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Unformatted text preview: HOME PAGE CHAPTER 8 Air-Entrained Concrete
One of the greatest advances in concrete technology was the development of air-entrained concrete in the mid1930s. Today air entrainment is recommended for nearly all concretes, principally to improve freeze-thaw resistance when exposed to water and deicing chemicals. However, there are other important benefits of entrained air in both freshly mixed and hardened concrete. Air-entrained concrete is produced by using either an air-entraining cement or adding an air-entraining admixture during batching. The air-entraining admixture stabilizes bubbles formed during the mixing process, enhances the incorporation of bubbles of various sizes by lowering the surface tension of the mixing water, impedes bubble coalescence, and anchors bubbles to cement and aggregate particles. Anionic air-entraining admixtures are hydrophobic (repel water) and are electrically charged (nonionic or nocharge admixtures are also available). The negative electric charge is attracted to positively charged cement grains, which aids in stabilizing bubbles. The airentraining admixture forms a tough, water-repelling film, similar to a soap film, with sufficient strength and elasticity to contain and stabilize the air bubbles and prevent them from coalescing. The hydrophobic film also keeps water out of the bubbles. The stirring and kneading action of mechanical mixing disperses the air bubbles. The fine aggregate particles also act as a three-dimensional grid to help hold the bubbles in the mixture. Entrained air bubbles are not like entrapped air voids, which occur in all concretes as a result of mixing, handling, and placing and are largely a function of aggregate characteristics. Intentionally entrained air bubbles are extremely small in size, between 10 to 1000 m in diameter, while entrapped voids are usually 1000 m (1 mm) or larger. The majority of the entrained air voids in normal concrete are between 10 m and 100 m in diameter. As shown in Fig. 8-1, the bubbles are not interconnected; they are well dispersed and randomly distributed. Non-airentrained concrete with a 25-mm (1-in.) maximum-size aggregate has an air content of approximately 11/2%. This same mixture air entrained for severe frost exposure would require a total air content of about 6%, made up of both the coarser "entrapped" air voids and the finer "entrained" air voids. Video PROPERTIES OF AIR-ENTRAINED CONCRETE
The primary concrete properties influenced by air entrainment are presented in the following sections. A brief summary of other properties not discussed below is presented in Table 8-1. Freeze-Thaw Resistance
The resistance of hardened concrete to freezing and thawing in a moist condition is significantly improved by the use of intentionally entrained air, even when various deicers are involved. Convincing proof of the improve- Fig. 8-1. Polished section of air-entrained concrete as seen through a microscope. (67840) 129 Design and Control of Concrete Mixtures N EB001
2000 1800 1600 Table 8-1. Effect of Entrained Air on Concrete Properties
Effect Little effect; increased strength increases abrasion resistance Absorption Little effect Alkali-silica reactivity Expansion decreases with increased air Bleeding Reduced significantly Bond to steel Decreased Compressive strength Reduced approximately 2% to 6% per percentage point increase in air; harsh or lean mixes may gain strength Creep Little effect Deicer scaling Significantly reduced Density Decreases with increased air Fatigue Little effect Finishability Reduced due to increased cohesion (stickiness) Flexural strength Reduced approximately 2% to 4% per percentage point increase in air Freeze-thaw Significantly improved resistance to resistance water-saturated freeze-thaw deterioration Heat of hydration No significant effect Modulus of elasticity Decreases with increased air (static) approximately 720 to 1380 MPa (105,000 to 200,000 psi) per percentage point of air Permeability Little effect; reduced water-cement ratio reduces permeability Scaling Significantly reduced Shrinkage (drying) Little effect Slump Increases with increased air approximately 25 mm (1 in.) per 1/2 to 1 percentage point increase in air Specific heat No effect Sulfate resistance Significantly improved Stickiness Increased cohesion--harder to finish Temperature of wet No effect concrete Thermal conductivity Decreases 1% to 3% per percentage point increase in air Thermal diffusivity Decreases about 1.6% per percentage point increase in air Water demand of wet Decreases with increased air; concrete for equal approximately 3 to 6 kg/m3 (5 to 10 slump lb/yd3) per percentage point of air Watertightness Increases slightly; reduced water-cement ratio increases watertightness Workability Increases with increased air
Note: The table information may not apply to all situations. Properties Abrasion Cycles of freezing and thawing for 50% reduction in dynamic modulus of elasticity 1400 1200 1000 800 600 400 200 0 Symbols: Non-air-entrained Air-entrained 0 1 2 3 4 5 6 Air content, percent Fig. 8-2. Effect of entrained air on the resistance of concrete to freezing and thawing in laboratory tests. Concretes were made with cements of different fineness and composition and with various cement contents and water-cement ratios (Bates and others 1952, and Lerch 1960). ment in durability effected by air entrainment is shown in Figs. 8-2 and 8-3. As the water in moist concrete freezes, it produces osmotic and hydraulic pressures in the capillaries and pores of the cement paste and aggregate. If the pressure exceeds the tensile strength of the paste or aggregate, the cavity will dilate and rupture. The accumulative effect of successive freeze-thaw cycles and disruption of paste and
130 aggregate eventually cause significant expansion and deterioration of the concrete. Deterioration is visible in the form of cracking, scaling, and crumbling (Fig. 8-3). Powers (1965) and Pigeon and Pleau (1995) extensively review the mechanisms of frost action. Hydraulic pressures are caused by the 9% expansion of water upon freezing; in this process growing ice crystals displace unfrozen water. If a capillary is above critical saturation (91.7% filled with water), hydraulic pressures result as freezing progresses. At lower water contents, no hydraulic pressure should exist. Osmotic pressures develop from differential concentrations of alkali solutions in the paste (Powers 1965a). As pure water freezes, the alkali concentration increases in the adjacent unfrozen water. A high-alkali solution, through the mechanism of osmosis, draws water from lower-alkali solutions in the pores. This osmotic transfer of water continues until equilibrium in the fluids' alkali concentration is achieved. Osmotic pressure is considered a minor factor, if present at all, in aggregate frost action, whereas it may be dominant in certain cement pastes. Osmotic pressures, as described above, are considered to be a major factor in "salt scaling." Capillary ice (or any ice in large voids or cracks) draws water from pores to advance its growth. Also, since most pores in cement paste and some aggregates are too Chapter 8 N Air-Entrained Concrete Fig. 8-3. Effect of weathering on boxes and slabs on ground at the Long-Time Study outdoor test plot, Project 10, PCA, Skokie, Illinois. Specimens at top are air-entrained, specimens at bottom exhibiting severe crumbling and scaling are nonair-entrained. All concretes were made with 335 kg (564 lb) of Type I portland cement per cubic meter (cubic yard). Periodically, calcium chloride deicer was applied to the slabs. Specimens were 40 years old when photographed (see Klieger 1963 for concrete mixture information). (69977, 69853, 69978, 69854) small for ice crystals to form, water attempts to migrate to locations where it can freeze. Entrained air voids act as empty chambers in the paste where freezing and migrating water can enter, thus relieving the pressures described above and preventing damage to the concrete. Upon thawing, most of the water returns to the capillaries due to capillary action and pressure from air compressed in the bubbles. Thus the bubbles are ready to protect the concrete from the next cycle of freezing and thawing (Powers 1955, Lerch 1960, and Powers 1965). The pressure developed by water as it expands during freezing depends largely upon the distance the water must travel to the nearest air void for relief. Therefore, the voids must be spaced close enough to reduce the pressure below that which would exceed the tensile strength of the concrete. The amount of hydraulic pressure is also related to the rate of freezing and the permeability of the paste. The spacing and size of air voids are important factors contributing to the effectiveness of air entrainment in concrete. ASTM C 457 describes a method of evaluating the air-void system in hardened concrete. Most authorities consider the following air-void characteristics as representative of a system with adequate freeze-thaw resistance (Powers 1949, Klieger 1952, Klieger 1956, Mielenz and
131 others 1958, Powers 1965, Klieger 1966, Whiting and Nagi 1998, and Pinto and Hover 2001): 1. Calculated spacing factor, , (an index related to the distance between bubbles but not the actual average spacing in the system)--less than 0.200 mm (0.008 in.) (Powers 1954 and 1965) 2. Specific surface, , (surface area of the air voids)--24 square millimeters per cubic millimeter (600 sq in. per cubic inch) of air-void volume, or greater. Current U.S. field quality control practice usually involves only the measurement of total air volume in freshly mixed concrete; this does not distinguish air-void size in any way. In addition to total air volume, Canadian practice also requires attainment of specific spacing factors. Fig. 8-4 illustrates the relationship between spacing factor and total air content. Measurement of air volume alone does not permit full evaluation of the important characteristics of the air-void system; however, air-entrainment is generally considered effective for freeze-thaw resistance when the volume of air in the mortar fraction of the concrete--material passing the 4.75-mm (No. 4) sieve--is about 9 1% (Klieger 1952) or about 18% by paste volume. For equal admixture dosage rates per unit of cement, the air content of ASTM C 185 (AASHTO T 137) mortar would be about 19% due to the standard aggregate's properties. Design and Control of Concrete Mixtures
Spacing factor, micrometers N EB001 resistant with a low water-cement ratio. Fig. 8-5 illustrates the effect of water-cement ratio on the durability of nonair-entrained concrete. Concrete elements should be properly drained and kept as dry as possible as greater degrees of saturation increase the likelihood of distress due to freeze-thaw cycles. Concrete that is dry or contains only a small amount of moisture in service is essentially not affected by even a large number of cycles of freezing and thawing. Refer to the sections on "Deicer-Scaling Resistance" and "Recommended Air Contents" in this chapter and to Chapter 9 for mixture design considerations. Non-air-entrained mixtures Air-entrained mixtures 900 800 700 600 500 400 300 200 100 1 2 3 4 5 Air content in concrete, % 6 7 Deicer-Scaling Resistance
Deicing chemicals used for snow and ice removal can cause and aggravate surface scaling. The damage is primarily a physical action. Deicer scaling of inadequately air-entrained or non-air-entrained concrete during freezing is believed to be caused by a buildup of osmotic and hydraulic pressures in excess of the normal hydraulic pressures produced when water in concrete freezes. These pressures become critical and result in scaling unless entrained air voids are present at the surface and throughout the sample to relieve the pressure. The hygroscopic (moisture absorbing) properties of deicing salts also attract water and keep the concrete more saturated, increasing the potential for freeze-thaw deterioration. However, properly designed and placed air-entrained concrete will withstand deicers for many years. Studies have also shown that, in absence of freezing, the formation of salt crystals in concrete (from external sources of chloride, sulfate, and other salts) may contribute to concrete scaling and deterioration similar to the crumbling of rocks by salt weathering. The entrained air voids in concrete allow space for salt crystals to grow; this relieves internal stress similar to the way the voids relieve stress from freezing water in concrete (ASCE 1982 and Sayward 1984). Deicers can have many effects on concrete and the immediate environment. All deicers can aggravate scaling of concrete that is not properly air entrained. Sodium chloride (rock salt) (ASTM D 632 or AASHTO M 143), calcium chloride (ASTM D 98 or AASHTO M 144), and urea are the most frequently used deicers. In the absence of freezing, sodium chloride has little to no chemical effect on concrete but can damage plants and corrode metal. Calcium chloride in weak solutions generally has little chemical effect on concrete and vegetation but does corrode metal. Studies have shown that concentrated calcium chloride solutions can chemically attack concrete (Brown and Cady 1975). Urea does not chemically damage concrete, vegetation, or metal. Nonchloride deicers are used to minimize corrosion of reinforcing steel and minimize groundwater chloride contamination. The use of deicers containing ammonium nitrate and ammonium sulfate should be strictly prohibited as they rapidly attack and disintegrate concrete.
132 Fig. 8-4. Spacing factor as a function of total air content in concrete (Pinto and Hover 2001). The air content of concrete with 19-mm (3/4-in.) maximumsize aggregate would be about 6% for effective freezethaw resistance. The relationship between air content of standard mortar and concrete is illustrated by Taylor (1948). Pinto and Hover (2001) address paste air content versus frost resistance. The total required concrete air content for durability increases as the maximum-size aggregate is reduced (due to greater paste volume) and the exposure conditions become more severe (see "Recommended Air Contents" later in this chapter). Freeze-thaw resistance is also significantly increased with the use of the following: (1) a good quality aggregate, (2) a low water to cementing materials ratio (maximum 0.45), (3) a minimum cementitious materials content of 335 kg/m3 (564 lb/yd3), (4) proper finishing and curing techniques, and (5) a compressive strength of 28 MPa (4,000 psi) when exposed to repeated freeze-thaw cycles. Even non-air-entrained concretes will be more freeze-thaw 100
Relative dynamic modulus, % 80 60 40 20
ASTM C 666 Water to cement ratio 0.30 0.35 0.40 0.45 0.50 0 0 50 100 150 200 250 Number of freezethaw cycles 300 350 Fig. 8-5. Durability factors vs. number of freeze-thaw cycles for selected non-air-entrained concretes (Pinto and Hover 2001). Chapter 8 N Air-Entrained Concrete Magnesium chloride deicers have come under recent criticism for aggravating scaling. One study found that magnesium chloride, magnesium acetate, magnesium nitrate, and calcium chloride are more damaging to concrete than sodium chloride (Cody, Cody, Spry, and Gan 1996). The extent of scaling depends upon the amount of deicer used and the frequency of application. Relatively low concentrations of deicer (on the order of 2% to 4% by mass) produce more surface scaling than higher concentrations or the absence of deicer (Verbeck and Klieger 1956). Deicers can reach concrete surfaces in ways other than direct application, such as splashing by vehicles and dripping from the undersides of vehicles. Scaling is more severe in poorly drained areas because more of the deicer solution remains on the concrete surface during freezing and thawing. Air entrainment is effective in preventing surface scaling and is recommended for all concretes that may come in contact with deicing chemicals (Fig. 8-6). A good air-void system with a low spacing factor (maximum of 200 micrometers) is perhaps more important to deicer environments than saturated frost environments without deicers. The relationship between spacing factor and deicer scaling is illustrated in Fig. 8-7. A low water to portland cement ratio helps minimize scaling, but is not sufficient to control scaling at normal water-cement ratios.
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.4
2% 4% 6% 5 4
Visual rating Rating: 0 = no scaling 5 = severe scaling 3 2 1 0 0 200 400 Spacing factor, m 600 800
ASTM C 672 deicer scaling test Fig. 8-7. Visual rating as a function of spacing factor, for a concrete mixture with a water to cement ratio of 0.45 (Pinto and Hover 2001). ASTM C 672 deicer scaling test 0.3 0.2 0.1 0 10 20 30 Number of cycles 40 50 Fig. 8-6. Cumulative mass loss for mixtures with a water to cement ratio of 0.45 and on-time finishing (Pinto and Hover 2001). Table 8-2. Deicer Scaling Resistance (Visual Ratings) of Concrete with Selected Supplementary Cementing Materials
Mixture Mass replacement of cement, % Scale rating at 25 cycles Scale rating at 50 cycles Control 0 1 2 Fly ash (Class F) 15 1 2 Slag 40 1 1 Calcined shale 15 1 2 Calcined shale 25 1 1 Concrete had 335 kg of cementing materials per cubic meter (565 lb/yd3), a Type I portland cement, a water to cementing materials ratio of 0.50, a nominal slump of 75 mm (3 in.), and a nominal air content of 6%. Test method: ASTM C 672. Results are for specific materials tested in 2000 and may not be representative of other materials. Scale rating: 1 = very slight scaling (3 mm depth maximum) with no coarse aggregate visible, 2 = slight to moderate scaling. Cumulative mass loss, lb/ft2 Air content Fig. 8-8 illustrates the overriding impact of air content over water-cement ratio in controlling scaling. To provide adequate durability and scale resistance in severe exposures with deicers present, air-entrained concrete should be composed of durable materials and have the following: (1) a low water to cementitious materials ratio (maximum 0.45), (2) a slump of 100 mm (4 in.) or less unless a plasticizer is used, (3) a cementitious materials content of 335 kg/m3 (564 lb/yd3), (4) proper finishing after bleed water has evaporated from the surface, (5) adequate drainage, (6) a minimum of 7 days moist curing at or above 10C (50F), (7) a compressive strength of 28 MPa (4000 psi) when exposed to repeated freezethaw cycling, and (8) a minimum 30-day drying period after moist curing if concrete will be exposed to freezethaw cycles and deicers when saturated. Target air contents are discussed in "Recommended Air Contents" at the end of this chapter. Normal dosages of supplementary cementitious materials should not effect scaling resistance of properly designed, placed, and cured concrete (Table 8-2). The ACI 318 building code allows up to 10% silica fume, 25% fly ash, and 50% slag as part of the cementitious materials for deicer exposures. However, abuse of these materials along Cumulative mass loss, kg/m2 133 Design and Control of Concrete Mixtures
5 N EB001
1.0 0.8 0.6 Average mass loss, kg/m2 4 3 2 1 0 0.2 Average mass loss, lb/ft2 Air content 2% 4% 6% The effect of mix design, surface treatment, curing, or other variables on resistance to surface scaling can be evaluated by ASTM C 672. Sulfate Resistance
Sulfate resistance of concrete is improved by air entrainment, as shown in Figs. 8-9 and 8-10, when advantage is taken of the reduction in water-cement ratio possible with air entrainment. Air-entrained concrete made with a low water-cement ratio, an adequate cement content and a sulfate-resistant cement will be resistant to attack from sulfate soils and waters. ASTM C 672 deicer scaling test 0.4 0.2 0.25 0.3 0.35 0.4 Water to cement ratio 0.45 0.5 Fig. 8-8. Measured mass loss of concrete after 40 cycles of deicer and frost exposure at various water to cement ratios (Pinto and Hover 2001). Resistance to Alkali-Silica Reactivity
The expansive disruption caused by alkali-silica reactivity is reduced through the use of air-entrainment (Kretsinger 1949). Alkali hydroxides react with the silica of reactive aggregates to form expansive reaction products, causing the concrete to expand. Excessive expansion will disrupt and deteriorate concrete. As shown in Fig. 8-11, the expansion of mortar bars made with reactive materials is reduced as the air content is increased. with poor placing and curing practices can aggravate scaling. Consult local guidelines on allowable dosages and practices for using these materials in deicer environments as they can vary from ACI 318 requirements. Air Drying. The resistance of air-entrained concrete to freeze-thaw cycles and deicers is greatly increased by an air drying period after initial moist curing. Air drying removes excess moisture from the concrete which in turn Strength reduces the internal stress caused by freeze-thaw conditions and deicers. Water-saturated concrete will deterioWhen the air content is maintained constant, strength rate faster than an air-dried concrete when exposed to varies inversely with the water-cement ratio. Fig. 8-12 moist freeze-thaw cycling and deicers. Concrete placed in shows a typical relationship between 28-day compressive the spring or summer has an adequate drying period. strength and water-cement ratio for concrete that has the Concrete placed in the fall season, however, often does not recommended percentages of entrained air. As air content dry out enough before deicers are used. This is especially is increased, a given strength generally can be maintained true of fall paving cured by membrane-forming comby holding the voids (air + water) to cement ratio conpounds. These membranes remain intact until worn off by stant; this may, however, necessitate some increase in traffic; thus, adequate drying may not occur before the cement content. onset of winter. Curing methods, such as use of plastic Air-entrained as well as non-air-entrained concrete sheets, that allow drying at the completion of the curing can readily be proportioned to provide similar moderate period are preferable for fall paving on all projects where strengths. Both generally must contain the same amount deicers will be used. Concrete placed in the fall should be allowed at least 30 Cement days for air drying after the moist-curing With air Without air content period. The exact length of time for sufficient drying to take place may vary 222 kg/m3 with climate and weather conditions. 3
(375 lb/yd ) Treatment of Scaled Surfaces. If surface scaling (an indication of an inadequate air-void system or poor finishing practices) should develop during the first frost season, or if the concrete is of poor quality, a breathable surface treatment can be applied to the dry concrete to help protect it against further damage. Treatment often consists of a penetrating sealer made with boiled linseed oil (ACPA 1996), breathable methacrylate, or other materials. Nonbreathable formulations should be avoided as they can cause delamination. 306 kg/m3 (515 lb/yd3) 392 kg/m3 (660 lb/yd3) Fig. 8-9. Effect of entrained air and cement content (Type II) on performance of concrete specimens exposed to a sulfate soil. Without entrained air the specimens made with lesser amounts of cement deteriorated badly. Specimens made with the most cement and the lowest water-cement ratio were further improved by air entrainment. Specimens were 5 years old when photographed (Stanton 1948 and Lerch 1960). 134 Chapter 8 N Air-Entrained Concrete
Cement content, lb/yd3 517 35
Air-entrained concrete Cement: Type I Age: 28 days 1 376 5 2 Cement type V 28 4 Visual rating 3 II
Rating: 1 no deterioration 6 failure V 4 21
Air-entrained concrete Non-air-entrained concrete 3 5 II I
150 x 150 x 760-mm (6 x 6 x 30-in.) beams exposed for 11 years to soil containing approximately 10% sodium sulfate 0.50 0.55 0.60 Water to cement ratio, by mass 6 I 223 Fig. 8-12. Typical relationship between 28-day compressive strength and water-cement ratio for a wide variety of airentrained concretes using Type I cement.
390 307 Cement content, kg/m3 Fig. 8-10. Performance of a variety of air-entrained and nonair-entrained concretes exposed to sulfate soil. Sulfate resistance is increased with the use of Types II and V cements, a higher cement content, lower water-cement ratio, and air entrainment (Stark 1984). 70 60 Reduction in expansion at one year, percent 50 40 30 20
50 x 50 x 250-mm (2 x 2 x 10-in.) mortar bars 1: 2 mortar water to cement ratio = 0.40 19% of the sand by mass contained reactive siliceous magnesian limestone 10 0 2 4 6 8 10 Air content, percent 12 14 Fig. 8-11. Effect of air content on the reduction of expansion due to alkali-silica reaction (Kretsinger 1949). 135 of coarse aggregate. When the cement content and slump are held constant, air entrainment reduces the sand and water requirements as illustrated in Fig. 8-13. Thus, airentrained concretes can have lower water-cement ratios than non-air-entrained concretes; this minimizes the reductions in strength that generally accompany air entrainment. At constant water-cement ratios, increases in air will proportionally reduce strength (Fig. 8-14). Pinto and Hover (2001) found that for a decrease in strength of 10 MPa (1450 psi), resulting from a four percentage point decrease in air, the water-cement ratio had to be decreased by 0.14 to maintain strength. Some reductions in strength may be tolerable in view of other benefits of air, such as improved workability. Reductions in strength become more significant in higher-strength mixes, as illustrated in Fig. 8-15. In lower-cement-content, harsh mixes, strength is generally increased by entrainment of air in proper amounts due to the reduced water-cement ratio and improved workability. For moderate to high-strength concrete, each percentile of increase in entrained air reduces the compressive strength about 2% to 9% (Cordon 1946, Klieger 1952, Klieger 1956, Whiting and Nagi 1998, and Pinto and Hover 2001). Actual strength varies and is affected by the cement source, admixtures, and other concrete ingredients. Attainment of high strength with air-entrained concrete may be difficult at times. Even though a reduction in mixing water is associated with air entrainment, mixtures with high cement contents require more mixing water than lower-cement-content mixtures; hence, the increase in strength expected from the additional cement is offset somewhat by the additional water. Water reducers can offset this effect. Compressive strength, 1000 psi Compressive strength, MPa Design and Control of Concrete Mixtures N EB001
6 60 400 Cement content, lb/yd3 600 800 100 40 5 40 Water reduction, kg/m3 8% air 30 6 4 20 2 10 80 Water reduction, lb/yd3
Compressive strength, MPa
364 kg/m3 (613 lb/yd3) 4
308 kg/m3 (519 lb/yd3) 60 20
252kg/m3 (425 lb/yd3) 3 40 20 2 10 1
150 x 300-mm (6 x 12-in.) cylinders 0 250 300 350 400 400 Cement content, kg/m3 500 0 75 Sand reduction, dm3/m3 400 Cement content, lb/yd3 600 800 0 0 1 2 2.0 3 4 5 6 Air content, percent 7 8 9 1.5 50 8% air 1.0 6 25 4 2 0 250 300 350 400 400 Cement content, kg/m3 500 0 0.5 Sand reduction, ft3/yd3 Fig. 8-15. Relationship between air content and 28-day compressive strength for concrete at three cement contents. Water content was reduced with increased air content to maintain a constant slump (Cordon 1946). Workability
Entrained air improves the workability of concrete. It is particularly effective in lean (low cement content) mixes that otherwise might be harsh and difficult to work. In one study, an air-entrained mixture made with natural aggregate, 3% air, and a 37-mm (11/2-in.) slump had about the same workability as a non-air-entrained concrete with 1% air and a 75-mm (3-in.) slump, even though less cement was required for the air-entrained mix (Cordon 1946). Workability of mixes with angular and poorly graded aggregates is similarly improved. Because of improved workability with entrained air, water and sand content can be reduced significantly (Fig. 8-13). A volume of air-entrained concrete requires less water than an equal volume of non-air-entrained concrete of the same consistency and maximum size aggregate. Freshly mixed concrete containing entrained air is cohesive, looks and feels fatty or workable, and can usually be handled with ease; on the other hand, high air contents can make a mixture sticky and more difficult to finish. Entrained air also reduces segregation and bleeding in freshly mixed and placed concrete. Fig. 8-13. Reduction of water and sand content obtained at various levels of air and cement contents (Gilkey 1958). Compressive strength at 90 days, MPa 70 60 Water to cement ratio 0.25 0.30 0.35 0.40 0.45 0.50 10 8 50 40 0 1 2 3 4 5 Air content, percent 6 7 6 Fig. 8-14. Relationship between compressive strength at 90 days and air content (Pinto and Hover 2001). Compressive strength at 90 days, 1000 psi AIR-ENTRAINING MATERIALS
The entrainment of air in concrete can be accomplished by adding an air-entraining admixture at the mixer, by using an air-entraining cement, or by a combination of these
136 Compressive strength, 1000 psi 30 Chapter 8 N Air-Entrained Concrete methods. Regardless of the method used, adequate control and monitoring is required to ensure the proper air content at all times. Numerous commercial air-entraining admixtures, manufactured from a variety of materials, are available. Most air-entraining admixtures consist of one or more of the following materials: wood resin (Vinsol resin), sulfonated hydrocarbons, fatty and resinous acids, and synthetic materials. Chemical descriptions and performance characteristics of common air-entraining agents are shown in Table 8-3. Air-entraining admixtures are usually liquids and should not be allowed to freeze. Admixtures added at the mixer should conform to ASTM C 260 (AASHTO M 154). Air-entraining cements comply with ASTM C 150 and C 595 (AASHTO M 85 and M 240). To produce such cements, air-entraining additions conforming to ASTM C 226 are interground with the cement clinker during manufacture. Air-entraining cements generally provide an adequate amount of entrained air to meet most job conditions; however, a specified air content may not necessarily be obtained in the concrete. If an insufficient volume of air is entrained, it may also be necessary to add an airentraining admixture at the mixer. Each method of entraining air has certain advantages. On jobs where careful control is not practical, airentraining cements are especially useful to ensure that a significant portion of the required air content will always be obtained. They eliminate the possibility of human or mechanical error that can occur when adding an admixture during batching. With air-entraining admixtures, the volume of entrained air can be readily adjusted to meet job conditions by changing the amount of admixture added at the mixer. Variations in air content can be expected with variations in aggregate proportions and gradation, mixing time, temperature, and slump. The order of batching and mixing concrete ingredients when using an air-entraining admixture has a significant influence on the amount of air entrained; therefore, consistency in batching is needed to maintain adequate control. When entrained air is excessive, it can be reduced by using one of the following defoaming (air-detraining) agents: tributyl phosphate, dibutyl phthalate, octyl alcohol, water-insoluble esters of carbonic acid and boric acid, and silicones. Only the smallest possible dosage of defoaming agent should be used to reduce the air content to the specified limit. An excessive amount might have adverse effects on concrete properties (Whiting and Stark 1983). FACTORS AFFECTING AIR CONTENT Cement
As cement content increases, the air content decreases for a set dosage of air-entraining admixture per unit of cement within the normal range of cement contents (Fig. 8-16). In going from 240 to 360 kilogram of cement per cubic meter (400 to 600 lb of cement per cubic yard), the dosage rate may have to be doubled to maintain a constant air content. However, studies indicate that when this is done the airvoid spacing factor generally decreases with an increase in Table 8-3. Classification and Performance Characteristics of Common Air-Entraining Admixtures (Whiting and Nagi 1998)
Classification Wood derived acid salts Vinsol resin Chemical description Alkali or alkanolamine salt of: A mixture of tricyclic acids, phenolics, and terpenes. Tricyclic acids-major component. Tricyclic acids-minor component. Fatty acids-major component. Tricyclic acids-minor component. Coconut fatty acids, alkanolamine salt. Notes and performance characteristics Quick air generation. Minor air gain with initial mixing. Air loss with prolonged mixing. Mid-sized air bubbles formed. Compatible with most other admixtures. Same as above. Slower air generation. Air may increase with prolonged mixing. Smallest air bubbles of all agents. Compatible with most other admixtures. Slower air generation than wood rosins. Moderate air loss with mixing. Coarser air bubbles relative to wood rosins. Compatible with most other admixtures. Quick air generation. Minor air loss with mixing. Coarser bubbles. May be incompatible with some HRWR. Also applicable to cellular concretes. Primarily used in masonry mortars. All these are rarely used as concrete air-entraining agents in current practice. Wood rosin Tall oil Vegetable oil acids Synthetic detergents Synthetic workability aids Miscellaneous Alkyl-aryl sulfonates and sulfates (e.g., sodium dodecylbenzenesulfonate). Alkyl-aryl ethoxylates. Alkali-alkanolamine acid salts of lignosulfonate. Oxygenated petroleum residues. Proteinaceous materials. Animal tallows. 137 Design and Control of Concrete Mixtures N EB001
6 3/8 1/2 12 3/4 1 Maximum-size aggregate, in. 11/2 2
Air-entrained concrete Non-air-entrained concrete (entrapped air only) Cement: Type I Slump: 50 to 80 mm (2 to 3 in.) 21/2
Air-entrained concrete 10 4 8
Air content, percent Air content, percent 3 Cement: Type I 280 to 335 kg/m3 (470 to 565 lb/yd3) Slump: 80 to 100 mm (3 to 5 in.) 6 222 kg/m3 (375 lb/yd3) 2 4 306 kg/m3 (515 lb/yd3) 388 kg/m3 (655 lb/yd3) 1 Non-air-entrained concrete (entrapped air only) 2 222 kg/m3 (375 lb/yd3) 388 kg/m3 (655 lb/yd3) 0 20 0 9.5 12.5 24 28 32 36 40 Fine aggregate content, percent of total aggregate 44 19.0 25.0 37.5 50 Maximum-size aggregate, mm 63 Fig. 8-16. Relationship between aggregate size, cement content, and air content of concrete. The air-entraining admixture dosage per unit of cement was constant for airentrained concrete. PCA Major Series 336. Fig. 8-17. Relationship between percentage of fine aggregate and air content of concrete. PCA Major Series 336. cement content; and for a given air content the specific surface increases, thus improving durability. An increase in cement fineness will result in a decrease in the amount of air entrained. Type III cement, a very finely ground material, may require twice as much air-entraining agent as a Type I cement of normal fineness. High-alkali cements may entrain more air than low alkali cements with the same amount of air-entraining material. A low-alkali cement may require 20% to 40% (occasionally up to 70%) more air-entraining agent than a high-alkali cement to achieve an equivalent air content. Precautions are therefore necessary when using more than one cement source in a batch plant to ensure that proper admixture requirements are determined for each cement (Greening 1967). the amount of fine aggregate causes more air to be entrained for a given amount of air-entraining cement or admixture (more air is also entrapped in non-airentrained concrete). Fine-aggregate particles passing the 600 m to 150 m (No. 30 to No. 100) sieves entrap more air than either very fine or coarser particles. Appreciable amounts of material passing the 150 m (No. 100) sieve will result in a significant reduction of entrained air. Fine aggregates from different sources may entrap different amounts of air even though they have identical gradations. This may be due to differences in shape and surface texture or contamination by organic materials. Mixing Water and Slump
An increase in the mixing water makes more water available for the generation of air bubbles, thereby increasing the air content as slumps increase up to about 150 or 175 mm (6 or 7 inches). An increase in the water-cement ratio from 0.4 to 1.0 can increase the air content by four percentage points. A portion of the air increase is due to the relationship between slump and air content. Air content increases with slump even when the water-cement ratio is held constant. The spacing factor, , of the air-void system also increases, that is, the voids become coarser at higher water-cement ratios, thereby reducing concrete freeze-thaw durability (Stark 1986). The addition of 5 kg of water per cubic meter (8.4 lbs of water per cubic yard) of concrete can increase the slump by 25 mm (one inch). A 25-mm (1-in.) increase in slump increases the air content by approximately one-half to one
138 Coarse Aggregate
The size of coarse aggregate has a pronounced effect on the air content of both air-entrained and non-air-entrained concrete, as shown in Fig. 8-16. There is little change in air content when the size of aggregate is increased above 37.5 mm (11/2 in.). Fine Aggregate
The fine-aggregate content of a mixture affects the percentage of entrained air. As shown in Fig. 8-17, increasing Chapter 8 N Air-Entrained Concrete percentage point for concretes with a low-to-moderate slump and constant air-entraining admixture dosage. However, this approximation is greatly affected by concrete temperature, slump, and the type and amount of cement and admixtures present in the concrete. A lowslump concrete with a high dosage of water-reducing and air-entraining admixtures can undergo large increases in slump and air content with a small addition of water. On the other hand, a very fluid concrete mixture with a 200 to 250-mm (8 to 10-in.) slump may lose air with the addition of water. Refer to Tables 8-4 and 8-5 for more information. The mixing water used may also affect air content. Algae-contaminated water increases air content. Highly alkaline wash water from truck mixers can affect air con- Table 8-4. Effect of Mixture Design and Concrete Constituents on Control of Air Content in Concrete
Characteristic/Material Alkali content Effects Air content increases with increase in cement alkali level. Less air-entraining agent dosage needed for high-alkali cements.
Portland cement Guidance Changes in alkali content or cement source require that air-entraining agent dosage be adjusted. Decrease dosage as much as 40% for high-alkali cements. Air-void system may be more unstable with some combinations of alkali level and air-entraining agent used. Fineness Decrease in air content with increased fineness of cement. Decrease in air content with increase in cement content. Smaller and greater number of voids with increased cement content. Air content may be altered by contamination of cement with finish mill oil. Air content decreases with increase in loss on ignition (carbon content). Use up to 100% more air-entraining admixture for very fine (Type III) cements. Adjust admixture if cement source or fineness changes. Increase air-entraining admixture dosage rate as cement content increases. Cement content in mixture Contaminants
Supplementary cementitious materials Verify that cement meets ASTM C 150 (AASHTO M 85) requirements on air content of test mortar. Changes in LOI or fly ash source require that airentraining admixture dosage be adjusted. Perform "foam index" test to estimate increase in dosage. Prepare trial mixes and evaluate air-void systems. Fly ash Ground granulated blast-furnace slag Silica fume Metakaolin Water reducers Air-void system may be more unstable with some combinations of fly ash/ cement/air-entraining agents. Decrease in air content with increased fineness of GGBFS. Decrease in air content with increase in silica fume content. No apparent effect. Air content increases with increases in dosage of lignin-based materials. Spacing factors may increase when water-reducers used. Effects similar to water-reducers. Minor effects on air content. Moderate increase in air content when formulated with lignosulfonate. Spacing factors increase. Air content requirement decreases with increase in maximum size. Little increase over 37.5 mm (11/2 in.) maximum size aggregate. Air content increases with increased sand content. Middle fractions of sand promote airentrainment. 139 Use up to 100% more air-entraining admixture for finely ground slags. Increase air-entraining admixture dosage up to 100% for fume contents up to 10%. Adjust air-entraining admixture dosage if needed. Reduce dosage of air-entraining admixture. Select formulations containing air-detraining agents. Prepare trial mixes and evaluate air-void systems. Adjust air-entraining admixture dosage. No adjustments normally needed. Only slight adjustments needed. No significant effect on durability. Decrease air content. Chemical admixtures Aggregate Retarders Accelerators High-range water reducers (Plasticizers) Maximum size Sand-to-total aggregate ratio Sand grading Decrease air-entraining admixture dosage for mixtures having higher sand contents. Monitor gradation and adjust air-entraining admixture dosage accordingly. Design and Control of Concrete Mixtures N EB001 Table 8-4. Effect of Mixture Design and Concrete Constituents on Control of Air Content in Concrete (Continued)
Characteristic/Material Water chemistry
Mix water and slump Water-to-cement ratio Slump Effects Very hard water reduces air content. Batching of admixture into concrete wash water decreases air. Algae growth may increase air. Air content increases with increased water to cement ratio. Air increases with slumps up to about 150 mm (6 in.). Air decreases with very high slumps. Difficult to entrain air in low-slump concretes. Guidance Increase air entrainer dosage. Avoid batching into wash water. Decrease air-entraining admixture dosage as water to cement ratio increases. Adjust air-entraining admixture dosages for slump. Avoid addition of water to achieve high-slump concrete. Use additional air-entraining admixture; up to ten times normal dosage. Table 8-5. Effect of Production Procedures, Construction Practices, and Environment on Control of Air Content in Concrete
Procedure/Variable Batching sequence Effects Guidance Simultaneous batching lowers air Add air-entraining admixture with initial water or content. on sand. Cement-first raises air content. Air increases as capacity is approached. Run mixer close to full capacity. Avoid overloading. Central mixers: air content increases Establish optimum mixing time for particular mixer. up to 90 sec. of mixing. Truck mixers: air content increases Avoid overmixing. with mixing. Short mixing periods (30 seconds) Establish optimum mixing time (about 60 seconds). reduce air content and adversely affect air-void system. Air content gradually increases up to Follow truck mixer manufacturer recommendations. approx. 20 rpm. Air may decrease at higher mixing Maintain blades and clean truck mixer. speeds. Accuracy and reliability of metering Avoid manual-dispensing or gravity-feed systems system will affect uniformity of air and timers. Positive-displacement pumps interlocked content. with batching system are preferred. Some air (1% to 2%) normally lost during transport. Loss of air in nonagitating equipment is slightly higher. Normal retempering with water to restore slump will restore air. If necessary, retemper with air-entraining admixture to restore air. Dramatic loss in air may be due to factors other than transport. Optimize delivery schedules. Maintain concrete temperature in recommended range. Retemper only enough to restore workability. Avoid addition of excess water. Higher admixture dosage is needed for jobsite admixture additions. Production procedures Transport and delivery Mixer capacity Mixing time Mixing speed Admixture metering Transport and delivery Haul time and agitation Retempering Long hauls, even without agitation, reduce air, especially in hot weather. Regains some of the lost air. Does not usually affect the air-void system. Retempering with air-entraining admixtures restores the air-void system. 140 Chapter 8 N Air-Entrained Concrete
Table 8-5. Effect of Production Procedures, Construction Practices, and Environment on Control of Air Content in Concrete (Continued)
Procedure/Variable Belt conveyors Pumping Effects Reduces air content by an average of 1%. Reduction in air content ranges from 2% to 3%. Does not significantly affect air-void system. Minimum effect on freeze-thaw resistance. Generally reduces air content in wetprocess shotcrete. Air content decreases under prolonged vibration or at high frequencies. Proper vibration does not influence the air-void system. Air content reduced in surface layer by excessive finishing. Air content decreases with increase in temperature. Changes in temperature do not significantly affect spacing factors.
9.0 Placement techniques Guidance Avoid long conveyed distance if possible. Reduce the free-falling effect at the end of conveyor. Use of proper mix design provides a stable air-void system. Avoid high slump, high air content concrete. Keep pumping pressure as low as possible. Use loop in descending pump line. Air content of mix should be at high end of target zone. Do not overvibrate. Avoid high-frequency vibrators (greater than 10,000 vpm). Avoid multiple passes of vibratory screeds. Closely spaced vibrator insertion is recommended for better consolidation. Avoid finishing with bleed water still on surface. Avoid overfinishing. Do not sprinkle water on surface prior to finishing. Do not steel trowel exterior slabs. Increase air-entraining admixture dosage as temperature increases. Shotcrete Internal vibration
Finishing and environment Finishing Temperature tents. The effect of water hardness in most municipal water supplies is generally insignificant; however, very hard water from wells, as used in rural communities, may decrease the air content in concrete. 25-mm (1-in.) immersion-type vibrator. All mixes contained same amount of air-entraining admixture. 8.0 Slump and Vibration
The effect of slump and vibration on the air content of concrete is shown in Fig. 8-18. For a constant amount of airentraining admixture, air content increases as slump increases up to about 150 or 175 mm (6 or 7 inches); then it begins to decrease with further increases in slump. At all slumps, however, even 15 seconds of vibration (the ACI 309 limit) will cause a considerable reduction in air content. Prolonged vibration of concrete should be avoided. The greater the slump, air content, and vibration time, the larger the percentage of reduction in air content during vibration (Fig. 8-18). However, if vibration is properly applied, little of the intentionally entrained air is lost. The air lost during handling and moderate vibration consists mostly of the larger bubbles that are usually undesirable from the standpoint of strength. While the average size of the air voids is reduced, the air-void spacing factor remains relatively constant. Internal vibrators reduce air content more than external vibrators. The air loss due to vibration increases as the volume of concrete is reduced or the vibration frequency is significantly increased. Lower vibration frequencies (8000 vpm) have less effect on spacing factors
141 7.0 6.0 Air content, percent 137-mm (5.4-in.) slump 5.0
96-mm (3.8-in.) slump 4.0 3.0
46-mm (1.8-in.) slump 2.0 1.0 0 10 20 30 Vibration time, seconds 40 50 Fig. 8-18. Relationship between slump, duration of vibration, and air content of concrete (Brewster 1949). Design and Control of Concrete Mixtures N EB001 concrete increases, particularly as slump is increased. This effect is especially important during hot-weather concreting when the concrete might be quite warm. A decrease in air content can be offset when necessary by increasing the quantity of air-entraining admixture. In cold-weather concreting, the air-entraining admixture may lose some of its effectiveness if hot mix water is used during batching. To offset this loss, such admixtures should be added to the batch after the temperature of the concrete ingredients have equalized. Although increased concrete temperature during mixing generally reduces air volume, the spacing factor and specific surface are only slightly affected. and air contents than high vibration frequencies (14,000 vpm). High frequencies can significantly increase spacing factors and decrease air contents after 20 seconds of vibration (Brewster 1949 and Stark 1986). For pavements, specified air contents and uniform air void distributions can be achieved by operating within paving machine speeds of 1.22 to 1.88 meters per minute (4 to 6 feet per minute) and with vibrator frequencies of 5,000 to 8,000 vibrations per minute. The most uniform distribution of air voids throughout the depth of concrete, in and out of the vibrator trails, is obtained with the combination of a vibrator frequency of approximately 5,000 vibrations per minute and a slipform paving machine forward track speeds of 1.22 meters per minute (4 feet per minute). Higher frequencies of speeds singularly or in combination can result in discontinuities and lack of required air content in the upper portion of the concrete pavement. This in turn provides a greater opportunity for water and salt to enter the pavement and reduce the durability and life of the pavement (Cable, McDaniel, Schlorholtz, Redmond, and Rabe 2000). Supplementary Cementitious Materials
The effect of fly ash on the required dosage of air-entraining admixtures can range from no effect to an increase in dosage of up to five times the normal amount (Gebler and Klieger 1986). Large quantities of slag and silica fume can double the dosage of air-entraining admixtures (Whiting and Nagi 1998). Concrete Temperature
Temperature of the concrete affects air content, as shown in Fig. 8-19. Less air is entrained as the temperature of the
175-mm (7-in.) slump Admixtures and Coloring Agents
Coloring agents such as carbon black usually decrease the amount of air entrained for a given amount of admixture. This is especially true for coloring materials with increasing percentages of carbon (Taylor 1948). Water-reducing and set-retarding admixtures generally increase the efficiency of air-entraining admixtures by 50% to 100%; therefore, when these are used, less airentraining admixture will usually give the desired air content. Also, the time of addition of these admixtures into the mix affects the amount of entrained air; delayed additions generally increasing air content. Set retarders may increase the air-void spacing in concrete. Some water-reducing or set-retarding admixtures are not compatible with some air-entraining admixtures. If they are added together to the mixing water before being dispensed into the mixer, a precipitate may form. This will settle out and result in large reductions in entrained air. The fact that some individual admixtures interact in this manner does not mean that they will not be fully effective if dispensed separately into a batch of concrete. Superplasticizers (high-range water reducers) may increase or decrease the air content of a concrete mixture based on the admixture's chemical formulation and the slump of the concrete. Naphthalene-based superplasticizers tend to increase the air content while melaminebased materials may decrease or have little effect on air content. The normal air loss in flowing concrete during mixing and transport is about 2 to 4 percentage points (Whiting and Dziedzic 1992). Superplasticizers also affect the air-void system of hardened concrete by increasing the general size of the entrained air voids. This results in a higher-than-normal
142 Concrete temperature, F 60 70 80 90 6
125-mm (5-in.) slump 5 75-mm (3-in.) slump Air content, percent 4 25-mm (1-in.) slump 3 2 1
Cement: 335 kg/m3 (565 lb/yd3 ) Aggregate: 37.5-mm (11/2-in.) max. size 0 5 10 15 20 25 Concrete temperature, C 30 35 Fig. 8-19. Relationship between temperature, slump, and air content of concrete. PCA Major Series 336 and Lerch 1960. Chapter 8 N Air-Entrained Concrete spacing factor, occasionally higher than what may be considered desirable for freeze-thaw durability. However, tests on superplasticized concrete with slightly higher spacing factors have indicated that superplasticized concretes have good freeze-thaw durability. This may be due to the reduced water-cement ratio often associated with superplasticized concretes. A small quantity of calcium chloride is sometimes used in cold weather to accelerate the hardening of concrete. It can be used successfully with air-entraining admixtures if it is added separately in solution form to the mix water. Calcium chloride will slightly increase air content. However, if calcium chloride comes in direct contact with some air-entraining admixtures, a chemical reaction can take place that makes the admixture less effective. Nonchloride accelerators may increase or decrease air content, depending upon the individual chemistry of the admixture, but they generally have little effect on air content.
5 11 rpm 4 Air content, percent 4 rpm 3 2 1 Cement: 305 kg/m3 (510 lb/yd 3 ) Mixer: 4 m3 (6 yd3 ) transit mixer Mixing time: starts after charging completed 0 0 10 20 30 40 Mixing time, minutes 50 60 Fig. 8-20. Relationship between mixing time and air content of concrete. PCA Major Series 336. Mixing Action
Mixing action is one of the most important factors in the production of entrained air in concrete. Uniform distribution of entrained air voids is essential to produce scaleresistant concrete; nonuniformity might result from inadequate dispersion of the entrained air during short mixing periods. In production of ready mixed concrete, it is especially important that adequate and consistent mixing be maintained at all times. The amount of entrained air varies with the type and condition of the mixer, the amount of concrete being mixed, and the rate and duration of mixing. The amount of air entrained in a given mixture will decrease appreciably as the mixer blades become worn, or if hardened concrete is allowed to accumulate in the drum or on the blades. Because of differences in mixing action and time, concretes made in a stationary mixer and those made in a transit mixer may differ significantly in amounts of air entrained. The air content may increase or decrease when the size of the batch departs significantly from the rated capacity of the mixer. Little air is entrained in very small batches in a large mixer; however, the air content increases as the mixer capacity is approached. Fig. 8-20 shows the effect of mixing speed and duration of mixing on the air content of freshly mixed concretes made in a transit mixer. Generally, more air is entrained as the speed of mixing is increased up to about 20 rpm, beyond which air entrainment decreases. In the tests from which the data in Fig. 8-20 were derived, the air content reached an upper limit during mixing and a gradual decrease in air content occurred with prolonged mixing. Mixing time and speed will have different effects on the air content of different mixes. Significant amounts of air can be lost during mixing with certain mixtures and types of mixing equipment.
143 Fig. 8-21 shows the effect of continued mixer agitation on air content. The changes in air content with prolonged agitation can be explained by the relationship between slump and air content. For high-slump concretes, the air content increases with continued agitation as the slump decreases to about 150 or 175 mm (6 or 7 in.). Prolonged agitation will decrease slump further and decrease air content. For initial slumps lower than 150 mm (6 in.), both the air content and slump decrease with continued agitation. When concrete is retempered (the addition of water and remixing to restore original slump), the air content is increased; however, after 4 hours, retempering is ineffective in increasing air content. Prolonged mixing or agitation of concrete is accompanied by a progressive reduction in slump. 8
Agitating speeds: 2 or 4 rpm Transit mixer: 4.5 and 6.1 m3 (6 and 8 yd3 ) Initial mixing: 70 rev. at 10 rpm Air content, percent 7 225-mm (9-in.) initial slump 6 5
100-mm (4-in.) initial slump 4 10 20 30 40 50 60 70 80 Agitating time, minutes (after initial mixing) 90 Fig. 8-21. Relationship between agitating time, air content, and slump of concrete. PCA Major Series 336. Design and Control of Concrete Mixtures
N EB001 When aggregates larger than 50 mm (2 in.) are used, they should be removed by hand and the effect of their removal calculated in arriving at the total air content. 3. Gravimetric method (ASTM C 138 or AASHTO T 121, Standard Test Method for Unit Weight, Yield, and Air Content [Gravimetric] of Concrete)--requires accurate knowledge of relative density and absolute volumes of concrete ingredients. 4. Chace air indicator (AASHTO T 199, Standard Method of Test for Air Content of Freshly Mixed Concrete by the Chace Indicator)--a very simple and inexpensive way to check the approximate air content of freshly mixed concrete. This pocket-size device tests a mortar sample from the concrete. This test is not a substitute, however, for the more accurate pressure, volumetric, and gravimetric methods. The foam-index test can be used to measure the relative air-entraining admixture requirement for concretes containing fly ash-cement combinations (Gebler and Klieger 1983). Transporting and Handling
Generally, some air--approximately 1 to 2 percentage points--is lost during transportation of concrete from the mixer to the jobsite. The stability of the air content during transport is influenced by several variables including concrete ingredients, haul time, amount of agitation or vibration during transport, temperature, slump, and amount of retempering. Once at the jobsite, the concrete air content remains essentially constant during handling by chute discharge, wheelbarrow, power buggy, and shovel. However, concrete pumping, crane and bucket, and conveyor-belt handling can cause some loss of air, especially with high-air-content mixtures. Pumping concrete can cause a loss of up to 3 percentage points of air (Whiting and Nagi 1998). Finishing
Proper screeding, floating, and general finishing practices should not affect the air content. McNeal and Gay (1996) and Falconi (1996) demonstrated that the sequence and timing of finishing and curing operations are critical to surface durability. Overfinishing (excessive finishing) may reduce the amount of entrained air in the surface region of slabs--thus making the concrete surface vulnerable to scaling. However, as shown in Fig. 8-22, early finishing does not necessarily affect scale resistance unless bleed water is present (Pinto and Hover 2001). Concrete to be exposed to deicers should not be steel troweled. 5
Time of finishing 4 Visual rating early on-time 3
Bleed water 2 TESTS FOR AIR CONTENT
Four methods for determining the air content of freshly mixed concrete are available. Although they measure only total air volume and not air-void characteristics, it has been shown by laboratory tests that these methods are indicative of the adequacy of the air-void system. Acceptance tests for air content of freshly mixed concrete should be made regularly for routine control purposes. Samples should be obtained and tested in accordance with ASTM C 172 (AASHTO T 141). Following are methods for determining the air content of freshly mixed concrete: 1. Pressure method (ASTM C 231 or AASHTO T 152, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method)--practical for fieldtesting all concretes except those made with highly porous and lightweight aggregates. 2. Volumetric method (ASTM C 173 or AASHTO T 196, Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method)--practical for fieldtesting all concretes, but particularly useful for concretes made with lightweight and porous aggregates.
144 1 0 0.50 0.45 0.40 0.35 Water-to-cement ratio 0.30 0.25 5
Time of finishing early on-time 4 Visual rating 3 2 1 0 0.50 0.45 0.40 0.35 Water-to-cement ratio 0.30 0.25 Fig. 8-22. Effect of early finishing--magnesium floating 20 minutes after casting--on scale resistance for: (top) 6% airentrained concrete; (bottom) non-air-entrained concrete. Chapter 8 x Air-Entrained Concrete The air-void characteristics of hardened concrete can be determined by ASTM C 457 methods. This test is used to determine void spacing factor, specific surface of voids, and number of voids per length of traverse. Air-Void Analysis of Fresh Concrete
The conventional methods for analyzing air in fresh concrete, such as the pressure method noted above, measure the total air content only; consequently, they provide no information about the parameters that determine the quality of the air-void system. These parameters--the size and number of voids and spacing between them--can be measured on polished samples of hardened concrete (ASTM C 457); but the result of such analysis will only be available several days after the concrete has hardened. Therefore, test equipment called an air-void analyzer (AVA) has been developed to determine the standard ASTM C 457 air-void parameters in fresh samples of airentrained concrete (Fig. 8-23). The test apparatus determines the volume and size distributions of entrained air voids; thus an estimation of the spacing factor, specific surface, and total amount of entrained air can be made. In this test method, air bubbles from a sample of fresh concrete rise through a viscous liquid, enter a column of water above it, then rise through the water and collect under a submerged buoyancy recorder (Fig. 8-24). The viscous liquid retains the original bubble sizes. Large bubbles Fig. 8-24. Air bubbles rising through liquids in column. (67962) rise faster than small ones through the liquids. The change in buoyancy is recorded as a function of time and can be related to the number of bubbles of different size. Fresh concrete samples can be taken at the ready mix plant and on the jobsite. Testing concrete before and after placement into forms can verify how the applied methods of transporting, placing, and consolidation affect the airvoid system. Since the samples are taken on fresh concrete, the air content and air-void system can be adjusted during production. Currently, no standard exists for this method. The AVA was not developed for measuring the total air-content of concrete, and because of the small sample size, may not give accurate results for this quantity. However, this does not mean the AVA is not useful as a method for assessing the quality of the air-void system; it gives good results in conjunction with traditional methods for measuring air content (Aarre 1998). RECOMMENDED AIR CONTENTS
The amount of air to be used in air-entrained concrete depends on a number of factors: (1) type of structure, (2) climatic conditions, (3) number of freeze-thaw cycles, (4) extent of exposure to deicers, and (5) the design life of the structure. The ACI 318 building code states that concrete that will be exposed to moist freezing and thawing or deicer chemicals shall be air entrained with the target air content of Table 8-6 for severe exposure. Furthermore, the water to cementitious materials ratio should not exceed 0.45. ACI 318 allows a one percentage point reduction in target air contents for concretes with strengths over 34 MPa (5,000 psi) and presumably very low water-cement ratios.
145 Fig. 8-23. Equipment for the air-void analyzer. (67961) Design and Control of Concrete Mixtures N EB001 ACI 318 limits the amounts of pozzolans and slag-- 10% for silica fume, 25% for fly ash, 50% for slag--as part of the cementitious material for deicer exposures. However, mix designers should consult local practices as to allowable dosages to prevent frost and deicer damage. Combinations of materials without historical record can be analyzed using ASTM C 666 (AASHTO T 161) and ASTM C 672. Pinto and Hover (2001) evaluate the applicability of the ACI 318 requirements for frost resistance of portland cement concrete mixtures with water to cement ratios from 0.25 to 0.50. Fig. 8-26 illustrates the effect of increased air content with respect to aggregate size on reducing expansion due to saturated freezing and thawing; it emphasizes the need to follow the requirements of Table 8-6 for severe exposure. When entrained air is not required for protection against freeze-thaw cycles or deicers, the target air contents for mild exposure given in Table 8-6 can be used. Higher air contents can also be used as long as the design strength is achieved. As noted earlier, entrained air helps to reduce bleeding and segregation and can improve the workability of concrete. More information on air-entrained concrete can be found in Whiting and Nagi (1998). Fig. 8-25 illustrates how deicer-scaling resistance is impacted by air content and low water to portland cement ratios (strengths ranging from 40 to 59 MPa [5,800 to 8,600 psi]). This illustrates that concretes with very low water to portland cement ratios are more frost and deicer resistant; hence, they may allow use of lower air contents. This relationship (Fig. 8-25) was not established for concretes containing supplementary cementitious materials as they were not studied (Pinto and Hover 2001). Table 8-6. Recommended Total Target Air Content for Concrete
Nominal maximum size aggregate, mm (in.) <9.5 (3/8) 9.5 (3/8) 12.5 (1/2) 19.0 (3/4) 25.0 (1) 37.5 (11/2) 50 (2) 75 (3) Air content, percent* Severe exposure** 9 71/2 7 6 6 51/2 5 41/2 Moderate Mild exposure exposure 7 6 51/2 5 41/2 41/2 4 31/2 5 41/2 4 31/2 3 21/2 2 11/2 * Project specifications often allow the air content of the concrete to be within -1 to +2 percentage points of the table target values. ** Concrete exposed to wet-freeze-thaw conditions, deicers, or other aggressive agents. Concrete exposed to freezing but not continually moist, and not in contact with deicers or aggressive chemicals. Concrete not exposed to freezing conditions, deicers, or aggressive agents. These air contents apply to the total mix, as for the preceding aggregate sizes. When testing these concretes, however, aggregate larger than 37.5 mm (11/2 in.) is removed by handpicking or sieving and air content is determined on the minus 37.5 mm (11/2 in.) fraction of mix. (Tolerance on air content as delivered applies to this value.)
Expansion, percent 0.20
Freezethaw cycles: 300 Specimens: 75 x 75 x 280-mm (3 x 3 x 111/4-in.) concrete prisms Cement: Type I, 310 kg/m3 (517 lb/yd3) Slump: 50 to 75 mm (2 to 3 in.) 0.18 0.16 0.14 0.12 Maximum-size aggregate 9.5-mm (3/8-in.) 19.0-mm (3/4-in.) 37.5-mm (11/2-in.) Average mass loss at 40 cycles, kg/m2 3.0 ASTM C 672
Water-to-cement ratio 0.40 0.35 0.30 0.25 0.6 Average mass loss at 40 cycles, lb/ft2 0.10 0.08 2.0 0.4 0.06 0.04 1.0 0.2 0.02 0.0 1 2 3 4 5 Total air content, percent 6 7 0.0 0 0 2 4 Fig. 8-25. Measured mass loss after 40 cycles of deicers and freeze-thaw exposures at various air contents (Pinto and Hover 2001). 6 8 10 Air content, percent 12 14 Fig. 8-26. Relationship between air content and expansion of concrete test specimens during 300 cycles of freezing and thawing for various maximum aggregate sizes. Smaller aggregate sizes require more air (Klieger 1952). 146 Chapter 8 x Air-Entrained Concrete REFERENCES
Aarre, Tine, " Air-Void Analyzer," Concrete Technology Today, PL981, Portland Cement Association, http://www. portcement.org/pdf_files/PL981.pdf, April 1998, page 4. ACI Committee 201, Guide to Durable Concrete, ACI 201.2R92, reapproved 1997, ACI Committee 201 Report, American Concrete Institute, Farmington Hills, Michigan, 1992. ACI Committee 308, Standard Practice for Curing Concrete, ACI 308-92, reapproved 1997, ACI Committee 308 Report, American Concrete Institute, Farmington Hills, Michigan, 1992. ACI Committee 309, Guide for Consolidation of Concrete, ACI 309R-96, ACI Committee 309 Report, American Concrete Institute, Farmington Hills, Michigan, 1996. ACI Committee 318, Building Code Requirements for Structural Concrete and Commentary, ACI 318-02, ACI Committee 318 Report, American Concrete Institute, Farmington Hills, Michigan, 2002. ACPA, Scale-Resistant Concrete Pavements, IS117, American Concrete Pavement Association, Skokie, Illinois, 1996. 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