# Water Demand (or Specific Surface) of Aggregate as a Dominating Factor for SCC Composition Design
## 1 Introduction  
Self-compacting concrete (SCC) has been known for about 30 years, but its composition design is still a serious technical problem: in the works [1; 2; 3; 4; 5] over 60 methods of composition design of SCC have been collected (till 2018). Due to the application nature of this publication, the literature query is deliberately limited. All examples given below have been deliberately selected from the entire 30-year period of SCC use (also newer ones than those mentioned in Refs. [1; 2; 3; 4; 5]), usually citing 3 characteristic methods illustrating a given phenomenon or feature. Such a wide spectrum of reported design methods indicates, on the one hand, great research interest in this regard. On the other hand, however, it indicates a lack of agreement on the basic rules governing self-compacting ability. As an example, one can give the simplest and probably most obvious distinguishing factor regarding the selection of aggregate composition for SCC: whether the aggregate should be composed assuming its compacted or loose state as the basis. The cited methods are divided in this respect roughly 50:50. Thus, half of them are based on the traditional method transferred directly from normal fluidity concretes (e.g., all methods described in Ref. [2]; modern examples: Refs. [6; 7; 8]). The second half is based on the assumption of a loose state formulated for the first time in Ref. [9]; other examples can be found in Refs. [3; 10]. It should not be forgotten that the universal application of a given technical solution is always the ultimate goal of all scientific research in the field of building industry. That is why there is a fundamental question, unfortunately basically rhetorical: which methods are used by designers of SCC mixtures for industry. In this respect, the author is not aware of any comprehensive source data. In general, in engineering practice, it is easiest to rely on something already known which only needs to be modified in some way--see for instance an interesting attempt to adopt the standard ACI method [11]. Therefore, the author has decided to base his own research on the Polish laboratory method of designing a traditionalconcrete mix according to [PERSON] [12]. The method of determining the aggregate composition described in Ref. [3] was adopted from this method. In the further development of the method, it turned out [13; 14] that the minimum paste content necessary to achieve self-compacting ability (V\({}_{\text{paste min}}\)) depends on the specific surface area determined by the method of [PERSON] et al. [15]. This observation is somewhat \"against the flow\" of the main trends in SCC design, as only four other methods are based on this assumption [9; 15; 16; 17]. The rest of methods are based on:  
* The voids of the aggregate (overfilling or paste/mortar thickness methods), e.g., [17; 18; 19] etc. ;
* The packing of particles (CPM method [20] and its modern derivatives, e.g., [7; 8] etc. );
* Blocking criterion for obtaining good passing ability, e.g., [21; 22] etc. ;
* Nomograms or fixed aggregate content, e.g., [21; 23; 24], the UCI method [2; 5] and its derivatives--e.g., [25]--etc. Therefore, specific surface of aggregate assessment for a SCC composition design is generally underestimated. Neglecting specific surface of aggregate in SCC design is based on the use of super-efficient superplasticizers (SP) based on polycarboxylates. In the case of the occurrence of problems resulting from the greater water demand of the granular phase, typically it is enough to use more admixture. This approach is proposed in almost all cited SCC design methods. It results from the tendency of use of high dosages of fine-grained additions in SCC composition. In such a case, paste phase water demand dominates the water demand of the aggregate. Unfortunately, the approach requires troublesome adjustment of the SP content on the test mixtures and results in obtaining not always satisfactory (i.e., too high) paste content.