Study On The Application Of Prestress Technology In Crack Controlling For Outer-walls Of Ultra-long Basements

With the increasing of the number of various ultra-long basements, leakage and seepage problems caused by the cracks in the R.C. ultra-long basements are very common, which have already attracted more and more attention in the field of construction engineering. As an effective means of crack controlling for R.C. structures, prestress technology has been widely used in outer-wall of basements. However, due to various reasons, there are still several problems in the use of prestress technology in this field. It has important theoretical significance and engineering value to carry out research on these problems.This paper aims to find the cause of cracking in outer-walls of ultra-long basement and to establish a corresponding design method for prestressing in outer-walls. The results of the research can provide references for designers to control cracks using prestress technology properly.Firstly, the reasons of cracking in outer-walls of ultra-long basements is been analyzed, and the calculation methods for the Second Effect which is commonly considered as the main cause of cracking in outer walls are introduced.Secondly, considering the stress characteristics of outer-walls, the constraint applied by the foundation is taken into account in the design method provided by this paper. The formulas are set up with the assumption of Long Walls on Elastic Foundation, which are proved to have enough accuracy by ABAQUS. A further study is carried out on the technical measures that effect the prestress distribution in outer-walls, and main conclusions are drawn as follows:(1) The distribution of columns in the outer-walls can affect the prestress of outer-walls. However, the influence can be ignored with proper arrangement of columns.(2) In order to control the prestress loss along the length of the outer-wall, it is necessary to sectionalize prestress tendons. Section length is provided by this paper for general conditions.(3) Sectionally placed prestress tendons need to be joined properly by lapping joints to ensure prestress transferring continuously along outer-walls. Taking lapping joints with short tendons as an example, the effect produced by secondary tension on the prestress distribution has been studied and corresponding formulas are established. The calculation results show that lapping joints with short tendons will lead to a smaller prestress loss, and this is recommended to be used in engineering practice.Finally, the monitoring results of the prestress in the outer-wall of ultra-long basement of a project in Hangzhou are presented in the paper. A comparison is made between the measured data and the calculated results by the formulas provided by this paper. It also shows that the new design method has enough accuracy.