Study On Cracking Law And Mechanism Of Precast Concrete Box Girder Under Different Curing Methods In Dry-Cold And Large Temperature Difference Areas

With the rapid development of basic transportation in Northwest China,precast concrete box girder is widely used in highway construction.The Northwest region has large temperature difference and dry-cold climate,the concrete poured in this environment is easy to crack which will affect the quality and safety of concrete structures and the present maintenance methods of precast concrete box girder can not be fully applicable to this special climate.Therefore,it is necessary to study the cracking law and cracking mechanism of precast concrete box girder under different curing methods in large temperature difference and dry-cold area.In this paper,relying on theBaiMing highway project,we monitored the climate environment,detected the raw materials,investigated the maintenance methods,and selected some maintenance methods for the test,then analyzed and studied the strength,crack resistance,impermeability,micro pore structure,early strain and hydration temperature field of the precast concrete box girder under different curing methods.Specific research contents are as follows:(1)By testing and analyzing the tensile strength of concrete box girder under different curing ages,the following conclusions are drawn: at the age of 28 days,the tensile strength of concrete decreased gradually with the sequence of steam curing,standard curing,rubber and plastic plate package,geotextile package,film wrapping,sprinkler curing and natural curing;and the ratio of tensile strength to compressive strength of concrete under different curing modes ranged in 1/18-1/14 which decreased gradually with the above sequence.(2)By testing and analyzing the tensile strength and compressive strength of precast concrete box girder under different curing methods,the following conclusions are drawn: At the age of 28 days,the compressive strength of concrete under steam curing and standard curing are the highest,and the strength are greater than 50 MPa,while other curing methods are less than 50MPa;the ratio of the rebound strength of the box girder and the strength of the same test block is between 0.96-0.98,and the strength of the outer side of the box girder is 1.2-1.4 times of the inner strength;when the steam curing time is more than 4 days,the strength can reach 50 MPa at the age of 28days;when the steam curing time is 7 days,the strength can reach 50 MPa at the age of 7days;Finally,in this paper,the formula for calculating the strength of concrete at different ages,such as steam curing,standard curing,rubber and plastic plate curing and different steam curing days,is obtained.(3)Based on the analysis and research on the crack width,cracking time,crack length and crack number of concrete anti cracking ring and concrete box girder under different curing methods,the following conclusions are drawn:the cracking resistance of concrete decreased gradually with the sequence of standard curing,steam curing,rubber and plastic plate package maintenance,geotextile maintenance,film curing sprinkler curing and natural curing;the crack width distribution of the test box girder is mainly in the 0-0.02 mm and 0.03-0.06 mm two intervals,indicating that the early concrete cracking is mainly caused by various types of shrinkage deformation;the temperature difference caused by hydration reaction under different curing methodsis far less than the standard requirement;20hoursafter pouring,the concrete strain value is no longer changed;in the late period,the influence of temperature and solar radiation is sinusoidal within a small range.(4)Based on the study of the effect of sunlight in large temperature difference,dry-cold and strong wind area on the concrete box girder,the conclusions are drawn:sunshine can promote the hydration reaction speed and strength of cement,but it will accelerate the cracking of concrete;the variation of the temperature field of the precast concrete test box beam in the beam field is in the standard requirements,but the temperature change in the floor is slightly different from the specification;(5)By testing and studying the micro pore structure and permeability of concrete under different curing methods,the following conclusions are drawn: concrete impermeability and strength weakenedgradually whilethe pore space coefficient,the average pore size,the number of holes and the gas content increased gradually withthesequence ofstandard curing,steam curing,rubber and plastic plate package curing,film wrapped maintenance,geotextile package maintenance,sprinkler curing and natural curing;under different curing methods,the coefficient of concrete pore spacing is less than 0.300 mm,so the concrete frost resistance grade can reach F300.Combined with all factors,steam curing is the best maintenance method in large temperature difference and dry-cold regions.