What causes freeze-thaw damage in concrete?

Water in the concrete freezes and expands. FHWA gives the expansion as approximately 9 percent. The expansion builds up hydraulic pressure in the cement paste. FHWA states that concrete suffers this damage only when the capillaries in the paste are more than 91.7 percent full.

How does air entrainment protect concrete from freeze-thaw damage?

The tiny air voids act as pressure relief sites, and each void protects the paste around it. FHWA gives the usual target as 6 ± 1 percent total air with a spacing factor of 0.20 mm (0.008 inch) or less. The spacing factor is the maximum distance that water has to travel to reach a void.

How is the freeze-thaw resistance of concrete tested?

The standard laboratory test is ASTM C666 (AASHTO T 161). It cycles concrete prisms between 4 °C and -18 °C, and each cycle lasts 2 to 5 hours. The test tracks the relative dynamic modulus and usually runs for 300 cycles. ASTM C672 tests the surface scaling resistance with a deicing solution, and ASTM C457 measures the air voids in hardened concrete.

How can freeze-thaw damage be prevented?

Prevention starts in the mix design and in construction. For airfield pavement, FAA Item P-501 sets an air content for the exposure level and a water to cementitious ratio of 0.38 to 0.45. It also limits the aggregate to sources without D-cracking and requires a 7-day cure that protects new concrete from freezing. Joint sealing and drainage then keep water out of the slab.

What is a freeze-thaw resistant aggregate?

It is a coarse aggregate that does not cause D-cracking when the concrete freezes. The FAA P-501 specification accepts an aggregate with a 20-year service record without D-cracking. It also accepts one with a durability factor of 95 or more in ASTM C666. The FAA lists crushed granite, quartzite and basalt among the aggregates that meet the D-cracking test requirement.

Does freeze-thaw damage asphalt pavement?

Yes, but it acts differently. FAA AC 150/5380-6C says that severe freeze-thaw cycles make pothole development worse in northern climates. Asphalt has no air-void protection like air-entrained concrete. FAA Item P-401 asks for freeze-thaw conditioning in the tensile strength ratio test in areas subject to freeze-thaw.

How much air must the concrete of an airfield pavement contain?

The FAA does not give one value. In Item P-501 of AC 150/5370-10H the engineer chooses the air content for the exposure level and the largest aggregate size. For 3/4 inch (19.0 mm) aggregate the table gives 3.5 percent for mild exposure. It gives 5.0 percent for moderate exposure and 6.0 percent for severe exposure.

How is D-cracking different from scaling?

D-cracking is a pattern of cracks that runs parallel to a joint or crack, with dark colour around the fine cracks. Scaling is the loss of the slab surface. On one slab the PAVER manual counts only the D-cracking, not the scaling.

Freeze-Thaw Damage in Concrete Pavements

Freeze-thaw damage is the deterioration that repeated freezing and thawing of water causes in concrete. It shows on airfield slabs as scaling, D-cracking and popouts.

How freezing damages concrete

Cement paste holds water in gel pores and in larger capillary pores. Water in the gel pores cannot freeze above -78 °C. The capillary pores are much larger. Water expands by approximately 9 percent when it freezes. Three effects turn that expansion into damage.

Hydraulic pressure. As ice forms, unfrozen water is pushed through the capillary pores, away from the freezing site. The pressure rises when freezing is faster, when the flow path is longer and when the paste is less permeable.
Osmotic pressure. Ice rejects dissolved ions, so the water that remains becomes more concentrated. This draws more water toward the ice and adds pressure.
Critical saturation. Concrete suffers this damage only when the capillaries in the paste are more than 91.7 percent full. Partly filled pores can take the expansion. Critical saturation can occur locally within the paste.

Saturation is therefore the first factor. Concrete that stays dry does not suffer. FAA AC 150/5320-6G lists a high number of freeze-thaw cycles among the causes of concrete durability problems. Poor mix design, poor construction and nondurable aggregate are others.

How air entrainment limits it

Concrete holds two kinds of air. Entrapped air is 1 to 3 percent of the volume, in irregular voids of 1 to 10 mm or more. Entrained air is a system of much smaller bubbles with a typical average size of about 0.10 mm. An air-entraining admixture creates these bubbles.

Entrained voids act as pressure relief sites. Water moves to a void, and ice can form there without a build-up of pressure, if the concrete is not completely saturated. Each void protects a shell of paste around it. The spacing factor is an empirical value for the maximum distance that water has to travel in the paste to reach a void.

FHWA gives these rules of thumb:

6 ± 1 percent total air
a specific surface of at least 24 mm²/mm³
a spacing factor of 0.20 mm (0.008 inch) or less

FHWA adds that only about 1.3 percent total air is needed to take the expansion of fully saturated concrete. It also states that air volume alone is not enough, because smaller voids protect better. These limits may not be adequate for some synthetic admixtures.

The FHWA course manual gives the usual ranges that ASTM C457 tests find in air-entrained concrete. The paste-air ratio is 4 to 10, the specific surface is 25 to 45 mm⁻¹ and the spacing factor is 0.1 to 0.2 mm. The spacing factor is generally the most significant indicator. The maximum for moderate exposure is usually 0.20 mm. Mild exposure may allow larger values, and severe exposure may need smaller ones.

Air content that FAA Item P-501 recommends

For airfield pavement, Item P-501 of FAA AC 150/5370-10H lets the engineer choose the air content. The choice depends on the exposure level and the largest aggregate size. The table gives the recommended values in percent.

Exposure level	2 in (50 mm)	1-1/2 in (37.5 mm)	1 in (25.0 mm)	3/4 in (19.0 mm)	1/2 in (12.5 mm)
Mild	2.0	2.5	3.0	3.5	4.0
Moderate	4.0	4.5	4.5	5.0	5.5
Severe	5.0	5.5	6.0	6.0	7.0

Mild exposure is a pavement that freezing or deicing agents do not reach.
Moderate exposure is a climate with freezing. The concrete is not wet for long periods before it freezes, and it sees no deicing agents.
Severe exposure means deicing chemicals or other aggressive agents. It also covers concrete that may become highly saturated by continual contact with moisture or free water before it freezes.

The contractor tests the air content of fresh concrete to ASTM C231. For slag and other highly porous coarse aggregate the test is ASTM C173.

What it looks like on a slab

Scaling is the loss of the surface of the slab. FAA AC 150/5380-6C says that a surface that improper curing or finishing weakened, together with freeze-thaw cycles, can lead to scaling.
D-cracking is a pattern of cracks parallel to a joint or crack, with dark colour around the fine cracks. It can lead to disintegration within 1 to 2 feet (0.3 to 0.6 m) of the joint or crack.
Popouts are small pieces that break out of the surface. They are about 1 to 4 inches (25 to 100 mm) across and 1/2 to 2 inches (13 to 50 mm) deep.

Durability problems are most severe along joints and at panel corners, because the moisture level is highest there.

How to tell the cases apart

Distress	Where and how it looks	What the FAA documents give as the cause
Scaling	Delamination or disintegration of the slab surface to the depth of the defect. Construction defects usually affect part of a slab. Material defects often affect several slabs.	Construction defects, material defects or environmental factors. Freeze-thaw is one of them.
D-cracking	Fine cracks parallel to a joint or linear crack, with dark colour.	The concrete cannot withstand freeze-thaw cycles because of variable expansive aggregates.
Popouts	Single small pieces missing from the surface.	Freeze-thaw action with expansive aggregates. A large single aggregate piece or a clay ball can also cause a popout.
Alkali silica reaction (ASR)	Map cracking, cracks perpendicular to the joint face, gel or staining, and signs of expansion.	A chemical reaction between alkalis and reactive silica in the aggregate. It is not freeze-thaw damage.

The PAVER manual separates ASR from D-cracking by the cracks perpendicular to the joint face. It separates ASR from map cracking and scaling by the signs of expansion. Only coring and petrographic analysis confirm ASR.

Not the same as frost heave

Frost heave is a soil problem under the slab. FAA AC 150/5320-6G says it needs frost-susceptible soil, freezing temperature in that soil and free moisture that forms ice lenses. Freeze-thaw damage is a problem of the concrete itself.

What makes it worse

Inadequate air entrainment for the climate. The PAVER manual names this as a material defect that causes scaling. It affects several slabs that came from the same concrete batches.
Freezing before the concrete gains strength. The PAVER manual lists this as an environmental cause of scaling.
Poor finishing and curing. The PAVER manual lists over-finishing, water added to the surface during finishing and lack of curing as construction defects that cause scaling.
Expansive aggregate. FAA AC 150/5380-6C ties D-cracking and popouts to expansive aggregates.
Water that cannot drain. AC 150/5380-6C says that poor subsurface drainage can contribute to secondary damage such as D cracking.
Deicing chemicals. Item P-501 puts concrete that sees deicing chemicals in the severe exposure level.
An asphalt overlay. AC 150/5320-6G says the severity of durability distress often increases after an asphalt overlay, because more moisture is trapped at the interface.

How freeze-thaw resistance is tested

Test	What it does	Result
ASTM C666 or AASHTO T 161	Cycles concrete prisms between 4 °C and -18 °C in a temperature chamber. Each cycle takes 2 to 5 hours. Procedure A freezes the specimens in water. Procedure B freezes them in air.	The relative dynamic modulus falls as microcracks grow. The durability factor is the cycles completed divided by the planned cycles (usually 300), multiplied by the relative dynamic modulus.
ASTM C672	Covers a concrete surface with 6 mm of calcium chloride solution (4 g in 100 mL of water). It freezes for 16 to 18 hours and thaws in air for 6 to 8 hours each cycle.	An inspector flushes the surface and rates the scaling visually after every five cycles. The test runs about 50 cycles.
ASTM C457	Examines a section of hardened concrete under a microscope. It uses the linear-traverse or the point-count method.	The result gives the void frequency, the specific surface, the spacing factor and the paste-air ratio.

FHWA says that testing normally stops when the relative dynamic modulus falls below 50 to 60 percent of its initial value. The relative dynamic modulus is calculated from the resonant frequency of the prism. Item P-501 uses the ASTM C666 result to accept coarse aggregate. See the section on prevention.

Prevention in airfield pavement

The FAA Item P-501 specification and its notes to the engineer set these measures. The notes are guidance and the engineer adapts them to the project.

Air content. Use the table above for the exposure level and the aggregate size.
Water to cementitious ratio. The ratio is between 0.38 and 0.45 by weight. The engineer specifies a maximum of 0.45 unless severe freeze-thaw or deicer exposure needs a lower value. A ratio below 0.38 needs approval of the Engineer and the FAA.
Cementitious content. A minimum of 470 pounds per cubic yard (280 kg/m³) is advised. Where severe freeze-thaw, deicer or sulfate exposure is expected, the minimum should be at least 517 pounds per cubic yard (310 kg/m³).
Aggregate. The engineer selects the largest aggregate size with freeze-thaw vulnerability in mind. Where D-cracking is a concern, the coarse aggregate needs a durability factor of 95 or more in ASTM C666. A 20-year service record in similar conditions without D-cracking also qualifies.
Chert. Chert popouts are a known problem in freeze-thaw areas. P-501 limits chert lighter than 2.40 specific gravity to 0.1 percent by mass. The limit is 1.0 percent in areas without severe freeze and thaw.
Cold weather work. The contractor stops mixing when a falling air temperature reaches 40 °F (4 °C). Mixing restarts at 35 °F (2 °C). The RPR may authorize work in writing. Concrete must be at least 50 °F (10 °C) when placed, and never on frozen material.
Curing. The cure period is 7 days. In cold weather the concrete stays at 50 °F (10 °C) or more for 72 hours. It then stays above freezing for the rest of the 7 days.
Joint sealing. P-501 requires the joints to be sealed to Item P-604 or P-605. AC 150/5380-6C says that sealing cracks keeps surface moisture out of the pavement structure.
Drainage. AC 150/5380-6C says that the criteria for a subsurface drainage system under runways and taxiways are more stringent than for aprons.

The FAA lists crushed granite, calcite cemented sandstone, quartzite, basalt, diabase, rhyolite and trap rock as aggregates that meet the D-cracking test requirement. They must still meet the other P-501 quality tests.

Freeze-thaw damage in asphalt

The air-void mechanism above belongs to concrete. For airfield asphalt, FAA AC 150/5380-6C ties freeze-thaw to potholes. In northern climates with severe freeze-thaw cycles, pothole development gets worse and may not relate to traffic alone. The AC adds that potholes are not a common distress on airfields. It also says that moisture in the asphalt can strip the binder from the aggregate.

For mix design, Item P-401 of AC 150/5370-10H gives a rule for the tensile strength ratio test. In areas subject to freeze-thaw, use freeze-thaw conditioning. The test method is ASTM D4867.

How the PAVER manual rates it

The PAVER manual counts distress per slab. It rates scaling by the FOD hazard and D-cracking by the area and the disintegration.

Scaling, low: a minimal loss of surface paste with no FOD potential.
Scaling, medium: loss of surface paste with some FOD potential, such as loose mortar or loose coarse aggregate.
Scaling, high: low durability concrete that continues to pose a high FOD hazard. Routine sweeping does not stop the FOD.
D-cracking, low: hairline cracks in a limited area of the slab, such as one or two corners along one joint. Little or no disintegration has occurred.
D-cracking, medium: the cracking covers a considerable amount of slab area with little disintegration, or covers a limited area with missing pieces.
D-cracking, high: the cracking covers a considerable amount of slab area with disintegration or FOD potential.
Popouts: the manual defines no severity levels. It counts the slab only when the average density exceeds approximately three popouts per square yard (square meter).

If D-cracking is counted on a slab, the scaling on that slab is not recorded. If corner or joint spalling comes from D-cracking, only the D-cracking is recorded. Low severity scaling, which is crazing, is counted only if scaling is likely within 2 to 3 years.

What to do about it

The PAVER manual notes that sweeping removes the FOD from scaling, but the concrete keeps scaling until the affected depth is gone. For popouts, AC 150/5380-6C gives one action: remove the FOD.

The repair method for rigid pavement depends on the type of damage, the climate, experience and the materials available. The AC lists crack and joint sealing, partial depth repair and full depth repair. Partial depth repair suits joint spalling, which is the breakdown of the slab edge within 2 feet (0.6 m) of the joint. A repair thinner than 2 inches (5 cm) usually deteriorates quickly on an airfield pavement.

In airfield inspection

A pavement survey records the distress and its severity. It does not record the cause. D-cracking and scaling are separate distress types, and on one slab the PAVER manual counts only the D-cracking. TarmacView plans pavement survey flights and captures the photos today. Crack detection and the ASTM D5340 rating are in development and offered as early access.

This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.

What is Freeze-Thaw Damage in Concrete Pavements?