This study analyzes the large-scaled sewers deterioration database published by NILIM, Government of Japan and estimates statistically the defect rates concerning concrete sewers construction. With regard to six kinds of deterioration, axial crack, circumferential one, the rates with the 95％ confidential interval are estimated at 66.19％±11.62％, 18.49％±10.15％, 34.41％±11.32％, which are not small.

 Though newly constructed sewers ought to have no deterioration theoretically, the analysis points out the fact different from it. It is reported in the paper published in 1982 by Isamu KATO that many cracks were brought to concrete sewers surrounded by refilled and tamped sand when steel sheet piles in two rows were pulled up one after another sequentially from the row ends by vibrating hammer under the open cut method construction. It is probable that this phenomenon has deteriorated sewers widely in Japan.

 In this study, it is shown statistically and numerically that sewers contain deterioration to some ratio not only in concrete sewers but also in ceramic ones and PVC ones immediately after construction namely at the sewer age of 0 year.

 First, every span data selected from the national large-scaled sewer deterioration database is classified into one of the four grades ordinarily used in the deterioration inspection in Japan. Second, with regard to every sewer material type, the coefficients of Weibull performance curves, which are the upper boundary of each grade sphere on the graph of sewer age vs accumulating ratio, and the vertical axis intercepts are calculated statistically.

 If the usual curves with no deterioration at the sewer age of 0 year are replaced in the planning of sewer asset management by the proposed ones with some deterioration, it is expected that the future deterioration can be predicted with higher accuracy than before.