current research on air-entrained concrete
TRANSCRIPT
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Current Research on
Air-Entrained Concrete
Tyler Ley, P.E., Ph. D Oklahoma State University Peter Taylor, P.E., Ph. D.
CP Tech Center
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Air-Entrained Concrete…
The Continuing Saga!!!
Tyler Ley, P.E., Ph. D Oklahoma State University Peter Taylor, P.E., Ph. D.
CP Tech Center
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Summary
• Introduction to air-entrained concrete • Can air-entrained voids lose their ability
to provide frost resistance in concrete? • Can inadequate curing impact the void
system in fresh concrete? • Closing statements
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Why Do We Add Air to Concrete?
• Air-entrained bubbles are the key to the freeze-thaw resistance of concrete • Smaller bubbles are more effective in providing freeze-thaw resistance than larger bubbles
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What Do You Want in an Air-Void System?
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• Volume of air provided is the same for both circumstances.
• Case B has a lower spacing factor and a higher specific surface.
A B
What Do You Want in an Air-Void System?
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A B
• Volume of air provided is the same for both circumstances.
• Case B has a lower spacing factor and a higher specific surface.
What Do You Want in an Air-Void System?
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Current Measuring Techniques
PCA photo
ASTM C 231 PCA photo
ASTM C 173 ASTM C 138
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Current Measuring Techniques
PCA photo
ASTM C 231 PCA photo
ASTM C 173 ASTM C 138
These only measure volume!!!
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µCT Scanner
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µCT Scanner
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Micro-CT
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Micro-CT
Void
0.42 w/c paste 48 mL/100 kg cm wood rosin AEA
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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Micro-CT
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What can we do with it?
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150 µ
450 µ 600 µ
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Air bubbles in the material
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Air bubbles and a 20 micron protected paste shell
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Air bubbles and a 20 micron protected paste shell
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Can air voids lose their ability to provide frost resistance?
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Core from a JPCP
• All concrete images courtesy of Gerard Moulzoff
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Core from a JPCP
Elevation Plan
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Core from a JPCP
Elevation Plan
failed joint
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Core from a JPCP
Elevation
saw cut
Plan
deteriorated concrete
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Core from a JPCP
Elevation
saw cut
Plan
deteriorated concrete
A A
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Cut and lapped x-section
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Cut and lapped x-section
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2
3 4
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Image 1 – light image taken near the top surface near the saw cut. Note the large number of air voids.
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Cut and lapped x-section
1
2
3 4
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Image 2 – light image taken at 60 mm from the surface near saw cut. Note the lack of small voids.
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Image 1 Near surface at saw joint
Image 2 60 mm depth at saw joint
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Cut and lapped x-section
1
2
3 4
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Image 3 – light image taken at 90 mm from the surface near saw cut. Note the lack of small voids.
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Cut and lapped x-section
1
2
3 4
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Image 4 – light image taken at 90 mm from the surface away from the saw cut. The voids are back!
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1
2
3 4
very few small air voids small air voids
are present
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Image 3b – light image taken at 90 mm from the surface near saw cut with higher resolution. A number of spherical white regions are shown.
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Image 3c – polarized light image taken at >60 mm from the surface near saw cut with higher resolution. The voids appear to be full of ettringite and portlandite.
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Observations
• A well distributed bubble system was observed:
• Away from the joint in the bulk paste • At the joint near the surface
• A more coarse bubble system was observed:
• At the joint in two different locations away from the surface
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Observations
• Based on high magnification polarized light it appears that the small voids are being filled with ettringite and some portlandite • This was found along the joint near the failure and also at further depths in the concrete
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What could have happened?
• Since the sealant had failed the joint may have filled with water • This water may have saturated the paste surrounding the joint • Ettringite and portlandite likely formed in the water filled space • Whether these voids are filled with hydration products or water this is bad for frost durability (Weiss, 2011)
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OSU Research
• Mortar samples from an air entrained concrete mixture were consolidated in a 5 mm (¼”) diameter straw • One sample was stored in water for 50 days at 73oF • The other sample was stored in water for one day and then in air for 49 days at 73oF •Both samples were then investigated with the µCT scanner
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1 day wet cure
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50 day wet cure
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1 day wet cure 50 day wet cure
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0
20
40
60
80
100
120
140
160
180N
umbe
r of v
oids
Void Diameter (microns)
1 day of curing
after approximately 50 days of wet curing
Equal volumes were compared
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Observations
• Needle like hydration products are seen filling the voids in the 50 day water cure sample •The sample that had been wet cured for 50 days had 60% less voids then the sample taken from the same mixture that had only been cured for 1 day
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Observations
• This behavior is similar to what is hypothesized to happen in the field air-entrained concrete • A number of others have observed ettringite in mature concrete
• Iowa DOT • Folliard et al., 1998 • Ley et al., 2009 + Others…
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Does inadequate curing impact the air-void system?
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Does inadequate curing impact the air-void system?
• When rapid CSH formation occurs water is consumed in the reaction and outside water may be needed • This can be seen in the field as the bleed water above the concrete is sucked back inside • This contributes to the disappearance of the bleed water and is the signal of setting
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OSU Research
•An air-entrained 0.42 w/cm paste mixture was made and consolidated in a 5 mm (1/4”) straw • Some water was placed on top of the surface of the sample • Images were taken with the µCT scanner at 0.5, 2.5 and 4.5 hours after mixing • The sample was never moved from the machine
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OSU Research
• The water level dropped over time until there was no water present after 4.5 hours
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0.5 hours 2.5 hours 4.5 hours
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0.5 hours 2.5 hours 4.5 hours
If one looks closely you can see the water level
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2.5 hours
If one looks closely you can see the water level
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0.5 hours 2.5 hours 4.5 hours
water level no water
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0.5 hours 2.5 hours 4.5 hours
Notice the change in the void system!!
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0.5 hours 2.5 hours 4.5 hours
Lets compare the void systems at these spots
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Equal volumes were used for all comparisons
0
100
200
300
400
500
600
700
Num
ber o
f Voi
ds
Void diameter
0.5 hr-Top
0.5 thr-Btm
4.5 hr-Top
4.5 hr-Btm
4.5 hour near the surface!
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Observations
• Between 2.5 and 4.5 hours there was a significant change in the water level and the air-void system near the top of the sample
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What does this mean?
• If water is not available at the surface of the sample to satisfy the chemical suction of hydration then it appears that this could compromise the void system at the surface • This could be a contributor to the surface scaling of concrete
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The World of Air-Entrained Concrete…
• Providing a small and well distributed air-void system is crucial to frost durability
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The World of Air-Entrained Concrete…
• Are we getting the needed air-void system when we first make the concrete? • Are we keeping that air-void system through delivery and placement? • Do we have it after setting? • Do we have it after years of service?
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The World of Air-Entrained Concrete…
• Are we getting the needed air-void system when we first make the concrete?
• Current Mix Track Research - 2012 • Are we keeping that air-void system through delivery and placement? • Do we have it after setting? • Do we have it after years of service?
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The World of Air-Entrained Concrete…
• Significant research is needed to better understand how to produce, maintain, and keep a quality air-void system in our concrete for years of service • It is unclear where these needed resources will come from
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Questions???