High-strength concrete behavior in post-limit conditions

The objects of study were specimens of various shapes and aspect ratios made of high-strength modified concrete B90-B100 with a modified elastic modulus of 55,000 MPa. This modulus significantly exceeds the normative values specified in the building code SP 63.13 330 when the concrete is loaded beyond its ultimate state. The need for this study stems from insufficient research on the deformation characteristics of high-strength concretes in extreme states (after reaching the ultimate load — with or without subsequent unloading), as well as the inapplicability of classical microcracking theories (Berg, Winter) to describe their behavior, which requires the development of new evaluation methods. Since high-strength concretes fail brittly, the research methods included two approaches to loading specimens — loading by stresses (standard method) and additionally by deformations up to the peak failure load with subsequent unloading and holding (from 1 hour to 8 days); the elastic modulus was determined according to GOST 24 452 before and after loading, while microcrack development was monitored using ultrasonic testing methods (through-transmission and surface sounding). Based on the research results, it was established that during short-term holding (1 hour), the elastic modulus increased by 40−71% (reaching 73,602−101,192 MPa) — this is explained by crack closure during specimen compression and the inertia of the stress relaxation process, while strength decreased by 20%. After holding ≥1 day, the elastic modulus (49,684−57,683 MPa) and strength approached the initial values (±6%), despite visible damage to specimens after initial peak load attainment. At the same time, the ultrasonic wave travel time and Poisson’s ratio (0.21−0.26) remained practically unchanged up to 90% of the failure load, which does not correspond to classical microcracking development models. The main conclusions of the work: high-strength concretes retain nearly linear deformation behavior even after reaching the ultimate state. These results cast doubt on existing theoretical models describing crack formation processes and emphasize the importance of accounting for stress relaxation in structural assessments, as well as the necessity for comprehensive research on high-strength concretes.