Statistical relationship between mix properties and the interfacial transition zone around embedded rebar

The relationship between concrete mix properties and the properties of the interfacial transition zone (ITZ) formed around embedded rebar was investigated. Multiple samples of various mix compositions and bar orientations were prepared so as to represent common concrete technology. Water-to-cement ratios varied from 0.40 to 0.65 and powder (cement + limestone filler) contents ranged from 362 kg/m3 to 564 kg/m3. Over 1300 BSE images of the steel–concrete interface were taken and analyzed automatically. Statistical methods were used to identify correlations between ITZ properties and mix composition or fresh mix properties.A single large void was identified beneath all horizontal bars regardless of concrete composition. The ITZ around vertical bars was more uniform and extended around the entire rebar. No clear relationship was found between ITZ thickness and mix composition or fresh mix properties for either vertical or horizontal bar orientations. The degree of ITZ variability beneath horizontal bars clearly depends, however, on the bleeding properties of the mix. The distance from steel surface to the closest concrete solid, which influences the chemistry over the surface of the steel, is affected by precipitation of hydration products in horizontal bars, but not by mix composition.