Strain gradient effects on flexural strength design of normal-strength concrete beams

In the flexural strength design of normal-strength concrete (NSC) beams, the resultant concrete force and its centroid within the compression zone is generally expressed in an equivalent rectangular stress block. The equivalent concrete stress is expressed as , where is the uni-axial concrete cylinder strength. Currently, the value of a stipulated in various design codes for NSC is taken as 0.85. Nonetheless, in an experimental study conducted earlier by the authors on NSC columns subjected to concentric and eccentric axial loads, it was found that the value of a significantly depends on strain gradient, which generally increases as strain gradient increases until it reaches a maximum value. Therefore, strain gradient is a critical factor and should be considered in the flexural strength design of NSC members. In this paper, a new flexural strength design method that incorporates the effects of strain gradient is developed for NSC beams. An equivalent rectangular concrete stress block, which is strain-gradient dependent, is proposed and applied to produce a set of equations for the flexural strength design of singly and doubly reinforced NSC beams with various concrete strengths. Lastly, these equations are converted into charts for practical design application. Copyright (c) 2010 John Wiley & Sons, Ltd.