avoiding cold weather paving mistakes · 2019-10-31 · during periods of hot weather when the...
TRANSCRIPT
Avoiding Cold Weather Paving
Mistakes
Gary L. Mitchell, P.E.
Vice President—Airports & Pavement Technology
American Concrete Pavement Association
Cold Weather – ACI 306 working
definition
Average daily temperature <5ºC (40ºF) for 3 successive days
Stays <10ºC (50ºF) for more than ½ of any 24h period
Materials and equipment for adequate curing must be available
Cold weather paving plan – part of the QC plan
a. Cold weather. Unless authorized in writing by the Engineer, mixing and concreting operations
shall be discontinued when a descending air temperature in the shade and away from artificial heat
reaches 40°F (4°C) and shall not be resumed until an ascending air temperature in the shade and away
from artificial heat reaches 35°F (2°C).
The aggregate shall be free of ice, snow, and frozen lumps before entering the mixer. The temperature
of the mixed concrete shall not be less than 50°F (10°C) at the time of placement. Concrete shall not be
placed on frozen material nor shall frozen aggregates be used in the concrete.
When concreting is authorized during cold weather, water and/or the aggregates may be heated to not
more than 150°F (66°C). The apparatus used shall heat the mass uniformly and shall be arranged to
preclude the possible occurrence of overheated areas which might be detrimental to the materials.
************************************************************************************
Information regarding cold weather concreting practices may be found in ACI
306R, Cold Weather Concreting.
************************************************************************************
The Concerns
Little strength gain in cold weather
Reduced (or eliminated) use of SCMs
Keep concrete temperature above freezing
Continued hydration
Control cracking through joint placement
Chemical admixtures?
Restrained shrinkage with little strength gain
Retain moisture through proper curing
Potential for damage from early freezing
Damage potential is reduced as water pore saturation
levels decrease to <90%
f’c > 500 psi, typically after 24 hours or less for normal concrete
Early-Age Strength
Source: Portland Cement Association
Effect of Temperature on
Concrete Strength Development – Pt 1
Source: Portland Cement Association
Effect of Freezing Fresh Concrete
Retards hydration, strength gain and hardening
Up to 50% reduction of ultimate strength can occur if frozen —
Within a few hours
Before reaching a strength of 500 psi
Frozen only once at an early age —
With curing nearly all strength can be restored
Less resistance to weathering
More permeable
Air-entrained concrete –
Less susceptible to damage
Source: Portland Cement Association
Durability of Non-Air-Entrained Concrete
Source: Portland Cement Association
Approaches for Mitigating
Cold-Weather Effects
Methods to accelerate strength gain (reduce
temporary protection time):
• Mixtures that gain strength more quickly
• Use high-early strength cement (e.g., Type III)
• Use additional cement (100 – 200 lb/c.y.)
• Use of chemical admixtures
• Shrinkage and durability concerns?
• Heated enclosures
Methods of Protection
• Insulation
• Wind breaks and curing
Component Material Effects on
Batched PCC Temperature
Aggregate temperature
Varies with weather and type of storage
Contains frozen lumps and ice when temp < freezing
Must be thawed prior to mixing
Mixing water temperature
Easiest and most practical to heat
Can store ~5 times the heat of cement and aggregate
Effect of Temperature of Materials on
Concrete Temperatures
0.22(TaMa + TcMc) + TwMw + TwaMwa
0.22(Ma + Mc) + Mw + Mwa
T =
T = temperature of the freshly mixed concrete, °C (°F)
Ta, Tc, Tw, and Twa = temperature in °C (°F) of
aggregates, cement, added mixing water, and free
water on aggregates, respectively
Ma, Mc, Mw, and Mwa = mass, kg (lb), of aggregates,
cementing materials, added mixing water, and free
water on aggregates, respectively
Source: Portland Cement Association
Cold Weather Paving Operations
ACI 306:
Protect concrete from
freezing if air temperature
is expected to fall below
4°C (40°F) in any of the
three days following
paving
Insulating blankets
Retaining Heat of Hydration
Insulation blankets
Retain heat of hydration
efficiently
Keep concrete warm as
possible
Thermometer reading tell
if covering is adequate
More important as heat of
hydration slows
Cold Weather Paving Operations
Insulating blankets:
Removal of blankets can cause thermal shock
Blankets should be removed after concrete
temperature has cooled sufficiently
Paving Under High Temperatures
What are key
differences?
What changes in the
mixture?
How do I minimize the
risks of problems?
Hot Weather “ACI 306 Working
Definition”
No absolute threshold value for “hot weather” placement.b. Hot weather. During periods of hot weather when the maximum daily air temperature exceeds 85°F
(30°C), the following precautions shall be taken.
The forms and/or the underlying surface shall be sprinkled with water immediately before placing the
concrete. The concrete shall be placed at the coolest temperature practicable, and in no case shall the
temperature of the concrete when placed exceed 90°F (32°C). The aggregates and/or mixing water shall be
cooled as necessary to maintain the concrete temperature at or not more than the specified maximum.
The finished surfaces of the newly laid pavement shall be kept damp by applying a water-fog or mist with
approved spraying equipment until the pavement is covered by the curing medium. When necessary, wind
screens shall be provided to protect the concrete from an evaporation rate in excess of 0.2 psf (0.98 kg/m2 per
hour) per hour. When conditions are such that problems with plastic cracking can be expected, and
particularly if any plastic cracking begins to occur, the Contractor shall immediately take such additional
measures as necessary to protect the concrete surface. Such measures shall consist of wind screens, more
effective fog sprays, and similar measures commencing immediately behind the paver. If these measures are
not effective in preventing plastic cracking, paving operations shall be immediately stopped.
************************************************************************************
Information regarding hot weather concreting practices may be found in ACI 305R, Hot
Weather Concreting.
************************************************************************************
Hot weather concrete placement
Must focus on 2 primary issues:
Mix temperature (as batched and during placement).
Ambient conditions.
The biggest question for the contractor (and the owner) is: What can I control and how?
Existing Specifications
Specifications control upper limits typically
Mix temperatures 85 – 90 degrees
Ambient temperatures 90 degrees – precaution
Rarely an upper limit on ambient conditions under
which concrete can be placed.
The new AASHTO Interim Mechanistic-Empirical
Pavement Design Guide (M-E PDG) may
influence this in the future (curling and warping).
As Placed Maximums
Attempt to Control:
Strength
Durability
Plastic shrinkage cracks
Thermal crack
Drying shrinkage
Much too Complex – If strength, curing, air are
adequate – durability should follow
Why Limit Concrete Temperature?
Early stiffing of the mix is likely.
Placement issues
Poor consolidation
Potential difficulties in finishing
and texturing.
Impact on strength.
The reason: early stages of
forming hydration products.
The Stages of Hydration and Why
They Matter
Hydration reactions are chemical.
Heat is a catalyst
Water – cement reaction is sped up
Higher placement (or mix) temperature –
reactions will happen faster
Concrete Characteristics
Construction
Implications
of Stages
High heat Plastic, workable Significant heat Stress exceeds strength Strength gain
Rapid cooling No heat generation Setting, hardening, strength gain Reduced permeability
Stress development
Mix adequately Transport, place, finish Begin thorough curing ASAP Saw joints Insulate slab
Protect cure compound
Effects and Control
Potential problems include:
Rapid slump loss (during construction).
Increased water demand (during mixing and construction).
Reduced air content (during and post construction).
Premature stiffening (during construction).
Plastic shrinkage cracking (post construction).
Thermal cracking (post construction).
Can’t control the weather, BUT, you can compensate for
higher temps.
Approach to the “Problem”
Two different, but complimentary,
approaches:
Mix control.
Placement control.
Concepts overlap (example: wetting the
stockpiles).
High temps may require a combination of
“fixes”.
Laboratory/Field Mixture Design
and Proportioning
Evaluate concrete mixes at anticipated
construction temperatures.
Anticipate issues and test for them.
Consider:
SCM’s,
retarders and
methods to reduce concrete
temperature.
Check for compatibility problems.
Anticipate the unexpected.
Hot Weather Options – Concrete
Mix
Don’t exceed the maximum w/cm ratio.
Use a retarding admixture.
Use Class F fly ash and/or slag cement if available.
Reduce concrete temperature.
Increase air entraining admixture dosage.
Retarding Admixtures
Don’t exceed the maximum w/cm ratio.
Use a retarding admixture.
Use Class F fly ash and/or slag cement if available.
Reduce concrete temperature.
Increase air entraining admixture dosage.
Effect of Retarder
• Lengthened dormancy
• Slowed hydration
• Reduced heat peak
• Extended heat generation
• Increased long-term strength
• Reduced permeability
Supplemental Cementitious
Materials (SCM)
Don’t exceed the maximum w/cm ratio.
Use a retarding admixture.
Use Class F fly ash and/or slag cement, if available.
Reduce concrete temperature.
Increase air entraining admixture dosage.
ACI 226.1 Blast Furnace Slag
Significant retardation at low temperatures
Initial set increased 1 to 11/2 hours at 73°F
Little change in initial set above 85°FTemperature concrete
cured has a great effect on strength, especially at early
ages
At normal or low temperatures, strength reduction at early
ages – higher at 28 days
At higher temperatures, early strength is increased – lower
at 28 days
ACI 226.3 Fly Ash
Generally causes an increase in setting time
Class F generally retards
Class C may reduce, increase, or have no effect
Some Class C fly ashes may contribute to early
temperature rise, especially in hot weather
Reduce Concrete Mix Temperature
Don’t exceed the maximum w/cm ratio.
Use a retarding admixture.
Use Class F fly ash and GGBFS if available.
Reduce concrete temperature.
Increase air entraining admixture dosage.
Effect of Curing Temperature on
Strength
This means that 4000 lbs design concrete, cured at
83°F, would only have cylinder breaks reach 3800 psi
in the field
1° F increase in
curing temperature(above standard lab conditions) in the
first 24 hours
0.5% decrease in 28-day strength
Results in
Luther, Mark, etal, Effect of Temperature on Cylinder Strength, 2009
Effect of Change in Material
Temperature on Concrete
Temperature
Change in Temperatures of Individual Components of a
Typical Concrete Mix to Affect a 1o F Change in Concrete
Temperature
Cement Water Aggregates (coarse and fine)
8o F (4o C) 4o F (2o C) 2o F (1o C)
ACI 305
Methods to Reduce the Temperature of
the Concrete – Crushed Ice
Maximum replacement 75%
Must account for ice as replacement for mix water
Chillers and ice makers for Batch Plant
Other Options to Reduce Concrete
Mix Temperature
▪ Chilled mix water
▪ Fog the aggregate stockpiles
▪ moisture control
▪ evaporative cooling.
▪ For smaller plants, stockpiles or bunkers in the
shade.
▪ Others?
Air Entraining Admixtures
Don’t exceed the maximum w/cm ratio.
Use a retarding admixture.
Use Class F fly ash and GGBFS if available.
Reduce concrete temperature.
Increase air entraining admixture dosage.
Hot Weather Options - Construction
Early and adequate curing is critical.
Consider nighttime paving operations.
Carefully evaluate the sawing window.
Use HIPERPAV to estimate stress build-up in the
slabs.
Curing is Critical
Apply curing as soon as possible.
Apply adequate amount.
Apply liberally to edges
Fogging
Night Paving
Early and adequate
curing is critical.
Consider nighttime
paving operations.
Carefully evaluate the
sawing window.
Use HIPERPAV to
estimate stress build-up
in the slabs.
Cooler Temperatures
Evaporation Chart
IMCP Manual, 2007
The goal is to reduce the
rate of evaporation.
www.acpa.org
Resources: ACPA Web apps library -Construction/Analysis Apps; Evaporation Rate Calculator
Saw Cutting
Early and adequate curing is critical.
Consider nighttime paving operations.
Carefully evaluate the sawing window.
Use HIPERPAV to estimate stress build-up in the
slabs.
Early Entry Joint Sawing
Predicting Stresses in the Slabs
Early and adequate curing is critical.
Consider nighttime paving operations.
Carefully evaluate the sawing window.
Use HIPERPAV to estimate stress build-up in the
slabs.
Crack Prediction with HIPERPAV
Visit FHWA website to download
Many factors considered in the program.
Input parameters are job specific.
www.hiperpav.com
Problems
▪ Can occur at any time;
▪ mixing,
▪ hauling,
▪ during placement, or;
▪ post-paving.
▪ Can range from minimal disruptions to REAL problems.
▪ There is almost always a solution available……figure it out BEFORE it is a problem.
Hot Weather Concrete Problems
Can Take Other Forms
Questions or Discussion?
Gary L. Mitchell, P.E.Vice President - Airports and Pavement Technology,
American Concrete Pavement Association
Email: [email protected]
Phone: 704-766-0851