Effect of Water Condensate on Corrosion of Wires in Ungrouted Ducts

In the case of existing prestressed concrete structures, information about the actual state of prestressing is an important basis for determining their load-carrying capacity, as well as remaining service lifetime. This is even more important in the case of existing prestressed concrete bridges, which are exposed to a more aggressive environment than the other prestressed concrete structures. The level of prestressing is affected and reduced by prestress losses at a given time. In calculating the internal forces and stresses, required for the assessment of the Ultimate Limit State and the Serviceability Limit State, it is necessary to know not only the prestressing level but also the cross-sectional area of the prestressing steel (wire, strand or cable), which can change in time due to corrosion. In practice, in the case of the pre-tensioned concrete members, it has often happened in the past that cable ducts have been grouted only partially, or not at all, due to poor grouting technology. Experts did not realize what this could cause in the future—the penetration of water with aggressive agents directly into the cable duct and consequently corrosion of the prestressing steel, which means not increased protection of the steel, but rather acceleration of degradation. On the other hand, in many cases, corrosion also occurs in ducts that are not grouted and no water has entered them. This paper deals with this phenomenon—the formation of corrosion of prestressing steel in cable ducts in ungrouted ducts due to moisture. This problem was investigated experimentally and numerically in the simulation program ESP-r. Experimental measurements and numerical simulations have shown that the water vapor condenses in the cable ducts, which can subsequently cause corrosion of the prestressing steel.

The Influence of Wire Type Indentation on Longitudinal Splitting in Pre-Stressed Concrete

The important characteristic in the creation of longitudinal splitting cracks in pretensioned concrete members has found to be the geometry of the pre-stressing wire indents. Longitudinal splitting along prestressing tendons can result in severe splitting of prestressed member in the field under loading over time. The research evaluated the influence of wire type indentation on the longitudinal splitting in prestressed concrete members fabricated with different concrete mixtures and different compressive strength of concrete. A key objective was to find the best type of wire to avoid failures in the field. A study was conducted at Kansas State University to understand the effect of wire type on the longitudinal splitting between prestressing steel and prestressed concrete. Three different types of wires will be presented in this paper denoted as “WB”, “WF” and “WQ”. The wires have different parameters which include indent depth, indent width, indent sidewall angle, indent pitch and indent volume.

Role of Non-Metallic Inclusions in the Fracture Behavior of Cold Drawn Pearlitic Steel

In this paper an exhaustive scientific work is performed, by means of metallographic and scanning electron microscope (SEM) techniques, of the microstructural defects exhibited by pearlitic steels and their evolution with the manufacturing process by cold drawing, analyzing the consequences of such defects on the isotropic/anisotropic fracture behavior of the different steels. Thus, the objective is the establishment of a relation between the microstructural damage and the fracture behavior of the different steels. To this end, samples were taken from all the intermediate stages of the real cold drawing process, from the initial hot rolled bar (not cold drawn at all) to the heavily drawn final commercial product (prestressing steel wire). Results show the very relevant role of non-metallic inclusions in the fracture behavior of cold drawn pearlitic steels.

Detection, Protection, Repair and Rehabilitation of Corroded Prestressed Concrete Structures

Corrosion is a natural and unavoidable process and its control is a global challenge. The civil engineers of 21st century are facing a major problem for corrosion of prestressed concrete as they maintain an aging infrastructure. It affects various public and private economic sectors including infrastructure, transportation, production, manufacturing and utilities. Corrosion of prestressing steel is much more severe than corrosion of mild steel reinforcement. This is due to higher strength of the prestressing steels, and the high level of stressing in the steel. Usually prestressing steels are stressed about 70%-80% of their ultimate strength which is much lower in mild steel reinforcement. The loss of cross-sectional area of the reinforcing steel due to corrosion is likely lead to tensile failure. The cross-sectional area of prestressing steel is less than mild steel reinforcement due to its higher strength. As a result, the loss of one prestressing strand or bar will have a tremendous effect on the capacity of the member than the loss of an equivalent size mild steel bar. The corrosion of prestressing steel in concrete is an electrochemical reaction that is influenced by various factors including chloride-ion content, pH level, concrete permeability, and availability of moisture to conduct ions within the concrete. Normally steels in concrete are protected from corrosion by a passive film of iron oxides resulting from the alkaline environment of the concrete. For the corrosion process to be initiated, the passive oxide film on the prestressing steel must be destroyed. Passivation of the steel may be destroyed by the carbonation or by the presence of the chloride ions. In Canada, one of the reasons of this problem is due to the huge amount of deicing chemicals to combat the cold climate. Once corrosion occurs, the corrosion products occupy up to six times as much volume as steel, leading to cracking and disruption of the concrete. The ACI limit on chloride in prestressed concrete members is half of that for conventionally reinforced concrete. Prestressing steel is also more inclined to other forms of corrosion related deterioration that do not occur in mild steel reinforcement. These forms are stress corrosion cracking, hydrogen embrittlement, fretting fatigue and corrosion fatigue. These types of deterioration are very difficult to detect, and can lead to brittle failure with little or no sign of warning. This report presents the mechanisms, causes and effects of corrosion in North American design and construction and the proper detection and protection systems.

Detection, Protection, Repair and Rehabilitation of Corroded Prestressed Concrete Structures

Corrosion is a natural and unavoidable process and its control is a global challenge. The civil engineers of 21st century are facing a major problem for corrosion of prestressed concrete as they maintain an aging infrastructure. It affects various public and private economic sectors including infrastructure, transportation, production, manufacturing and utilities. Corrosion of prestressing steel is much more severe than corrosion of mild steel reinforcement. This is due to higher strength of the prestressing steels, and the high level of stressing in the steel. Usually prestressing steels are stressed about 70%-80% of their ultimate strength which is much lower in mild steel reinforcement. The loss of cross-sectional area of the reinforcing steel due to corrosion is likely lead to tensile failure. The cross-sectional area of prestressing steel is less than mild steel reinforcement due to its higher strength. As a result, the loss of one prestressing strand or bar will have a tremendous effect on the capacity of the member than the loss of an equivalent size mild steel bar. The corrosion of prestressing steel in concrete is an electrochemical reaction that is influenced by various factors including chloride-ion content, pH level, concrete permeability, and availability of moisture to conduct ions within the concrete. Normally steels in concrete are protected from corrosion by a passive film of iron oxides resulting from the alkaline environment of the concrete. For the corrosion process to be initiated, the passive oxide film on the prestressing steel must be destroyed. Passivation of the steel may be destroyed by the carbonation or by the presence of the chloride ions. In Canada, one of the reasons of this problem is due to the huge amount of deicing chemicals to combat the cold climate. Once corrosion occurs, the corrosion products occupy up to six times as much volume as steel, leading to cracking and disruption of the concrete. The ACI limit on chloride in prestressed concrete members is half of that for conventionally reinforced concrete. Prestressing steel is also more inclined to other forms of corrosion related deterioration that do not occur in mild steel reinforcement. These forms are stress corrosion cracking, hydrogen embrittlement, fretting fatigue and corrosion fatigue. These types of deterioration are very difficult to detect, and can lead to brittle failure with little or no sign of warning. This report presents the mechanisms, causes and effects of corrosion in North American design and construction and the proper detection and protection systems.

Experimental and Numerical Investigations on Flexural Behaviour of Prestressed Textile Reinforced Concrete Slabs

Nowadays, concrete is mostly prestressed with steel. But the application of prestressing steel is restricted in a highly corrosive environment area due to corrosion of prestressing steel, leading to a reduction in strength and may cause sudden failure. Carbon textile is considered an alternate material due to its corrosive resistance property, high tensile strength, and perfectly elastic. Prestressing is also the only realistic way to utilize fully ultra-high tensile strength in carbon textile material. In this study, experimental and numerical analyses were carried out for the flexural behaviour of prestressed and non-prestressed carbon textile reinforced concrete slabs. This study also focuses on the influences of textile reinforcement ratios, prestressing grades on the flexural behaviour of carbon textile reinforced concrete (TRC). Fifteen precast TRC slabs were tested, of which six were prestressed to various levels with carbon textile. The obtained results show that prestressing textile reinforcement results in a higher load-bearing capacity, stiffness, and crack resistance for TRC slabs. The first-crack load of the prestressed specimens increased by about 85% compared with those of non-prestressed slabs. Three-dimensional finite element models were developed to provide a reliable estimation of global and local response. The modeling techniques accurately reproduced the experimental behaviour. Doi: 10.28991/cej-2021-03091712 Full Text: PDF

Initiation of stress corrosion cracking in cold-drawn prestressing steel in hardened cement mortar exposed to chlorides

Cold-drawn, high strength, prestressing (PS) steel strands are widely used in pretensioned concrete (PTC) structures. This paper discusses the stress corrosion cracking (SCC) of PS steel embedded in cement mortar and gradually exposed to chlorides. Various stages of the passive to active (P-to-A) transition, which marks the onset of SCC, were investigated using EIS technique. The key mechanisms were identified and confirmed using SEM/EDAX, XRD and Confocal Raman Spectroscopy. It was found that the passive film on unstressed PS steel has better electrochemical characteristics than that on conventional steel rebars. However, the residual tensile stress at the surface of PS steels can assist passive film cracking after chloride attack - contrary to the pitting corrosion without cracking of passive film in conventional steels. Further, tests indicated that the concentration of chlorides required to crack the passive film in PS steels can reduce by about 50% when prestressed – as in field structures. Chemical composition, stress state and microstructural features at the PS steel surface were identified as possible factors influencing the initiation of SCC in PTC structures.