Distribución estratégica de los proyectos alineados al Eje Transversal

Recycled aggregates stand out by their heterogeneity, and as such the determination of the different components becomes highly relevant. For the recycled coarse aggregates, the identification of constituents were performed following the procedure described in UNE EN 933-11 (2009).

A sample in keeping with the requirements described in UNE EN 932-1 (1997) and UNE EN 932-2 (1999) was selected per each type of recycled aggregate and dried (40±5 ºC) until constant mas.

Then, by means of a manual classification seven fractions were separated: concrete, mortar and natural aggregates with mortar attached (Rc), unbound natural aggregates without mortar attached (Ru), ceramics such as brick, or tiles (Rb), glass (Rg), asphalt (Ra), gypsum (X1) and other impurities such as wood, plastic or metals (X2). Lastly, the estimation of the relative weight proportion of each fraction was determined as a percentage by mass.

This test not only made possible to detect the different components of a recycled aggregate but also, when used in conjunction with the results arising from other tests, to determine the influence of their presence on the physical, mechanical and chemical behaviour of the recycled aggregates and the recycled concrete.

2.2. PARTICLE SIZE DISTRIBUTION

A granulometric analysis was carried out with an electromechanical sieve shaker (Figure 3.2) for all natural and recycled aggregates. The testing procedure was performed in consonance with UNE EN 933-1 (2012). The maximum (D) and minimum (d) sizes of aggregates were calculated respectively as the minimum and maximum opening in a UNE EN 933-2 (1996) sieve which satisfies the general requirements set out in the Spanish Code on Structural Concrete (EHE-08) (Permanent Commission on Concrete, 2008), and their relationship known as the D/d ratio was evaluated within the limitations (<1.40) of the EHE-08 (Permanent Commission on Concrete, 2008).

Figure 3.2: Sieve shaker

73 In addition, the results were represented graphically as grading curves to easily verify the compliance with the upper and lower limits of the optimal particle size distribution of coarse and fine aggregate specified in the requirements of the EHE-08 (Permanent Commission on Concrete, 2008).

2.3. GEOMETRY

Although the geometrical differences between natural and recycled coarse aggregates were apparent to the naked eye, a more thorough analysis was performed. The shape of coarse aggregates was determined through the flakiness index (FI) expressed as the percentage of flaky or needle-like particles in the total of the sample (UNE EN 933-3, 2012).

The determination comprised two screening steps, a sample selected according to UNE EN 932-2 (1999) was first sieved in accordance to UNE EN 933-1 (2012), and each retained granulometric fraction was subsequently manually sieved with the corresponding rectangular slot sieve (Figure 3.3) as indicated in UNE EN 933-3 (2012). Thereby, the FI value was calculated as the percentage in mass of the particles that pass through from the total tested in the second sieving operation.

Finally, the obtained values were assessed according to the 35 wt% upper limit stipulated by EHE-08 (Permanent Commission on Concrete, 20EHE-08).

Figure 3.3: Rectangular slot sieve for flakiness index determination

2.4. FINES ASSESSMENT

A fines assessment, comprising both quantity and quality parameters, was carried out based on the requirements stipulated in the EHE-08 (Permanent Commission on Concrete, 2008). In the first place, the percentage of fines passing through a 0.063 mm sieve was determined and compared with the limits of 1.50 wt% for coarse aggregates and 6.00 wt% for fine aggregate; and if this limit was exceeded, compliance with the limitation for the total fines of concrete - coming both from the aggregates and the cement - (<175 kg/m³) was sought (Permanent Commission on Concrete, 2008).

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In terms of quality, a sand equivalent (SE) test was performed according to UNE EN 933-8 (2012) in order to determine the presence of clay-like fines. The equipment consisted of a sand equivalent shaker, 2 measuring cylinders with rubber stoppers, a weighted foot, a solution with flocculent properties in a bottle assembled with a siphon and an irrigator tube, and a graduated rule (Figure 3.4).

For the determination, two samples selected according to UNE EN 932-2 (1999) and within the 0/2 mm size range were air dried (<2.00% saturation degree) and then flocculated with a solution of calcium chloride (CaCl2), glycerol (C3H8O3), formaldehyde (CH2O) and distilled water in the proportions indicated in UNE EN 933-8 (2012) to promote the release and suspension of the clay coatings. After a waiting time of 20 minutes in the measuring cylinder, the SE value was calculated as the percentage of the total height of flocculated material (Figure 3.5).

The evaluation of this index was made depending on the concrete general class of exposure, having in mind that the SE values should exceed 70 vol% for I, Ila or llb general exposures or 75 vol% for the remaining classes of exposure (Permanent Commission on Concrete, 2008). Appendix A can be consulted for equivalence between Spanish and European environmental exposures classes.

Figure 3.4: Sand equivalent equipment (a) automatic shaker and (b) measuring cylinders, weighted foot, and solution bottle.

Figure 3.5: Sand equivalent determination (a) (b)

h1

h2

Weighted foot Metal lid

Suspension height Flocculated

height

Measuring cylinder Flocculent

75 Finally, if some uncertainty arose as to whether the fines contained any clay, its presence was identified and qualitatively calculated using an X-ray diffraction test (described in section 3.1 of this chapter) allowing the use of the fine material if the clays were of the kaolinite or illite type (Permanent Commission on Concrete, 2008).

2.5. DENSITY AND WATER ABSORPTION

Density and water absorption (WA) values were determined simultaneously by means of a pycnometer (Figure 3.6) according to UNE EN 1097-6 (2014) standard. The pycnometer method is a well-known method for determining the volume of irregularly formed samples based on Archimedes' Principle.

Figure 3.6: Pycnometer

A sample in accordance with UNE EN 932-2 (1999) was submerged in water inside a pycnometer taking care that no air bubbles were present. After 24 hours in a thermostatic bath (22±3 ºC), the existing air bubbles were eliminated before refilling with water and weighing the pycnometer with stopper (Msat). Afterwards for coarse aggregates, the test portion was cloth-dried in order to get a moisture state of saturation with dried surface and then was weighed (M1) and oven-dried (110±5 ºC) to constant mass (Mrd). In the case of fine aggregates, the test portion was air-dried and the collapse behaviour (Figure 3.7) after removing from a conical mould (Dbottom=90 mm, Dtop=40 mm, H=75 mm) was used to assess the moisture condition until a saturation state with dried surface was reached, and at this moment it was weighed (Mssd) and oven-dried (110±5 ºC) to constant mass (Mrd). Finally, the pycnometer with stopper completely filled with water was weighed (Mpyk).

Figure 3.7: Collapse criterion for fine aggregates (a) wet aggregates, (b) slightly wet aggregates, (c) saturated with dried surface aggregates and (d) oven dried aggregates (UNE EN 1097-6, 2014)

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Following this procedure three main specific gravities were determined: apparent specific gravity (3.1), dry bulk specific gravity (3.2) and saturated with dry surface bulk specific gravity (3.3), and also water saturation (3.4) was obtained.

ρ_a=ρ_w∙ M_rd

M_rd- M_sat-M_pyk (3.1)

ρ_rd=ρ_w∙ M_rd

M_ssd- M_sat-M_pyk (3.2)

ρ_ssd=ρ_w∙ M_ssd

M_ssd- M_sat-M_pyk (3.3)

WA₂₄=100×(M_ssd-M_rd)

M_rd (3.4)

with ρa the apparent specific gravity [Mg/m³], ρrd the dry bulk specific gravity [Mg/m³], ρssd the saturated with surface dried bulk specific gravity [Mg/m³], WA24 the water absorption after 24 hours of immersion [wt%], ρw the density of the water at test temperature (0.9982 Mg/m³ at 20ºC) [Mg/m³], Mssd the mass of the saturated aggregates with dry surface [g], Msat the mass of the pycnometer with the test sample - saturated aggregates - and the water [g], Mpyk the mass of the pycnometer only with water [g] and Mrd the mass of the test sample -oven dried aggregates- [g].

Although the EHE-08 (Permanent Commission on Concrete, 2008) does not establish any limitation for the density of aggregates, strict specifications are laid down for water absorption.

Natural and recycled aggregates shall not exceed 5 wt% and 7 wt% respectively, while the combination of natural and recycled coarse aggregate shall obey the 5 wt% limit for recycled concrete with more than 20 wt% of recycled aggregates.

2.6. MECHANICAL RESISTANCE

The mechanical resistance of the aggregates to degradation by abrasion and impact was determined in accordance with UNE EN 1097-2 (2010) by means of the Los Angeles test (LA) as recommended by EHE-08 (Permanent Commission on Concrete, 2008).

The tested sample was produced in accordance to UNE EN 932-2 (1999) and as specified in the Los Angeles test standard presented a 10/14 mm size with a 65 wt% proportion of the total sample passing through the 12.50 mm sieve. After drying (110±5 ºC) to constant mass and cooling down in a desiccator, the coarse aggregates were introduced in the testing machine (Figure 3.8) with 11 steel balls of 400 g and were subjected to 500 revolutions at a constant speed between 31 and 33 rpm. The resulting material was screened through a 1.60 mm sieve and the retained portion was dried (110±5 ºC) until constant mass was achieved. Finally, the LA coefficient was calculated as the percentage of the test portion passing a 1.60 mm sieve after completion of the test.

77 Figure 3.8: Los Angeles test machine

Regarding the evaluation of this property, EHE-08 (Permanent Commission on Concrete, 2008) sets a 40 wt% upper limit but also extends the previous limitation to 50 wt% for those aggregates whose previous field experience deemed them as suitable for mass or reinforced concrete with a characteristic strength not exceeding 30 MPa.

2.7. ADHERED MORTAR

The presence of adhered mortar is a common occurrence in coarse recycled aggregates coming from construction and demolition, as certain amount of paste from the original concrete inevitably remains attached to natural aggregates or other particles present (mostly bricks) after the demolition occurs.

Currently, there is no standardized method for assessing the amount of adhered mortar in recycled aggregates. Thus, two experimental methods previously described in the literature have been employed.

One of the research techniques conducted in this PhD involved the immersion of recycled aggregates in different acidic environments in order to dissolve the attached mortar. In the first test implemented, a coarse recycled aggregate sample was washed with demineralized water to remove fine particles and dried at 75±5 ºC for 24 hours. Then, the recycled aggregate was submerged and carefully stirred in a hydrochloric acid solution (HCl) at 0.10 M at 20±2 ºC. After 24 hours, the sample was washed with demineralized water and dried at 75±5 ºC for 24 hours. In the same conditions, a second treatment was performed to verify the level of efficacy of the method. The mortar attached was then calculated as the percentage of total weight loss. The second test employed was also based on the dissolution of the cement paste in a hydrochloric acid solution. However, the aforementioned procedure was slightly adjusted to increase the treatment aggressiveness, in such a way that made it more relatable to methods found in the literature (Duan and Poon, 2014). First, the temperature of drying was risen up to 105±5 ºC.

Moreover, the HCl concentration was modified up to 1.00 M and the immersion period was reduced to 8 hours. To conclude, the particles presenting adhered mortar were gently brushed and the sample was sieved though a 4 mm sieve to ensure that only the recycled coarse aggregates were weighed.

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Additionally, the mortar evaluation method based on daily freeze-thaw cycles of soaked coarse recycled aggregates in a salt solution was also performed. This technique combines mechanical stresses and chemical degradation to remove the mortar from the aggregate surface. The procedure was first employed by (Abbas et al., 2008) as a modification of (ASTM C88-13, 2013).

The sample was washed with demineralized water and dried at 105±5 ºC for 48 hours. Then, the aggregates were immersed in a 26 wt% sodium sulphate solution (Na2SO4) for 24 hours. After that time period, the still immersed samples were subjected to five daily freeze-thaw cycles, i.e. 16 hours at -15 ºC and 8 hours at 80 ºC. Lastly, the sample was drained, washed again with demineralized water, sieved though a 4 mm sieve and dried at 105±5 ºC for 24 hours before weighing. The ratio of attached mortar was calculated as percentage of differential weight before and after the treatment.

3. MICROSTRUCTURAL CHARACTERIZATION OF AGGREGATES AND