Air Entrained Concrete: What Is It and Why Do We Use It?
- Increased resistance to freeze-thaw conditions
- Increased workability
- Increased durability
- Increased resistance to chemical attack
- Increased cohesion Disadvantages of Air Entrained Concrete:
- Strength loss
- Decreases in unit weight
- Potential honeycombing
What is the effect of air content on concrete?
Air content reduces as the fineness or surface area of fly as increases. Increase of fly ash per unit concrete reduces air content of concrete and increase in fly ash carbon content reduces concrete air content. 8. Effect of Vibration on Air Content
What is the purpose of air bubbles in concrete?
In fresh concrete, the air bubbles reduce the water demand of the concrete and make the mix stickier, which helps to reduce segregation and reduces bleeding when air content is around 3%. If the air content is higher, the increased stickiness makes the concrete more difficult to finish,...
What is the purpose of entraining air in concrete?
The primary purpose of entraining air in concrete is to increase its durability when exposed to freeze/thaw cycles with water present. Air-entraining admixtures for concrete create small, uniform, and stable air bubbles from around 0.01mm to 1mm diameter in the cement matrix throughout a concrete mix.
What are the advantages of air-entrained concrete?
Air voids, however, also improve the workability of concrete. Therefore, air-entrained concrete at a lower w/cm can be prepared to provide similar workability to non-airentrained concrete, thus compensating to some extent for the reduction in strength.
What is a good air content in concrete?
Testing tip: A typical air content for concrete with a ¾-inch maximum-size aggregate is about 6%, and specified ranges in air content are typically minus 1 ½% and plus 1 ½% of the target value.
What does low air content do to concrete?
Any air voids reduce the strength of concrete, with about a 5% reduction in strength for each 1% increase in the volume of air voids. Air voids, however, also improve the workability of concrete.
What is the benefit of air entrained concrete?
The main reason for using air-entrained concrete is for the increased resistance of concrete against the alternating freezing and thawing cycles of water, which can occur during those periods when the surrounding temperature drops below 0 ° C.
What happen if concrete air is high?
A: The major effect of too much air is a reduction in concrete strength. Each percent increase in air content can reduce compressive strength by 2% to 6% for moderate-strength concrete. The degree of strength loss depends on the cement, admixtures, and other concrete ingredients used.
Does adding water to concrete increase air content?
A.: Adding water at the jobsite increases both slump and air content. The rule of thumb for slump is that 1 gallon of water added to a cubic yard of concrete increases slump by 1 inch. Air entraining agents form air bubbles by a frothing action. The wetter the concrete, the more frothing action you get.
When should you add air to concrete?
It is beneficial to use air entrained concrete when placing concrete in areas with freeze-thaw conditions. Freeze-thaw conditions occur when the temperature of an environment fluctuates between above freezing temperatures and below freezing temperatures.
What does full air mean in concrete?
Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes.
Should concrete footings be air entrained?
The building code requires that the minimum compressive strength of concrete for footings be 2500 psi, for foundation walls 3000 psi. The code also specifies that the concrete be air entrained.
What is the difference between air entrained and non air-entrained concrete?
However, air entrained concrete has a higher air content than concrete without entrained air. In this manner, the well-distributed air voids act as expansion chambers for the frozen water. Thus, the air voids within air entrained concrete reduce the probability of microcracks.
How is air content measured in concrete?
Air Content – Air content is calculated by subtracting the unit weight, ρ, from the theoretical unit weight, T, divided by the theoretical unit weight, T, multiplied by 100 as shown below. 151.7 lbs/cu. ft.
What is the meaning of air content?
Air Content - The air content is the amount of air contained within a concrete element, usually expressed as a percentage.
Is code for air content in concrete?
Air-content of freshly mixed concrete shall be determined by the pressure method given in IS 1199. Time of setting, initial and final, shall be determined as given in IS 8142.
What is the difference between air entrained and non air entrained concrete?
However, air entrained concrete has a higher air content than concrete without entrained air. In this manner, the well-distributed air voids act as expansion chambers for the frozen water. Thus, the air voids within air entrained concrete reduce the probability of microcracks.
What is air void concrete?
The air voids provide empty spaces within the concrete that act as reservoirs for the freezing water, relieving pressure and pre- venting damage to the concrete. Forming Air in Concrete. Entrained air is produced during mechanical mixing of concrete that contains an air-entraining admixture.
What is air content test in fresh concrete?
As the pressure rises the concrete becomes compressed and the water level in the graduated cylinder falls. When the pressure has reached a specified level, the reading of the water level is recorded. The pressure is then released and the water level rises. The difference in the two levels is the apparent air content.
What factors influence the air content of concrete in the plastic state?
Factors Affecting Air Content of ConcreteWater content.Fine aggregates grading.Coarse aggregates.Temperature of concrete.Mixing action of concrete.Use of admixtures other than air entrained admixture.Cement content of concrete.Fly ash content.More items...•
Why is concrete air entrainment important?
Generally, concrete is required to be air entrained event if it is not exposed to freezing and thawing due to important benefits that concrete enjoys in other ways such as improving concrete workability. Concrete air entrainment is not an easy process since it is influence by several factors.
How long does it take for air to increase in a concrete mixer?
Air content increase by mixing for up to 15 minutes and mixing beyond this time will decrease air content. Entrained air will vary with type, physical condition, speed of the mixer and quantity of concrete that is being mixed. Seriously worn mixer will increase difficulty of air entrainment operation. Similarly, mixer with significant amount of hardened concrete buildup on mixer blades or in a drum increase air entrainment operation.
What size sieve for air entraining?
Air entrainment will be carried out easily as the percentage of fine aggregate is increased. Fine aggregate sizes ranges from sieve No. 30 to sieve No. 100 will create small voids that can contain air bubbles. However, other fine aggregate sizes will require greater quantity of air entraining admixture to reach the same air content.
How does vibration affect concrete?
Vibration reduces concrete air content. For example, half of air content will be lost if vibration is applied for more than three minutes.
Why would hard water reduce air content?
This is because the increase of water content would increase fluid mixture into which air bubbles could be integrated easily through mixing operation. Hard water such as well or quarry water, which contain minerals, would reduce air content if they are used to dilute air entraining admixture. 2.
Does fly ash reduce air content?
Air content reduces as the fineness or surface area of fly as increases. Increase of fly ash per unit concrete reduces air content of concrete and increase in fly ash carbon content reduces concrete air content.
Does dust affect air quality?
Presence of dust on coarse aggregate surface decreases air content. Crushed aggregate would entrain lesser air compare with gravel aggregate.
Why use air entrained concrete?
Additionally, air entrained concrete provides scaling resistance and a prolonged service life. It is important to use air entrained concrete to avoid cracks in the concrete which later leads to increased deterioration of the structure.
How much air is needed to freeze thaw concrete?
For freeze-thaw protection, the air content of a concrete mix will typically be in the 5-7% range. Depending on the amount of air specified, it will then be necessary to increase the cement content to achieve desired strengths. Although these air bubbles are microscopic, when the amount of air in a concrete mix is increased it will weaken the concrete. Generally, the use of air entraining admixtures will lead to a strength reduction of about 5% for each 1% increase in the volume of air voids in a mix. The reason why this occurs is because as more air is introduced to the mix the concrete becomes less dense as well as more of the concrete being air. With more cement (the “glue” in concrete) a concrete mix will have more strength. Therefore, if a concrete mix requires air entrainment there will typically be more cement in the mix to compensate for the strength loss.
Why is air entraining important in concrete?
The primary purpose of entraining air in concrete is to increase its durability when exposed to freeze/thaw cycles with water present. Air-entraining admixtures for concrete create small, uniform, and stable air bubbles from around 0.01mm to 1mm diameter in the cement matrix throughout a concrete mix. The bubbles become air voids in the hardened concrete, creating a relief system that allows expansion of water as it freezes without damaging the concrete.
What are the benefits of a concrete relief system?
Additional benefits include: Water demand for mixing and curing decrease. Workability increases. Bleeding and segregation reduce.
What is a type A concrete air meter?
Type A and Type B concrete air meters use different techniques to apply air pressure to a concrete specimen and compress the air voids in the sample. Differences in volume before and after compression indicate apparent air content, which is corrected to exclude air voids in aggregate particles. The pressure method cannot be used on mixes containing lightweight aggregates, blast furnace slag, or other aggregates with high porosity. Detailed test procedures for both types of meters are detailed in ASTM C231 and AASHTO T 152. Both meters use similar methods to consolidate fresh concrete into the measuring bowl, or pot. Concrete is deposited in three equal layers, and each layer is rodded 25 times with a tamping rod. The concrete gets struck off with a strike-off bar or plate, so the volume in a given bowl is the same each time.
What is the gravimetric method for calculating air content?
The gravimetric method for calculating air content is part of a procedure in ASTM C138. AASHTO T 121 also determines unit weights and yield of a concrete batch. Equipment for performing this test includes unit weight measures, strike-off plates, tamping rods, and field weighing scales. Fresh concrete consolidates into a unit weight measure, then carefully struck off and weighed. From there, a series of calculations are performed using the specimen weight and actual batch weights of the mix to determine the unit weight, density, yield, air content, cement content, and other properties of the mix.
How many times is concrete rodded with a tamping rod?
Concrete is deposited in three equal layers, and each layer is rodded 25 times with a tamping rod. The concrete gets struck off with a strike-off bar or plate, so the volume in a given bowl is the same each time.
What is a super air meter?
Super Air Meter (SAM) is a modified Type B concrete air meter invented by Tyler Ley, Ph.D., PE, who promises the SAM will change the way we view concrete’s freeze-thaw durability. (To read more about Tyler, see our Tyler Ley Interview blog post with Dr. Ley himself).
How long does it take to get results from air void test?
Test results in 10 minutes versus weeks or months for other air void evaluations, such as ASTM C457
Why is air space important in concrete?
The air space allows hardened concrete to better accommodate the pressures caused from expansion and contraction of freeze-thaw cycles, helping to prevent fracturing.
What are some common misconceptions about pressure air meter?
Some common misconceptions include (1) assuming that these factors are not required for the type of pressure air meter being used, (2) believing that the aggregate correction factor is zero or negligible, and (3) presuming that an aggregate correction factor is not required by a certain project or location.
Is aggregate correction factor important in concrete testing?
Although often ignored, aggregate correction factors are an important aspect of concrete testing. A pressure air content test is not valid without applying this correction factor. Aggregate correction factors vary from region to region, source to source, and mix design to mix design. Ensuring this process is accounted for in testing can be vital to the life of the concrete.
