Showing papers by "Zdenek P. Bazant published in 1978"

Pore Pressure and Drying of Concrete at High Temperature

Abstract: A mathematical model for water transfer in concrete above 100°C is developed. Drying tests of heated concrete are reported and material parameters of the model are identified from these tests as well as other test data available in the literature. It is found that water transfer is governed principally by the gradient of pore pressure, which represents the pressure in vapor if concrete is not saturated. Permeability is found to increase about 200 times as temperature passes 100°C, which could be explained by a loss of necks on migration passages. The pore volume available to free water increases as dehydration due to heating progresses and as the pore pressure is increased. The temperature effect on pressure-water content (sorption) relations is determined. Thermodynamic properties of water are used to calculate pore pressures. A finite element program for coupled water and heat transfer is developed and validated by fitting test data.

Statistical Stability Effects in Concrete Failure

Abstract: Using a uniaxial localization model, it is shown that parallel elastic restraint increases ductility, while an increase in support flexibility or length of specimen reduces ductility. Statistical macroscopic nonhomogeneity of the specimen is modeled by a system of uniaxial parallel elements of random properties following the normal distribution. The stability analysis and Monte Carlo simulations explain that in such a system an increase of length or support flexibility reduces not only ductility but also the strength of the system. The effect on strength depends on the number of elements (width of specimen), which represents a new non-classical statistical size effect, and on the standard deviation of peak stress values within the parallel system. Existence of an inflection point and the prolonged tail on the descending branch is explained by the nonhomogeneity of the specimen, and the shape of the descending branch, along with the location of the inflection point, is obtained as a function of machine stiffness, parallel elastic restraint, and specimen length and width.

Constitutive Equation for Cyclic Behavior of Cohesive Soils

Abstract: Endochronic theory is employed to develop a relatively general constitutive relationship to model the dynamic behavior of cohesive soils subjected to multi-dimensional stress or strain paths. The proposed constitutive law is capable of describing (a) strain softening and hardening, (b) densification and dilatancy, (c) frictional aspects, and (d) rate dependence of the stress-strain behavior; it also accounts for pore pressure response in undrained conditions by considering saturated soils as two-phase media. The theory is based on a series of new internal state variables that are defined in terms of semi-empirical intrinsic material relationships, and it is able to handle elastic and inelastic strain histories for rate-dependent materials from the very beginning of their cyclic stress-strain path. The intrinsic relations involve ten material parameters, including the initial elastic modulus, which must be determined from a quasi-static and cyclic tests. Although limited data preclude the development at this time of specific correlations for the material parameters in terms of soil characteristics, this model offers a significant improvement in the interpretation and analysis of the cyclic behavior of cohesive soils. The mathematical model is applied to describe data from low-frequency cyclic constant-strain-rate tests on undisturbed samples and low-frequency cyclic constant-load-amplitude tests on slurry-consolidated samples of kaolin; both types of test were conducted under undrained triaxial conditions with pore pressure measurements. Emphasis is directed toward the behavior of cohesive soils under low-frequency, large-strain cyclic conditions, such as those associated with earthquakes. /Author/

Part III: Drying Creep

Abstract: The practical model for predicting creep and shrinkage developed in Parts I and II is extended to creep at drying environment and constant temperature. The increase of creep due to drying is related to shrinkage. Formulas for determining material parameters from concrete strength and mix composition are presented and verified by extensive comparisons with test data from the literature. INTRODUCfION The prediction models for shrinkage and basic creep, developed in Parts I and II, must now be extended to cover creep in a drying environment. The expressions describing this behavior were developed in a preceding work [4] e), and they were shown to allow good fits of test data. However, each data set was fitted indivi­ dually, and no formulas that correlate the material parameters and predict them from the given concrete strength and mix composition were derived. This will be done in this part, in which data analysis of un­ precedented scope (24 different mixtures) will be undertaken.