avoiding cold weather paving mistakes · 2019-10-31 · during periods of hot weather when the...

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.

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Information regarding cold weather concreting practices may be found in ACI

306R, Cold Weather Concreting.

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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


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


Durability of Non-Air-Entrained Concrete


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


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.

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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



Use Class F fly ash and/or slag cement if available.



Effect of Retarder

• Lengthened dormancy

• Slowed hydration

• Reduced heat peak

• Extended heat generation

• Increased long-term strength

• Reduced permeability

Supplemental Cementitious

Materials (SCM)



Use Class F fly ash and/or slag cement, if available.



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



Use Class F fly ash and GGBFS if available.



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



Use Class F fly ash and GGBFS if available.



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

http://www.acpa.org/

Saw Cutting





slabs.

Early Entry Joint Sawing

Predicting Stresses in the Slabs





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

mailto:[email protected]

avoiding cold weather paving mistakes · 2019-10-31 · during periods of hot weather when the...

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