The disintegration of improperly air-entrained concrete by weathering is often due to the effects of freezing and thawing of water in the capillary pores of concrete. In critically saturated concrete, freezing of water in the capillary pores leads to an increase of approximately 9% in the volume of the water. If no relief is provided to accommodate the expansion, this can lead to the development of high tensile stresses within the concrete matrix. Repeated exposure to freezing and thawing cycles under these conditions will eventually lead to cracking and deterioration of the concrete. Therefore, concrete that will be exposed to cycles of freezing and thawing in a critically saturated state should be air-entrained. Entraining air in concrete provides an outlet and relief for water in the capillary pores during freezing, thereby protecting the concrete matrix from damage. Air-entraining admixtures are used primarily to create a stable air-void system in concrete and properly air-entrained concrete can withstand years of exposure to freezing and thawing without damage. Air-entraining admixture formulations are typically based on natural wood resins (vinsol resin) and rosins, and synthetic detergents. Due to significant raw material cost increases over the years, the use of vinsol resin-based air-entraining admixtures is on the decline.
A universal concrete durability problem is the corrosion of embedded steel reinforcement. If left unchecked, this leads to cracking, spalling, and delamination of the concrete; loss of steel area; and, ultimately, loss of structural integrity. Corrosion is often due to depassivation of the reinforcing steel by chlorides that may come from deicing salts, brackish water, seawater, or spray from these sources.
Reinforcement in concrete structures that will be exposed to chlorides in service should be protected against chloride-induced corrosion. Corrosion protection measures include good design practice (such as adequate concrete cover to protect the reinforcement), the use of corrosion-resistant concrete, and external measures (coatings and sealers) that require periodic maintenance. Corrosion-resistant concrete includes the use of low-permeability concrete and corrosion-inhibiting admixtures.
Several corrosion-inhibiting admixtures are commercially available, but the predominantly used products are either inorganic 30% solutions of calcium nitrite or an amine-ester organic corrosion inhibitor. Calcium nitrite inhibitors provide protection to embedded reinforcement by forming a protective oxide layer at the surface of the steel and are used at dosages ranging from 2 to 6 gal/yd3 of concrete. Dosages of 3 to 4 gal/yd3 are typical in parking structures and upwards of 5 gal/yd3 in marine structures and some prestressed bridge girders. Dosages are determined by taking into account the design corrosion service life of the structure, the expected chloride exposure, the quality of the concrete, and other design details such as concrete cover over the reinforcement.
Because calcium nitrite accelerates the time-of-set of concrete, a retarding admixture is often needed in the concrete mixture to facilitate concrete placement, consolidation, and finishing. A novel workability-retaining admixture introduced into the marketplace in 2009 is also being used to maintain the workability and consistency of calcium nitrite-treated concretes, as was the case during construction of two cooling towers at Brayton Point in Fall River, Mass.
The amine-ester organic corrosion inhibitor provides protection to steel via a two-fold mechanism, by reducing chloride ingress and by forming a protective film at the steel surface. It is used at a fixed dosage of 1 gal/yd3 but, unlike calcium nitrite, it does not accelerate the time-of-set of concrete. In air-entrained concrete, however, it often requires an increase in the dosage of air-entraining admixture. It may also result in a slight decrease in compressive strength, depending on the proportions of the concrete mixture. The amine-ester organic inhibitor was used in the Denver International Airport parking structures.
Lithium for ASR Mitigation
Alkali-silica reactivity (ASR) is a potentially destructive phenomenon that can occur in concrete resulting from a reaction between dissolved alkalis and certain types of siliceous aggregates that contain reactive silica. The reaction produces an absorptive gel that can expand and lead to abnormal cracking and disintegration of concrete. The net result is a reduction in strength (load-carrying capacity) and reduced service life of the concrete structure.
Lithium compounds have been shown since the early 1950s to be effective in mitigating ASR in concrete, and lithium nitrate-based admixtures are currently available for use in concrete mixtures that may be susceptible to ASR. The lithium ion interferes with the ASR process by combining with reactive silica to form a lithium-silica gel that does not absorb water and, therefore, does not expand. Lithium-based admixtures can be used synergistically with some SCMs to provide cost-effective mitigation of ASR in concrete. Dosages are based on the alkali content of cement and the reactivity of the aggregate, and are typically established or verified by testing.