La reutilización de estructuras antiguas. Beneficios y técnicas

The reuse of old structures stands as one of the most effective emission reduction strategies in the construction sector. According to the Preservation Green Lab report by the National Trust for Historic Preservation (2011, updated 2016), rehabilitating an existing commercial building avoids between 50% and 75% of the embodied carbon emissions that demolition and equivalent reconstruction would generate. A 4,000 m² office building constructed in the 1960s contains approximately 1,200 tonnes of embodied CO₂ in its reinforced concrete structure; demolishing and rebuilding it would emit an additional 800 to 1,100 tonnes from the manufacture of new materials and transportation of debris, compared with the 200-400 tonnes required by a deep renovation.

Data from the European Environment Agency (2022) indicate that the construction and demolition sector generates 374 million tonnes of waste annually in the EU, representing 37.5% of total waste. The Waste Framework Directive 2008/98/EC, revised in 2018, sets a target of 70% recovery by weight of construction and demolition waste by 2020, a target that only 18 of the 27 Member States achieved. On-site structural reuse eliminates at the source between 60% and 80% of this waste, according to data from the BAMB (Buildings as Material Banks) program, funded by Horizon 2020 with 9.5 million euros, which developed material passports for 300 buildings across 7 European countries.

Reusing an old structure requires a comprehensive structural diagnosis to determine its residual load-bearing capacity and the interventions needed. Non-destructive testing (NDT) techniques include the Schmidt rebound hammer (estimation of concrete surface strength with accuracy of ±15-20%), core extraction of 75-100 mm diameter cylinders for compression testing per EN 12504-1:2019, ground-penetrating radar (GPR) for locating reinforcement and voids at depths up to 600 mm, and infrared thermography for detecting moisture and thermal bridges. The cost of a comprehensive diagnostic survey for a 3,000 m² building ranges from 8,000 to 25,000 euros, according to the Asociación Científico-Técnica del Hormigón Estructural (ACHE, 2022).

Eurocode 8 Part 3 (EN 1998-3:2005) establishes the criteria for seismic assessment of existing structures, defining three performance levels: Damage Limitation (DL), Significant Damage (SD), and Near Collapse (NC). In Spain, the seismic design code NCSE-02 was partially superseded by the National Annex to Eurocode 8 in 2023, which tightens requirements for zones with basic seismic acceleration above 0.04 g (covering 70% of peninsular territory). Finite element models used for assessment simulate the nonlinear behavior of the structure under seismic loads, with pushover analyses requiring computation times of 4 to 48 hours depending on model complexity and the software employed (SAP2000, ETABS, OpenSees).

Reinforcement with carbon fiber reinforced polymers (CFRP) has become the predominant technique for increasing the load-bearing capacity of existing structures. CFRP sheets, with tensile strengths of 2,800 to 4,900 MPa and elastic moduli of 120 to 240 GPa, are bonded to concrete beams and columns using epoxy resins, increasing flexural capacity by 40% to 90% and shear capacity by 30% to 60%, according to data from the FIB Bulletin 14 (2001, updated 2019). The cost of CFRP reinforcement ranges from 120 to 350 euros/m² of reinforced surface, compared with 80-200 euros/m² for reinforced concrete jacketing, but with the advantage of adding thicknesses of only 1-3 mm and weights below 2 kg/m², thereby preserving the original dimensions of structural elements.

The base isolation technique, applied to historic buildings, involves inserting elastomeric or friction pendulum devices between the foundation and the superstructure, decoupling ground motion. The seismic rehabilitation project for the Christchurch Town Hall (New Zealand), completed in 2019 at a cost of 64 million NZD (approximately 36 million euros), installed 41 friction pendulum isolators that reduce seismic forces transmitted to the structure by 80%. In Italy, the Sismabonus program provides fiscal incentives for seismic reinforcement of existing buildings with tax deductions of 70% to 85% of the investment (up to a maximum of 96,000 euros per housing unit), having mobilized over 3.2 billion euros in interventions between 2017 and 2023 according to the Agenzia delle Entrate.

The conversion of the Bankside Power Station in London into the Tate Modern, completed in 2000 by architects Herzog & de Meuron, stands as a global benchmark for structural reuse. The original 1947 steel and brick structure, with a floor area of 34,500 m² and a turbine hall 155 meters long and 35 meters high, was adapted for museum use while preserving 95% of the existing structural volume. The Switch House extension (2016) added 22,000 m² at a cost of 260 million pounds, whereas demolition and equivalent reconstruction was estimated at 450 million pounds, 42% more. The museum receives 5.9 million visitors annually and generates an estimated economic impact of 100 million pounds per year for the Southwark neighborhood.

In Spain, the transformation of the former El Águila brewery in Madrid into the headquarters of the Archivo Regional y Biblioteca Joaquín Leguina (completed in 2002, project by Tuñón y Mansilla) reused the exposed brick wall structure dating from 1914 across 12,000 m² of floor area. Structural reinforcement using 200 mm micropiles and distribution beams allowed adaptation to storage-use loads (imposed loads of 7.5 kN/m²) while preserving the perimeter walls in their entirety at 60 cm thickness. The project received the Spanish Architecture Prize 2003 and the EU Mies van der Rohe Prize 2003. More recently, the rehabilitation of the Docks de Marseille (2015, project by 5+1AA) converted 52,000 m² of 1858 port warehouses into offices and retail, preserving the limestone wall and brick vault structure, at a cost of 175 million euros and an estimated saving of 28,000 tonnes of CO₂ compared with the demolition and new construction alternative.