  This paper presents an investigation of the age-dependent size effect and
fracture characteristics of an ultra high performance concrete (UHPC). The
study is based on a unique set of experimental data connecting aging tests for
two curing protocols of one size and scaled size effect tests of one age. Both
aging and size effect studies are performed on notched three point bending
tests. Experimental data is augmented by state of the art simulations employing
a recently developed discrete element based early-age computational framework.
The framework is constructed by coupling a hygro-thermo-chemical (HTC) model
and the Lattice Discrete Particle Model (LDPM) through a set of aging
functions. The HTC component allows taking into account variable curing
conditions and predicts the maturity of concrete. The mechanical component,
LDPM, simulates the failure behavior of concrete at the length scale of major
heterogeneities. After careful calibration and validation the mesoscale
HTC-LDPM model is uniquely posed to perform predictive simulations. The
ultimate flexural strengths from experiments and simulations are analyzed by
the cohesive size effect curve (CSEC) method, and the classical size effect law
(SEL). The fracture energies obtained by LDPM, CSEC, SEL, and cohesive crack
analyses are compared and an aging formulation for fracture properties is
proposed. Based on experiments, simulations, and size effect analyses, the
age-dependence of size effect and the robustness of analytical size effect
methods are evaluated.
