powerpoint presentation€¦ · pumping, and vibrating air entrained concrete. this seminar,...

3/9/2020

1

PLACING, CONSOLIDATING AND PUMPING AIR

ENTRAINED CONCRETEKEN HOVER, P.E.

CORNELL UNIVERSITY, ITHACA, NY

THANK YOU FOR BEING HERE!

Ken Hover

Cornell University, Ithaca, New

York

[email protected]

I will be happy to say in the room afterwards

to respond to your questions and comments!

Description: There is a lot of confusion surrounding placing,

pumping, and vibrating air entrained concrete. This seminar,

presented by one of the industry's leading experts, will dispel

those myths and get to the facts to take advantage of

effective placing methods. Understand all the factors such as

the concrete mixture itself, types of admixtures, slump,

combined aggregate grading, pumping rate and pressure, the

rate of depressurization, angle of the boom, the length of

free-fall, and the manner in which the concrete exits the line.

LEARNING OBJECTIVES

1. Know when changes in air content mean changes in freeze-thaw durability.

2. Plan pumping operations to minimize effects on air entrained concrete.

3. Speak-up in pre-pour conferences about how non-standard, point-of-placement test-methods change the test results.

COPYRIGHT MATERIALS

This presentation is protected by US and International

copyright laws. Reproduction, distribution, display and

use of the presentation without permission of the

speaker is prohibited.

© Kenneth C. Hover, P.E.

mailto:[email protected]

3/9/2020

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Pumping

Air Entrained

Concrete

Variable Air & Yield

Hot & Cold weather

Admix compatibility

Flyash & Carbon

Strength and Stiffness

Scaling Resistance and Freeze-Thaw Durability

Pumps, conveyors, concrete buckets, & vibrators

Air volume & Bubble Size

Sampling & Testing

OUTLINE

• Pores are the problem

• Why we need air: Freeze-Thaw

• How to get the air

• How to keep the air we want

• Getting rid of air we don’t want

• Measuring what we got

• Air content is variable in the best of cases

• Air bubbles are fragile

• Can knock air bubbles out with

• Cranes & buckets

• Conveyors

• Tremies

• Pumps

• Vibrators!

PRE-NAP POINTS

• Loss of air DOES NOT always meanLoss of freeze-thaw durability.

• Nobody knows the correct the way to sample and test air content at the end of the pump line.

• Because there is no correct way to do it.

PRE-NAP POINTS General Rule of Impact of Concrete Pumps on Air Content

No Exceptions!

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ASTM C231 PressureAir Meter

ASTM C173Volume Air Meter

15 18 8 28 31

7 14 4 24 51

Concrete Composition

Air Entrained Concrete

Rich Mix

Lean Mix

15 18 8 28 31

AIR/PASTE Ratio


Rich Mix

100%

Air

Volume

24%

Paste

Volume

7 14 4 24 51

AIR/PASTE Ratio


Lean Mix

100%

Air

Volume

19%

Paste

Volume

½ mm 5/1000

inch

Straight Pins

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½ mm 5/1000

inch


• Concrete to which a detergent has been

intentionally added for the purpose of stabilizing

air bubbles formed during the concrete-mixing

process.

• Improved Workability

• Water Reduction

• Reduced Segregation

• Increased Cohesiveness

• Less Sand

REASONS: FRESH CONCRETE

• General benefit of lower w/c

• Improved water-tightness

• Improved sulfate resistance

• Improved Freeze-Thaw resistance

• Improved Scaling Resistance

REASONS: HARDENED CONCRETE

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When Freeze-Thaw Damage

is the problem,

Air Entrainment is the solution

Along with mix -design, aggregates, placing, vibrating, finishing, and curing

When Freeze-Thaw Damage

is NOT the problem,

Air Entrainment

is not necessarily a good idea…

• Strength Reduction• Porous Paste

• Aggregate Interface

• Required Compensations• Lower water content

• Admixtures

• Higher cement content

• Cost

• Heat

• Shrinkage

DISADVANTAGES:

HARDENED CONCRETE

200-250 psi per 1% air

Clustering of Air Bubbles

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• Stickiness

• Finishing Problems

• Bleeding, Plastic Cracking, Delaminations

• Variable Volume – Yield problems

• Interaction with other admixtures

DISADVANTAGES:

FRESH CONCRETE

Hard-troweled surface on

Air Entrained Concrete—Problem!

Hard-troweled surface on

Air Entrained Concrete → Delamination!

“…but only with the acknowledgment that the risk associated with delamination or blistering and the changes in hardened air void parameters are entirely the responsibility of the specifier.”

“ASCC concrete contractors will hard-trowel air-entrained concrete if required by specification, …”

ACI 302 - Construction of Floors

• Entrained air not recommended when smooth, dense, hard troweled finish required

ACI 301 - Specification

• Intentionally entrained air should not be incorporated in normalweight concrete slabs to be given a smooth,

dense, hard troweled finish.

Exposed concrete?Deicing salt

Scaling resistance?Freeze-Thaw?

StrengthStiffness

Abrasion resistance

Delayed bleedingSlab blisters

Delaminations

Clustering of air bubbles

DisadvantagesAdvantages

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• General benefit of lower w/c

• Improved water-tightness

• Improved sulfate resistance

• Improved Freeze-Thaw resistance

• Improved Scaling Resistance

REASONS: HARDENED CONCRETE

❑Control porosity by controlling water

❑Control Finishing

❑Wet Curing

❑Temperature Control

❑Frost-Resistant Aggregate

❑ Air-Entrained Paste

FROST AND SCALE RESISTANCE

✓

✓✓

✓

✓

Air VoidsDo NotProtectAggregate

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Frost Resistant Concrete Requires

•Durable, Frost Resistant Aggregate

•Air-Entrained Paste

❑Control porosity by controlling water

❑Control Finishing

❑Wet Curing

❑Temperature Control

❑Frost-Resistant Aggregate

❑ Air-Entrained Paste

FROST AND SCALE RESISTANCE

✓

✓✓

✓

✓

✓

Air Entrained

Paste

Why does this happen?

And how do air bubbles

prevent it?

Concrete is a Porous Material

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Concrete is Porous

Penetration of Salts, Liquids, and Gases

into the ConcreteCO2

O2

H2O

Sulfates

Salts

Why does this happen?

And how do air bubbles

prevent it?

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http://datagenetics.com/blog/d

ecember22013/index.html

Air Void Air Void

SIZE(of the air bubbles)

MATTERS

The Protected Paste Shell

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Air Void

“0.010 INCHES

OR

THEREABOUTS”

10 mil poly slab underlayment

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Frost

protection

Air

volume

Frost

protection

Air volume

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•Large number of small voids

•Well distributed in paste

•Prefer bubbles around 5 mils

•< About 10 mils to nearest bubble

•Air vol about 18% of paste vol

WANTED: Characteristics of AE Concrete

•4-8% air bubbles by vol. of concrete

•1 to 2 ft3 Air bubbles / CY

•10-15 billion air bubbles / CY

•2 million in2 air bubble surface area/CY

•1500 SY of bubble surface area / CY

•Bubble Size Matters!

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BUBBLE MANAGEMENT

Obtaining Air Retaining Air

•Air Entraining

Admixtures

•Thorough Mixing

•Re-dosing AEA

•Transport

•Placing

•Consolidating

•Finishing

AIR-ENTRAINING

ADMIXTURES

The Fishing Bobber

The Fishing Bobber

Operates at the “Air-Water Interface”

The Air-Entraining

Admixture

Operates at the “Air-Water Interface”

Repelled by water (oils, fats, resins)

Attracted to water (salts)

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Air trapped

here!

Air Bubble

Stabilized

By Air-Entraining Admixture

Air Bubbles vary in Size

From smaller than cement to larger than coarse aggregate

0

20

40

60

80

100

100 mm 10 1 0.1 0.01 0.001 0.0001

4 in. # 325# 200#81/2 1/4 #30

SilicaFume

CementSlag, Ash

Air

FineAgg

CoarseAgg.

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0

20

40

60

80

100

100 mm 10 1 0.1 0.01 0.001 0.0001

4 in. # 325# 200#81/2 1/4 #30

Air

0

20

40

60

80

100

100 mm 10 1 0.1 0.01 0.001 0.0001

4 in. # 325# 200#81/2 1/4 #30

“Entrained”

Air“Entrapped”

Air

#1 mm is arbitrary dividing line

“Entrapped”• Coarse

• Rain drops

• Inefficient

• Less protection per unit volume

• More impact on strength

“Entrained”• Fine

• Fog droplets

• Efficient

• More protection per unit volume

• Less impact on strength

WHAT THE NAMES REALLY MEANBut Bubble Size Matters!

But Bubble Size Matters!

TESTS OF FRESH CONCRETE ESTIMATE

TOTAL AIR VOLUME ONLY—THESE

TESTS DO NOT PROVIDE INFORMATION

ON BUBBLE SIZE!

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TESTS OF FRESH CONCRETE ESTIMATE

TOTAL AIR VOLUME ONLY—THESE

TESTS DO NOT PROVIDE INFORMATION

ON BUBBLE SIZE!

FACTORS AFFECTING THE AIR VOID SYSTEM

Mix Ingredients

Batching/Mixing/Transport

Construction Practices• Carbon-Bearing Materials

• Portland Cement

• Fly Ash

• Carbon can neutralize the AEA and reduce air content

• Carbon can neutralize the defoamer in HRWRA or Latex and increase air content

ADMIXTURE INTERACTION WARNING

• Temperature

• Hardness (calcium)

• Treated water

• Well water

• W/C

• Water Content

INFLUENCE

OF WATER

Batching Mixing Transport

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Placing

•There is almost always too much or too little air. Air volume meets specs only when no testing is performed.

FIRST LAW OF AIR ENTRAINMENT

•On those rare and happy occasions in which there is just about the right amount of air, further investigation will show that the bubbles are either too big or too small.

SECOND LAW OF AIR ENTRAINMENT

•Air Bubbles Obey the Laws of Physics.

THIRD LAW OF AIR ENTRAINMENT

•The Laws of Physics are the responsibility of the contractor, and shall be submitted for approval.

FOURTH LAW OF AIR ENTRAINMENT

•Air and Water Don’t Mix

LAWS OF PHYSICS

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•Bubbles expand when heated

•Bubbles shrink when cooled

LAWS OF PHYSICS

• Bubbles are fragile:

Do not drop, shake, or vibrate!

• Little guys are tougher than big guys

LAWS OF PHYSICS

Effects of Free Fall

Break air bubbles on impact

Inside or outside of line

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Impact at Elbows

Impact at Placement

Effect of Pump Boom Configuration Pressure Decrease Consolidation by Vibrator: Air Bubble Hunter-Killer

0 2 4 6 8 10

Distance from vibrator insertion (inches)

2

3

4

5

6

Ha

rde

ne

d A

ir C

on

ten

t (%

)

24"

24"

5"

8"

10"

Effects of Finishing

Finishing

Removal of Air Bubbles Near Surface

(Rapid depressurization?)

• Bubbles compress when squeezed

• Bubbles expand when pressure released

LAWS OF PHYSICS

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Pressure Increase

Pressure Decrease

Air voids before

compression

Pump pipe

Most air bubbles at atmospheric pressure

Compressed air voids

Smaller air bubbles - Lower air content

Higher compression

Smaller air bubbles - Lower air content

Controlled decompression

Larger air bubbles - Greater air content

Back to normal pressure

Larger air bubbles - back to normal

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National Ready-Mixed Concrete Assn. & American Concrete Pumping Assn. Test Program-Baltimore

0

1

2

3

4

5

6

7

8

9

Chute Pump

Free Fall, Slow Discharge < 50 CY/hrNRMCA TEST - 2A

Air

Co

nte

nt

(%)

Free Fall

0

1

2

3

4

5

6

7

8

9

Chute Pump

Free Fall, Rapid Discharge < 100 CY/hrNRMCA TEST - 3A

Air

Co

nte

nt

(%)

Free Fall

0

1

2

3

4

5

6

7

8

9

Chute Pump

Free Fall, Faster Than 3ANRMCA TEST - 3B

Air

Co

nte

nt

(%)

Free Fall

0

1

2

3

4

5

6

7

8

9

Chute Pump

Free Fall, Slow DischargeNRMCA TEST - 4A

Air

Co

nte

nt

(%)

Free Fall

0

1

2

3

4

5

6

7

8

9

Chute Pump

Free Fall, Rapid DischargeNRMCA TEST - 4B

Air

Co

nte

nt

(%)

Free Fall

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The

‘‘Loop”

0

1

2

3

4

5

6

7

Chute Pump

4-Elbow Loop, Slow Discharge

NRMCA TEST - 4C

Air

Co

nte

nt

(%)

4-Elbow

Loop

5.2

5.4

5.6

5.8

6

6.2

6.4

Chute Pump

4-Elbow loop, Rapid Discharge

NRMCA Test - 4D

Air

Co

nte

nt

(%)

4-Elbow

Loop Schwing Tests, White Bear, Minnesota

• Schwing America

• Cemstone / Wyatt

• American Engineering

• Concrete Microscopy, Inc.

•Minnesota DOT

• Cornell University

PARTICIPANTS: PARTICIPANTS:

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0

1

2

3

4

5

6

7

8

9

10

108a Chute 108a Pump 77a Chute 77a Pump

Air Meter

Microscope

Air Content (%)

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0

1

2

3

4

5

6

77b Chute 77b Kink 77b Free Fall

Air Meter

Microscope

Air Content (%)

0

1

2

3

4

5

6

7

8

108b Chute 108b Kink

Air Meter

Microscope

Air Content (%)

0

1

2

3

4

5

6

7

8

285 Chute 285 Conveyor

Air Meter

Microscope

Air Content (%)

0

1

2

3

4

5

6

7

8

9

10


Avg. Bubble Dia. (mils.)

Smaller is better

0

1

2

3

4

5

6

7

8



Smaller is better

0

1

2

3

4

5

6

7

8



Smaller is better

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0

1

2

3

4

5

6

7

8



Smaller is better

0

1

2

3

4

5

6

7

8

9

10


Spacing Factor (mils)

Lower (closer) is better

0

1

2

3

4

5

6

7

8

9

10




0

1

2

3

4

5

6

7

8

9

10




0

1

2

3

4

5

6

7

8

9

10




0%10%20%30%40%50%60%70%80%90%

100%

108

Chute

77 Chute 77 Kink 77 Free

Fall

Conveyor

Durability Factor (%)

5.7% 5.2% 4.0% 3.4% 5.2%

ASTM C666

Freeze-Thaw

Testing

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Air at the Chute (%)

0

1

2

3

4

5

6

7

8

9

10

Federal Highway-National Highway Institute

Testing Program 1992-2008









3/9/2020

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Apparent Rate of Air-

Loss Rate AT THE

CHUTE:

2% / Hour

CiDRA Air-Trac, July, 2019 (Real Time Air in Drum)

• How much paste do you have to protect?

• How severe is the exposure?

• How small are the bubbles?

• Who are you asking?

HOW MUCH AIR DO YOU NEED? HOW MUCH AIR DO YOU NEED?

Legal responsibility of designer/specifier to include Exposure Category in contract

documents.

250 260 270 280 290 300 310 320 330 340 350

Water Content lb/CY

1

2

3

4

5

6

7

8

9

10

11

Re

qu

ire

d A

ir C

on

ten

t in

Co

ncre

te (

%)

90 95 100 105 110 115 120

Water Content kg/m3

Average Bubble Radius = 7 mils





= 430 in2 / in

3

= 500 in2/in

3

= 600 in2/in3

= 750 in2/in3

= 1000 in2/in3

w/c = 0.45

Ave

rag

e B

ub

ble

Ra

diu

s

w/c = 0.45

270 lb/CY

4 mil diameter

3.0% air OK!

• How much paste do you have to protect?

• How severe is the exposure?

• How small are the bubbles?

• Who are you asking?

• Fresh (pre-loss) or Hard (post-loss)?

HOW MUCH AIR DO YOU NEED?

3/9/2020

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Bubble Accounting

Mixing Stabilizing Break/float

Discharge Trapping Break

Handling Expand Contract/break

Placing Trapping Break

Vibrate

and Finish Break/float

Total

Transaction Gains Losses Total

Bottom Line

Bubble Accounting





Vibrate


Total


Bottom LineBottom Line

U.S. ASTM C172

Bubble Accounting





Vibrate


Total


Bottom LineNo US Sampling method

No US Std. Code or Specification

U.S. ASTM C72

U.S. ACI 318/301

Traditional

interpretation

Bubble Accounting





Vibrate


Total


Bottom LineCSA A 23.1-04

4.3.3.3

CSA A23.1-04

4.3.3.1

CSA A23.1-04

Concrete materials and

methods of concrete construction

The concrete will be considered to have a satisfactory

air-void system when the average of all tests shows

a spacing factor not exceeding 230 μm (0.009 inches),

with no single test greater than 260 μm (0.010 inches),

and air content greater than or equal to 3.0% in the

hardened concrete.

For concrete with a water-to-cementing materials

ratio of 0.36 or less, the average spacing factor shall not

exceed 250 μm, with no single value greater

than 300 μm.(0.0010 inches)

SAMPLE AT CHUTE OR POINT-OF-PLACEMENT?

C172-14 Standard Practice

for Sampling Freshly Mixed

Concrete

NO Standard Method Exists

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POINT-OF-PLACEMENT SAMPLING COMPLICATIONS

1. Before or after finishing?

2. Before or after consolidation?

3. Before or after placing?

4. Before or after concrete hits form?

5. “Clean catch?”

6. Composite sample?

7. Representative?

8. Safe?Warning: Sampling at placement can be dangerous, and

NO STANDARD APPROVED METHOD EXISTS FOR DOING SO

A Story Problem:

How long does it take to fill

bucket if pumping rate is

60 CY/HR?

5 gal. x = 0.133680556 cubic foot / gal0.66840276 cubic feet / (60x27.000)

= 0.000412594 hours

About a second and a half Warning: Slowing pump rate normally INCREASES air loss

If point-of-placement sampling is required:

• It must be specified, not assumed

• It should be in addition to at-the-chute tests

• An unbiased sampling method must be developed and described by the specifier

• Point-of placement acceptance criteria should be different from

point-of-discharge acceptance criteria

IN-PLACE CRITERIA FOR AIR CONTENTNOT NECESSARILY SAME ASDELIVERY CRITERIA!

5.6.5.4 — Concrete in an area represented by

core tests shall be considered structurally

adequate if the average of three cores is equal

to at least 85 percent of f ‘c and if no single

core is less than 75 percent of f ‘c.

ACI 318 Chapter 5, Strength Evaluation

3/9/2020

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Sampled per ASTM C172

at the chute unless specified 1-1/2

percent air

Need 18% of paste volume, in-place

CSA A23.1-04

Concrete materials and

methods of concrete construction

The concrete will be considered to have a satisfactory

air-void system when the average of all tests shows

a spacing factor not exceeding 230 μm (0.009 inches),

with no single test greater than 260 μm (0.010 inches),

and air content greater than or equal to 3.0% in the

hardened concrete.

For concrete with a water-to-cementing materials

ratio of 0.36 or less, the average spacing factor shall not

exceed 250 μm, with no single value greater

than 300 μm.(0.0010 inches)

• Air is a moving target

• Air measurements approximate

• Multiple tests required

• Air loss expected with handling

• Air loss expected with vibration

CONCLUSIONS:

• Loss of larger bubbles most likely

• Air Loss not equal to durability loss

• Expect different effects with different mixes

• Beware of non-standard sampling methods

• Specify hardened air on critical placements

CONCLUSIONS (CONT'D.)

• Minimum pressure increase

• Avoid rapid decrease in pressure.

• When possible and safe, do not slow pump rate to take samples.

• Take multiple samples at the chute for every sample taken at the hose.

• Consider different air criteria at end of hose (3 or 4%).

SUGGESTIONSTHANK YOU FOR BEING HERE!

Ken Hover

Cornell University, Ithaca, New

York

[email protected]

I will be happy to say in the room afterwards

to respond to your questions and comments!

mailto:[email protected]

3/9/2020

32

Questions?

Please complete the session evaluation on the CONEXPO –CON/AGG Mobile App. This is the same place you can log Professional Development Hours (PDH).

powerpoint presentation€¦ · pumping, and vibrating air entrained concrete. this seminar,...

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