5. Guidelines for the Estimation of W_nom

This Chapter describes the categories of possible internal discontinuities of masonry; it is because of such discontinuities (pores, local interface detachments, local sliding’s, cracks) that masonry strength may be reduced. The filling of these discontinuities by means of an appropriate grout may increase masonry strength, provided that the grout was able to penetrate the body of masonry, to reach most of these discontinuities and flow along each of them. In order to decide the necessary “penetrability” capacity of the grout, a rough evaluation of a critical value “W_nom” of the opening of these discontinuities of masonry, is needed. This chapter examines several possibilities of quantification of such a representative opening value for several categories of masonry. Finally, an easy to apply practical approach of “opening classes” is proposed, and relevant examples are given. Thus, for each specific case, the selection of grout solids’ grading is facilitated, in order to satisfy penetrability requirements established in Chap. 2.