Practical Solution to Diminish Restrained Shrinkage Effects in Bridge Decks

摘要本课题主要研究减少桥面早期约束收缩裂缝的实际解决方案。目前的实践表明，钢筋混凝土面板的使用寿命可能比桥梁的设计寿命短得多。观察到的劣化情形表明，桥面劣化过程往往是由收缩裂缝引起的。这些早期的小裂缝由于冻融循环、动荷载的应用和约束收缩的演变而及时发展。提出了解决问题的主要来源，即抑制收缩，这是裂缝产生的主要原因。建议在甲板上创建初始应力，与约束收缩引起的应力相反，从而消除问题。初始应力将由施工初期在梁上产生的弯曲引起，并在混凝土成熟初期收缩迅速增加时对其进行调整；然后，弧度会逐渐释放。在浇筑混凝土板之前，使用液压千斤顶诱导弯曲。提出的研究将首先集中在开发一个数学模型，用于计算混凝土桥面与钢梁组合的约束收缩。该模型将基于混凝土自由收缩的最新知识，典型公路桥梁复合截面的设计，剪切连接件的行为，以提供复合截面变形的连续性或剪切螺栓可能的滑移。该模型将允许在混凝土成熟的早期阶段进行纵向应变/应力计算。这些值将根据与梁连接引起的自由收缩发展的约束来计算。在对可能的约束收缩应力进行评估后，提出切实可行的解决方案，以降低由收缩引起的甲板早期拉应力。理论分析之后将进行实验室测试，以验证所提出的解决方案。该方法经济可行，技术可行，可大幅度降低约束收缩引起的拉应力，有效消除混凝土桥面早期裂缝。所提出的方法可以大大节省桥梁维修费用。限制混凝土甲板早期张拉裂缝的方法将易于使用，并且可能不改变典型的甲板设计。与典型的甲板施工过程相比，它不会造成更长时间的交通中断。

AbstractThe project will focus on a practical solution to diminish early-restrained shrinkage cracks in bridge decks. Current practice shows that service life of a reinforced concrete deck slab can be much shorter than the design life of the bridge. Observed deterioration scenarios indicate that degradation process of bridge decks is very often initiated by shrinkage cracks. These early, small cracks develop in time due to freeze and thaw cycles, application of dynamic load and evolution of restrained shrinkage. It is proposed to address the main source of problems, a restrained shrinkage, which is the main reason for the initiation of cracks.It is proposed to create initial stresses in the deck, of opposite sign to those caused by restrained shrinkage and thus to eliminate the problem. The initial stress will be induced by a camber created in the girders at the beginning of construction process, and by adjusting it during the first period of concrete maturing when shrinkage rapidly increases; than, the camber will be gradually released. Prior to pouring of concrete slabs, camber is induced using hydraulic jacks. The proposed research will be initially focused on developing of a mathematical model for calculation of the restrained shrinkage in concrete bridge decks composite with steel girders. The model will be based state-of-the art knowledge on free shrinkage of concrete, the design of a typical highway bridge composite cross section, the behavior of shear connectors to provide the continuity in deformation of composite cross section or possible slip at shear studs. The model will allow for longitudinal strain/stress computations in the early phase of concrete maturing. These values will be calculated according to restrains induced to the free shrinkage development by a connection with girders.After the evaluation of possible restrained shrinkage stresses, a practical solution will be proposed to decrease the early tensile stresses in deck caused by shrinkage. The theoretical analysis will be followed by lab tests to validate the proposed solution.This economical and technically viable method could substantially decrease tensile stresses caused by restrained shrinkage and effectively eliminate early cracks in concrete bridge decks. The proposed method can result in significant savings in bridge maintenance costs. The method to limit early tensile cracks in concrete decks will be easy to use and possible without any changes in typical deck design. It will not require any longer traffic interruptions than during a typical deck construction process.