The characterisation of the porous system of all the manufactured concretes is analysed and discussed here from macroscopic, at first sight (Figure 4 to Figure 5) and microscopic points of view, using direct observation via optical microscopy (Figure 6 to Figure 9). Table 2 and Figure 10 to Figure 11 show the quantitative results of porosity analysis and pore distribution by size and shape.
3.2. Microscopic level
The microstructure of the interfacial zone varies depending on the type of cement, the nature of the aggregates and the moisture present during the dosing process, among other factors (27). According to the literature, there are approximately the same number of failed interface cases in a conventional concrete as in a recycled one (9, 41); however, given the higher water absorption capacity of RA compared to NA, it becomes evident that the creation of an effective interface is less likely. Thus, the behaviour observed in an analysis of the union of the aggregates, both natural (Figure 5a) and recycled with adhered mortar (Figure 5b-f), with the new cement paste fundamentally depends on their moisture state and water absorption, and on their nature. This is due to the fact that the excess water in the aggregate causes it to move towards the paste, generating a union characterised by being more porous.
On the other hand, if the aggregate is dry, it absorbs part of the water from the mixture, and in turn releases the air occluded inside its pores. This is distributed around the surface of the mortar adhering to the aggregate (isolated bubbles), since the interfacial bond is stronger in terms of its resistance (Figure 5d). In the particular case of the aggregate with asphalt (Figure 5e) covered with an impermeable bitumen film (6-9 μm), this generated an ineffective aggregate-paste bond, giving rise to an interfacial bond that was porous and less dense than the rest of the matrix. Finally, the ceramic aggregate (Figure 5f), being more porous, behaved in a similar way as if it had been used in the dry state. These observations agree somewhat with those obtained in other studies (9, 24, 27, 42).
In view of the above, the importance of the humidity state of the RA in the formation of the microstructure of the manufactured concrete containing it makes it necessary to analyse its internal structure in greater detail in relation to the pre-soaking method used, and to better understand its mechanical behaviour.
For this purpose, the most important qualitative aspects (aggregate-paste/mortar bonds, porosity, fissures and particle packing and distribution) of each of the evaluated concretes were analysed after a digital analysis was carried out on the images captured with the optical microscope (Figure 6 and Figure 9). The reference concretes, N-Series C and R-Series 0 (manufactured with NA and with non-pre-soaked RA, respectively), and the rest of recycled concretes, R-Series 1 to R-Series 5 (manufactured with RA pre-soaked) were analysed separately, and then grouped according to the similarities observed in each of the pre-soaking methods studied.
Finally, it analysed the quantitative aspects obtained, for which the average values are summarised in Table 2; these included the 2D porosity of the subsamples used (four for each type of concrete (Figure 10), and the distribution of the percentage of pores as a function of their size and shape.
3.2.1. Microstructure of concrete containing NA (N-Series C)
In natural concrete (Figure 6), the discontinuity between the NA and the cement paste (Figure 6a, b), stands out, sometimes marked by the accumulation of surface water in the lower zone of the particles of the larger aggregate (Figure 6b-c); this indicates a point of weakness in the aggregate-paste contact zone (water band surrounding the aggregate with a maximum thickness of 35 μm). Although this area is the most vulnerable in conventional concrete (14), its quality will also depend on the surface characteristics of the particles of the granular material such as its texture, irregularity and porosity (43) or on the degree of exudation of the fresh concrete (9, 12).
The porosity observed in natural concrete is generally isolated, and is randomly distributed through the cement paste, corresponding to the occluded air (Figure 6a, d); however, the formation of air pores produced during the operations of mixing and compacting the concrete (28) and trapped under aggregates of more than 1 mm2 of irregularly shaped surface (Figure 6a) can also be observed.
Similarly, the images show the presence of simple and irregular cracks with an average width of about 8 μm at the weakest points, such as pores (Figure 6b). These irregularities are also observed in the form of isolated cracks, which are typical of crushed aggregate (Figure 6b,e). These cracks pass through the paste until they find denser areas, possibly generated during hardening of the concrete.
3.2.2. Microstructure of recycled concrete without pre-soaking (R-Series 0)
The internal structure of the R-Series 0 concrete is shown in Figure 7. In this case, the formation of small bubbles of air trapped at an average distance of 150–200 μm from the larger coarse aggregates (Figure 7a-c) is observed; the bubbles were isolated from each other (Figure 7c), although they were concentrated as an equidistant band around the adhered mortar (Figure 7b). This porosity is explained to a large extent by the lower w/c ratio in the aggregate-paste bond, since the RA was used without pre-soaking; this gave rise to a dry consistency (Table 2), therefore absorbing much of the water in the mixture and in turn releasing the air trapped inside.
The formation of other types of pores is also observed; these are randomly distributed throughout the cement matrix, and are of variable size and circular morphology (Figure 7e). In addition, there is the presence, in specific areas, of water accumulated in the union between mortar and paste, as well as in the RCA-paste interface (indicated in Figure 7d), and this gives rise to a minimum discontinuity between the different phases present in the RAC.
In turn, it is observed that within the random arrangement of the aggregate particles within the paste, the smaller ones are aligned towards the larger ones (Figure 7d). This was observed in all the concretes analysed, and has been discussed in the literature as being a consequence of the natural packaging of the granular structure of concrete (24, 44).
The results indicate that in the particular case of concrete manufacture with RA without pre-soaked, the mechanical strength obtained (55.54 MPa at 28 days), similar to that of the reference concrete (55.42 MPa at 28 days, Table 1) was due to the lower effective w/c ratio caused by using non-pre-soaked RA. The existence of poorly intercommunicated porosity not located in the transition zone also contributed to the mechanical strength of this RAC. In addition, the fine detachments from the adhering mortar, the rougher surface of these aggregates and their irregularity created a good coupling of these aggregates within the cement matrix, and consequently the creation of a more homogeneous and dense microstructure. This result is corroborated by taking into account that the load transfer mechanism in concrete depends on the properties of the existing contact surfaces between its constituents (aggregates, cement matrix and transition zones) (41, 45). In the specific case of R-Series 0 concrete, the union between the non-pre-soaked RA and the paste is very effective.
Finally, cracks are generally observed in RACs, since they are inherent in granular material processed by crushing, regardless of the pre-soaking method used (23, 46, 47). However, their presence is particularly frequent at the most fragile points of the RA, with variable thicknesses and lengths that depend on the scale of observation used. The cracks already existing inside the RA generally started in the mortar-aggregate ITZ when the original concrete was subjected to loading (48), and their propagation may extend from the interfacial zone to more solid regions, passing through all the phases of the material, as shown in Figure 7e.
3.2.3. Microstructure of R-Series 1, R-Series 2 and R-Series 4 pre-soaked recycled concrete
The analysis of the porous structure of R-Series 1 (100% pre-soaking water and 8.9% total water), R-Series 2 (50% pre-soaking water and 9.1% total water), and R-Series 4 (70% pre-soaking water and 8.9% total water), concretes is very similar (Figure 8), and highlights the porosity associated with the accumulation of water on the surface of the aggregates, with a thickness of between 7 μm and 55 μm (Figure 8a-c).
The higher pre-soaking water content used in these mixtures, combined with the increase in their total water (Figure 2), generated an excess of hydration water, which migrated towards the surface of the aggregates in an exudation process, causing the w/c ratio in the aggregate-paste transition zone to be higher than in the rest of the matrix. This fact is observed in the interfaces characterized by their high porosity and greater width (Figure 8b) which has provided a discontinuity between phases.
Likewise, the porosity present in these concretes is also formed by air voids distributed by the paste and those of the recycled material, as shown in the Figure 8a, c.
In relation to the shape of the aggregates, as mentioned above, it is observed that the RCA have a less angular shape compared to natural ones, due to the greater friability of the former caused by the detachment of a certain amount of fines; this characteristic, together with the better coupling of the particles (Figure 8a), provided greater compactness to the whole (49). This fact, in a way, offset the weakness in the interfacial joints, which has been reflected in its resistance to 28 days, with decreases with respect to the value obtained in the reference concrete (55.42 MPa) of 7% for the pre-soaked R-Series 1 (51.75 MPa) and 16% for R-Series 2 (46.67 MPa) and R-Series 4 (46.09 MPa) concretes.
Lastly, the fissures observed in these concretes, as in the previous case, belong in their great majority to those generated inside the RA. Still others formed during the concrete hardening process (Figure 8d,e) because of the difference between the modulus of elasticity of the components at the interface, resulting in higher tangential, radial and/or shear stresses in this zone (50, 51). It is for this reason that during the compression rupture test the RAC breaks, more frequently, by the internal cracks of the RA, while the natural one can do it also by the ITZ between aggregate and paste.
3.2.4. Microstructure of R-Series 3 and R-Series 5 pre-soaked recycled concrete
Figure 9 shows the internal structure of R-Series 3 (12% pre-soaking water and 9.1% total water), and R-Series 5 (80% pre-soaking water and 8.0% total water), concretes, which is characterized by the presence of a certain amount of air bubbles around the RCAs (Figure 9a-c), aligned as an equidistant band around their surface, with an average value of 100 μm (Figure 9c), although less frequently than in non-pre-soaked concrete (R-Series 0).
This type of porosity is caused, in the case of the R-Series 3, because the aggregates were pre-soaked with only 12% of the total water for 10 minutes (interfacial bond with low w/c ratio), which resulted in a plastic consistency (Table 1). While this phenomenon occurs in the R-Series 5 because the RCA were pre-soaked with 80% of the mixing water, but without adding additional water to the mixture (8.0% total water), thus obtaining a dry consistency (Table 1).
It is also observed the accumulation of a small layer of water in random areas of the surface of some aggregates (Figure 9d,e), which gave rise to a minimum discontinuity between the cement paste and the adhered mortar; even pores distributed throughout the matrix that corresponds to the air occluded generated during the mixing and pouring of the concrete, whose shape tends to become less spherical as its diameter increases (Figure 9d), or those generated inside the recycled mortar (Figure 9a, b). In spite of this, the porous structure, together with other qualities of the aforementioned RA, gives rise to adequate mechanical behaviour of these concretes that is similar to concrete manufactured with NA (55.42 MPa, Table 1) in the case of the pre-soaked R-Series 5 series, or with a decrease of only 9% in the R-Series 3 concrete.
In terms of cracks and coupling of aggregate particles, the images show insignificant changes with respect to what was observed in the other recycled concretes; therefore, these qualitative aspects of concrete are not conditioned by the pre-soaking method used.
3.3. Quantitative analysis
3.3.1. Porosity
The average percentages of pores quantified from the analysis of the four subsamples studied are presented in Table 2. This shows that the porosity of natural concrete (5.4%) is lower than that of RACs, which are mainly concentrated between sizes of 10 and 100 μm (88.62%) and whose morphology is presented in mostly elliptical forms (68.57%).
Of the values obtained in recycled concretes, the R-Series 0 (RA without pre-soaked) series shows one of the lowest porosities (8.3%), with pores that are mostly between sizes 10 and 100 μm (80%) and have elliptical shapes (60.69%), as shown in Figure 7b and c. In the case of the pre-soaked recycled concretes, the percentage of pores measured varied from 7.8% (R-Series 5, with 8% total water) to 11% (R-Series 3, with 9.1% total water), with both irregular (between 20% and 35%) and elliptical (above 60%) shapes. Their distribution is also concentrated in the range 10–100 μm (approximately 80%), including both the pores formed inside the cement matrix and those inherent in the RA.
Figure 10 compares the data obtained on the porosity of each of the concretes tested, together with the values of the variables that differentiate the pre-soaking method applied in each case (Table 1) and the deviation obtained. The results presented show the data of the reference series (N-Series C and R-Series 0) followed by the data of the pre-soaking series (R-Series 1 to R-Series 5), ordered from the lowest value to the highest, to facilitate an analysis of the influence of these variables on the results.
A greater content of pores can be observed in the RAC compared to that obtained in the reference concrete (5.4%), with increases ranging from 44% to 54% for R-Series 5 (pre-soaked, but without additional water) and R-Series 0 (without pre-soaking) concretes, respectively, to more than double for the remainder of the pre-soaked concretes (R-Series 1 to R-Series 4). These porosities increase with the total water content, thus confirming the influence of the increase in the w/c ratio on the porosity obtained; results that have also been obtained by other authors (25). This is confirmed for the concrete series R-Series 1 to R-Series 4, in which a greater accumulation of water was observed, after image analysis, on the surface of the pre-soaked aggregates, and a worsening of their mechanical behaviour. In relation to the rest of the variables analysed (water and pre-soaking time), Figure 10 shows the weak influence of these variables on the results obtained.
3.3.2. Pore distribution by size and shape
Figure 11a shows that the distribution of pores in the concretes evaluated here shows a similar trend in all of them, regardless of the type of aggregate and pre-soaking method used.
The maximum range of pores is concentrated between 10 and 100 μm, with a maximum value of 91% (R-Series 2, with 50% pre-soaking water and 9.1% total water) and a minimum of 75% (R-Series 4, with 70% pre-soaking water and 8.9% total water). These correspond to the porosity generated in the cement matrix (both new and old), which is usually spherical and arises from air bubbles that have become trapped during mixing and compacting of the concrete, with a random distribution throughout the material. In addition, small bubbles are generated in the vicinity of the RCA (Figure 7b, Figure 9c and Figure 8b), due to the large difference in humidity between the aggregate and the paste, although due to the fact that these are closed pore systems, their presence does not intervene in transport phenomena such as permeability or capillarity (28).
The pore content quantified as sizes greater than 100 μm (up to > 1000 μm), is less significant, with percentages ranging from 7% (R-Series 4, with 70% pre-soaking water and 8.9% total water) to 20% (R-Series 5, with 80% pre-soaking water and 8% total water). This range includes those pores that were retained between or under the larger aggregates by the same exudation of water, and which left these elliptical shaped voids in their place on evaporating (Figure 6a, Figure 7d, Figure 8d and Figure 9a). Considering that this porosity negatively affects the mechanical resistance due to its large size, and plays an important role in frost resistance (28), its lower content will reduce the degradation of the concretes studied.
Lastly, The percentage of pores with the smallest sizes (1–10 μm) is also unrepresentative, with values between 1% (R-Series 5) and 17% (R-Series 4); this is related to capillary pores of medium size, whose influence on permeability and resistance is expected to be minimal due to their low proportion (28).
In terms of morphology of the pores (Figure 11b), the data obtained show the tendency of these to form elliptical shapes, independently of the type of concrete analysed, with a minimum value of 60% (R-Series 0) and a maximum value of 72% (R-Series 5) for the series without pre-soaking and with pre-soaking but without additional water, respectively.
According to the images analysed, this porosity corresponds to the air bubbles and pores trapped inside the hydrated cement paste. These have variable radius (Figure 7c, Figure 8d and Figure 9c), and occupy the intergranular spaces in the matrix with little or no interconnection between them, thus reducing their influence on permeability (52).
The percentage of irregular pores obtained to a certain extent represents the accumulation of water in the interfacial zones. This is observable in practically all types of concrete, and has a minimum value of close to 20% in R-Series 5 concrete and a maximum of 34% in R-Series 1 (corresponding to the pre-soaked series with the lowest w/c ratio and the highest water content used for pre-soaking of the RA, respectively).
Finally, circular pores make up the smallest percentage, between 3% (R-Series 1) and 16% (R-Series 0); these are formed from spherical air bubbles with a radius greater than 10 μm. These results contrast with those previously obtained via image analysis.