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AbstractReturned concrete, a waste product of ready-mixed concrete (RMC), is generally washed to reclaim coarse aggregates from the finer portion consisting of fine aggregate, hydration products of cement, and un-hydrated cement particles, if any. In searching for environmentally sustainable construction materials, this finer part, termed as washed fines (WF), may substitute natural sand in producing mortar and concrete. An extensive study was conducted to determine the feasibility of replacing sand with WF by testing mortar cubes (50 mm x 50 mm x 50 mm) and prisms (285 mm x 25 mm x 25 mm) at replacement levels of natural sand by WF at 5%, 10%, 15%, and 20% for compressive strength, sorptivity, and drying shrinkage. The specimens were made with different types of cement. The microstructure of hardened mortar around WF was also examined using scanning electron microscope (SEM). Field trials reusing WF in concrete were conducted at an RMC plant to validate the laboratory results. Findings indicate compressive strength of mortar increased up to 19% with partial sand replacement by WF. The incorporation of WF reduced shrinkage and sorptivity coefficient. SEM images revealed a dense microstructure around WF in hardened mortar. [ABSTRACT FROM AUTHOR]

This paper deals with the macrocell and microcell corrosion of steel bars in cracked concrete exposed to a marine environment. For this, short-term accelerated laboratory and long-term exposure tests were carried out. Test items include electrochemical and physical evaluations of corrosion, chloride ion content in concrete, and oxygen permeability through concrete. Analyses by scanning electron microscopy and electron probe microanalyzer were also carried out to confirm the deposit in the healed cracks. For an unhealed crack, the portion of the steel bar located at the root of the crack acts as an anode and other portions as a cathode. Also, microcell corrosion at the cracked region is higher than the uncracked region. Macrocell corrosion process and the degree of macro and microcell corrosion are significantly influenced by the water/cement. In a natural marine environment, narrower cracks heal due to the deposition of ettringite, calcite, and brucite in the crack. This reduces the corrosion rate significantly at the cracked region. DOI: 10.1061/(ASCE)0899-1561(2003)15:5(460) CE Database subject headings: Chlorides; Concrete; Corrosion; Steel; Cracking; Coastal environment.

A study on the influence of crack widths and type of bars (plain and deformed) on corrosion of steel bars in cracked concrete is presented here. Microcell and macrocell corrosions of plain and deformed steel bars were investigated on 10 X 10 X 40 cm single crack specimens with crack widths of 0.1, 0.3, and 0.7 mm. Water-to-cement ratios were 0.3, 0.5, and 0.7. Electrochemical investigations were also conducted on 15 X 15 X 125 cm multicrack specimens with plain and deformed bars. For these specimens, water-to-cement ratios were 0.5 and 0.7 and crack widths were varied from 0.1 to 0.4 mm. After electrochemical investigations, chloride ions in concrete, corroded areas, weight losses, and pit diameters of the steel bars were investigated. The entire study was carried out in an artificially created chloride ion-induced corrosion environment. The study concludes that the relationship between crack widths and corrosion rate is observed at the very early age of exposure. Water-to-cement ratio and corrosion rate relationship is clearer than crack widths and corrosion rate relationship. Deformed bars are more prone to corrosion than plain bars.

The article presents a study which investigates the corrosion of steel bars and steel-concrete interface in a given a variety of surface conditions including mill-scaled (M), polished (P), brown-rusted (BR), black-rusted (BL), and pre passivated (PP). The objective of the study is to look for possible ways of improving steel-concrete interface. The study showed that the chloride threshold level for M, BR and P were 0.4, 0.5 and 0.8% respectively and that PP was observed to be the densest.

Presents a detailed study of concrete specimens made with portland cement and alumina cement exposed to the marine environment for a period of 30 years. Use of both tap water and seawater as the mixing medium; Evaluation of the compressive strength, chloride ingress, microstructure, mineralogy, concrete interfaces and steel bar corrosion; Revelation of the higher strength of alumina cement and saltwater against the same using tap water; Absence of significant transformation of hydration passes; Microstructure density.

Investigates 30-year-old concrete specimens made with different cements. Analysis on the results of chloride ingress and corrosion of steel bars in concrete; Examination of carbonation depth, water-and acid-soluble chloride concentrations in concrete and corrosion of steel bars in concrete; Effect of the porous steel-concrete interface on alumina cement; Benefit offered in using slag cement.

Investigates properties of concrete subjected to long-term exposure under the marine environment. Examination on the mineralogy and microstructure of mortar portions; Comparison of the microstructure of concrete for alumina cement and Type B slag cement between outer and inner region; Characterization of AL in the inner region.