Admixtures for Concrete - What is an Air Entraining Admixture?

Summary

In this video, Tyler Lai discusses the importance of air-entraining admixtures in concrete, especially in freeze-thaw environments. These admixtures are surfactants that stabilize small air bubbles within concrete, enhancing its durability. Tyler explains how these surfactants work at the molecular level, creating a shell around air bubbles that prevents them from coalescing or escaping. He demonstrates this with experiments showing how air bubbles behave in air-entrained and non-air-entrained cement paste. The video emphasizes that not only the volume but the distribution and size of the air bubbles are crucial for freeze-thaw resistance, highlighting the scientific basis and practical implications of air-entrained concrete.

Takeaways

• 🧼 Air-entraining admixtures are anionic surfactants or soaps that stabilize small air bubbles in concrete, essential for freeze-thaw durability.
• ❄️ In freeze-thaw environments, air-entraining admixtures are crucial for the concrete's ability to resist freezing and cracking.
• 💧 Surfactants have a hydrophilic head that loves water and a hydrophobic tail that hates water, which helps them align around air-water interfaces to stabilize bubbles.
• 🫧 The admixtures prevent bubbles from coalescing, forming a shell that maintains the bubble size and prevents air from escaping.
• 🔬 The user conducted experiments showing that air-entrained bubbles are stable over time and resist changes, unlike non-air-entrained bubbles that change size.
• 👁️ Air-entrained bubbles appear to have a visible shell that makes them more durable and prevents air exchange, crucial for concrete stability.
• 📉 Air-entrained bubbles show resistance to size changes under pressure, while non-air-entrained bubbles shrink or grow due to air transfer between them.
• 🔄 The shell around air-entrained bubbles can self-repair when cracked, showing the material's resilience under changing pressure conditions.
• 🧲 Cement particles adhere strongly to air-entrained bubbles, which enhances stability by preventing bubble floatation and maintaining their position within the concrete.
• 📏 The spacing factor, which measures the distance between air voids, is crucial for freeze-thaw protection, as well-distributed small bubbles provide more protection than large bubbles.

Q & A

What are air-entraining admixtures, and why are they important in concrete?

Air-entraining admixtures are anionic surfactants (soaps) added to concrete to stabilize small air bubbles during the mixing process. They are critical for providing freeze-thaw durability in environments where concrete may freeze and thaw repeatedly.

How do surfactants stabilize air bubbles in concrete?

Surfactants have hydrophilic (water-loving) heads and hydrophobic (water-hating) tails, which align at the air-water interface. This alignment helps stabilize air bubbles, making them thermodynamically possible, and preventing them from coalescing or escaping the concrete.

What is the role of cement grains in stabilizing air bubbles?

Cement grains, attracted by the negative charges of the surfactants, help hold the air bubbles in place. This prevents the bubbles from floating out of the concrete, similar to how foam forms on ocean waves but stays within the concrete due to air-entraining agents.

How do air-entrained bubbles differ from non-air-entrained bubbles in concrete?

Air-entrained bubbles are stable, maintain their size, and do not coalesce, thanks to a shell that forms around them. Non-air-entrained bubbles change in size over time, with larger bubbles getting larger and smaller ones shrinking due to air interchange between them.

What is the significance of the shell surrounding air-entrained bubbles?

The shell around air-entrained bubbles prevents air exchange between bubbles and helps resist the transfer of gas. This shell also helps the bubbles maintain their structure and size, contributing to the overall stability of the concrete.

What happens when the shell around an air-entrained bubble is damaged?

When the shell around an air-entrained bubble is damaged, air interchange can occur, and the bubble can shrink in size. However, these shells can self-heal over time, reforming to maintain the bubble's stability.

How does the size and distribution of air bubbles affect concrete's freeze-thaw durability?

Small, well-distributed air bubbles provide better protection against freeze-thaw cycles. The water in the paste expands when it freezes, and the bubbles provide space for the water to move into, preventing cracking and damage to the concrete.

What is the spacing factor, and how is it related to freeze-thaw protection?

The spacing factor measures the distance between air bubbles in the concrete. A lower spacing factor, with more closely spaced bubbles, provides better freeze-thaw protection because more of the paste is protected by the air voids.

How is the spacing factor determined in hardened concrete?

The spacing factor is determined through a linear traverse technique, where a polished section of concrete is examined under a microscope. The distances between air bubbles (chords) are measured, and these values are used to calculate the spacing factor.

How many air bubbles are typically present in air-entrained concrete, and why does their volume matter?

Air-entrained concrete typically contains 4-8% air by volume, which can translate to 10-15 billion bubbles per cubic yard. The volume and distribution of these bubbles are crucial for ensuring freeze-thaw durability, as they allow space for water to expand without damaging the concrete.

Outlines

00:00

🌬️ Introduction to Air-Entraining Admixtures

The speaker, Tyler Lai, introduces air-entraining admixtures and highlights their importance in creating durable concrete in freeze-thaw environments. These admixtures are crucial for stabilizing bubbles in concrete, which helps prevent freezing damage. The air-entraining admixtures, made from surfactants (soap), work by creating and stabilizing small air bubbles in the concrete mix.

05:01

🔬 Air Bubbles in Cement Paste: Video Analysis

Tyler explains a video showing the behavior of air bubbles in cement paste. The cement paste expands and pushes air bubbles, which leads to the formation of cracks and bubble growth. The speaker analyzes how bubbles, when pushed, do not merge but move around each other. This demonstrates the behavior of air-entrained cement paste and the formation of protective shells around the bubbles.

10:01

⚙️ Non-Air-Entrained vs. Air-Entrained Systems

A comparison of non-air-entrained and air-entrained systems is presented. Non-air-entrained bubbles change in size over time due to air interchange, while air-entrained bubbles remain stable. The difference lies in the protective shell formed around air-entrained bubbles, which helps resist changes in size and movement. Pressurization tests further demonstrate how these shells react under stress.

15:08

🔧 Self-Healing of Air Bubble Shells

Tyler explains the phenomenon of bubble shell self-healing, where cracks formed in the shell over time repair themselves. He describes a pressurization and depressurization test showing how damaged shells can reform, highlighting the resilience of air-entrained bubbles in concrete. This property ensures the durability of air-entrained systems over time.

20:08

📏 Measuring Air Void Spacing and Freeze-Thaw Durability

The speaker discusses the importance of air void spacing in determining freeze-thaw durability in concrete. The ideal spacing of bubbles allows water to move during freezing, preventing damage. Tyler also explains the process of measuring the spacing factor using hardened air void analysis, which involves cutting and analyzing the concrete to evaluate bubble distribution.

Keywords

💡Air Entraining Admixtures

Air entraining admixtures are additives used in concrete to create tiny air bubbles that enhance the material’s durability, particularly in freeze-thaw environments. In the video, Tyler explains that these admixtures are crucial in places where concrete gets wet and freezes because they prevent cracking caused by expansion of ice inside the concrete.

💡Surfactants

Surfactants are special molecules that have both hydrophilic (water-loving) and hydrophobic (water-repelling) parts. In the video, surfactants are described as critical components in air entraining admixtures, as they help stabilize the air bubbles in concrete by surrounding the air-water interface and preventing the bubbles from collapsing.

💡Freeze-Thaw Durability

Freeze-thaw durability refers to the ability of concrete to withstand cycles of freezing and thawing without deteriorating. The video emphasizes that air entraining admixtures are essential in environments where freeze-thaw cycles occur, as they allow space for water to expand as it freezes, protecting the concrete from damage.

💡Bubbles

Bubbles play a central role in the video, as they are formed by air entraining admixtures and are stabilized by surfactants in the concrete. The video explains how small, stable bubbles are desirable because they help prevent the concrete from cracking under freeze-thaw conditions, with larger bubbles being less effective for durability.

💡Hydrophilic and Hydrophobic

Hydrophilic means 'water-loving' and hydrophobic means 'water-repelling.' These terms describe the two ends of a surfactant molecule. In the video, Tyler explains that surfactants stabilize air bubbles in concrete by aligning their hydrophilic heads toward water and hydrophobic tails away from it, creating a stable environment for the bubbles.

💡Bleeding

Bleeding in concrete refers to the process where water rises to the surface of the mix, often carrying small air bubbles with it. Tyler mentions bleeding when discussing how he observed air bubbles in concrete rising and being captured under a microscope in one of his experiments.

💡Coalescing

Coalescing refers to the merging of smaller bubbles into larger ones. The video discusses how air entraining admixtures prevent this by forming a shell around the bubbles, which stops them from coalescing. This is important because smaller bubbles are more effective at improving concrete durability.

💡Shell

The shell is a protective layer that forms around air bubbles in concrete, thanks to the air entraining admixtures. The shell prevents bubbles from coalescing and exchanging gas, making the concrete more durable. Tyler highlights the importance of this shell in ensuring the stability and longevity of the bubbles.

💡Cement Grains

Cement grains are tiny particles in the concrete mix that play a role in forming and stabilizing air bubbles. According to Tyler, surfactants in air entraining admixtures attract cement grains, which helps keep the bubbles in place, preventing them from rising to the surface and escaping the concrete mix.

💡Self-Healing

Self-healing refers to the ability of the bubble shells to repair themselves when damaged, such as when the bubbles are pressurized and depressurized. In the video, Tyler demonstrates how, over time, cracks in the bubble shells can disappear, restoring the bubble's structure and preserving the durability of the concrete.

Highlights

Air-entraining admixtures are crucial for freeze-thaw durability in wet environments, where concrete gets wet and freezes.

Air-entraining admixtures are made from anionic surfactants or soaps, which stabilize bubbles in concrete.

The surfactants align at the air-water interface, creating small, stable bubbles and preventing air from escaping the concrete.

These surfactants also attract cement grains, helping to hold the bubbles in place, ensuring they don't float out of the mix.

Air-entraining agents create a shell around the bubbles, which prevents them from coalescing and aids in maintaining freeze-thaw resistance.

The shell structure also blocks air exchange between bubbles, further stabilizing them within the concrete.

Experiments showed that cement paste expands as it hydrates, cracking the shell of air bubbles, but these bubbles don't merge with others.

Non-air-entrained systems show air interchange between bubbles, leading to changes in bubble size over time.

Air-entrained systems exhibit stable bubbles over time, which don’t change in size, making them ideal for concrete stability.

When the shell of an air-entrained bubble is broken, air interchange begins, causing smaller bubbles to shrink and larger bubbles to grow.

The shell of air-entrained bubbles helps resist external pressure, and in some cases, these shells can self-heal after damage.

Air-entrained concrete typically contains 4-8% air by volume, which equates to billions of small bubbles per cubic yard.

The critical factor for freeze-thaw durability is not just air volume but the size distribution and spacing of air voids in the concrete.

Smaller, well-distributed air voids offer greater protection against freezing than a few larger bubbles.

Hardened air void analysis, like the linear traverse technique, can measure the spacing factor to assess the effectiveness of air-entrained concrete.

Transcripts

hello concrete kiddies my name is Tyler

Lai and this is an exciting episode

because I get to talk about air

entraining admixtures my favorite

admixture and arguably in my opinion and

actually most people most people's

opinion the most important admixture

that we use inside of our concrete air

entraining admixtures they're critical

and so critical than if you are in a

free-stall environment if you're in a

place where concrete gets wet and

concrete freezes you got to have them if

you don't have them you won't have

freeze-thaw durability okay so

they're essential if you're in these

environments what are they they are

anionic surfactants or soaps soaps yes

we add soap to concrete as we're making

it yeah

because we want to stabilize these small

bubbles you add the soap it makes

bubbles and through the mixing process

you whip air you trap air into the

concrete and you break and split those

bubbles over and over and over and over

and over again and

the air entraining admixtures they help

stabilize the bubble so I'm all

explained what I mean by that

first they are surfactants what's a

surfactant well it's a special molecule

that has a

hydrophilic

hydro

philic what's that mean hydro means

water

philic means love it loves water a part

that loves water

and

then it's got a hydro

phobic

tail

that means it hates water

and these surfactants align themselves

around the air water interface

alright and they help stabilize bubbles

they help make bubbles that stable or

thermal thermodynamically possible that

wouldn't usually be there because we

want a certain size but we want small

bubbles not not big bubbles but there's

another huge benefit of these

surfactants they actually attract cement

grains and

these negative charges actually attract

cement grains and this helps hold the

bubble in place it helps keeping it from

floating out of the concrete like if you

ever go to the ocean and you see a wave

come in and crash down on the surface

that wave traps air

right and we see it come out as foam

well the same things happening when you

make concrete but those bubbles don't

escape they're held into the concrete by

the air and Trainer not amazing

these surfactants not only do they

attract cement cranes and I'll show you

that I'll prove that to you coming up

but they also form this kind of shell

material around the void that helps the

bubbles from coalescing and it also

stops air from exchanging between the

bubbles so this shell is really really

important I'm so excited to share it

with you because this is work from my

PhD yeah I know it's super old right now

just kidding I started out to try to

prove learn more about air entrained

concrete and I started out using these

bottles that were optically clear you

can you can see right through them and

then I filled these bottles full of air

entrained cement paste I didn't fill

them all the way up I filled them fill

them up fill them up fill them up fill

them up and then I put the lid on them

and turn them over on their side okay

just like that and over time

the cement is gonna go to the bottom and

the water is gonna go to the top and

this is called bleeding right we know

this okay and that water when it comes

up to the top it's gonna bring with it

air bubbles and I'm gonna be able to

watch these air bubbles in the surface

and I'm gonna use this stereo microscope

which is hooked up to a computer that I

wrote a program to to take pictures over

time and this was the very first video I

ever made let's watch

so over time not much is happening every

second in this movie is about five

minutes in real life okay so it's kind

of boring and then look at this

seems to be an impact on the bubble from

below this large bubble there's a split

and watch

the bubble seems to

be emerging from the inside of the shell

and watch the bubble just takes over

like the blob I

know

pretty awesome video right let's watch

it again I want you to keep your eyes

down here what happens is the cement

pace is expanding because it's heating

up while it hydrates and as expand

expands you'll see it impacts or pushes

and once it starts to push watch

there'll be a crack that forms right in

the surface of the bubble and as it

keeps expanding it keeps forcing this

bubble and crushing this bubble the

bubble doesn't Bend like a basketball it

actually cracks on the outside and watch

look at this this inner thing emerges

holy cow and watch when it goes to touch

these other bubbles they don't coalesce

they don't join to become larger bubbles

it pushes them watch push push push push

push

shoves them out of the way

pretty cool video huh

let's talk about what happened that was

Aaron and cement paste the paste seems

to rise up and push on this large bubble

and the diameters do not change over

time the diameters are pretty constant I

don't know if you notice that or not the

diameters didn't change until the bubble

was pushed on the bubble seems to have

some kind of shell surrounding them okay

and the bubbles and because we saw it

crack and its bubble emerge from the

inside and the bubbles don't coalesce

they just kind of push each other around

and that was air entrained cement paste

I know awesome right now

I didn't want the pace to keep crushing

my bubbles so I had to modify my

experimental set up I had to use it now

where my paste was below and my bubble

was above right so the pace can rise up

and down and it's never gonna touch my

bubble it's not gonna touch it and let's

just see what happens

these are non air entrained

bubbles and notice I have letters on

several of these okay and I'm gonna talk

about my show you a graph how they

change over time but let's just watch

look let's just watch

then we can see over time again every

second is five minutes and the larger

bubbles are getting larger right you see

him and these smaller bubbles look at

that one get smaller look at that one

over there look at that

what it's going on there's actually air

interchange happening between the

bubbles okay this is widely known in

physics okay I've got different letters

for the different sizes of bubbles and

we can see some of these bubbles got

small some of these bubbles stayed

around for a while then they got small

and some of these bubbles just kept

getting larger and larger and larger

remember that is a non air entrained

bubble system

let's talk about what happened the voids

were not affected by the paste the paste

didn't didn't crush them right the

bubbles appear to be transparent you

noticed that let's go back and look the

bubbles were transparent I don't quite

have that shell around them talk more

about that coming up and the bubbles

change in size with time even though

nothing is touching them that is non air

entrained bubbles let's go back let's

look at air entrained bubbles

start this movie Mazal let it go for a

while

because it goes for hours and hours and

there's no change in the bubbles it goes

and goes and goes it's the most boring

movie I ever made so we can see the

bubbles look totally different they're

not changing in size nothing is changing

it's boring that

might be what you want if you're making

concrete you might want to make a bubble

system and then have it stay there so

this was air entrained paste and the air

voids were not affected by the paste the

paste didn't come up and crush them the

bubbles appear to be covered in a shale

do you notice the difference there's the

shell around them and the bubbles don't

change size with time

let's do another one

now in this system I've actually

pressurized the bubbles then

depressurized them really quickly I used

air pressure and if you notice there's a

split in this large bubble see this

split right there okay let's watch what

happens so as we start the movie if we

watch look look at these smaller bubbles

they're changing in size

they're decreasing in size

you'll see that again let's watch it

again

these bubbles were larger notes watch

let's keep an eye on them as this one

cracks and open nuts almost like an

eyelid opening up the bubbles gonna get

smaller watch

yep it shrinks in size

when this shell is broken this air

interchange can then begin to start to

happen

okay

so air entrained pastes that been

pressured that's what we're looking at

the shell of the large air voids Dam and

the smaller ones probably are well as

well we just can't see them the smaller

bubbles decrease in diameter and the

larger bubble increases in diameter and

kind of opens up this shell around the

outside so it appears that this shell is

created when an errant trainer is used

and the shell seems to be important in

resisting the transfer of gas from the

surrounding fluid and if this shell

becomes damaged and it seems to be

possible this transfer of gas can then

start occurring again now let's talk

about some properties of the bubble

shells we're gonna talk about physical

properties today and we'll talk about

transparency adhesion and this air void

shell responds to pressure and then

self-healing huh yeah I know amazing

right

we can see these two air void systems

the stuff I showed you before they look

different they have different

transparency this one we can't see

through the bubble we can see a shell or

a texture on the outside and this one we

can see right through it pretty crazy

right so they just look different

talk about adhesion of cement particles

now in this movie every second is about

16 seconds and I'm gonna actually change

my depth of focus I'm gonna start out

looking at just the surface of the

cement paste and as you can see there

there are a couple bubbles that are just

poking their head out of the surface and

what I'm gonna do is I'm gonna let the

time go and over time I'm gonna change

my my focus and I'm gonna watch these

bubbles

emerge

here they come

here they come are you scared yet look

at these bubbles they've risen out of

the cement paste so let's watch that

again they rise out of the cement paste

and they're like the creature coming out

of the Blue Lagoon right yeah and they

bring up with them a huge chunk of

cement paste look at those divots they

leave behind like if that was a golf

divot people would be mad right look at

that that's a lot of adhesion that's a

lot of attraction between those cement

grades and the surface of the bubble

now let's talk about the air void shell

response to pressure now different air

aryn trainers behave differently let's

show a Vince Hall resident first and as

you increase the pressure on the Vince

Hall resin it gets smaller and then as

you start to decrease the pressure they

crack and that's kind of what we showed

you before the bubble shell cracks and

you can see inside of it

crazy right now let's look at a

different system a synthetic air

entraining agent okay or a sodium Allah

Nate when you increase the pressure it

doesn't just shrink like a normal bubble

it bends in twists and buckles almost

like a Walmart bag being squeezed and

then when you decrease the pressure it

comes back and if it's a pristine bubble

you can't see any damage in it at all

now I was talking about self-healing

so this is a whole bunch of bubbles and

I was pressurized in and depressurising

them over and over again and we're gonna

focus on this bubble right here okay

we're gonna zoom in on it and I'm gonna

watch the movie I'm gonna play the movie

here we'll play it through once then

I'll talk about it the second time

because it's that amazing

start it going here again every second

about five minutes and you can see the

bubble shells been cracked and it's kind

of losing its shell okay and then over

time

look closely

check that out

the cracks going away

what happened what happened

well let's do it again

overtime bubbles cracked because I

pressurized and depressurized it it

opens up and you can see the inner

bubble but over time it seems to be that

shale is reforming on the surface again

and again and again and it finally it

gets to a point

where it dips itself back up it

heals itself isn't that amazing

that's just crazy right so

in conclusion we see bubbles that are

air entrained they have a different

transparency than non Energon because

they have a shell around them these

cement particles adhere to the outside

of the bubbles it's true I've seen it

different air entraining agents respond

differently to outside pressures okay

and the bubble shell have been observed

to repair themselves

pretty crazy right

typically in concrete we try to entrain

about four to eight percent air now if

you just make concrete without any air

entrainer at all it will still have air

in it about one to two usually went 1-2

percent by volume but when you wee one

about four to eight percent air and how

much air do we need it really depends on

the bubble size distribution and

at four to eight percent there's

anywhere between 10 to 15 billion

billion with a B air bubbles per cubic

yard per cubic yard and a truck of

concrete holds about ten cubic yards

that's a hundred to a hundred and fifty

billion

bubbles floating around in that concrete

truck that's the same thing in a cubic

yard that's two hundred and fifty

bottles of champagne pods a lot of

bubbly right that's amazing

but people love to ask this question is

the volume of air enough to determine

freeze-thaw durability and I say no it's

not then it's about the air void spacing

I'll tell you about that what do I mean

by that well I'm showing a picture here

where I have an air void and around that

is paste and these are these little

bitty pores that are filled with liquid

and upon freezing there the water tries

to expand there's about a 9% volume

change that tries to happen when water

starts to freeze and it freezes in these

very very small pores okay and it

actually shoves water to the large

bubbles they actually travel to the

large bubble there's only a certain

distance that this water can travel over

this is this is this water I'm showing

these arrows is water movement on

freezing and this moisture ends up

entering the void and ends up forming

ice crystals people have imaged it it's

pretty cool but there's only a certain

region or zone that this water can

travel over and this is called the

protected paste volume and the volume

around the air entrained void is where

freezing liquid can escape so because of

this if we can make a small well

distributed air void system we're gonna

have much better protection what I mean

by that if I have these just a couple

large bubbles I'm only gonna have a

certain amount of protection and if you

notice the same thickness of this

protection is the same thickness over

here because the protection zone doesn't

have anything to do with the size of the

bubble the protection zone only has to

do with the properties of the paste so I

get a lot more protection out of these

small well distributed bubble system a

whole lot more of my paste is protected

how do we know what this is how do we

measure this well the spy sizing the

spacing the voids can be measured in the

hardened concrete with something called

the spacing factor get out of a hardened

air void analysis I'll tell you about

that in just a second but the spacing

factor if I have this idealized spacing

of my bubbles this is kind of like the

longest distance that a water molecule

would have to travel in this idealized

spacing or system and this diagonal

distance would be the largest one and

this would be twice something called L

bar or the spacing factor so how do we

get that spacing factor well we have to

do a hardener board analysis we actually

have to cut the concrete polish it and

then we have to go over it in a

systematic manner and count bubbles the

technique I'm going to talk about today

is a linear Traverse technique where it

goes over in a linear line and every

single time that line hits a bubble you

measure how much of the bubble that it

cuts through it's called a chord okay

and we can make a plot of the chords on

the x-axis versus the frequency or how

often they end up occurring and these

are the small voids over here and these

are the large voids over here now

they're chords and voids are two

different things okay but they're

somewhat related to one another somewhat

related number so we would say the green

one is what we want and the blue one is

not as much of what we want the green

one would have a low spacing factor and

the blue one would have a higher spacing

factor