1. Introduction
The environmental impact of concrete constructions is huge. For this reason, recycling concrete construction waste to obtain recycled concrete aggregate can lower the environmental impoverishment. Indeed, the use of Construction and Demolition Waste (C&DW) as alternative aggregates for new concrete production improves natural resource preservation, reduces landfill disposal, and promotes construction sustainability [
1].
The physical properties of Recycled Aggregates (RA) depend on the quality and amount of the adhered cement mortar [
1,
2]. Actually, the quantity of adhered mortar increases with the decrease of the RA size [
1,
2]. Furthermore, the crushing procedure modifies the amount of adhered mortar. Due to this mortar, RA have higher water absorption and lower density in comparison to natural ones. In addition, the un-hydrated cement on the RA surface can modify the properties of concrete [
3] and crack propagation [
4,
5].
It is observed that the mechanical properties (compressive strength, splitting tensile strength, and modulus of elasticity) of concrete with recycled concrete aggregates decrease with the increase of the replacement percentages of Natural Aggregates (NA) [
6,
7,
8]. The different mechanical performances can be explained considering the different stress distribution and failure mechanisms caused by the different micro-structures of concrete with RA in comparison to the ones with NA. The failure mechanism of the concrete with RA is complex and it is influenced by the geometrical and mechanical properties of the aggregates but also by two different interfacial transition zones. Indeed, one is located between the original NA and the old mortar and the other one is between the old and the new mortar. Clearly, the situation is different in the case of normal concrete with NA where there is only one interfacial transition zone [
9].
Often, RA have been used for concrete block pavements [
10,
11], but other research [
9,
12,
13,
14] has shown how it is possible to produce structural concrete with RA. Limbachiya et al. [
14] found that flexural strength and modulus of elasticity of concrete containing recycled aggregates are similar to the ones of concrete made with NA. Recently, many researchers have investigated the influences of polymer additives on Self Compacting Concrete (SCC) cast with recycled and natural aggregates [
15]—see [
16] for a review—proving how it is possible to employ RA in the production of structural elements casted with SCC.
The durability properties of concrete with RA (chloride diffusion, freeze thaw resistance, and abrasion resistance) are still under investigation, since a wide variability in the results is reported [
17]. The durability of concrete with recycled concrete aggregates is generally lower in comparison with traditional concrete [
17,
18,
19,
20,
21]. Pereira et al. [
22] suggest that the concrete containing recycled concrete aggregates should be avoided in aggressive environments. Actually, the adhered mortar that remains attached to the recycled concrete aggregates also influences the durability properties of concrete [
19]. Saravanakumar and Dhinakaran [
23] show that resistance to chloride ion penetration, water absorption, and acid attack resistance of concrete decrease with addition of recycled concrete aggregates. Kwan et al. [
24] report that using recycled concrete aggregates as partial replacement of NA yields to low Water Absorption (WA) and low intrinsic permeability compared to the control concrete mix. Medina et al. [
25] show that concrete with higher ratios of RA have higher freeze–thaw resistance. This can be explained considering the high mechanical quality of RA and the intrinsic properties of the new aggregates. Olorunsogo and Padayachee [
26] reveal that the durability characteristics of concrete with RA are reduced by the increase in RA content. However, the durability of concrete with recycled concrete aggregates can be improved by the addition of pozzolanic materials, such as superfine phosphorus slag and ground granulated blast-furnace slag [
27]. Xiao et al. [
28], considering a Chinese experimental database, summarizes that the resistance of chloride penetration of Recycled Aggregate Concrete (RAC) is lower compared to that of Normal Concrete (NC), and that the resistance of chloride penetration of RAC decreases with the increase of RA replacement percentage. Similar studies confirm these conclusions; see [
29,
30]. Kurda et al. [
31], considering both literature experimental data and their new experimental campaign, show that water absorption increases, and the electrical resistivity decreases with increasing replacement percentage of RA. An opposite result is obtained if fly ash is added to concrete for both tests. The reduction of water absorption is higher in mixes with both RA and fly ash in comparison to the mixes with only RA or fly ash. In addition, the benefit of incorporating fly ash and RA in concrete increases even more when superplasticizers are used. In addition, Lima et al. [
32] prove that the presence of fly ash in the mixture improves the concrete workability, and compressive and tensile strengths.
In order to develop the marketing of recycled aggregates and the management of recycling plants, it is important to know whether their chemical, physical, and mechanical characteristics are influenced by parent concrete and also whether it influences the properties and performance of the concrete with RA. The experimental data representing the properties of RAC are characterized by high dispersion [
33]. According to some authors [
34,
35], the quality of RA is mostly influenced by the quality of original demolished concrete. Even if more research is needed, some general statements can be drawn. For example, RAC with low to medium compressive strength can be produced independently from the characteristics of parent concrete [
1,
36,
37,
38,
39]. On the other hand, Tabsh and Abdelfatah [
40] state that the influence of the parent concrete is more significant in a weak concrete than in stronger one. Actually, this can be explained considering that the strength of concrete depends on both coarse aggregates and cement. Therefore, if more cement is used, then the effect of the coarse aggregate is reduced.
Given that non conclusive statements have been proved on this issue, in this paper, an extensive experimental campaign was carried out to evaluate the mechanical performance and durability of concrete with coarse recycled concrete aggregates obtained through the demolition of concrete with quite low compressive strength (Rck ≤ 20 MPa). In this case, the old football stadium located in Cagliari (Italy) has been used as an artificial “quarry”. Indeed, in the future, the stadium will be demolished and rebuilt with a modern design. Thus, the RA are obtained from its concrete cantilever beams and foundations. Tests were carried out to evaluate the concrete mechanical performance of these concrete structures. Parts of cantilever beams and foundations have been separately demolished and crushed in order to obtain two types of coarse RA with a size range between 4 and 16 mm. Three different replacement percentages (30, 50, and 80%) of NA with RA have been used to produce different six concrete mixes. Three of them were casted using the RA obtained from the beams and the others were produced using the RA obtained from the foundations. An additional mix of NC with only NA was produced as a benchmark. Further tests were carried out to obtain a full description of physical and mechanical properties and durability of these concretes.
The aim of this work is twofold: to verify the feasibility of using concrete debris of the old Cagliari stadium for new structural concrete and to investigate the influence of the parent concrete on the new concrete obtained with RA.
After this brief introduction,
Section 2 presents the experimental program, while
Section 3 describes the characteristics of the RA.
Section 4 deals with the mechanical and durability properties of the concrete with RA, discussing the influence of the parent concrete. Some discussions and conclusive remarks are presented in
Section 5.
3. Recycled Aggregates
Taking into account that two different parent concretes have been considered, two kinds of RA have been produced: Recycled Aggregates obtained from crushed Foundations (RA_F), and Recycled Aggregates obtained from crushed Beams (RA_B). In both cases, the aggregates size range is 4–16 mm.
The tests following the indications of UNI EN 12620: 2008 [
41] and UNI 8520-1: 2015 [
42] have been performed on both types of RA.
Table 2 presents the main test results while
Figure 3 depicts the RA size distribution. It is interesting to point out that both RA types have very similar characteristics even if they have been obtained by crushing two different concretes. Indeed, only four parameters (content of acid-soluble sulfate and water-soluble sulfates, percentage of fines, shape index) out of twenty-one are different.
The physical properties, workability, mechanical performances, and durability of concrete with RA is strongly influenced by the Residual Mortar Content (RMC) attached onto the original NA particles [
2,
29,
43,
44,
45,
46,
47]. Indeed, previous studies have proved that the reduction in compressive strength of concrete with RA [
43,
44,
45,
46,
47,
48] and in modulus of elasticity [
49] are related to the presence of RMC. Thus, in order to evaluate the properties of concrete with RA, the determination of the RMC is critical. However, currently no standard method is available. In this research, the authors follow the strategy proposed by Abbas et al. in [
50]. RA samples were exposed to daily cycles of freezing and thawing in a sodium sulphate solution.
Table 3 presents the RMC obtained in RA_F and RA_B considering two fraction sizes (retained by a 4 and 10 mm sieve) and it highlights that RMC is almost similar for RA_B and RA_F.
5. Discussion and Conclusions
In this paper, an experimental campaign has been developed in order to assess the mechanical and durability properties of concrete with recycled concrete aggregates. Two different parent concretes have been used to produce the recycled aggregates. In this way, it was possible to investigate what is the influence of the parent concrete on the performance of recycled concrete. RC_F and RC_B denote the concrete with recycled concrete aggregates respectively obtained from the foundation and the beam of the old Cagliari Stadium. The foundation concrete showed better mechanical performance in comparison to the beams one. The following conclusions can be drawn from the results:
- -
Recycled concrete produced with coarse recycled aggregates has shown similar mechanical performances to normal concrete produced with natural aggregate, even when the natural aggregates replacement percentage reaches 80%.
- -
The mechanical performance of recycled concrete is not related to the parent concrete mechanical characteristics.
- -
Concerning the durability, experimental results show that:
- -
The resistance to pressured water penetration is not reduced by the presence of recycled aggregates.
- -
The chloride penetration resistance of concrete with RA is lower than that of normal concrete (NC). In addition, it appears to be influenced by the parent concrete. Indeed, the theoretical service life of RC_F is 40% higher than that of RC_B, regardless of the percentage of recycled aggregate replacement.
- -
The resistance to the frost–thawing cycle is higher in concrete with RA_B. Instead the Sn value of the concrete with RA_F is lower or approximately equal to the value of the normal concrete NC. The results obtained do not show a relationship between the replacement percentage of recycled aggregates and the resistance to frost and thaw.
These results highlight the importance of the mix design that can allow the obtaining of structural concrete even with concrete demolition waste with different mechanical characteristics.
Recycled aggregates can represent an efficient way to lower the buildings’ impact on the environment, improving their sustainability. At the same time, RA can create new opportunities for the companies that re-design their production workflow. For instance, the processing scraps of precast concrete elements should be used to create recycled aggregates, reducing losses and maximizing earnings with a beneficial effect on the environment.
Actually, the transportation costs of construction materials have a paramount relevance in the economic analysis. Thus, recycled aggregates can be very effective when the source of the parent concrete is near the location of the construction, as happens in the case of demolition and re-building, or in the case of retrofitting of existing structures and infrastructures (see [
62,
63]). Finally, it should be considered that if the environmental impact of the retrofitting intervention is taken into account (see [
64,
65]), the equivalent CO
2 cost is reduced by the use of RA. Furthermore, the combined use of RA and alternative bio-natural aggregate [
66] and structures [
67] represent an effective approach to lower the environmental impact of constructions.
Further developments of this work are expected considering whole structural elements like those presented in [
68,
69,
70,
71].