What is Scaling in Concrete Pavements?

Scaling is the loss of the upper surface of a concrete pavement slab, normally 3 to 13 mm deep. It can occur anywhere on the slab. The FHWA Long-Term Pavement Performance (LTPP) manual codes it as distress 8b on jointed concrete pavement and 4b on continuously reinforced concrete pavement. The FAA PAVER manual for concrete airfields codes it as distress 70.

The word means the same on a road, a parking area or an airfield pavement. Rigid pavement is normally concrete pavement. PAVER describes scaling as delamination or disintegration of the slab surface to the depth of the defect.

What it looks like

Scaling leaves a rough, pitted surface. FHWA describes deicer scaling as a disintegration of the outermost layer of cement paste. What remains is a layer of powdered paste, freed aggregate, or aggregate that is clean of paste. Plowing or other work can remove this loose layer. Then only the scaled surface is visible.

FHWA adds that scaling may be progressive, so new layers of paste are lost in later cycles. In many cases the scaling stays in the outermost layer during one winter and does not progress in later winters.

Causes

The PAVER manual groups the causes in three classes.

• Construction defects: over-finishing, water added to the surface during finishing, lack of curing, and surface repairs of fresh concrete with mortar. These usually affect part of one slab.
• Material defects: inadequate air entrainment for the climate. These usually affect several slabs made from the same concrete batches.
• Environmental factors: concrete that freezes while it is still weak, and thermal cycles from certain aircraft. Freezing affects a large area. Thermal effects affect isolated areas.

FAA AC 150/5380-6C gives the same two main reasons in short form. A surface weakened by improper curing or finishing and then exposed to freeze-thaw cycles can scale. Crazing, a network of shallow hairline cracks, may lead to scaling.

Deicer scaling

FHWA calls deicer scaling a form of freeze-thaw damage that stays at the surface of the slab. Recent research shows that the surface layer of paste delaminates from the slab. The cause is the tensile force in that layer. Expansion of the ice layer on the concrete surface creates the force. The force is highest when the solution that freezes on the surface holds about 3 percent salt. The type of salt does not matter.

A well air-entrained paste resists deicer scaling. FHWA therefore links scaling to finishing:

• Finishing while bleed water is still rising can trap water just below the surface. When the water freezes, scaling or spalling can follow.
• Finishing while bleed water is present works the water into the surface. This raises the water to cementitious materials ratio at the surface and makes a weaker paste.
• Overworking the surface removes the entrained air near the surface. FHWA calls this loss of air a common cause of scaling.

Air entrainment is a mix specification. FAA AC 150/5370-10H, Item P-501, recommends a higher air content as the exposure gets harsher. For 3/4 inch maximum aggregate it recommends 3.5 percent for mild exposure, 5.0 percent for moderate exposure and 6.0 percent for severe exposure. Severe exposure includes concrete that meets deicing chemicals. The page on freeze-thaw damage explains the air-void mechanism in full.

How to tell scaling from similar distresses

Severity in PAVER

PAVER notes that sweeping removes the FOD but the concrete keeps scaling until the affected depth is gone.

How it is counted

In a PAVER survey the surveyor counts a scaled slab as one slab. If a slab shows two severities, the surveyor counts the higher one. Where the surveyor counts D cracking or alkali-silica reaction on the slab, scaling is left out. If a patch sits on a scaled area, only the scaling counts. Low severity scaling counts only if the surveyor expects more scaling within 2 to 3 years.

The LTPP manual records no severity for scaling. It records the number of occurrences and the square meters affected.

How scaling resistance is tested

ASTM C672 tests the scaling resistance of a concrete surface that freezes and thaws under deicing chemicals. The judgement is by visual examination. The FHWA course manual describes the method:

• The laboratory covers the surface with 6 mm of calcium chloride solution, 4 g in 100 mL of water.
• Each cycle freezes the specimen for 16 to 18 hours, then thaws it in air for 6 to 8 hours.
• After every five cycles the laboratory flushes the surface, examines it and replaces the solution.
• The test usually runs for about 50 cycles or more, until the surface can be evaluated.

The test shows the effect of mix proportions, surface treatment and curing on scaling resistance. For the general freeze-thaw resistance of concrete, see ASTM C666 on the page about freeze-thaw damage.

How to prevent scaling

FHWA gives this guidance for jointed concrete pavement.

• Air-void system. Add an air-entraining agent that creates a protective air-void system. For deicer exposure, FHWA quotes ACI as 6 ± 1.5 percent total air and a w/cm below 0.45.
• Mix. Use a minimum cement content of 335 kg/m³ and a w/c of 0.45 or less. FAA Item P-501 sets a water to cementitious ratio between 0.38 and 0.45.
• Finishing. Finish after the bleed water has risen and dissipated. Do not overwork the surface.
• Curing. Cure the concrete properly. Ideally it would cure for 28 days and then dry for another 28 days before it freezes. With fly ash or slag cement, FHWA suggests 56 days of curing.
• Deicers. Provide a minimum 30-day drying period after curing before deicers touch the surface. Use deicers sparingly on young concrete, especially in the first year.
• Sealers. Penetrating sealers such as silane and siloxane help by reducing water ingress, when applied properly.
• Joints. A working joint seal keeps water and deicer solution out of the slab. A damaged seal lets the solution in and raises the saturation of the concrete at the joint.

Item P-501 also sets a 7-day cure period for airfield concrete.

Repair and treatment

The choice depends on how deep the damage goes. FHWA guidance for materials-related distress gives these options.

• Diamond grinding. It removes a thin layer of concrete. Surficial scaling is a good candidate. Grinding only buys time, because it does not address the cause and the new surface will deteriorate. Do not use it for deeper damage.
• Partial-depth repair. Scaling and deicer deterioration are candidates when the damage stays in the upper one-third of the slab. It is not recommended for damage that goes deeper.
• Surface sealer. For salt scaling FHWA suggests grinding, then a surface sealer. A sealer also treats widespread deterioration.
• Full-depth repair, overlay or slab replacement. FHWA lists these, with partial-depth repair and grinding, as possible rehabilitation options for deicer scaling.

FAA AC 150/5380-6C says that a concrete repair thinner than 2 inches (5 cm) usually deteriorates quickly on an airfield pavement. Sweeping removes the FOD from scaled concrete, but it does not stop the scaling.

On an airfield

Scaling matters on an airfield because the loose material is FOD. This is why PAVER rates each severity by its FOD potential. ICAO Annex 14 Volume I, paragraph 10.2.1, requires aerodromes to inspect and monitor the surfaces of the movement area. The surfaces are checked as part of a maintenance programme that avoids and eliminates FOD.

Scaling is also part of the distress inventory behind the Pavement Condition Index (PCI) under ASTM D5340. FAA AC 150/5380-6C says that ASTM D5340 determines the PCI. 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.