Hidenori Hamada

CHLORIDE DIFFUSION, MICROSTRUCTURE, AND MINERALOGY OF CONCRETE AFTER 15 YEARS OF EXPOSURE IN TIDAL ENVIRONMENT

This research describes a detailed study conducted into the chloride diffusion, microstructure, and mineralogy of 15-year-old concrete specimens of dimensions 150 mm in diameter and 300 mm in height. These specimens were exposed to a tidal environment. The cement types were ordinary portland cement, slag cements of Types A, B, and C, and Type B fly ash cement. Water-cement ratio was 0.45. Compressive strength, carbonation depths, pulse velocity, water- and acid-soluble chloride profiles, microstructure, and mineralogy of concrete were evaluated. Results are provided.

Corrosion of Steel Bars in Concrete with Various Steel Surface Conditions

Marine concrete structures are prone to deterioration by the corrosion of the internal steel reinforcing bars. This article reports on a study carried out to investigate the corrosion of steel bars and steel-concrete interface in concrete comparing various surface conditions of steel bars, such as mill-scaled (M), polished (P), brown-rusted (BR), black-rusted (BL), and pre-passivated (PP). Other variables were considered as possible ways to improve the steel-concrete interface, including PP, revibration of the concrete after casting, and insertion of steel bars in concrete after casting. The specimens were exposed to an artificially created accelerated marine exposure and tested for the corrosion of steel bars and chloride ingress into concrete at a regular interval. Steel-concrete interfaces were examined by a scanning electron microscope (SEM) to check the nature of the steel-concrete interface with the variation of surface condition and casting method. Results showed that application of a cement paste coat over the steel bar created a remarkably dense steel-concrete interface compared to the other types investigated, and therefore significantly improved the chloride threshold level. M bars produced the most corrosion compared to the other bars investigated. Revibration causes a slight improvement of mechanical properties of concrete; however, it causes relatively earlier corrosion over the steel bars due to the formation of micro-crevices at the steel-concrete interface.

MICROSTRUCTURES AND INTERFACES IN CONCRETE AFTER 15 YEARS OF EXPOSURE IN TIDAL ENVIRONMENT

Detailed investigations on the properties of concrete specimens after long-term exposure to a marine environment are very scarce in the technical literature. In this paper, a detailed study of microstructure and aggregate-matrix and steel-matrix interfaces in concrete specimens after 15 years of exposure to a tidal environment was conducted. Cement types were ordinary portland, slag cements, and fly ash cement. The results of this research will be useful in understanding the changes in microstructure and interfacial conditions after a long-term exposure, and comparing them with the short-term laboratory test results.

Corrosion of horizontal bars in concrete and method to delay early corrosion

A detailed investigation was carried out to clarify the macrocell and microcell corrosion of horizontally oriented steel bars in concrete. To find an effective way to control corrosion of horizontal bars in concrete, three alternatives, such as the application of a cement paste coat over the steel bars, revibration, and the use of expansive admixture were studied. For this, prism specimens 200 mm in length, 150 mm in width, and 500 mm in depth were made with steel bars at three horizontal levels at 100, 250, and 400 mm from the bottom. The specimens were exposed to artificially-created wetting and drying cycles using natural seawater. Electrochemical and physical evaluations of corrosion, chloride ingress, and SEM investigation on the steel-concrete interfaces were carried out. A porous steel-concrete interface under the bottom half of the steel bar was found. The weaker interface caused early corrosion under the bottom half of the steel bar. Generally, the macro-corrosion cell is formed between the top half (anode) and bottom half (cathode) of the steel bar. A significant amount of micmcell corrosion was found over the bottom half. The application of a cement paste coat over the steel bars before embedding into concrete was found to be an effective way to delay early corrosion under the bottom half of the steel bars. The use of an expansive admixture was also found as another alternative. No improvement of steel-concrete interface was obtained by revibration; revibration creates more corrosion over the steel bars.

Corrosion of Steel Bars in Cracked Concrete Made with Ordinary Portland, Slag and Fly Ash Cements

Corrosion of steel bars in pre-cracked prism specimens exposed in marine environment for 15-years is presented here. The size of the specimens was 100x100x600 mm. The specimens were made with ordinary portland, slag (Type A, B and C) and fly ash (Type B) cements. A round steel bar of diameter 9 mm was embedded in each specimen. Water cement ratios were 0.45 and 0.55. Crack widths were varied from 0.1 to 5 mm. Chloride-ion concentrations in concrete and corrosion of steel bars were evaluated. Narrower cracks (< or = 0.5 mm) heal irrespective of the cement types. Chloride ingress and corrosion of steel bars in concrete are highest for the specimens made with ordinary portland cement and lowest for the specimens made with slag cement of Type C. Locations of the maximum corroded area as well as the deepest corrosion pit are not necessarily at or near the cracked region. Wider cracks are not healed and maximum corroded area and deepest corrosion pit are observed at the cracked region. The presence of voids at the steel-concrete interface results in corrosion pits even for chloride-ion concentration less than 0.4% of cement by weight.

Marine Durability of 23-Year-Old Reinforced Concrete Beams

This paper gives findings of marine durability tests of 23-yr-old reinforced concrete beams exposed to marine tidal and atmospheric environments. The study mainly focused on corrosion of steel bars with respect to orientation in concrete, carbonation and chloride-ion induced corrosion of steel bars, quantitative evaluation of corrosion based on the electrochemical data, and the influence of stirrups on macrocell formation. Experimental work covered electrochemical investigations, chloride ion profile and carbonation depth measurements, microscopic steel concrete interfaces, and visual observation of steel bar corrosion. The study found that chloride-ion induced corrosion is significantly influenced by the orientation of steel bars in concrete. Cracks and interfacial gaps between steel and concrete must be accounted for in order to predict corrosion of steel bars in concrete.

Oxygen permeability in cracked concrete reinforced with plain and deformed bars

Abstract Deformed bars are commonly used as reinforcing bars because of its good bonding ability compared to plain (smooth or round) bars. However, only a few studies were carried out comparing the corrosion performance of plain and deformed bars. With this background, the authors carried out detailed investigations on the corrosion of plain and deformed bars. Here, the results of oxygen permeability of multicracked concrete beams reinforced with plain and deformed bars are presented. For this, reinforced concrete beams of size 150×150×1250 mm were made with plain and deformed bars separately. In each beam, a steel bar specially made with segmented steel elements was embedded to measure the distribution of oxygen permeability along the beam after cracking. The cracking load was kept the same for the beams. W/C was 0.5. It was found that the oxygen permeability of cracked concrete reinforced with deformed bars is significantly higher than one reinforced with plain bars. The results indicate that deformed bars are more prone to corrosion than plain bars in cracked concrete.

A discussion of the paper “Influence of pre-rusting on steel corrosion in concrete” by P. Novak, R. Mala, and L. Joska ☆

The authors of this paper deserve to be congratulated for their study on the influence of pre-rusting on steel corrosion in concrete. The report of this kind of study is very scarce in the technical literatures. The topic of the study is very interesting as in the actual construction practice generally pre-rusted steel bars are used. We also are planning to do some study on this matter in the near future. After reading this article, we found that the authors did not provide/explain enough information/discussion related to the experimental setup and the experimental results. Further explanation on these matters will be very useful to us and also to other interested readers of the cement and concrete research journal.

Long-Term Performance of Alumina Cement Concrete Mixed with Tap Water and Seawater

This work describes the detailed study of concrete specimens (150 mm in diameter and 300 mm in length), made with normal portland cement (NPC) and alumina cement (AL), carried out after 30 years of exposure to the marine environment. Mixing water was tap water and seawater. Compressive strength, chloride ingress, microstructure, mineralogy, interfaces of concrete, and corrosion of steel bars (electrochemically and physically) were evaluated after 30 years of exposure. AL concrete mixed with seawater shows higher strength compared with the same mixed with tap water. No significant transformation of hydration phases from CAH-sub-10 to C-sub-3AH-sub-6 is found for AL concrete mixed with seawater. It shows a dense (almost nonporous) microstructure at inner and outer regions of specimens and also a dense (almost nonporous) steel-concrete interface compared with that mixed with tap water.

Experimental study on anticorrosive effect of new developed sacrificial point anode method

A new sacrificial anode method was developed in this study. This sacrificial point anode method has three main features: the anode raw material is zinc, material covering the anode is mortar containing a lithium nitrite solution, and site casting is possible using a system to inject mortar on site. This method was applied to a 36-year-old RC beam that had been exposed to marine environment conditions for more than 20 years. Cracks due to steel bar corrosion were found especially on the tension side. Moreover, corrosion (rust and loss of cross-section) of steel bars embedded in the RC beam was confirmed by a partial chipping survey. After six holes 30 mm in diameter were drilled in the RC beam, this point anode was set in a position of 50 mm from the tensile steel bar, and a special mortar was injected into the hole. After this method was applied, the condition of steel bars in the RC beam was measured with an electrochemical measurement method such as electric current, potential, 24-hour depolarization, anodic polarization curve, and polarization resistance. From corrosion current density evaluated by both anodic polarization curve and polarization resistance after exposure in dry conditions for 20 months, it was confirmed that corrosion was prevented on steel bars.