Post on 14-Jan-2020
transcript
Pumping Air
Entrained Concrete
Marllon “Dan” Cook, Ph. D
Research Associate
Civil & Environmental Engineering
Oklahoma State University
Research Team
Principle Investigator:
Tyler Ley, PE, PhD
Graduate Students:
Justin Becker, Nick Seader, Chad
Staffileno
Acknowledgements• Oklahoma DOT
• FHWA
• Kansas DOT
• Nebraska DOT
• Iowa DOT
• Minnesota DOT
• Idaho DOT
• N. Dakota DOT
• Penn. DOT
• Connecticut DOT
• Illinois DOT
• Indiana DOT
• Michigan DOT
• Wisconsin DOT
• New Jersey DOT
• RMC Foundation
• American Concrete
Pumping Association
Mixture Batched Over Time
Air
Do
sag
e (
oz./
cw
t)
Dosage Required for
6% Air volume
Air Dosage Over Time
Parameters Effecting Air Dosage
• Weather & temperature
• Sand gradation
• Fly ash production changes
• Water temperature
• Inadequate mixing
• Admixture/cement incompatibility
• Alkali content of binder
• Material source change
• Pumping concrete…………….list goes on and on
1. Air volume will go down
2. Air volume will go up
3. Air volume will stay the same
- Ken Hover
When Pumping Air-Entrained Concrete 1 of 3 Things Happen:
What Does the Engineer Say?
• Test at point of placement
• Engineering logic says:
“This will represent
hardened properties”.
What Does the Producer Say?
• Test at point of discharge
• Supplier logic says:
“Concrete met specs
before the pump. That’s
what I can control”.
Experimental Investigation
• Investigate the following before and after pumping
• Both field and laboratory testing
Testing Methods:
• Air volume with SAM (AASHTO TP 118)
• Air void spacing with SAM (AASHTO TP 118)
• Spacing factor (ASTM C 457)
• Freeze-thaw performance (ASTM C 666)
Super Air Meter (SAM)
• AASHTO TP 118
• Measures vol. & bubble distribution
• Modified pressure meter with larger
pressure steps
• The test takes 8 - 10 minutes
• Uses SAM number to indicate bubble
distribution.
www.superairmeter.com
digital
gauge
six
clamps!
different
bleeder
valve
Comparing Meters
• AASHTO TP 118
• Measures vol. & distribution
• Multiple pressure steps
Type B Meter SAM
• AASHTO T152
• Measures volume
• 1 pressure step
• Small bubbles contribute to freeze-thaw resistance and improve overall durability
• Large bubbles form inconsistently, don’t contribute to f-t, decrease strength, and create inconsistencies for dosage rates.
Large & Small Air Bubbles
• Mixing process: “entrapped” air (0.5% to 2.5%)
• Inadequate mixing
• Admixture and/or cement incompatibilities
• Cement grinding aids
• Alkalis in cementitious mat.
• Sand gradation
• Changes in temperature
• Pumping
Large Air Bubbles Created By
Large Bubbles
• Provides air volume and air distribution of hardened concrete.
• Takes roughly 6 weeks for lab.
• Usually completed if there's a problem.
Harden Air Void Analysis
Take Core
Cut Core
Harden Air Void Analysis
• Measures cored length and SSA.
• Calculate the spacing factor from cored length
Scan Path
on Sample
• 0.45 w/cm
• 20% Class C fly ash
• 6.5 sacks (611 lbs.)
• Limestone & natural sand
• 6% air content
• 20 laboratory mixtures
Mixture Design in Laboratory
Concrete Pump Network
• 4” diameter pipe
• 60’ of steel pipe
• 10’ Rubber hose
• pumping pressures from 55 to 110 psi
Approximately 20% air loss after one circulation through the concrete pump.
Air Volume & Pumping Cycles
Discussion of Fresh Concrete
• After one cycle:
➢Air content decreased
➢SAM Number shows loss of small bubbles when you pump.
• Air void system in the fresh concrete changed due to pumping with these materials and equipment.
Discussion
• After pumping the hardened air content was on average 1.15x higher in the fresh air content.
• For example – After pumping 6% fresh and about 7% in hardened concrete
• The fresh measurements after pumping do not seem to represent the performance or properties of the hardened concrete.
Discussion
• Since the air volume is going down and SAM Numbers are going up this means that we are losing small bubbles in the fresh concrete during pumping.
• BUT! ➢Concrete not pumped: fresh air = hardened air➢Concrete pumped: fresh air < hardened air
• Satisfactory freeze thaw performance of pumped concrete was observed even though there were low air contents and high SAM Numbers after pumping.
Discussion
• The pressures during pumping causes the bubbles to dissolve and so they are not present in the fresh concrete when it discharges from the pump.
• This is why the air volume decreases and the SAM Number increases.
Mechanism: What is Happening???
• The pressures from pumping causes the small bubbles to temporarily dissolve.
• Good performance in the petrographic analysis, freeze-thaw testing, and reducing SAM Number over time suggests that the dissolved air comes back before the concrete hardens.
• When the air comes back it seems to be well dispersed and provides a similar spacing factor to what went into the pump.
What does this Mean for Concrete?
• Air Content and SAM testing after pumping may not be representative of the hardened concrete.
• If this is true then concrete should not be rejected for low air or high SAM Number after pumping.
• It appears that sampling the concrete prior to pumping is a good indicator of the air void system in the hardened concrete.
How to Avoid Air Loss?
Air loss occurs more in the following:•Higher pumping pressures•Smaller diameter lines•Mixtures with poor aggregate gradations
•Lower slump mixtures•A-frame and arch configurations
What do I think needs to happen?
• Testing air at the point of discharge from a pump is dangerous and it is not representative of the properties of the hardened concrete.
• We need to test concrete with the SAM before pumping and not require testing at the point of placement.
• We need to have local discussions about how we can change our specifications and construction practices.
Questions?
Marllon.cook@okstate.edu
Tyler.ley@okstate.edu
www.tylerley.com
www.superairmeter.com
Field Pumping Information
• 62 different mixtures tested
• Bridge decks, walls, sidewalk, parking lot, drilled shaft
• 18 Different Types of Pumps
• Boom lengths ranged from 100’ to 180’
• Pipes from 4” to 6” in diameter
• Used three different boom configurations
• Oklahoma, Kansas, Missouri
Discussion of Field Data
*Most flat configurations in the lab showed a change in SAM
Configuration
% of mixtures
that lost > 1%
air
A-frame 39%
Arch 37%
Flat* 0%
Discussion of Field Data
Configuration
% of mixtures
that SAM
changed > .05
A-frame 45%
Arch 60%
Flat* 40%
*Most flat configurations in the lab showed a change in SAM
Discussion
• We are seeing similar things in the lab and the field!
• For A frame or Arch we are losing > 1% air ~ 40% of the time
• For all configurations we are losing SAM > 0.05 > 40% of the time.
Measuring field concrete
To convince our industry that this is real then we need more data.
We need SAM measurements and hardened air void analysis before and after pumping.
97
SAM + cylinder
Discussion
• The hardened and fresh measurements closely matched prior to pumping.
• After pumping the hardened air content was on average 1.15x higher than the fresh air content.
• For example – After pumping 6% fresh and about 7% in hardened concrete
Discussion
• These are the same findings from the lab but from 62 different mixtures and 18 different types of pumps using a variety of pump configurations!!!!
• The fresh measurements after pumping do not seem to represent the performance or properties of the hardened concrete.
Pumping Changes The Concrete
• 60% of ready-mix concrete is pump.
• Pumping effects:• Slump
• Segregation
• Air
Air Volume & Distribution Over Time
• In order to learn more we decided to do a
pump mixture in the lab and then measure the
change in the SAM Number over time.
• While discharging from the pump concrete was
placed in separate containers.
• These containers were sampled over time.