Evaluating the Continuous Deck Placement Sequence Approach at Steel Bridges Using a Time-Dependent Numerical Analysis

Abstract

Concrete bridge deck cracking can cause serious serviceability issues during a bridge’s design life and compromise a bridge’s structural strength. Early age concrete bridge deck cracking occurs within a month of the concrete deck being placed and prior to live load application. It can typically be attributed to two factors: (1) construction practices; and (2) concrete shrinkage. This article examines the continuous deck placement sequence approach and its influence on early age concrete bridge deck cracking. Although the continuous deck placement process is commonly used, studies related to stresses induced during this process and the potential for deck cracking when using this construction method are very limited to work conducted overseas. Consequently, a time-dependent analysis considering the continuous bridge deck placement sequence at a steel composite bridge is presented in this article to help explain possible causes of early age bridge deck cracking. The modular ratio approach discussed in the Eurocode 4 is incorporated to approximate the early age concrete elastic modulus. To show the significance of using a continuous placement sequence, tensile stresses along the bridge length centerline are compared with the allowable tensile stress at two times, six and twenty-seven days after the bridge deck construction is initiated. Additionally, the article includes a simplified approach for calculating maximum bridge thermal stresses. The study results in this article will serve to aid engineers in understanding possible factors resulting in early age bridge deck cracking when a continuous deck placement sequence is used for bridge deck construction. Early age bridge deck cracking can typically be attributed to construction practices and concrete shrinkage. Most bridges are constructed using a sequential deck placement where the concrete bridge deck is placed in discontinuous segments in an order to minimize tensile stresses developing in the concrete bridge deck. Conversely, a continuous deck placement entails placing the concrete bridge deck from one end of the bridge to the other end continuously. Contractors prefer using a continuous deck placement because of the method’s reduced construction time. The Arkansas Department of Transportation (ARDOT) has identified early age concrete bridge deck cracking at a disproportionate number of continuous steel girder bridges constructed using a continuous deck placement. Early age concrete bridge deck cracking develops when tensile stresses in the concrete bridge deck exceed the concrete’s tensile strength. During the bridge construction phase, concrete self-weight, construction loads, and thermal shrinkage are the primary load contributors to early age concrete bridge deck tensile stress. Since most continuous girder steel bridges consist of multiple short spans, early age concrete bridge deck tensile stress is primarily due to resistance to thermal shrinkage. An equation is derived in the article to calculate an approximate value for the thermal shrinkage stress. Thermal shrinkage cracking is averted by implementing proper curing methods to ensure the concrete tensile strength exceeds the induced tensile stress. Proper curing includes moist curing and minimizing moisture loss by using concrete curing blankets.