Air Entrained Concrete

Introduction Air-entrainment in concrete is the process by which numerous small air bubbles are incorporated into fresh concrete to become the part of the matrix that binds the aggregate together in the hardened concrete. Air entrainment is attained by use of air-entraining agents (AEA), added during mixing, under careful engineering super vision as the concrete is mixed on the job. Entrained air is produced during mechanical mixing of concrete containing AEA. The shearing action of mixer blades continuously breaks up the air into a fine system of bubbles. AEA stabilizes these air bubbles.

Air-Void system Once the concrete has set, the casts of the original air bubbles are left behind in the hardened concrete as voids. This is commonly referred to as the “air-void system” in hardened concrete. The major parameters of air-void system are the total air content, average spacing factor between the voids, & specific surface. They determine the quality of air void system. The majority of the voids in normal air-entrained concrete are between 10μm and 1mm in diameter. These bubbles are uniformly distributed throughout the concrete. The relative distance between the voids is termed as the spacing factor. A smaller spacing is better. The specific surface indicates the relative number and size of air bubbles for a given volume of air. A larger specific surface is better because it indicates a larger number of small bubbles. A spacing factor of less than 0.2 mm & specific surface greater than 24mm2/mm3 are considered as prime requirement of air-void system.

Durability vs. strength Air entrainment greatly increases concrete durability but reduces concrete strength. Compressive strength is generally reduced by 2 to 7% for each % increase in air content. Therefore adequate strength & maximum durability are achieved by establishing optimum air content & spacing factors.

Air-entraining admixtures ™ Natural wood resin and their soaps ™ Animal or vegetable fats and oils such as fallow and olive oil ™ Wetting agents such as alkali salts of sulphonated or sulphated organic compounds

Factors influencing air entrainment in concrete Cement- An increase in the fineness of cement will decrease the air content. Fine aggregateWell rounded particles are conductive to air entrainment. Coarse aggregate- Crushed stone concrete will entrain less air than gravel concrete. Water- Hard water is used to dilute the AEA prior to batching; the air content will be reduced. Slump- an increase in slump from 75 mm to 150 mm will increase the air content. Temperature-an increase of concrete temperature will decrease the air content. An increase in temperature from 21oC to 40oC may reduce the air content by 25%. Mixing time- The air content will increase with increased time of mixing up to 2 minutes in stationary or paving mixtures & to about 15 minutes in most transit mixtures. Vibration- Excess vibration will reduce the air content. The quantity of air entrained increased with reduction in cement content. Addition of super plasticizers in concrete diminished the air entraining effect of AEA.

Test Procedure. An AEA does not generate air & the entrained air exists with in the paste portion of the mix. For the purpose of quality control test on AEA the air produced in motor by given Portland cement in the presence of AEA can be measured in accordance with ASTM C 185. Test involves mixing 350g of test cement, 1200 g of specially graded sand & enough water to produce a mortar whose flow is in designated range. A portion of mortar then weighed & its air content is calculated. As per ASTM C 457, standard test method for microscopical determination of parameters of the air void system can also be conducted on polished sample of hardened concrete.

Details of Experimental work. Materials used in work were typical 53 grade OPC-IS 12269(tested as per IS 4031; initial setting time-85 mins, final setting time 300 mins, fineness of 3050 cm2/g, 28th day compressive strength of 56 Mpa) river sand (fineness modulus 2.8, G-2.6), C.A (granite; G2.75) max size of 20mm. Air content measured based on the densities of cement composites with & without AEA as per ASTM C 185, water-cement ratio- 0.4, mineral admixtures like fly ash( class F, low calcium type, fineness 3010cm2/g, G-2.14, bulk density 1010kg/m3, GGBS-fineness 3200cm2/g, silica fume (SiO2 95%, G-2.24,bulk density-680kg/m3) Air content= 100 x (Do-Da)/Do Where, Do= density of air free cement composite, Da= density of cement composite containing air.

Test Results & discussion Investigations were made on the influence of various ingredients of concrete on the air entrained, by conducting test on binder paste, mortar & concrete. For concrete mortar AEA was used as a dosage of 0.32% by weight of binder (i.e. Cement). This dosage of AEA caused a 15% air in a 1:1.4:2.3 concrete, in a leaner mix of 1:2:4, there was an enhanced air entrainment of 21%. Similarly in mortars for cement sand ratio of 1:1.4, there was an air entrainment of only 5%; where as for leaner mixes of 1:2.0 the air content increased to 12%. Further increase in sand, still increased air content. For 1:2.5 and 1:4.0, air content of 17% & 21% obtained. For cement paste alone a mere 1% entrained air was noticed. Above results indicated that presence of aggregate is required for air entrainment. Its size & quantity also influence air entraining capacity of AEA. A comparison of air content of concrete and mortar having same cement, sand ratio of concrete & mortar having same cement/sand ratio indicated concrete entrains considerably more air than the mortar. 1:1.4:2.3 concrete contains 15% entrained air, 1:1.4 mortar have only 5% entrained air. 1:2 mortars contain 12% and 1:2:4 concrete it was 21%. The coarser the aggregate, more will be the entrained air.

Effect of super plasticizers on air entrainment Carboxylic acid based super plasticizers were utilised in this experiment. Air entrained decreases with addition of super plasticizers except when silica fume is used for partial replacement of cement. The reduction of air entrained can be attributed to the fact that with the addition of super plasticizers, the mix will be more fluid & at this stage entrained bubble tend to become unstable & hence the air content.

Effect of ambient temperature on air-entrainment AEA being a chemical admixture, ambient temperature can be expected to influence its behaviour in cement composites. To understand this, concrete mixes with AEA were prepared at two different ambient temperatures of 15oC and 35oC in a humidity temperature controlled walk in type chamber; installed at SERC, Chennai. It was found, a reduction in the quantity of air entrained with the temperature. The controlled concrete with AEA had indicated 14.1% of air at 15oC, where as this was reduced to 12.7% at 35oC. Similarly entrained air content in concrete containing mineral admixtures was found to reduce generally with increase in ambient temperature.

Effect of air-entrainment on compressive strength Observations on different concrete mixes (cementitious material: sand: coarse aggregate: water; 1.0:1.4:2.3:0.4; it was seen that the compressive strength of concrete was reduced with enhanced air entrainment. There was about 4.6% reduction in compressive

strength of concrete with each 1% of air entrained. This aspect should be considered while designing concrete with air entraining agent.

Effect of air entrainment on concrete freeze-thaw durability Concrete in a mix of many different materials and is inherently porous, not solid & impervious like a material such as steel. In hardened concrete, ice will form from free water in the larger pores. The ice crystals will grow by drawing water from the walls of the additional voids. This then causes water to be drawn from the smallest pores until eventually as the ice crystals grow large enough to where the pressure build up in dense pastes will result in rupture; due to repetitive freezing and thawing. It is characterised by cumulative damage in the form of cracking & scaling. Air entrainment is a necessary component of concrete mixtures exposed to freezing & thawing environment. The incorporation of unfilled welldistributed voids in the cement paste prevents excessive pressures from developing due to water crystal formation during freezing by providing spaces for these developing pressures to disperse, reducing disruptive damage.

Advantages and disadvantages ™ This technique primarily used to improve the freeze-thaw resistance of concrete ™ Intentional entraining of air into concrete enhances the cohesion of fresh concrete and also durability of hardened concrete ™ Entrained air has a pronounced effect on workability. This phenomenon is caused by the micro bubbles act as a kind of lubricant, better described as behaving like miniature ball bearings where these well dispersed bubbles act as air cushions between aggregate particles, reducing friction and interlocking. So a slight reduction in water content is possible. ™ Entrained air eliminates segregation and subsequent bleeding. It is because, the entrained air bubbles provide a certain degree of buoyancy to the aggregates and cement, there by reducing their sedimentation and also effective volume through which the differential movement of water may occur. ™ Up to 6% air is beneficial, but beyond this range the amount of entrained air can be deleterious to the structural properties of the hardened concrete. ™ Partial replacement of cement by mineral admixtures having same grain size distribution & shape of cement(ensuring total surface area of particles of in the same order) improves properties of concrete, fresh & hardened, economy & makes more environment friendly. ™ Aerated blocks, which are light weight and have excellent thermal insulation can be manufactured using this

Conclusion The quality of air void system is essential. Means an average of 4% reduction of compressive strength occurred for every 1%entrained air content. Adequate strength and maximum durability are achieved by establishing optimum air contents and spacing factors. Partial replacement of cement is possible, there by causing economy and environment friendliness.