La  “Generación  X”  y  su  relación  con  la  novela  y  el  autor

Given that the recycled aggregates used in this research work came from construction and demolition wastes (CDW), their common composition is a mixture of unbound aggregates (Ru), cement based materials (Rc), ceramics (Rb), asphalt (Ra), glass (Rg), gypsum (X1) and other impurities (X2) such as wood, plastic, and metal in varying proportions.

• Unbound aggregates: Apart from their natural origin, the crushing procedure at the CDW management plant causes not only the reduction in size of the concrete lumps but also serves as a cleaning procedure for the adhered mortar on the surface of some natural aggregates appearing in the recycled aggregate. It is generally acknowledged that this fraction has the same behaviour as natural aggregates; and as such, no investigation on their effect on the quality of mixed recycled aggregates has ever been performed.

• Cement based materials: Concrete rubble and particles with attached mortar are one of the most generated materials in the demolition works. This fraction has been identified as responsible of the increased porosity in comparison with the natural aggregates and the derived variation in physical (density and water absorption) and mechanical properties of the mixed recycled aggregates (Katz, 2003; Etxeberria et al., 2007; Padmini et al., 2009;

Sánchez de Juan and Alaejos Gutiérrez, 2009; Paine and Dhir, 2010; Silva et al., 2014).

• Ceramic: The clay based fraction is composed by different ceramic materials such as bricks, tiles, stoneware and sanitary ware. Except for sanitary ware waste (Medina et al., 2012), the ceramic component is as well recognised as accountable for the higher porosity of the mixed and ceramic recycled aggregates (Khalaf and DeVenny, 2005; Silva et al., 2014)

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• Asphalt: Because of the predominant use of this material in road construction works, the likelihood of this component in the mixed recycled aggregates is associated to the origin of the construction and demolition wastes. Moreover, given its high potential for reuse, the recovered material is generally taken to an asphalt plant instead to a CDW plant in order to be reused in road base layers or even the wearing course of pavements. Medina et al. (2015) studied the effect of asphalt on the quality of the mixed recycled aggregates and two recycled concrete mixes (30 MPa) with a 25% and 50% substitution of the natural coarse aggregate, and their results showed a higher density and lower water absorption of the mixed recycled aggregates without asphalt content and therefore a higher density and compressive strength of the recycled concrete.

• Glass: In order to facilitate its reuse and recycling, the common practice dictates removing this material before the demolition of the building; thus, the quantities of glass in the CDW are usually not significant. In addition, research works indicate that the negative effect of using glass waste as coarse recycled aggregate in concrete increases with the substitution rate; for instance, Topçu and Canbaz (2004) registered decreases in compressive, flexural and indirect tensile strength for concrete with recycled glass aggregates up to 15% and Shayan and Xu (2004) concluded that no deleterious alkali-silica reaction (ASR) effects were detected up to a 30% replacement of the conventional coarse aggregate.

• Gypsum: As the construction industry uses gypsum for stucco, plasterwork, cardboard-plaster panels, etc. its occurrence in the mixed recycled aggregates as an independent component or associated to other particles is fairly common. The presence of gypsum has a negative effect on the quality of mixed recycled aggregates due to its solubility, low density and hardness (Vrancken and Laethem, 2000); moreover, as gypsum is a source of sulphates its incorporation could cause expansions in the recycled concrete due to the delayed formation of ettringite (Neville, 1995; Odler and Colán-Subauste, 1999) and contaminate the environment through the leaching of water soluble sulphate (Barbudo et al., 2012b).

• Other impurities: In general terms, the separation process depends on the simplicity of the procedure and the economic benefits achieved. Paper, wood and plastic are normally separated by air blowers after the crushing step and by hand-picking during their transport in the conveyor belt. Likewise, magnetic belts and eddy currents are used to remove ferrous (steel) and non-ferrous (aluminium, stainless steel and cooper) metals after the crushing procedure and the aforementioned hand-picking step is also employed to separate this component from the final recycled aggregate. Hence, the occurrence of these impurities in the mixed recycled aggregate is low and it is associated to the level of effectiveness of the treatment process in the CDW management plant. The research conducted by Medina et al. (2015) also studied the influence of the floating particles on the quality of the mixed recycled aggregates and the recycled concrete, and their results showed that the floating particles were accountable for lower density and higher water absorption of the recycled aggregate and lower density and a decline in the compressive strength of the recycled concrete.

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Table 4.3 and Figure 4.6 display the proportion in weight of the different components being part of the selected recycled aggregates. The results show that the predominant component was concrete (44.11% and 37.05% respectively) followed by materials of ceramic nature (33.56% and 38.44% respectively) in RA-L(S) and RA-L(B) samples, while in RA-M(S) the situation was reversed with a majoritarian ceramic component (66.38%) followed by the occurrence of cement based materials (17.31%). For these three, unbound aggregates was the third most common component with values ranging between 15.07% and 22.46%., In regard to sample RA-H(S), the recycled aggregate was solely composed by ceramic materials (bricks and tiles principally) as expected

Table 4.3: Composition of the recycled aggregates RA-L(S) RA-L(B) RA-M(S) RA-H(S)

Ru 17.51 22.46 15.07 0.00

Rc 44.11 37.05 17.31 0.00

Rb 33.56 38.44 66.38 100.00

Ra 0.44 1.74 0.43 0.00

Rg 0.75 0.00 0.19 0.00

X1 3.48 0.30 0.22 0.00

X2 0.16 0.00 0.41 0.00

Figure 4.6: Composition of the recycled aggregates

As the Spanish EHE-08 (Permanent Commission on Concrete, 2008) only allows the use of crushed concrete as recycled aggregate for structural and non-structural applications of concrete, the standard considers the ceramic component as an impurity, and as such it is limited up to 5%.

However, as the aim of this research work is to characterize the suitability of mixed recycled aggregates with significant contents of ceramic in the concrete manufacture, this requirement would be ignored. Nonetheless, based on the aforementioned objective, the determination of the component proportions allows the classification of the recycled aggregates according to their ceramic content. Hence in this context, the sample RA-H(S) is considered as ceramic recycled aggregate and the rest of the samples, (RA-L(S), RA-L(B) and RA-M(S)), are classified as mixed recycled aggregates due to the variability in their composition.

135 A more detailed analysis of the existing classifications of the recycled aggregates in function of their ceramic percentages was discussed in Chapter 2. Concerning to the rest of impurities, EHE-08 (Permanent Commission on Concrete, 20EHE-08) lays down a 1% limit for asphalt and the so-called other impurities (glass, metals, plastic, wood…). On one hand, for the asphalt limitation, only sample RA-L(B) exceeded the requirement; however, the Spanish limit is stricter when compared to other European requirements (LNEC E-471, 2009; CEN/TC 104/SC 1/TG 19, 2013; BS 8500-2, 2015). In this regard, the Spanish Association of CDW management plants (GERD) carried out a national wide study on the quality and performance of 65 CDW treatment plants and recommended a 5% limit for the asphalt constrain in mixed recycled aggregates (Güell-Ferré et al., 2012). On the other hand, the content requirement for the other impurities (glass, metal, plastic and wood among others) was only not complied by the sample RA-L(S), which suggests some failure in the separation procedure at the TEC-REC management plant. Special mention should be made of the high gypsum content of samples RA-L(S), as Agrela et al. (2011) recommended the rejection of recycled aggregates containing more than 1.67% of gypsum due to lack of compliance with the EHE-08 sulphate requirements (Permanent Commission on Concrete, 2008).

Section 3.2.1.2 deals with the results and discussion of the sulphate content of the analysed samples.

In addition, the content of so-called other impurities (wood, paper, plastic) is considered to provide a good estimation of the content of organic substances in mixed recycled aggregates (Vegas et al., 2011). Despite that the Spanish Code on structural concrete (Permanent Commission on Concrete, 2008) appoints the UNE EN 1744-1 (2010) as the reference method for the determination of organic matter, the suitability of this technique is further on criticized in the annex 15 of recommendations for the use of recycled aggregates in concrete. Likewise, the effectiveness of other recognised methods for the determination of organic matter, such as the potassium permanganate (KMnO4) method and the loss on ignition at 500°C have also been questioned when applied to recycled aggregates (Vegas et al., 2011).

Hence, the aforementioned approach was used to evaluate the organic matter content of the four samples (Table 4.3). All the recycled aggregates exhibited contents inferior to 0.80%, which falls in line with the recommendation of Sherwood (1995) to avoid the deleterious effects of this component in the concrete manufacture, and within the normal range of values found in the literature (Table 4.4). In the previous fashion, the data from the literature review is expressed by the average and interval values of all the recycled aggregates assessed in each research work.

Table 4.4: Literature review on the organic matter content of the mixed recycled aggregates Organic matter (%) Barbudo et al. (2012a) 0.43 (0.24-0.95) 0.45 (0.21-0.89) Jiménez et al. (2012a) 0.50 Jiménez et al. (2012b) 0.40

a 100% ceramic recycled aggregate

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Nevertheless, as organic substances were detected in some samples (RA-L(S) and RA-M(S)), their influence on the setting time and the compression strength of the recycled concrete was further assessed and will be discussed in chapter 6 and 8, respectively, in accordance with the requirements laid down in the EHE-08 (Permanent Commission on Concrete, 2008).

Since the approval of the standard for the classification of the constituents of coarse recycled aggregates (UNE EN 933-11, 2009), scientific works not only incorporate the components classification but try to infer quality properties of the recycled aggregate based on the proportions of a determined constituent. Further details of such attempts will be discussed in this chapter for each one of the properties analysed.

Although comparatively inferior to the attention received by the recycled aggregates from crushed concrete, several studies on the composition of the mixed recycled aggregates were found in the literature. The compositional data of 146 mixed recycled aggregates pertaining to 29 research works can be consulted in Table 4.5, where the mean value and the interval range of each set of recycled aggregates are presented. Nonetheless, in all further analysis the data of each individual sample of recycled aggregates was employed. Note that, although the categories of components outlined in UNE EN 933-11 (2009) were followed, some studies presented together the content of cement based materials and unbound aggregates (Rc+Ru).

Table 4.5: Literature review on the composition of mixed recycled aggregates

Rc Ru Rc+Ru Rb Ra Rg X1 X2

137 Table 4.4: Literature review on the composition of mixed recycled aggregates (continued)

Rc Ru Rc+Ru Rb Ra Rg X1 X2

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Table 4.4: Literature review on the composition of mixed recycled aggregates (continued)

Rc Ru Rc+Ru Rb Ra Rg X1 X2

Figure 4.7 illustrates the margin in which the compiled compositional results fluctuate. The percentage of cement based materials (concrete lumps and natural aggregate with attached mortar) ranged between 9.33% and 81.40% with a mean value of 43.04%, the natural aggregates without mortar attached present an average of 27.94% ranging between 2.50% and 80.90%, and the ceramic particles content ranged between 1% and 67.30% with an average of 25.06%. Regarding to the asphalt, this component constituted on average 2.77%, which is in keep with other European less strict standards. However, approximately the 20% and the 5% of the mixed recycles aggregates surpassed the 5% (LNEC E-471, 2009; CEN/TC 104/SC 1/TG 19, 2013) and 10% (LNEC E-471, 2009; CEN/TC 104/SC 1/TG 19, 2013; BS 8500-2, 2015) limit respectively; the asphalt content reaching a maximum of 19.33% for one of the samples.

Figure 4.7: Literature review on the composition of mixed recycled aggregates

Content (%)

139 Similar results can be seen for the rest of impurities (glass, gypsum and others), and approximately 56% of the samples exceeded the 1% conjoint limit established in the EHE-08 (Permanent Commission on Concrete, 2008). Nonetheless, the RILEM TC 121-DRG (1994) recommended less severe limitations that depend on the type of waste materials generating the recycled aggregates;

for example, the restriction of foreign materials for aggregates with a ceramic origin (Type I) is fixed at 5%, which translates in roughly 8% of samples not complying with the recommendation.

These results suggest the necessity to implement more strict separation methods in order to remove impurities that could hinder the quality of the mixed recycled aggregates.

A Pearson product-moment correlation analysis was performed in order to detect possible linear correlation between the different components present in the recycled aggregates of the experimental study and the literature review. Table 4.6 and Table 4.7 show the Pearson coefficient (top row) and the p-value (bottom row) for each pair of variables evaluated. Significant linear correlations (p-value<0.05) are displayed in grey coloured cells.

Table 4.6: Pearson correlation results for the components of the recycled aggregates (correlation coefficient (top row) and p-value (bottom row))

Rc Rb Ra Rg X1 X2

Ru 10.882 -0.9340 0.7930 0.2970 0.3310 0.2020

0.1180 0.0660 0.2070 0.7030 0.6690 0.7980

Rc -0.9920 0.6000 0.6230 0.7050 0.0106

0.0082 0.4000 0.3770 0.2950 0.9890

Rb -0.6560 -0.5600 -0.6280 -0.0779

0.3440 0.4400 0.3720 0.9220

Ra -0.2510 -0.1230 -0.2860

0.7490 0.8770 0.7140

Rg 0.9700 0.3100

0.0302 0.6900 X1

0.0799 0.9200

The quantity of natural unbound aggregates shows a linear dependence with the ceramic content in the sample and as the natural aggregates tend to decrease the ceramic materials tend to increase, a normal tendency when dealing with mixed recycled aggregates. In addition, a linear relationship became apparent between the glass and gypsum content of the samples. This connection suggests that both contaminants increase simultaneously with a decrease in the cleanliness of the sample or the effectiveness of the CDW treatment process.

In addition to the already established relationships between concrete/mortar and ceramic content and glass and gypsum content from the experimental data, five more linear interrelations were detected when analysing the results of the literature review. An almost perfect correlation (p-value=0.00) was found for two pairs of compositional variables, unbound aggregates and concrete/mortar content and asphalt and ceramic content.

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In the first case, an inverse relationship is evident since the amount of unbound aggregates tends to decrease when the concrete/mortar content increases, which responds to a substitution between two of the main components of the mixed recycled aggregates. The same reasoning applies to the correlation found between the content of unbound aggregates and the ceramic presence in the recycled aggregates. For the dependency between the asphalt and ceramic content, although the previous argument is also valid, the correlation is also explained by the fact that the presence of these components in the sample responds to two opposed origins of the CDW, since the ceramic is associated to the demolition of buildings and the asphalt is linked to maintenance works of roads.

Table 4.7: Pearson correlation results for the literature review

Rc Rb Ra Rg X1 X2

Ru -0.6960 -0.3110 0.0904 0.0599 0.2300 -0.1480

0.0000 0.0039 0.4140 0.6920 0.0659 0.2160

Rc -0.4340 0.0387 -0.1420 -0.3570 0.1860

0.0000 0.7270 0.3450 0.0036 0.1180

Rb -0.3660 0.0474 0.2810 0.0534

0.0000 0.6830 0.0020 0.5960

Ra -0.0064 0.0592 -0.0346

0.9560 0.5220 0.7310

Rg 0.3900 0.2010

0.0011 0.1920 X1

-0.0525 0.6400

All the remaining correlations connected the gypsum content with the relative presence of other materials (concrete/mortar, ceramic and glass) in the recycled aggregates sample. Based on the absolute value of the Pearson coefficient of the different relationships, it is possible to identify the glass content and the ceramic content as the variables with higher and lower influence respectively. While the association gypsum-glass has been explained as a matter of effectiveness in the CDW treatment process, the positive correlation of gypsum with the ceramic is due to the common constructive practice of plastering the masonry walls and finally the negative correlation of gypsum with the concrete/mortar content could be justified by the underlying fact previously stated between the concrete/mortar and ceramic content substitution.

In spite of the detected dependencies between compositional variables, the regression analysis performed showed low R² values (Figure 4.8) for the pairs of variables selected from the Pearson correlation analysis in order to fit the results from the literature into a straight line, due to the wide dispersion of the results found in the literature.