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Diagnosis of reinforced concrete balconies

In recent years, tragedies related to the collapse of reinforced concrete balconies have multiplied. A striking example in Angers in 2016 deeply shocked public opinion and raised legitimate concerns about the safety of our buildings.

Vulnerability of reinforced concrete balconies

Balconies, mostly built with reinforced concrete, are particularly vulnerable to aggressive environmental agents such as chlorides and carbon dioxide, which cause corrosion of their reinforcements. Moreover, mechanical incidents can also occur due to design errors or improper positioning of the steel.

A study by the Quality Construction Agency reveals that in France, there are at least a dozen of these accidents every year, raising questions about the ability of property managers to guarantee their durability. This concern is all the more pressing as our real estate stock is aging rapidly: more than 60% of collective housing units were built over 50 years ago, suggesting an increase in such incidents.

Reinforced concrete balconies collapse
Figure 1 : Examples of collapses of reinforced concrete balconies in recent years (1) Angers in 2016 (2) Wimereux in 2022 (3) Rouen in 2020 (4) Villeneuve-Saint-Georges in 2016

However, these tragedies are not inevitable, and regular inspections allow property managers or social landlords to dispel doubts about the solidity of the balconies under their responsibility, and thus comply with regulations. The methodology implemented by BlueSpine experts allows for a precise diagnosis of the strength of the balconies and recommends a treatment adapted to their longevity.

Pathologies of reinforced concrete balconies

Most reinforced concrete balconies are anchored to the main structure of the building, often as an extension of the interior floor slab. In these cantilevered balconies, the natural bending under their own weight causes tension in the upper part of the slab, where the structural reinforcements are positioned.

Despite calculation and control of deflection, reinforced concrete balconies systematically present natural cracking at their anchorage. This cracking parallel to the support can be accentuated in case of insufficient steel section or improper positioning of the reinforcements in height. This latter case is frequently observed, with bending reinforcements often placed too low in the vertical section of the slab.

Various types of cracks can occur on a reinforced concrete balcony over time: 

  • Cracks appearing on the upper part of the slab and perpendicular to the support, often observed on continuous balconies without expansion joints.

  • Cracks on the concrete guardrails, resulting from concrete shrinkage and thermal expansion variations between parts more or less exposed to the sun.

  • Horizontal cracks at the junction between the slab and the guardrail, due to a lack of stitching reinforcements in the concrete pouring plane.

  • Vertical cracks, regularly distributed, caused by a lack of horizontal reinforcements or excessive spacing between expansion joints.

These cracks promote water infiltration and accelerate the penetration of aggressive environmental agents such as chlorides and carbon dioxide, leading to early initiation of reinforcement corrosion.

Moreover, reinforced concrete balconies are particularly sensitive to the "top bar effect", which alters the quality of the steel-concrete interface and facilitates corrosion propagation. This phenomenon is caused by concrete bleeding, creating a void under the reinforcement, more pronounced for upper bars when multiple layers are superimposed. In cantilevered balconies, the main bending reinforcements, located in the upper part of the slab, are therefore more likely to be affected.

Corrosion propagation inevitably induces structural deterioration of the reinforced concrete balcony. Expansive oxides first generate bursts in the concrete cover. Although these bursts do not immediately represent a danger to the stability of the balcony, they pose a risk of injury to people exposed to their fall.

Moreover, steel corrosion leads to a reduction in reinforcement sections, weakening the load-bearing capacity of the balcony, which can no longer support the originally planned loads. A corroded balcony also becomes more fragile due to the decrease in steel ductility, making it more vulnerable to vibratory solicitations such as earthquakes, exposing occupants to a risk of sudden collapse.

Pathologies of reinforced concrete balconies
Figure 2 : Example of pathologies on reinforced concrete balconies

Diagnosis of reinforced concrete balconies

Cracking, concrete bursts, or the presence of efflorescence under the slab should alert property managers to a real risk of balcony collapse. A diagnosis must then be carried out without delay to determine the origin of the pathology and assess its extent.

This diagnosis is carried out on a representative sample depending on the number of balconies, their typology, or the exposure of the structure. Initially, a detailed visual inspection allows assessing the extent of the damage and defining the scope of repairs as part of a building rehabilitation.

As mentioned earlier, insufficient steel section or improper positioning of upper reinforcements can have a crucial impact on the load-bearing capacity of the balcony. It is therefore essential to accurately measure the position and size of the reinforcements, as well as the concrete cover. Various methods are used for this purpose, including non-destructive methods such as the cover meter and radar, combined with destructive testing.

Non destructive methods of recognition of reinforcement on reinforced concrete
Figure 3 : Recognition of reinforcement of a reinforced concrete element with a non-destructive method (radar)

A verification calculation of the load-bearing capacity is then carried out to ensure the balcony's compliance with its use. It is essential to integrate into this calculation the impact of corrosion on the reduction of steel ductility and section. Based on the calculated load-bearing capacity, BlueSpine experts may recommend possible structural reinforcement.

Structural recognition is indispensable but not sufficient. Indeed, it provides no information on the steel corrosion pathology, which is nevertheless the main cause of their collapse. A specific diagnosis must therefore be systematically carried out to identify the origin and stage of corrosion, which may vary from one balcony to another depending on exposure conditions.


Corrosion diagnosis : Measurement of depth of carbonation on concrete sample
Figure 4 : Measurement of the depth of carbonation on a concrete core using a coloured indicator

Onsite, mapping the steel potential is a reliable method for detecting corrosion activity and differentiating between balconies affected by corrosion and those still intact. Concrete samples are also taken to assess the depth of carbonation and chloride penetration, which are then compared to the minimum reinforcement cover.

In case of advanced corrosion, a curative approach becomes essential to stop its propagation and associated structural damages. With decades of favorable experience, cathodic protection emerges as the most effective treatment to extend the lifespan of our real estate assets. Its early implementation avoids costly repairs later on. This treatment is now regulated by international standards, attesting to its technological maturity. BlueSpine experts have all the necessary certifications to design cathodic protection according to the diagnosis performed.


Faced with the risks of collapse of reinforced concrete balconies, it is crucial to adopt a proactive approach to diagnosis and treatment of pathologies. Past tragedies remind us of the urgency to act to ensure the safety of users and the sustainability of our infrastructure.

The precise diagnosis of these sensitive elements, including on-site measurements and laboratory analyses, represents a fundamental step in detecting structural and corrosion problems. Based on this data, our experts can recommend appropriate solutions, such as structural reinforcement or the implementation of cathodic protection.

Whether you are a condominium association, social landlord, or representative of a community, if you observe signs of deterioration on your balconies, do not hesitate to contact us for a thorough diagnosis. Our competent and certified team will be by your side at every step of your rehabilitation project.


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