LOW POROSITY HIGH STRENGTH CONCRETE FOR BRIDGE DECKS
Abstract
The early deterioration of concrete bridge decks in most areas of North America where de-icing salts are used is a major concern. Corrosion of reinforcing steel in the concrete has generally been singled out as the major cause of this problem. The corrosion is a consequence of penetration of chloride from the de-icing salt through the concrete to the vicinity of the reinforcing steel. This study is based on the idea that low porosity concrete prepared from so-called "low porosity cement" may provide an appropriate preventive measure against such difficulties. The thrust of the study was first to prepare and characterize appropriate mix design for this purpose using low porosity cement with and without fly ash additions. Subsequently, the durability-related characteristics of low porosity concretes were compared with those of conventional bridge deck concrete as normally specified. A further comparison was made with a special high density low slump mix design incorporating conventional cement and used successfully in bridge deck rehabilitation. Low porosity concretes prepared at water:cement ratios of 0.24 to 0.30 were found to have high slump (6-10 in.) but were difficult to work in the fresh state and had a relatively short working life. Fly ash incorporation was found to remove both these difficulties. Measurements of the resistance of concretes to freeze-thaw action, the resistance of de-icer scaling, the ingress of chloride ions into the concretes, the water permeability, the extent of carbonation, the development of compressive strength, the rates of drying shrinkage, the pore size distribution of the concrete mortar, and the apparent corrosion rates of reinforcing steel embedded in test concrete specimens were secured. The performance of the low porosity concrete formulations was found to be highly superior to conventional bridge deck concrete mixes in essentially all the laboratory tests carried out, and to the special high-density mixes as well. Incorporation of fly ash resulted in only modest reduction in the overall superiority of the hardened low porosity concrete while markedly improving workability and handling characteristics.
Degree
Ph.D.
Subject Area
Civil engineering
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