Retrofitting and strengthening concrete structures for a sustainable future

In a world increasingly focused on sustainability, the call to reuse and adapt our built environment has never been louder. While tearing down and building anew can seem like a clean slate, there’s a compelling, and often underappreciated, case for retrofitting existing concrete structures. It isn’t just a technical challenge, it's a smart bet on cost, heritage, and the planet.

Prevention over collapse

Across the world, this need is becoming urgent. Ageing bridges and buildings, many designed to outdated standards and pushed far beyond their original service life, now demand reassessment and revitalization. In Europe, North America, and the Asia-Pacific region, tens of thousands of structures are already classified as being in a poor or deficient condition, with many more awaiting detailed investigation. And recent collapses such as Genoa’s Morandi Bridge or the partial failure in Dresden serve as stark reminders of what happens when strengthening comes too late. International codes like ISO 13822, AASHTO’s Bridge Evaluation Manual, and emerging Eurocode guidance are driving a shift: evaluate first, reinforce what matters, and extend the useful life of the structures we rely on every day.

Carola/Elbe bridge partial collapse (Dresden, 11 Sep 2024) and Ponte Morandi (Genoa, Italy, 14 Aug 2018) 

But it doesn’t always have to be about significant structures, sometimes a small detail can carry enormous weight and prevent major problems.

See, for example, a short cantilever modeled in IDEA StatiCa Detail using the existing reinforcement parameters but assessed for new loads according to current standards. In the first image, you can see the reinforcement layout, utilization, and stress in the bars. The structure fails, with the capacity limited by excessively high tensile stresses that the existing reinforcement cannot accommodate.

The next image shows the strengthened version, now using unbonded bars. This almost entirely eliminates tensile stress in the reinforcement and safely carries the ultimate limit state loads, demonstrating how targeted retrofitting can dramatically improve performance even in a seemingly minor element.

We’ll get into IDEA StatiCa Detail and the range of features it offers for strengthening later, but for now, let’s focus on the question that this example raises...

Old concrete, new rules

Concrete structures built decades ago were designed according to very different standards. Material properties were assumed rather than measured. Reinforcement was often smooth instead of ribbed. Anchorage lengths were shorter. Detailing in D-regions was simpler, sometimes too simple. Cracks might have developed gradually over years of service, yet they now have to be considered in a new loading reality.

Smooth bars – picture from Experimental behaviour of anchored smooth rebars in old type reinforced concrete buildings, by Giovanni Fabbrocino, Gerardo M. Verderame, Gaetano Manfredi

All these uncertainties pile up. Engineers often discover that the existing material simply does not fit the assumptions required by today’s codes. Specially when the verification is based mainly on following empirical patterns that were determined through tests and experience, but under completely different conditions! Standard linear analysis loses relevance. And the question suddenly becomes: What is the actual capacity of this existing structural member today, not decades ago?

Retrofitting vs. rebuilding

When comparing strengthening an existing structure to constructing a new one, the answers are increasingly leaning toward retrofit, both financially and environmentally. It’s funny how people focus on substitutability mainly in the context of new materials. That might be nice, but only as long as we’re building on a greenfield site and don’t have to deal with demolition. Honestly, sometimes the most eco-friendly approach is simply reuse.

Demolition and new construction create large amounts of waste, destroy the embodied carbon already invested in the structure, and require the production of new concrete and steel. Retrofitting, on the other hand, preserves what is already there, substantially reducing carbon emissions. 

From a cost perspective, strengthening can often be done faster and cheaper than rebuilding. Existing foundations remain. Structural frames remain. Disruption is lower. In many cases, a well-designed retrofit achieves not only compliance with modern standards but even improved performance and service life.

Bridge strengthening – picture from Bridge Engineering - Selected Issues, by Tomasz Siwowski

IDEA StatiCa Detail

Rather than relying on conservative assumptions or oversimplified models, the Compatible Stress Fields Method helps engineers realistically simulate how stress redistributes. 

The Compatible Stress Fields Method (CSFM) is an extension of the Strut-and-Tie (S&T) and Stress Fields Methods. It is a modern nonlinear method for the analysis of D-regions and elements whose behavior can be simplified to plane stress, i.e., a 2D model. See the following article to understands the basics: CSFM explained

By using IDEA StatiCa Detail, engineers can model and analyze any geometry, regardless of complexity, and obtain results such as stress and strain fields, crack widths, and load paths. This tool supports both ACI and EN standards, making it versatile for various projects. 

Recent advances in IDEA StatiCa Detail make retrofitting much more precise and effective. Two particularly relevant features are:

1. Unbonded tendons: Unlike traditional bonded post-tensioning, unbonded tendons don’t rely on bond with the concrete along their length.

• This is especially useful for strengthening D-regions: for example, vertical tendons in corbels or dapped ends can close cracks, redistribute stresses, and reinforce without invasive bonding.

• When retrofitting very old concrete (already partially creeped), the creep coefficient for the prestress should be adjusted lower to avoid overestimating effects. IDEA StatiCa allows for that nuance.

2. Smooth reinforcement bars: IDEA StatiCa Detail also supports smooth rebars (i.e., un-deformed) in design and analysis.

• Such bars may more closely resemble older reinforcement or provide simpler anchorage details.
• Combining smooth bars with unbonded tendons can make the strengthening of aged precast beams, ribbed slabs, or D-regions more efficient, precise, and economical.

3. Anchorage of steel structure into concrete: During rehabilitation or strengthening projects, it is very common to combine materials by anchoring new steel structures into existing reinforced concrete. With IDEA StatiCa Detail (3D module), these connections can be modeled using realistic material behavior, capturing the nonlinear load transfer between steel and concrete.

Respecting the past

Retrofitting isn’t just an engineering problem, it’s an act of stewardship. Many of the concrete structures standing today are more than just building blocks; they carry historical, social, and cultural value. By strengthening, we preserve their narrative, extend their functional life, and tie sustainability to legacy. In the end, retrofitting is not just about repairing. It is about transforming what already exists into something that can keep serving society for decades to come.

Example for the assessment and retrofitting of an existing building - picture from New European Technical Rules for the Assessment and Retrofitting of Existing Structures, by P. Luechinger, J. Fischer, Christis Z Chrysostomou, Peter Tanner

Many of you know all the benefits, yet still prefer to go for demolition and designing new, and we know why. Such a task is very difficult to grasp, whether due to poor documentation, inaccurate methods, or the effects of time on the structure. Engineering work is often unique and requires a deep understanding of the issue with only a very few examples you can rely on, and that’s where CSFM, (available in IDEA StatiCa Detail) helps.

Conclusion

Retrofitting and strengthening existing concrete structures is not merely a second-best option, it’s a forward-looking choice. With the right tools and analysis, engineers can overcome the technical challenges posed by old reinforcement, cracked regions, and uncertain material behavior. Meanwhile, the others benefit from reduced environmental impact, and the preservation of heritage. As for the saved cost… I’ll leave that to your imagination.