Steel And Recycled Concrete Slab (SARCOS) project is aimed to extend the use of recycled aggregates to steel–concrete composite slabs, which actually are exclusively realized by using concrete composed with natural aggregates. As well established {1, 2, 3, 4, 5}, steel–concrete composite slabs represent a very efficient floor solution combining the key performance of two different materials: the concrete, able to efficiently resist to compressive stresses and the steel of the hi-bond thin profiled steel sheeting, supporting tensile forces (Figure 1). Unlike the case of reinforced concrete beams, where rebars are totally embedded in the cross-section, the composite action depends on the interface behaviour between the steel sheeting and the concrete, i.e. by the chemical bond and the mechanical interlock between the concrete and steel components {6}.

Circular Economy

All the research studies developed in last decades on composite slabs have been carried out by considering only concrete made with natural aggregates, focusing mainly on the ultimate behaviour of the slabs and paying a very limited attention to both in-service and long-term responses {7}. It is meaningful that despite the steel-concrete composite slab solution is worldwide recognized as a cheap constructional technique, the environmental impact associated with the extensive use of natural aggregates is absolutely non-negligible. In the actual design prescriptions of EN1994-1-1 {8} there is no reference about the use of concrete slabs with recycled aggregates. Concrete production with natural aggregates yields to a progressive reduction of the raw material, due to need of opening new quarries of natural aggregates, and to the associated production of a very high quantity of waste. The rational use of available materials and the recycling of waste can partially solve the problem and represent an important element of the circular economy. According to the data reported in the Annual Review 2019-2020 of the European Aggregates Association {9}, in 2018 in the 28 countries of the European Union (EU) the production of aggregates is 3x1012 t, of which 85% is extracted from natural quarries and the remaining part comes from recycling reuse processes. Several studies on the properties of coarse recycled aggregates, obtained from concrete construction and demolition waste, and the effect of their replacement in new recycled concretes {10, 11, 12, 13} have been already published. In the literature there are also studies {14, 15, 16} on the use of recycled fine aggregates combined with coarse recycled aggregates. The recycled fine aggregates come both from the demolition of concrete only and from the processing of construction and demolition mixed waste {17, 18, 19, 20}.

National and international standards

Looking at the national and international standards (UNIEN206 {21}, RILEM {22}) different prescriptions and limitations have been developed in the use of concrete with recycled aggregates. Total replacement (100%) of natural coarse aggregates with recycled coarse aggregates is allowed only in Australia, China, Denmark and Switzerland and in the RILEM Recommendations. In China and Switzerland there are no limits on the strength class of concrete made with 100% recycled coarse aggregates. In Australia, on the other hand, the concrete obtained belongs to the C_25/30 class, in Denmark the resistance class reaches C_40/50 and according to the RILEM Recommendations the resistance class can reach C_50/60. Instead, the use of fine aggregate, coming from the crushing of concrete only, can be used, for the structural concrete, in few countries like, China, with replacement percentages ranging from 30% to 100%, Denmark and Japan with replacement percentages of 100%, Korea up to 30% and the United Kingdom up to 20%. It can be said that most of the considered Countries limit the use of recycled aggregates to the coarse fraction only and to replacement percentages from 20% to 30%, while the use of fine aggregates produced by crushing concrete is considered only from 20% of the countries examined here. The different limitations in the standards and the extreme heterogeneity of the results reported in the scientific literature, require an urgent commitment from the scientific community and construction market stakeholders in order to improve the knowledge of the behaviour of composite slab casted with concrete with recycled aggregates.

Sarcos Project

SARCOS project would determine the feasibility of composite slab with recycled aggregate which would otherwise be considered as waste {23, 24}. According to the EU-policies the project works for the sustainable development of the EU based on balanced economic growth and market stability, and on a highly competitive social market economy, aiming at safeguarding and increasing employment and thus, the social progress. SARCOS is expected to contribute to the improvement of the life quality of the European residents and protects the environment. Detailed description of the project Steel And Recycled Concrete Slab (SARCOS) research project, with an expected duration of 24 months, is dived into 6 Work Packages (WP), plus the WP0 dealing with the coordination of the project. SARCOS will improve the state-of-knowledge of composite steel-concrete slabs realized with sustainable recycled concrete. More research is needed in this field. The results of the previous discussed state-of-the-art review can be summarized as follows:

• Recycled aggregates, produced from concrete construction and demolition waste of concrete, have different physical-chemical characteristics depending on the strength and state of conservation of the parent concrete. In particular, the crushing technique and their storage are decisive for fine recycled aggregates. All these issues determine variations on the performance of the recycled concrete aggregates that require further investigation.
• The mechanical performances of concrete, in particular the compressive and tensile strength and the modulus of elasticity, depend on the content of recycled aggregate, on the production procedures of the new concrete, on the type of recycled aggregate used (concrete or mixed), on the particle size fraction (coarse and / or fine). The compressive strength of concrete is not significantly affected (10% resistance decreases compared to conventional concrete) when the substitution ratio is 30% of coarse recycled aggregates and 20% of fine recycled aggregates. The presence of coarse recycled aggregates has a lower impact on tensile strength of concrete, since it only decreases by 5% when the replacement percentage of the coarse aggregate is between 15% and 50% and the fine aggregate is replaced up to 35%.
• Actual design procedures of steel-concrete slabs (see EN1994-1-1{8}) are not exhaustive and are based on highly approximate hypothesis:
  1. at the ultimate limit state (ULS), it is assumed that the steel sheet works always in the plastic range independently on the stress level reached in the cross-section. Moreover, the concrete inside the ribs gives no contribution to the final resistance. These assumptions affect the resistance domain and could leads to a non-reliable estimation of the level of interaction between concrete and steel;
  2. there are no information regarding the serviceability limit state (SLS). In fact, it is not clearly indicated if engineers should consider the cracked or non-cracked cross-section in the evaluation of the vibration and displacements. The very important problem of vibration is completely neglected;
  3. lastly, no information nor indications are reported on the use of concrete made with recycled aggregates.
• In addition, in the codes there is no reference related to the use of recycled aggregate in the concrete. As discussed, the mechanical response of concrete made with recycled aggregates is noticeably different than the one made with natural aggregates and for this reason specific design rules for the proposed sustainable concrete-steel slabs are necessary.
• Actually, the loss of performance due to the replacement of the natural aggregate with the recycled one could be balanced by increasing the cement content in the mix design of the new concrete. However, this is not the most recommendable strategy, as it would increase the cost/m3 of the final product and its environmental footprint, as cement accounts for 85.6% of CO2 emissions related to production from concrete {19, 20}. Actually, some Countries, have enacted legislation that limits the use of recycled material for the production of structural concrete, but no information has been found about composite slab casted with recycled concrete.

Advancement of Knowledge

SARCOS will bring further contributions of knowledge and results in the following main macro areas:

1. Characterization of the concrete with recycled aggregates. An extensive physical-mechanical characterization of the concrete with recycled aggregates (both fine and coarse), including durability (water penetration, absorption chloride, carbonation, resistance to freezing and thawing), will be carried out within the project.
2. Experimental and theoretical response of composite slabs. Steel-concrete sustainable composite slabs, i.e. hi-bond steel metal decking combined with a recycled concrete, will be tested by varying the total length of the specimens. Moreover, advanced analytic models will be proposed.
3. Economic and environmental field. Within the project, a complete life-cycle assessment will be proposed on the selected concrete typologies. The analyses will clearly show the economic and environmental benefits in the use of recycled materials in the concrete.
4. Engineering design practice. At the end of the project, refined design rules/approaches to be used for both standard and sustainable slabs will be proposed.

General objectives

The SARCOS project has the general objective of encouraging, through participatory and applied research, the use of recycled aggregates in the concrete of new sustainable composite slab. In the SARCOS project, the sustainability of the composite slab is seen in a broader and wider way, in fact the use of recycled materials yields to a better management of natural resources and at the same time produces efficient materials, with consequent economic and environmental added value. The importance of SARCOS project is underlined by its main objectives, which can be summarized as:

1. estimation of the influence of construction and demolition waste recycled aggregates, in partial replacement of the natural coarse and fine aggregate on the rheological, mechanical and durability performance of concrete;
2. development of a reliable experimental test protocol on sustainable concrete slabs, in accordance with the general principles for the limit state design;
3. evaluation of the influence of different types of fine recycled aggregates on the structural behaviour of composite slab;
4. development of advanced numerical and theoretical models which will be used to directly carry out a technical guidelines;
5. evaluation of the whole life cycle of the composite slab considering both the economic and environmental costs.

It should be underlined that the European Union (EU) expects high recycling rates for construction and demolition waste. From {20} it appears that the percentages of recycled construction waste vary between the 38 countries of the European community in a range between 54% and 100%. To develop a real Circular Economy, it is necessary to obtain suitable recycling procedures for commercial products that are also subject to new recycling processes (zero waste). EU legislation sets key principles in waste management:

1. reducing the amount of waste produced;
2. reduce or eliminate toxic substances present in building materials;
3. raise the quantity and quality of recycling;
4. reduce greenhouse emissions produced by the management of construction waste;
5. promote selective demolition to facilitate high-quality reuse and recycling.

The use of recycled aggregates is in line with the principles of LEED certification (Leadership in Energy and Environmental Design) according to which ecological systems are envisaged for the construction and renovation of buildings and infrastructures. Energy and pollution are the poles around which the future must be built not only in industrialized countries, but also in developing countries. Furthermore, recycled aggregates are materials that meet the Minimum Environmental Criteria (CAM). Pending national legislation, the difficulties linked to the technical limitations of the recycling processes, the quality of recycled products, the general lack of trust in recycled products and the lack of coherence and coordination at the governance level should be emphasized. The attention is particularly high on the part of all the players involved in the construction world and the enhancement of the waste produced becomes the center of economic activities and applied research.