Tracing hail stone impact on external thermal insulation composite systems (ETICS) – An evaluation of standard admission impact tests by means of high-speed-camera recordings

Hail impact damage on External Thermal Insulation Systems (ETICS) is increasingly recognised by insurance companies owing to increased storm occurrence frequency and storm intensity as well as more widespread installations of ETICS. In order to develop hail resistant ETICS for houses and to evaluate existing admission tests, high-speed-camera recordings of ice ball impacts at an angle of 45° and steel ball impacts at angles of 90° and 45° were used to characterize the impact process and to derivate the damage mode of hail impacts on facades. The major differences in the impact process of the European steel ball impact test (90°, ETAG 004) and the Swiss ice ball impact test (45°, VKF P. No. 8) are identified to be (1) a 20-40 % higher maximum indentation depth in the case of the 90° steel ball tests leading to more damage, (2) a shorter impact duration caused by the higher impact speed of the ice balls resulting in a higher strain rate and (3) the shattering of the ice ball at impact energies exceeding 6 Joule. Considerable surface parallel shear movements of the ball are observed for 45° impacts. Resulting shifts in the impact stress field cause the formation of an elongated damage pattern. The rebound of the impactor, an indicator for the elasticity of the system, is found to be 10 % higher in the 45° setups compared to the 90° setup. High strain caused by deep indentation depths is identified as the main reason for damage . First sub-surface fractures already occur shortly (tenth of milliseconds) after impact. In contrast, visible surface fractures form later during the impact processes at average indentation depths of 3-4 mm, i.e. at a time when strain localizes at the depression shoulders. Hence to avoid the observed brittle failure behaviour, the development of flexible materials with the ability to elastically accommodate impact strains is favourable to reduce hail stone impact damage. The European steel ball test is suitable to evaluate the hail resistance of materials in laboratory studies; however, ice ball tests provide more realistic conditions (impactor material, impact speed) and are therefore advisable for final admission of ETICS regarding hail resistance on-site

The microstructural evolution of cementitious, flexible waterproofing membranes during deformation with special focus on the role of crazing

The initiation and evolution of deformation-induced (micro)structures in one-component, cementitious, flexible waterproofing membranes is investigated combining normed crack bridging (EN14891) with image analysis of in-situ photos acquired by optical and scanning electron microscopy. The permanent deformation of the polymer matrix concentrates in a trapezoidal deformation volume and subdivides into an active and a passive part. During the active part, the polymer matrix stretches and fibril-void microstructures (FVM) form. The ubiquitously present heterogeneities (quartz grains, cement particles and air pores) act as stress concentrators. After FVMs reach their maximum density inside the deformation volume, straining by passive stretching until final rupturing takes over. Starting at the substrate-membrane interface, the cracking propagates through the membrane along aforementioned heterogeneities and the spatially distributed FVMs. Linking mechanical behaviour and deformation structures is therefore crucial to (i) understand the complex elasto-plastic deformation and to (ii) develop new products increasing the durability of the protective system

Influences of temperature and opening rate of substrate cracks on the mechanical behaviour, crack–bridging ability and deformation mechanisms of one–component, cementitious, flexible waterproofing membranes

One–component, cementitious, flexible waterproofing membranes bridge cracks because their polymer-dominated matrix takes up the deformation. The sensitivity of the deformation of polymers to temperature and strain rate raises interest to the influence of these two parameters on the crack–bridging ability of the waterproofing membranes. Temperature–controlled crack–bridging and dog-bone experiments with in–situ monitoring of the sample's surfaces show a decrease in the crack–bridging ability (CBA) of the waterproofing membranes and a decrease in plastic deformation at testing temperatures above the glass transition temperature (Tg) of the used polymer. At temperatures below the Tg, the membranes deform in a brittle manner. Rate–controlled crack–- bridging and dog-bone tests with in–situ monitoring of the sample's surfaces reveal a decrease in crack–bridging ability and plastic deformation with decreasing displacement rate. Hence, temperature and displacement rate are crucial parameters governing the crack-bridging performance of waterproofing membranes

Combined Methods to Investigate the Crack-Bridging Ability of Waterproofing Membranes

Polymer modification of dry mortars with redispersible polymer powders is a common approach to increase the mortar’s mechanical properties. The use of polymer-modified mortars (PMMs) as waterproofing membranes requires a well performing crack-bridging ability (CBA) to avoid infiltration of water into the construction owing to shrinkage or expansion. In order to evaluate the performance of PMMs, we present below a combination of crack bridging tests with optical analysis. Latter is exerted with digital image analysis on photos acquired during the tensile tests and pictures of CBA-samples, which have been fixated after a certain displacement. Additionally, thin-sections of fixated CBA-samples are investigated using transmitted light-, UV- and SEM-microscopy. These methods are developed to understand the (micro)mechanical initiation and evolution of cracks in order to improve the crack-bridging ability of waterproofing membranes

Evidences for Strain Localisation on Surfaces of Waterproofing Membranes - A link between Structures and Mechanics

Polymer-modified mortars are commonly used as waterproofing membranes to prevent crack propagation between a substrate and a tiling. In this study, a cementitious mortar modified with redispersible polymer powder is investigated in two crack bridging testing series. Testing the influence of the polymer content on load-displacement behaviour, reveals a positive correlation between the displacement the membrane can bridge and increasing polymer content. Contrarily, the maximum load only increases with polymer content until the polymer is the volumetrically dominant phase in the membrane. Above this threshold, the maximum load remains constant despite increasing polymer content. The second experimental series investigates the evolution of the strain localisation behaviour on the surface of the waterproofing membrane for a formulation with constant polymer content and relates it to the load-displacement behaviour. Strain localisation starts to appear as dispersed spots on the surface and correlates to a strain hardening behaviour. With increasing deformation, the incipient strain localisation spots grow, interconnect and eventually forming a continuous zone, in which further strain is accumulated. The mechanical evolution of this continuous zone can be correlated to the change from strain hardening to strain softening process of the membrane

Skin and crust formation at mortar surfaces-mechanisms and influencing factors (Part 2)

Cementitious mortar layers have a high surface-volume ratio and their evolution is influenced by intrinsic formulation parameters and extrinsic environmental parameters. The latter often cause an early drying of the mortar surface while deeper mortar parts are still wet and soft. This dry surface is called skin and crust, respectively, when thinner than 0.1 mm and having a thickness of ∼0.5 mm. Investigation by a newly developed micro-rheology method allows to follow the buildup of a crust during the first hour after mortar application. The crust is growing in thickness and reaches a critical strength between 10 and 15 mN/mm2. When stronger, the crust will not break anymore during embedding of the tile. As a result, the tile is not properly wetted by fresh mortar and adhesion strength is below 0.5 MPa. According to EN1346 the tile adhesive has lost in this case its open time performance. Laser-scanning microscopy of hardened mortars containing cellulose ether, polyvinyl alcohol and redispersible polymer powders, which were stained by fluorecein-5-isothiocyanate isomer I, indicate that these organic additives can be enriched in a thin skin. Our observations reveal that additive enrichment as a mechanism to build up a 0.5 - 1.0 mm thick crust can be excluded. Conclusively, crusting is mainly driven by a drying front which first affects the surface and then retreats into the mortar. The study reveals a second mechanism which is limiting open time performance. Instead of forming a crust, a mortar rib can thicken uniformly across its entire volume. But when the mortar becomes stronger than 20 mN/mm2, then the tile embedding pressure is too low to press the combed mortar ribs completely and hollows remain in between the ribs