Multi-scale Evolution Characteristics Of Aging Behavior For External Thermal Insulation Composite System With Thin Rendering

The deformation and failure of the external thermal insulation composite system with thin rendering（ETICS-TR）is a progressive aging process during service,which is closely related to its service environmental factors.Moreover,various damages appear easily,in particular,under the action of continuous heat-rain cycles.At present,the research on aging mainly focuses on the influence of aging on its hygrothermal performance and deformation characteristics.However,the change mechanism of microstructure of the ETICS-TR before and after aging is still unclear,and it is extremely difficult to establish the relationship among the deformation characteristics,microstructure,and hygrothermal performance for the deep exploration of the aging mechanism of the ETICS-TR.In this study,inorganic light aggregates（i.e.,glazed hollow bead（GHB）,expanded perlite）with thin rendering were utilized as the research object.Further,the pore distribution law was analyzed under different moisture transport conditions with the limit pore size proposed to judge the moisture transport performance for the ETICS-TR.Then,a connection link among the pore structure of the insulation layer,protective layer,and macro hygrothermal performance was established.Moreover,the aging mechanism and interaction relationship between the properties of the ETICS-TR at different scales were thoroughly explored.The main steps and outline of this study are as follows:1.The overall and stratified T₂ spectrum signals of the system made of thin-layer inorganic insulation mortar were tested by nuclear magnetic resonance（NMR）technique under vacuum-saturation,free water absorption and vapor adsorption process.Moreover,the water content and pore size distribution characteristics of structural layer of the system were studied under different moisture transport conditions.The results indicated that the pore size of the system test sample was in the range of 0.01 to 1000μm.The pore volume showed a bi-modal distribution with the pore size.The first peak appeared at around 0.1μm and the second peak appeared at around 20μm.The vapor adsorption of the system sample mainly occurred in pores with the size smaller than 0.1μm and free water absorption mainly occurs in pores with the size smaller than 10μm.The smaller the pore volume percentage corresponding to the pores of size smaller than 0.1μm,the less the hygroscopic pores in the material and thus the material’s hygroscopic effect would not be obvious in the humid environment.The larger the pore volume percentage in the range of 0.1 to 10μm,the stronger the free water absorption capacity.2.Aiming at investigating the evolution of pore structure and moisture transport characteristics of external thermal insulation composite system with expanded perlite mortar（ETICS-EP）during the aging process,an artificially accelerated aging test with 160 heat-rain cycles was performed.The pore distribution characteristics of samples after different heat-rain cycles under vacuum saturated water absorption,unidirectional free water absorption,and vapor adsorption equilibrium were evaluated by NMR spectroscopy.Moreover,the fractal dimensions were calculated for characterizing the pore structure,and the influences on moisture transport characteristics during the aging process were concluded.Results showed that the peak diameter of the pores with a diameter smaller than 1μm first decreased and then increased,while that of the pores with a diameter larger than 1μm changed quite slightly.However,the volume first decreased and then increased.The unidirectional water absorption capacity was first enhanced and then weakened during the aging process,while the vapor adsorption capacity varied linearly with the pore volume percent corresponding to the pores smaller than 0.1μm in size.The fractal dimensions of both the vapor adsorption pores and water absorption pores first increased and then decreased;nonetheless,the former had a more exhibited variation range.3.Further,the main objective was to evaluate the thermal deformation damage of the ETICS caused by extreme weather（i.e.,sunlight and rain）,which is closely related to the interaction between insulating layer and substrate wall.The hygrothermal deformation characteristics of the ETICS with GHBs thermal insulation mortar was carried out under heat-rain cycles.Thermometers and strain gages were placed into different wall layers to gather thermal and strain data,and the temperature and deformation of the protective layer,insulation layer,and substrate wall were analyzed.At the same time,the finishing coat crack dimensions were observed using instrument measuring after every 4 cycles,and the deterioration characteristics of the protective layer were quantitatively analyzed based on the number,length,and width of the cracks.The results showed that the finishing coat shrank at early stage（elastic deformation）and then the finishing coat tended to expand and become damaged at later stage（plastic deformation）.The deformation of insulation layer was found to be similar to that of the finishing coat;however,its variation amplitude was smaller.Deformation of substrate expanded with the heat and contracted with the cold due to the small temperature variation.The number and total length of cracks on the finishing coat increased linearly,and the width of the cracks grew faster in the later aging period.4.A non-contact thermal deformation measuring system,combining the heat-rain simulation device of a large-scale weathering test chamber and the digital image correlation（DIC）technique,was proposed for the thermal deformation measurement of finish coat materials of the ETICS under the harsh climate.The digital images of the system were obtained at different aging stages of the ETICS finish coat under the heat-rain cycle through a CMOS industrial digital camera.Both the local deformation field and strain field and the crack initiation and evolution process of the surface for the finish coat of the ETICS were obtained by the DIC method.The results revealed that the phenomena of localized displacement field are very significant especially at the cracks,and there are obvious regional boundaries.The localization of the strain field is highly correlated with the initiation and evolution of cracks,with a significant local distribution band,which may be used for quantitative analysis of the surface aging stage of the protective layer.The crack width of the protective layer shrinks or even closes with the drying time,and the strain changes uniformly along the line.However,the shrinkage during the rain and resting process causes the crack width to increase,and the strain along the line changes suddenly.The range of the crack-affected area increases gradually with the aging;however,the increasing rate decreases gradually.5.Based on the heat-moisture-coupled transfer model with temperature and relative humidity as the continuous driving potential and taking into account the effect of fractal characteristics of the porous material on the conductivity,permeability,and diffusivity;a heat-moisture-coupled transfer model for the ETICS was established and applied to study the distribution of temperature,moisture,and average moisture content in the system subjected to different number of hygrothermal cycles,ie.,20,60,100,130 and 160.From the above-mentioned theoretical analyses and experimental investigations,some conclusions may be obtained:1)during the drying process,the temperature in the protective layer decreased with the increase in the water absorption porosityφ_l during the rain and rest process,while it increased with the reduction in the hygroscopic porosityφ_v;.The temperature of the insulation layer was negatively correlated with the water absorption porosityφ_l.The temperature pattern of the substrate wall was found to be analogous to that of the insulation layer,but lagged and attenuated from the insulation layer.2)during the drying process,the relative humidity of the protective layer increased with the increase of the hygroscopic porosityφ_v,in the initial stage of rain,the relative humidity decreased with the reduction in the diffusion coefficientδ_pof the water vapor,and increased with the increase ofφ_v in the later stage.During the drying process,the relative humidity of the insulation layer increased with the increase ofδ_p,and in the rain stage,it decreased with the decrease of the water permeability D_w.In the rest stage,the relative humidity increased with the increase ofφ_v.In the drying process,the relative humidity of the substrate wall was relatively less influenced by the number of the hygrothermal cycles,but reduced with the passing time in the rain stage.The relative humidity of the substrate wall changes in positive correlation with theδ_pof the insulation layer in the rain stage,until it reaches a stable value in the rest stage.3)the average moisture content of the protective layer increased with the increase ofφ_l,and is correlated to itsφ_v.The average moisture content of the insulation layer changes in positive correlation to itsδ_p,showing no correlation with itsφ_v,φ_l,and D_w.The average moisture content of the substrate wall increases gradually with the drying time with the changing amplitude evidently larger than that in the insulation layer.In the rain stage,the water content absorbed by the substrate wall shows a single-peaked graph of time;in the rest stage,the substrate wall presents certain moisture storage.The research results may provide theoretical support for the material optimization design of the ETICS-TR from a multi-scale perspective,and sufficient reference for the disease assessment and repair of the ETICS-TR in service.