Stability of hydraulic grouts for masonry strengthening

Besides its sufficient penetrability and fluidity, a hydraulic grout in order to be injectable should also exhibit an appropriate stability, i.e. to be able to keep its homogeneity during the injection process, up to its setting. The paper is part of a broader attempt to establish a rational methodology for the design of hydraulic grouts for strengthening of masonry historical buildings, based on their discrete injectability characteristics. The first and second part of this holistic methodology regarding the penetrability and fluidity were published elsewhere. This paper deals with stability, the third basic injectability characteristic. The most prevailing parameters shaping stability characteristics, are water content and percentage of ultrafine materials. After a brief literature survey, an oversimplified predictive model of bleeding is firstly proposed and then its validity is confirmed using the results of an experimental study. The role of superplasticizers is also discussed. In both cases, with and without superplasticizer, semi-empirical formulae are proposed, that may be useful for the design of a grout composition. Finally, the paper presents experimental results demonstrating the role of water and superplasticizer content in the appearance of segregation; some empirical formulae are also proposed for the estimation of the critical water content initiating segregation.