Flowable and Stable Concrete : Design, Characterization and Performance Evaluation

Abstract

Flowable concretes, while possessing remarkable workability properties, are inherently susceptible to sedimentation and segregation, especially under the influence of external stress such as vibration and pumping pressure. This situation is further aggravated by the fact that the concrete production, transportation and casting processes are liable to fluctuations in the quality of the raw materials and the environmental conditions. Consequently, their application in the concrete construction sector is currently very limited. However, when the geometrical complexity, reinforcement density and dimensional enormity of today’s modern structures are considered, there remains no plausible option other than to use flowable concretes. Hence, dealing with the bottlenecks beforehand is of paramount importance for a reliable application of such concretes. Within the scope of this dissertation, three main aspects with regard to flowable and stable concrete are addressed: mix-design, characterization and performance evaluation. The newly developed Water Balance Mix-Design method (WBMD) guarantees not only the flowability and pumpability but also stability (under vibration and pressure) and robustness of concrete. This is achieved through a systematic design strategy which includes optimization of the aggregate compositions to enhance the lattice effect, determination of the minimum paste demand of the aggregates and quantifying the effective water demand of fines compositions by integrating the effects of superplasticizers (SP). The water balanced concretes (WBC) were composed by making use of different paste and aggregate compositions. Moreover, extra water was added to the mixtures in order to evaluate their robustness. The characterization of the fresh concrete properties was carried out using standard and new investigation methods. The flowability was investigated using slump flow tests (with and without tapping). The stability under vibration was evaluated using a modified wash-out test (WT), sedimentation - sieve - test (SST) and visual assessment of the sedimentation behavior on hardened concrete specimen. The pumpability and pump-stability were quantified by means of a pumping resistance simulator (PuReSi) and high pressure filter press (HPFP). Moreover, rheological characterization of the concretes was conducted using a rotational rheometer Viskomat XL while the extracted mortar and paste compositions were tested using Viskomat NT. A high level of shear loading was applied for the rheological investigations to reproduce the structural breakdown process that takes place when concretes are exposed to external stress. Based on the results of the investigations, a detailed analysis is presented with regard to the effects of the different constituent materials and design parameters on the fresh concrete as well as the rheological properties. Moreover, through a systematic assessment of the rheological properties of the subsequent phases of paste, mortar and concrete, a multiscale rheological model is developed for quantifying the structural breakdown process. The rheological studies are also applied for the characterization of the sedimentation behavior during the structural breakdown process and the quantification of the pumpability and pump-stability properties. Furthermore, new performance evaluation criteria are defined for flowable concretes on the basis of the results of the stability, rheological and flowability investigations, especially with regard to the stability properties under vibration and pressure. To this end, the rheological performance criteria as applied to the paste, mortar and concrete phases are integrated with the performance criteria derived from the stability and flowability investigations to produce a multiscale performance evaluation strategy. A combined analysis of the water balance criteria (WB) with the performance evaluation criteria has confirmed the adequacy of the WBMD for designing flowable concretes of reliable stability. Finally, a comprehensive model for flowable and stable concrete comprising micro, meso and macro scales is presented that encompasses the WBMD, the relevant characterization methods and the corresponding performance evaluation criteria.