Study On Bleeding And Early Cracking Law Of Fresh Concrete

Because of its own advantages,concrete has become the most widely used construction material in the field of construction engineering,but it also has certain disadvantages.The bleeding and early cracking of fresh concrete are particularly common.Therefore,through the slump,bleeding,early cracking and compressive strength tests of concrete with different proportions,this paper studies the bleeding law,early cracking law of fresh concrete and the effect of bleeding on early cracking,the factors affecting the bleeding and early cracking were studied.Pass the bleeding test of fresh concrete,measure the cumulative bleeding at each time,and obtain the parameters such as the bleeding rate at each time,the latent time t0,and the coefficient k related to the bleeding.Refine the parameter value of the bleeding type division,namely early fast type(t0 <8.5,k?0.01),early slow type(t0 <8.5,k <0.01),late fast type(t0?8.5,k?0.01),late Slow type(t0?8.5,k <0.01);It is found that the 240 min bleeding rate increases with the increase of slump,and the 240 min bleeding rate of C20 concrete at each slump is significantly higher than that of C30 and C40 concrete;concrete The latent time of bleeding gradually became shorter with the increase of slump,and the relationship between the latent time of bleeding and slump was fitted under each test condition.Through the early cracking test of concrete,measure initial cracking time,crack width and length at each moment,and obtain parameters such as total cracking area per unit area.The total cracking area per unit area of the concrete gradually increases with time.According to the crack development rate,the early cracking is divided into a rapid growth period(initial cracking time-8h),a steady growth period(8h-20h)and a slow growth period(20h-24h)Three stages;the time of the first crack of concrete is mainly concentrated between 5h-8h,the initial cracking time of concrete and the total cracking area per unit area gradually increase or increase with the increase of slump;the initial cracking time of concrete And the total cracking area per unit area gradually becomes longer or larger with the increase of the bleeding rate of 240 min,and the average value of the total cracking area per unit area of the four types of bleeding is from slow to late,late fast,and early slow,Early fast type,late slow type bleeding concrete has the best crack resistance,and early fast type bleeding concrete has the worst crack resistance.With different water-cement ratio,sand ratio,slurry aggregate ratio,coarse aggregate particle size range,water-reducing agent content as control variables,the bleeding and early cracking tests were carried out to obtain the concrete bleeding latency t0,and the water-related coefficient k value,240 min bleeding rate,early cracking and initial cracking time,and the total cracking area per unit area vary with various influencing factors.The refinement of the parameters for the classification of bleeding types and the research on the bleeding law of fresh concrete make the bleeding become controllable and help reduce the hazards of concrete bleeding.The study of the early cracking law of concrete combined with the bleeding test can provide guidance for the construction of concrete structures in actual projects to avoid or reduce the occurrence of early cracks.The study of the factors affecting bleeding and early cracking can provide suggestions for the control of concrete bleeding and early cracking in engineering construction.