  Ultra high performance concretes (UHPCs) are cementitious composite materials
with high level of perfor- mance characterized by high compressive strength,
high tensile strength and superior durability, reached by low water-to-binder
ratio, optimized aggregate size distribution, thermal activation, and fiber
reinforcement. In the past couple of decades, more and more UHPCs have been
developed and found their ways into practice. Thus, the demand for
computational models capable of describing and predicting relevant aging
phenomena to assist design and planning is increasing. This paper presents the
early age experimental characterization as well as the results of subsequent
simulations of a typical UHPC matrix. Performed and simulated tests include
unconfined compression, splitting (Brazilian), and three-point-bending tests.
The computational framework is formulated by coupling a hygro-thermo-chemical
(HTC) theory and a comprehensive mesoscale discrete model with formulated aging
functions. The HTC component allows taking into account various types of curing
conditions with varying temperature and relative humidity and predicting the
level of concrete aging. The mechanical component, the Lattice Discrete
Particle Model (LDPM), permits the simulation of the failure behavior of
concrete at the length scale of major heterogeneities. The aging functions
relate the mesoscale LDPM mechanical properties in terms of aging degree,
defined in this work as the ratio between the quasi-static elastic modulus at a
certain age and its asymptotic value. The obtained results provide insights in
both UHPC early age mechanisms and a computational model for the analysis of
aging UHPC structures.
