Finite element model of masonry structures on expansive soils

Abstract

The ancient construction of masonry is still a prominent feature in the industry. Masonry is used particularly as a construction material for homes, schools, offices, and industrial buildings. To ensure that masonry structures are safe, they must comply with the Code of Practice for Unreinforced Masonry (SANS 0164-1:1980 as amended in 1986 and 1987), which states the necessary limitations for masonry design work. Therefore, the SANS devised several tests for different material choices. As these tests are mostly performed on physical structures, they are destructive, time-consuming, labour intensive and can be costly. Thus, alternative methods that will provide equivalent information faster, more accurately and less costly will be advantageous. This study was thus undertaken to design a finite element model to simulate the behaviour of masonry wall panels subjected to soil deformation so that rational material choices can be made to improve the susceptibility of cracking in masonry. In Phase 1, the material properties of five different mortar mixes were analysed in a controlled environment at the Central University of Technology Free State laboratory. In Phase 2, a finite element model was designed with the Prokon software program and refined using the mesh refinement method. The performance of different mortar material properties was analysed using the designed finite element model. Three experiments were performed on the five different mortar mixes. Three of the mortar mixes were sourced from the SANS 10164-1, and two from the ASTM C270- 1T. The experiments performed included the Young’s modulus test, determination of the bond strength test, and the application of the deformation test on physical wall panels. After conducting the experimental tests on the five different mortar mixes, an initial finite element model was designed using the finite element method approach based on mesh refinement. The mesh of the initial finite element model was refined to create five different mesh refinement models, each consisting of a different size mesh. The five mesh refinement models were used to design five finite element wall panels. The dimensions of the finite element wall panels were the same throughout, namely 8 × 10 bricks (1 830 × 790 mm). Theoretical parameters were assigned to each of the five different finite element wall panels to determine which wall panel supplied the most consistent results pertaining to the maximum and minimum stress in a wall panel, as well as the overall flow of stress. One finite element wall panel with a specified mesh refinement was selected from the five finite element wall panels for the analysis on the performance of different mortar material properties and standard cement brick properties. Each of the different mortar material properties and standard cement brick material properties was applied to the selected finite element model with specified mesh refinement. A deformation of 1 mm was applied to each finite element wall panel to determine the magnitude of stress found in a finite element wall panel designed using a specified mortar mix and standard cement brick properties. The tensile stress in each finite element wall panel was compared to the results of the three experiments performed. The finite element wall panels showed significant similarity to the experimental wall panels built. The stress distribution through each of the finite element wall panels during the mesh refinement process proceeded to illustrate stress differences among the brick and mortar elements. In the finite element wall panel where the mortar material properties with the least amount of strength was implemented the stress distribution in both the brick and mortar elements was remarkably low. In contrast to this result, the finite element wall panel with the higher mortar material properties of higher strength revealed a higher stress distribution in between the brick and mortar elements. The experimental wall panels that were built showed similar comparisons between the mortar with a high strength and a mortar with a lower strength. The experimental wall panel constructed with the higher strength mortar failed at a lower applied deformation than the experimental wall panel constructed with the mortar with lower compressive strength and higher flexibility. The results of these tests indicated that a mortar with a lower Young’s modulus, combined with a brick of higher strength, showed higher flexibility coefficient and lower stress distribution than a mortar with a higher Young’s modulus. Therefore, the use of a flexible mortar would reduce the stress in the wall panel and assist in masonry flexibility. The designed finite element model could make a significant contribution to the masonry industry in the construction of masonry structures on expansive soils. The finite element model could provide some level of confidence on the effect of masonry constructed on heaving soil, and thus determine the choice of adequate materials in such situations.