International Concrete Abstracts Portal

The effectiveness of Ca(NO2 )2 and NaN02 as corrosion inhibitors has been confirmed. However, the behavior of NO2 - ion and its stability in the pore solution in concrete containing the inhibitors is a matter of concern for their effective use as a corrosion-inhibiting admixture. Furthermore, it may be important from the viewpoint of the corrosion of reinforcement in concretes contaminated with Cl- ion to examine the behavior of NO2 - ions in relation to Cl-ion concentration in the pore solution in concrete. In this study, the analyses of pore solutions expressed from NaCl containing-mortars with and without the inhibitors were carried out to elucidate effects of the inhibitors on the chloride binding capacity of mortars. Simultaneously, the corrosion rate of steel bars embedded in the mortars was monitored by measurements of the potential and polarization resistance. The N02 - ion concentrations in the pore solutions in NaCl-containing mortars treated with 1% Ca(N02 )2 by mass of cement were lower than those in the corresponding mortars with NaN02 . The addition of NaN02 raised the OH- ion concentration slightly, but the OH- ion concentration was somewhat reduced in the mortars with Ca(NO2 )2 . The combined addition of NaCl and NaNO2 on the other hand, greatly raised the OH- ion concentration. However, the Cl’ /NO2 - ratios in the pore solutions in mortars containing Ca(N02 )2 were slightly lower than in mortars containing NaNO2 because of the dissipation of greater amounts of Cl- ions from the pore solutions in the former than in the latter.

A new family of comb polymers has been developed that have been found to overcome many performance shortcomings typically exhibited by conventional high range water reducing agents, commonly referred to as superplasticizers. This paper discusses some theoretical considerations associated with comb polymer type super-plasticizers, and reports on several key performance advantages such as extended slump life without prolonged set time, minimal variation in slump and air performance as a function of cement brand, and linear dosage response in mortar flow tests.

Heat curing is the most common method used for accelerating the strength development in concrete. Accelerated curing finds large applications i in the precast industry for quick turnaround of forms and casting beds. The increase in the initial strengths is simply a result of increased rate of hydra-ion caused by higher temperature. However, later strengths are often lower than those of the same concrete ured at 20o C. The causes of the strength loss are of physical and chemical nature. The physical cause results in increased porosity and cracking because the concrete constituents have different thermal expansion, (air has the highest). The chemical causes are the differences in the hydration products, microstructure and degree of hydration. Generally, physical causes are the dominating factors for strength loss in heat cured concrete. Results of extensive laboratory and field tests are presented showing that equivalent compressive strengths at 18 hours are obtained with concrete containing the new generation super-plasticizers and heat cured concretes at 60° C. The 28 day strengths of concretes with admixtures are substantially higher. Thus, with the use of these new generation super-plasticizers it’s possible to overcome the negative effects of steam curing such as strength loss, permeability, shrinkage, creep and frost resistence.

The influence of silane-based hydrophobic products - used as concrete chemical admixtures - on the corrosion of steel rebars was studied. Reinforced concrete specimens with and without a silane admixture were exposed to seawater or to aqueous solutions of de-icing salts containing chlorides. Sound and uncracked or deliberately pre-cracked concrete specimens were manufactured and cured before the exposure to aggressive environments. In the pre-cracked specimens the concrete crack tip was in contact with the steel reinforcement. The results - in terms of corrosion electrochemical potential, short circuit electric current and visual corrosion observed on the steel reinforcement - were compared with those obtained on the corresponding uncracked specimens. In uncracked specimens any corrosion process was completely blocked independently of the water to cement ratio and concrete cover provided that hydrophobized concrete was used. This effect was due to lack of water penetration, and then of the chloride ingress, through the pores of the hyrophobized cement matrix. In uncracked specimens without the silane admixture, there was corrosion risk when high water to cement ratio and/or thin concrete cover were adopted. On the other hand, corrosion of steel rebars was surprisingly more severe in cracked specimens manufactured by hydrophobized concrete rather than in the corresponding reference concrete specimens without the hydrophobic admixture. These results can be interpreted by admitting that oxygen diffusion -which is needed to feed the corrosion process - can occur directly as a gaseous phase through the open concrete voids in hydrophobized concrete, whereas in concrete without silane oxygen can diffuse much more slowly only through the water filled concrete voids.

Galvanized steel and corrosion inhibitors added to concrete are considered methods to protect reinforcement from corrosion. In present paper the simultaneous and separated use of both methods are considered. Concrete specimens have been made for the study. For depassivation the spray salt chamber was used. The results show that if NO2 is used the resistance of galvanized steel to chloride attack is improved. Bare steel embedded in concrete with NO2 - resists well chloride attack.