Cold Weather Concreting Presentation

3/4/2015
COLD WEATHER
CONCRETE PRACTICES
C R M C A S O UT H E R N M A R KE T I N G
O
SUCCESSFUL
COLD WEATHER
CONCRETING
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ACI DEFINITION OF COLD WEATHER
Cold Weather - A period when, for more than 3 consecutive days, the
following conditions exist:
• The Average daily air temperature is less than 40 ºF (5 ºC), and
• The air temperature is not greater than 50 ºF (10 ºC) for more than
one-half of any 24 hour period.
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U.S. FREEZE / THAW ZONE MAP
Annual average number
of days temperatures
fall below 32 °F (0 °C)
Less than 30
30 to 90
91 to 150
151 to 210
More than 210
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Setting Time of Concrete at Various
Temperatures
Temperature, °F
70
60
50
40
32
Approx. Setting Time, hrs.
6
8
11
14
Concrete Freezes
CONCRETE FREEZES!
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Effect of Early Freezing
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Rule of Thumb
A 10 degree drop in
concrete temperature
will DELAY set time by
approximately
2 – 2 ½ hours
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Increased Contractor
Labor Costs
Extended Concrete Setting Times
Increased Contractor Labor Costs
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OBJECTIVES OF SUCCESSFUL
COLD WEATHER CONCRETING
• Prevent damage to concrete due to freezing at early stages
• Assure that the concrete develops the required strength for safe
removal of forms
• Maintain curing conditions that foster normal strength
development
• Limit rapid temperature changes
• Provide protection consistent with the intended serviceability of
the structure
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What Can We Influence
•
•
•
•
•
Materials
Mix Design
Placement Conditions
Curing conditions and length of time
Protection
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Materials & Mix Design
• GOAL is to ACCELERATE and INCREASE
Maximum Temperature
• Provide concrete with predictable setting
times
• Maintain Air entrainment and workability
• Minimize Plastic and Drying shrinkage
Cold Weather Concreting (Producer)
Speed up the early hydration (heat gain) of
concrete can be obtained by using one or more
of the following:
•
•
•
•
•
•
Additional portland cement
Use Type III Cement
Hot water
Heat Aggregate
Use of calcium chloride
Use of a non-chloride accelerating admixture
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Effect of Cement Content on Setting Time
Performance
Concrete and Ambient Temperature: 50 °F (10 °C)
Initial Time of Set - Hours
14
12
12:35
10
10:40
8
8:55
6
4
2
0
420 (250)
520 (310)
600 (355)
Cement Content, lb/yd³ (kg/m³)
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Rule of Thumb
An increase of 1
sack of cement, will
improve the set time
by about 1 hour
Why?
Lower unit water
content
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Early Age Compressive Strength for
Type I and Type III Cement
80
80
73°F (23°C) Cured Concrete
Compressive Strength, % of 28 Day,
73°F (23°C) Cured Concrete
Compressive Strength, % of 28 Day,
60
60
40
40
MixedMixed
and and
cured
at(13°C)
55°F (13°C)
cured at 55°F
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0
20
0
1
1
3
3
7
7
Test, Days
Age of Age
Test,ofDays
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Early Age Compressive Strength for
Compressive Strength For
Type I and Early
TypeAge
III Cement
Type I and Type III Cement
Compressive Strength, % of 28 Day,
73°F (23°C) Cured Concrete
80
60
40
Mixed and
cured at 40°F (4°C)
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III
Type
0
1
Type I
3
7
Age of Test, Days
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Chemical Admixtures for Concrete
ACI 212.3
“An accelerating admixture is a material
added to concrete for the purpose of
reducing the time of setting and
accelerating early strength development.”
“Accelerating admixtures are useful for
modifying the properties of concrete,
particularly in cold weather, to:”
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Chemical Admixtures for Concrete
ACI 212.3
• Expedite the start of finishing operations
• Reduce the time required for proper curing
and protection
• Increase the rate of early strength
development to permit earlier removal of
forms and earlier opening of construction
for service
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Accelerating Admixtures
Accelerated setting time characteristics
•
•
•
•
Earlier finishing of slabs
Increased early and ultimate strength
Reduced protection time in cold weather
Earlier stripping and reuse of forms
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Classes of Accelerating Admixtures
Calcium Chloride
Accelerating admixtures containing calcium chloride
Non-chloride accelerating admixtures
Non-chloride accelerating admixtures for use in
concrete placed in sub-freezing temperatures
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Placing Concrete on Ground
• Concrete should never be placed on a
frozen subgrade
• On a frozen subgrade, heat will migrate
rapidly away from the bottom of the
concrete retarding setting time
• Thaw the subgrade not just the surface
• Reschedule for a warmer day!!
Plastic Concrete Problems in Cold Weather
Concrete bleeding
Bleed water can freeze on surface
Bleed water capillary channels can freeze within
the concrete
Cold subgrades
Rapid migration of heat from concrete will affect
setting time
Uneven settlement may occur, causing cracking
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Finishing Problems in Cold Weather
Surface crusting which can cause:
Blisters
Delamination
Scaling (premature/over finishing)
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Cold Weather Concreting Above Ground
All snow, ice and frost must be removed from
forms, reinforcement and other embedments
Must protect from all sides
Use of Type III cement, additional cement and/or
non-chloride accelerators will reduce the length
of protection period, HOWEVER, know the effects
each will have on the plastic and hardened
properties
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Table 3.1 – Recommended Concrete
Temperatures
<12 in.
12-36 in.
Air Temperature
Minimum Concrete temperature as placed and maintained
1
55 °F
50 °F
Minimum concrete temperature as mixed for indicated air temperature
2
Above 30 °F
60 °F
55 °F
3
0 to 30 °F
65 °F
60 °F
4
Below 0 °F
70 °F
65 °F
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DURATION OF RECOMMENDED
PROTECTION FOR % OF
STANDARD-CURED 28-DAY
STRENGTH
Percentage of
Standard-cured
28-day strength
50
65
85
95
At 50 °F (10 °C), days
Type of Cement
I
II
III
6
11
21
29
9
14
28
35
3
5
16
26
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Concrete Curing
8.1 – Introduction ― “Newly placed concrete
must be protected from drying so that adequate
hydration can occur. Normally, measures must
be taken to prevent evaporation of moisture
from concrete. During cold weather, when the
air temperature is below 50 ºF(10 ºC),
atmospheric conditions in most areas will not
cause excessive drying.”
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Concrete Curing
8.2 - Curing during the protection period
When dry heating is used, the concrete should
be covered with an impervious material or
curing compound.
Water curing is not recommended.
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Heating
Things to watch
Carbonation CO2
Vent or have heat source from outside
Watch Carbonation from other trades equipment
Watch blowing hot air across a fresh slab
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Concrete Curing
8.3 - Curing following the protection
period “…if a curing compound is applied
during the first period of above-freezing
temperature after protection is removed,
the need to conduct further curing
operations if the temperature should rise
above 50 ºF(10 ºC) is eliminated.”
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Effect of Curing on Compressive
Effect of Curing on Compressive
StrengthStrength
Compressive strength, percent of 28-day
moist-cured concrete
150
entire time
Moist-cured
125
In air after 7 days
100
In air after 3 days
75
In air entire time
50
25
0
3 7
28
90
180
Age, Days
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Testing
Cylinders should be protected and stored in
protective area between 60°F and 80°F for the first
24 hours.
Record field storage conditions with a high / low
thermometer.
Field cure cylinders for form removal should be
representative of the structure and not used for
quality assurance.
Recommend using maturity meter for in place
strength data.
Standard 28-Day Compressive Strength
Effect of Freezing on 28-Day
Compressive Strength
110
100
80
60
40
20
0
3
6
9
12
18
24
Age at which specimens were frozen, in hours
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Sample Cold Weather Spec.
“Work on your project named here shall
conform to all requirements of ACI
306.1,Standard Specification for Cold Weather
Concreting, published by the American
Concrete Institute, Detroit, Michigan, except
as modified by the requirements of these
Contract Documents.”
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Successful Cold Weather Concreting
Practices
The entire team must:
Plan ahead
Be prepared
Be concerned
Schedule work
Instruct and inspect
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Conclusions and Recommendations
Quality concrete can be successfully placed at
low ambient temperatures
ACI 306R Guide, ACI 306 specification and ACI
308 Guide should be followed
More cement, Type III Cement, heat and/or
accelerating admixtures may all be used to
accelerate concrete setting time and increase
early strength development
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Resources
“Cold Weather Concreting”
Reported by
ACI Committee 306
“Standard Specification for Cold Weather Concreting”
Reported by
ACI Committee 306
“Standard Practice for Curing Concrete”
Reported by
ACI Committee 308
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