Construction and Building Materials 28 (2012) 549–556

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Construction and Building Materials

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Feasibility study of using recycled fresh concrete waste as coarse aggregatesin concrete

Kou Shi-cong, Zhan Bao-jian, Poon Chi-sun ⇑Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hong Kong, China

a r t i c l e i n f o

Article history:Received 3 November 2010Received in revised form 30 July 2011Accepted 1 August 2011Available online 14 December 2011

Keywords:RecyclingFresh concrete wastePropertiesNon-structure concrete

0950-0618/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.08.027

⇑ Corresponding author.E-mail address: cecspoon@polyu.edu.hk (C.-s. Poon

a b s t r a c t

In Hong Kong, a large amount of fresh concrete waste (FCW) is generated from ready mix concrete plantsevery day. Up to now, these wastes are usually delivered to landfills for disposal. The landfill areas inHong Kong will be saturated in 6–8 years, as a result there is a need to develop a new technique for uti-lizing the FCW. In this study, FCW was crushed into coarse aggregate, and then it was used to replace nat-ural coarse granite at percentages of 0%, 15%, 30% and 50%, in producing new concrete mixes. Theconcrete were produced with water/cement ratios of 0.35 and 0.50. The effect of using the normal mixingapproach and the two-stage mixing approach on the properties of concrete was also compared. Theresults indicated that the density, strength and static modulus of elasticity of new concrete weredecreased with an increase in FCW content. Due to the lower density and higher water absorption ofFCW, the water absorption, chloride ion permeability and dry shrinkage of the new concrete wasincreased with the increase in FCW content. Moreover, the two-stage mixing approach (TSMA) onlyimproved the strength of the FCW concrete when the concrete was prepared with a lower water-to-cement of 0.35. The results demonstrated that the FCW can be used to replace natural aggregates forthe production of non-structural concrete.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

In ready mixed concrete batching plants, fresh concrete waste(FCW) is generated by several processes; including from reclaimingof over-ordered concrete and cleaning of mixing equipment. It isknown that about 8–10 tons fresh concrete waste can be producedevery day from a concrete batching plant with a daily output of1000 m3 of concrete. As a result, about 3000 tons of FCW wouldbe produced from one batching plant each year. The plant withouta reclaiming system will produce even more waste.

As Hong Kong is subject to insufficient landfill space for wastedisposal [1], recycling and reusing the FCW from concrete batchingplants should be actively explored.

In the past two decades, research on the reuse and recycling ofconstruction and demolition waste has been conducted extensivelyin many countries [2–9]. Furthermore, the construction industry ofHong Kong has acquired a lot of experience in reusing andrecycling construction and demolition waste for new concreteand concrete blocks production [10–13]. However, very littleinformation is available on the reuse the FCW.

So far, the only reported study on FCW was carried out byCorreia et al. who conducted a factorial design experiment to

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).

recycle FCW, as a replacement of natural fine aggregates, in newconcrete, [14]. It was found that the fresh concrete workabilityworsened with the increase of FCW content, but the specified 28-day compressive strength (32–44 MPa) could still be achieved atwith a replacement contents of <30 wt.% [14].

As regards the effect of mixing methods on the properties ofrecycled aggregate concrete, Otsuki et al. [15] suggested that adouble mixing method was able to enhance the compressivestrength of the concrete prepared with coarse recycled concreteaggregate (RCA) by improving the interfacial transition zone be-tween the RCA and the cement paste. Poon and Chan [16] foundthat the adverse effects (i.e. strength reduction) due to the use offine recycled aggregates could be minimized by the deploymentof the double mixing method, which can be easily implementedin pre-cast concrete production. Tam and Tam [17] reported thatthe additions of silica fume and the use of the two-stage mixing ap-proach can fill up the weak areas in the RA which helps to developa stronger interfacial layer around the recycled aggregates, andhence a higher strength of the concrete.

This paper focuses on studying the feasibility of using FCW inthe new concrete. The fresh concrete wastes were crushed to theparticle sizes of coarse aggregates normally used for ready mixedconcrete. The workability, compressive strength, tensile splittingstrength, static modulus of elasticity, chloride ion penetrabilityand drying shrinkage of the new concrete were determined.

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Moreover, the effect of using the normal mixing approach and thetwo-stage mixing approach on the properties of concrete was alsocompared.

2. Experimental program

2.1. Materials

2.1.1. CementAn ordinary Portland cement (ASTM Type I) sourced locally was used in this

study, and the density and specific surface areas of the cement was 3.16 g/cm3

and 3500 cm2/g, respectively. The chemical composition of the cement is presentedin Table 1.

2.1.2. Natural aggregateCrushed granite with nominal sizes of 10 mm and 20 mm were used as natural

coarse aggregate, and natural river sand with a fineness modulus of 2.11 was usedas the fine aggregate in this study. The 10 mm and 20 mm crushed granite wereused a weight ratio of 1:2. The grading of the sand satisfied the grading limit ofBS 882 [15] for fine aggregate.

2.1.3. Fresh concrete waste aggregateThe FCW were sourced from the residue fresh concrete reclaiming system of a

local ready mixed batching plant. The generating process of the fresh concretewaste is shown in Fig. 1. It can be seen that some of the natural aggregates are actu-ally recycled directly after fresh concrete waste is reclaimed (e.g. by washing thefresh cement paste the coarse aggregates could be recuperated and used as conven-tional aggregates). In this study, FCW used is composed of cement paste and a smallpart of fine aggregate. The FCW were crushed manually by using a hammer, andthen sieved in the laboratory to produce coarse aggregates with nominal sizes of10 mm and 20 mm. Similar to the case of the natural coarse aggregate, the10 mm and 20 mm FCW aggregates were blended with weight ratio of 1:2 beforetheir use. The grading of the blended aggregates satisfied the requirement of BS882 [18] for coarse aggregate.

The properties of the natural and FCW aggregate were determined according toBritish Standard methods and are shown in Table 2.

2.1.4. AdmixtureIn this study, a superplasticizer ADVA 109 was used to control the workability

(slump values) of the concrete mixes, especially for the mixes prepared with a low-er W/C ratio. The amount of superplastizicer used in each concrete mix is shown inTable 3.

Table 1Chemical composition of the cement (%).

CaO SiO2 Al2O3 Fe2O3 MgO SO3 L.O.I

63.15 19.61 7.32 3.32 2.54 2.13 2.94

Fig. 1. (a) Generating process of fresh concrete wa

2.2. Mix proportions and mixing methods

Two series of concrete mixes (Series I and II) were prepared at free water/ce-ment ratio (W/C) of 0.35 and 0.50, respectively, for investigating the effect ofFCW aggregate on the properties of both normal and high performance concrete.Four FCW aggregate replacement levels of 0%, 15%, 30% and 50% by weight of nat-ural river sand were used in each of the mix series. In addition, the normal mixingapproach (NMA) and two-stage mixing approach [19] (TSMA) were used in prepar-ing the concrete mixes for selected FCW aggregate replacement levels of 30% and50% (which were denoted as ‘‘N’’ and ‘‘T’’ at end of the mix codes, respectively).The details of all the concrete mixes are presented in Table 3. In this study, theaggregates were designed at a saturated surface-dried (SSD) condition. Adjustmentswere sequentially made according to the moisture contents and water absorptioncapacity of the respective aggregates.

2.2.1. CuringAll concrete specimens were demolded after 24 h of casting and cured in water

at 27 ± 2 �C until the test ages.

2.3. Testing

2.3.1. WorkabilitySlump test was used for evaluating the workability of fresh concrete in accor-

dance with BS EN 12350-2:2009 [20].

2.3.2. Density and water absorptionThe concrete cube specimens (100 � 100 � 100 mm) at the saturated-surface-

dry (SSD) condition were used for testing SSD density, in accordance with BS EN12390-7:2009 [21], and then the rate of water absorption was determined usingthe same specimens, based on ASTM C 1403-06 [22].

2.3.3. Mechanical propertiesIn this study, mechanical properties such as compressive strength, tensile split-

ting strength and static modulus of elasticity of concrete were measured. Cubicspecimens with size of 100 � 100 � 100 mm were used for testing of the compres-sive strength at the ages of 1, 3, 7, 28 and 90 days in according with BS EN 12390-3:2009 [23]. Cylindrical specimens with a diameter of 100 mm and a height of200 mm were prepared for testing of the tensile splitting strength at the ages of7, 28 and 90 days, and for testing of the static modulus of elasticity at ages of 28and 90 days in accordance with BS EN 12390-6:2009 [24] and BS 1881-121 [25],respectively.

2.3.4. Chloride ion permeabilityRapid chloride ion permeability test was used to evaluate the permeability of

concrete. This test was based on the standard test method of ASTM C 1202-09[26]. The specimens with 100 mm diameter and 50 mm thick were obtained fromthe u100 � 200 mm concrete cylinders. And the tests were conducted twice, atthe ages of 28 and 90 days.

ste; (b) slurry cake; (c) fresh concrete waste.

Table 3Mix proportion of concrete mixes (kg/m3).

Mix code W/C FCW (%) Cement Water Sand Coarse aggregate ADVA 109 (L/m3)

NA FCW

0-I 0.35 0 450 158 697 1046 0 4.515-I 0.35 15 450 158 685 873 154 4.530 N-I 0.35 30 450 158 673 707 303 4.530T-I 0.35 30 450 158 673 707 303 4.550 N-I 0.35 50 450 158 657 493 493 4.550T-I 0.35 50 450 158 657 493 493 4.50-II 0.50 0 395 198 664 1083 0 2.615-II 0.50 15 395 198 653 906 160 2.130 N-II 0.50 30 395 198 643 735 315 2.130T-II 0.50 30 395 198 643 735 315 2.150 N-II 0.50 50 395 198 622 508 508 2.150T-II 0.50 50 395 198 622 508 508 2.1

Table 2Properties of aggregates.

Properties Size of BS test sieve (mm) Percentage passing (%)

20 mm granite 10 mm granite 20 mm FCW 10 mm FCW Sand

Sieve analysis 37.5 100 – 100 - –20 97 – 99 – –14 18 100 19 100 –10 4 96 4 98 1005 – 21 – 20 992.36 – 4 – 4 961.18 – – – – 870.6 – – – – 700.3 – – – – 260.15 – – – – 2

Density (kg/m3) 2620 2620 1826 1826 2630Oven-dried 2588 2587 1328 1328 2589

Strength (10% fines KN) 179 106 –Water absorption (%) 1.11 1.12 29.5 37.2 0.89

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2.3.5. Dry shrinkage and weight lossConcrete prism specimens (75 � 75 � 285 mm) were prepared for the determi-

nation of dry shrinkage and weight loss of the concrete according to BS ISO 1920-8[27]. The length and mass change measurements were carried out at the dryingperiod of 1, 3, 7, 28, 56, 90 and 112 days.

2.3.6. Ultrasonic pulse velocity (UPV)The concrete cube specimens (150 � 150 � 150 mm) were used for the deter-

mination of the velocity of propagation of pulses of ultrasonic longitudinal wavesin the hardened concrete according to BS 12504-4 [28]. The test was conductedtwo times using the direct transmission method for each specimen at the curingages of 1, 3, 7, 28, 56 and 90 days.

Fig. 2. Slump value of concrete mixes with NMA and TSMA.

3. Results and discussion

3.1. Workability

The workability of concrete, expressed in terms of slump, variedwithin the range from 170 mm to 280 mm due to keep the dosagesof ADVA 109 of 4.5 L/m3 and 2.1 L/m3 in Series I and II concretemixes, respectively. The slump values of all concrete mixes areshown in Fig. 2. It can be seen that slump value increased withthe increase of FCW aggregate content compared with the controlmixes. This is due to the FCW aggregate was used at air-dried con-dition, and extra water was added to reach a SSD condition of FCWin the mixtures corresponding to the absorption of FCW aggregateduring concrete preparation. As more FCW aggregates were used,more extra water had to be added into the mixtures due to thehigher water absorption capacity of FCW aggregate compared tonatural granite.

Moreover, the slump value was decreased when the mixturewas prepared with TSMA. During the first stage of TSMA, half of re-quired water was added into the mixer before the cement wasadded, and as a result, some water was absorbed by multi-porousFCW, which meant less free water was available for mixing. Whenthe NMA was used, dry-mixing of cement and aggregates was con-ducted before water was introduced. Less water was absorbed bythe FCW aggregate due to blockages of the pores on the surfaceof FCW aggregate by the cement particles. Consequently, therewas more free water available in mixing with NMA, than withTSMA.

Fig. 3. (a) Density and (b) water absorption of concrete.

Fig. 4. Effect of mix approach on (a) density and (b) water absorption of concrete.

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3.2. Density and rate of water absorption

Fig. 3 shows the variation of density and water absorption of theFCW concrete. For both W/C ratios used, the density of the concretemixes was linear decreased in approximately the same proportionwith the increase of FCW aggregate content. Meanwhile, the waterabsorption capacity of concrete mixes was linear increased as moreFCW aggregates were added (Fig. 3b). This might be attributed tothe lower density of FCW than the crushed granite, which meantwhen more FCW was used, the higher the concrete porosity.

The effect of the method of mixing on the density and waterabsorption of the concrete mixes are shown in Fig. 4. It is seen thatthe density of the concrete mixes with NMA was similar to TSMA.Moreover, TSMA slightly decreased the water absorption of theconcrete mixes due to half of required water was added into themixer before the cement was added, and as a result, lower effectiveW/C ratio in the new mortar, which means lower porosity was con-ducted in the concrete.

3.3. Mechanical properties

3.3.1. Compressive strengthThe results of compressive strength of all the concrete mixes at

1, 3, 7, 28 and 90 days are summarized in Table 4. Every presenteddata is an average of three measurements. It is clear that the con-trol mixes had the maximum compressive strength at all the curingages. The compressive strength decreased with the increase in theFCW aggregate content in both series of concrete mixes. Fig. 5shows the relative compressive strength, which is defined as com-pressive strength of the concrete containing FCW to that of thecontrol mix at the same curing age. Fig. 5a indicates that at28 days, the compressive strength of the concrete mixes with15%, 30% and 50% FCW aggregate and prepared with NMA in SeriesI (W/C = 0.35) was about 37%, 47% and 57% lower than that of thecontrol. Similar results were obtained at 90 days. Fig. 5b indicatesthat at 28 days, the decrease of compressive strength of the con-crete mixes with 15%, 30% and 50% FCW aggregate and preparedwith NMA in Series II was about 26%, 40% and 55% lower comparedwith the control. At 90 days, the compressive strength of concretemixes with 15%, 30% and 50% FCW aggregate and with NMA de-creased by 25.0%, 34.8% and 51.4% compared with the correspond-ing control concrete.

Moreover, at all test ages, the compressive strength of concretemix prepared with 30% FCW aggregate and with TSMA in Series Iwas higher than the corresponding concrete mix with NMA dueto TSMA decreased the porosity of the new mortar. However, TSMAhas less effect of the compressive strength of concrete mixes pre-pared with 50% FCW in both series.

The relative compressive strength (defined as the ratio of com-pressive strength of concrete containing FCW to that of the controlmixes at the same curing age) of the concrete mixes prepared withNMA and TSMA are shown in Fig. 6. It can be seen that at 28 and90 days the compressive strength of the concrete mix 30T-I in SeriesI with TSMA and 30% FCW aggregate was higher by 22.4% and 15.5%than the corresponding concrete mixes with NMA, respectively.

Moreover, although at 28 days, the compressive strength of theconcrete mixes prepared with 30% FCW aggregate and TSMA inSeries I (W/C = 0.35) were still lower than that the control mixes,the strength was higher than that the control in Series II (W/C = 0.50). Hence, by adjusting the W/C ratio or using the two stagemixing approach, it was possible to match the designed compres-sive strength of the concrete containing 30% FCW aggregate withthat of the corresponding natural aggregate concrete.

The compressive strength of the concrete mixes prepared withFCW aggregate was significantly decreased compared with thecontrol. This should be attributed to weaker FCW aggregate, lower

Table 4Compressive strength of concrete mixes (MPa).

Mix code 0-I 15-I 30 N-I 30T-I 50 N-I 50T-I 0-II 15-II 30 N-II 30T-II 50 N-II 50T-II

1-day 32.2 20.4 13.3 16.9 9.3 8.4 16.5 11.0 7.6 7.2 4.6 4.63-day 46.0 38.2 22.2 32.9 22.2 22.1 29.8 21.8 16 16.4 11.5 13.07-day 66.5 50.3 30.5 38.2 25 26.4 36.6 26.9 21.7 19.2 15.6 16.328-day 85.7 53.6 45.0 55.1 36.6 34.8 48.3 35.8 29.1 27.8 21.8 22.390-day 98.1 78.3 52.3 60.4 40.1 40.0 52.9 39.7 34.5 31.5 25.7 25.6

Fig. 5. Relative compressive strength of concrete mixes (a) W/C = 0.35; (b)W/C = 0.50.

Fig. 6. Relative compressive strength of concrete mixes with NMA and TSMA: (a)W/C = 0.35; (b) W/C = 0.50.

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density and higher water absorption capacity than the naturalgranite.

Fig. 7. Tensile splitting strength of all concrete mixes.

3.3.2. Tensile splitting strengthThe tensile splitting strength values of all the mixtures are pre-

sented in Fig. 7. Every presented data is an average of three mea-surements. A sharp decrease in tensile splitting strength for themixes incorporating FCW aggregate was observed at all the curingages when only 15% FCW was used. With the increase in FCW con-tent, the further decrease of tensile splitting strength was not obvi-ous. Comparing the results of concrete mixes prepared with TSMAwith that with NMA, it was found that using TSMA did not improvethe tensile splitting strength significantly. Fig. 8 shows the correla-tion between tensile splitting strength and compressive strength ofall the concrete mixes at 28-day. It is seen that there was a verygood exponential correlation between tensile splitting strengthand compressive strength in the both series concrete mixes [29].

Fig. 8. Correlation between tensile splitting strength and compressive strength at28-day.

Fig. 9. Correlation between modulus of elasticity and compressive strength at 28-day.

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3.3.3. Static modulus of elasticityThe static modulus of elasticity for all the concrete mixtures at

the curing ages of 28 and 90 days are listed in Table 5. Everypresented data is an average of three measurements. The elasticmodulus decreased with the increase of FCW content. The concretemade with 15% of FCW in Series I and suffered a reduction in theelastic modulus of 12.5% with respect to control concrete. Thereduction was up to 50% at the curing age of 90 days when FCWcontent was increased to 50%. This may be attributed to the FCWaggregate had more pores than the natural granite. It could be ex-plained by Spriggs’ equation [30], which indicated more pores in-duced more reduction of static modulus of elasticity.

The correlation between elastic modulus and compressivestrength for all the mixtures at 28-day are presented in Fig. 9. Itcan be expressed with a Power-law. However, this relationshipwas different from the ACI equation [31]; but the correlation ofthe results of this study was still good.

3.4. Chloride-ion diffusion

The test results of rapid chloride ion permeability of the con-crete mixes at 28 and 90 days are shown in Fig. 10. Every presenteddata is an average of two measurements. It is seen that the resis-tance to chloride ion penetration of concrete mixes decreased withan increase in FCW content. The total charge passed through themixes incorporating with 50% FCW aggregate in Series I and II wereall more than 7000 C (Coulombs) at 28 days. However, the chargepassed through the mixes of 0-I, 15-I, 30 N-I and 30T-I were lowerthan 4000 C at the age of 90 days, which equated to the level of‘‘Moderate’’ according to the ASTM C1202-09 [26]. Apparently,the use of FCW aggregate decreased the resistance of chloride ionpenetration of the concrete. This may also be related to FCW aggre-gate had more porosity than natural granite aggregate.

3.5. Dry shrinkage

The test results of dry shrinkage (DS � 10�6) and weight loss(WL%) of the concrete mixes at the drying period of 112 days aresummarized in Table 6. Every presented data is an average of three

Table 5Static modulus of elasticity of concrete at 28 and 90 days (GPa).

Mix code 0-I 15-I 30 N-I 30T-I 50 N-I 50T-

28-day 35.8 31.3 21.7 24.2 16.8 15.590-day 40.3 33.9 22.2 28.3 19.9 17.4

measurements. It is seen that the control mixes (0-I and 0-II) havethe lowest dry shrinkage and weight loss.

For mixes using W/C of 0.35, the dry shrinkage values were notsensitive to the FCW content until the FCW content was more than30%. And the dry shrinkage values of all the mixes were below750E-06 which were acceptable according to Australia StandardAS3600 [32]. On the contrary, for the W/C = 0.50 mixes, the shrink-age values were high. Most of the shrinkage occurred during thefirst 7 days, and then slowed down. Moreover, even though theFCW content was only 15%, there was a sharp increase in dryshrinkage value (up to 35%, compared to that of the control 0-II)and the value was far higher than the limits given by AS3600. Fur-thermore, there was very little difference in dry shrinkage resultsbetween the mixes prepared with NMA and TSMA.

Weight loss and dry shrinkage value of all the mixes had a sim-ilar trend. The weight loss (WL%) represents weight change of theshrinkage prisms due to egress of moisture from the samples. Dry-ing shrinkage in the concrete was induced by external dryingthrough diffusion from an exposed surface, so moisture loss hadan immediate effect upon the dry shrinkage [33]. In fact, not onlythe ultimate results, but also for the results at each testing time,there was a linear relationship between DS and WL during themeasurement period and the correlation coefficient was 0.753,which was given by Fig. 11.

3.6. Ultrasonic pulse velocity (UPV)

The measurement results of UPV of the concrete mixes at agesof 1, 3, 7, 28 and 90 days are presented in Figs. 12 and 13. Everypresented data is an average of six measurements. It can be seenthat there was a sharp decreased of UPV for all the concrete mixeswith the use of FCW aggregate during the initial 7 days, which wassimilar to the trend of compressive strength. The UPV decreasedwith the increase in FCW aggregate content. This is also attributedto lower density and higher water absorption capacity of FCWaggregate than that natural granite aggregate. Fig. 14 shows therelationship between compressive strength and UPV at each differ-ent curing ages with a correlation coefficient equal to 0.9366 byusing an exponential model. This result is consistent with thosepresented in other related literatures [34–36].

I 0-II 15-II 30 N-II 30T-II 50 N-II 50T-II

29.6 21.2 18.4 20.0 9.7 12.032.6 23.7 20.2 21.1 12.7 16.3

Fig. 10. Total charge passed in Coulombs of concrete at 28 and 90 days.

Table 6Dry shrinkage and weight loss of concrete at drying period of 112 days.

Mix code 0-I 15-I 30 N-I 30T-I 50 N-I 50T-I 0-II 15-II 30 N-II 30T-II 50 N-II 50T-II

DS (� 10�6) 548 559 568 594 1130 1070 644 1010 1220 1160 1280 1250WL (%) – – 5.10 4.41 5.91 6.38 3.24 4.35 6.86 6.99 8.15 8.38

Fig. 12. UPV of concrete mixes in Series I.

Fig. 11. Correlation between drying shrinkage and weight loss.

Fig. 14. Correlation between compressive strength and ultrasonic pulse velocity.

Fig. 13. UPV of concrete mixes in Series II.

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4. Conclusions

A comprehensive study on the properties of concrete incorpo-rating FCW as coarse aggregates was presented in this paper. Thefollowing conclusions can be drawn:

1. At a fixed W/C, the slump of fresh concrete increased with theincrease in FCW aggregate content.

2. The utilization of FCW reduced the density, and increased thewater absorption of the hardened concrete. But with no morethan 30% replacement of granite by FCW aggregate and with aW/C of 0.35, it is still possible to produce concrete with a target28-day compressive strength of 40 MPa.

3. There was a degradation of mechanical properties of the con-crete made with FCW.

4. The penetration of chloride ion and dry shrinkage of concretemixtures increased with the increase in FCW content. However,at a curing age of 90 days, the results of the two indexes wereacceptable according with relevant specifications when theconcrete specimens were prepared with W/C = 0.35, and witha replacement of FCW of not more than 30%.

5. When the concrete mixes were prepared with a W/C of 0.35 andreplacement level of FCW was lower than 30%, the two stagemixing approach increased the compressive strength of theFCW concrete.

6. Nevertheless, further research is required in the direction ofimproving the quality of FCW as aggregate.

Acknowledgement

The authors wish to thank Gammon Construct ion Ltd for pro-viding the fresh concrete waste samples. Sun Hung Kai PropertiesLtd and The Hong Kong Polytechnic University are thanked for pro-viding funding support.

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