Retarded Bonded PC Strands in Constructions

Retarded bonded prestressing is a new type of prestressing technology following unbonded prestressing and bonded prestressing. It has the characteristics of convenient construction of unbonded prestressing and good mechanical properties of bonded prestressing, and has a wider range of applications.

The slow-bonding prestressing technology is a new type of prestressing technology developed after the unbonded prestressing and bonded prestressing technologies. During the construction stage, the steel strands do not bond with the adhesive, and the construction is as convenient as the unbonded prestressing. After the construction is completed, the slow-bonding adhesive solidifies to make the prestressed steel strands bond with the concrete, thereby achieving the mechanical effect of bonded prestressing.

Japan began to develop slow-bonding prestressed steel strands in the 1980s and successfully applied them in actual projects in the 1990s. my country first began to study slow-bonding prestressing in the early 1990s. The slow-bonding material used slow-bonding mortar and was made on-site by hand-painting and winding. It was not put into mass production and engineering application. At the beginning of the 21st century, my country began to develop slow-bonding prestressed steel strands with epoxy resin as a slow-bonding agent and began to apply them in engineering projects.

The performance advantages of both unbonded and bonded prestressing are achieved through retarding materials, which are the core of retarding prestressing technology. There are two types of retarding materials: retarding mortar and retarding agent. Compared with retarding mortar, retarding agent based on epoxy resin can adjust the tensioning applicable period and curing time by adjusting the proportion of ingredients to meet the needs of the project. At the same time, the prestressed tendon has a small cross-section and weakens the concrete cross-section less. Current research and application are mainly carried out on retarding agents based on epoxy resin. The mechanism of action of retarding prestressing with retarding agent as the medium is to combine the retarding agent with steel strand and outer sheath through extrusion coating process. The early retarding agent is equivalent to the grease of unbonded prestressing. The processing, production and installation construction are similar to those of unbonded prestressing. The tensioning construction is completed within the tensioning applicable period of the retarding agent. As time goes by, the retarding agent is completely cured, and bonding force is generated between the prestressed steel strand and the outer sheath. Through indentation, it is tightly interlocked and bonded with the concrete to achieve the bonding effect and mechanical properties of bonded prestressing.

The slow-setting adhesive is the core of the slow-bonding prestressed steel strand, which is corrosion-resistant and has high strength after curing. The strength after curing is greater than 50Mpa. The tensioning applicable period has various specifications such as 60 days, 90 days, 120 days, and 240 days, and the curing period has various specifications such as 180 days, 270 days, 360 days, and 720 days. The tensioning applicable period and curing period are adjustable according to the characteristics of the project. In general, each project corresponds to a slow-setting adhesive formula based on the construction period and the temperature characteristics of the area. Different construction periods, different temperatures in different areas, and different formulas. During the tensioning applicable period, the slow-bonding prestressed steel strand can be tensioned freely with a small friction coefficient. After the tensioning applicable period, the friction coefficient increases and is not suitable for tensioning. Therefore, the slow-bonding prestressed steel strand must be tensioned within the tensioning applicable period.

The technology of slow-bonding prestressing has undergone many years of research and development, and has been applied in actual projects, accumulating a lot of experience. There are still some aspects that need to be further developed, improved, and innovated:

• (1) Optimization of slow-bonding agent materials, improving the performance of slow-bonding agents by improving materials and formulas to meet the needs of multi-faceted and diversified design and construction.
• (2) Development and application of diversified models of supporting products such as steel strands, anchors, anchor pads, etc., suitable for different projects.
• (3) Based on the research results of slow-bonding prestressing, further improve the industry standards in the fields of engineering design, construction and material production related to it to ensure the orderly and stable development of slow-bonding prestressing technology.
• (4) Further study the mechanical properties of slow-bonding prestressing under complex working conditions, its performance under earthquake action, fatigue performance, etc. The development and application of slow-bonding prestressing technology has broad prospects, but there is still a lot of work to be done, which requires the joint efforts of industry personnel.