Research On The Influence Of Steel Bar Shape On The Normal Cracks Of Concrete Beams

The crack width of reinforced concrete structure is one of the main factors affecting its structural durability.The progress of our country’s metallurgical technology and the improvement of steel production technology have made the strength of steel bars higher and higher.At the same time,our country’s steel bar standards have also undergone many revisions.According to the previous experimental research of the research group,the crack spacing of the concrete beam with HRB400 steel bar is obviously smaller than that of the HRB335 steel beam under the same conditions.The new steel bar standards for concrete have different requirements on the external dimensions of steel bars compared with the standards in the 1970s,and the establishment of the calculation method of crack width in our country’s concrete structure design code is basically based on the experimental data of this period.The shape of the steel bar is one of the main factors that affect the bonding performance between the steel bar and the concrete,and it is also the main factor that affects the distribution and size of the cracks in the reinforced concrete structure.In order to explore the changes in the crack spacing and crack width of the concrete members with high-strength steel bars compared with the previous use of lower-strength steel bars,the above work was carried out according to the work plan and special research of the compilation team of"Specification for Design of Hydraulic Concrete Structures"（DL/T5057-2009）.This research analyzes and compares the average crack spacing,average crack width,maximum crack width and effective tensile cross-sectional area of concrete axial tension members configured with HRB335 steel bars under the same test conditions based on the test of 27 concrete axial tension members with HRB400 steel bars,and preliminarily explores the influence of the shape factors of steel bars on concrete cracks starting from the normal cracks of concrete beams combining with the collected crack test data of a large number of concrete components in the past 40 years through comparison,fitting regression and statistical analysis.The main work and conclusions are as follows:（1）The measured values of average crack width and maximum crack width of the concrete with HRB400 steel bars are generally larger than those with HRB335 steel bars,and the crack width value increases with the increase in the stress of the steel bars;The measured value of the average crack spacing of the component with HRB335 rebar is larger than the measured value of the average crack spacing of the component with HRB400 rebar.（2）After comparing and analyzing the results of cracking tests on axial tension members with HRB355 and HRB400 steel bars,it shows that the effective receiving area of concrete is basically the same.And a circular area with a radius of 7.5d（d is the diameter of the steel bar）or a square area with a side length of 6.5d can be used.（3）The shape of the steel bar affects the average crack spacing of the concrete beams.The analysis of the test results shows that under the same conditions,the average crack spacing of the members of the steel with a larger relative rib height h/d and relative rib spacing l/d is smaller,but the stress of the steel bar is still the decisive factor for controlling the width development of concrete cracks,and the crack width of members with higher steel bar stress levels is also larger.（4）On the basis of comparative analysis,it is proposed to adopt the influence coefficient k of the shape of the steel bar reflecting the influence of the relative rib height and the relative rib spacing on the average crack spacing.According to the regression analysis of the test data,k=1.065-18.38h/d+2.72l/d,which is in good agreement with the test results.（5）Based on the grey theory,a simple calculation model of correlation degree is established to show that:the shape of the steel bar is the same as the thickness of the concrete protective layer c,the ratio of the diameter and the effective reinforcement ratio d?ρ_te has the same level of influence on the crack spacing.Among the four parameters of the shape of the steel bar,the relative rib height and the relative rib spacing have a relatively higher impact on the crack spacing of concrete members than the tooth-rib-ratio and the base circle area rate.Part of the experimental research of this subject is based on Open Fund Project of State Key Laboratory of Hydroscience and Engineering-Tsinghua University（sklhse-2018-C-04）.