Yield Stress in Injection Grouts for Strengthening of Stone Masonry Walls

Stone masonry is a simple and durable constructive technique that was exhaustively used until the mid 20th century. However, stone masonry has particular weaknesses which, associated with the absence of maintenance, increase the vulnerability of its structural integrity. For this reason, masonry walls often need consolidation to improve mechanical performance, such as adhesion between elements and load bearing capacity. Grout injection is a frequently used technique for the consolidation and strengthening of old stone masonry walls. The grout can be seen as suspension of binder particles in aqueous media that has a considerable fluidity in order to be pumped into voids and cracks within the masonry. For this reason, the rheology appears as a very useful tool in the design and quality control of the injection grout. The rheological behavior of hydraulic grouts for masonry consolidation is complex and some rheological properties (such as yield stress) are problematic to determine because of the combined effect of the hydration reactions of the binder and the interactions between the particles, present in the suspension. Despite the relevance of yield stress for injection grouts, no standard protocols are available and, therefore, the yield stress is often determined as an isolated parameter without taking into account phenomena such as thixotropy and hydration. In this study, the determination of yield stress of natural hydraulic lime-based grouts was performed with different measurements techniques using a rotational rheometer. The change in yield stress with time due to hydration was determined. Two yield stress, static and dynamic, and the critical shear rate were also measured, which can be used in the grouts design in order to achieve better grouting operation.