MICROSTRUCTURAL ASPECTS OF THE MECHANICAL RESPONSE OF PLAIN CONCRETE

It is generally acknowledged that mechanical properties of concrete ar ch as strength and stiffness are influenced directly by the physical p roperties of material microstructure. However, this relationship is ra rely addressed in constitutive models of material response to mechanic al loads. Instead the most frequently Sought alternative for developme nt of such models is by curve-fitting of the database of experimental stress-strain results Sor each range of normal cylinder strength, this being related to the water-cement ratio. A simplified constitutive mo del that recognizes and incorporates the properties of microstructure and its influence on mechanical response is developed in this paper. T he model evaluates uniaxial compressive stress at arty level of axial strain, using a strain-dependent estimate of material stiffness. The i nitial elastic modulus of uncracked concrete is evaluated, based on wa ter-cement ratio, age, volume fraction of aggregates, paste porosity, degree of hydration, and paste-aggregate interface properties. Reducti on of the initial modulus with increasing load is modeled by the appli cation of a factor that depends on the natural porosity of the materia l and mechanically induced porosity as it is assessed by the area stra in that develops in the cross section supporting the load. Using this approach, it is possible to model the change effected on the initial r esistance of the material from progressive microcrack growth and inter nal damage occurring in the concrete. The sensitivity of the proposed model to several variables was evaluated from parametric studies and b y comparisons with available experimental data.