So far, creep reversibility of concrete has been tested either under single unloading or stepwise (''staircase'') stress histories. However, these investigations do not provide a complete understanding of the creep behavior under repeating stress histories, similar to variable live load histories in usual concrete structures. Typical examples are parking garages, bridges or storage buildings with frequent but still longer-term loadings and unloadings. Therefore, the paper attempts to extend the knowledge on the creep under frequently repeated stresses by testing concrete specimens under various loading and drying conditions. The creep-recovery versus creep ratio over time, considered here as a measure of creep reversibility, was studied within two separate experiments. The first experiment aims to assess the influence of different service stress levels with 30% and 45% of the concrete compressive strength f c as well as different unloading levels of full and partial unloading. The second one focuses on the influence of different hygral conditions of the specimens, namely sealed and unsealed conditions. Regardless of the stress level, the creep-recovery versus creep ratio tends to approach unity after a sufficient number of loading and unloading cycles. Drying conditions show negligible effects on the creep recovery. On the other hand, they have remarkable influence on the proportion of the creep reversible in each loading cycle. Moreover, the basic creep component shows high reversibility under repeating stress histories. The results demonstrate that the recovery behavior under repeating stresses pronouncedly differs from the ones under just sustained stresses. Keywords Creep recovery Á Repeating load history Á Creep-recovery versus creep ratio Á Basic creep Á Drying creep Abbreviations COV Coefficient of variation d max Maximum aggregate size E cm , E(t 1 ), E 28 (Mean, time-dependent, 28 days) modulus of elasticity of concrete f c , f cm (Ultimate, mean) concrete compressive strength J, J rec (Creep, creep recovery) compliance function n Number of stress increments or decrements
Abstract:The objective of this paper is to study the influence of repeated variable action on long-term behavior of concrete structural elements using quasi-permanent combination of actions, for the assessment of long-term effects (e.g., effects due to creep and shrinkage in concrete structures), as it is proposed in Eurocodes. Extensive experimental program and analytical research using model B3 and AAEM (age adjusted effective modulus) method was performed in order to define quasi-permanent factor ψ 2 for two specific loading histories. These loading histories were consist of long-term permanent action "G" and repeated variable action "Q". The variable load was applied in cycles of loading/unloading for 24 h and 48 h in period of 400 days appropriately for one series of concrete elements. 24 reinforced concrete beams, dimensions 150 mm × 280 mm × 3,000 mm, were tested. Twelve beams were made of concrete class C30/37 and 12 of concrete class C60/75.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.