Excerpt from the instructional book Self-Compacting Concrete: Materials, Properties and Applications.
3.2.3 Surface Finish of Self-Compacting Concrete (SCC)
Surface-compacting concrete (SCC) is generally used for architectural concrete because the surface finish of SCC is of high quality, often more appealing with sharp edges compared to traditional concrete. The improved surface finish is attributed to the self-leveling and filling capabilities of SCC, which allows concrete to flow smoothly, and thereby fill holes. The surface finish of traditional concrete often has discoloration because of hydration by-products and segregation. Other imperfections such as sand textured areas, honeycombing (aggregate bridging), and some problems caused by mortar loss can also occur (de Schutter et al., 2008). Using SCC can increase the chance of eliminating these surface imperfections. However, a balanced concrete mixture with optimized rheological properties is required to achieve a high-quality surface finish for SCC, i.e. aesthetic appeal for exposed architectural use. Mixtures with lower viscosity, i.e. higher slump flow allow for entrained air to escape more efficiently and thereby provide a better surface finish. The quality of formwork surfaces, type and amount of release agent, as well as production and placement methods also affect the surface finish.
Three main types of surface imperfections of SCC are bugholes, honeycombing and surface cracking. These issues are explained below and some solution to rectify these problems are provided.
Bugholes are small cavities which result from air bubble entrapment between the concrete and formwork, or the trace of bubbles escaping from the free surface of concrete during the hardening stage, whereby the self-leveling property is no longer available. It was discussed previously that the method of placement, formwork and mixture design of concrete is essential to reduce air entrainment. There is a higher chance of air escape while the concrete is still plastic, which can recover its surface. The interrupted delivery of concrete, very long or very short flow length, very high viscosity, the rough internal surface of formwork, inappropriate application or choice of release agent are the main causes of bugholes. Image analysis has been developed for the quantitative evaluation of bugholes on the surface of concrete (Liu and Yang, 2017). Different grades of bugholes on the concrete surface, based on the Concrete Industry Board of American Concrete Institute, are shown in Fig. 3.4.
Imperfections in the internal surface of the formwork can cause severe flaws in the surface finish of SCC compared to traditional concrete whereby these imperfections are less noticeable. In order to achieve a better surface finish, a permeable lining inside the formwork is typically used for escaping air bubbles and for limiting bugholes in the surface finish (Kothandaraman et al., 2016). However, the excellent flow properties of SCC can produce a good surface finish even in the case of using steel formwork with an impermeable surface. Dry, uncoated wooden formwork can excessively absorb water, which may result in staining discoloration or retarding of the surface of the concrete. The surface cleanliness of the formwork and use of an appropriate type and amount of release agent are also critical in order to achieve a better surface finish. Dirty formwork can increase the chance of producing a rough surface finish.
Also, using wax- or oil-based release agents, as well as the application of a thick layer of a release agent, increase the formation and retainment of air bubbles on the surface of concrete, which creates bugholes. A thin layer of water-based release agent, which is applied evenly on the surface of formwork, is highly recommended to achieve the highest quality surface finish. Release agents should fully adhere to the internal surface of formwork. Detachment of the release agent and then blending it with SCC can cause severe reductions in mechanical properties and durability, as well as issues with the surface finish of concrete.
The incompatibility of hydrophobic release agents (oil-based) with water can cause stabilization of air bubbles in concrete after casting, which can potentially increase the air content and porosity of the concrete. The amount of release agent should be adequate in order to easily remove the formwork and to create a perfect surface finish but should be controlled to prevent other issues. The excess amount of release agent can also run to the bottom of the formwork due to gravity (especially for a sprayed release agent) and then blend with the concrete mixture. In this case, air entrapment will be considerably higher because of the high pressure of SCC placement and bubble stabilization of release agents.
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