Macro-meso properties and constitutive behavior of concrete affected by early-age loading.

• The effect of early-age loading on the mechanical properties of concrete is related to meso-structure. • The mechanical properties of concrete deteriorate with the increase of early-age loading. • The pore structure characteristics of concrete were obtained by nuclear magnetic resonance. • A constitutive behavior which can reflect the initial compaction stage is established. To explore the influence of early-age loading on the mechanical properties and internal structures of concrete, loads equivalent to 10%, 25%, and 40% of the concrete current compressive strength were applied to 1 day and 3 days specimens. The results of mechanical properties and mesoscopic scanning tests show that the development of uniaxial compression damage of concrete affected by early-age loading is divided into three stages. With an increase in early-age loading, the elastic modulus decreased and the Poisson's ratio first increased and then decreased. When the loading was 10% of current compressive strength of specimens, the number of macropores and cracks would decrease, and the subsequent mechanical properties of concrete would not be affected considerably. A continuous increase in early-age loading, the proportion of mesopores and cracks increased considerably, the cementitious structure loosened, and the mechanical properties of concrete deteriorated in the subsequent stage. By analyzing the relationship between the total deformation and the deformation of primary flaws and skeleton, the concept of void strain ratio was proposed. Accordingly, a constitutive behavior that reflects the nonlinear characteristic of primary flaws compaction stage during uniaxial compression process was established, the theoretical calculation is in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]