SECTION IV. ADMIXTURES
DEFINITION AND PURPOSE
2-51. An admixture is any material other than cement, water, or aggregate that is added to concrete in small quantities--either immediately before or during mixing--to modify such properties as workability, strength, durability, watertightness, or wear resistance. Admixtures can also reduce segregation, the heat of hydration, and entrained air, and can either accelerate or retard setting and hardening. Similar results are obtainable by changing the concrete-mix proportions instead of using admixtures (except air-entraining ones). When possible, examine all alternatives before using an admixture to determine which is more economical or convenient.
2-52. A major advance in concrete technology in recent years is the introduction of tiny disconnected air bubbles into concrete called air entrainment. Air-entrained concrete results from using either an air-entraining cement or an air-entraining admixture during mixing. Adding entrained air to concrete provides important benefits in both plastic and hardened concrete, such as resistance to freezing and thawing in a saturated environment. Air entrapped in nonair-entrained concrete fills relatively large voids that are not uniformly distributed throughout the mix.
2-53. The following are properties of air-entrained concrete:
Workability. The improved workability of air-entrained concrete greatly reduces water and sand requirements, particularly in lean mixes and in mixes containing angular and poorly graded aggregates. In addition, the disconnected air bubbles reduce segregation and bleeding of plastic concrete.
Freeze-thaw durability. The expansion of water as it freezes in concrete can create enough pressure to rupture the concrete. However, entrained air bubbles serve as reservoirs for the expanded water, thereby relieving expansion pressure and preventing concrete damage.
Deicers resistance. Because entrained air prevents scaling caused by deicing chemicals used for snow and ice removal, air-entrained concrete is recommended for all applications where the concrete contacts deicing chemicals.
Sulfate resistance. Entrained air improves concrete's resistance to sulfate. Concrete made with a low W/C ratio, entrained air, and cement having a low tricalcium-aluminate content is the most resistant to sulfate attack.
Strength. The voids to cement ratio basically determines air-entrained concrete strength. For this ratio, voids are defined as the total volume of water plus air (both entrained and entrapped). When the air content remains constant, the strength varies inversely with the W/C ratio. As the air content increases, you can generally maintain a given strength by holding the voids to the cement ratio constant. To do this, reduce the amount of mixing water, increase the amount of cement, or both. Any strength reduction that accompanies air entrainment is minimized because air-entrained concrete has lower W/C ratios than nonair-entrained concrete having the same slump. However, it is sometimes difficult to attain high strength with air-entrained concrete, such as when slumps remain constant while the concrete's temperature rises when using certain aggregates.
Abrasion resistance. Air-entrained concrete has about the same abrasion resistance as that of nonair-entrained concrete of the same compressive strength. Abrasion resistance increases as the compressive strength increases.
Watertightness. Air-entrained concrete is more watertight than nonair-entrained concrete since entrained air prevents interconnected capillary channels from forming. Therefore, use air-entrained concrete where watertightness is a requirement.
2-54. Air-entraining admixtures are usually liquid derivatives of natural wood resins, animal or vegetable fats or oils, alkali salts of sulfated or sulfonated organic compounds, and water-soluble soaps intended for use in the mixing water. The manufacturer provides instructions to produce a specified air content. Some manufacturers market automatic dispensers that accurately control the quantities of air-entraining agents in a mix. Air is incorporated in concrete by using air-entraining cement, an air-entraining admixture at the mixer, or both methods. Air-entraining cements should meet the specifications in ASTM C175. Add commercial air-entraining admixtures at the mixer. They should comply with ASTM C260. Use adequate controls to always ensure the proper air content. Factors affecting air content are:
Aggregate gradation and cement content. Both significantly affect the air content of both air-entrained and nonair-entrained concrete. For aggregate sizes smaller than 1 1/2 inch, the air content increases sharply as the aggregate size decreases due to the increase in cement volume. As cement content increases, the air content decreases but remains within the normal range of the cement content.
FA content. This affects the percentage of entrained air in concrete. Increasing the FA content incorporates more air in a given amount of air-entraining cement or admixture.
Slump and vibration. These affect the air content of air-entrained concrete because the greater the slump, the larger the percentage reduction in air content during vibration. At all slumps, even a 15-second vibration causes reduced air content. However, properly applied vibration mainly eliminates large air bubbles and little of the intentionally entrained tiny, air bubbles.
Concrete temperature. Its effects become more pronounced as slump increases. Less air is entrained as the concrete's temperature increases.
Mixing action. This is the most important factor in producing air-entrained concrete. The amount of entrained air varies with the mixer type and condition, the amount of concrete mixed, and the mixing rate. Stationary and transit mixers may produce concrete having very different amounts of entrained air. Mixers not loaded to capacity can increase air content, whereas overloading can decrease air content. Generally, more air is entrained as the mixing speed increases.
Admixtures and coloring agents. These can reduce the amount of entrained air, particularly fly ash having high percentages of carbon. To prevent a chemical reaction with certain air-entraining admixtures, you must add calcium-chloride solutions to the mix separately.
Premature finishing operations. This can cause excess water to work itself to the concrete surface. If this occurs, the surface zone may not contain enough entrained air and be susceptible to scaling.
RECOMMENDED AIR CONTENT
2-55. Air contents for frost-resistant concrete must be as stated in Table 2-6. Such concrete must be used when there is a danger of concrete freezing while saturated or nearly saturated with water. Air content produced by air-entraining admixtures meeting ASTM requirements will give about 9 percent air in the fraction of the concrete mixture passing the 4.75 millimeter (mm) number 4 sieve; that is, the mortar fraction.
Table 2-6. Total air content for frost-resistant concrete
TESTS FOR AIR CONTENT
2-56. Tests that determine air entrainment in freshly mixed concrete measure only air volume, not air-void characteristics. This indicates the adequacy of the air-void system when using air-entraining materials meeting ASTM specifications. Tests should be made regularly during construction, using plastic samples taken immediately after discharge from the mixer and from already placed and consolidated concrete. The following are the standard methods to determine the air content of plastic concrete.
Pressure method. This method is practical for field testing all concrete except those containing highly porous and lightweight aggregates.
Volumetric method. This method is practical for field testing concrete, particularly concrete containing lightweight and porous aggregates.
Gravimetric method. Impractical for field testing because it requires accurate knowledge of specific gravities and absolute volumes of concrete ingredients. It is satisfactory for laboratory use.
2-57. When added in small qualities, admixtures change the uniformity of the concrete. The following are admixtures that modify concrete:
Water-reducing admixtures. These reduce the quantity of mixing water required to produce concrete of a given consistency. The slump is increased for a given water content.
Retarding admixtures. These are sometimes used to reduce the rate of hydration to permit placing and consolidating concrete before the initial set. They also offset the accelerating effect of hot weather on the set. These admixtures generally consist of fatty acids, sugars, and starches.
Accelerating admixtures. These hasten the set and strength development. Calcium chloride is the most common. Add in solution form as part of the mixing water, but don't exceed 2 percent by weight of cement. Do not use calcium chloride or other admixtures containing soluble chlorides in prestressed concrete or concrete containing embedded aluminum, in permanent contact with galvanized steel, subject to alkali-aggregate reaction, or exposed to soils or water containing sulfates. Table 2-7 shows the limitations.
Pozzolans. These materials contain considerable silica or much silica and alumina. They are combined with calcium hydroxide to form compounds having cement-like properties. Pozzolans should be tested first to determine their suitability, because the properties of pozzolans and their effects on concrete vary considerably.
Workability agents. These improve the workability of fresh concrete. They include entrained air, certain organic materials, and finely divided materials. When used as workability agents, fly ash and natural pozzolans should conform to ASTM C618.
Dampproofing and permeability-reducing agents. These are water-repellent materials that are used to reduce the capillary flow of moisture through concrete that contacts water or damp earth. Pozzolans are also permeability-reducing agents.
Grouting agents. These are various air-entraining admixtures, accelerators, retarders, and workability agents that alter the properties of portland cement grouts for specific applications.
Gas-forming agents. When added to concrete or grout in very small quantities, they cause a slight expansion before hardening in certain applications. However, while hardening, the concrete or grout decreases in volume in an amount equal to or greater than that of normal concrete or grout.
Table 2-7. Maximum chloride ion content for corrosion protection
|David L. Heiserman, Editor||
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Revised: June 06, 2015