New insights into the load sequence effect: Experimental characterization and incremental modeling of plain high-strength concrete under mode II fatigue loading with variable amplitude

Reinforced concrete (RC) infrastructure typically endures repeated subcritical loading throughout its operational lifespan, potentially compromising structural integrity. A realistic loading condition of these structures consists of constantly changing maximum and minimum load events. Characterizing the fatigue response of concrete proves challenging due to its heterogeneous and quasi-brittle nature, leading to varying fatigue lifetimes depending on the loading sequence. Understanding the impact of this so-called load sequence effect stands as a critical facet in ensuring structural longevity and reliability of current infrastructure. This paper offers insights into this phenomenon, focusing on unraveling the intricate behavior of plain concrete under multistage fatigue loading. Since the experimental investigation of the load sequence effect is highly complex due to the large scatter of test results, a newly developed, mode II test setup was employed. The advantage of this test setup resides in the precise specification of stress state and fracture zone. The systematic investigation of over 50 specimens elucidates the pronounced load sequence effect and provides insight into the underlying mechanisms. Based on these experimental results, a deformation-based incremental modeling approach was developed which considers the effect of loading sequence and provides not only the fatigue life but also the entire evolution of deformation. The experimental recalculation underscores the efficacy of this approach over the conventional Palmgren-Miner (P-M) rule.