RWTH Aachen University, Institute of Building Materials Research


Fresh renderings applied to substrate lose water by suction and evaporation and start shrinking, simultaneously hardening and stiffening. The deformation is restrained by a rigid wall and stresses start to develop, often resulting in crack formation. This can be avoided by optimal adaptation of mechanical and physical properties of the render and its background.
Within the scope of a project financed by the Deutsche Forschungsgemeinschaft DFG (German Research Foundation) and currently carried out at the Institute of Building Materials Research (ibac) of RWTH Aachen University, extensive practical tests and numerical simulations of bonded systems (masonry/rendering) are being conducted with the objective to predetermine several influencing variables concerning crack formation in external renderings and to develop recommendations for common crack-safety practice.
Experimental investigations are conducted on several renderings and backgrounds under varying moisture. The contact between the rendering and background is taken into account. To simulate the drying and hardening of the rendering a 3-dimensional Finite Element Model is used. As a result, moisture distribution is defined. Using experimentally measured moisture strain coefficients of the render, the strain distribution field can then be determined. After that, in the mechanical analysis, the resultant stresses within the rendering and at the intersection with the masonry are calculated.
This paper reports on actual results that describe formation and propagation of cracks in drying rendering.

Key words
Masonry, external rendering, crack, strain, stress, numerical simulation