摘要
徐变对于钢筋混凝土构件尤其是大跨度预应力桥梁的影响已经得到广泛的重视,但是在过去的理论及试验研究中,更多的是从混凝土材料和力学模型的角度研究徐变性质,对实际工程结构还缺乏较深入的研究和良好的结合。现行的混凝土徐变预测模型和计算理论绝大部分是基于混凝土标准养护28d以上的试件,对于早龄期混凝土的徐变性质和理论还没有系统的进行研究过。
本文从实际工程入手,采用室内试验和有限元计算相结合的方式,设计了C20～C50四种强度等级的混凝土试件进行早龄期徐变试验,通过试验结果从理论上总结早龄期混凝土的徐变特性,并建立早龄期混凝土徐变计算模型。在对早龄期混凝土的徐变理论充分了解的基础上,创建了在早龄期张拉部分预应力,待强度增长达到设计要求后再张拉剩余的预应力的张拉方案,并通过与传统张拉方案进行比较,发现新的张拉方案对于降低混凝土徐变方面能取到良好的效果。主要研究成果如下:
1、设计了C20～C50四种强度等级的混凝土试件进行早龄期徐变试验,针对60天记录的数据进行归纳总结,得出C20～C50早龄期混凝土的徐变特点和规律。为了了解长期徐变趋势,对徐变系数建立与时间的关系曲线,并进行拟合,得到早龄期混凝土徐变系数的计算式,这是国内外首次对早龄期混凝土徐变所做的试验和研究。
2、经过对早龄期混凝土的徐变理论的建立,创建了在早龄期张拉部分预应力,待强度增长达到要求后再张拉剩余的预应力的张拉方案,并通过与传统张拉方案进行比较,发现新的张拉方案对于降低预应力构件的徐变有很明显的作用。该张拉方法是首次尝试的降低徐变的措施,在室内试验和有限元计算的条件下已经验证了该方案的有效性。
3、以实际工程的各种施工参数和条件作为参考,基于有限元分析对钢筋混凝土预应力桥梁进行仿真计算,对于桥梁未来十年的收缩徐变进行了考虑,在此基础上得到该桥的内力值和变形值。这是该新型施工方法通过计算首次在实际工程中的应用,为将来该方法进行推广和使用提供了计算数值上的依据。
The impact of creep on reinforced concrete structures especially long span prestressed bridge has been valued widely, but the creep nature was studied from the perspective of concrete material and mechanical model in past theoretical and experimental research, the deeper research and close combination are lacked in actual project structure. Most of existing concrete creep prediction models and theoretical calculations are based on above 28d of concrete standard curing specimen, while earlier creep nature and theory of concrete have not been studied systematically.
This paper starts from actual project, the combination of laboratory tests and finite element calculation is adopted, four concrete specimens with strength grades ranging from C20 to C50 are designed, the creep characteristics of early-age concrete are summarized theoretically from test results, the creep model for early-age concrete is set up. The prestresss of early-age tension part on the basis of full understanding on creep theory of early-age concrete, and then take tensioning program of remaining prestress until the strength increases and meets design requirements, and good results of new tension program in reducing concrete creep is found by comparing it with traditional tension program. The main research results are as follows:
1.The concrete specimens with four strength grades ranging from C20 to C50 to designed to take early-age creep test, the data recorded for 60 days are summarized, the creep characteristics and laws for C20 ~ C50 early-age concrete are derived. In order to understand the long-term creep trend, the relation curve between creep coefficient and time is set up, and the fitting is made, the formula for creep coefficient of early-age concrete is obtained, this is the first experiment & research for early-age concrete made at home and abroad.
2.The prestress of early-age tension part is created through establishment of creep theory for early-age concrete, the tension program of remaining prestress is tensioned until the strength increases to meet requirements, and it is found that new program in reducing the creep of prestress component has a significant role by comparing it with traditional tension program. The tension program is the first measure firstly attempting to lower the creep, and the effectiveness of the program has been verified under the conditions of laboratory test and finite element calculation.
3. Take construction parameters and conditions of actual works as reference, take simulative calculation on prestress bridge of reinforced concrete based on finite element analysis, take the bridge shrinkage creep in the following decades into consideration, and the internal force value and deformation value of this bridge were on this basis are obtained. The new construction method is firstly applied in actual construction by calculation, providing the basis of numerical calculation for promotion and application of this method in the future.
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