Mix-Design and Application of Hydraulic Grouts for Masonry Strengthening

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.

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.

This Chapter deals with grout-mix design issues related to the targeted strength of the masonry to be grouted. Only two parameters enter the discussion: targeted f_wc-value and corresponding required f_gr,c-value. The chapter explores how a range of required f_gr,c-values suffices to be related to a targeted f_wc-value. This loose correlation is due to the fact that grout-to- stone bond properties are shaping the final structural behaviour of grouted masonry. Thus, tensile rather than compressive strength of the grout is the relevant parameter. Besides, dehydration of grout entering the masonry takes place; consequently, some additional rules regarding mix-composition are respectively derived. Finally, several empirical relationships are offered predicting masonry compressive strength before and after grouting. Obviously, among the grout compositions resulting in f_gr,c-values within the required strength range, those mixes will be retained respecting the other required performances. The Chapter ends with a long Appendix presenting detailed strength results (both tensile and compressive) for several grout compositions, described in literature.

This Chapter is related to durability issues. Physical effects are considered first, referring to the consequences of water introduced in masonry by the grouting (freezing and dissolution of soluble phases). Subsequently, chemical effects are considered, such as sulphate reactions, alkali-silica reactions, possible chlorides’ attack and leaching. The chapter ends with a brief presentation of literature results of durability tests, and with a guide for the selection of binders vs durability.

This Chapter refers to some inevitable contradictory requirements of a grout, especially related to ultrafines’ content (penetrability against fluidity), superplasticizer’s content (fluidity against stability), type and content of binders to achieve sufficient bonding with masonry materials, without however imparting to the masonry disproportionally high stiffness (strength against ductility) and without jeopardizing durability (strength against durability) or level of grouting pressure to avoid local rupture of very low masonry strength. Optimization of grout performances is needed in most cases. To this end, a simple first procedure is proposed following a step-by-step selection of mix-design parameters (Sect. 8.2). First, the type of binder is selected, based on strength and durability demands. An appropriate percentage of fines is then selected, based mainly on penetrability criteria, in combination with strength and durability aspects as well. Using experimental results already presented in Chapters 2–6, diagrams combining penetrability, fluidity, stability and strength characteristics versus W/S ratio are traced. Thus, a “usable” range of W/S-values appears; its compatibility with the required grout-strength range will be checked. Normally, several satisfactory solutions are thus found. Otherwise, the necessary correcting measures are discussed in the chapter, for each specific case, mainly by means of increased percentages of fines and addition of superplasticizers. This way, the rationality of the proposed methodology is understood. Thus, the designer is in better position to handle the final design of the grout, by means of a better knowledge of the interplay of the intervening numerous parameters.

In this Chapter, the scientific approach followed in this book finds its justification: The rational and detailed examination of all properties of a grout, offers now the possibility to follow a practical step-by-step procedure of mix-design, permitting to handle numerous parameters in a logical sequence. Thus, this chapter contains more practical guidelines for the mix-design of grouts used in masonry strengthening. The use of Tables and empirical formulae included in previous Chapters, greatly facilitate the selection of (i) the type of the binders and the final grading of the solids, (ii) the minimum acceptable fluidity factor, depending on the finest discontinuities width class (W_nom), (iii) the zero-bleeding and the critical-bleeding (W/S) expressions (with or without superplasticizers), as a function of the calculated average specific surface S_A of the solid phase, and (iv) the limit value W/S against segregation. Subsequently, a practical procedure for the selection of the final (W/S) ratio of the mix is described, respecting all the aforementioned limits. Corresponding remedy-measures are presented in case such a complete respect is not feasible. Moreover, for masonries of minor historical importance and with representative W_nom ≥ 0.25 mm, a Table is offered, containing approximate compositions of grouts for three different required grout strength ranges. Experimental verification of the required grout-performances will be in any case necessary.