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Mathematical model for the flow of cement grouts in masonry |
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Grout injection for the consolidation of masonry has become a frequently used technique. Some research has been done about the composition of the grouts. Important requirements are: injectability, stability, shrinkage, mechanical performance, adhesion to the old building materials. Studies carried out in France showed the importance of the granularity of the used hydraulic material and how this granularity can be altered using ultra fines such as silica fume to produce an injectable grout. Many case studies are described and a lot of fundamental research is carried out by different research groups in Italy and Greece about the use of cementitious grouts or lime grouts for consolidation works. In many other countries scientists are studying the injection technique. In the USA the Atkinson-Noland Associates, inc. are working in this field. As a part of the research within the field of consolidation injection of ancient masonry, the development of a mathematical model to predict the flow of the grout inside the masonry is my personal subject of research. The penetration of the grout through the masonry is hindered by different properties of the masonry and of the grout. The overall situation of the building and masonry, the degree of weathering, the moisture content, the width of the cracks, the "crack size distribution" are data that can hardly be modified in case of a restoration job. These data can be considered as being input to the model and depending on the values of these parameters a proper grout has to be developed in combination with a suitable injection hole pattern and a suitable injection pressure. Most important are the rheological properties of the grout. Depending on the rheological parameters that are considered one can use different Rheological models: The Newton model ( constant viscosity, no yield value), the Bingham model (constant viscosity, yield value) or Herschel-Bulkley model ( viscosity is depending exponentially on the shear rate, yield value). The truth is even more complicated since all of the above parameters depend strongly on the water content of the grout. Since water is absorbed by the dry masonry, the water content is continuously decreasing. Furthermore a grout is not a pure liquid, but a dispersion of fine particles in water. The rheological behavior of a dispersion is a broad research field on it own. A superplasticizer is helping to keep these particles in suspension and to reduce the initial water content. Often a stabilizing admixture gives the grout time dependent rheological properties (thixotropy). Aim is to incorporate as many as possible from the above interfering parameters to obtain a good prediction of the injection process. Research started in August 1994. A new developed test is able to monitor the stability of the grout in function of time. This test enables to evaluate stabilizing agents, mixing procedures, cement types and compositions with regard to the stability. A permeability test method for isolation materials was adapted to measure the permeability of a masonry structure. Both tests were presented during the RILEM Joint International Workshop on Evaluation and Strengthening of Existing Masonry Structures in Padua 1995. A simple finite difference model is developed that takes into account the relation between water content and viscosity of a grout, the water absorption by the masonry and the permeability of the masonry. In the model the fact that the main resistance for the grouts to penetrate the unsaturated masonry is located at the front is characterized by the "front resistance" Research continues and focusses on the mechanisms that block injection. These mechanisms will then be brought in into the model to predict the penetration depth. Once the model gives reliable predictions it can be used for in situ injection works to decide the process parameters and the composition of the grout depending on the masonry properties. |
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