates in making blocks for a beam-and-block floor system. The blocks were440=215=100 mm in dimension and were produced ata block factory. Recycled aggregates were used toreplace 2575% by weight of the natural aggregate(including coarse and fine aggregates). A compressivestrength of 6.75 MPa and a transverse strength of 1.23MPa were reported for the blocks with 75% of thenatural aggregates replaced by recycled aggregates.This paper presents a recent study at the Hong Kong

Polytechnic University, which aims to develop a tech-nique for using recycled aggregates in molded concrete

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Table 5Properties of recycled aggregates

Properties Recycled aggregates BS limit

TKOS TKOL KASw1217x

Fine Coarse Fine Coarse Fine Coarse

Dry surface density (gm )3 2.39 2.47 2.26 2.35 2.28 2.31 Water absorption (%) 11.23 4.19 14.21 7.60 13.55 7.16 Ten percent fine value (kN) 145.1 109.3 130.9 )100Impact value (%) 10.02 14.51 14.76 -30Crushing value (%) 21.57 24.54 22.18 -30Acid soluble materials (%)0.65 mm 9.37 18.90 -0.6 mm 15.05 22.46

Table 6Sieve analysis of combined recycled aggregate containing 45% fineaggregate and 55% coarse aggregate

BS test sieve Percentage passing by weight

TKOS KTS TKOL

10 mm 98.35 98.95 1005 mm 62.25 54.62 51.832.36 mm 45.41 39.79 42.691.18 mm 28.38 26.49 27.38600 mm 22.28 18.69 16.1300 mm 13.32 10.6 5.61150 mm 6.96 4.55 2.29

bricks and paving blocks. The bricks are expected toachieve a 28-day compressive strength of not less than7 MPa according to the requirement of BS 6073 formasonry units w13x. The paving blocks are expected toachieve either a 28-day compressive strength of not lessthan 49 MPa according to the requirement of BS 6717w14x. Laboratory trials were carried out to prepare themolded specimens with dimensions of 225=105=75mm, with up to 100% by weight of the natural aggre-gates (both coarse and fine) replaced by recycled aggre-gates. Attempts were also made to incorporate fly ashinto the bricks and blocks. The density, compressivestrength, transverse strength, shrinkage and skid resis-tance of the specimens were determined. In addition tothe laboratory trials, a plant trial was conducted to testthe feasibility of using recycled aggregates in producingmolded paving blocks in real industrial productionconditions.

2. Materials

2.1. Cement, fly ash and natural aggregates

The cementitious materials used were an ordinaryPortland cement complying with BS 12, a fly ashcomplying with BS 3982, and a fly ash that failed tofulfill the requirements of the BS standard on finenessand loss on ignition. In this paper, the fly ash complyingwith the requirements of the BS standard is referred toas fine fly ash whilst the fly ash with poorer qualityis referred to as coarse fly ash. Both the cement andthe fine fly ash are commercially available in HongKong. The coarse fly ash was the reject of an airclassifying process of a local power plant and wasregarded as waste and was required to be disposed of.This it referred to as reject ash. The properties of thecement and fly ash are shown in Table 1. The naturalaggregates used included a natural river sand as the fineaggregate and a crushed granite with a maximum nom-inal size of 10 mm as the coarse aggregate. Thesenatural aggregates were mixed in a ratio of 35% fine

aggregate and 65% coarse aggregate by weight. Theproperties of the natural aggregates are shown in Table2, determined according to British Standard methodsw1520x. The combined grading of the natural aggre-gates is shown in Table 3.

2.2. Recycled aggregates

The recycled aggregates used in this study were C&Dwastes sourced from the two public filling areas inTseung Kwan O (TKO) and the old Kai Tak (KT)airport in Hong Kong. The C&D waste underwent afurther process of mechanized crushing and sieving toproduce fine aggregate and coarse aggregate accordingto the particle size requirements of British Standard BS812 w17x. In addition, selected crushed concrete rubblewas taken from TKO and was further crushed in ourlaboratory using a hammer (hand) to obtain a secondtype of fine and coarse aggregate. In this paper, therecycled aggregates obtained from the mechanized pro-cess are referred to as TKOS and KTS recycled aggre-gates, respectively. The recycled aggregates sourcedfrom TKO but processed in our laboratory are referredto as TKOL recycled aggregates. The maximum nominalsize of the recycled coarse aggregate used in this studywas 10 mm.The KTOS and KTS recycled coarse aggregates con-

tained mainly old concrete rubble, but also contained a


Table 7Mix proportions for masonry bricks

Mix notation Proportion (kg)

Cement Fine fly ash Natural Recycled Addedaggregate aggregate water

Control 3200 0 85BR-TKOL-25 2400 800 90BR-TKOL-50 1600 1600 98BR-TKOL-75 200 50 800 2400 110BR-TKOL-100 0 3200 122BR-TKOS-100 0 3200 122BR-KTS-100 0 3200 122

Table 8Mix proportions of paving blocks without fly ash


Cement Natural Recycled Addedaggregate aggregate water

BL-Control 670 3030 0 250BL-TKOS-50 670 1515 1515 280BL-TKOS-100 670 0 3030 320BL-KTS-50 670 1515 1515 290BL-KTS-100 670 0 3030 330

Table 9Mix proportions for paving blocks incorporated with fly ash


Cement Fly ash Natural Recycled Added

Fine Coarseaggregate aggregate water

BLF-Control 3000 0 250BLF-TKOS-25 2250 750 260BLF-TKOS-50 1500 1500 280BLF-TKOS-75 750 2250 300BLF-TKOS-100 550 120 30 0 3000 320BLF-KTS-25 2250 750 270BLF-KTS-50 1500 1500 290BLF-KTS-75 750 2250 310BLF-KTS-100 0 3000 330

small amount of natural stones, clay bricks, and otherimpurities such as small pieces of wood, bricks, tilesand metals. The constituents of the recycled aggregatesare shown in Table 4. The impurities were not removedbefore the experiment. The TKOL recycled coarseaggregate contained only old concrete. The propertiesof the recycled aggregates were tested according toBritish Standard methods w1520x and are given inTable 5. In the present study, the recycled fine andcoarse aggregates were used in a ratio of 45% to 55%by weight. The combined grading of the recycled aggre-gates is shown in Table 6.

3. Laboratory trials

3.1. Mix proportioning

A total of three series of mixtures were prepared inthe laboratory trials. Series I mixtures were prepared forproducing masonry bricks according to the requirementsof BS 6073 w13x; series II mixtures were prepared forproducing paving blocks according to the requirementsof BS 6717 w14x without using fly ash; and series IIImixtures were prepared for making paving blocks withthe two types of fly ash to partly replace the cement.The details of these three series of mixes are given inTables 79. The mix notations indicate the differenttypes of mixes (with BR for bricks and BL for blockswithout fly ash and BLF for blocks with fly ash), thenotations of recycled aggregates, and the percentages(by weight) of the recycled aggregates in the totalamount of aggregates used.The seven mixtures in series I included a control

mixture using only natural aggregates, and six mixtureswith recycled aggregates (four with TKOL, one withTKOS and one with KTS) and the replacement ofnatural aggregates was at levels from 25 to 100% by


Fig. 1. Fabrication of concrete paving block specimens: (a) before compaction, (b) after compaction.

Fig. 2. Concrete paving block specimens after curing.

weight. All these seven mixtures were prepared withcement, fine fly ash and aggregates in the proportion of200:50:3200 by weight, and were expected to achieve atarget compressive strength of not less than 7 MPa atthe age of 28 days.The five mixtures in series II included a control

mixture using only natural aggregates, and four mixtureswith recycled aggregates (two with TKOS and two withKTS) and the replacement of natural aggregates was atthe levels of 50 and 100% by weight. All these fivemixtures were prepared with cement and aggregates inthe proportion of 670:3030 by weight, and were expect-ed to achieve a target compressive strength of not lessthan 49 MPa at the age of 28 days.The nine mixtures in series III included a control

mixture using only natural aggregates, eight mixtureswith recycled aggregates (four with TKOF and fourwith KTF) and the replacement of natural aggregatesby recycled aggregates was at the levels from 25 to100% by weight. The concrete mixtures were preparedwith cement, fine and coarse fly ash, and aggregates inthe proportion of 550:120:30:3000 by weight, and werealso expected to achieve a target compressive strengthof not less than 30 MPa at the age of 28 days.The proportioned materials were mixed in a pan

mixer. The amounts of water added to each mix variedfrom 86 to 122 kg for preparing the mixtures of thebricks, and from 250 to 330 kg for preparing themixtures of the paving blocks. The prepared mixtureshad a zero slump and a V.B time within 2030 s.

3.2. Fabrication of bricks and paving blocks

Concrete bricks and paving blocks were fabricated insteel moulds with internal dimensions of 225 mm inlength, 105 mm in width, and 75 mm in depth. Themixed materials used were approximately 3.4 kg foreach brick and 3.84 kg for each paving block. Withthese amounts of materials, the steel moulds were over

filled and the initial depth of materials covering themould was approximately 8085 mm. A compressionforce of 600 kN was applied for 1 min to compact thematerials in the mold (Fig. 1). The formed bricks andpaving blocks were then removed from the moulds andcured under specified conditions as follows: the brickswere cured in air at room temperature for 28 days,whilst the blocks were first cured in a steam bath at 658C for 6 h, and then further cured in air at roomtemperature until the age of 28 days (Fig. 2).

3.3. Tests methods

A series of tests were conducted to determine thedensity, compressive strength, transverse strength (flex-ural strength), and drying shrinkage of the bricks andpaving blocks. For the paving blocks the skid resistancewas also determined.The density of the specimens was determined using a

water displacement method. The compressive strengthof the specimens was determined using a compressiontesting machine with a maximum capacity of 3000 kN,according to standard methods. For the bricks, thecompression load was applied to the face with a nominalarea of 225=75 mm according to the local application


Fig. 3. Compression tests of brick and block specimens: (a) masonry bricks, (b) paving blocks.

Table 11Test results of paving blocks without fly ash

Mix notation Compressive Transverse Drying shrinkage Skid Densitystrength (MPa) strength (MPa) (%) resistance (kgym )3

BL-Control 58.6 3.31 0.027 98 2328BL-TKOF-50 62.1 3.74 0.030 110 2281BL-TKOF-100 51.2 3.81 0.038 108 2258BL-KTF-50 60.4 3.79 0.032 114 2244BL-KTF-100 50.9 3.89 0.039 112 2215

Table 10Test results of bricks

Mix notation Compressive Transverse Drying shrinkage Densitystrength (MPa) strength (MPa) (%) (kgym )3

BR-Control 16.2 1.76 0.040 2210BR-TKOL-25 15.9 1.80 0.042 2195BR-TKOL-50 16.7 1.87 0.044 2150BR-TKOL-75 15.0 1.95 0.046 2120BR-TKOL-100 11.8 1.99 0.052 2060BR-TKOF-100 12.9 1.92 0.050 2070BR-KTF-100 11.4 1.94 0.051 2054

requirements, and for the paving blocks, the compressionload was applied to the face with a nominal area of225=105 mm according to BS 6717 w14x. Beforeloading, the specimens were packed with two pieces ofplywood (Fig. 3a,b). The compressive strength wasdetermined by dividing the maximum load by the loadarea of the specimen.The transverse strength (flexural strength) of the

specimens was determined by a three-point bending testwith a supporting span of 180 mm and a height of 75mm, using a material-testing machine with a maximumload capacity of 30 kN.The drying shrinkage of the specimens was deter-

mined using the method specified in BS 6073 w13x witha modification that an environmental chamber withcontrolled temperature and humidity was used for theconditioning rather than an oven. The specimens werefirst immersed in water at room temperature for a

minimum of 4 days and a maximum of 7 days, andwere then conditioned in the environmental chamber for13 days. The temperature and relative humidity insidethe chamber were controlled at 55 8C and not less than95% according to the BS requirements. The lengthchange of the specimen before and after drying wasmeasured and the drying shrinkage was determined.The surface frictional properties of the paving blocks

were determined using a British Pendulum Skid Resis-tance Tester. The skid resistance of the specimen surfacewas expressed as the measured British Pendulum Num-ber (BPN), as specified in ASTM E303 w21x.

3.4. Test results

The results of the tests of the laboratory trials areshown in Tables 1012, where drying shrinkage is givenas the averages of two measurements and other proper-


Table 12Test results of blocks containing fly ash

Mix notation Compressive Transverse Drying shrinkage Skid resistance Densitystrength (MPa) strength (MPa) (%) (BPN) (kgym )3

BLF-Control 46.6 3.30 0.026 95 2285BLF-TKOF-25 44.7 3.30 0.026 112 2257BLF-TKOF-50 46.5 3.32 0.029 112 2245BLF-TKOF-75 45.4 3.53 0.034 114 2193BLF-TKOF-100 40.1 3.63 0.036 106 2167BLF-KTF-25 45.3 3.32 0.028 110 2250BLF-KTF-50 47.5 3.47 0.030 106 2231BLF-KTF-75 42.5 3.56 0.035 109 2170BLF-KTF-100 41.0 3.68 0.037 106 2136

Fig. 4. Paving blocks produced in the plant trial. Fig. 5. Shape of paving block produced in the plant trial.

ties as the averages of three measurements. For masonrybricks, all the mixes satisfy the requirements of BS 6073w13x for compressive strength (G7 MPa), transversestrength (G0.65 MPa), and shrinkage (G0.06%),although the replacement of the natural aggregates byrecycled aggregates resulted in lower strength valuesand slightly higher shrinkage values (Table 10). Itshould be noted that for the bricks, the compressivestrength was obtained with load applied on the 225=75mm face. The results should be slightly lower than thoseobtained with the load applied on the 225=105 mmface due to the effect of aspect ratio w22x.For paving blocks, apart from those specified in BS

6717 that requires a minimum 28-day compressivestrength of 49 MPa, a type of paving block for footwayuse is specified by a local standard in Hong Kong w23x,requiring a 28-day compressive strength of 30 MPa.Table 11 shows that the paving block specimens pre-pared without fly ash satisfy the requirement of BS6717 with the average 28-day compressive strengthhigher than 49 MPa w14x. Table 12 shows that thepaving block specimens prepared with fly ash incorpo-ration give lower strength values that the blocks preparedwith only cement but they are well fitted as the pavingblocks for footway uses ()30 MPa) w23x. (The authorshave developed a novel method to compensate for thedecrease in compressive strength of the paving blocks

due to the incorporation of fly ash. This will bediscussed in a separate paper.)Besides, both the paving blocks prepared with recy-

cled aggregates, with and without fly ash show higherskid resistance than those prepared with natural aggre-gates. Also, the paving blocks prepared with recycledaggregates satisfy the drying shrinkage requirement inBS 6073 w13x for masonry bricks, although this param-eter is not required for paving blocks.It should be noted that a 25% or 50% replacement of

natural aggregates by recycled aggregates had littleeffect on the compressive strength, but a higher per-centage of replacement resulted in lower compressivestrength for both the bricks and the paving blocks.However, the transverse strength of the specimensincreased as the percentage of recycled aggregates usedin the mixes increased.

4. Plant trial

A plant trial on the production of the paving blocksusing the developed mix proportions for local applica-tion was conducted at a local brick and block manufac-turing plant. A mix as given in Table 9 with 100% ofthe natural aggregates replaced by TKOS recycled aggre-gates and the incorporation of fly ash was used toproduce a batch of 384 paving blocks (Fig. 4). Theproduced paving blocks had a shape given in Fig. 5 and


Table 13Test results of plant trial

Mix notation Compressive Transverse Drying shrinkage Skid resistance Densitystrength (MPa) strength (MPa) (%) (BPN) (kgym )3

BLF-TKOF-100 43.8 3.66 0.037 113 2185

the equivalent dimension was 220=110=80 mm, whichwas specified as shape 4 by Knapton w24x. The materialswere mixed in a drum mixer. Water was added to meetthe requirements of molding. The blocks were moldedin an automatic mechanized block-making machine,under a combined vibration and compacting force of 80psi for 12 s. The molded blocks were cured in steam at60 8C for 12 h and then were air cured. Tests werecarried out at the age of 28 days and the results areshown in Table 13. A slightly higher compressivestrength and a similar transverse strength were observedfor the blocks produced in the plant trial when comparedwith the specimens prepared in the laboratory trialsusing the same mix proportion (Table 11). Thus, thismix proportion can be used for producing paving blocksfor footway use by an industrial process.

5. Summary and conclusions

This study aimed to develop a technique for producingconcrete bricks and paving blocks using C&D wastesas recycled aggregates. Laboratory trials were carriedout on three series of mixes with recycled aggregatesobtained from different sources in Hong Kong, as thereplacement for both coarse and fine natural aggregatesat levels of up to 100% by weight, with or without theincorporation of fly ash. Tests were carried out todetermine the compressive strength, transverse strength,shrinkage, skid resistance and density. In addition to thelaboratory trials, a plant trial was conducted to test thefeasibility of this technique in an industrial setting.The test results showed that the replacement of coarse

and fine natural aggregates by recycled aggregates atthe levels of 25% and 50% had little effect on thecompressive strength of the bricks and blocks, but higherlevels of replacement reduced the compressive strength.However, the transverse strength of the specimensincreased as the percentage of the replacement increased.Using recycled aggregates as the replacement of naturalaggregates at the level of up to 100%, concrete pavingblocks with a 28-day compressive strength of not lessthan 49 MPa can be produced without the incorporationof fly ash, while paving blocks for footway uses with alower compressive strength of 30 MPa and masonrybricks can be produced with the incorporation of flyash. The performance of the bricks and blocks were alsosatisfactory in the shrinkage and skid resistance tests.

Acknowledgments

The authors wish to acknowledge the financial supportof The Hong Kong Polytechnic University, through thefunding of IGARD and ASD-Advanced Building Tech-nology. The Civil Engineering Department of the HongKong SAR Government is thanked for providing therecycled aggregates samples. Special thanks are also dueto K. Wah Stones Co. Ltd for its assistance in carryingout the plant trial.

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Use of recycled aggregates in molded concrete bricks and blocksIntroductionMaterialsCement, fly ash and natural aggregatesRecycled aggregates

Laboratory trialsMix proportioningFabrication of bricks and paving blocksTests methodsTest results

Plant trialSummary and conclusionsAcknowledgementsReferences

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