Analysis and Improvement Measures for Efflorescence of Concrete Columns with Outsourced Steel Pipes

Concrete efflorescence frequently occurs around the exhaust vents of steel-encased concrete columns. The alkali-leached products not only corrode the steel pipes but also significantly reduce the apparent quality. Currently, there is limited research on the prevention and control of efflorescence in steel-concrete composite members in China. This paper proposes a comprehensive measure combining material selection, mix proportion, and construction process, which involves "selecting low-alkali materials, adding mineral admixtures and reducing the water-binder ratio, compacting and polishing the surface", and verifies it through tracking in physical components. Through research, the following measures to reduce efflorescence are proposed: (1) strictly controlling the content of harmful salts such as sulfate and chloride in aggregates, water, and admixtures, selecting low-alkali cement, limiting the alkali content, and ensuring it is below 3.0 kg/m³ for Class II environments; (2) maintaining a water-binder ratio of no more than 0.45, using a water reducer, and recommending a mixing ratio of 0.25%; (3) appropriately adding pozzolanic materials, fly ash, and ground slag; (4) vibrating and compacting the concrete densely, initially leveling the concrete exposed at the top of the steel pipe before initial set, and performing final troweling between initial and final sets, especially at the circular interface between the steel pipe and concrete, to reduce cracking and water ingress after the top concrete cures and shrinks. The above measures significantly reduce efflorescence in the steel structure project of this ceremonial building, with primary efflorescence reduced by 80% and secondary efflorescence almost eliminated. The integrated approach of materials, mix proportions, and processes can significantly inhibit efflorescence in steel-encased concrete columns, providing a feasible solution for the durability and appearance quality control of steel-concrete composite members.