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Applications  > High Performance Admixtures
Waterproofing, self/environment-cleaning and other performance properties
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Admixtures are those ingredients in concrete other than portland cement, water, and aggregates. They are generally used to enhance the properties of the concrete.
why

 

Liquid admixtures, from left to right: anti-washout, shrinkage reducing, water reducing, foaming agent, corrosion inhibitor, and air-entraining. (PCA No. 12188)Concrete should be workable, finishable, strong, durable, watertight, and wear resistant. These qualities can often be obtained easily and economically by the selection of suitable materials (except air-entraining admixtures where needed). However, admixtures are sometimes beneficial.
The major reasons for using admixtures are:
 
 
  • To reduce the cost of concrete construction.
  • To achieve certain properties in concrete more effectively than by other means.
  • To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions.
  • To overcome certain emergencies during concreting operations.
     

Admixtures should be used in conjunction with, and not a substitute for, good concreting practice.

The foundation is 16 ft below sea level at the Waterfront Marriott Hotel in Seattle. Waterproof concrete is used because tolerances did not allow for excavation around the outside to apply a membrane. The waterproof concrete protects the rebar from corrosion, even in salt water. (Photo Courtesy of Glacier Northwest. www.glaciernw.com )

Waterproofing and Permeability Admixtures:
Permeability-reducing admixtures reduce the rate at which water under pressure is transmitted through concrete. One of the best methods of decreasing permeability in concrete is to increase the moist-curing period and reduce the water-cement ratio to less than 0.5. Most admixtures that reduce water-cement ratio consequently reduce permeability. Some supplementary cementing materials, especially silica fume, reduce permeability.

Some admixtures act to block the capillaries in concrete. These are also effective in reducing corrosion of steel reinforcement. Waterproofing admixtures can create built-in waterproofing that does not require the additional cost of a waterproofing membrane. Since it is part of the concrete, it cannot be punctured, torn or damaged in the construction phase, in service, or deteriorate with age. These can be used for concrete basements, tunnels, roofs, floors, and foundations.
 
Self Cleaning and Environment-Cleaning Concrete. While technically not an admixture, photocatalytic cements are available for producing self-cleaning and environment-cleaning concrete and cementitious materials such as stucco (portland cement plaster). Photocatalysis is a natural phenomenon whereby a substance called a photocatalyst uses light to accelerate the rate of a chemical reaction. In the presence of ultraviolet light, these cements used in concrete break down pollutants, such as particulate matter, volatile organic compounds (VOC), and smog-forming nitrogen oxides into harmless nitrate, sulfate, and carbonate salts.
 

(Photo courtesy of TX Millennium, Italcementi Group )

Some cements are only self-cleaning. These cements, available in white or gray, are specifically designed for the construction of prestigious architectural works. Concrete made with white cement in particular has an extraordinarily brilliant white surface. The self-cleaning effect and concrete durability can be further enhanced by a smooth dense finish. Some cements can be used to produce self-cleaning and environment-cleaning concrete and cementitious materials. When used as the binder in concrete and other materials such as stucco (portland cement plaster) and concrete pavers, its increased level of photocatalysis is capable of abating noxious substances produced in urban areas by sources such as cars and power plants.
 
Air-Entraining Admixtures: These are used to purposely introduce microscopic air bubbles in concrete. Air-entrainment will dramatically improve the durability of concrete exposed to cycles of freezing and thawing.

Water-Reducing and Plasticizing Admixtures: These are used to reduce the quantity of mixing water required to produce concrete of a certain slump, to increase slump, to reduce the water-cement ratio, or to reduce cement content. An increase in strength is generally obtained with water-reducing admixtures since the water-cement ratio is reduced. The 28-day strength of concretes using water-reducers can be 10 to 25% greater than normal concrete. Plasticizers can be used to produce flowing concrete that requires little or no compaction.

Retarding Admixtures: These are used to delay the rate of setting of concrete. High temperatures of fresh concrete (above 85°F) are often the cause of an increased rate of hardening that makes placing and finishing concrete difficult. Concrete can be cooled by cooling the aggregates or mix water. Retarding admixtures are used to extend the setting time (when the concrete hardens).

Hydration-Control Admixtures: These make it possible to reuse concrete returned in a ready-mix truck by suspending setting. These consist of a stabilizer or retarder and an activator. The stabilizer can suspend hydration for 72 hours and the activator is added to the mixture just before the concrete is used.

Accelerating Admixtures: These are used to increase the rate of setting of concrete. They are often used in cold weather applications.
Corrosion Inhibitors: These are used in concrete for parking structures, marine structures, and bridges where chloride salts are present. Chlorides can cause corrosion of steel reinforcement in concrete.
 
Other Admixtures: Other admixtures are available for reducing shrinkage, adding color, reducing washout, and aiding pumping.
considerations

Admixtures being considered for use in concrete should meet applicable specifications. Trial mixtures should be made with the admixture and the job materials at the temperatures and humidities anticipated on the job. In this way the compatibility of the admixture with other admixtures and job materials, as well as the effects of the admixture on the properties of the fresh and hardened concrete, can be observed. The amount of admixture recommended by the manufacturer or the optimum amount determined by laboratory tests should be used.
BOOKMARK
Resources
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Located at BookstoreDesign and Control of Concrete Mixtures, 14th Edition (2002)
S.H. Kosmatka, B. Kerkhoff, and W.C. Panarese, Portland Cement Association, Item Code EB001, 372 pages
Available for $80 Definitive reference on concrete technology covers fundamentals and detailed information on freshly mixed and hardened concrete. Extensively updated and expanded, this new edition discusses materials for concrete, such as portland cements, supplementary cementing materials, aggregates, admixtures and fibers; air entrainment; procedures for mix proportioning, batching, mixing, transporting, handling, placing, consolidating, finishing, and curing concrete; precautions necessary during hot- and cold-weather concreting; causes and methods of controlling volume changes; commonly used control tests for quality concrete; special types of concrete, such as high-performance, lightweight, heavyweight, no-slump, roller-compacted, shotcrete, mass concrete and many more. Applicable ASTM, AASHTO, and ACI standards are referred to extensively.
Located at External Web SiteBuilding Even Better Concrete (2007)
Originally printed in the December 2007 of Architectural Record, this article by Joann Gonchar, AIA of McGraw-Hill looks at the current trends in cement and concrete construction that improve performance and reduce environmental impact. One hour of AIA Continuing Education Credit is available on-line through McGraw-Hill by reading the article and completing a brief test. http://construction.com/CE/articles/0712edit-1.asp#
Located at External Web SiteSmog-Eating Cement (2006)
Managing Green Newsletter, Leonardo Academy/The Ashkin Group
"Italians Unleash Smog-Eating Cement Revolutionary Product Can Reduce Pollution By Over 60 Percent" Overview article on TX Active