## 2 Materials and Methods
### Constructive Approach  
Considering the requirements and complex geometry of the MMFAR, it was necessary to design an efficient structural system to (i) allow adequate adjustment of the reinforcing elements within the module and ensure uniform cross-sectional stress distribution as well as a minimum nominal reinforcement cover; (ii) ensure continuity of the internal metallic reinforcement elements and between elements, to facilitate electrical conduction for added functionalities (e.g., self-healing techniques); and (iii) the need to adopt a modular and adaptable construction system to allow the practical and simple assemblage, compatible to underwater operations.  
Algorithmic thinking using parametric modelling tools as well as structural design approaches was used for the iterative design and analysis, as shown subsequently. Concrete was chosen for its versatility and cost-effectiveness, as well as its proneness to serve as a colonizing substrate [41; 43]. Its brittle nature was countered by the incorporation of ductile reinforcements, such as hot-dip galvanised (HDG) steel which is technically and financially attractive while offering high corrosion resistance. Other options may include  
Figure 2: Geometric design using novel approaches and design processes for the 3D network of individual modules of the MMFAR (adapted from [57]).  
self-healing cementitious composites and fibre-reinforced polymers, although the design of these materials was not addressed in this study.  
In this sense, different structural systems for the reef module were proposed, as shown in Figure 3: (i) a reinforced concrete structure approach (RC) based on the use of galvanized steel rebars along each limb of the unit and (ii) a composite steel and concrete structure (CS) where a HDG steel tube was adopted, aligned with the geometric centroid of each limb. The tube was perforated to allow the concrete to flow and fill the inner part of the tube, increasing the bond between the materials and providing a dead weight to counteract buoyancy.  
The adoption of straight reinforcement elements was tried; however, the accommodation of the reinforcement elements in the geometry was found to be hard, in particular trying to guarantee the minimum concrete cover. Curved reinforcement elements, although requiring additional forming procedures, were easier to adapt to the complex geometry of the module, ensuring better distribution of the reinforcement area along the cross-section and providing adequate reinforcement cover.  
The extension of the tube or rebar beyond the edges of each module was considered to support the connection system, particularly favourable in the case of the solution adopting the steel tubes as shown in Figure 4.  
Figure 3: Structural systems: (**a**) RC unit with four galvanized steel rebars interconnected using external rods by welding, (**b**) CS unit based on the use of four galvanized steel tubes with holes along the length, interconnected using external rods by welding, (**c**) RC with three central galvanized steel rebars interconnected at the centre of the unit by welding or tying, and (**d**) CS based on the use of three central galvanized steel tube profiles with holes along the length, interconnected by welding at the intersection.