Novel approaches for monitoring effects of steel corrosion in reinforced concrete

Licentiate thesis, 2023

Corrosion of steel reinforcement is a common deterioration mechanism in reinforced concrete structures.
This deterioration impacts the safety of the structure and may require extensive repair and maintenance work.
There is currently no method available for accurately measuring internal corrosion damage non-destructively.
Therefore, establishing correlations between outer signs of corrosion, such as crack pattern and width, and the corresponding internal corrosion morphology and level is of significant interest.
However, prior research has shown that relying solely on crack width as a performance indicator for corrosion damage is inadequate.
Hence, further research is needed to study the corrosion and cracking processes in reinforced concrete to be able to identify and quantify underlying physical processes and factors.
To contribute to knowledge in the field, this thesis focuses on novel approaches for non-destructive monitoring of the effects of steel corrosion in small-scale reinforced concrete samples.

In this work, time-resolved neutron and X-ray Computed Tomography were applied to link the evolution of material phases to kinematics.
Further, two independent studies, one using neutron and the other using X-ray Computed Tomography, were used to quantify corrosion-induced deformations within concrete.
These deformations were successfully quantified, and the identified locations of concrete cracking correlated well with the observed strain localisation.
Interestingly, the kinematics quantified allowed for the detection of strain localisation in areas where concrete cracks were too small to be visually identified from the image data, indicating the potential for early-stage concrete crack detection.
Additionally, an expression for the average volumetric strain in the compressed corrosion layer was derived based on the evolution of material phases within the sample.

Further, an experimental setup was designed to monitor corrosion-induced deformations adjacent to the steel-concrete interface using an open-ended steel tube instrumented with an optical fibre for distributed strain sensing.
X-ray Computed Tomography allowed for quantitative and qualitative assessment of corrosion level and concrete cracking.
The corrosion-induced deformations in the steel tube were found to be non-uniform, indicating a non-uniform distribution of radial stress around the steel.
This non-uniformity correlated well with the location of the corrosion-induced cracks, with extension hoop strains observed in the steel tube at the location of these cracks, and contraction hoop strains observed in between them.
Corrosion was more severe in bands along the steel, coinciding with the position of the longitudinal cracks.

The research conducted in this work demonstrated the potential of non-destructive monitoring of steel corrosion in reinforced concrete.
For future research aiming at increasing the fundamental understanding of corrosion-induced concrete cracking, it is essential to integrate advanced experimental techniques with numerical modelling.