Shrinkage And Creep Effects On The Beam Performance Analysis Of RPC-NC Composite Beam

ABSTRACT:Reactive Powder Concrete (RPC) is a kind of ultra-high performance concrete material with high intensity, high ductility and high durability which can effectively overcome common problems in concrete structures. Because of different basic preparation of RPC and normal concrete, it needs a high requirement to each component and production process. It willimprove the cost of RPC with the use of silicon powder, high efficiency water reducing agent, steel fiber and higher molding and curing condition.There this paper uses a combination of ordinary concrete and RPC components, namely the tension zone using RPC and compression zone pouring ordinary concrete in order to take advantage of the material properties of the two materials.In the cases of RPC and ordinary concrete bonding well,the deformation difference caused by the casting time difference and the material difference will make members produce section stress redistribution and structural internal force redistribution, and influence performance such as deformation, the emergence and development of cracks, and even affect the ultimate bearing capacity. To address this situation, the paper carried the following research and get relevant conclusions:1. By analyzing the compositon of concrete strain under constant stress and varying stress, the paper discusses the effects of concrete creep calculated using initial stress method, and derive strain increment formula.2. The creep stress of one-way center compressive member was calculated with an example. By the calculation results, the creep makes stress of concrete decreases and stress of reinforcement increases, which means that the stress borned by concrete transfer to the steel. In the actual project, it should be noted when checking reinforcement stress.3. Based on the section, the entire process of13different composite are analyzed and each curve is under comparison. The results show that,(1)RPC-NC ultimate flexural strength of composite beams equals to the whole pouring NC beam’s ultimate load. The deviation between the results using the commonequivalent rectangular stress diagram method to calculate the ultimate flexural strength of the composite beam and the numerical calcultion is very small. Therefore, using the original beam member flexural strength formula to calculate the ultimate flexural strength of composite beams is feasible.(2)The presence of pre-RPC component only improves the cracking moment of the overall section to the degree of about4to5times. There are two kinds of cracking form, one is beginning form the ordinary concrete tension destruction, the other is beginning from the RPC tension destruction on the bottom of composite beams. In order to make better use of RPC durability and tensile strength, it is recommended to use the half of the full beam height as the height of prefabricated components.4. The paper make a least squares fit to the creep coefficient given by the specifications to three indices formula. The result shows that the three exponential type and the multi-factor product formula fit well.5. Through the analysis of composite beam only counting shrinkage, only taking into account the creep and counting shrinkage and creep at the same time, the results show that creep and shrinkage influence greatly to the stress in the tension zone for reactive powder concrete and the stress for the tensile reinforcement, and increse the stress to varing degrees. Thus it should be paid attention when designing and checking.6. Through the calculations of the long-term deflection, the long-term stiffness and craking moment of the composite beam,combined with the results of ordinary beams, the results show that:(1) Short-term stiffness of composite beams is greater than ordinary concrete beams, and the short-term deflection is less than ordinary concrete beam, but the difference between the upper and lower composite shrinkage and creep characteristics results in its long-term deflection increasing coefficient greater than ordinary concrete beams. Its value is close to20%larger, though, the final deflection of composite beams is less than ordinary concrete beam.(2) Shrinkage and creep make the cracking moment of composite beams decrese, but the final moment is still greater than the ordinary cracking concrete beams cracking moment, which is also from the side guaranteeing that the new composite beam can be effectively improve the cracking moment.7. Through the comparion analysis of the factors which affect the shrinkage and creep, the results show that, RPC height, concrete grade and reinforcement ratio for shrinkage and creep all have varing degrees of impact. The higher RPC height, the larger the concrete strength, and the larger the reiniforcement ratio, the smaller the final deflection, the smaller the long-term deflection incresing coefficient. RPC height and strength of the concrete make great influences to the development of shrinkage and creep, so it should be paid attention particularly in the design.8. The paper optimize the long-term deflection prediciton method in the desig and propose multi-parameter correction factor method. In addtion, the paper makes multiple linear regression for the influence factor and derive expressions of each parameter. The actual results and the model predictions are similar.