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Mass Concrete
1
CVLE 519
Concrete Technology
Dr. Adel El Kordi
Professor
Civil and Environmental
Engineering Department
Faculty of Engineering
Outline
• Definition
• General Overview
• Material and Mix design
• Construction Practices
• Conclusion
Bridge Piers Dams Mat Foundation
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Definition
•Concrete in a massive
structure, a
beam, columns, pier, lock, or
dam where its volume is of such
magnitude as to require special
means of coping with the
generation of heat and
subsequent volume change.
•When dimensions are > 1m or
3ft, temperature rise
should be considered. Mass concrete columns and footings
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General Overview
• The design of mass concrete structures
is generally based on
durability, economy, and thermal action.
• Strength is secondary in the design
process
• Different from other concrete only in its
distinguished thermal behavior.
• Because the cement-water reaction is
exothermic by nature, the temperature
rise within a large concrete mass, where
the heat is not quickly dissipated can be
quite high
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The temperature rise depends on:
1. Concrete placing temperature.
2. Cement composition, fineness, and content.
3. Aggregate content and CTE (Coeff. Pf Thermal Expansion).
4. Section thickness.
5. Formwork type and time of removal.
6. Ambient conditions.
7. Supplementary cementing materials.
Temperature Rise
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Temperature Rise
Many factors are influential in
the temperature rise of concrete:
1- The heat of hydration being
the most important factor. The
hydration of cement generates
heat (500J/g of CEM I).
2- The amount of cement in the
mix. As a general rule a 5°C to
9°C temperature rise per 45 kg of
Portland cement can be expected
from the heat of hydration (ACI
Committee 211 1997).
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Materials and Mix Design
• Mass concrete is composed of:
1. Cement
2. Aggregates
3. Water
4. Pozzolans and admixtures
• The combination of these materials
should be adjusted to meet the
requirements of use of mass concrete:
economy; workability; dimensional
stability and freedom from cracking;
low temperature rise; adequate
strength; durability
Cement
• The heat of hydration of cement is a function of its
compound composition and fineness.
• Cements used for Mass Concrete should have a low C3A
and C3S content to reduce excessive heat during
hydration.
• Most Mass Concrete structures do not require early
strength, so slower hydration is usually not harmful to
construction.
Cement
In this figure: Temperature rise in mass
concrete containing 223 kg/m3 cement of
different types
• Between (Type I) and a low-heat cement
(Type IV) the difference in temperature
rise was 9°C in 7 days and 23°C in 90
days
• The total temperature rise was above
30°C even with the low-heat cement.
• Temperature rise and the relative
temperature drop of the order of 30°C is
judged too high from the standpoint of
thermal cracking
Cement
• One way to lower it would be by reducing the cement
content of the concrete provided that this can be done
without compromising the minimum strength and workability
requirements needed for the job. By using several methods
it is possible to achieve cement contents as low as 100
kg/m3 in mass concrete for dams. With such low cement
contents, even ASTM Type II portland cement is considered
adequate.
• Substitution of 20 percent pozzolan by volume of portland
cement produces a further drop in the adiabatic
temperature rise.
Admixtures
With cement contents as low as 100 kg/m3, it is essential to use a
low water content to achieve the designed 1-year compressive
strength (in the range 13 to 17 MPa) :
1. 4 to 8 percent entrained air incorporated into the concrete.
2. Water-reducing admixtures are simultaneously being
employed for the same purpose.
3. Pozzolans used primarily as a partial replacement for
portland cement to reduce the heat of hydration.
4. Most fly ashes when used as pozzolans have the ability to
reduce the water content by 5 to 8 percent.
Aggregates
With concrete mixtures for dams, every possible method of
reducing the water content that would permit a reduction in the
cement content (i.e., maintaining a constant w/c ratio) has to be
explored.
1. The choice of the largest possible size of coarse aggregate.
2. The selection of two or more individual size groups of
coarse aggregate that should be combined to make a dense
mix (less voids).
Aggregates
U.S. Bureau of
Reclamation’s
investigations on
mass concrete dam:
=> As max size of
aggregate
increase, both water
and cement contents
decrease!
Aggregates
Normal Concrete
Mass Concrete
Larger Max Size
• Less cement
• Less Heat
Aggregates
Also, choosing an
aggregate with a low
CTE can cut thermal
stresses in half!
Provide resistance
to thermal cracking!0
1
2
3
4
5
6
7
8
9
5 6 7 8 9 10 11
CTE
of
Co
ncr
ete
CTE of AGG
Limestone
Basalt
Granite
Blastfurance Slag
Dolerite
Sand and Gravels
QuartziteCTE = coefficient of thermal expansion
Mix Design
The objective:
1. Economical mixtures of proper strength
2. Durability
3. Permeability
4. Workability for placement
5. Least practical rise in temperature after placement.
procedure is the same as used for determining the
concrete mix proportions for normal weight concrete but
some points must be taken into consideration!
Mix Design
Steps for arriving at the actual batch weights:
1. Determine the max size of aggregates.
2. Determine the water content to achieve a required
slump which can be as low as (35 to 50 mm).
For 6 in. (150 mm) maximum-size aggregate, water
contents for air-entrained, minimum-slump concrete may
vary from approximately 71 to 89 kg/m3 for natural
aggregates, and from 83 to 113 kg/m3 for crushed
aggregates.
Mix Design
3. Determination of w/c It is guided by the relation
between w/c ratio and strength.
Mix Design
4. Determine the cement content by dividing the total
weight of water content by w/c ratio or, when workability
governs, it is the minimum weight of cement required to
satisfactorily place the concrete.
5. Assume air content of 3 to 5%.
Now the remainder will only be the aggregates!
The only remaining decision is to select the
relative proportions of fine and coarse aggregate!
Mix Design
6. The optimum proportions depend on aggregate grading
and particle shape, and they can be finally determined
only in the field.
=> For 6 in. (150 mm) aggregate concrete containing
natural sand and gravel, the percentage of fine
aggregate to total aggregate by absolute volume
may be as low as 21%.
=> With crushed aggregates, the percentage may be in the
range of 25 to 27%.
Construction Practices
In addition to reducing the cement content
of the concrete, some practices are done
to control the temperature rise in massive
concrete structures:
1. Postcooling
2. Precooling
3. Surface Insulation
4. Expansion Reinforcement
1- Postcooling
It is the process of cooling the core of the concrete to
reduce the temperature differential:
• Mostly by circulating a cool liquid (usually water)
through thin-walled pipes embedded in the
concrete, these pipes are usually made from aluminum
or thin steel.
• The rate of heat removal depends on:
• Size of the pipe,
• Volume of fluid circulated
• Temperature of the fluid
Postcooling
Embedded pipe grid
Area cooled by one pipe
Note: Must be spaced in a
manner which achieves the
desired temperature
differential!
Postcooling
Postcooling
• Smaller pipes with colder fluid create a more severe
local condition than larger pipes with a less cold fluid
=> CRACKS!
• First cooling period can take from some days to a
month.
• The concrete temperature will rise again, If the increase
is significant, one or more additional cooling periods will
be necessary!
Postcooling
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Postcooling
The first critical period of
cooling continues until:
• The temperature of the
concrete decreases to
about 30 F (17 C)
below the initial peak
value.
• The concrete has been
cooled to its final stable
temperatureTimes in days
Temp (F)
0
20
40
60
80
100
120
140
0 5 10 15 20 25
Temp (F) (with cooling pipes)
Temp (F) (no cooling pipes)
Postcooling
=> Steel Pipes are
the most effective
at extracting the
heat from the core!
Times in days
Temp (F)
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25
Steel Pipes
Polythene pipe
Adiabatic
Post-cooling
• It is important to emphasize again that significant
internal and surface thermal cracking can result if
post-cooling is improperly designed or performed.
• If properly designed, a post-cooling system can
significantly reduce concrete temperatures and the
amount of time required for cooling.
2- Precooling
Concrete components can be precooled in several ways:
• The batch water can be chilled or ice can be substituted
for part of the batch water.
• Aggregate stockpiles can be shaded.
.
• Fine aggregates can be processed in a classifier using
chilled water.
Precooling
Cooling the coarse aggregate the most important among
these practices, because it provides the greatest potential
for removing heat from the mixture:
1. Sprinkling stockpiles with water to provide for
evaporative cooling.
2. Spraying chilled water on aggregates on slow-moving
transfer belts.
3. Immersing coarse aggregates in tanks of chilled water
4. Use of liquid nitrogen to cool aggregates.
Precooling
Metal cover over fine aggregates Chilled water spray to cool aggregates
Precooling
A new aspect of Precooling is Flushing the mix is
Liquid Nitrogen:
• Costs ~$75 to cool a
truckload of concrete
by 25F
• Local availability is a
big concern!
Precooling
Pre-cooling using Ice :
The most common, yet perhaps least
understood, cooling method is replacing mix water
with ice. This cools concrete in two ways:
• It first lowers the mix-water temperature
• Second it lowers the mix temperature by extracting
heat during the phase change from ice to water
A part of mixing water was introduced into concrete as
crushed ice so that the temperature of in-place fresh
concrete was limited to 6°C. Generally, the lower the
temperature of concrete when it passes from a plastic state
to an elastic state, the less will be the tendency toward
cracking.41
Cooling of water or using ice
3- Surface Insulation
Insulating formwork after placement is another technique
to reduce the temperature gradient:
By limiting the heat loss on the surface we limit the
temperature differential between the surface and the core!
Surface Insulation: The purpose of surface insulation is not
to restrict the temperature rise, but to regulate the rate of
temperature drop so that the stress differences due to steep
temperature gradients between the concrete surface and the
interior are reduced.
Surface Insulation
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Surface Insulation
Typical Insulation Blanket:
4 - Expansion Reinforcement
Expansion Reinforcement can be used to lessen thermal
cracking:
• Designed in addition to loads placed onto the structure.
• Distributes thermal stresses to minimize crack widths.
Impractical for very large pours! Very expensive!
Thermal Control Planning
• The implementation of the thermal
control plan saved money and kept the
project on schedule
• No thermal cracking in concrete was
reported
A severe case of thermal cracking in a concrete footing.
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To minimize thermal stresses:
1. Aggregate with low coefficient of thermal expansion
2. Cement with low C3S and low C3A
3. Insulating forms
4. Cast concrete at night / early morning
5. Use ice instead of water
6. Pre-cooling aggregate and cement
7. Post cooling – embedded pipes
8. Provide joints (for expansion and movement)
9. Less amount of cement
10. Use liquid nitrogen
11. Use thin layers
12. Fly ash and slag can reduce the heat of hydration
Thermal Stresses in Concrete
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Temperature gradient
In the case of casing mass concrete, the internal
temperature rises and drops slowly, while the surface
cools rapidly to ambient temperature.
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Surface contraction due to cooling is restrained by the
hotter interior concrete that doesn’t contract as rapidly as
the surface. This restraint creates tensile stresses that can
crack the surface of the concrete. The width and depth of
cracks depends upon the temperature differential, physical
properties of the concrete, and the reinforcing steel.
Temperature gradient
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The key to reducing thermal cracking is to:
• Reduce the peak temperature
• Control the temperature differential between
the hot core and the cool surface.
•Reducing the peak temperature effects the time it
takes for an element to reach a stable temperature
and effects the temperature differentials.
Excessively high internal concrete temperatures
(>70°C) may also lead to DEF (Delayed Ettringite
Formation) and durability issues.
Temperature gradient
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• A temperature differential limit
attempts to minimize excessive cracking
due to differential volume change. A
limit of 20°C is the industry room.
So, our target is that:
•The core temperature should be ≤ 70°C.
•The temperature of top surface of the
concrete should be ≥ 50°C, Thus the
differential temperature will be < 20°C
Our Target
Concrete insulating a column.
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Conclusion
Mass Concrete is just ordinary normal
concrete but at a huge scale of
practice, where thermal stresses caused by
the hydration of cement, becomes very
considerable.
This behavior requires some special
actions in:
1. The mix design (decreasing the cement)
2. The construction (cooling and
insulation)
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Example 1
A massive concrete structure with 28 days design strength of 17 MPa is required for a
dam to be exposed to freezing and thawing, deicers, and very severe sulfate soils. A slump
of 50 mm is required. The ratio of sand to all in aggregate = 0.27 and the water/ cement
ratio = 0.30 .
The following materials are available:
• Type HS, silica fume modified PC. Relative density of 3.14. Silica fume content of 7%.
• Fly ash: Class F . Relative density of 2.60.
• Slag: Grade 120 . Relative density of 2.90.
• Air entrainer: Synthetic. Dosage of 0.50 g per kg of cementing materials.
• Retarding water Type D. Dosage of 3 g per kg of cementing materials.
• Plasticizer: Type 1. Dosage of 30 g per kg of cementing materials.
• Shrinkage reducer: Dosage of 15 g per kg of cementing materials.
Coarse aggregate: Well-graded 150-mm crushed with an ovendry relative density of
2.60, absorption of 0.5%, and ovendry density of 1650 kg/m3. The laboratory sample has
a moisture content of 1.50%. This aggregate has alkali-silica reactivity in the field.
Fine aggregate: crushed with an ovendry relative density of 2.60 and an absorption of
0.50%. The laboratory sample has a moisture content of 4%. The fineness modulus is
2.60.
Calculate:
The concrete mix proportions.
Adjust mix proportions.
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Example 2
Concrete with a structural design strength of 40 MPa is required for a bridge to be
exposed to freezing and thawing, deicers, and very severe sulfate soils. Acoulomb value
not exceeding 1500 is required to minimize permeability to chlorides.
1.Water reducers
2.Air entrainers
3.Plasticizers are allowed.
4.A shrinkage reducer is requested to keep shrinkage under 300 millionths. Some
structural elements exceed a thickness of 1 meter, requiring control of heat development.
The concrete producer has a standard deviation of 2 Mpa for similar mixes to that
required here. For difficult placement areas, a slump of 200 mm to 250 mm is required.
The following materials are available:
•Type HS, silica fume modified PC. Relative density of 3.14. Silica fume content of 5%.
•Fly ash: Class F . Relative density of 2.60.
•Slag: Grade 120 . Relative density of 2.90.
Coarse aggregate: Well-graded 19-mm crushed with an ovendry relative density of
2.68, absorption of 0.5%, and ovendry density of 1600 kg/m3. The laboratory sample has
a moisture content of 2.0%. This aggregate has alkali-silica reactivity in the field.
Fine aggregate: Natural sand with an ovendry relative density of 2.64 and an absorption
of 0.7%. The laboratory sample has a moisture content of 6%. The fineness modulus is
2.80.
•Air entrainer: Synthetic. Dosage of 0.50 g per kg of cementing materials.
•Retarding water Type D. Dosage of 3 g per kg of cementing materials.
•Plasticizer: Type 1. Dosage of 30 g per kg of cementing materials.
•Shrinkage reducer: Dosage of 15 g per kg of cementing materials.
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