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SP173 In October 1997, the Council for the Orginazation of CANMET/ACI Conferences in association with American Concrete Institute and several cement and concrete organizations in Italy sponsored the fifth conference on the subject in Rome, Italy. This conference was aimed at transferring technology in the fastmoving field of chemical admixtures. A total of more than 60 papers from more than 20 countries were received and reviewed by the ACI review panel and 49 were accepted for publication in the proceedings of the conference. The proceedings were published as ACI special publication SP-173.

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.

A technology to impart high fluidity to concrete with an extremely low W/C range of about 0.2 is required, in order to place ultra high- strength concrete with a compressive strength of over 100 MPa. For this purpose, the authers have developed a methacrylic water- soluble polymer as a superplasticizer (SSP) that imparts adequate workability and excellent cement dispersing capability to concrete mixtures with low W/C. Also, the high cement content of ultra high- strength concrete containing normal portland cement leads to high concrete temperature due to heat of hydration, posing problems of thermal cracking and low long- term strength. In this study, the authers used a low- heat, belite- rich portland cement recently developed in Japan, together with powder silica fume to produce ultra high- strength concrete containing the SSP with a low water- binder ratio of approximately 0.2. As a result, the belite-rich portland cement was foud to reduce the adiabatic temperature rise without causing set retardation when compared with normal portland cement. In addition, the concrete showed high fluidity as well as a high long- term compressive strength of over 150 MPa. These results suggest that the SSP is highly compatible with low- heat cement and is very effective in producing high-performance concrete when used in combination with this type of binder.