**Economic and Technical Viability of Using Shotcrete with Coarse Recycled Concrete Aggregates in Deep Tunnels**

#### **Gonçalo Duarte 1, Rui Carrilho Gomes 1, Jorge de Brito 1,\*, Miguel Bravo <sup>2</sup> and José Nobre <sup>1</sup>**


Received: 10 February 2020; Accepted: 3 April 2020; Published: 14 April 2020

**Abstract:** This work analyzes the technical and economic viability of using coarse recycled aggregates from crushed concrete in shotcrete, as a primary lining support in tunnels. Four incorporation ratios of coarse natural aggregate (CNA) with coarse recycled concrete aggregates from concrete (CRCA) were studied: 0%, 20%, 50% and 100%. The mechanical properties of the dry-mix shotcrete were obtained in an independent experimental campaign. Initially, the technical viability of CRCA shotcrete was validated for deep rock tunnels, based on the convergence-confinement method. Two cases were studied to determine the equivalent thickness for each combination of replacement ratio using CRCA shotcrete: (i) similar stiffness and (ii) similar yield stress. Subsequently, an economic assessment was performed. The stiffness criterion increased the thickness below 10% in both the 20% and 50% replacement ratios, which shows their technical viability with very marginal cost increase (<5%). On the other hand, the maximum pressure criteria required higher increments, close to 30% in the 50% replacement ratio. A full replacement was proven to be impracticable in both analyses.

**Keywords:** shotcrete; deep tunnels; convergence-confinement method; coarse recycled concrete aggregate; dry-mix process

#### **1. Introduction**

The construction sector is considered one of the main players in the generation of environmental impact. Besides cement production, which is responsible for more than 6% of all global CO2 emissions, the total amount of construction and demolition waste (CDW) represents a third of all waste generated in the European Union. However, there are barriers to its reuse, one of the main of which lies in the lack of trust in the quality of CDW [1].

The incorporation of recycled aggregates (RA) in concrete, by replacing a percentage of natural aggregates (NA), usually leads to poor mechanical and durability-related performances [2,3].

It is therefore important to understand to what extent the behavior of the RA differs from that of the NA. According to Bravo et al. [2], the main differences between these aggregates are:


Chan [4] and Bravo et al. [2] evaluated the performance of recycled aggregate concrete (RAC), finding that the nature of RA strongly influences their binding capacity to the new cement paste, thus affecting the mechanical properties of the resulting concrete. Nevertheless, these authors also concluded that the use of these aggregates in concrete production is feasible. However, considering that their use affects the mechanical and durability-related properties of RAC, imposition of limitations and cautions on their inclusion is required.

Shotcrete is widely used in the underground mining and tunneling industry, slope stability, rehabilitation works, and in many other areas.

The tunneling industry is growing because the available space in dense urban environment tends to be used for nobler purposes than transit subways, parking lots, electric lines, water supply and sewer lines [5].

Nowadays, the widespread use of the New Austrian Tunneling Method (NATM) has increased the use of shotcrete for linings [6]. In recent years, several studies on the use of fibres to replace the traditional electro-welded meshes have been published [7–10].

To the authors' knowledge, the incorporation of recycled aggregates from CDW in deep tunnels has not yet been studied in detail. This work studies the viability of using CDW in urban tunnels, because CDW is usually generated in urban environments. A technical and economic assessment of different dry-mix shotcrete compositions, incorporating coarse recycled concrete aggregates (CRCA) in substitution of coarse natural aggregates (CNA), intends to contribute to the sustainability of the construction sector.

For generalization purposes, the analysis is made for a reference case of a deep tunnel where analytical equations proposed by Carranza-Torres and Fairhurst [11] are valid (see Appendix A), based on the convergence-confinement method, which is broadly used to describe the ground-support interaction in design practice [12,13].

It is demonstrated that limited incorporation ratios of CRCA can be a valid alternative from a technical and economic perspective.

#### **2. Experimental Program**

#### *2.1. Tests*

Firstly in this experimental campaign, a detailed characterization of all aggregates used in the production of concrete was made.

The physical characterization of the aggregates involved the following tests: particle density and water absorption [14], bulk density and volume of voids [15], shape index [16], Los Angeles wear [17] and humidity content [18].

Table 1 shows the tests performed to characterize the mechanical properties of the shotcrete mixes with RA.


**Table 1.** Hardened properties of shotcrete.

It should be referred that the mechanical tests were always performed on core specimens (extracted from slabs produced with shotcrete) with a diameter of 98 mm. The ultrasound pulse velocity determination was carried out in core specimens which were then tested for compressive strength at 28 days.

#### *2.2. Materials*

In this research, limestone gravel and alluvial rolled sand were used as NA, respectively, as coarse and fine aggregates. RA from concrete were obtained by crushing a C30/37 strength class concrete with a maximum aggregate size of 14 mm. All the aggregates were added to the mixes completely dry. Type I 42.5 R cement was also used.

Table 2 shows the physical properties of the aggregates used. When compared to NA, it is confirmed (as stated in the introduction) that RA have lower density and higher water absorption capacity. This derives from the nature and porosity of the RA from concrete. For the same reason and because they possess more elongated shapes, RA present a lower bulk density than NA.


Table 2 also shows that the RA display a higher shape index than NA. This can lead to a lower workability of the concrete mix with RA compared to that of the reference concrete (RC), which has to be balanced by a higher w/c ratio.

It was also found that, due to their composition, RA show a lower resistance to fragmentation than NA.

#### *2.3. Mixes Design*

The composition of the reference concrete was determined by the Faury method for a target strength belonging to class C30/37 (Figure 1). The maximum aggregate size used was 12.5 mm. Concrete compositions with RA were based on that of reference concrete. However, in order to keep the concrete workability constant (125 ± 25 mm slump), the w/c ratios of the concrete mixes with RA were increased. Table 3 shows the final composition of the concrete mixes studied.

**Figure 1.** Faury method for reference concrete.


**Table 3.** Composition of the concrete mixes studied (m3/m3) and estimated w/c ratios.

#### **3. Mechanical Properties**

Regarding the mechanical properties, as stated, the bond strength (by pull-off), compressive and splitting tensile strengths, secant modulus of elasticity, abrasion resistance and ultrasound pulse velocity were determined. Table 4 provides a summary of the hardened state mechanical properties.


**Table 4.** Mechanical properties of shotcrete mixes (adapted from Duarte et al. [25]).

Legend: Green—Better result in comparison with reference concrete (RC); Yellow—Worse result than RC, with reductions lower than 10%; Red—Worse result than RC, with reductions higher than 10%.

Regarding bond strength, no clear variation of this property with the incorporation of CRCA in concrete was found. As for compressive strength, it decreased as the percentage of the replacement of CNA with CRCA increased. Maximum reductions between 20% and 30% were obtained. This result is in agreement with those of several authors who analyzed cast RAC and obtained maximum reductions within this range. Even so, at seven days, the strength values of (RAC) shotcrete compared to cast RAC were lower, which supports the hypothesis that hardening is slower in the former than in the latter.

With respect to tensile strength, the incorporation of CRCA led to non-significant influence, as also reported by other authors, with maximum decreases of 15%, which are clearly lower than those in compressive strength. The reason for this is that the bond between the cement paste and CRCA is improved (compared to CNA) due to their shape and higher water absorption capacity.

The ultrasound pulse velocity decreased as the CRCA incorporation increased, which is consistent with the higher porosity of these aggregates that leads to a concrete mix with lower quality.

The modulus of elasticity was one of the properties mostly affected by the replacement of CNA with CRCA, with reductions of up to 31%. This stems from the higher deformability of the CRCA and lower strength of the bonded cement paste, the latter promoting further degradation of stress transmission capacity between the various concrete constituents. With respect to cast concrete, the reductions obtained (by the incorporation of CRCA) were more significant. The maximum aggregate size adopted also played a role, since the smaller the size the higher the porosity, and thus the higher the deformability and the lower the modulus of elasticity.

Finally, the abrasion resistance increased with the incorporation of CRCA as a result of the higher roughness and porosity of these aggregates, which yielded a better bonding to the cement paste and thus a better adhesion to concrete. Hence, the prevailing tendency was the improvement of this property with the incorporation of this type of aggregate. This result is in agreement with those of other researches, in which reductions in depth loss between 10% and 15% were obtained, showing that this property indeed improves when CRCA are incorporated in concrete.

#### **4. Technical Viability**

#### *4.1. Case Study*

Nowadays, the analysis and design of tunnels in urban environments require the use of numerical simulations to simulate with accuracy complex geometries of the tunnel and ground layering. For simplicity's sake, the case study selected is a deep tunnel in a rock massif (hydrostatic stress field of 5 MPa), witha3m radius circular cross-section, where the analytical method proposed by Carranza-Torres and Fairhurst [11] is valid and is briefly described in Annex 1. In this example, stress relief factor λ is taken equal to 85% and lining thickness, tc, is equal to 200 mm for CNA shotcrete.

For the lining of the tunnel, shotcrete with different CRCA incorporation ratios (IR) was considered, whose mechanical properties were determined experimentally by Duarte et al. [25]. Table 5 shows the values of the design compressive strength and modulus of elasticity obtained, including the percentage of reduction of the mechanical properties in relation to the shotcrete mix without incorporation of CRCA (IR0).


**Table 5.** Design compressive strength and modulus of elasticity of each composition.

Table 5 shows that incorporating CRCA in the mix results in lower mechanical properties, ranging from about −3% for IR20 to −31% for IR100. For tunnels, the reduction of strength and stiffness in the shotcrete with recycled aggregates can be balanced by an increase of the lining's thickness. The lining's equivalent thickness for different CRCA incorporation ratios was computed for two criteria: (i) lining with similar stiffness and (ii) lining with similar yield stress.

#### *4.2. Elastic Radial Sti*ff*ness Criterion, Ks*

In this section, it is assumed that each shotcrete mix considered has a similar stiffness, namely the elastic radial stiffness, Ks, is equal to 636 MPa/m (tc = 200 mm for CNA shotcrete).

Table 6 quantifies the differences in yield pressure and equivalent thickness. For IR20 and IR50, the thickness increase is small (<15 mm, 3% and 7%, respectively). The thickness increase is significant (42%, 83 mm) for IR100 only.

**Table 6.** Variation of yield stress and equivalent thickness when a similar stiffness criterion is adopted.


Table 6 supports the conclusion that both IR20 and IR50 are competitive solutions from a technical point of view, with small increases of the lining's thickness (<7%).

#### *4.3. Maximum Pressure Criterion, pmáx s*

In this section, it is assumed that each shotcrete mix considered has a similar yield stress: 1.43 MPa (tc = 200 mm for CNA shotcrete). Table 7 quantifies the differences in stiffness and equivalent thickness. For IR20, the equivalent thickness increases 12% (24 mm), which can be considered acceptable, while IR50 and IR100 require thickness increments over 28% (more than 50 mm).


**Table 7.** Results for the maximum pressure criterion.

For cases where yield stress criterion is relevant, only the IR20 mix can be considered equivalent to IR0, as the other options may be compromised by an increase of the lining thickness over 50 mm.

#### **5. Economic Viability**

Adopting the equivalent thicknesses determined in Section 3, the economic viability analysis of CRCA shotcrete is presented in this section. Based on data collected from Portuguese tunneling companies, the estimation of the current costs range to produce dry-mix shotcrete is shown in Table 8.

Figure 2 shows the unit costs, in €/m3, of the different shotcrete scenarios, assuming three possible costs for CRCA to assess its impact:



**Table 8.** Material costs per m3 of dry-mix shotcrete produced.

**Figure 2.** Cost of shotcrete for each composition, assuming different cost scenarios of the recycled aggregates.

Figure 2 shows that incorporating recycled aggregates in shotcrete generates small variations is cost per cubic meter (<8%), for the three scenarios analyzed.

Table 9 shows the cost of the lining per unit length of the tunnel for the two cases for which equivalent thickness was determined: similar stiffness and similar yield stress.

In all cases, for simplicity's sake, rebound losses equal to 25% of the shotcrete's theoretical volume were assumed.

For the case in which CRCA cost is 50% of the cost of CNA, the lining's cost per unit length of the tunnel with CRCA is slightly higher than without CRCA for IR20 (2.3%) and IR50 (5.3%) for Case 1. For the other IR's and cases, the increase in cost is higher than 10%.

Because the recycled aggregates' cost represents less than 15% of the total production costs, an increase in thickness due to the lower mechanical properties leads to higher consumption of the other components, and thus costs increase.


**Table 9.** Lining cost per unit length of the tunnel for Scenario 1 (50% Coarse Natural Aggregates).

Figure 3 shows the variation of the lining's cost per unit length of tunnel for the three hypothetical costs of CRCA.

**Figure 3.** Variation of lining's cost per unit length of tunnel for three different costs of Coarse Recycled Concrete Aggregates.

Figure 3 shows that the lining's cost per unit length of the tunnel is fairly insensitive to the cost of CRCA. In particular for Case 1, similar stiffness criterions-mixes IR20 and IR50 lead to marginal increases in the lining's cost (between 1.2% and 5.3%), while the other cases studied lead to variations higher than 10%.

So, IR20 and IR50 are interesting alternatives for CRCA incorporation in dry-mix shotcrete solutions. The remaining cases require a larger increase in lining's thickness, which is economically unattractive.

#### **6. Conclusions**

The technical economic analysis of dry-mix shotcrete incorporating CRCA was performed considering three different replacement ratios of CNA with CRCA, for two cases: similar stiffness criterion and similar yield stress criterion.

For deep tunnels, a small increase in the lining's thickness is required to have linings of similar stiffness for the IR20 and IR50 mixes, which shows their technical viability. An economic assessment proved that both mixes can be competitive against a conventional shotcrete, with very small increases in the lining's cost per unit length of the tunnel.

A sensitivity study to assess the impact of CRCA cost on lining's cost per unit length shows that CRCA cost is very small, and therefore its influence on the overall costs is also small.

Based on this study, it can be concluded that using coarse recycled concrete aggregates in tunnels can be an interesting solution for the IR20 and IR50 mixes, with marginal impact on thickness and the lining's cost per unit length of tunnel.

The viability of using CRCA in tunnel construction practice should be analyzed considering the specific data from each project, such as costs to produce dry-mix shotcrete and the strength and deformability study.

**Author Contributions:** Conceptualization, G.D., R.C.G., J.d.B., M.B., J.N. methodology, G.D., R.C.G, J.d.B., M.B., J.N. formal analysis, G.D., R.C.G., J.d.B. validation, J.d.B. investigation, G.D., R.C.G., J.N. data curation, G.D., R.C.G., J.N. writing—original draft, G.D., R.C.G. writing—review and editing, G.D., R.C.G., J.d.B., M.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors wish to thank CERIS (Civil Engineering Research and Innovation for Sustainability) research centre and FCT (Foundation for Science and Technology).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **Appendix A The Convergence-Confinement Method**

The stress redistribution and three-dimensional deformation induced by tunnel excavation and lining installation during construction is a very complex three-dimensional problem. To simulate tunnel construction, the convergence-confinement method in plane strain conditions is widely used. The method relies on the ground reaction curve, which describes the response of the ground around the tunnel [26,27], and the support reaction curve, which describes the lining response.

The two-dimensional tunneling problem is illustrated in Figure A1a. The tunneling process is represented by increasing the λ value from zero to one. Figure A1b shows the main characteristics of the convergence-confinement method. The convergence curve corresponds to the internal pressure versus the tunnel radial displacement. The radial displacement of the tunnel increases as the internal pressure decreases. The tunnel can be self-stabilized without a liner (Curve [a]) in Figure A1b) or the surrounding ground can fail which leads to an increase in the ground load acting on the tunnel lining (Curve [b] in Figure A1). In the latter case, a liner must be installed to keep the stability of the tunnel.

Based on the convergence-confinement method, Carranza-Torres and Fairhurst [11] proposed a set of analytical equations valid for deep circular tunnels in rock. These equations are used to ensure the technical viability of the shotcrete solutions proposed in a simplified manner, bypassing complex numerical simulations necessary for shallow tunnels and non-circular shapes, but without loss of generalization capacity of the conclusions.

The problem is studied as a two-dimensional plane strain case, in which the pressure, *pi*, and the radial displacement, *ur*, are constant in the tunnel walls.

The ground reaction curve is obtained from the elastoplastic solution of a circular opening subjected to a hydrostatic stress field, σ*0*. Equations (A1) and (A2) present the elastic and plastic portions of the curve, respectively.

$$
\mu\_r^{cl} = \left(\frac{S\_o - p\_i}{2G\_{rm}}\right) \text{R} \tag{A1}
$$

where *ur el* is the elastic radial displacement of the massif, *So* the uniform tension around the tunnel, *pi* the pressure of the walls at the tunnel, and *Grm* the shear modulus of the rock massif.

$$\begin{split} \boldsymbol{u}\_{r}^{\text{pl}} &= \boldsymbol{u}\_{r}^{\text{el}} \Big\{ \frac{\boldsymbol{K}\_{\psi} - 1}{\boldsymbol{K}\_{\psi} + 1} + \frac{2}{\boldsymbol{K}\_{\psi} + 1} \Big( \frac{\boldsymbol{R}\_{pl}}{\boldsymbol{R}} \Big)^{\boldsymbol{K}\_{\psi} + 1} + \frac{1 - 2\boldsymbol{\nu}}{4(\boldsymbol{S}\_{\boldsymbol{o}} - \boldsymbol{P}\_{i}^{\text{er}})} \Big[ \ln \Big( \frac{\boldsymbol{R}\_{pl}}{\boldsymbol{R}} \Big) \Big]^{2} - \Big[ \frac{1 - 2\boldsymbol{\nu}}{\boldsymbol{K}\_{\psi} + 1} \frac{\sqrt{\boldsymbol{P}\_{i}^{\text{er}}}}{\boldsymbol{S}\_{\boldsymbol{o}} - \boldsymbol{P}\_{i}^{\text{er}}} + \\ & \frac{1 - \boldsymbol{\nu}}{2} \frac{\boldsymbol{K}\_{\psi} - 1}{(\boldsymbol{K}\_{\psi} + 1)^{2}} \frac{1}{\boldsymbol{S}\_{\boldsymbol{o}} - \boldsymbol{P}\_{i}^{\text{er}}} \Big] \Big[ \left( \boldsymbol{K}\_{\psi} + 1 \right) \ln \Big( \frac{\boldsymbol{R}\_{pl}}{\boldsymbol{R}} \Big) - \left( \frac{\boldsymbol{R}\_{pl}}{\boldsymbol{R}} \Big)^{\boldsymbol{K}\_{\psi} + 1} + 1 \right] \Big\} \end{split} \tag{A2}$$

where *K*<sup>ψ</sup> is the dilatancy coefficient, *Pi cr* the critical pressure, related with the transition of an elastic to plastic behavior of the tunnel walls, and *Rpl* the radius of the failed region, developed when *pi* < *Pi cr*.

The shotcrete support is modeled with perfect elastoplastic behavior, with an elastic radial stiffness *ks*, and maximum pressure, *ps,max*. Equations (A3)–(A5) describe the support behavior [28].

$$p\_s = \mathbb{K}\_s u\_r \tag{A3}$$

$$p\_s^{\text{mfix}} = \frac{f\_{cd}}{2} \left[ 1 - \frac{\left(R - t\_c\right)^2}{R^2} \right] \tag{A4}$$

$$K\_{\varepsilon} = \frac{E\_{\varepsilon}}{(1+\nu\_{\varepsilon})R} \frac{R^2 - (R - t\_{\varepsilon})^2}{\left(1 - 2\nu\_{\varepsilon}\right)R^2 + \left(R - t\_{\varepsilon}\right)^2} \tag{A5}$$

where *ps* is the pressure in the support, *fcd* the design compressive strength of the shotcrete, *ur* the radial displacement of the support, ν*<sup>c</sup>* the Poisson's coefficient of the shotcrete, *Ec* the modulus of elasticity of the shotcrete, *R* the radius of the shotcrete ring and *tc* representing its thickness.

**Figure A1.** Convergence confinement method: (**a**) evolution of tunnel convergence with face advance; (**b**) ground reaction curve and support reaction curve (adapted from Orestes [13]).

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **High Performance Self-Compacting Concrete with Electric Arc Furnace Slag Aggregate and Cupola Slag Powder**

#### **Israel Sosa, Carlos Thomas \*, Juan Antonio Polanco, Jesus Setién and Pablo Tamayo**

LADICIM (Laboratory of Materials Science and Engineering Division), University of Cantabria, 39005 Santander, Spain; sosai@unican.es (I.S.); polancoa@unican.es (J.A.P.); setienj@unican.es (J.S.); pablo.tamayo@unican.es (P.T.)

**\*** Correspondence: thomasc@unican.es

Received: 17 December 2019; Accepted: 19 January 2020; Published: 22 January 2020

**Abstract:** The development of self-compacting concretes with electric arc furnace slags is a novelty in the field of materials and the production of high-performance concretes with these characteristics is a further achievement. To obtain these high-strength, low-permeability concretes, steel slag aggregates and cupola slag powder are used. To prove the effectiveness of these concretes, they are compared with control concretes that use diabase aggregates, fly ash, and limestone supplementary cementitious materials (SCMs, also called fillers) and intermediate mix proportions. The high density SCMs give the fresh concrete self-compacting thixotropy using high-density aggregates with no segregation. Moreover, the temporal evolution of the mechanical properties of mortars and concretes shows pozzolanic reactions for the cupola slag. The fulfillment of the demands in terms of stability, flowability, and mechanical properties required for this type of concrete, and the savings of natural resources derived from the valorization of waste, make these sustainable concretes a viable option for countless applications in civil engineering.

**Keywords:** self-compacting concrete; high-performance concrete; EAFS; cupola slag; electric arc furnace slag; mechanical properties

#### **1. Introduction**

The production of crude steel in Europe in the year 2017 was 168.3 Mt, almost 4% higher than in 2016, while the production by electric arc furnaces stands at 40.3% of total production (67.8 Mt) [1]. This steel production determines the amount of electric arc furnace slag (EAFS) generated during the fusion processes of scrap, totalling 18 Mt for the year 2016 according to the Euroslag. In Europe, 12 Mt of cast iron was generated in 2017. These values of production reveal the volume of slag resulting from the steel and cast iron industries that ends in landfills.

Several authors have incorporated different types of industrial waste as SCM's to concrete to modify its properties [2–4]. The pozzolanic properties of the slag generated in steel processes depend on the cooling process. With rapid cooling (as is the case of cupola slag) using water, the vitrification of the slag occurs, leaving the silica in an amorphous form and, therefore, susceptible to reaction. On the contrary, slow cooling (case of EAFS) promotes the complete crystallization of the phases and the inertization of the final product, thus not compromising its dimensional stability. The pozzolanicity of EAFS has been studied, but its reactivity has been reported to be rather weak [5] although it can be improved by remelting treatments [6]. The content of periclase (MgO) in the slag causes a risk of potential expansion because the process of transformation into brucite [Mg(OH)2] by hydration is slow or even delayed, putting the dimensional stability at risk. Given the possible expansive reactions, it is important to verify the efficiency of the stabilization treatments [5]. Another problem present in these

aggregates is the significant deficit of particle sizes that pass through the smaller sieves in the sands. Therefore, the manufacture of mortars and concretes with EAFS sand entails mixing by combining them, either with natural sand or with inert filler [7,8].

Another interesting practice in recovery/recycling strategies (in addition to incorporating EAFS aggregates) is to incorporate recycled aggregates from Construction and Demolition Wastes (C&DW) to self-compacting concrete [2,9,10]. It has been demonstrated that their mechanical properties and durability [2,11–13] are suitable for structural concrete and they can be recycled several times [14].

SCMs are important for the self-compacting concrete (SCC), since they reduce the intergranular interaction [14], increase the cohesion and the flowability of the mixture, improve the hydration of the paste [15,16], and strengthen the resistance to segregation. In hardened concrete, SCMs typically reduce capillarity and permeability but also mechanical properties can be reduced [14,17]. The cements most used for the manufacture of high strength SCC are Portland type I, however, blends containing one or more SCMs [18] and 350 kg/m<sup>3</sup> of CEM can be used readily [19]. It is recommended not to exceed 500 kg/m3 (to avoid shrinkage problems) and to use SCMs to improve the workability of the fresh concrete. Several authors [20,21] have established that drying shrinkage increases with drying speed and is proportional to the volume of cement paste, while the opposite occurs by increasing the lime filler content.

The use of cupola slag as a concrete SCM is not a common application despite of its sustainable benefits. Nevertheless, the use of granulated cupola furnace slag (GCFS) as fine and coarse aggregate (0–16 mm) in concrete does not seem a viable option [22]. However, some authors have demonstrated by the manufacture of mortars with various replacements [23] that is suitable to be used as SCM with the right activation process. Nevertheless, great reactivity has been reported when acting as substitution of ordinary Portland cement (OPC), showing 30% compressive strength gains for 15% replacements at 28 days [22].

The use of SCMs increases the concrete strength, and in greater proportion, by using SCMs with pozzolanic properties. According to Domone [23], the type and proportion of SCM has greater influence on the compressive strength than the water/filler ratio (cement + SCMs). Rozière et al. [20], reported the rise in compressive strength by increasing the limestone filler content, keeping the effective water/cement ratio and the amount of cement.

The use of SCC brings with it a series of advantages with respect to the conventional one such as better adhesion between the paste and the aggregates or the uniform distribution of the stresses during load applications [19], even having the same w/c ratio [24,25]. Some studies claim that indirect tensile strength is higher in SCC [25] due to its packed structure, other studies argue that there are no differences because this property does not depend on the paste content [23,24] and other studies argue that this greater paste content affects negatively [20]. Tensile splitting strength of SCC and conventional concrete with the same amount of cement and water/cement ratio is directly influenced by the type of aggregate used [19] and elastic modulus in SCC is lower because it has a lower proportion of coarse aggregate [21], and for the same compressive strength the SCC presents a bigger strain [19]. The stiffness of SCC can be 40% lower but in high strength concrete (the concrete considered in this study) the difference is reduced to 5%.

The mechanical properties of concretes with EAFS are superior to those obtained with conventional aggregate concretes [26,27], mainly due to an improvement in the bond with the cement paste owing to the quality of the paste-aggregate interfacial transition zone (ITZ), which can be observed by means of a scanning microscope [23,28]. Experimentally, it has been found that the coarse fraction of EAFS contributes to the increase in compressive strength, tensile splitting strength, and elastic modulus. Similarly, the total substitution of fine aggregate leads to a reduction in compressive strength [29]. In terms of durability, concrete with EAFS is more vulnerable to frost and freeze–thaw cycles [30].

The use of EAFS in SCC is a challenge and very few studies have been done to date, mainly due to a decrease in flowability, also due to intergranular friction and a slight increase in density, although it has been possible to obtain stronger concretes than conventional ones [31], always using a

significant amount of superplasticizer additive. Recently, Santamaría et al. [32,33] have demonstrated the feasibility of manufacturing SCC using EAFS as both coarse and fine, obtaining consistency classes of S4 and SF2 and reasonably good mechanical properties. Likewise, Qasrawi [34] advises not to use replacements greater than 50% of EAFS so as not to negatively affect the properties in the fresh state, mainly density, air content, and stability. With regard to these studies and broadly speaking, in this paper new concrete mixes are developed through the use of two different wastes, to obtain high-performance concrete. Analyzing in detail the specific differences with the works found in the literature, this paper presents these main novelties:


The aim of this study is to demonstrate that it is possible to obtain a high-performance concrete (HPC), considering this to have a strength between 70 and 150 MPa, which is self-compacting and also uses steel slag aggregate in all fractions (coarse, fine, and SCMs), obtaining a concrete with countless potential applications. For this purpose, the work consisted of two phases; in the first, mortar mixes were produced with different cement replacements for cupola slag, thus demonstrating the pozzolanicity of this material. In the second phase, self-compacting concretes have been compared with different types of coarse, fine, and locally available SCMs, demonstrating the improvement of mechanical performance with the use of EAFS aggregates and cupola slag (as SCM) with respect to conventional materials such as diabase (high quality aggregate) or siliceous sand.

#### **2. Materials and Methods**

#### *2.1. Materials*

Standardized siliceous sand, CEM I 52.5 R and cupola slag have been used for the manufacture of conventional mortars. For the manufacture of the different concrete proportions, coarse 6/12 (DC) diabase and coarse 6/12 (SC) slag have been used, limiting their quantities to avoid problems of blockage and segregation. Diabase sand 0/6 (DS), electric arc furnace slag 0/6 (SLS), and silica sand 0/2 (SIS) have been used as fine aggregates. The SCM used include limestone filler (LF), fly ash (FA), and cupola slag filler (CS). The granulometric distribution of the aggregates used (coarse and fine) according to EN 933-1 is shown in Figure 1.

**Figure 1.** Aggregates granulometric grading.

The physical-mechanical properties of the aggregates have been determined by characterizing the bulk specific gravity and porosity according to EN 1097-3, the apparent specific gravity and water absorption according to EN 1097-6, the resistance to fragmentation determining Los Angeles coefficient according to EN 1097-2 and the aggregates crushing value according to UNE 83112. The fine aggregates (cement and SCM) have been characterized by determining the actual density according to UNE 80103 and by the specific Blaine surface according to EN 196-6. The results obtained are shown in Tables 1 and 2. Both types of aggregates have excellent mechanical properties and a 35% higher density in the case of EAFS.


**Table 1.** Main properties of the coarse aggregates used in the self-compacting concrete (SCC).

**Table 2.** Main properties of the sands, supplementary cementitious materials (SCMs) and cement used in the SCC.


The chemical characterization of the slags used was carried out by means of X-ray fluorescence (XRF), in order to determine the semi-quantitative concentration of compounds in oxides represented by percentages (Table 3). For this characterization an ARL-ADVANT-XP Thermo-spectrometer was used. It was observed that the main components of slags were iron, calcium, and silicon oxides, although there were traces of chromium and titanium oxides, probably generated during the injection of oxygen. On the other hand, the cupola slag shows high concentrations of silicon, calcium, and aluminum oxides, the first being an indicator of the possible reactivity of the material, since it is in an amorphous state. The compositions of the rest of the materials were obtained by Energy-dispersive X-ray spectroscopy (EDX) using a Zeiss EVO MA15 scanning electron microscope (SEM) equipped with an Oxford Instruments X-ray detector, selecting different representative areas of particles chosen randomly.


**Table 3.** Chemical composition of the materials used.

The expansiveness of the EAFS is one of its main problems, and can cause cracking of concrete in the medium/long term. To avoid this phenomenon, EAFS aggregates have been submerged in pools for 24 h and have remained wet in storage stacks for 3 months, in order to hydrate free lime and magnesia. The expansiveness of the aggregates was determined according to EN 1744-1, obtaining values of 0.16 vol.% at 24 h and 0.17% vol. at 168 h, the first value being those required when the MgO content is less than 5%.

The mortar proportions (Table 4) were carried out with CEM I 52.5 R, CEN standardized sand and in accordance with the amounts proposed by EN 196-1 (except the amount of sand, slightly higher). The mortars were cured submerged in water at a temperature of 20 ± 1 ◦C. The manufacture of these mortars (M) was carried out using replacements of 0, 10, 20, 30 vol.% of cement by cupola slag filler, in order to establish the pozzolanity of the cupola slag compared to cement.

**Material [g] M-0% M-10% M-20% M-30%** CEM I 52.5 R 450 405 360 315 Silica sand 1450 1450 1450 1450 Cupola slag filler - 42 84 126 Water 225 225 225 225 w/c ratio 0.5 0.6 0.7 0.8

**Table 4.** Mix proportions of the mortars [g].

For the mix proportions of the self-compacting control concrete, the methodology proposed by Dinakar et al. [35] was used, based on compressive strength. The design goal was to obtain a high-performance concrete with 100 MPa strength at 90 days, for which an amount of cement of 450 kg/m3, and the use of 2% (of the cement weight) of a superplasticizer additive (enabling a more viscous paste to be obtained) was selected. A limestone filler quantity of 100 kg/m3 was used, thus using a total amount of cement, SCMs, and filler of 550 kg/m3, less than the maximum 600 kg/m3 recommended by EHE-08 [36] and EFNARC [37]. An attempt was made to maximize the coarse content (50 vol.%) to obtain greater use of the by-product without affecting segregation or blocking. Likewise, the amount of water used was optimized so as not to adversely affect the strength without affecting the flowability or segregation.

Four concrete mixes were made: three control mixes that use diabase coarse with three different filler materials (limestone, fly ash and cupola slag) and a fourth that uses EAFS coarse with cupola slag filler. The first three dosages enable the comparison of the SCM used, while the fourth enables the comparison of high strength natural aggregates with siderurgical aggregates. The three control dosages are analogous, while the fourth had to be modified because it presented notable deficiencies in the fresh state. The sand content was increased because SLS has less fine aggregates than DS and the latter has a much more cavernous and angular geometry. In addition, the reduction of SC enabled slump to be improved and prevented concrete blockage. A slightly lower w/c ratio was used in this case due to the limitation due to the segregation of the EAFS aggregate.

The preparation of these self-compacting concretes was similar to that of conventional concretes, with the exception of kneading time (12 min), considerably increased to ensure the complete distribution of the superplasticizer additive. The mixtures were made in a 120 L rotating drum mixer, with 30 L batches. The samples were been demolded at 24 h and were cured in a moisture chamber, at a constant temperature of 20 <sup>±</sup> 2 ◦C and constant humidity of 95 <sup>±</sup> 5%. The final proportions, in kg/m3, of the mixtures appear in Table 5.


**Table 5.** Concrete mix proportions (kg/m3).

#### *2.2. Properties of Fresh Concrete*

The self-compactability of a concrete depends essentially on its filling ability, its passing ability, and its static and dynamic stability or segregation resistance. The characterization of the properties in the fresh state of self-compacting concrete is different from conventional concretes, using in this study the slump flow, the L-box and the V-funnel tests. The slump flow test has been carried out according to EN 12350-8, to obtain the flow capacity of the concrete, as well as its stability. To carry out this test, the Abrams cone (EN 12350-2) and a 900 × 900 mm metal steel plate have been used, on which three concentric circles are marked, measuring the conjugated diameters of the drained concrete and determining the average diameter (SF). This test also determines the time it takes for the concrete to cover the 500 mm circle (t500), which allows the relative viscosity of the concrete to be evaluated, as well as the flow rate. According to EN 206-9 the permissible range for this test is between 550 and 850 mm for the SF and the t500 must be less than or equal to 8 s.

The L-box test was carried out according to EN 12350-10 using 3 bars. It measures the ability to pass through the reinforcements, as well as stability. The concrete was poured through the upper opening, measuring the height of the concrete in the vertical section (H1) as well as the height at the end of the horizontal section (H2). The "passing ability" of concrete for the PL test is defined as the H2/H1 ratio. According to EN 206-9, there are two kinds of passing capacity delimited by a PL of 0.8. The V-funnel test has been carried out according to EN 12350-9 to assess the viscosity, the capacity to pass through confined spaces and the mold filling capacity. The test measures the time it takes for the concrete to flow through the V-mold, from the opening of the lower gate until the mold is emptied. According to EN 206-9, the permissible tv range is 0–25 s, with 9 s being the limit between the two existing categories.

#### *2.3. Physical Properties of the Concrete Mixes*

The apparent, bulk and saturated-surface-dry (SSD) specific gravities were determined following EN 12390-7 and, additionally, both accessible porosity (vol.%) and water absorption (wt.% ) were obtained according to UNE 83980 applying air vacuum. The physical properties were determined on nine thirds of standard cylindrical specimens of 150 × 300 mm at 28 days per mix (a total of 36 specimens), specimens in which the upper and lower ends were cut in order to avoid edge effects.

#### *2.4. Mechanical Properties of the Mortars*

The mortars underwent the mechanical tests proposed by and according to EN 196-1. These tests consisted first of all in the determination of flexural strength and then in the determination of compressive strength on each of the halves obtained in the first test. Three specimens were manufactured per age and per mix (64 specimens in total), with tests being performed at 7, 28, 60, and 90 days. The tests were carried out on a servohydraulic machine with a capacity of 250 kN at rates of 0.05 mm/s for flexural tests and 0.1 mm/s for compression tests.

#### *2.5. Mechanical Properties of the Concrete*

The uniaxial compressive strength was determined according to EN 12390-3 on four cubic specimens of 100 mm at 7, 28, 90, 180 and 365 days per mix (a total of 80 specimens). The use of these specimens is compatible with the maximum aggregate size and correction coefficients were applied if necessary. This test was carried out on a servohydraulic machine with a capacity of 1500 kN at a rate of 0.5 MPa/s, after removing the specimens from the moisture chamber and waiting for their surface drying.

The compressive elastic modulus was determined following EN 12390-13 on 1 standardized cylindrical specimen at 7, 28, 90, 180, and 365 days of age per mix (a total of 20 specimens). Method A was applied, which includes 3 preload cycles to check the correct positioning of the specimens and subsequently 3 loading/unloading cycles were applied. The initial and final stress and strain values enable the initial elastic modulus (first cycle) and the stabilized elastic modulus (third cycle) to be obtained. The specimens were capped with sulfur on their upper face and fitted with strain gauges 120 mm in length. The test was carried out on a servohydraulic machine with a capacity of 2000 kN at a load/unload rate of 0.7 MPa/s.

The splitting tensile strength test was carried out according to EN 12390-6, on 9 thirds of standard cylindrical specimens of 150 × 300 mm at 90 days per mix (a total of 36 specimens). A servohydraulic testing machine of 1500 kN capacity and a load rate of 0.05 MPa/s was used for this purpose.

#### **3. Results and Discussion**

#### *3.1. Properties of Fresh Concrete*

The properties in the fresh state of the different mixes of SCC appear in Table 6. The mixes that incorporate diabase coarse and limestone filler or slag filler have an average slump flow of 750–850 mm corresponding to the upper category or SF3 according to EN 206-9, equivalent to category AC-E3 according to EFNARC. In the case of using slag coarse or diabase aggregates with fly ash, the average slump flow is reduced to 650–750 mm, corresponding to category SF2 according to EN 206-9 or AC-E2 according to EFNARC, ideal workability for conventional applications as pillars and walls. The use of slags as coarse aggregate penalizes slump flow by 10%, which is compatible with the results obtained in conventional concretes, where the use of slags leads to a general decrease in workability [29,30]. The decrease in the flowability of concrete with slags is due to their increased friction, as well as their angular geometry and their high density. The t500 for the SCC-DC-FA and SCC-DC-CS mixes are classified as VS2 according to EN 206-9 and AC-V1 according to the EFNARC, with values at the limit of the recommendations. On the other hand, the mixes with limestone filler and slag coarse fall outside the categories proposed by the recommendations, the latter due to the viscosity of the paste itself, since a greater amount of fines is used. The slump test of the SCC-SC-CS mix can be seen in the following link: https://www.youtube.com/watch?v=rZUtHjiP4dw.


**Table 6.** Rheological properties of the self-compacting concrete in fresh state.

The flowability is at odds with viscosity, incorporating high-density aggregates requires a viscous paste (high fine content and low w/c ratio) that prevents the segregation of aggregates. Increasing viscosity also means obtaining a higher volume of occluded air or a poorer surface finish, which is why it is important to establish a compromise between both variables. In Figure 2, the correct distribution of aggregates in all mixes can be observed. In the case of SCC-DC-LF, areas with higher aggregate concentration and a mortar band of about 10 mm thick and slight bleeding are observed on the perimeter of the disk. The behavior of the SCC-DC-FA is more stable, favored by the shape of the fly ash particles that provide greater homogeneity in slump. Although it also has a mortar band about 10 mm thick, no bleeding is reported. In the case of SCC-DC-CS there is a homogeneous distribution of aggregates but a decrease in the viscosity of the paste and consequently greater bleeding and exudation due to incorporating the vitreous powder. The disk of the SCC-SC-CS shows a very homogeneous, symmetrical, and stable distribution. There is no segregation or concentration of coarse aggregates despite the high density of slags although there is a mortar band of about 10 mm around the disk.

**Figure 2.** Slump disks detail of all mixes.

The values obtained from PL are consistent with the slump measured. The results of the L-box test (Table 6) show a passing capacity (PL) greater than 0.8 in all cases, which allows the mixes to be classified as PL1 according to EN 206-9 and as AC-RB2 according to EFNARC. In none of the cases has there been blockage in the bars and it should be noted that the SCC-SC-CS mix has shown a flow rate through confined spaces well below the other mixes, taking twice as long to stabilize the final height H2. This low-passing speed is associated with the high friction in the sliding between layers of paste with slag sand and cupola filler.

The results for the V funnel test also appear in Table 6. As was already apparent in the L-box test, the greater times correspond to the mixes that use cupola slag filler and slag sand. The difference between using limestone filler and cupola slag filler is almost negligible, although the use of fly ash improves the mobility of the mixture due to its spherical shape. On the other hand, the values obtained with the SCC-SC-CS are 70% higher than with SCC-DC-LF and SCC-DC-SC, a value that rises to 300% higher in comparison with SCC-SC-FA. The SCC-SC-CS is the most affected in this parameter due to the high viscosity of the paste, this viscosity also generates thixotropy gelation, observed after keeping the fresh mixture at rest. In all mixes the values are higher than the 25 s established by EN 206-9 for conventional SCC due to the high coarse content, which makes it difficult to pass through the funnel. Therefore, the designed mixtures are not recommended for highly confined areas, obliging modification of the mixture.

#### *3.2. Physical Properties of the Concrete Mixes*

The physical properties of the concrete mixes are shown in Table 7. The high density of EAFS gives the concrete with slags and the mixture of cupola slag around 15% more density than the reference concrete with diabase aggregates, except the mix that contains fly ash, where the ratio goes up to 20%. This is partly due to the low actual density of the fly ash (2.13 g/cm3).


**Table 7.** Physical properties of the concrete mixes.

Due to the difference in densities between the mixes, it is ideal to perform a comparison between the open porosity (vol.%) and not the water absorption (wt.%). The mix with the lowest open porosity is the one that uses EAFS and cupola slag due, in part, to having a slightly lower w/c ratio and due to the higher density of aggregate particles, which facilitate the expulsion of trapped air (greater self-compaction). Conversely, the mix with greater open porosity is the one that uses diabase coarse and fly ash (220% greater than the previous one), this is due to a reaction between the plasticizer additive and the fly ash, which generates bubbles of around 1 mm.

In the comparison between SCC-DC-LF and SCC-DC-CS, it can be concluded that there are no significant differences in any of the properties shown between the limestone filler and the cupola slag SCM, it being necessary to analyze the mechanical properties of both mixes to establish the benefits of each filler. The visual aspect of each mix in the hardened state is shown in Figure 3, where a better orientation of the aggregates in the filling can be seen in the SCC-SC-CS mix.

**Figure 3.** Appearance of the concrete mixes.

#### *3.3. Mechanical Properties of the Mortars*

Figure 4 shows the results of the mechanical properties of mortars that incorporate several replacements of Portland cement. Compressive strength increases with the age for all replacements following a logarithmic trend (R<sup>2</sup> <sup>≈</sup> 0.85), while intermediate replacements shows intermediate behavior between maximum replacement and reference mortars. After 7 days, mortars with a 30% replacement show a 27% loss in compressive strength with respect to the reference mortar, showing a slow rate of hydration reactions. This loss is reduced to 12% at 28 days and 5% at 60 days. At 90 days, the strength shown for all replacements tends to converge because the start of hydration of the alite and the peak of hydration occur much later when using cupola slag SCM than when using Portland cement (the speed of the reactions is slower when using cupola slag) [38].

**Figure 4.** Mechanical properties of mortars with several cement replacements with cupola slag.

Flexural strength (Figure 4) also increases with age for all replacements and analogously to compressive strength. After 7 days, the strength shown by 30% replacements is 25% lower than that obtained with the reference mortars. For 28 and 60 days the losses reach 10% and 6% respectively. Again, there is a development of strength with age, more pronounced when cupola slag SCM is incorporated into the mortar. For ages over 90 days the values tend to converge again, showing the slowness of the reactions and demonstrating the strong pozzolanic character of the cupola slag.

#### *3.4. Mechanical Properties of the Concrete Mixes*

The evolution of the compressive strength over time for the different mixes is shown in Figure 5. Regarding the SCM's, comparing the 3 mixes that use natural aggregates (identical proportions), it is observed that the cupola slag (SCC-SC-CS and SCC-DC-CS mixes) provides more strength than the limestone filler and the fly ash (SCC-DC-LF and –SCC-DC-FA), both at short and long ages, despite the instability of the mix. This demonstrates the pozzolanic character of the cupola slag, showing a 13% greater compressive strength at 28 days and 11% greater at 360 days than both mixes and showing once again the importance of the promotion of the pozzolanic reaction by the SCM's in high-performance self-compacting concrete [39]. Of these last two (SCC-DC-LF and –SCC-DC-FA), it is surprising that with the fly ash similar strength is achieved as with the limestone filler, despite the greater porosity of the first, due to the high alkalinity of concrete, which favors the reaction of the SCM. In the comparison of the SCC-SC-CS with the rest of the mixes, great improvements in compressive strength are observed at all ages due mainly to three factors: better compressive strength of EAFS, higher coefficient of friction between particles due to the EAFS roughness, and a better bond between paste and aggregates. Comparing SCC-SC-CS with SCC-DC-CS, there is an increase in resistance of 5% at 28 days and 13% at 360 days due to the incorporation of the EAFS aggregate and a slight reduction in the w/c ratio. Comparing SCC-SC-CS with SCC-DC-LF and SCC-DC-FA, the increase is 20% at 28 days and 25% at 360 days. Figure 6 (obtained by scanning electron microscopy) shows the appearance of the cupola slag particles at 5000× after reacting, generating compounds in the form of parallel hexagonal plates, perfectly integrated in the paste and in combination with the cement hydration products. The composition of these plates, determined by EDX, corresponds to hydrated calcium aluminosilicates.

**Figure 5.** Evolution of the compressive strength for the different mixes: the first at 28 days (left) and the second at 360 days (right).

**Figure 6.** Appearance of the cupola slag after reacting on the cement paste (5000×).

In absolute terms, the mixes that incorporate cupola slag exceed 100 MPa at 28 days while the rest of the mixes are over 90 MPa, all of which can be considered high-performance concretes. From 28 to 360 days, the most evolved mix is SCC-SC-CS (16.5%) followed by SCC-DC-FA (12%) and SCC-DC-LF (7.5%) and SCC-DC-CS (7.5%). Also, at 360 days, the mix with cupola SCM and EAFS reaches 130 MPa. Comparing Figures 4 and 5, a remarkable increase in strength can be observed when cupola slag is used as a filler, rather than a cement substitute, which indicates cupola slag's potential as a reactive filler.

In Figure 7, the appearance of some of the cracking of the cubic specimens tested at 365 days is presented. As can be seen, due to the high stress applied to these concretes and the excellent paste–aggregate interface, the crack planes have propagated through the paste and aggregates, releasing all their energy through explosive cracks.

SCC-DC-CS SCC-SC-CS

The analysis of the cracking behavior of these materials is important, and in addition to the visual analysis, there are methodologies to determine the cracking load from deflection–load curves in flexural tests [40].

The evolution of the elastic modulus over time is shown in Figure 8. Among the mixes that use diabase aggregate, it is observed that SCC-DC-FA has the smallest elastic modulus (37 and 40 GPa at 28 and 360 days), which is due to the great porosity of the mix, allowing greater deformations. The SCC-DC-CS mix shows a modulus 10% higher than SCC-DC-LF, having a slightly lower porosity, due to the pozzolanic character and the greater Blaine surface of the cupola slag SCM. On the other hand, the SCC-SC-CS mix shows an elastic modulus far superior to the rest of the mixes (56 and 59 GPa at 28 and 360 days), being 24% and 18% higher than for SCC-DC-CS at 28 and 360 days respectively. This is due to the high EAFS modulus (and its iron nature), a slightly lower w/c ratio and a 25% lower porosity (Table 1). Compared with compressive strength, the elastic modulus has increased less with age, obtaining at 7 days approximately 90% of the final elastic modulus in all mixes.

**Figure 8.** Evolution of the initial elastic modulus (**left**) and the stabilized elastic modulus (**right**) for the different concrete mixes.

On the other hand, the stabilized elastic modulus is slightly higher in all cases, since the material undergoes small permanent deformations for loads below the elastic limit in all cycles

The results of tensile splitting (Brazilian) strength for all mixes at 90 days are presented in Figure 9. Among all the mixes that use natural aggregate, it is observed that SCC-DC-FA has the lowest tensile splitting strength (4.5 MPa), due to a high porosity and a lower bonding in the interface transition zone favored by the smooth surface of the fly ash particles. The SCC-DC-LF mix is the next one with the highest tensile splitting strength (5.4 MPa), slightly lower than SCC-DC-CS (5.7 MPa), where once again the reactive character of the cupola slag SCM shows an improvement of the mechanical properties. For this property, there are no differences between using natural aggregate or siderurgical aggregate, as shown by the comparison between SCC-DC-CS and SCC-SC-CS (5.6 MPa), showing that the bonding strength in the interfacial transition zone (ITZ) is similar (good quality) for both aggregates.

**Figure 9.** Tensile splitting strength at 90 days for all mixes.

The main mechanical properties of these mixes have been established, however, as future work it is proposed to check the functionality of conventional mechanical models for these materials and the creation of new models if necessary. These models will allow predicting the post-crack behavior of the material and the damage mechanisms for complex structures, using finite-element numerical approaches, as they are widely addressed in the literature [41].

#### **4. Conclusions**

This research deals with the recycling of two by-products, cupola slag and electric arc furnace slag, to obtain high-performance self-compacting concrete and the comparison of these with similar concrete mixes using high-quality natural aggregates (diabase) and the most common SCMs. After analyzing the rheological and physical-mechanical results obtained, the following conclusions can be drawn:


modulus of mixes that use slag cupola powder is 10% higher than the mixes with traditional fillers. In the case of the elastic modulus, practically 90% of the value at 360 days is obtained after 7 days. No significant differences were found in the tensile splitting strength at 90 days for all the mixes.

**Author Contributions:** Conceptualization, J.A.P. and C.T.; methodology, C.T. and I.S.; validation, J.A.P. and J.S.; formal analysis, I.S. and P.T.; investigation, I.S.; data curation, I.S.; writing—original draft preparation, I.S. and P.T.; writing—review and editing, C.T. and P.T.; visualization, I.S.; supervision, J.A.P. and C.T.; project administration, C.T. and J.A.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors of this research would like to thank GLOBAL STEEL WIRE for the EAF slag supply and Saint Gobain Pam España for the Cupola Furnace Slag as well as ROCACERO for providing the cement, natural aggregates and superplasticizer additive.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

#### *Article*
