2012 Volume 66 Issue 1 Pages 251-258
The enhancement of structural performance of placing joint in concrete is one of the most critical issues for concrete structure in terms of interfacial bonding strength and toughness. There are well-known purposes of surface treatment, such as a removal of the weak layer and a roughening of the surface for the better adhesion. The authors re-investigated whether or not there are any other reasons in an attempt to achieving high structural performance for the joint.
The authors conducted fracture toughness test with concrete specimens including observations of the placing joint with Electron Probe Microscope Analyzer. The observations made it clear that there is another important reason of making the rough surface, which is the dispersion of the weak layer generated by bleeding from the concrete placed afterwards along the substrate concrete.
The authors proposed new methods of image-analysis；one is observing a weak layer by emphasizing the quantity of Ca and another is revealing a wall effect of aggregate by accumulating the quantity of Si near the interface. These two methods not only helped reveal the mechanisms of formation of the weak layer but also helped determine quantitatively the minimum depth of roughening the surface for the vertical placing joint.
