Research On Compressive Creep Model And Stress-strain Relationship Of Concrete

Creep of concrete is the phenomenon where the deformation of concrete increases with the increase of the holding time under a constant load.Creep behavior not only impacts the normal performance of the concrete structure,but the high stress level of creep also affects the bearing capacity of the concrete structures.The scope of an existing compressive creep models for concrete is rather narrow,and the existing models cannot be applied to high strength concrete or high stress structures.The influence of creep has not been considered in the present compressive stress-strain curves of concrete,and the influence of creep on the peak compressive strain of concrete is unable to be measured.The study on the evolution of the instantaneous recovery strain and the lag recovery strain after creep is still lacking.The effects of the cross section of stress redistribution and the compressive steel bar in the axial compressive column under creep stress on the creep behavior of the column still cannot be accurately analyzed.Based on the above factors,the research work of this thesis has been conducted.The creep tests and compressive failure tests were conducted after creep on 188 concrete specimens.The axial compressive strength of the specimens with curing age of 28 days varied from 20.9 MPa to 40.9 MPa,the creep stress levels varied from 0.154 to 0.725,the loading age varied from 30 days to 512 days,and the load holding time varied from 39 days to 589 days.The text improved the model of concrete creep,explored the strain recovery law of concrete creep after unloading,and established the equation of compressive stress-strain curve for considering the concrete creep basing on the test data.Definite description as following:(1)Based on the experimental data from domestic and overseas,the coefficient formulae regarding the influences of the cubic compressive strength,the loading age,the environmental temperature,the environmental relative humidity,and the theoretical thickness of the specimen on the creep were established.Under the standard conditions using the proposed formulae,the linear creep test data of 54 concrete specimens were transformed into the creep test data.In the standard conditions,the axial compressive strength of the specimens with curing age of 28 days is 30 MPa,the loading age is 28 days,the environmental temperature is 20 ?,the environmental relative humidity is 60 %,and the theoretical thickness of the specimen is 50 mm.A linear creep calculation model under the standard conditions was then proposed.The cut-off point between the linear creep and the nonlinear creep was set to 0.35,and the cut-off point between the convergent nonlinear creep and the divergent nonlinear creep was set to 0.76.An enlargement coefficient calculation formula for the nonlinear creep,which was varied with the stress level in the exponential function(the stress level varied from 0.35 to 0.76),was proposed based on the experimental data of 134 specimens.Therefore,the scope of the proposed creep model was enlarged.The model can be used to analyze and calculate the creep strain in the condition of the cubic compressive strength from 15 MPa to 180 MPa,the loading age from 1 day to 3650 days,the environmental temperature from-20? to 80?,the environmental relative humidity from 3% to 100%,the theoretical thickness of the specimen from 20 mm to 270 mm,and the stress levels from 0 to 0.76.(2)Based on the unloading after creep test data of 114 specimens,it was found that the instantaneous recovery strain coefficient decreased with the increase of the stress level,and the stable time of the lag recovery strain enlarged with the increase in stress levels.The lag recovery strain was proportional to the creep strain before unloading.The variation of the lag recovery strain diminished with the increase of time.The calculation formulae for the instantaneous recovery strain and the lag recovery strain were established,and were then used to analyze the influences of the cross section stress redistribution and the compressive steel bar on the creep behavior of the column.(3)Based on the compressive failure test data of 188 concrete specimens after creep,a four stage stress-strain curve for the compressive test after creep without unloading,as well as a six stage stress-strain curve for compression test with unloading to zero after creep were proposed.The total strain corresponding to compressive failure peak force with experience of different stress levels of creep was about 2400×10-6~4500×10-6.The total strain increased by 20~120 % compared to the same batch of shrink test specimens.The peak compressive strain calculation formula was established in reasonable consideration of the influence of creep.The researchers provide some references for reasonable application of the high strength hot rolled steel bar as compressive steel bar.