Donald J Janssen

INFLUENCE OF EARLY AGE VOLUME CHANGES ON LONG-TERM CONCRETE SHRINKAGE

Volume changes can occur in concrete during the first 24 hr and are generally missed in laboratory shrinkage evaluations. Unfortunately these early age volume changes are present in real pavements and structures and can contribute to the cracking behavior of the concrete at later ages. Early age volume changes can occur in two forms: drying shrinkage before the start of curing and autogenous volume changes. Although these early age volume changes are often dismissed as being insignificant, recent work in Europe has identified magnitudes for early age volume changes of some concretes that are equal to or greater than 28-day drying shrinkage measurements. Expansions have also been identified in some cases. The results of some investigations of volume changes in concrete during the first 24 hr under both drying and nondrying conditions are presented. An example of potential long-term cracking under partially restrained conditions (concrete slab-on-grade modeled by a concrete ring cast around a hollow steel ring) is used to illustrate the magnitude of influence of early age volume changes on concrete cracking. Both test procedures employ nonstandard methods to quantify the cracking potential of concrete.

Methodology for Identifying Zero-Stress Time for Jointed Plain Concrete Pavements

This study focused on identifying the zero-stress time (TZ) in jointed plain concrete pavements (JPCPs). TZ is the time when the concrete slab is sufficiently strong to deform (thermally expand or contract and thus curl) despite the existing external restraints, including the friction at the base-slab interface. It is critical to be able to identify TZ so that the temperature gradient present in the slab at TZ, known as the built-in temperature gradient, can be characterized. In this study, TZ was established through the instrumentation of 36 concrete slabs in four JPCP construction projects. Strain-temperature behavior in each slab was used to identify TZ. The slabs in each project were paved at different times of the day (morning, noon, early afternoon, and late afternoon) to investigate the effects of the ambient curing conditions on TZ. The degree of hydration at TZ (αTZ) was established for each slab. The field data were used in the development of a model for predicting αTZ as a function of the concrete water-to-cement ratio, unit weight, early-age elastic modulus, and slab thickness.

Material Characterization of Silicone Sealants

Silicone sealants were analyzed using dynamic shear rheometry (DSR) and numerical analysis to determine if a method could be developed that would provide the basis for a performance-based specification. The research used a three-phase approach including DSR analysis of aged and unaged sealants, numerical model development using the DSR data as input, and laboratory tension experiments for model verification. Results of the investigation indicated that the average material properties determined through DSR and laboratory tensile testing appeared to be representative of the “true” material properties for elongations of up to 25 percent. The results were less accurate for 50 percent elongation but still acceptable. The DSR testing could be related to field performance; however, conducting several tests on multiple samples to develop a discrete stress-relaxation spectrum for numerical analysis would not be feasible for most users. Instead, two test temperatures should be selected for DSR testing based upon the maximum and minimum in-use temperature the sealant would be exposed to for a given application.