Recommendations for the design of ultra-high performance concrete structures

Abstract

New materials frequently require modifications or rewrites of existing construction codes. They may also need new methods for their manufacture and installation. DUCTAL, a new ultra-high performance concrete (UHPC) with enhanced tensile, compressive, and deflective behavior offered by LaFarge, is one such material, and current guidelines for concrete do not sufficiently account for these improved properties. Research by other universities and professional institutions has produced sequential recommendations, beginning with the experiment-based set from the Association Francaise de Genie Civil (AFGC) through the analytically-based set from MIT. In this thesis, the MIT approach is further developed into a coherent method for hardened UHPC design. The first two sections familiarize the reader with the analytical model for UHPC and the evolution in design codes and their philosophical bases. Essential concepts, such as the two-phase matrix-fiber behavior of the material and the use of a maximum crack width criterion to govern design, are explained. Next, the most current design guidelines are presented in full, with attention paid to bending and shear resistance. Comparisons with previous codes demonstrate the ability of these guidelines to produce more structurally efficient sections which consume less material.

(cont.) Analysis of the recommendations themselves will demonstrate the existence of a size effect and the cross-sectional parameters that affect structural efficiency most. Optimization based on the one-dimensional analytical model closes with an analysis of different cross-sections for their structural efficiency, span-to-height ratios, required prestressing, and amount of material consumed. The one-dimensional model is then extended to three-dimensions, providing the framework and relations needed to perform non-linear finite element analysis. Practical consequences of the differences between the 1-D and 3-D models allows for the proposed MIT guidelines to be validated and their safety ensured. A dynamic analysis of a box section optimized according to the proposed guidelines is then performed with the aid of the 3-D model, and the results demonstrate its safety. Overall, the reader shall be given an outline of how to design for hardened UHPC.