Durable Concrete--Current Practice and Future Trends

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Title: Durable Concrete--Current Practice and Future Trends

Author(s): P. C. Aitcin

Publication: Special Publication

Volume: 144

Issue:

Appears on pages(s): 85-104

Keywords: concretes; covercrete; curing; durability; environments; high-performance concretes; microclimates; microstructure; permeability; strength; water-cement ratio; Design

Date: 3/1/1994

Abstract:
The durability track record of concrete is in some respects disappointing. It is possible to find adjacent concrete elements that have performed differently, although they were exposed to the same environmental conditions. As with any other material, concrete durability is governed by the material's quality and the harshness of the environmental conditions to which it is exposed over its service life. While the engineer has little or no influence over environmental conditions, generating disastrous microclimatic conditions can be avoided through appropriate design. In the case of concrete, the engineer has to "tailor" concrete composition and characteristics to provide the properties required to best suit the expected environmental conditions to which the structure will be exposed. In the past, too much emphasis was placed on structural criteria only without much regard for environment. For example, while it is structurally possible to build a parking garage with 20-Mpa concrete, environmental conditions that normally prevail with this type of structure can lead to disastrous consequences. In the future, when determining concrete characteristics, it will be important to give priority to environmental exposure conditions over structural requirements to insure that structures remain durable. From the materials standpoint, concrete durability is closely linked to concrete microstructure, more specifically to its impermeability. The microstructures of normal strength and high-strength concretes are discussed. Data reported in the literature linking the permeability of low water-cement concrete to various aggressive agents are presented, showing, for example, that often the best and most inexpensive way to protect concrete against aggressive agents is to make it as dense as possible. If the strength of the resultant impermeable concrete exceeds the design strength, the designer should then learn how to take advantage of the enhanced strength provided rather than reduce the strength and therefore the impermeability to meet the design specifications. Recent studies show that concrete mixtures for use is severe environments can be made essentially impermeable to air, water, and chloride ions if they possess a minimum of 50 MPa compressive strength at 28 days.