Improvement of Self-Compactability of Air-Enhanced Self-Compacting Concrete with Fine Entrained Air

The purpose of this study is to clarify the enhancement in self-compactability of self-compacting concrete with entrained air with respect to the size of entrained air bubbles. Air bubbles entrained with a simple mixing method in which all the materials were poured at once was not suitable for enhancement in self-compactability. The authors developed a new mixing method called water-dividing mixing method, which enabled entrained air bubbles to be suitable for enhancement in self-compactability. The water-dividing method with excessive dosage of air entraining agent (AE) entrained high proportion of smaller air bubbles. The larger total surface area of bubbles due to high amount of fine entrained air reduced internal friction of fresh mortar during deformation, resulted in higher level of self-compactability of self-compacting concrete. This usefulness leads to the successful achievement of self-compacting concrete (SCC) with lower cement content with the air content of approximately 10%. The authors have named the concrete “air-enhanced self-compacting concrete (air-SCC). ” 1. Necessity for low cement content with maintaining level of self-compactability and air-Enhancement for lower cement content and higher aggregate content Self-compacting concrete (SCC) was first developed in Japan in 1988 in order to improve durability of concrete structures (Okamura et al. 1993). It is usually wellknown treated as high performance concrete that can flow into every corners of formwork by its own weight without any vibration. However, SCC has not been extensively used because of its high cost due to high unit cement content. According to “JSCE Recommendation for SelfCompacting Concrete” (JSCE 1999), the water to binder ratio (W/B) in conventional SCC is recommended to be in the range of 28-33% by weight in order to ensure that segregation will not occur. Sand to mortar ratio (s/m) is also limited approximately at 45% by volume. The selfcompactability of SCC is very sensitive to the unit volume of coarse aggregate in the mix. Accordingly, the volume of coarse aggregate is recommended as approximately in the range of 30-33%. According to the limitation of materials amount used for SCC mentioned previously, high amount of cement is necessary to achieve SCC with sufficient self-compactability. The unit cement content in SCC is approximately 2 times higher than that of ordinary concrete and aggregate content is lower than that of ordinary concrete, as shown in Fig. 1. Moreover, low-heat cement which is not so common in the market is necessary for SCC if cement is to be used as the only powder material in SCC. This results in high unit cost of SCC. SCC has been mostly used in structures with a confined zone of reinforcing bars, to which vibrating compaction is difficult or impossible. The authors have proposed an Air-enhanced SelfCompacting Concrete (air-SCC) based on a concept in which self-compactability of SCC can be enhanced by entraining air. Specific gravity of air-SCC significantly depends on air content, especially for air-SCC includes air content of 5 to 20%. Self-compactability seems to reduce when the specific gravity reduces due to the reduction of its gravity force (self-weight). On the contrary, air-SCC with air content of 10% exhibited high self-compactability evaluated according to Japanese standard. The suitable quality of entrained air which enables fine aggregate content in the mortar can be increased resulting in a reduction in cement content leadPostdoctral researcher, Department of Infrastructure Systems Engineering, Kochi University of Technology, Kami-city, Kochi Pref. , Japan. *Corresponding author, E-mail: anuwat.atta@gmail.com PhD candidate, Department of Infrastructure Systems Engineering, Kochi University of Technology, Kamicity, Kochi Pref. , Japan. Engineer, Giken Ltd., Tokyo, Japan. Professor, Department of Infrastructure Systems Engineering, Kochi University of Technology, Kamicity, Kochi Pref. , Japan. *Corresponding author, E-mail: ouchi.masahiro@kochi-tech.ac. jp Ce m en t