The Research On Early Cracking Mechanism Of Concrete Beam Bridge And Crack Control

As the throat of traffic engineering,bridge engineering has played a key role in improving people’s living environment and promoting economic development.However,in recent years,as the application of high-strength and high-performance concrete in concrete bridges has become more and more widespread,while the bridge span has increased,the problem of early cracking during bridge construction has become more and more serious.It affects the appearance of the structure,and even affects the safety and applicability of the beam.How to effectively control or even prevent the early cracking of concrete bridges is still the focus and difficulty of current scientific research.Based on the existing research results of early temperature stress and early shrinkage and creep of concrete structures,this thesis takes a yellow river bridge in Ningxia as the main engineering background,and takes fabricated t-beam and large-span cast-in-place box beam as the research objects.According to the distribution characteristics of early cracks on the beam surface and field investigations,preliminary explorations of the reasons for the cracks of the beam were carried out.Subsequently,through the method of combining finite element simulation and field test,the main reasons and influence factors of cracks were reasonably analyzed.Considering the factors of material,structure,construction and other aspects,the prevention methods and control measures for the early cracking of two types of precast T-beam and continuous box girder bridges are proposed.The main research content and results of this thesis are as follows:(1)On the basis of collecting and studying relevant references,through on-site investigation and investigation of beam cracks,and discussion with managers,the causes of cracking of prefabricated Tbeams and continuous box beams were analyzed respectively.And results show that the early vertical cracks of precast T-beams were mainly distributed in the web position 2～4m away from the end of the precast T-beam.It is preliminary believed that excessively high hydration heat temperature is main reason for the early cracking of the precast T-beam.Unfavorable weather environment is the inducing factor of beam end cracking;the vertical cracks in the web of the cast beam section on the top of the box girder pier are the most serious.Analysis of the causes shows that there are many influencing factors for the formation of vertical cracks.The main reasons are the temperature of concrete hydration heat,early shrinkage deformation and the age difference between two castings of concrete,the bedrock restraint effect,etc.(2)The finite element simulation method is used to explore the two aspects of the hydration process inside the T-beam concrete and the external climate environment.Results show that in the initial stage of T-beam concrete pouring,the position near the beam end has a higher risk of cracking due to the rapid accumulation of temperature during the hydration process;However,the increase of the mold entry temperature and cement content before concrete pouring will increase the temperature extremes of the heat of hydration inside the T-beam,and increase the structure heating rate and cooling rate,which is not conducive to controlling the development of temperature stress and preventing the occurrence of cracks;before the concrete is poured,the method of controlling the entering temperature and reducing the amount of cement can be adopted to control the maximum temperature rise and delay the heating rate;the temperature gradient on the surface of the structure will increase drastically with the increase of wind speed,leading to the risk of cracking of the concrete on the surface of the structure;changes in ambient temperature will directly affect the temperature stress generated during the hydration process of the Tbeam,leading to a very high risk of cracking inside the concrete structure.(3)Based on the principle of the controlled variable method,the finite element simulation method is used to explore the environmental temperature,the entry temperature and the bedrock restraint effect.The results show that compared with the factors that directly affect the heat of hydration of continuous box girder,such as ambient temperature and mold entry temperature,the effect of bedrock restraint on the tensile stress of concrete is more significant.The influence of the bedrock restraint effect on the pouring girder sections on the top of the piers of a large-span continuous box girder bridge mainly lies in the time interval between two concrete pouring and the height of the first concrete pouring.Specifically,as the time interval between the two layers of concrete pouring is greater,the degree of cracking of the concrete web is more serious;as the height of the first concrete pouring of the beam section is higher,the cracks are easier to be controlled.(4)Prefabricated T-beams and continuous box girder can consider crack control measures from the perspective of reducing the temperature effect and early shrinkage deformation of the concrete hydration heat process.Both types of bridges can be used to control the molding temperature and reduce the amount of cement before the concrete is poured to reduce the maximum temperature rise and delay the temperature rise rate.However,compared with continuous box girder,because external environmental factors have a more significant impact on the temperature stress of prefabricated T-beam.Therefore,measures can be taken to strengthen the wind shielding work after the T-beam cooling and demolding,and to choose the pouring time in the evening or early morning,so as to reduce the influence of external environmental factors on the temperature stress of the precast T-beam.At the same time,in order to avoid the significant influence of the bedrock confinement effect on the early stress of the continuous box girder.It is recommended that the time interval between two concrete pouring should be controlled within 7 days as much as possible,and adopt the construction plan that the concrete needs to be poured to the lower edge of the roof for the first time,and the roof is poured for the second time.