Excerpt from the December 2012 article from Concrete Plant International by Ludger Lohaus and Karen Fischer
Promising apparatus successfully tested in the laboratory
Facing concrete can be perceived in its complexity only by taking into account the original materials and the entire construction method. Yet, this construction material, concrete, is subjected to varying influences beginning with the composition of its source materials, continuing with processing right up to hardening. Careful planning and vigilant execution of construction work can counteract such influences but they are not always controllable. As a consequence, undesired effects crop up repeatedly. Among these are to be found: surface pore holes, marbling, clouding, ridges, misalignments and coloration deviations which can occur over broad areas, locally, in fill layers or as a result of leaking formwork.
The smooth facing concrete surfaces which currently find approbation (see Fig. 1) are manufactured with non-absorbent formwork skin material which favors certain effects, like pores and coloration differences for example (see Fig.2), in contrast to absorbent formwork material. In addition, it is almost impossible to draw on past experience with stiff to plastic freshly mixed concrete consistencies, since the tendency to slim down construction component geometry and to create curved molds demands very soft to running fresh concrete consistency.
There are still no separate standards available for facing concrete in Germany. The leaflet "Sichtbeton"  (facing concrete) provides the only national recommendations. Testing facilities are necessary for assessing facing concrete construction methods and their further development. For this purpose, at the institute of construction materials at Leibniz University, Hanover, two sets of experimental formwork were developed  and put to the test in the research department . Their benefits in possible fields of application in laboratory and practice are described in the following report.
Experimental formwork for facing concrete
The experimental formwork types developed and tested by the institute of construction materials at Leibniz University, Hanover, are represented in figures 3 and 4. Their objective is to create test conditions in the laboratory which reflect conditions in practice as closely as possible. Differentiation is made between column formwork and wall formwork. For the column formwork, "column" type test specimens measuring HxWxD = 60cm x 20cm x 20cm were produced, and for the wall formwork, "wall" type test specimens with measurements of HxWxD = 150cm x 60cm x 20 cm. The experimental formwork types are basically set up in the same way. The front side consists of a transparent plastic panel for observing the behavior of the concrete while it is poured. The other three sides can be arranged with different formwork skin materials and release agents, and also exhibit various predefined surface defects. Horizontal formwork skin joints which have not been sealed off and a kerf in the formwork board are all to be found on each side. In addition, there are tie bars with sheathing and tie bar cones, and a reinforcement cage with spacers in the specimens.
Experience from research and practice with facing concrete
Facing concrete construction methods produce concrete surfaces whose outer appearance is almost irreparably determined by the entire manufacturing process. Blemishes like pores and coloration variations can only be rectified by removing the surface or applying further layers. For this reason, risks of error must be confronted as far as is possible at both a concrete engineering, organizational level (operations planning, quality management, protection measures, etc.) and a sociological level (identification of employees with their company, creating a team spirit, etc.).
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