As a result, several process steps (spraying, extrusion, endless fibre reinforcement) can be implemented in an automated process with the spray-extrusion multi-tool nozzle without changing the tool or position of the component. In this publication, reinforcement with endless fibres for better comparability was not used.
2.2. Materials
The material composition bases on a complex of technological restrictions and requirements of the precast concrete element. For example, a high-strength and durable fine-grained concrete with short-fibre reinforcement was developed, so these could be processed using spraying and extrusion. When characterizing the material, the green strength as well as the material and component-related strength and failure behaviour under compressive and flexural stress were of particular interest. In addition to high strength in the use state, the concrete for this process combination of spraying and extrusion also requires application-specific properties in the uncured state, such as high green strength, thixotropy, low shrinkage, adjusted processing time and good bonding behaviour of the individual layers as well as the contact zone between the sprayed and extruded layers.
To evaluate the durability, crack and failure behaviour of the reference components, climate change tests and angle-dependent insert pull-out tests were also carried out. Furthermore, the influence of the recipe and the process parameters on the material characteristics in the fresh and hardened state was examined and described.
Another focus was on the exposed concrete properties of the sprayed concrete shell for facade elements. For this purpose, the air void content in the fresh concrete, the exposed concrete surface according to the DBV/VDZ data sheet in porosity class and deviations from evenness and colour requirements according to DIN 18202 were determined and evaluated [7, 8].
The concrete recipe was determined empirically with constant testing of the consistency and is summarized in table 3. The mixing process was using an Eirich R05T.
Table 3. Concrete mixture