Advances in Mass Concrete Technology – The Hoover Dam Studies

Advances in Mass Concrete
Technology – The Hoover Dam
Studies
Timothy P. Dolen, P.E.

Evolution of Mass
Concrete Dam
Construction

Introduction
• The Problem – Strength vs. Heat
• Investigations of Portland Cements
– Chemical Composition of Portland Cements
– Strength Development
– Heat of Hydration / Temperature Rise
• The Mass Concrete “Recipe for Success”
– Mixture Proportioning and Properties of Fresh Concrete
– Compressive Strength and Elastic Properties
– Size Effects
– Permeability
• Thermal Properties of Mass Concrete
• Bond Strength of Lift Lines
• Construction Firsts
• Conclusions

The giant leap forward!
Buffalo Bill
1905-1910
350 ft
87,515 yd 3
Owyhee
1928-1932
417 ft
537,500 yd 3
Hoover
1931-1936
726 ft
4,400,000 yd 3

The Problem - Strength vs. Heat
Portland cement gains strength through
the chemical process of “hydration.”
But, hydration of cement is an exothermic
reaction that generates heat.
The inability of such a massive structure
to dissipate heat leads to the potential for
thermal cracking.

Cement Investigations
Great collaboration between government,
academia, manufacturers and trade
associations.
• Chemical composition of cement
• Heat of hydration of cement
• Physical properties and strength of mortar
• Durability investigations
• Manufacturing processes
Berkley cement investigations – 15,429 tests!

Composition of Portland Cement
Component Elements (raw feed)
Clay, limestone, iron + = “”clinker”
“Clinker”
Compound Oxides (CaO, SiO 2 , Al 2 O 3 , Fe 2 O 3 )
Cement Compounds
C 3 S C 2 S C 3 A C 4 AF
binder – “gel”
(strength)
Hydration Products
Ca(OH) 2 – lime
HEAT

Hydration of Portland Cement
Cement Compounds
C 3 S C 2 S C 3 A C 4 AF
+ = +
Cement “gel” + lime

Strength Development of Portland Cement
C 3 A (

Heat of Hydration of Portland Cement
C 3 A ( 7 days)
C 4 AF

Chemical Composition of Cements
C 3 S C 2 S C 3 A C 4 AF
Low Heat 23 50 5 14
Standard 50 25 10 8
“Modified
GCD”
46 30 5 13

Physical Properties of Mortar
Compressive
Strength (psi)
Heat of Hydration
(cal/g)
7 day 28 day 7 day 28 day
Low Heat 1,770 3,760 55 64
Standard 2,660 3,350 85 97
“Modified
GCD”
2,720 5,030 75 82

Concrete Mixture Proportioning
Investigations
0.5 Water : 1 Cement : 2.45 Sand : 7.05 Aggregate
• Nominal maximum size aggregate (NMSA)
• Aggregate grading / sand – aggregate ratio
• NMSA vs. paste volume
• Properties of fresh concrete
• Properties of hardened concrete

Nominal Maximum Size Aggregate
6 ft !
Elephant Butte Dam
“plum stones” 1916

Arizona
Gravel
Deposit
NMSA = 8-9 in

Mass Concrete Fresh Properties
Slump – 3 in.

NMSA vs.
Paste
Content
Paste
(Cement + Water)
Fine
Aggregate
Coarse
Aggregate

Effect of NMSA on Paste Content
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0.53 0.49 0.47
0.28
0.27
0.25
0.19 0.24 0.28
8 in NMSA
380 lb / cy
3 in NMSA
460 lb/cy
1-1/2 in NMSA
560 lb/cy
Coarse Aggregate Volume
Sand Volume
Paste Volume
3 inch slump
Cement content
lb per cubic yard

Compressive Strength and Elastic
Properties of Mass Concrete
• Effect of cement type
• Effect of NMSA
• Test age
• Water / cement (W/C) ratio
• Curing condition
• Placing temperature
• Cylinder size (effect of wet screening)

Compressive Strength - psi
Compressive Strength vs. W/C ratio
5000
Compressive Strength of Mass Concrete
Hoover Dam Laboratory Test Program
4000
3000
2000
1000
0
Note - average core compressive
strength at 60 years age - 7,410 psi
0 100 200 300 400
Test Age - days
W/C - 0.66
W/C - 0.54
W/C - 0.47
Power (W/C - 0.66)
Power (W/C - 0.54)
Power (W/C - 0.47)

NMSA vs. Test Specimen Size

Test Specimen Size

Compressive Strength of Mass Concrete
Size Effects

NMSA vs. Test Specimen Size
NMSA
(in)
Test Cylinder
Size (in)
Volume
(ft 3 )
Mass
(lb)
“9” 36 x 72 42.4 6,530 *
6 24 x 48 12.6 1,935
3 12 x 24 1.6 240
1-1/2 6 x 12 0.2 30
3/4 3 x 6 0.03 4
* Approximately 250 shovels full of aggregate

Batching Mass Concrete in the
Laboratory

6,530 lbs. - approximately 250 shovels
full of aggregate per test specimen!
Grant Wood - 1930

Elastic Properties of Concrete

Modulus of Elasticity - 10 6 psi
Modulus of Elasticity of Mass Concrete
7
6
5
4
3
2
1
0
Modulus of Elasticity of Mass Concrete
Hoover Dam Laboratory Test Program
Note - average coremodulus of elasticity
at 60 years age - 6.65 X 10 6 psi
0 100 200 300 400
Test Age - days
W/C - 0.66
W/C - 0.54
W/C - 0.47
Power (W/C - 0.66)
Power (W/C - 0.54)
Power (W/C - 0.47)

Thermal Properties
• Conductivity
• Specific heat
• Diffusivity
• Thermal Expansion
• Adiabatic Temperature Rise

Test Conditions
Conductivity
(Btu/(ft-hr- o F)
Thermal Properties
Specific Heat
(Btu/(lb- o F)
Diffusivity
(ft 2 /hr)
Type of coarse
aggregate varied
through range of tests.
1.2 to 2.0
Varied as much
as 42 percent
0.23 to 0.245
Varied as much
as 8 percent
0.032 to 0.058
Varied as much
as 47 per cent
Water content
increased from 4 to 8
per cent of the
concrete by weight.
(1.7 to 1.55)
Decreased as
much as 10
percent
(0.22 to 0.24)
Increased as
much as 12
percent
(0.049 to 0.042)
Decreased as
much as 16
per cent
Mean temperature of
concrete increased
from 50 to 150 o F.
Increased as
much as 12 per
cent and
decreased as
much as 6
percent
Increased as
much as 24
percent
Decreased as
much as 21
per cent

Thermal Conductivity Testing Equipment

Thermal Expansion 10 -6 in/in per o F
Thermal Expansion of Mass Concrete
5.5
5
4.92
Quartzose
Sandstone
4.8
5.18
5.01
5.2 5.27
4.5
4
3.9
Granite
3.98 4
4.3
4.4
3.97
3.5
3
Vesicular
Basalt
3.01
3.33
2.5
2
2.1
1.5
1
Thermal Expansion Mass
Thermal Expansion RCC

Adiabatic *
Temperature
Rise of Mass
Concrete
* No heat loss or gain

Temperature Rise - o F
Temperature Rise of Mass Concrete
70
60
50
40
30
20
10
0
Adiabatic Temperature Rise
Hoover Dam Mass Concrete
0 5 10 15 20 25 30
Test Age - days
Low Heat Cement
Standard Cement
Modified Cement

Permeability of Mass Concrete

Bond Strength of Mass Concrete Lift
Lines
• Effect of concrete
strength
• Time between placing
lifts
• Curing of lift lines
• Temperature during lift
exposure
• Method of lift line
cleaning
• Bonding mortar

Recipe for Success - The solution – Low
heat cement for Hoover Dam mass
concrete – 8 in NMSA – Post cooling
Strength (psi) to Heat (cal/g) ratio
3 days 7 days 28 days
Low Heat 32 46 65
Standard 41 46 50
“Modified
GCD”
42 53 60

Hoover Dam Historic Firsts

Concrete batching and placing

Over 100 years to cool down!

“Chuting” concrete into place - ~1908 – 1928)
(the flatter the slope – the more water added)

Highline – bucket placing

Block placing and post cooling

Post cooling and grouting
Closure
Slot

Black Canyon Dam - 1924
Bleeding
Segregation

Lower slump mass concrete

Unsung hero – the internal vibrator
6 in > 2-3 in slump
10 – 20 % less water
25 – 50 lb less cement
10 – 15 °F less heat

Conclusions – Mass Concrete
• Unprecedented in scope
Investigations
• Unprecedented in complexity
• Unprecedented in difficulty
• Verification followed each stage of laboratory testing
and large scale mass concrete construction
• The systematic methodology of mixture
proportioning investigation still followed today
• Database of materials thermal properties still used
today
• Construction innovations still followed today

Special thanks to all of the engineers and laboratory
technicians for their achievements, especially anyone
who has ever had to manipulate a 12 inch diameter
(or larger) test specimen!

Thank you
for your
attention