Load-deformation behaviour of reinforced concrete structures affected by local corrosion

Author: Severin Häfliger
Language: English
external pageDOI: 10.3929/ethz-b-000602335call_made

Abstract

A growing number of ageing structures is affected by pitting corrosion due to the ingress of chlorides or structural defects such as honeycombs. The resulting local damage reduces their load-carrying capacity and – even more pronouncedly – their deformation capacity due to the corresponding strain localisation. The latter is particularly critical for statically indeterminate structures whose structural safety relies on plastic load redistributions or for structures whose main loading is deformation-dependent, such as the earth pressure in the case of retaining walls. In fact, many design rules in current codes are based on the lower bound theorem of the plasticity theory (though often implicitly, e. g. by neglecting initial internal or external restraint stresses), which requires a sufficient deformation capacity as commonly available in uncorroded elements. These rules are, however, no longer applicable to locally corroded structures unless their deformation capacity is carefully assessed. Unfortunately, and despite much research conducted over the past decades, no mechanically consistent, generally applicable assessment strategies in case of local corrosion exist. This thesis addresses this knowledge gap by investigating the influence of local corrosion on the load-deformation behaviour of reinforced concrete structures, focusing on the practical case of corroding cantilever retaining walls.

A comprehensive series of tensile tests on artificially damaged bare reinforcing bars revealed the influences of (i) strain rate, (ii) varying microstructural layers over cross-section, and (iii) the pit geometry on their load-deformation behaviour. Whereas the varying strain rate (i) along the bar axis tends to increase the tensile strength at the corrosion pit, it is potentially reduced for increasing cross-section loss in modern reinforcing bars exhibiting a varying microstructure (ii) over the cross-section, as it is characteristic for quenched and self-tempered (“Tempcore”) reinforcing bars. Depending on the pit geometry (iii), the apparent uniaxial tensile strength and the deformation capacity in the pit and in its vicinity increase due to a triaxial stress state. This effect counteracts strain localisation and leads to a significantly higher deformation capacity of affected bars than assumed by common strain localisation models.

A series of large-scale tests on cantilever retaining wall segments with artificially damaged reinforcing bars confirmed a pronounced influence of the effective corrosion distribution among the reinforcing bars, as anticipated based on a preliminary theoretical analysis: the load-carrying and deformation capacity of structures containing many slightly corroded bars differs significantly from that of structures with only a few but severely corroded bars, even if the total cross-section loss is equal in both cases. Hence, merely indicating the mean cross-section loss is inappropriate to conclude on the load-deformation behaviour of a structure. Two hybrid tests, where the corrosion damage was increased at simultaneously decreasing load simulating the earth pressure, revealed that the deformation increase caused by an increasing cross-section loss is very limited even for considerable damage (approximately 1 mrad rotation for 40% cross-section loss). Deformations might notably increase only very close to failure, which challenges the successful application of monitoring systems relying on deformation measurements.

Finally, a mechanically consistent model enabling the reliable assessment of the structural safety and the load-deformation behaviour of locally corroded reinforced concrete structures was developed: the Corroded Tension Chord Model. In its basic version, this model combines the effects of tension stiffening and strain localisation. Based on the experimental observations, it was enhanced to account for the effects of a triaxial stress state at the corrosion pit, considering axisymmetric damage. The model predictions of the experimental results are very promising, with the comparison indicating an additional softening effect – exceeding that of the triaxial axisymmetric stress state – at the corrosion pit, probably caused by superimposed bending stresses due to unilateral corrosion. The deformation capacity of the specimens was thus clearly less impaired than predicted by established strain localisation models.