An experimental study was undertaken to investigate aspects of durability of reinforced concrete elements subjected to the combined action of mechanical pre-cracking and alkali-silica reaction (ASR). Concrete beams with a high alkali content were pre-cracked in cyclic loading and subsequently submerged partially in water, or exposed to high humidity according to ASTM C1293-07 conditions (long-term method). Aggregates of relatively high, and aggregates of low reactivity were used. Reference beams with both reactive and less reactive aggregate were placed in the ASR chambers to compare the responses of specimens exposed to the combined action and specimens subjected to the ASR action only. The crack widths were monitored in pre-cracked specimens under both conditions (humidity, partially submerged). The test results indicate that the expansion due to the combined action at the middle of samples (mechanical crack zone) was significantly different in specimens exposed only to a high alkaline environment. The micro and macro cracks, induced by cyclic load, form ingress paths leading to increased ASR rate, while the macro-mechanical cracks provide regions of relatively free expansion, as deduced from decreasing crack widths.
Previous studies demonstrated that alkalis can considerably affect the volumetric properties of hardened cementitious binders, including shrinkage, creep, and micro-cracking. The objective of this paper is to reexamine the effect of alkalis on the composition, nanostructure, phase stability, and morphology of calcium-silicate-hydrate (C-S-H) in cementitious materials, and to further propose a conceptual model bridging the C-S-H characters with its volume change mechanisms in alkali-enriched systems. The proposed microstructural model is an extension of the colloidal model of C-S-H considering the effect of alkalis. It is suggested that the presence of alkalis makes C-S-H more thermodynamically unstable and structurally disrupted, and easier to reorganize and redistribute upon drying-induced internal stresses or external loading. In addition, new experimental results regarding the influence of lithium on shrinkage and micro-cracking of alkali-activated blast-furnace slag are discussed. It shows that lithium addition can dramatically improve the cracking resistance and volumetric stability of alkali-activated slag.
The shear behavior of corroded reinforced concrete (RC) members is an urgent problem to be solved. The study carried out an experimental program on the shear behavior of 21 beams, which included 3 un-corroded RC beams and 18 corroded ones. Both shear span-to-depth ratio and corrosion degree influencing the shear strength as well as failure patterns were discussed in detail. This paper aims to contribute to a better understanding about the effect of corrosion on the shear behavior of corroded RC beams. The test results show that corrosion of stirrups reduces the aggregate interlock capacity of concrete, also the ductility and shear strength of the flexural members. In shear-compression zone, cracks caused by corrosion of stirrup have a predominant influence on shear strength; Failure modes of RC beams depend largely on shear span-to-depth ratio and are little affected by corrosion of stirrups. Based on the results of experiments and the limit equilibrium theory, the calculation model of shear capacity for corroded simply supported RC beams is proposed, which has a simple form and clearly physical conception.
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