All air voids in concrete are not created equal. Find out the difference between beneficial air entrainment and accidental, or detrimental, air entrapment.
By Claude Goguen, P.E., LEED AP
In regions where temperatures can frequently fluctuate near freezing, freeze-and-thaw cycles pose one of the greatest challenges to concrete durability. Capillary voids will form during concrete production, so protection against capillary ice damage must be designed into the precast concrete product. Air-entraining admixtures are one part of the solution that will prevent damage from freeze-thaw conditions.
The importance of entrained air was first noticed during the 1930s, when certain highway sections were found to be more immune to the effects of freezing and thawing than others. Studies traced it to cement that was milled at plants using beef tallow as a grinding agent. The beef tallow was an unintended air-entraining agent, and it improved the durability of the concrete.
During hydration, the reaction of water and cement leaves capillary cavities, or voids, that become filled with water when a precast product is exposed to wet conditions. As the capillary water freezes inside concrete, it expands about 9% in volume. The water-to-ice volume change exerts internal pressure inside the concrete that exceeds its tensile strength, causing cracking, spalling and eventual disintegration. Entrained air pockets provide a relief system for internal ice pressure by providing internal voids to accommodate the volume expansion caused by freezing water.
Entrapped air is problematic
It is important to note that entrained air is not the same as entrapped air. Entrapped air is created during improper mixing, consolidating and placement of the concrete. Air pockets, or irregularly sized air voids, are spread throughout the concrete and have negative effects on product appearance, strength and durability. Proper vibration techniques can be helpful in removing entrapped air.
Entrained air is intentionally created by adding a liquid admixture specifically designed for this purpose. The goal is to develop a system of uniformly dispersed air voids throughout the concrete. Proper use of air-entraining admixtures ensures the development of the correct spacing, size (usually measured in micrometers) and amount of these voids. These voids absorb the pressure created by the expansion of the freezing water.
Spacing and size of entrained air make a difference
The criterion for spacing is defined as the maximum distance the water would have to move during freezing before reaching the safety valve, or capillary void, of an air reservoir. This recommended average “spacing factor” should not be greater than 0.008 in., according to ASTM C457, “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete.”
The size of these voids is also important. The “specific surface” is the average surface area of the voids in hardened concrete per unit volume of air. The specific surface necessary for adequate resistance to repeated freezing-and-thawing cycles is recommended to be greater than 600 sq in./cu in.
One of the concerns with air-entraining admixtures is that they can decrease the strength of the concrete. Typically, an increase of 1% in air content will decrease concrete’s compressive strength by approximately 5%. Therefore, it is important that air content be closely controlled. The NPCA Quality Control Manual for Prestressed and Precast Concrete Plants recommends air content tests be conducted for at least every 150 cu yd of concrete produced and not less than once a day when air entrainment is used. However, state and local specifications may require more frequent tests. The air content test should be conducted in accordance with either ASTM C173-10b, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method,” or C231-10, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method.” Note that ASTM C173 requires all samples for acceptance testing be taken from the middle third of the batch.
The recommended air content varies with the severity of exposure and aggregate size. For instance, 3/8-in. aggregate with a severe exposure (frequent freeze-thaw cycles) requires 7.5% air content, while the same aggregate with moderate exposure requires 6% air content. A 1-in. aggregate requires 6% for severe exposure and 4.5% for moderate exposure. Air-entraining agents are generally added to the mix in a range from 0.25 to 2 fl. oz./100 lb. of cementitious materials. This is a broad range, and proper dosage should be determined after consulting the admixture supplier, and considering mix design, materials and trial-batch test results.
Many mechanisms affect air content
Many factors affect entrained-air stability:
• Finer cements with low alkali content
• Fly ash mixtures and an increase in fine aggregates passing the No. 100 sieve
• Dust and very fine material on coarse aggregates
• Hard mixing water (high mineral content)
• Detergents that can add soap bubbles
• Other chemical or mineral admixtures such as water-reducing agents and superplasticizers used in the concrete mix
• Batch size and mixer settings (rotation rates and times)
Every mix component – from cementitious material to aggregates and water, as well as methods used for batching, mixing, placing, consolidating and finishing – can affect the final air content, so production steps must be monitored closely. All these production variables mean that trial batches are necessary to determine the appropriate amount of air entrainment.
What goes into air-entraining admixtures?
Today’s air-entraining admixtures are primarily liquids produced from byproducts (salts) of wood resins. However, there are new products made from synthetic detergents, sulfonated lignins, petroleum acids, proteinaceous materials and sulfonated hydrocarbons. There are also particulate air-entraining admixtures composed of hollow plastic spheres and crushed brick. Although outside the scope of this article, there are air-entraining cements that meet ASTM C150, “Standard Specification for Portland Cement.” These cements have an “A” identifier, such as Type IA or IIIA.
The specifications for air-entraining admixtures are covered in the ASTM C260-10a, “Standard Specification for Air-Entraining Admixtures for Concrete.” This specification sets limits on the impact of the admixture for concrete bleeding, set time, strengths, compressive and flexural strengths, freeze-thaw resistance, and length change during drying. The manufacturer of the admixture should guarantee that its product meets this specification.
Entrained air is one of the critical techniques available to precasters to reduce the impact of freeze-thaw processes and ensure the durability of precast concrete in severe climates. Because so many production variables can affect air entrainment, careful monitoring and trial batch testing are the foundation for proper air entrainment and a long service life for exposed precast concrete products.
By Claude Goguen, P.E., LEED AP is NPCA’s director of Technical Services.
Ralph Ramirez says
My question is what is considered to be the average entrapped air in concrete ?
A reading of 2.5% is what ?
satya paul says
Entrained air also helps highway concrete slabs,where large area is exposed to sun in countries where day temperature reaches 48 degree centigrade and night temperature reaches 15 degree centigrade.steel fibers up to 35 kg/m3 are used to control micro cracking. only aggregates which have least coefficient of expansion shall be used for exposed to sun slabs.further research is required to introduce uniform spread of air bubbles(entrained air) in top 100 mm thick slabs.
Kris Koenig says
Does vibrating reduce the air entrainment in precast concrete? And if so how do I control the air entrainment but still get the air bubbles out of the face of the precast?
Sara Geer says
Thank you for the comment Kris. Phil Cutler, director of quality assurance programs, provided the following response:
“The quick, easy answer to your first question is probably not. Proper vibration to consolidate a typical precast concrete mix would not reduce air entrainment in precast. However, over vibration may reduce entrained air, but detrimental segregation would occur prior to significant loss.
NPCA technical staff suggests you consider reviewing the following two references for additional information.
PCA’s Design and Control of Concrete Mixtures 16th Edition, Chapter 17, “Placing and Finishing Concrete,” pages 422-427, mentions consequences of improper vibration defects from over vibration include:
1. Segregation as vibration and gravity causes heavier aggregates to settle while lighter aggregates rise
2. Sand streaks
3. Loss of entrained air in air-entrained concrete
And ACI 309R-05 Guide for Consolidation of Concrete, Section 7.7.8, “Over Vibration Effects,” states, “Loss of entrained air in air-entrained concrete—This can reduce the concrete’s resistance to cycles of freezing and thawing. The problem generally occurs in mixtures with excessive water contents. If the concrete originally contained the amount of entrained air recommended by ACI Committee 211 and the workability is in the proper range, serious loss of entrained air is highly unlikely. A properly proportioned concrete mixture with a dosage of air-entraining admixture to give a 7% air content can be vibrated until the air content is 3% with no reduction in its frost resistance because the bubble spacing factor, .005 to .007 inch, is still less than .008 inch. Too many insertions of the vibrator too close together in high-slump concrete can cause a coalescing of the entrained-air system, which may cause a reduction in resistance to freezing and thawing.”
To answer your second question. The following factors all contribute to successfully releasing entrapped air leading to a quality finish.
– Appropriate mix design
– Proper batching and mixing
– Good forming materials and techniques
– A good form release agent
– Proper placement of concrete
– Adequate concrete consolidation”
Thomas DeGaetano says
This is an odd question:
What would happen to the overall air content (thus strength and durability) of a batch of concrete if you mix two different mix designs of concrete (one air entrained with addmixture, one air entrapped)? This actually happened during an important air-required (6% +/-1.5%) structure pour.
I am trying to tackle how this may negatively affect the product. My guidance has been PCA’s 15th edition of Design and Control of Concrete Mixtures by S. Kosmatka and M. Wilson – however i cannot garner any advice that this may have on my situation. I am assuming the admixture to induce proper air entrainment will perhaps negatively affect or be reduced by volume to the air entrapped mix?
Would appreciate anyone’s thoughts.
Sara Geer says
Thank you for the comment Thomas. Eric Carleton, P.E., director of codes and standards, provided the following response:
“Your predicament brings up a few questions; however, the answer is the same as for standard concrete. Mixing an air entrained mix with a non-air entrained mix will lead to a reduced air entrainment effectiveness. You did not mention the circumstances that led to this “mix mixing.” If this occurred from two different batching operations and the mixes were within normal slump range and then cast into the formwork, the final concrete structure would have a stratified concrete condition dependent upon how the concrete was poured and vibrated. Assuming the air entrainment requirements provided durability to the expected environmental conditions, the stratified concrete would deteriorate early and uneven.
Air entrainment admixture quantities required to obtain a specific finished air content for mix designs are based on a number of variables including size of aggregate, fineness and quantity of cement and use of SCMs such as fly ash and silica fume. Therefore, the constituents of the non-air entrained mix would affect the quantity required of the air entrained admixture for the combined mix. However, assuming each mix design was identical except for the addition of the air entrainment admixture, you would immediately realize only half of the chemical has been added to the batch to meet the required air entrainment requirements.
During batching, air testing by traditional ASTM methods should have been conducted to determine the actual concrete air content for that mix. At that point additional air entrainment admixture could have been added to correct the anticipated low reading. One last thought on your dilemma. If the concrete cast was using zero-slump or dry-cast concrete, testing and antidotal evidence from pipe and box culverts exposed to freeze/thaw conditions without deterioration have shown air entrainment requirements are not needed for dry cast mixed and highly consolidated precast concrete products. If this is the case, prescriptive requirements for air entrainment may be voided. However, that is a discussion between the contractor/producer and the design engineer to determine.”
HI , I am making concrete pipe vertical cast, zero slump concrete, water cement ratio 0.39
all good, but bug holes is coming surface. please guide how to remove bug holes from pipe surface.
Sara Geer says
Thank you for your comment vijay. I forward your question to our Technical Services engineers. The following response is from Eric Carleton, P.E., director of codes and standards. This response has been emailed to you as well.
You did not mention if your pipe process was using packerhead consolidation or standard dry cast with an external vibration (either attached to the form work, vibrating core or both). Regardless, when casting a zero-slump concrete as you describe, the internal friction within the cement paste and fine aggregate is greater than normal concrete with measurable slump. Consequently, when entrapped air is pressed against the form, many times the paste and fines do not move to fill the void with vibration.
Some things to check would be to ensure the vibrators are functioning correctly. Sometimes in the noise and commotion of pipe production a vibrator may stop working, but it is difficult to identify without checking. Also, ensure the vibrator is adequate for the formwork. You may need to request assistance with your vibrator vendor to ensure the frequency and amplitude of the external vibrator is appropriate for the placement and product cast. If the pipe is consolidated on a vibrating table that worked well for smaller diameter pipe, it may not work well for a larger size.
For packerhead production, you may experiment to see if reducing the speed rate of the packerhead or if two passes of the roller head greatly decreases the bug holes on the exterior surface. Additionally, check if the distribution fins at the top of the packerhead are functioning properly.
For both production methods, the easiest solution is to slow down the rate of concrete placement to allow the vibration or packerhead time to consolidate the mix. For dry-cast forms, it is good practice to load the form uniformly and continuously along the perimeter (described as “ribbons” or layers of concrete) rather than in larger pockets moved intermittently along the form which can trap substantial amounts of air within the mix.
Lastly some precasters find the addition of liquid surfactant admixture provides an enhanced surface texture. It provides a slight reduction of internal friction of the concrete components and increases movement into those trapped air voids. It also provides a slick surface which the form can “trowel” as it is stripped from the product to remove smaller bug holes. However, experimenting to provide the correct amount is critical since too much will create entrapped bubbles.
What is the reason behind getting high air voids on concrete cores when we’re testing according to the ASTM-457 versus the in-situ air-entraining measurement ( some time we measure the air 4% on-site and after core testing is done as per ASTM 457 will get 10% or even more?
Joe Frollo says
You mention in-situ air-entraining measurement. There are no common field tests to accurately measure just the entrained air, but to measure the total air content, the two most common methods used in situ are ASTM C173 – Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method and ASTM C231 Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method.
You could also use ASTM C138 Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete where the air content can be calculated based on the theoretical and measured densities. This method’s accuracy depends on knowing the exact mixture proportions and ingredient specific gravities.
Typically, air void content determined by ASTM C457 – Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete should correspond closely to air void content determined by the previously mentioned in situ test methods; however, there could be some slight difference by a percentage point or two. The 6 percentage points you mention seems excessive.
Differences in fresh concrete and hardened concrete sample testing can often be attributed to how the samples are handled and finished. Fresh concrete samples are not subjected to the same conditions as the concrete placed in formwork. A scoop and tamping rod cannot accurately mimic the effects of field placement and consolidation of concrete in its final position. Placing and consolidating concrete in the form can have a significant impact on air voids in the cementitious paste. Dropping the concrete from greater heights can entrap air. Inadequate consolidation can also leave large entrapped air in the mixture.
Another factor comes from the sampling of the hardened concrete structure. You should take samples from different parts of the structure so they are representative of the entire structure.
There is a note in ASTM C457 that mentions: For concrete with a relatively high air content (usually over 7.5 %), the value determined microscopically may be higher by one or more percentage points than that determined by Test Method C231/C231M.
I would start by examining how your air testing in-situ is performed. What test method are you using?
I would then examine how the concrete is placed, consolidated and finished in the field. I would also look into how hardened concrete samples are obtained in the field.
These tests should not produce 6% difference, so it comes down to finding where source of the inconsistencies in the fresh and hardened concrete samples.
What if we’re getting a high spacing ( 0.467) factor even though the air content on the same test is 4.4%?
How come we’re getting this SF almost 2.5 times more than allowable of 0.200 mm?/
Thanks in advance
Joe Frollo says
First you may want to review how you are conducting the test method, to ensure the test method is not flawed and is not contributing to the unexpected spacing factor. Next, it would be wise to consult with your admixture supplier and/or whoever has developed this concrete mix design. The admixture supplier in particular will be the best equipped to provide input and guidance on how the admixture is performing in your particular mix design and in your specific scenario. The supplier will be able to help you troubleshoot and find a solution.