Investigation Of Robustness And Shrinkage Of Self-compacting Concrete

During recent decades of years,Self-Compacting Concrete?SCC?,also known as the?Self-Consolidating Concrete?,is reported to be one of the most important inventions and applications in construction industry.Comparing to traditional vibrating concrete?VC?,SCC presents excellent performances of flowability,passingability and segregation resistance.Meanwhile,the unique of vibration-free significantly reduces construction noise and saves labor,making it gradually become an irreplaceable type of high performance concrete.However,due to the complexity of its components,there are inevitable fluctuations of component content in practice,possibly resulting in dramatically variations of fresh properties and leading to the deterioration of hardened properties and durability of structures.In addition,the use of large quantity of binder materials also increases the volumetric deformation and the risk of cracking of hardened SCC.This study has focused on the robustness and volumetric stability problems of SCC mentioned above which refers to the following aspects:the investigation of rheological properties of cement-based materials at different scales,the analysis of key factors affecting the robustness of SCC,the establishment of predicting model for robustness,the optimum design of low-binder SCC and its robustness analysis,as well as the shrinkage of cement-based materials and the mechanism of Shrinkage-reducing admixture?SRA?,etc.The main research details and conclusions of this study are as follows:?1?Multi-scale study of rheological properties of SCCThe influence of variations of component content on the yield stress of SCC was systematically studied at the scales of cement paste,mortar and concrete,respectively.At the scale of cement paste,the isothermal adsorption behavior of polymer-based high range water-reducing agent?HRWRA?on different cement particles,as well as the variation of yield stress of cement pastes with different cement content and HRWRA dosages were studied;at the scales of mortar and concrete,the effect of sand volume content on the yield stress of mortar and the effects of aggregate volume content and proportion of coarse and fine aggregates on the yield stress of concrete were studied,respectively.It is shown that the yield stress of cement paste is inversely proportional to the mean particle size and the surface to surface separation distance of cement particles,and increases as the increase of cement volume fraction in cement pastes.Furthermore,the yield stress of mortar and concrete increases as the increase of the relative volume fraction of aggregates contained therein,and as the relative volume fraction approaches the critical value of 0.80,the yield stress of the mixture starts to vary by orders of magnitude.?2?Establishment of multi-scale model of SCC robustnessBased on the multi-scale study of rheological properties of SCC,the potential factors that affect the robustness of SCC from the yield stress's point of view are summarized as follows:the ratio of the actual dosage of HRWRA to its saturated dosage??,the volume fraction of cement particles in cement pastes(_?8?and the relative volume fraction of aggregates(_?6?.Accordingly,the prediction model of SCC robustness was established.Based on the combined model,we conclude that among the variations of component properties,the variations of water and aggregate volume fractions in the SCC mixtures seem at present to be the critical factors that restricting the robustness of SCC in the concrete plants.The results of this conclusion are schematically shown by the proposed SCC robustness index as a function of water to cement ratio and either matric volume fraction or relative aggregate volume fraction.?3?Optimum design of low-binder SCC and its robustnessThe optimum design of low-binder SCC based on particle packing theories was investigated.It is first demonstrated that a multi-scale approach can be considered and the mortar with particle sizes smaller 0.60 mm can be regarded as the matrix of the concrete mixtures.It is shown that in order to design a low-binder SCC,it is necessary to consider the thresholds of the inclusion volume fraction,the relative matrix thickness and the yield stress of the matrix that respectively relating to flowability,passing ability and stability of concrete mixtures.For the mechanical properties,it is shown that the strength of concrete decreases with the increase of the total surface area of aggregates,hence a lower sand volume fraction is suggested to achieve higher mechanical properties as long as the fresh properties are fulfilled.Finally,in order to simplify the mix design of the low-binder SCC,recommendations of the values of distribution modulus in the optimum packing curves are proposed for both the aggregate mixtures and the total particle mixtures?including powder?in SCC,respectively.In terms of robustness,both fresh and hardened properties of a low-binder SCC were studied in the cases of the quantity variations of components.The results show that the robustness of the hardened properties is higher than that of the fresh-related properties.Meanwhile,a new robustness assessment method based on the concepts of linear algebra?using norm analysis of vectors?was proposed in order to define and evaluate the robustness of SCC.It is shown that the robustness of SCC can be described and evaluated in a more intuitive way by the proposed method.Additionally,a good consistency was found between the proposed method and a previous robustness evaluation method.?4?Investigation of the deformation properties of SCCThe shrinkage-reducing effects of a traditional polyether-type shrinkage-reducing admixture?SRA?and a novel polymer-type SRA with water-reducing function on cement paste and SCC were experimentally studied.It is shown that the addition of both types of SRA can visibly reduce the autogenous shrinkage and drying shrinkage of cementitious materials.For the polyether-type SRA,it is concluded that the combined effect of decrease of capillary pressure and increase of expansion stress at early age results in a significant reduction of shrinkage development of the hardened mixtures,while for the polymer-type SRA,it is suggested that the delay and regulation effects on cement hydration results in a higher internal relative humidity of mixtures thus plays an inhibitory effect on shrinkage deformation of the system.Furthermore,the addition of polyether-type SRA shows no barrier on the fresh and mechanical properties of cementitious materials while the addition of novel polymer-type SRA obviously cuts down the requirement of HRWRA to fulfill certain flowability and slightly increases the air content and decreases the strength of the hardened mixtures.