Premature failure of prestressing steel in prestressed concrete structures is usually induced by corrosion. The most obvious type of failure occurs when high-strength steel fails because, in areas of corrosion pitting, its notched bar tensile strength is exceeded. In such a case, the failure is brittle in character (Section 6.4.1). The effect of local corrosion on the strength and deformation characteristics of a component is related to the type of prestressing steel used. Drawn steel is stronger than hot rolled steel which in turn is stronger than quenched and tempered steel.14 In practice, pure brittle failures in prestressed concrete are rare,3 and are not involved in the cases of spectacular failure discussed in Section 6.5.2. Pitting depths of at least 1 mm are required for notched-bar tensile strength to be lower than the actual stress applied to the steel.
The failure of prestressing steel in structures is predominantly attributable to hydrogen-induced stress corrosion cracking (pages 200±208). Stress corrosion in steel susceptible to hydrogen is promoted by conditions that enable the formation of atomic hydrogen on the steel surface. These conditions include:
· Activation or depassivation of certain regions of the steel surface.
· Conditions that promote pitting corrosion and the formation of shallow pits (e.g., sulfate and chloride attack).
· Presence of certain substances (so-called promoters), which prevent the harmful atomic hydrogen recombining to form harmless molecular hydrogen.
Substances such as sulfur, arsenic, thiocyanate and selenium compounds, which are occasionally found in concrete constructions, are particularly active as promoters.
In the case of prestressed structures, such conditions can occur before the injection of concrete or mortar. It is most important to take suitable preventative measures against corrosion before the tendons are grouted. Conditions that favour corrosion may also appear later, and hence corrosion protection must be permanently maintained for the structure’s lifetime.
Failure during service, appearing after several years or decades of use, is typically a consequence of inadequate protection. Insufficient alkaline protec- tion due to carbonation and/or de-passivation after chloride attack are the major causes of eventual deterioration and even of steel failure.1±3,5±7,43 In the following discussion, the reasons for corrosion of post-tensioning tendons in internally grouted ducts in particular will be reviewed. As noted in Section 6.1, major issues which strongly influence the level of durability achieved are:1±3,5,7,44
· inadequate design (poor construction)
· incorrect execution of planned design (poor workmanship)
· unsuitable mineral composition of building materials
· unsuitable post-tensioning system components, including prestressing steel.
Inadequate design (and poor construction)
Planning errors include mistakes made during design as well as an insufficient assessment of the behaviour of the structure. The following are some examples of faults occurring during the building phase:
· inefficient drainage systems
· missing or inefficient waterproofing systems
· poor construction and joints
· cracking in the concrete.
The drainage system in structures such as bridges or multi-storey car parks must remove water efficiently not only from the surface, but also the water that has passed through the surfacing down to the deck waterproofing system. The design of the drainage path should be such that, if there is a leak or a blockage, water does not find access to the prestressing system. The use of waterproofing systems, e.g. on concrete bridge decks, provides a protective barrier against ingress of salt-containing water, in particular from the bridge surface. In the past, there were often no systems available that could provide a complete seal or which could be guaranteed to remain waterproof throughout the lifetime of the construction. As a consequence, construction joints leaked. Modern high-quality liquid-applied membranes are more effective than earlier systems.
Poorly made construction joints may leak. It is therefore advisable to keep anchorages, e.g. in the deck slabs of bridges, away from construction joints, and to prevent any access for leaks from joints into these sensitive systems. Where the prestressing anchorages are inevitably located at construction joints, care should be taken in design. A high proportion of expansion joints leak and their effectiveness and life span are very dependent on the quality of installation and maintenance. Appropriate drainage paths for leakages should be provided to prevent leaks accessing the prestress anchorages or the bearings and to ensure that the water cannot collect at these points.
Cracking in concrete occurs for a number of reasons. Cracks occur mainly in regions with higher stresses due to loading and higher deformations. Wide cracks significantly increase the risk of corrosion. Crack widths should be limited in accordance with normal design practice. Special care is required to minimise the risk of cracking, particularly in the vicinity of anchorages.
Incorrect execution of planned design
Major mistakes in work execution continue to be:
· failure to inject mortar adequately into ducts around tendons in post- tensioned concrete
· manufacture of an insufficiently protective concrete cover.
Fundamental mistakes made by the construction workforce cannot be excluded completely in any type of construction, but the most effective way to avoid these mistakes is to employ better trained personnel. Poorly executed grouting of post-tensioned members, which can also be associated with the use of unstable cement grouts, is a major reason for corrosion damage where the prestressing steel is exposed to water polluted by chlorides from de-icing salt or at a coastal location. Ungrouted tendons in dry structures located in regions with a dry continental climate have been found with no corrosion at all.3,5
However, poor workmanship can cause wider problems, ranging from poor compaction of concrete and high permeability to cases where the specified level of covering of steel members has not been achieved. Experience has shown that neither the steel sheath nor even a well-compacted grout can form a sufficiently tight barrier, if aggressive water percolates through porous concrete or if the concrete cover is too small and loses its passivating effect due to chloride ingress or carbonation. Although defective grouting of the ducts is a primary condition for the development of corrosion, it is not enough on its own to generate damaging corrosion.
Unsuitable mineral building materials
The systematic use of unsuitable building materials, resulting in significant component failures, has damaged the image of prestressed concrete construction considerably. Particular problems have been caused by high alumina cement and chloride-containing curing accelerators, which, for economic reasons, were used for the manufacture of ceiling cross-sections and pretensioned prefabricated girders.24,26 In the case of high alumina cement, damp heat in building such as stables can lead to increased porosity and carbonation, as well as strong decay of the concrete due to conversion of the cement matrix. The sulfide contained in some high alumina cements used in Germany in the 1950s also functions as a promoter for hydrogen uptake in prestressing steels, leading to hydrogen- induced stress corrosion cracking. A series of ceiling collapses due to corrosion decay (pitting corrosion) and stress corrosion cracking in prestressing steels led to the removal of all components in stable ceilings and above other wet rooms that had been manufactured with high alumina cement or with chloride- containing curing accelerators. In 1958 and 1962 respectively, chloride- containing curing accelerators and high alumina cement were banned from use in reinforced concrete in Germany.
Another problem arose when thiocyanate was added in low concentrations to plasticisers used as additives for concrete.25 Thiocyanate acts similarly to sulfide in encouraging hydrogen-induced stress corrosion cracking. In a typical damage assessment for prestressing steel failures with post-tensioning tendons not yet injected, Table 6.3 shows the presence of chloride, sulfate and thiocyanate found in the concrete where failures in prestressed steel components had occurred. Sulfates and thiocyanates in particular are capable of collecting in water from bleeding concrete. Dissolved in water, they penetrate non-injected ducts, favouring pitting corrosion (sulfate) or hydrogen adsorption (thiocyanate) respectively in prestressing steels.
Unsuitable (sensitive) prestressing steels
The susceptibility of prestressing steels to corrosion attack is of great signific- ance for the durability of prestressed concrete structures. All types of corrosion (e.g., pitting corrosion, hydrogen embrittlement, stress corrosion cracking, corrosion fatigue, fretting corrosion) must be taken into account. A prestressing steel is considered to be susceptible to hydrogen-induced stress corrosion cracking (pages 200±208) where minimal hydrogen quantities stemming from corrosion are sufficient to cause irreversible damage to the prestressing steel. It took years to recognise that some steels were unsuitable due to, among other things, the absence of suitable test procedures to recognise susceptibility to hydrogen-induced stress corrosion cracking at an early stage. The most notorious examples of steels that are not sufficiently resistant to hydrogen are hot-rolled bars with bainitic structure and the old type of quenched and tempered wires3,5,23 (Section 6.5.2).
