Phosphate treatment of TMT rebar bundle to avoid early rusting: An option for single step process

Abstract

Thermo-mechanically treated (TMT) rebars are extensively used for reinforcement of cement concrete. In a large tropical country like India, where transit times are long and moisture levels are high, the rebars get easily rusted leading to a poor appearance. The present paper documents the work done by the authors to alleviate this problem. The work involved phosphate treatment on the surface of TMT rebars in bundle without additional process steps such as degreasing, rinsing and pickling. Rebars were treated in two alternate phosphate solutions one containing nitric acid and another nitric acid free. The phosphate layer obtained on the rebar surface was dense and black in appearances. When treatment was done in phosphate solution containing nitric acid, Zinc phosphate (hopeite) and zinc–iron phosphate (phosphophyllite) compounds formed. On the other hand, zinc phosphate (hopeite) in combination with iron phosphate (ludllamite) compounds formed when surface treatment was done in nitric acid free phosphate solution. The treated rebars were tested for extended exposure in normal atmosphere as well as under high humidity conditions. Also, Salt spray and Tafel study were conducted. In all cases the results were compared with those for untreated bars as control. Surface treatment carried out in nitric acid free phosphate solution showed 3–4 times improved resistance to red rust formation compared to bars treated in phosphate solution containing nitric acid. The bond strength of the rebar with concrete also increased marginally when treated in nitric acid free phosphate solution, whereas it reduced 10–15% for treatment in phosphate solution containing nitric acid.