What is ECF strand?
Epoxy coated and filled (ECF) strand is a prestressed concrete (PC)* steel stranded cable filled with epoxy resin. The cable is also coated with epoxy resin to enhance its corrosion resistance. Reflecting the recent situation where the service lives of concrete structures degrade due to the corrosion of the reinforcing steel bars or PC steel cables used in the structures, market needs for higher corrosion-resistant PC steel cables are increasing.
To meet the above market needs, Sumitomo (SEI) Steel Wire Corp. has developed a new ECF strand having superior corrosion resistance and durability. The new strand is made by coating the surface of a PC steel stranded cable and at the same time filling the spaces between the element wires of the cable with a durable and ductile resin we developed jointly with a resin manufacturer. Since the 1990s, ECF strands have been used in bridges, slope stabilizers, wind power stations, and many other structures that support our social infrastructure. Our ECF strand lineup consists of a concrete adhesive type that ensures high adhesion with concrete, a standard model having a smooth surface, and a polyethylene (PE)-coating type that is heavily protected from corrosion, allowing the selection of an appropriate type depending on purposes.
Strength comparison between standard concrete and PC reinforced by steel members
What are the features of the newly developed ECF strand?
Standard-size (12.7 mm, 15.2 mm) ECF strands specified in the Design and Construction Guidelines (draft) drawn up by the Japan Society of Civil Engineers have been used mainly in the longitudinal direction after two or more strands are tied up in a bundle. We have recently strengthened our ECF strand lineup by adding larger-diameter strands and a thinner strand in order to expand their use in other parts of bridges.
1. High-tensile strength ECF strand (15.7 mm)
We have developed a new high-strength ECF strand. Its tensile strength per strand exceeds the level specified in JIS Standards currently in force. The new ECF strands have been used to support a bridge in the longitudinal direction and saved labor and resources.
Recently, we have also developed a tension stabilization system that can be used for a single strand. The new system enables the use of ECF strands also as transverse prestressing cables that unify bridge floor slabs and bridge girders into one body.
2. Large-diameter, high-strength ECF strands (17.8 mm, 21.8 mm)
Using our original high-strength wire stranding technique, we have developed large-diameter ECF strands to assure the further high tensile strength of individual strands. Owing to its high tensile strength, the new strand can be used independently without combining with another strands when transversely prestressing floor slabs and bridge girders. Coating epoxy resin over the ECF strand enhances the durability of PC members even at bridge girder joints.
3. Thin ECF strand (9.3 mm)
The thin ECF strand is used in a thin concrete member. Arranging many strands in a concrete member dissipates stress and prevents surface cracking of the member.
What difficulties did you face in developing the new ECF strands?
We had a hard time achieving uniform epoxy resin coating film thickness. When we make ECF strands, we perform in-line measurement of epoxy resin coating film thickness along the entire length of each strand. An advanced technique is required to control the film thickness of epoxy resin coating large-diameter ECF strands, because these strands use larger amounts of epoxy resin than conventional strands. We designed a device that can control the position where epoxy resin should be applied, and put it to practical use to optimize the process conditions of coated strands in response to their diameter. As a result, we now can achieve the same uniformity level of epoxy resin coating film thickness as that of conventional strands over the entire length of the new strands.
Where are ECF strands particularly used?
Our ECF strands enjoy a good reputation particularly in regions where weather affects the durability of structures. In the Tohoku Region, for instance, construction materials are required to be resistant to sea air damage and frost damage and superior in durability. Our ECF strands have been used for bridges that were newly constructed as part of the Tohoku disaster recovery project, contributing to the enhanced durability and extended service lives of concrete structures in the region.