Study On Structural Design And Construction Technology Of Large Area Basement Without Expansion Joint

In order to avoid the problem of cracks in large-area concrete structures due to excessive temperature stress,current specifications recommend permanent expansion joints to release temperature stress.However,too many expansion joints are not conducive to the seismic and fire performance of the structure,caulking material is also prone to aging and failure,which lead to increasing the cost of engineering maintenance.For the basement structure,the leakage of expansion joint is even more difficult to cure.In view of the above problems,this paper explores the feasibility of large-scale basement structure without permanent expansion joints,and relevant technical measures are proposed to realize this concept.The main research work and conclusions are as follows:(1)Based on concrete unsteady heat conduction equation and theory of solve temperature stress,ABAQUS was used to study the temperature field and temperature stress of the large-area basement structure of New City Safe Housing Project in Nanjing Yanziji under the most unfavorable conditions in summer and winter.The results show that: the basement roof is subjected to the maximum compressive stress of 15.1 MPa during summer daytime and the maximum tensile stress of 2.71 MPa in winter night.When considering creep and reinforcement,the maximum tensile and compressive stresses are less than the tensile and compressive strength of the concrete,and no cracking risk.Therefore,the largearea basement structure without permanent expansion joint is theoretically feasible in theory.(2)Through research on the influencing factors of restraint stress,the anti-cracking concept of “releasing and resisting combination” of large-area basement structure without expansion joint is proposed.The measures of “releasing”(skip casting,setting the induction joint,stretching the post-casting belt)are equivalent to reducing the length-to-height ratio of the structure and reducing the constraint stress;applying pre-compression of 0.2~0.7MPa in the structure by compensating shrinkage concrete,which reflects the idea of “resisting”.Combined with the use of the post-casting belt,an expansion-enhanced post-casting belt is formed to enable seamless construction of a large-area basement structure.(3)The effects of mineral admixtures and additives(fly ash,blast furnace slag powder,expansion agent)on concrete hydration rise and fall,(dry)shrinkage stress and effective compensation for shrinkage stress were studied.The results show that the above materials can be formed by rational deployment of low thermal expansion multi gel material composite system.It has the advantages of low hydration heat release,long temperature peak duration,slow temperature rising and cooling rate,which can reduce the temperature shrinkage stress and increase the ultimate elongation of concrete.At the same time,it can form expansion stress inside concrete to compensate shrinkage and realize "releasing and resisting combination".(4)Based on the constraint stress formula,the formula for calculating the retention distance of the post-expansion belt is deduced.Combined with the law of material crack resistance test,the conservative retention distance of post-casting belt in large area basement telescopic is estimated to be 25 m.And the closing time should be determined according to the requirements of construction flow operation,but not earlier than 14 d.The relevant results of this study provide theoretical support for the large-area basement structure without permanent expansion joints.The various anti-crack technical measures proposed in this paper can be used for reference in similar projects.The research of the subject will help to promote the design concept of non-expansion joints,form normative standards in the field,and have certain technical and economic benefits.