Reinforcement corrosion research

Northern Concrete Repair Afternoon.

The Netherlands faces a huge task when it comes to maintenance of concrete structures. By far the largest part of the maintenance problems on concrete is caused by corrosion of the reinforcement. Floor Thijssen, Structural Engineer at Solid Services, is currently completing her Master's degree at TU Eindhoven on the subject of corrosion, in particular chloride-initiated reinforcement corrosion. This topic has made headlines several times in recent decades due to incidents involving buildings. Thijssen will also give a presentation at the Northern Concrete Repair Afternoon.

On May 23, 2011, a gallery floor of the Antillenflat in Leeuwarden collapsed. In its fall, the fifth-floor gallery floor also took the galleries below with it. The subsequent investigation revealed that the gallery collapsed due to a combination of factors: increased permanent load, under-positioning of reinforcing bars and chloride-initiated reinforcement corrosion. Chloride-initiated reinforcement corrosion can be caused by the chlorides from de-icing salts. If cracks develop, they can penetrate relatively easily. As a result of this incident, the CUR 248 recommendation was issued and the cantilevered galleries of many pre-1975 apartments were examined and strengthened where necessary. Another type of concrete structure that is highly exposed to chlorides from de-icing salts are parking garages. However, less is known about the influence of chloride-initiated reinforcement corrosion on the structural safety and durability of this type of structure. 

Salts

Thijssen was already interested in existing construction during her undergraduate studies and did her master's internship at Solid Services, where she is now employed. She was surprised during her master's degree in Structural Engineering and Design that it was heavily focused on new construction. "While existing construction is just as interesting and actually needs much more attention. Fortunately, that has been improving in recent years." In her final assignment for her master's, Thijssen is specifically investigating chloride-initiated reinforcement corrosion in parking garages. "In this we distinguish, on the one hand, corrosion as a result of mixed chlorides, which we know, among other things, from the infamous Kwaaitaal and Manta floors in which calcium chloride was added as a hardening accelerator. On the other hand, we distinguish de-icing salts, sea water or chloride-containing air (coastal areas) as external influences that can easily penetrate through cracks and affect the reinforcement. Nowadays we know much better what the consequences are of the ingress or penetration of chloride into reinforced concrete structures and the requirements regarding concrete quality, concrete coverage and crack width control are better adapted to this." 

Pit formation

Whether structural safety is compromised by corrosion is then the next question. "Typical of chloride-initiated reinforcement corrosion is pitting. This results in very locally large surface decreases in the steel. In addition to a decrease in the capacity of the structure, this local pitting results in a decrease in the deformation capacity. This reduced deformation capacity can lead to reduced warning behavior when the structure fails. The extent to which pitting endangers the structural safety of a structure depends on many factors: the area decrease due to the pitting, the location of the pitting, any residual capacity of the structure, et cetera."  

Continued

In the event that a structure may be affected by chloride-initiated reinforcement corrosion, several investigations are possible to determine if the reinforcement is affected.

Thijssen: "Due to the localized nature of pitting corrosion, mere destructive testing often does not give an accurate picture of the extent of the damage and specialist investigation is required. To repair the structure, cathodic protection can be applied to stop the corrosion process. If the structural capacity of the structure is significantly reduced, the structure will be strengthened. Traditionally, this can be done by adding reinforcement. In addition, it is possible to reinforce the structure with alternatives such as carbon reinforcement."

"Various methods are available to investigate the extent of reinforcement corrosion and for repairing damaged structures. However, it is important to be able to make an informed statement about the structural safety of existing structures and for more scientific research to be done on the capacity and failure behavior of structures damaged by reinforcement corrosion. In any case, let's give maintenance the attention it deserves."