Effect of mixed recycled aggregates on mechanical

Magazine of Concrete Research Effect of mixed recycled aggregates on mechanical properties of recycled concrete Rodrı´guez-Robles, Garcıá-Gonza´lez, Juan-Valde´s, Morań-Del Pozo and Guerra-Romero

Magazine of Concrete Research http://dx.doi.org/10.1680/macr.14.00217 Paper 1400217 Received 07/07/2014; revised 17/09/2014; accepted 24/09/2014 ICE Publishing: All rights reserved

Effect of mixed recycled aggregates on mechanical properties of recycled concrete Desireé Rodrı´guez-Robles

Julia Ma Morań-del Pozo

Agricultural engineer, Department of Engineering and Agricultural Sciences, University of Leon, Leon, Spain


Julia Garcıá-Gonza´lez

M. Ignacio Guerra-Romero



Andre´s Juan-Valde´s Agricultural engineer, Department of Engineering and Agricultural Sciences, University of Leon, Leon, Spain

The use of construction and demolition waste in concrete manufacture allows the concept of sustainability to be included in the construction industry and helps to alleviate both the large consumption of natural resources and the high generation of waste. The present research assesses first the suitability of two different mixed recycled aggregates as coarse aggregates for concrete according to the code on structural concrete (EHE-08), and subsequently evaluates the effect of their use as a partial (50%) replacement of natural coarse aggregate (gravel) on the mechanical properties (compressive strength, splitting-tensile strength, flexural strength, stress–strain curves and modulus of elasticity) in concrete mixes of 25 MPa strength grade. The secondary material characterisation shows that, although presenting promising properties in terms of granulometry, particle size, density and shape, the recycled aggregates should be pretreated in order to comply with the quality of fines and water absorption, and their resistance to fragmentation limits their application in concrete mixes with a strength grade below 30 MPa. Regarding the results of the tests performed on the hardened concrete, the values obtained suggest that the use of mixed recycled aggregates is feasible, but at the expense of minor losses of the mechanical characteristics.

Introduction The European construction industry, both because of its intensive consumption of natural resources (more than 50% of European natural resources (Schultmann et al., 2010)) and the large amounts of waste generated (about 33% of the total waste generated annually (EEA, 2012)), is one of the productive sectors with a greater negative impact on the environment. Increased environmental awareness has led to the search for alternatives to include the concept of sustainability in the construction sector. One of the pillars supporting sustainable construction is the lifecycle management of the raw materials that are part of buildings; this has demonstrated the environmental benefits to be gained when the construction industry incorporates the waste generated in construction activities as a secondary resource in new construction works. Over recent years, the recycling potential of construction and demolition waste (CDW) has become a target of interest and the main focus of waste management policies encouraging minimisation, reuse, recycling and recovery of waste rather than final disposal in landfills. Research on the use of recycled materials, specifically the use of recycled aggregates in concrete manufacture, focuses on the

influence of the composition and quality of these secondary aggregates on the properties of the fresh and hardened recycled concrete. Most of the studies carried out to date have focused on the potential of recycled aggregate concrete (Dhir et al., 1999; Etxeberria et al., 2007a, 2007b; Evangelista and De Brito, 2007; Mefteh et al., 2013; Olorunsogo and Padayachee, 2002; Poon and Dixon, 2007; Rao et al., 2007; Tam et al., 2008; Xiao et al., 2006; Yang et al., 2011) or ceramic recycled aggregate (Cachim, 2009; Correia et al., 2006; De Brito et al., 2005; Debieb and Kenai, 2008; Go¨kçe and S¸ims¸ek, 2013; Khalaf, 2006; Khalaf and DeVenny, 2004, 2005; Medina et al., 2011, 2014; Pacheco-Torgal and Jalali, 2010; Senthamarai and Devadas Manoharan, 2005) in the manufacture of eco-efficient concrete. However, owing to construction practice (usually based on ceramic elements combined with mortar and concrete) and the current waste management situation (i.e. sorting on-site is not mandatory for all construction works) the recycled aggregates most produced at Spanish construction and demolition treatment plants are mixed recycled aggregates (70%) (GERD, 2012). These are composed of stone materials (bricks, roof tiles, ceramic materials and products, slabs, concrete and so on) and some 1

Magazine of Concrete Research

Effect of mixed recycled aggregates on mechanical properties of recycled concrete Rodrı´guez-Robles, Garcıá-Gonza´lez, JuanValde´s, Morań-Del Pozo and Guerra-Romero

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impurities such as asphalt, gypsum, glass, metals, plastic and wood. Thus, in an attempt to identify a possible means of reusing this waste, this research characterises the mixed recycled aggregates from CDW, in order to assess their quality as a partial substitute for conventional coarse aggregate in concrete according to the Spanish code on structural concrete EHE-08 (Permanent Commission on Concrete, 2008), and then analyses the effect of this replacement on the mechanical properties of the resulting recycled concrete.

Materials

(a)

In the manufacture of concrete Portland cement CEM III/A 42 .5 N/SR, natural river sand (0/4 mm), natural siliceous gravel (4/16 mm) and mixed recycled aggregates supplied by two different CDW recycling plants were used. The mixed recycled aggregates are the result of the processing of CDWs at the TEC-REC facilities in Madrid (RA1) (Figure 1) and Bierzo Recicla facilities in Castile and Leon (RA2) (Figure 2). For both cases, the treatment consisted of two distinct phases of crushing and sieving accompanied by a series of screening points (pneumatic, mag-

(b)

Figure 2. (a) Bierzo Recicla CDW treatment plant. (b) Mixed recycled aggregates from CDW

netic and manual separations) in order to obtain a size-classified material, free of most impurities. (a)

Mix proportions Three concrete mixes of 25 MPa strength grade were made – control concrete (CC), recycled concrete with RA1 substitution (RC1-50%) and recycled concrete with RA2 substitution (RC250%) – to compare the effect of partial replacement of conventional coarse aggregate. All the concrete mixes (Table 1) were designed with the same water/cement ratio (w/c ¼ 0 .55) with the aim that this parameter could serve as a point of comparison between the three batches.

Methods

(b)

Figure 1. (a) TEC-REC CDW treatment plant. (b) Mixed recycled aggregates from CDW

2

Characterisation of the recycled aggregate The characteristics of the aggregates used in the manufacture of concrete largely determine the properties of the final product. Thus, in order to obtain quality recycled concrete, it is necessary for the recycled aggregates used to meet the requirements of the standard UNE-EN 12620 (AENOR, 2009a) and Chapter VI:




Water: l/m3 Cement: kg/m3 Sand: kg/m3 Gravel: kg/m3 Recycled aggregate: kg/m3

Recycled concrete

Control concrete (CC)

RC1-50%

RC2-50%

215 .00 390 .91 482 .39 1238 .54 –

215 .00 390 .91 578 .56 505 .21 505 .21

215 .00 390 .91 617 .09 474 .06 474 .06

Table 1. Control and recycled concrete mixes

Materials of the Spanish code on structural concrete (Permanent Commission on Concrete, 2008). In order to perform the characterisation of the recycled aggregates, a representative quantity of each sample was selected following the specifications in UNE-EN 932-1 (AENOR, 1997), and was subjected to the following assays (Table 2). Characterisation of the concrete An experimental programme was carried out in order to evaluate the mechanical behaviour of the concrete. First, the consistency of the fresh concrete was evaluated with a slump test according to the UNE 12350-2 (AENOR, 2009b) as problems with workability have been reported (Poon et al., 2004; Yang et al., 2011) when recycled aggregates have been used in concrete manufac-

Parameter

Standard

Composition content

UNE-EN 933-11 (AENOR, 2010b) UNE-EN 933-1 (AENOR, 2012a) EHE-08 (Permanent Commission on Concrete, 2008) EHE-08 (Permanent Commission on Concrete, 2008) UNE-EN 933-8 (AENOR, 2012b) UNE-EN 933-3 (AENOR, 2012c) UNE-EN 1097-6 (AENOR, 2014b) UNE-EN 1097-6 (AENOR, 2014b) UNE-EN 1097-2 (AENOR, 2010c)

Particle size Particle size

Fines quantity

Fines quality (sand equivalent) Shape (flakiness index) Density Water absorption Resistance to fragmentation (Los Angeles test)

Table 2. Characterisation tests of recycled aggregates

ture. The density was studied in the hardened concretes using methods described in the standard UNE-EN 12390-7 (AENOR, 2009c). The compressive strength was measured at 7 and 28 d through 150 3 300 mm cylindrical specimens described in the UNE-EN 12390-3 (AENOR, 2011), also 150 3 300 mm cylindrical specimens were used for the determination of the modulus of elasticity according to the UNE 12390-13 (AENOR, 2014a) and the splitting-tensile strength as specified in the standard UNE-EN 12390-6 (AENOR, 2010a). In addition, 100 3 100 3 400 mm prismatic specimens were needed to carry out the flexural strength test described in UNE-EN 12390-5 (AENOR, 2009d).

Results and discussion Characterisation of the recycled aggregate As recommended in EHE-08 (Permanent Commission on Concrete, 2008), the granulometric curves (Figure 3) are continuous and non-uniform, indicating that the particle size distribution of both recycled aggregates is correct in all sizes, which allows a greater margin for interaction between the particles and produces a greater degree of compactness and mechanical strength in the concrete (Kraemer et al., 2004). Table 3 shows the different components in the recycled aggregates and their proportion by weight in the sample. The presence of a content of impurities (X1 + X2) greater than 1% in both mixed recycled aggregates, the limit established in the EHE-08 (Permanent Commission on Concrete, 2008), requires a pretreatment of the samples in order to remove the excess of impurities. Table 4 shows the results obtained in the physical and mechanical tests conducted in order to characterise the recycled aggregates, the limit value stipulated in the current Spanish legislation and the level of compliance of the mixed recycled aggregates. As indicated in the general particle size requirements of the 100 90

Percentage passing: %

Component

RA1

RA2

80 70 60 50 40 30 20 10 0 0·01

0·1

1 Sieve size: mm

10

100

Figure 3. Granulometric curve of the mixed recycled aggregates

3




Component Concrete and mortar (natural aggregates with mortar attached) Unbound aggregates (natural aggregates without mortar attached) Ceramics (brick, tiles and so on) Glass Asphalt Gypsum Others (wood, plastic, metals)

Rc Ru Rb Rg Ra X1 X2

RA1: %

RA2: %

44 .11 17 .51 33 .56 0 .75 0 .44 3 .48 0 .16

19 .39 9 .42 68 .27 0 .28 0 .29 2 .14 0 .20

Table 3. Composition of the recycled aggregates based on UNEEN 933-11 (AENOR, 2010b)

Test Particle size

Fines content Sand equivalent Density

Water absorption Flakiness index Los Angeles test

D: mm d: mm D/d FC: % SE: % ra: Mg/m3 rrd: Mg/m3 rssd: Mg/m3 WA: % FI: % LA: %

RA1

RA2

Limit

16 4 4 0 .04 38 .1 2 .53 2 .08 2 .26 8 .53 14 .75 40 .99

16 4 4 0 .05 24 .8 2 .49 1 .99 2 .15 13 .55 28 .26 43 .90

– – . 1 .4 , 1 .5 70–75 – – – , 7% , 35 , 40–50

Compliance – – Yes Yes No (pretreatment required) – – – No (pretreatment required) Yes Yes (under certain restrictions)

Table 4. Characterisation of the recycled aggregates

EHE-08 (Permanent Commission on Concrete, 2008), the relationship between the maximum (D) and minimum (d ) size of aggregates is greater than 1 .4. The fines assessment is an important determination, because a high amount of fines prevents the good adherence of the cement paste and facilitates concrete failure. Regarding the fines content, the limit imposed is not exceeded. However, in terms of quality of the fines determined by the sand equivalent, the recycled aggregates exceed the limitations due to the presence of mortars and ceramic elements, which behave as clays. Nonetheless, compliance could be achieved if the samples were pretreated through sieving to remove the fines fraction. The density values obtained lay within the range 1 .6–2 .65 Mg/m3 of other ceramic materials found by Sańchez de Juan and Alaejos (2006), which is lower than the natural coarse aggregates due to the greater porosity of the mixed recycled aggregate resulting from both its ceramic nature and the presence of adhered mortar. The determination of water absorption is a good indicator of the quality of the recycled aggregates, because these kind of recycled aggregates have a high absorption capacity that can be detrimental to the workability of the concrete mix (Agrela et al., 2011). In 4

view of the results obtained, which are similar to those reported by Agrela et al. (2011) and Mas et al. (2012), the EHE-08 (Permanent Commission on Concrete, 2008) requirements are not fulfilled. However, there is also the possibility to fix this lack of compliance if a pre-saturation of the aggregates is carried out before their use in the concrete manufacture (Agrela et al., 2011). In terms of shape, the mixed recycled aggregates present a different external morphology compared to natural aggregates, as the former have sharp edges, angular outlines, variable shapes and a more or less flat surface, which is reflected in a higher flakiness index. This external appearance is due both to the initial form of the waste and the crushing process, and the flakiness index values obtained are in agreement with those reported by Mas et al. (2012). Finally, the Los Angeles test results complied with the 50% limit of the code on structural concrete (Permanent Commission on Concrete, 2008), which allows their use for concrete with a strength grade below 30 MPa. This is a predictable outcome, as this property has been identified by Jimeńez et al. (2011) and Vegas et al. (2011) as a limiting factor in the use of mixed recycled aggregates, because the presence of ceramic and adhered mortar means less resistance to fragmentation (Martıń-Morales et al., 2011).



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution Mechanical properties of concrete Regarding the mechanical properties, Figure 5 shows the results obtained for compressive strength at 7 and 28 d of curing and tensile splitting strength and flexural strength at 28 d of age.

In general, and although both mixed recycled aggregates showed the same pattern of compliance and non-compliance with the EHE-08 (Permanent Commission on Concrete, 2008) requirements, the greater amount of ceramic particles in sample RA2 is the feature held responsible for the inferior quality of the recycled aggregate, as some margins of fulfilment are narrower and the non-conformities are higher than for RA1. However, the extent of the influence of the type of ceramic particle (brick, tiles and so on) on this reduced quality should not be overlooked among the properties tested, as not only the quantity of ceramic but also its quality plays an important role.

Compressive strength The control concrete and both of the recycled concrete samples exhibited compressive strengths over 25 MPa. On average, the 7-d compressive strength was between 17 .8% and 19 .0% lower for the recycled concrete than for the control concrete, whereas the 28-d compressive strength was 6 .34% lower for the recycled concrete than for the control concrete.

Physical properties of concrete Consistency The Abrams cone was used to perform the consistency measurements (Figure 4). The control concrete (CC) reached a slump value of 32 mm, the recycled concrete RC1-50% presented a slump value of 23 mm, which accounts for a 28 .13% loss in workability, and the recycled concrete RC2-50% exhibited a slump value of 19 mm, which represents a 40 .63% workability loss. As all concrete mixes were designed with the same w/c ratio, the slump difference between them is attributed to the use of recycled coarse aggregates with higher water absorption capacity. Martıńez-Lage et al. (2012) and Medina et al. (2014) observed an inverse relationship between the consistency loss in recycled concrete and the increase in the recycled coarse aggregate replacement ratio.

40

CC RC1-50%

35

25

RC2-50%

23·70 19·48 19·19

20 15 2·94 2·68 2·55

10 5

7·16 6·59 6·52

Stress: MPa

30

Density of hardened concrete The saturated density values for the control and recycled concretes were determined at 28 d of curing. Density was lower in the recycled concrete mixtures (2 .33 Mg/m3 and 2 .30 Mg/m3 for RC1-50% and RC2-50%, respectively) than in the control concrete (2 .38 Mg/m3). This 2 .1% decline was the result of the lower density of the recycled coarse aggregates (2 .53 Mg/m3 for RA1 and 2 .49 Mg/m3 for RA2) compared to the natural coarse aggregate (2 .66 Mg/m3). These results are in the lower range of the density loss described in the regression estimates of MartıńezLage et al. (2012) and Medina et al. (2014).

(a)

38·46 36·02 35·65

It is noteworthy that the loss of compressive strength is more pronounced in 7-d specimens, which is contrary to the findings of Medina et al. (2014), but similar to the results obtained by Mas

0 Compressive Compressive strength strength ( 7 d) ( 28 d)

Splittingtensile strength (28 d)

Flexural strength (28 d)

Figure 5. Compressive, tensile splitting and flexural strength of the control concrete (CC) and the recycled concretes (RC1-50% and RC2-50%)

(b)

(c)

Figure 4. Consistency of (a) the control concrete (CC) and the recycled concretes (b) RC1-50% and (c) RC2-50%

5




et al. (2012), who reported that the use of mixed recycled aggregate delayed the hardening of the recycled concrete. The effect of the ceramic materials as well as some impurities present in the mixed recycled aggregates was responsible for the poorer performance in the recycled concrete (Paine and Dhir, 2010); the results confirmed that an increase in the ceramic percentage of the recycled aggregate used in the substitution had a higher negative impact on the mechanical resistance of the recycled concrete; and for both ages of curing, RC2-50% performed worse (1 .0% and 1 .5% for 7 and 28 d of curing, respectively) than RC1-50%. This trend has been corroborated by many authors. Chen et al. (2003) observed declines in compressive strength up to 25% in 100% recycled concretes with 67% ceramic content. De Brito et al. (2005) argued that the decrease in compressive strength was on the order of 45% for 100% ceramic substitutions and up to 24% when one-third or two-thirds of natural aggregates were replaced by ceramic recycled aggregates. Debieb and Kenai (2008) stated that reductions in compressive strength were 30% when 100% of the natural coarse aggregates were replaced. Yang et al. (2011) observed that for concretes containing 20% recycled aggregate made from fired clay waste, compressive strength decreased by 11% and by 20% when the replacement rate was 50%. Martıńez-Lage et al. (2012) reported decreases in compressive strength up to 13% and 23% in recycled concrete with 50% and 100% replacements of mixed recycled aggregates, respectively. Mas et al. (2012) obtained reductions up to 13–39% of compressive strength in 40% substitutions. Medina et al. (2014) found that compressive strength for recycled concrete with 25% and 50% replacement ratios was 8 .7– 15 .9% and 15 .1–18 .4% lower than for the reference concrete at 7 and 28 d of curing, respectively, and that the removal of the floating particles diminished the strength differences between recycled and reference concrete.

Splitting-tensile strength The results show that a 50% replacement of the natural coarse aggregate produces a decrease between 8 .85% and 13 .37% in the splitting-tensile values of the recycled concrete (RC1-50% and RC2-50%, respectively). The present findings are consistent with the observations reported by other authors. Gonza´lez-Fonteboa et al. (2011) informed of declines in splitting-tensile strength up to 4 .80% even when a pre-saturation of the recycled aggregates and 20 kg/m3 of additional cement was used in the recycled concrete. Yang et al. (2011) reported declines up to 31% and 34% in 100% recycled concrete with 20% and 50% ceramic content, respectively. Mas et al. (2012) obtained 10–34% decreases of splitting-tensile strength in recycled concretes with a 40% replacement ratio. Medina et al. (2014) also found that in recycled concrete with 50% mixed recycled aggregates the splitting-tensile strength decreases up to 16 .5% if the recycled aggregate is untreated or up to 2 .9% if the floating particles are removed. 6

Flexural strength Although the angular shape of the crushed material and its surface roughness are generally beneficial for a good bond between the aggregates and the cement paste, which could hence increase the flexural strength performances (Devenny and Khalaf, 1999), a decrease in flexural strength of 7 .96% for RC1-50% and 8 .94% for RC2-50% was observed for the recycled concrete, which showed that for the same substitution percentage the increase in the ceramic content had a negative effect. Chen et al. (2003) reported decreases in flexural strength up to 20% in 100% recycled concretes with different contents of ceramic. De Brito et al. (2005) reported a decline in flexural strength around 26% for 100% ceramic substitutions and up to 15% if two-thirds of natural aggregates were replaced by ceramic recycled aggregates. Yang et al. (2011) informed that decreases of 6% were found in 100% recycled concrete with 50% of ceramic materials in the aggregate replacement. Mas et al. (2012) recorded declines between 13% and 30% in recycled concrete with replacement ratios of 40%. As discussed by Hoffmann et al. (2012), there is no significant difference in the relation between the compressive strength and flexural strength of the recycled concretes in comparison with the control concrete, which shows that the bond between paste and aggregate in the recycled concrete mixes was comparable to that in the control concrete, as confirmed by Kheder and Al-Windaw (2005). The analysis of the fracture zone (Figure 6) in the prismatic recycled specimens subjected to flexural tests showed that the ceramic particles present are fractured, as they represent

(a)

(b)

Figure 6. Fracture zone of the recycled concretes (a) RC1-50% and (b) RC2-50% subjected to flexural tests



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a weak point mechanically; thus the ceramic content is an important limiting factor in the mechanical resistance of the recycled concretes.

y ⫽ ⫺0·26301 ⫹ 0·02549x R ⫽ 0·99914

12

6 CC 2 0

0

14 12

50

100 150 200 250 300 350 400 450 500 Strain: μm/m (a) y ⫽ 0·21819 ⫹ 0·02543x R ⫽ 0·99904

10

Stress: MPa

Modulus of elasticity For structural concrete, the modulus of elasticity is an important mechanical parameter, reflecting the ability of the concrete to deform elastically. As the modulus of elasticity of the concrete is a function of the modulus of elasticity of the aggregates and the cement matrix and their relative proportions, when mixed recycled aggregates – whose modulus of elasticity is lower than coarse natural aggregates – are used in concrete manufacture, a decrease is expected. The test conducted reported a modulus of elasticity of 25 .49 GPa for control concrete, 25 .43 GPa for RC150% recycled concrete and 23 .57 GPa for RC2-50% recycled concrete, which only accounts for a 0 .23% and a 7 .53% reduction correspondingly. In order to describe better the modulus of elasticity, trend lines were also calculated. A first-order polynomial yielded a satisfactory fit for the control concrete and both of the recycled concrete mixtures (Figure 8).

8

4

8 6 4

RC1-50%

2 0

0

12

50

100 150 200 250 300 350 400 450 500 Strain: μm/m (b)

y ⫽ ⫺0.·6691 ⫹ 0·023577x R ⫽ 0·99938

10

Stress: MPa

Stress–strain curves The stress–strain behaviour of concrete was assessed by compressive tests on cylinders (Figure 7). With regard to the shape of the ascending branch of the stress–strain curve, the behaviour of the control and recycled concrete is similar. However, as mixed recycled aggregates are weaker than natural aggregates, differences in the stress–strain curves appear when the stress–strain curves become parabolic. For the same stress increase, the recycled concrete suffers a higher strain than the control concrete, and this phenomenon was increased by the increase in the ceramic percentage in the mixed recycled aggregate used in the substitution. Moreover, the presence of mortar adhered to the recycled aggregate, which is more compliant than conventional aggregate, allows the increase of strains in the ascending branch (Gonza´lezFonteboa et al., 2011).

Stress: MPa

10

8 6 4

RC2-50%

2 40 0

35

Stress: MPa

30 25

0

50

100 150 200 250 300 350 400 450 500 Strain: μm/m (c)

Figure 8. Modulus of elasticity of (a) the control concrete (CC) and the recycled concrete (b) RC1-50% and (c) RC2-50%

20 15 10 CC

5

RC1-50%

RC2-50%

0 0

500

1000 1500 Strain: μm/m

2000

2500

Figure 7. Stress–strain curves of the control concrete (CC) and the recycled concretes (RC1-50% and RC2-50%)

The results obtained in this research work are in the lower range of those described by other authors. Rilem specifications (Rilem, 1994) showcased that the E modulus of concrete produced with 70% of the aggregate consisting of crushed bricks is 55% lower than that of control concrete at an identical compressive strength. Chen et al. (2003) and Debieb and Kenai (2008) reported a decrease in the modulus of elasticity around 30% for 100% recycled concrete with mixed recycled aggregates. Gonza´lezFonteboa et al. (2011) informed of declines in the modulus of 7



Offprint provided courtesy of www.icevirtuallibrary.com Author copy for personal use, not for distribution elasticity up to 11 .30%, even when a pre-saturation of the recycled aggregates and 20 kg/m3 of additional cement was used in the recycled concrete. Martıńez-Lage et al. (2012) stated a 28% and 34% decrease of the modulus of elasticity when 50% or 100% mixed recycled aggregates are used in concrete manufacture.

Conclusions The recycled aggregate characterisation showed the suitability of the samples for use in concrete manufacture as coarse aggregate. Although the use of mixed recycled aggregates in concrete manufacture has yet to be allowed in Spain, according to the current Spanish legislation on the matter, namely the code on structural concrete (EHE-08), the mixed recycled aggregates selected meet the requirements for granulometry, particle size, density and shape. Also, the lack of compliance with the requirements for impurities, fines quality and water absorption could be fixed by subjecting the aggregates to treatment by screening, sieving and water saturation prior to use in concrete. However, the value obtained for resistance to fragmentation limited the application of such aggregates to concrete with a strength grade lower than 30 MPa. Nonetheless, it is necessary to emphasise that the promising results obtained are the product of the analysis of the mixed recycled aggregates collected on a specific day of sampling in each of the CDW treatment facilities. Thus, due to the changing nature of the CDW arriving at the management plant, the reproducibility over time within the same CDW treatment company may not be ensured. However, given the conventional constructive practice in Spain and under the assumption that the Spanish mixed recycled aggregates composition would largely range between the values of those in the selected samples, it is possible to assume that the results arising from other mixed recycled aggregates retrieved by another campaign of sampling in the same, or even different, CDW management plants could be inside the suitability margins of the EHE-08. Additionally, this reasoning indicates that the results obtained in this research work may be extrapolated outside national borders, if the composition of mixed recycled aggregates in other countries compares with the Spanish situation, which could undoubtedly happen in territories with similar constructive practice; only pending the verification of the national standards in these countries to assess the potential suitability of this type of recycled aggregates. Regarding the results of the mechanical tests performed on the hardened concrete, and despite the decrease in the values obtained compared to the control concrete, this research work suggests that the use of mixed recycled aggregate from CDW in the manufacture of concrete is feasible without a significant loss in the mechanical characteristics of the recycled concrete. Thus, recycled concrete with mixed recycled aggregates should be considered, both nationally and internationally, as an option to improve construction sector sustainability. Therefore, this alternative for reusing CDW as coarse aggregate 8

in concrete manufacture represents an environmental benefit as it reduces the landfill of inert wastes readily usable as secondary building materials and, simultaneously, prevents further extraction of natural resources. Moreover, this alternative is also advantageous economically, because it avoids some waste management costs and offers a lower cost option to conventional coarse aggregate.

Acknowledgements This paper was made possible by the university teacher training grant (FPU AP2010-0613) and the research staff training grant (FPI BES-2011-047159) awarded to Julia Garcıá Gonza´lez, associated with project BIA2010-21194-C03-02. The authors would like to acknowledge the material contribution (sample of mixed recycled aggregates) made by the CDW treatment plant TEC-REC S.L. and Bierzo Recicla S.L. REFERENCES

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