Strength Correction Factors and Stress-Strain Models for Injection Mortar Considering Specimen Size and Geometry

This study investigates the structural performance of injection mortar used in non-interlocked joints between steel girders and precast decks, and proposes a material model for composite bridge design and finite element analysis. Three mortar mixes of different sizes and shapes were tested for fresh-state properties (bleeding, flow, density, and air content) and hardened-state properties (compressive strength, elastic modulus, and Poisson’s ratio). The compressive strengths of different specimen geometries showed a high correlation (R<sup>2</sup> > 0.99), allowing the derivation of size-effect strength conversion factors. Existing code-based elastic modulus formulas overestimated the mortar stiffness; therefore, nonlinear regression on square-root and cubic-root models were used to propose a modified prediction equation. The resulting equation was incorporated into Eurocode 2 stress-strain relations, yielding nonlinear curves that agreed well with both laboratory and field test data.