Studies On Cracking Behaviour Of High Volume Slag And Fly Ash Concrete At Early Age

Fly ash and slag are two of the largest amount of industrial residue production,and the industrial residue aforementioned could be utilized widely in concrete,not only which could save the cement clinker and decrease the energy consumption,but also could decline the environmental pollution.However,the mechanism of the effect of industrial residue on the cracking behaviour of concrete is very complex and the disputes of researches exist in positive and negative effects.Correctly evaluating the impact of the superposition effect is meaningful to use the industrial residue effectively and promote the development of low carbon green society.From the perspectives of single factor,most researches published in journals focused on the early cracking behaviour of concrete under standard condition.But,there is a great difference between the laboratory research results and the practice of projects because the cast concrete of projects is in the environment coupled temperature,humidity and restrain,the cracking behaviour of that results from the these coupling factors.It is pointed that this study mainly investigates the effect of “hydration-temperature-humidity “coupled factor on the hardened properties of high volume industrial residue concrete.On the basis of the multi-field coupling “hydration – temperature-humidity-constraint” model,the cracking risk of high volume industrial residue concrete is analyzed.1.Study on the hydration process of high volume slag and fly ash cementitious system under coupling temperature and humidity fieldUsing calorimeter,TG and SEM experimental methods,the effect of high volume of slag and fly ash under different temperature,water to binder ratio and replacement amount condition on cement hydration is studied and results show that the cement hydration is accelerated by the high volume slag and fly ash.Hydration mechanism of slag and fly ash in cementitious system is summarized,it can be seen that the hydration degree of slag can reach to around 20% in age of 7 days while that of fly ash is less than 5%.The investigation of hydration degree of high volume slag and fly ash indicates that the hydration degree of high volume slag and fly ash can be increased but there is no obvious influence on hydration degree of cementitious materials when increasing the ratio of water to cement.On the basis of hydration degree of high volume slag and fly ash cementitious system and the modified hydration activation energy,the high volume slag and fly ash cement system hydration model coupling temperature and humidity is put forward.2.Study on the early cracking resistance of large volume fly ash and slag under coupling multi-fieldsThe study investigate the impact of high volume slag and fly ash on the evolution of compressive strength,flexural strength and the elastic modulus by cube compression,splitting tensile and ultrasonic testing methods.The results show that the hydration process of slag has played a significant role in promoting the compressive strength of high volume fly ash concrete in early period.Based on the experimental results,the mechanism of promoted early compressive strength is disclosed.Combining the modified early hydration activation energy,equivalent age and the hydration degree are utilized to modify the strength evolution model of high volume industrial residue concrete.3.Study on the early deformation and its driving force of large volume fly ash and slag under coupling muti fieldsThe effect of high volume slag and fly ash addition on autogenous shrinkage is investigated using non-contact electriceddy transducer,internal humidity and capillary pressure.And the results indicate that the autogenous shrinkage of high volume slag cement paste is reduced because of the decrease hydration degree of fly ash which can delay the occurrence of decreased internal humidity.On the other hand,slag can increase the autogenous shrinkage which can be attributed to the reason that the higher hydration degree results in higher consumption amount of capillary water and the decreased internal humidity.Early thermal deformation and thermal expansion coefficient of concrete was analyzed based on experiments.In order to modify the autogenous shrinkage model of high volume slag and fly ash concrete under the multi-field condition,on the basis of coupling mechanism of temperature and hydration,the early saturation coefficient of capillary is adopted by POWERS model and the multi-field deformation model of concrete is verified by the temperature stress tester experiments.To evaluate the influence of high volume fly ash and slag on early stress relaxation,the steel-ring tests are applied,which shows that large amount of fly ash obviously reduces the early stress relaxation coefficient of concrete.The exponential model is proposed based on the experimental results and it can be used to predict the stress relaxation coefficient of high volume slag and fly ash in early period.4.Study on evaluation model of early cracking of concrete and the simulation of cracking behaviour of concrete under coupled temperature and humidity fieldUnder the theoretical framework of equivalent stress,an evaluation model taking the restraint and the stress relaxation condition into consideration for early cracking of high volume slag and fly ash concrete is established.It is used to evaluate cracking risk of high volume industrial residue concrete in practical projects.