Admixtures for Concrete - What is an Air Entraining Admixture?
Summary
TLDRIn 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
🌬️ 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.
🔬 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.
⚙️ 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.
🔧 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.
📏 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.
Mindmap
Keywords
💡Air Entraining Admixtures
💡Surfactants
💡Freeze-Thaw Durability
💡Bubbles
💡Hydrophilic and Hydrophobic
💡Bleeding
💡Coalescing
💡Shell
💡Cement Grains
💡Self-Healing
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
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