What is Spalling?

Spalling is the breakdown of a concrete slab at a joint, a crack or a corner. It leaves loose pieces of concrete and an irregular void in the surface. The FAA rates it as a distress of concrete pavement.

Definition

The manuals word the definition in a similar way. FHWA calls spalling the “cracking, breaking, chipping, or fraying” of a slab edge. The FAA PAVER manual and FAA AC 150/5380-6C call it the breakdown of the slab edge. The zone differs. The airfield manuals count a spall within 2 feet (0.6 m) of the side of a joint. The FHWA highway manual counts it within 0.3 m.

A spall is a defect of the edge, not of the whole slab. NCHRP says that a joint spall may develop mainly in the top few inches of the slab, or at a greater depth. A deep spall needs a different repair (see Repair).

Types of spalling

The PAVER manual for concrete airfields has two spalling distresses.

• Joint spalling (distress 74) is the breakdown of the slab edge within 2 feet (0.6 m) of the side of a joint. A joint spall usually does not go vertically through the slab. It meets the joint at an angle. A crack in the slab can spall in the same way.
• Corner spalling (distress 75) is the raveling or breakdown of the slab within about 2 feet (0.6 m) of a corner. The spall angles downward to meet the joint.

The FHWA highway manual records spalling only at longitudinal and transverse joints. It has no separate corner spalling distress. FAA AC 150/5320-6G includes both airfield forms in the Structural Condition Index (SCI).

How to tell a spall from similar distresses

Each of these distresses has its own entry in the PAVER manual and has a different cause and repair.

• Corner break. A corner break is a crack that goes vertically through the whole slab thickness. A corner spall angles downward. The crack angle is often not clear at low severity. In that case, use the position. A crack that meets both joints more than 2 feet (600 mm) from the corner is a corner break. A crack closer than 2 feet is a spall, unless you verify that the crack is vertical.
• Scaling. A spall is not scaling. Scaling is the delamination or disintegration of the slab surface. It can happen anywhere on the slab. FHWA gives its depth as normally 3 to 13 mm.
• Fraying. A frayed joint is a joint where material is no longer in place along the joint or crack. Spalling means material may or may not be missing.
• Popouts. A popout is a small piece that breaks loose from the surface through freeze-thaw action and expansive aggregate. It is 25 to 100 mm across and 13 to 50 mm deep.
• D-cracking. D-cracking is a pattern of cracks parallel to a joint, with dark coloring around the fine cracks. It may lead to disintegration of the concrete within 1 to 2 feet (0.3 to 0.6 m) of the joint. It is a cause of spalls as well as a different distress.

Causes

Excessive stress at a joint or crack causes joint spalling. Several conditions create this stress.

• Incompressible material in the joint. Soil and rock collect in a joint when the sealant has damage. FHWA says that incompressible material keeps the joint from closing. The compressive stress can then cause spalling along the joint faces. FAA AC 150/5380-6C and the PAVER manual say the same about joint seal damage.
• Traffic loads on weak concrete. Weak concrete at the joint, from overworking during finishing, spalls under traffic.
• Dowels. Misaligned dowels, from bad placement or bad preparation, restrain slab movement and cause spalling. FHWA adds corrosion of dowel bars, metal joint inserts and reinforcing steel that sits too near the surface.
• Concrete durability. FHWA lists D-cracking and alkali-silica reaction. It also lists poor consolidation near the joints.
• Construction and maintenance damage. NCHRP lists joint sawing that is too early, and damage from cold milling, grinding or joint cleaning.
• Snow plows. AC 150/5380-6C lists snow plow damage. AC 150/5320-6G says that beveled joints may help minimize sliver spalls from snow plow damage.

Corner spalling has the same causes as joint spalling. AC 150/5380-6C says that it may appear sooner because of increased exposure. FHWA says that spalling can occur in any climate.

Severity and counting

The PAVER manual rates joint spalling at three severity levels. The rating depends on the length of the spall and on the state of the pieces.

For a spall under 2 feet (600 mm):

• Low: the spall is broken into pieces or fragmented. Little FOD or tire damage potential exists.
• Medium: some of the pieces are loose or absent. Considerable FOD or tire damage potential exists.
• The manual does not define a high level for a spall under 2 feet.

For a spall over 2 feet (600 mm):

• Low: the spall is broken into no more than three pieces by low or medium cracks, or the joint is lightly frayed. Little or no FOD potential exists.
• Medium: the spall is broken into more than three pieces by light or medium cracks. Alternatively, it is broken into no more than three pieces and one or more cracks are severe. Alternatively, the joint is moderately frayed. Some FOD potential exists.
• High: the spall is broken into more than three pieces and one or more cracks are high severity, or the joint is severely frayed. High FOD potential exists.

Corner spalling has three levels too.

• Low: the spall is broken into one or two pieces by low severity cracks. Alternatively, one medium severity crack defines it. Little or no FOD potential exists.
• Medium: the spall is broken into two or more pieces by medium severity cracks, and a few small fragments are loose or absent. Alternatively, one severe fragmented crack defines it. Alternatively, loose material gives some FOD potential.
• High: the spall is broken into two or more pieces by high severity fragmented cracks, and fragments are loose or absent. Alternatively, displaced pieces cause a tire damage hazard. Alternatively, loose material gives high FOD potential.

The manual counts spalling by slab, not by area. A slab with spalls on more than one edge counts once, at the highest severity. Two adjacent slabs that both have a joint spall each count. Two small cases do not count. One is a joint spall small enough to fill during a joint seal repair. The other is a corner spall under 3 inches (76 mm) wide that has a sealant fill.

The FHWA highway manual works in a different way. It rates joint spalls by width, measured to the face of the joint. A spall under 75 mm wide is low severity. A spall of 75 to 150 mm is moderate. A spall over 150 mm, a spall broken into two or more pieces, or a spall with patch material is high. FHWA records the length of longitudinal joint affected at each level, and only spalls of 0.1 m or longer count. For transverse joints it records the number of joints affected.

The PCI survey follows ASTM D5340, as AC 150/5380-6C requires. FAA AC 150/5320-6G counts joint spalling and corner spalling, at all severities, as two of the five distresses in the SCI.

Why it matters on an airfield

A spall releases concrete onto the pavement. FAA AC 150/5380-6C says that a pavement distress can generate loose material that may strike aircraft propellers or be ingested into jet engines. All pavement inspections should address this FOD. The PAVER ratings reflect it, because the FOD potential and the tire damage risk decide the severity.

EASA lists deteriorating pavement, with pieces of concrete that break loose, as a source of FOD. Its guidance for movement area inspections says to look for signs of pavement damage. This includes cracking and spall of concrete. The inspector also looks at the condition of joint sealing.

Repair

Choose the repair by the depth and the cause of the spall.

Table 6-2 of AC 150/5380-6C gives two treatments for joint spalling. One is to remove the loose material, refill with an approved product and reseal. The other is a partial depth repair. AC 150/5320-6G says that timely partial depth repair of spalling extends the functional life of rigid pavement. It says the same of timely resealing of joints, which keeps water and incompressible material out.

Partial depth repair suits a shallow spall. FHWA gives a limit. A spall deeper than the top one-third of the slab, from misaligned dowel bars or significant D-cracking, should not get a partial-depth repair. The repair is likely to fail. FHWA advises coring representative joints to find the depth of deterioration. Pavement Interactive gives a width limit. A spall that extends more than about 75 mm (3 in) from the crack face may reach the bottom of the joint. It needs a full-depth patch.

The partial depth repair in Appendix A8 of AC 150/5380-6C has these steps:

1. Mark the limits of the spall area.
2. Saw vertical cuts at least 2 inches (5 cm) deep and about 3 inches (8 cm) beyond the spall.
3. Remove all unsound concrete and at least 1/2 inch (13 mm) of sound concrete between the cut and the joint.
4. Clean the area and form a new joint sealant reservoir.
5. Place the patch, finish it and cure it.
6. Seal the joint.

Do not place concrete unless the air temperature is at least 40 °F (4 °C) and rising. Combine two repair areas that are less than 1.5 feet (45 cm) apart.

Where the concrete will get an asphalt overlay, item P-101-3.4 of AC 150/5370-10H allows an asphalt repair. The crew saws 2 inches (50 mm) outside the spall and 2 inches deep. It places the asphalt in lifts of no more than 4 inches (100 mm). The item limits an asphalt repair of concrete to depths less than one-third of the slab thickness.