Shrinkage Porperty Of Recycled Concrete Based On Hydration Characteristics

The environment issues such as sand and stone resource shortage,waste concrete excess which have been increasingly acute.The efficient use of waste concrete is an urgent need of sustainable development and environmental protection.The application of recycled concrete has become an important development direction in the reclamation of building rubbish.Because of containing adhered mortar,the water absorption,voidage and crushing index of recycled aggregate are higher than normal aggregate.Meanwhile,recycled aggregate also has the characteristic of water absorption and releasing,which may lead to significant difference in shrinkage porperty of recycled concrete in comparison to that of normal concrete.In this paper,the hydration characteristics,mechanical properties,autogenous shrinkage and drying shrinkage of recycled aggregate concrete were studied.The main tasks are summarized as follows:1?The hydration characteristics experiments of cement paste were carried out to reveal the effect of fly ash,slag and recycled powders on the hydration characteristics of cement paste.Based on the the test results,the comprehensive effect coefficient of recycled powders "?" is proposed.The formula to predict the cement hydration degree of complex binders is established by analyzing the effects of fly ash and slag.2?The hydration characteristics experimental study of recycled concrete hardened paste were conducted.By comparing with the hydration characteristics of cement paste,then the influence of water absorption and releasing of recycled aggregate on the hydration characteristics of recycled concrete hardened paste is investigated.The influence coefficient "A" is presented to reflect the relationship between hydration characteristics of recycled concrete hardened paste and cement paste.Through the influence coefficient "A",the calculation model to predict hydration degree of recycled concrete hardened paste is established.The hydration research supplies a foundation for understanding the relationship between hydration characteristics and shrinkage porperty of recycled concrete.3?Based on the hydration characteristics of recycled concrete hardened paste,in consideration of recycled aggregate features such as the containing adhered mortar,the calculation model of cubic compressive strength,axial compressive strength and elastic modulus of recycled concrete is proposed.4?Under the condition of constant temperature and humidity,27 groups of mix proportions were designed.The autogenous shrinkage and drying shrinkage of recycled concrete were measured by eddy current method and micrometer gauge respectively.The influence of mineral admixtures,recycled coarse aggregate replacement rates and varieties,recycled fine aggregate replaccement rates and water binder ratio on the autogenous shrinkage and drying shrinkage of recycled concrete were understood.Then on the basis of experiment results,the mix design method to reduce the autogenous shrinkage and drying shrinkage of recycled concrete is proposed,which makes sure the autogenous shrinkage and drying shrinkage of recycled concrete is close to that of normal concrete.5?The internal relationship beween hydration characteristics and autogenous shrinkage of recycled concrete is deeply analysed.Based on the theory of capillary tension,the theoretical model of autogenous shrinkage of recycled concrete is established by introducing recycled aggregate replacement rates and influence coefficient "A" of recycled aggregate water absorption and releasing.In order to be convenient for engineering application,the autogenous shrinkage empirical model along with hydration degree is proposed.6?The reaction mechanism of recycled aggregate on drying shrinkage of recycled concrete is discussed on the basis of micro-mechanics of composite materials theory.Through the comparison with different drying shrinkage calculation models of recycled concrete,the formula to predict drying shrinkage of recycled concrete is put forward by tacking correction coefficient of recycled aggregate replacement rates and influence coefficient of mineral admixtures into consideration.The models are proven to be effective.