Progress in creep and shrinkage prediction engendered by alarming bridge observations and expansion of laboratory database

This keynote lecture presents a broad, yet concise, overview of the advances at Northwestern University since ConCreep-8, triggered by the success in 2008 in enforcing the release of the (legally sealed) data on the tragic 1996 collapse and the preceding, grossly excessive, deflections of the record-span segmental box-girder bridge in Palau. Subsequent analysis put the main blame for the excessive deflections on wrong design codes or recommendations. This revelation stimulated an extensive effort, generously funded by DoT and NSF, to advance the knowledge of concrete creep. First, several advances, presented in detail in other papers at ConCreep-9, are briefly reviewed; they include: 1) assembly of a greatly enlarged database of laboratory creep and shrinkage data for various concretes, with and without admixtures; 2) collection of a database on deflections of 69 segmental bridges, mostly excessive; 3) development of an improved creep and shrinkage prediction model (labeled «B4») and its joint statistical calibration by both databases; 4) improved rate-type algorithm for creep analysis in which the inverse Laplace transform is used in each time step to obtain the current retardation spectrum; 5) a realistic viscoplastic constitutive law for prestressing steel, applicable to time-varying strain and temperature; and 6) an improved algebraic formula for converting the aging compliance function to the relaxation function. Development of new formula for cyclic creep is described next, in detail. The formula is derived from Paris' law for the growth of micrometer size cracks. Its consequences for segmental bridges of various spans are discussed. The final discussion outlines new theoretical results showing that one mechanism causing the hysteresis of nanopore water sorption-desorption isotherms consists of snap-through instabilities of water content, and of molecular coalescence. The nanopore water content matters for the disjoining pressure and the corresponding microprestress which destabilizes the bonds whose ruptures and restorations cause creep.