In this article we will discuss about the effects of air entrainment on the properties of concrete.
1. Effect on Resistance to Freezing and Thawing:
The high resistance developed in the hardened concrete to scaling due to freezing and thawing is the greatest advantage of air entrained concrete. It is observed that when an ordinary concrete is subjected to a temperature below freezing point, the water present in the pores of the concrete freezes and swells to a volume of 1.1 times of the original volume i.e. volume of ice is found 10% more than original volume of water.
Thus the ice formed in the pores of hardened concrete exerts considerable pressure on the hardened concrete, resulting scaling of the surface and disruption of the concrete at the weaker sections. Similar phenomenon of surface scaling and disruption of concrete takes place in plain concrete, when salts used for removing the ice from the concrete. Similar failure pattern is also observed in concrete structures at tidal and spray zones. Now it has been established that air entrainment increases the resistances to freezing and thawing about 3 to 7%.
The probable reason for the increased resistance to freezing and thawing seems to be the modification in pore structure of air entrained concrete. In ordinary concrete bigger voids interconnected by capillaries exist. The capillaries are formed due to bleeding. In case of air entrained concrete, though the total number of air voids is more, but the voids are minute.
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The bubbles are comparatively of uniform size and regular spherical shape. This air void system reduces the tendency for the development of large size ice crystals in concrete. Secondly the interconnected air voids system works as a buffer zone to relieve internal pressure. Fig. 6.3 shows the relative durability of plain and air entrained concrete.
The resistance of concrete to freezing and thawing was measured by “Blanks” by means of durability factor. He defined durability factor as the number of cycles of freezing and thawing till the failure of concrete divided by 100. From the curves it can be seen that good quality concrete with 4% air entrainment withstood 2000 cycles of freezing and thawing whereas poor quality concrete need about 14% air contents and disintegrated at about 200 cycles.
2. Effect on Workability:
The entrainment of air in fresh concrete improves its workability. The place-ability of an air entrained concrete having 7.5 cm slump was found better than a non-air entrained concrete having a slump of 12.5 cms. Better place-ability of air entrained concrete results in more homogeneous concrete with least segregation, bleeding and honeycombing.
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The air entrained concrete is more plastic and can be handled more easily than ordinary concrete. Air entrainment also increases the pump-ability of the concrete to a great extent.
For better workability of concrete, the aggregate particles should be spaced in such a way that they can move one over the other with ease during mixing and placing. In ordinary non air entrained concrete better workability can be achieved by including sufficient fine sand, cement and water to separate the particles of coarse aggregate and supply of matrix. In this condition the movement of particles may take place with minimum interference.
Improvement in workability by air entrainment may also be explained as follows:
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The proportioning of concrete mixes involves compromise between the requirements of workability, strength, durability and other properties of the hardened concrete. Workability requires that the voids in between the particles of coarse aggregates be more than which could be filled by cement paste, while the good quality hardened concrete needs that these voids should be such which just could be filled by cement paste.
The entrained air increases the effective volume of cement paste during mixing and placing eliminating the need for extra cement paste to improve workability. During the process of compaction the extra air will escape resulting hardened concrete of desired density. During compaction 50 to 66% of air may escape from the fresh concrete depending upon the water cement ratio and duration of compaction.
As a matter of fact the minute air bubbles introduced in the concrete by the use of air entraining agents behave as ball bearing over which the particles of aggregates slide easily. This is called a lubricating effect of air bubbles. They also provide a cushioning effect between the sand grains, reducing particle interference to a minimum.
The increase in workability by the use of air entrained agents is found more in the following type of concretes:
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1. The increase in workability is found more in concretes made with angular aggregates than rounded aggregates.
2. The increase in workability is found greater for wetter mixes than drier ones.
3. The increase in workability is found more in leaner mixes than richer ones.
The effect of air entrainment on compacting factor for different water cement ratios and different mixes is shown in Fig. 6.5 based on the results of experiments carried by “WRIGHT”. Road research Laboratory U.K, it is found that 5% air entrainment increases the compacting factor by 0.07, which would be equal to an increase in slump from 1.2 cm to 5.0 cms.
3. Effect on Strength of Concrete:
Generally it can be said that air entrainment in concrete reduces the compressive strength of concrete. On the basis of experiments carried out at Road Research Laboratory U.K., ‘WRIGHT’ has stated that each 1% of air entrained results in the loss of 5.5% in compressive strength of concrete. The actual strength obtained is shown Fig. 6.6. The mix proportion and water-cement ratio is shown on the curves on the right hand side from the curves it will be seen that strength decreases with the increase in air content.
Some results have been expressed as a percentage of the strength of concrete without air. These results forms a single straight line as shown in Fig. 6.7. This represents a decrease in strength of 5.5% per each 1% air entrained. He further states that entrapped air effects the compressive strength of concrete slightly more than the entrained air as shown in Fig. 6.7.
However the air voids caused by in complete compaction do not contribute other advantages of entrained air such as better workability and increased durability.
It has been observed that the effect of air entrainment on strength depends upon the following three factors:
1. The amount of air entrained
2. Richness of the mix
3. Type of air entraining agent
KLIEGER investigated the effects of entraining air on the strength and durability of concrete made with various, maximum size of aggregates. He made extensive study on the effect of amount of air entrained and Richness of the mix on the compressive as well as flexural strength of concrete. The water content was reduced to maintain the slump of the concrete constant as the air content was increased. The reduction in water content sometimes was more than compensated due to the increase in air content for the reduction in the strength.
On the basis of his experiments he stated as under:
1. For lean mixes, having maximum smaller size aggregate. When the reduction in water was greatest, the air entrainment actually increased the strength of concrete both in compression and bending.
2. In case of rich mixes, having largest maximum size aggregate, where greater reduction in water content could not be effected, there was reduction in both compressive as well as flexural strengths of concrete.
3. Between these two extremities the strength varied from an increase of about 10% in bending and about 17.5% in compression and a reduction of about 24% in bending and about 46% in compression. Thus the effect of air entrainment on flexural strength is less than the effect on compressive strength. The effect of air entrainment is more on compressive strength.
The above facts also have been confirmed by CORDON and BLANKS. CORDON carried out experiments to study the effect of air entrainment on the compressive strength of concrete. The results of his study have been shown in Fig. 6.8. From the above graph it can be seen that the compressive strength of concrete decreases uniformly with the increase in air content of fresh concrete. At a constant water/cement ratio each one percent of entrained air resulted decrease of about 5% on average in the compressive strength of concrete.
On the other hand, experiments carried out at the Military Engineering College, Pune using RITHA powder as an air entraining agent, show average reduction in compressive strength of about 2% for each 1% air entrained. In addition to the above main effects, following effects have also been found useful.
Effect on Segregation, Bleeding and Laitance:
The bleeding and segregation of concrete are different processes of loss of homogeneity. Segregation is defined as separation of cement paste from aggregates while bleeding is the emergence of water to the surface from freshly placed concrete. Usually, bleeding results due to the sedimentation of the solid particles due to compaction and self-weight of the solids.
Due to the bleeding phenomenon, a series of water channels are formed and some of them extend upto the surface. The channel of water emerging at the surface of concrete, often brings some quantity of cement with it and forms a layer of cement on the surface of concrete. This layer of cement formed on the surface is called laitance.
Air entrainment has been found effective in reducing segregation, bleeding and laitance formation. These actions probably result from physical phenomenon due to the introduction of minute air bubbles in the fresh concrete.
Air bubbles act as follows:
1. Air bubbles buoy up the aggregates and cement, resulting in the reduction of rate of sedimentation of these particles in the fresh concrete.
2. The bubbles decrease the effective area through which the differential movement of water takes place.
3. The bubbles increase the mutual adhesion between cement and aggregate particles.
4. The surface area of voids in the plastic concrete is sufficiently large to retard the rate of separation of water from the paste by drainage.
All researchers are unanimous regarding the reduction in bleeding by the use of entrained air, but they are divided about the role played by entrained air in reducing segregation.
RITHA Powder has been found an efficient air entraining agent in reducing the bleeding in cement concrete and cement mortars. Experiments have been carried out at the Military Engineering College, Pune to find the effect of vinsol resin and RITHA Powder on the reduction of bleeding in cement concrete and mortars.
The test results have been shown in Fig. 6.9. From the curves it can be seen that plain mortar i.e. without entrained air bled 15% of the mixing water in about 3 hours times whereas mortars containing RITHA Powder bled 7.5% of the mixing water and the mortars containing vinsol resin bled 11 % of the mixing water, both RITHA Powder and vinsol resin were mixed 0.1% by weight of cement.
The reduction in bleeding automatically reduces the formation of laitance as these two are interrelated. The reduced bleeding of concrete permits the early finishing of the concrete surface reducing the waiting period for the commencement of trowelling. Reduction in bleeding also improves the wearing quality of the concrete surface. If the bleeding does not reduce, the masons are in the habit of sprinkling of dry cement over the concrete surface to reduce the waiting period for troweling. The finished surface scales off after some time, rendering the surface unpleasant.
Effect on Permeability:
1. The entrained air increases the workability of concrete resulting in its greater uniformity.
2. The entrained air also modifies the pore structure of the concrete.
3. The entrained air also reduces the bleeding of concrete resulting in reduction of water channels.
4. The reduction of water/cement ratio also reduces the permeability of concrete.
Effect on Chemical Resistance:
Experiments conducted at Road Research Laboratory U.K. showed that air entrained concrete has greater chemical resistance than ordinary concrete.
Effect on Sand, Water and Cement Contents:
It has been observed that minute spherical air bubbles formed due to air entrainment act as fine aggregate and enable the reduction of sand (fine aggregates). The reduction of fine aggregate, results in the reduction of water requirement without any ill effect on the workability and slump. On the basis of experiments, researchers have found that the sand content by weight of total aggregate may be reduced by one percent for each percent increase in air entrainment upto about 8% without any appreciable change in workability or slump.
Cordon on the basis of his experiments, has shown that water requirement of an average concrete mix is reduced approximately 3.5 kg/m3 of concrete with rounded aggregates and 4.8 kg/m3 with angular aggregates for each percent air entrained. His results are shown in Fig. 6.10. The advantages of air entrained concrete regarding reduction of sand and water requirements are shown in Table 6.7 below.
Entrainment of air has been found useful in case of lean mixes. In lean mixes many a time increase in strength has been observed. In such cases for the given strength, a reduction in cement content may be effected. Blanks and Cordon found that concrete made with 160 mm maximum size aggregate gave satisfactory strength for mass concrete work using only 106 kg cement/m1 concrete with air entrainment, resulting in lower production of heat of hydration and lower rise of temperature. The lower rise in temperature resulted in reduced cracking and undesirable internal stresses in mass concrete.
Effect on Unit Weight:
Air entrained concrete of the same workability and strength was found to have 5% less solid materials hence of lower weight. This will result in an economy of about 5% in the cost of cement and aggregate.
Effect on Alkali-Aggregate Reaction:
Air entrained concrete is found more resistant to alkali-aggregate reaction.
Effect on Modulus of Elasticity:
With the same water/cement ratio and the same aggregate the reduction in modulus of elasticity of air entrained concrete is found 2 to 3% for each percent of air entrainment.
Abrasion Resistance:
It has been observed that concrete with air entrainment less than 6% has the same resistance to abrasion as that of ordinary concrete without air entrainment. If the air entrainment is increased beyond 6%, then abrasion resistance decreases, when the air content is about 10%, the abrasion resistance decreases greatly. For satisfactory abrasion resistance generally air entrainment for pavement concrete is specified not more than 3 to 6%.