## 5 Considerations for Plastic Design of Flat Slabs
### Discussion on Other Typologies and Potential Implications on Carbon Footprint  
As already pointed out by [PERSON] et al. [15], other structural typologies might be more efficient in terms of material consumption and overall carbon footprint. For instance, the use of waffle slabs can be identified as a promising alternative. This typology allows for the saving of large amounts of material in the inner panel regions. In addition, it is simple to cast in FRC, as flexural reinforcement may be arranged in the ribs, but shear reinforcement can be replaced by the fibers. In the following, a comparison of a flat slab and a waffle slab is presented both in ordinary reinforced concrete and FRC, following the methodology presented in [15] and by adopting the following reference values of the unitary CO\({}_{2}\)-eq footprint:  
* Concrete C30/37 (\(f_{\text{ck}}=30\) MPa) CEM II for reinforced concrete elements, considering 0.12 kg CO\({}_{2}\)-eq/kg as the reference value of the unitary CO\({}_{2}\)-eq footprint [15]. Such value and cement types are selected as representative of current construction practice. * Concrete C30/37 (\(f_{\text{ck}}=30\) MPa) CEM III for FRC elements, considering 0.05 kg CO\({}_{2}\)-eq/kg as the reference value of the unitary CO\({}_{2}\)-eq footprint [15; 20]. Such value is selected in order to highlight the potential of FRC to reduce the unitary carbon footprint. It shall be noted that other optimizations are possible, such as reducing the concrete strength if statically admissible. * Reinforcement steel B500 (\(f_{\text{yk}}=500\) MPa) for both reinforced concrete and FRC solutions, considering 0.5 kg CO\({}_{2}\)-eq/kg as a reference value of the unitary CO\({}_{2}\)-eq footprint [15]
* Polypropilene fibers, considering a ratio of 10 kg/m\({}^{3}\). For the fibers, it is adopted a reference value of the unitary CO\({}_{2}\)-eq footprint equal to 4 kg CO\({}_{2}\)-eq/kg according to [20]  
The results are plotted in Figure 8 for four representative cases, corresponding to two flat slabs (with reinforced concrete CEM II and with FRC CEM III) and to two waffle slabs (with reinforced concrete CEM II and with FRC CEM III). For the FRC solutions, flexural reinforcement was considered only in the support panel for the flat slab and in the support panel and ribs for the waffle slab. The results show a high potential to optimize the carbon footprint of the construction, with reductions in the amount of CO\({}_{2}\)-eq of more than two-thirds in several cases. It may be yet noted that this analysis is based on the same geometry for reinforced concrete and FRC members and that only flexural checks are performed. Also, the results  
Figure 8: Environmental footprint (embodied CO\({}_{2}\)-eq) of different structural solutions for a slab: flat slab and waffle slab in reinforced concrete (CEM II) and FRC (CEM III). may vary in the considered value of the unitary CO\({}_{2}\)-eq footprint, which may experience relatively high differences among the materials considered [15]. Nevertheless, a general trend can be observed, concluding that, with respect to current construction practice, strong reductions in the environmental impact can be expected if FRC CEM III is used in optimized structural shapes. Such reduction is, in fact, mostly due to the implementation of a CEM III cement in the FRC mix with polypropylene fibers. Concerning the type of fibers to be used, the higher degrees of toughness are achieved by using macro-polymer FRC and hybrid FRC (refer to Figure 4), whose use seems thus encouraged. It shall be noted that solutions where the amount of material is significantly reduced are economically interesting, as demonstrated in [20], provided that cost-competitive solutions are found for the formwork. Another relevant aspect for practice is the potential benefit of precasting when FRC is used.