Elastic modulus of high performance fiber-reinforced cement-based composites.

Abstract

The main objective of this research is to study and evaluate the various fundamental mechanisms that influence the modulus of elasticity and the stress-strain response of high performance fiber reinforced cement based composites such as SIFCON (Slurry Infiltrated Fiber Concrete). The research comprises an experimental and an analytical program. The experimental program focuses on a comprehensive series of uniaxial tests on fiber reinforced concrete (FRC) and SIFCON in both tension and compression. The analytical program includes first an extensive review and evaluation of existing models to predict the modulus of elasticity of fiber composites; second, a finite element analysis of a composite unit cell; and third, a semi-rational prediction model recommended for practical use. The experimental tests dealt with compression cylinders cast horizontally and vertically (to study fiber orientation effects), and dogbone shaped tension specimens. Variables included up to 10 different matrix mixes with strengths ranging from 3 ksi to 13 ksi, different types of steel fibers, different fiber volume fractions and aspect ratios, different fiber lengths including the case of continuous fibers, and bonded versus unbonded fibers (through the use of a greasing agent). Results from the experimental program confirmed the influence of some variables such as fiber volume fraction, fiber orientation, and fiber to matrix modular ratio, E$\sb{f}$/E$\sb{m}$. They also identified few other variables found uniquely important in cement based composites, namely the bond at the interface between the fiber and the matrix, matrix porosity, and fiber aspect ratio. It was also observed that the elastic modulus of fiber reinforced cement based composites increases with the compressive strength of the matrix, and that the presence of fibers, even when only matrix infiltration is used, greatly affects (in an adverse way) the porosity of the matrix. An extensive evaluation of seventeen existing analytical models to predict the elastic modulus of fiber composites showed that neither of these models is adequate enough to account for all the factors that influence most the elastic modulus of fiber reinforced cement composites such as bond and matrix porosity. Moreover, the finite element analysis revealed that the elastic modulus decreases with a decrease in bonded length, and that fiber end conditions (with and without end stress transfer capability) are critical especially for high fiber to matrix modular ratios. To account for the newly identified variables, namely the bond and the porosity, a semi-rational prediction model is proposed and is shown to be in good agreement with the experimental observations.