2. Penetrability

This Chapter deals with the first component of injectability, i.e., the penetrability of the grains of the grout through the effective (nominal minimum) width W_nom of masonry discontinuities (voids, microcracks, joints). As it is known, grouting is intended to fill voids, fissures and open joints of the masonry as a system, producing a “dendrite” (a three-dimensional skeleton), directly contributing to the strength of the masonry as a whole. However, to do so, the grout should be able to pass through the “narrowest” possible width of such discontinuities, in order to reach the maximum possible internal volume of masonry and open joints, avoiding most of possible blockages. In the specific case of three-leaf masonries, the most decisive result of the grouting is expected to be the strengthening of the bond along the interfaces between the external layers and the infill; the rather small voids, as well as pre-existing fissures along these interfaces, have to be penetrated. In this chapter the penetrability of hydraulic grouts is discussed, and relationships between (i) two characteristic diameters of the grains of the solid phase of the grout and (ii) the nominal minimum (effective) width of fissures and voids of the structure to be injected are proposed. Furthermore, the beneficial role of replacing part of the cement or hydraulic lime with ultrafine materials in order to improve penetrability is presented, and related criteria are proposed. Extensive experimental verifications of the proposed quantitative models are finally offered.