Corrosion failure of post-tensioning tendons in alkaline and chloride-free segregated grout: a case study

Abstract

This paper describes a case study of corrosion of post-tensioning strands in segregated grout that led to failure of one of the external tendons of a bridge after less than two years from the construction. Unexpectedly, severe corrosion attacks took place in areas where the ducts were filled by the injection grout, but segregation of the grout led to the presence of an unhardened whitish paste. A detailed inspection of the failed tendon, as well as other tendons removed afterwards, showed that corrosion occurred mainly in the inclined parts of the external post-tensioning tendons, in presence of whitish grout. Analysis on samples of segregated grout showed that it was highly alkaline and had a high content of sulphate ions, but was free of chloride ions, excluding thus the normal causes of corrosion of steel in cement-based materials. Microstructural analysis of failed wires also excluded the failure for hydrogen embrittlement

Assessment of Mechanical Properties of Corroded Prestressing Strands

“…The corroded strands had very similar corrosion shape to the actual corrosion shapes observed from existing Bridge B (Figure 2) in the 0-10% range of section loss. Before the corroded strands were introduced to the test beams, section loss was evaluated by Equations (1)- (3). The RB specimen ID refers to the reference member without corrosion.…”

“…Next, along the strand's length, the depth of the corrosion was measured every 20 mm using a pit-depth gauge, as shown in Figure 5. Excluding the core wire, which could not be evaluated for section loss, the degree of corrosion of each strand was evaluated as the summation of each section loss of the remaining six wires, as given in Equations (1)- (3). The highlighted part in Figure 5 is the section that was determined to be the most corroded (the highest section loss) by comparing the summations of the section losses.…”

Assessment of Mechanical Properties of Corroded Prestressing Strands

“…where is the sectional area of the wire, and is the loss of the sectional area from Equations (1)- (3).…”

“…Uncertain anchorage closure could allow the entrance of water, air, and chloride, and imperfect grout filling may permit the exposure of internal strands to pollutants, causing corrosion, as shown in Figure 1. According to the case study of Carsana et al [3], one bridge with tendon fractures caused by corrosion had enough grout filling but, owing to the segregation of the grout, severe tendon corrosion occurred within 2 years. Chloride was not detected; however, a high sulfate ion content was found on the surface of the grout, and it was assumed to be the cause of the weakening of the grout segregation and strength.…”

Residual Flexural Capacity of Corroded Prestressed Reinforced Concrete Beams

“…Recent collapses of prestressed RC structures due to corrosion put the safety of the elements under question [11][12][13][14][15][21][22][23][24][25]. Examples include the partial collapse of Berlin Congress Hall in 1980 due to the combination of corrosion and stress concentration in the tendons (stress corrosion cracking, SCC), the Ynys-y-Gwas Bridge in the UK, which failed in 1985, and the Saint Stefano Bridge in 1990, which collapsed due to the pitting corrosion of prestressing strands [11,13,22].…”