Showing papers by "Zdenek P. Bazant published in 2004"

Temperature Effect on Concrete Creep Modeled by Microprestress-Solidification Theory

Abstract: The previously developed microprestress-solidification theory for concrete creep and shrinkage is generalized for the effect of temperature ~not exceeding 100°C!. The solidification model separates the viscoelasticity of the solid constituent, the cement gel, from the chemical aging of material caused by solidification of cement and characterized by the growth of volume fraction of hydration products. This permits considering the viscoelastic constituent as non-aging. The temperature dependence of the rates of creep and of volume growth is characterized by two transformed time variables based on the activation energies of hydration and creep. The concept of microprestress achieves a grand unification of theory in which the long-term aging and all transient hygrothermal effects simply become different consequences of one and the same physical phenomenon. The microprestress, which is independent of the applied load, is initially produced by incompatible volume changes in the microstructure during hydration, and later builds up when changes of moisture content and temperature create a thermodynamic imbalance between the chemical potentials of vapor and adsorbed water in the nanopores of cement gel. As recently shown, this simultaneously captures two basic effects: First, the creep decreases with increasing age at loading after the growth of the volume fraction of hydrated cement has ceased; and, second, the drying creep, i.e., the transient creep increases due to drying ~Pickett effect! which overpowers the effect of steady-state moisture content ~i.e., less moisture—less creep !. Now it is demonstrated that the microprestress buildup and relaxation also captures a third effect: The transitional thermal creep, i.e., the transient creep increase due to temperature change. For computations, an efficient ~exponential-type! integration algorithm is developed. Finite element simulations, in which the apparent creep due to microcracking is taken into account separately, are used to identify the consti- tutive parameters and a satisfactory agreement with typical test data is achieved.

Size effect on flexural strength of fiber-composite laminates

Abstract: The size effect on the flexural strength (or modulus of rupture) of fiber-polymer laminate beams failing at fracture initiation is analyzed A generalized energetic-statistical size effect law recently developed on the basis of a probabilistic nonlocal theory is introduced This law represents asymptotic matching of three limits: (1) the power-law size effect of the classical Weibull theory, approached for infinite structure size; (2) the deterministic-energetic size effect law based on the deterministic nonlocal theory, approached for vanishing structure size; and (3) approach to the same law at any structure size when the Weibull modulus tends to infinity The limited test data that exist are used to verify this formula and examine the closeness of fit The results show that the new energetic-statistical size effect theory can match the existing flexural strength data better than the classical statistical Weibull theory, and that the optimum size effect fits with Weibull theory are incompatible with a realistic coefficient of variation of scatter in strength tests of various types of laminates As for the energetic-statistical theory its support remains entirely theoretical because the existing test data do not reveal any improvement of fit over its special case, the purely energetic theory-probably because the size range of the data is not broad enough or the scatter is too high, or both

Micro- and Nanomechanics

Abstract: It has been known for quite some time that materials and structures with small-scale dimensions do not behave in the same manner as their bulk counterparts. This aspect was first observed in thin films where certain defect structures were found to have deleterious effects on the film’s structural integrity and reliability. This became a significant concern because thin films are routinely employed as components in microelectronics and microelectromechanical systems (MEMS). Their properties frequently allow essential device functions and therefore accurate identification of these properties is key to the development of new technologies. Unfortunately, most of our knowledge is based on bulk material behavior, which many times fails to describe material response in small-scale dimensions because of the dominance of surface and interface effects, finite number