Research On Development For Brittleness Of Early-age Concrete Under Sustained Loads

In the process of high-speed railway construction,the construction progress of box girder prefabrication will directly affect the overall project progress.In order to speed up the construction progress,the precast box girders are often maintained by steam,so that the curing time is repeatedly shortened,and the initial tension time of the prestressed steel bars are further advanced.Under the early age loading,the brittleness of concrete generally increases.Aiming at this phenomenon,this paper selects the most commonly used two-parameter model to study the brittle development of concrete from the fracture properties of materials.In addition,combined with the tensile and compressive strength ratio method,compared with the fracture mechanics method,the influence law of curing mode and loading age on the brittleness performance of concrete was comprehensively explored.The main research works are as follows:(1)The fracture performance test of precast concrete notched beams are carried out.The results show that fracture toughness,critical crack tip opening displacement and critical strain energy release rate increase with age.However,early loading or steam curing will slow down the growth rate and reduce the ability to resist crack propagation.The characteristic length and material length decrease with the increase of age.The advance of loading age or steam curing increases the rate of reduction and obviously increases the brittleness of concrete in the later period.(2)The mechanical properties of concrete specimens were tested.The test results show that both the compressive strength and the splitting tensile strength are higher when the load is applied in the 3-day age than when the load is applied in the 28-day age.Steam curing can obviously improve the early strength of concrete,but the later strength development is limited.By the 90th day of age,the compressive strength and split tensile strength of the standard curing group are higher than those of the steam curing group.For the ratio of tensile and compressive strength,the application of load or steam curing in the early age makes the tensile and compressive strength lower than that in the control group.Although the load in the early age can increase the splitting tensile strength,the increase in compressive strength is smaller,so the brittleness is larger.(3)The brittle fracture development model of early age concrete is established.A qualitative mechanism explanation is given for the development trend of concrete properties with ages at different loading ages and different curing methods,which provides a theoretical basis for the development model of brittle fracture.The time-dependent development model of crack growth resistance of concrete under different loading ages and curing modes is established.At the same time,regression fitting of relevant parameters in the model was carried out by using test data,and the influence of loading age and curing mode on crack growth resistance was analyzed and discussed.Finally,model test was carried out for verification.The results show that the predicted values are in good agreement with the experimental results,and the new model has a good scope of application.