Mechanism study of effect of coarse aggregate size on permeability of concrete

The chloride penetration and capillary water absorption of concrete with different water-binder ratios (w/b) and different sizes of coarse aggregate were studied as well as the microstructure of the interfacial transition zone (ITZ). The mechanism of the size effect of coarse aggregate on the permeability of concrete was explored. The results show that the negative effect of porous ITZ in concrete with a lower w/b is more significant, while a larger size of the coarse aggregate can decrease the volume fraction of the ITZ and reduce the content of harmful pores in concrete. Based on this study, the impermeability of concrete with coarse aggregates of 0.19 to 1.00 in. (4.75 to 25.0 mm) is optimal. Conversely, the tortuosity effect of aggregate in concrete with a higher w/b becomes prominent so that aggregate that is too large can shorten the permeation path, increase the porosity and macropore content and the trend of the orientation growth of calcium hydroxide (CH) crystals in the ITZ. Under this condition, aggregate with a size of 0.19 to 0.75 in. (4.75 to 19.0 mm) is preferred.