Keys to Success for Hot and Cold Weather Concrete

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Keys to Success for Hot and Cold Weather Concrete
Canadian Concrete Expo ▪ Toronto, ON ▪ February 8, 2018
Laura Mammoliti, Ph.D., P.Eng. – Quality Control Manager, Hanson Ready Mix
Mike Stanzel – Technical Services Manager, Lehigh Cement Co.

 Portland cements are fine powders composed primarily of calcium
silicates and aluminates
 These cements set and harden by chemically reacting with water
through a process called hydration
 This reaction produces a phase (C-S-H) which forms on the surface of
each particle that gradually grows, spreads, and interlocks with
surrounding surfaces
 SCM’s, like slag, react in the presence of calcium hydroxide which also
forms during hydration, and convert into more C-S-H
02/08/2018
Keys to Success for Hot and Cold Weather Concrete – Mike Stanzel & Laura Mammoliti
Background
Cement Chemistry

 Hydration continues indefinitely as long as moisture and temperature
conditions are favourable with curing, and results in progressive
stiffening, hardening, and strength development
 Internally, strength development depends on cement chemistry and
fineness, w/c ratio, SCM proportions, water-reducing admixtures,
accelerators or retarders, and other concrete materials
 Heat accumulation in a concrete element is a function of the initial
concrete temperature, heat of reaction, ambient conditions, and
insulation or cooling effects
Background
Cement Chemistry
02/08/2018

 TIME and TEMPERATURE are the two biggest external factors in
concrete performance
 Proper preparations for hot- or cold-weather concreting, as well as for
weather events (rain, snow, winds, etc.) should be made well in
advance
 Temperature will affect the rate of chemical reaction:
– The reaction rate doubles for every 10 degree Celsius increase in
temperature
– The reaction rate halves for every 10 degree Celsius decrease in
temperature
Background
Concrete External Influences
02/08/2018

Background
Curing temperature effects on concrete strength gain
0
5
10
15
20
25
30
35
40
45
50
1 10 100
Strength(MPa)
Time (days)
32-C2 (25% Slag) Curing Temperature Effects
13C 23C 33C
02/08/2018

Background
The Golden Rule
Concrete shares a unique relationship with the human body in that both
operate at peak efficiency under the same conditions. If it’s too hot or too
cold for you outside… then you can be assured that the concrete
deserves the same consideration.
02/08/2018

 Spring: Rainy or dry, moderate
daytime temperatures, cool
nights, unpredictable.
 Summer: Hot with temperatures
reaching over 30oC (86oF), plus
humidity impact.
 Autumn: Rainy or dry, moderate
daytime temperatures, cool
nights, ground still warm.
 Winter: Cold and snowy with
temperatures dropping below
-30oC (-22oF) and harsh winds.
Source: Canadian Climate Normals 1971-2000 (Ottawa City)
Why do we care?
Ontario Climate Conditions
02/08/2018

Hot Weather
 ACI 305R and CSA A23.1
 Defined as any combination of:
– High ambient temperature (> 25oC)
– High concrete temperature
– Low relative humidity
– High wind velocity
– High solar radiation
 … that tends to impair the quality of
freshly mixed or hardened concrete by
accelerating moisture loss, rate of
cement hydration, or otherwise causing
detrimental results
Hot and Cold Weather Concrete
Definitions
Cold Weather
 ACI 306R and CSA A23.1
 Precautions need to be taken if
– Ambient temperature is expected to drop
below 5oC,
– If it will not be above 10oC for at least half of
a day
– If there is a probability that the temperature
will fall below 5oC within 24 hours of placing
the concrete
02/08/2018

Hot Weather
 Hot Weather Concreting concerns
include:
– Rapid setting times
– Surface evapouration and plastic cracking
– Rapid slump loss
– Reduced late age strengths
– Thermal cracking
– AVS Stability
Concerns
02/08/2018

Concerns
Cold Weather
 Cold Weather Concreting concerns
include:
– Slower setting times
– Reduced early age strengths and longer time
to form removal
– Reaction stalls around -10oC
– Concrete freezing under 3.5 MPa
– Freeze-thaw durability
– Thermal shock with form removal
02/08/2018
0
5
10
15
20
25
30
0 2 4 6 8
Strength(MPa)
Time (days)
Strength Progression
32-C2 mix at 10C 32-C2 mix at 22C

Concerns
Cold Weather
include:
to form removal
02/08/2018
KYUNG-TAEKKOH, et al, 2013
Compressive Strength
Chloride penetration
Compressive Strength
Freeze-thaw Resistance

Concerns
Cold Weather
include:
to form removal
02/08/2018

Hot Weather
 Reduce concrete temperatures
– Cool water, use of ice, keep aggregates at
SSD to promote evapourative cooling and
avoid mix water absorption
 Use of higher volume of SCM’s if
feasible to reduce heat generation
 Use of retarder, slump life extenders,
hydration stabilizers
 Ensure sufficient base water content to
protect against evapouration
What can the concrete producer do?
Material Control and Mix Designs
Cold Weather
 Heating of aggregates and water to
maintain healthy concrete temperatures
 Design specialized mixes with higher
cement contents and accelerators to
increase heat generation from hydration
reaction and to develop strength quicker
 Reduced SCM content if feasible, but
keep in mind durability specifications
 Use accelerator (it’s not an anti-freezing
agent!)
– Use non-chloride accelerator for pre-stressed
or reinforced concrete where corrosion is a
concern (i.e. WinterMix)
– 1% of calcium chloride will generally cut
reaction times by ½ and 2% will cut reaction
times to about ⅓ (i.e. WinterSet)
02/08/2018

Hot Weather
 Inform the supplier of your placing
schedule and needs
 Discuss needs for retarders, concrete
temperatures, ice, SCM’s, fiber, etc.
What can the contractor do?
Communication
Cold Weather
 Inform the supplier of your placing
schedule and needs
 Discuss needs for accelerators, concrete
temperatures, SCM’s, fiber, etc.
02/08/2018

Hot Weather
 Time is limited, so avoid delays on the
project and for delivery
 Schedule for a cooler day, or during a
cooler part of the day, if possible
 Ensure sufficient labour to minimise
time required to place and finish
concrete
 Order concrete with sufficient slump to
allow for rapid placement
Planning
Cold Weather
 In addition to the minimum curing period,
exterior concrete should be at least 25
MPa and have at least 30 days of air
drying before freeze-thaw exposure!
 Ensure availability of supplemental heat
to ensure forms, subgrades, reinforcing
steel, etc. are maintained above 5oC prior
to placement
 Schedule for a warmer day if possible
 Ensure sufficient labour as worker
efficiency is impacted by weather
 Order concrete with lowest practical
(water) slump
02/08/2018

Hot Weather
 Ultimate goal is to prevent moisture
loss to the ambient environment
– Dampen subgrades, steel reinforcement,
and formwork prior to placement but do not
place concrete over standing water
– Erect temporary wind breaks
– Erect sun shades to reduce surface
temperatures
– Protect concrete surface during placing
with plastic sheeting or evapouration
retarders to maintain initial moisture
– Consider fog misting area above the
concrete to raise humidity
 Provide appropriate curing as soon as
finishing is completed
Site Preparation
02/08/2018

Site Preparation
Cold Weather
 Remove all ice/snow from base/formwork
 Never pour on frozen base
 Protect concrete while placed,
consolidated, and finished
– Windbreaks, enclosures, supplementary
heat, thermal curing blankets
– Ensure combustion heaters are vented and
supervised to prevent carbonic acid attack of
concrete surface
– Beware of surface evapouration and wind
chill effect
 Limit rapid temperature changes (thermal
shock) by gradual removal of formwork
(CSA A23.1 Table 20)
 Provide appropriate curing as soon as
finishing is completed
02/08/2018

Hot and Cold Weather Precautions
 Verify proper facilities exist for testing and storage area (responsibility of contractor to
supply as per CSA A23.1 Clause 4.4.6.5.1.3 “Storage and curing facilities”)
 Ensure concrete testing is performed by ACI/CCIL certified technicians
 Protect concrete samples from moisture loss and weather conditions
 Specimens are stored in a controlled environment (20 ± 5oC)
Testing
02/08/2018

 Spring and Autumn weather can be tricky
 Weather conditions can be rapidly changing and both hot weather and
cold weather techniques may be needed
 Differences between air and subgrade conditions and temperatures
can result in differential setting of concrete
– Consider use of a part dose of accelerator in cases of differential set caused
by cool subgrade but warm, dry air to set the concrete before surface
crusting occurs
 Rapid drying conditions can be quite prevalent
 Communication and feedback on performance is critical
Transition Months
02/08/2018

 Prevent loss of moisture from the concrete so that hydration continues
– Strength development stops when the internal moisture (R.H.) of the
concrete drops to about 80%
– Low w/cm concrete (under 0.40) may not have sufficient internal moisture
and may require a wet curing method
 Maintain a favourable concrete temperature for hydration to occur
 Maintain proper conditions for a time period to ensure durability
properties develop
– Curing should begin immediately after finishing when concrete is sufficiently
set and strong enough that the surface won’t be damaged
Curing and Protection
Objectives
02/08/2018

 Does curing really matter?
Objectives
02/08/2018

 Does curing really matter?
Objectives
02/08/2018
Salt scaling pucks (7d/7d cure/dry and 1d/13d cure/dry, with 14 FT cycles)

Curing requirements dictated by:
 Expected exposure class and intended serviceability of concrete
 Concrete mix design (HVSCM mixes step up a level)
 Whether or not you want a phone call next year…
Requirements
02/08/2018

Moisture loss prevention methods
 Curing compounds
– Form a membrane over the surface to prevent
moisture evapouration
– Apply as per manufacturer’s suggestion – usually
two layers at right angles and uniform coverage
– May affect the bond of some floor coverings
 Plastic sheeting
– Ensure complete coverage of surface and sealed
edges
– Do not place directly on concrete surface (may
cause discolouration – green house effect)
– White for hot weather (reflect heat), black for cool
weather (absorb heat)
 Curing in formwork
– Effective for vertical elements (walls and columns)
– Schedule formwork removal accordingly
Types
Wet curing methods
 Water ponding
– Flooding the surface to provide moisture and uniform
curing temperature
– Water should be within 12oC of concrete
temperature to avoid thermal shock
 Water sprinkling
– Spraying water over the concrete surface
– Must ensure uniform coverage, drainage of run-off,
etc.
 Wet burlap
– Pre-soaked burlap (24 hours) either covered with
plastic or routinely re-wetted
– Burlap should be pre-rinsed to avoid potential
surface staining
02/08/2018

 Sealers on their own are not a curing replacement
 Follow manufacturer’s recommended application
 Surface should be cleaned of debris before application
 Generally, should not be applied until 1-2 months of drying
 Do not apply too thickly
– Two thin layers usually sufficient
 Do not apply under harsh weather conditions
– Excessive temperature (hot or cold), high humidity, etc.
 Use only breathable sealers on exterior concrete
– Unbreathable sealers may trap moisture and result in freeze-thaw damage
 Need to be routinely re-applied every 1 to 3 years (water beading test)
Concrete Sealers
02/08/2018

Troubleshooting
Factors Affecting Plastic Concrete *per cubic meter – typical results
• + 5 L (1 gal) water…………………………….
• + 5 L (1 gal) water…………………………….
• + 5 L (1 gal) water…………………………….
• + 5 L (1 gal) water…………………………….
• + 5 L (1 gal) water…………………………….
• + 5 L (1 gal) water…………………………….
• + 1% extra air………………………………….
• + 1% extra air………………………………….
• + 1% extra air………………………………….
• + 1% extra air………………………………....
• Overmixing/Delays……………………………
• + 10oC……………………………………..…..
• + 10oC…………………………………..……..
• + 10oC……………………………………..…..
• SCMs (Slag/Flyash)………………………….
• Admixtures……………………………………..
≈ + 25mm (1”) slump
≈ - 5% strength
≈ + 1% extra air (-1% if already over-slumped)
≈ Increased chance of segregation and cracking
≈ + 0.004% shrinkage (a 10% increase over normal)
≈ + 30 minutes set-time
≈ - 1% density
≈ - 5% strength
≈ + 15mm (1/2”) slump
≈ Increased workability, creaminess, and stickiness
≈ Temperature rise, Strength loss, Slump loss, AVS issues
≈ - 25mm (1”) slump
≈ - 2 hours final set
≈ - 10% final strength
≈ Increased workability, slower but higher strengths, reduced
permeability, increased set times, lower heat generation
≈ Various effects, need to be checked for compatibility
02/08/2018

Troubleshooting
Lehigh Hanson i.check app
02/08/2018

Concrete is the most resilient, flexible, sustainable, and widely-used
building material available, but…
 We need to have a basic understanding of the fundamental
mechanisms to understand what our product is doing
 We need to respect our product and treat it accordingly
 A little extra planning, forethought, and communication between parties
can make all the difference for a successful project
 The quality of your product depends on the quality of the whole
concreting system, from the specification, to raw material selection
and mix design, to batching and delivery, through to proper
construction practices and curing
Summary
02/08/2018

Questions?
Thanks!
“As a part of its commitment to Total Customer Satisfaction,
Lehigh Hanson offers free technical service to its customers.
We have made every effort to ensure the accuracy of the
information provided to you. While this advice is intended to
add value to your business, any actions taken must be the
responsibility of the customer. Customer acknowledges this
and agrees to accept Lehigh Hanson’s technical advice at its
own risk.
While the information contained within is mostly accurate,
appropriate simplifications may have been made to facilitate
conceptual understanding.”
02/08/2018

Keys to Success for Hot and Cold Weather Concrete

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