Research On Anti-cracking Technology For Concrete In Areas With Large Temperature Differences And Low Humidity

Due to its unique geographical location,the Qinghai-Tibet Plateau has formed climatic conditions characterized by low pressure,low humidity and large temperature difference,which makes the service life of infrastructure in this area face serious challenges.In this study,through the investigation of concrete structures such as G109 highway,Qinghai-Tibet railway,G214 and Lalin highway,it is found that the surface cracking phenomenon is more serious than that in the plain area,and it is more common in the early stage of construction.Due to the small variation range of air pressure,it will not have a significant impact on the shrinkage performance of concrete in general.In contrast,the influence of temperature and humidity on concrete is more significant due to the large variation range,which may lead to the expansion or shrinkage of concrete.Therefore,this paper mainly studies the influence of temperature and humidity changes on the mechanical properties and early age shrinkage properties of concrete in plateau climate environment,which has certain theoretical significance and practical value for exploring the evolution of concrete cracking and deterioration in plateau area.Firstly,an improved ring restrained shrinkage test device is proposed for the two shortcomings of the traditional ring restrained shrinkage test method,which is low sensitivity to the detection of concrete shrinkage cracking and random crack position.The multi-functional climate test chamber was used to simulate the environment of large temperature difference and low humidity in the plateau,and the macroscopic properties of concrete such as drying shrinkage and restrained shrinkage under different temperature and humidity conditions were studied.In order to explore the crack resistance technology of concrete under large temperature difference and low humidity environment,the effects of different internal curing and fiber materials on the mechanical properties,drying shrinkage and restrained shrinkage of concrete were studied.The main research contents and conclusions are as follows:The improved ring restraint shrinkage test method is to change the shape of the builtin steel ring in the ring method,so that the stress concentration occurs at one end of the restraint ring specimen.The steel ring is composed of a semicircle with a diameter of 280 mm and a semi-ellipse with a long axis of 355 mm and a short axis of 280 mm.The cracking time and crack location of cement mortar improved ring constraint specimens were statistically analyzed by mathematical statistics method.It was found that the average cracking time of cement mortar improved ring specimens was about 52.4 % faster than that of traditional ring specimens,which shortened the test observation period.And98 % of the crack offset is within 27.35 mm around the expected cracking position,and the offset distance is negligible compared with the length of the conductive coating,indicating that the improved ring constrained shrinkage test method can limit the cracking position of the specimen.The drying shrinkage and restrained shrinkage of concrete under different humidity(30 %,60 %,90 %)and temperature(20 ℃,40 ℃,60 ℃)and variable temperature and low humidity(-10-20 ℃ + RH40 %,-10-40 ℃ + RH40 %,20-50 ℃ + RH40 %)conditions were studied by simulating different curing environments in a multifunctional climate chamber.Low humidity curing environment will increase the shrinkage rate of concrete and accelerate the time of shrinkage cracking of cement mortar;variable temperature and low humidity curing environment will further increase the risk of cracking of mortar ring.Compared with a temperature of 20 ℃ and a humidity of 60 %,the 7-d shrinkage of concrete at 60 ℃ and a humidity of 30 % increases by 197 %,the cracking time of cement mortar increases by 62 % and the crack width increases by 47 %;the shrinkage of concrete at a temperature of-10 to 40 ℃ and a humidity of 40 %increases by 77 %,the cracking time of cement mortar increases by 71 % and the crack width increases by 72 %.The cracking time of cement mortar under positive and negative temperature variation of 50 ℃ was 87 % earlier than under positive and negative temperature variation of 30 ℃,and 105 % earlier than under positive temperature variation(20 ℃ to 50 ℃);the crack widths increased by 63 % and 41 % respectively.The grey correlation analysis shows that the influence of humidity on the cracking time is greater than the maintenance temperature.The effects of single and mixed super absorbent polymer(SAP),diatomite,polypropylene fiber(PP)and polyvinyl alcohol fiber(PVA)on the mechanical properties,drying shrinkage and restrained shrinkage of concrete were studied.The results show that under the environment of large temperature difference and low humidity,the compressive strength and splitting tensile strength of each group of concrete with different internal curing and fiber materials fluctuate within 8 % compared with the reference group,which can meet the strength design requirements.With the increase of SAP and diatomite content,the shrinkage rate of concrete decreases gradually,and the addition of fiber can further inhibit its shrinkage.Both diatomite and SAP can delay the cracking time of cement mortar ring specimens and reduce the crack width.The cracking time of 0.3 % SAP is about 450 % longer than that of the reference group,and the crack width is reduced by 26.5 %.The anti-cracking effect of 0.3 % SAP and 0.1 %polypropylene fiber is the best,the cracking time is about 10 times that of the reference group,and the maximum crack width is reduced by about 64.5 %.It can be seen that the combination of fiber and internal curing can significantly improve the early crack resistance of concrete.