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    Anti-Corrosion Measures For Post-Tensioned Prestressed Steel Strands

    This paper introduces the anti-corrosion measures of post-tensioned prestressed steel strands and the characteristics of related materials and points out the suitable environment for anti-corrosion of various materials.

    Anticorrosion of Post-tensioned Concrete Structures

    The anti-corrosion methods of post-tensioned concrete structures mainly include prefabricated pipeline grouting protection and non-bonding protection.

    1. Non-adhesive Protection

    Unbonded steel strands are specially developed and produced for post-tensioned structures and construction. Coat the common steel strand with anti-corrosion grease, outsource it with high-density polyethylene (polyvinyl chloride is prohibited), and eliminate the need for prefabricated pipes and grouting during construction. 

    Tension-free reinforcement is pre-arranged before concrete pouring, and pre-stressing is performed after curing. Tensioning, due to the sufficient grease in the plastic tube, the relative displacement of the stranded wire and the plastic tube occurs during tensioning, and the tensioning operation is completed.

    The prestressed anchor cables have been in the surrounding rock and soil for a long time under the state of tensile stress, and most of them have to pass through the stratum fracture zone or weak interlayer.

    This kind of corrosion includes electrochemical corrosion, stress corrosion, chemical corrosion, biological corrosion, etc. Therefore, in practical use, the corrosion of unbonded steel strands is almost always caused by improper sealing of the anchoring end. During the service process of the post-tensioned concrete structure, the plastic sheath and anti-corrosion grease separate the steel strand from the concrete and the surrounding medium, which plays a good protective role. 

    2. Grout Protection

    In theory, as long as the mortar composition is controlled, grouting sealing is an ideal anti-corrosion measure for prestressed steel. In the early days, the external cables were mostly protected by grouting in the opaque prefabricated corrugated pipes, while the prestressed tendons were poured into the mortar as a protective layer after tension construction. There are many air bubbles and cavities that cannot be monitored while grouting.

    In order to solve the problem of grouting compaction, Japan’s Anderson Technology Corporation(ATC) invented a transparent corrugated pipe and a new grouting method. During the whole grouting process, the enrichment of the mortar can be clearly observed. According to the data, the problem of difficulty in external cable grouting was solved with transparent corrugated pipes and a new grouting method, which was recognized by the Japanese construction industry.

    2.1 Sheath Composite Protection

    The steel strand protected by the sheath is plated(coated) on the surface of the steel strand with a metal coating or a non-metallic plated(coated) layer and then wrapped with a high-density polyethylene sheath. This product is widely used in the stay cables of cable-stayed bridges and the external cables of concrete box girders. According to the different plating(coated) layers, the sheathed steel strands are divided into PC steel strands, galvanized steel strands, and epoxy resin steel strands.

    2.2 PC Strand System

    The main body of this kind of cable material is made of smooth prestressed steel strands, coated with grease or wax, and then wrapped with HDPE sheath. 

    The advantages of the PC strand system are simple processing and low cost. Steel strands are low-relaxation products with strengths of 1 570, 1 860, 2 000, 2 100 MPa (up to 2 300 MPa abroad), and there is a large room for choice in design. Especially for cable-stayed bridges, when the span is large and the cable is long, choosing high-strength steel strands can reduce the number of strands and the weight of the structure. 

    The disadvantage of the PC strand system is that the strand itself has no anti-corrosion measures, and the anti-corrosion of the system depends on the integrity of the inner coating (grease or wax) and the HDPE sheath. However, under general environmental conditions, the designed corrosion resistance life of the inner coating + HDPE sheath + outer sheath of the whole cable can basically meet the design requirements.

    2.3 Galvanized Steel Strand System

    The early steel strand cables were all made of smooth steel strands, and cement mortar was poured into the casing for anti-corrosion treatment. Now, galvanized steel strands are generally used to process into separate sheathed steel strands, which greatly improves the anti-corrosion performance and makes the tensile strength. The durability of the cable is more reliably guaranteed.

    The main material of the cable is galvanized steel strand, which is coated with grease or paraffin and then covered with an HDPE sheath, and each steel wire is galvanized. The advantage of the galvanized steel strand system is that it has excellent anti-corrosion performance, and the sacrificial anode protection provided by the zinc layer can protect the steel base from corrosion for a long time even if the inner coating and sheath fail. The damage to the anti-corrosion system of the outer layer of the material caused by the material has a better compensation effect.

    2.4 Epoxy Steel Strand System

    This kind of cable material is made by splitting PC steel strands, and each steel wire is electrostatically coated with epoxy resin and melted and solidified to form a dense coating. 

    The advantage of the epoxy resin strand system is its excellent corrosion resistance. Because the epoxy resin is uniformly sprayed on each steel wire and the gap, the isolation between the steel wire and the atmosphere (corrosive medium) is realized, and the stable chemical properties of the epoxy resin provide good physical protection. The disadvantage of the epoxy steel strand system is that the manufacturing process is difficult and expensive.

    Several sheathed strand systems have their own advantages. The PC steel strand system has relatively low cost, mature technology, and low engineering cost; the galvanized steel strand system has a relatively high cost, mature technology, and severe corrosion environments. Obvious advantages; epoxy resin steel strand system has a relatively high cost and has unique advantages in the environment of severe corrosion, especially in the marine climate.

    The effectiveness of the prestressed steel strand anti-corrosion system is related to the durability of the structure. Choosing the appropriate structural form and anti-corrosion form according to the environment in which the project is located can take into account both economy and durability. Prestressed steel strands are also widely used in various large-scale bridge construction projects. If you would like to purchase prestressed steel strands for your company, please contact us.

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