Study On The Hydration Heat Analysis And Temperature Control Measures Of The Top Slab Of Prestressed Concrete Continuous Box Girder

In recent years,concrete box girder bridge has been widely used because of its advantages,but it has also exposed a lot of safety problems,of which early cracks are the most common.It is found that hydration heat of concrete is one of the important reasons for the early cracks of this kind of box girder bridge.The high risk area of box girder cracking caused by hydration heat is located in a relatively thick area,such as web and floor,and there are corresponding countermeasures.However,few people pay attention to harm of hydration heat on the roof,people generally think that the roof does not belong to the large volume of concrete,not due to hydration heat crack,in fact,there are a number of examples that the roof had early cracks,It endangers the safety of the bridge,so to explore the influence of hydration the heat on the roof is urgent.This paper is based on a highway connecting line project,which uses prestressed concrete continuous box girder bridge,one of which found a certain number of transverse cracks in the roof after removing the mould.In order to avoid the same problems in the later construction,this paper studies the hydration heat analysis and temperature control measures of the bridge by using Midas FEA software combined with the measured temperature data of the cast-in-place continuous box girder roof.(1)According to the example,the model of three span concrete box girder bridge is set up.The related parameters and the simulation method of the interior surface of the box are determined by consulting the relevant literature.The temperature field and stress field of concrete box girder are calculated and compared with the measured data of temperature.It is found that the two are in good agreement,and the validity of the model is verified.(2)This paper simulates the box girder hydration reaction by using Midas FEA,analyzes the temperature field and stress field of the hydration heat reaction of the roof of the box beam,and determines The peak temperature and peak time of each test point.And the temperature change between each measurement point,summarize the reasons for deep roof temperature change rule and led to the change of the stress field,in part,summarize the stress changes and explore the deep-seated reasons behind.In the end,it shows that the stress values of the measuring point at the bottom of the roof will basically exceed the allowable tensile stress values,it is in accordance with the cracks in the bottom of the roof in the actual project,so heat of hydration reaction box girder is an important cause of roof cracking.(3)The factors affecting the hydration heat of the roof of the box girder are analyzed,such as the temperature of the mould,the amount of cement,the variety of cement,the condensing pipe and the inner environment of the box chamber,and the corresponding control measures are discussed,and puts forward some practical suggestions based on these factors.The results showed that too much mold temperature,too large amount of cement in unit concrete,improper selection of cement varieties,or no air flow in the box room will increase the roof temperature,the stress,and the risk of roof cracking will increase.Therefore,we should use cold water mixing concrete,fly ash,low heat cement,setting condenser pipe,and ensuring ventilation inside the box room to relieve the temperature of hydration heat and prevent roof cracking in reality.