What is Map Cracking (Crazing)?

Map cracking is a network of shallow, interconnected hairline cracks that extends only through the upper surface of a concrete pavement. FAA AC 150/5380-6C uses the names map cracking and crazing for the same distress.

Names and look-alikes

• Map cracking and crazing. The FAA maintenance circular defines both names the same way. The FAA handbook on alkali-silica reaction says the two are often hard to tell apart.
• Pattern cracking. The FHWA ASR handbook says map cracking is also called pattern cracking.
• Crack mapping. This is not a defect. It is the act of recording where cracks are. The FHWA LTPP manual uses distress maps that show the exact location of each distress on a test section.
• Alligator cracking. The FHWA ASR handbook lists this as a third name for map cracking in concrete. In asphalt, AC 150/5380-6C uses it for fatigue cracking, which is a different distress. Repeated traffic loading causes fatigue cracking, and the cracks start at the bottom of the asphalt surface.

What it looks like

The FHWA LTPP manual describes a series of cracks that extend only into the upper surface of the slab. Larger cracks are often oriented along the pavement, and finer transverse or random cracks connect them. It is a surface distress. The cracks do not divide the slab the way a corner break or a longitudinal crack does.

The FAA ASR handbook adds that map cracking is never spread evenly over a pavement. It can range from a few cracks to isolated areas of pattern cracking to closely spaced, extensive cracking. FHWA notes that map cracking from ASR often appears first at the corners of slabs.

Causes

There are two groups of causes.

Curing and finishing

AC 150/5380-6C says crazing usually results from improper curing or finishing of the concrete. The PAVER manual lists the same construction defects for scaling: over-finishing, water added to the surface during finishing, and lack of curing. An FHWA curing brief says curing should start as soon as possible after placing, to prevent excessive water loss and drying of the surface.

Alkali-silica reaction

In alkali-silica reaction, alkalis in the cement react with certain reactive silica minerals and form a gel. The gel absorbs water and expands, which can damage the concrete. AC 150/5380-6C calls ASR another source of distress associated with map cracking. The FAA ASR handbook gives three conditions that must all be present:

1. A reactive form of silica in the aggregate.
2. Enough alkali, mainly from the cement.
3. Enough moisture, which means a relative humidity of not less than 85 percent in the pores of the concrete.

If one condition is missing, damaging expansion from ASR cannot occur.

How to tell the cases apart

The PAVER manual says ASR is differentiated from map cracking and scaling by visual signs of expansion. Its indicators for ASR are:

• white, brown, gray or other colored gel or staining at the cracks
• aggregate popouts
• an increase in concrete volume, such as slab faulting, joint misalignment or extruded joint seals

The manual gives more clues:

• ASR is generally present throughout the pavement section, because it depends on the materials.
• ASR distress is generally not seen in the first few years after construction. Plastic shrinkage cracking can occur on the day of construction and shows within the first year.
• ASR shows cracking perpendicular to the joint face. D cracking develops as parallel cracks along joint faces.

The FAA ASR handbook says that ASR and other mechanisms often look very similar. It calls positive identification virtually impossible from visual inspection alone, and it says laboratory investigation is essential. The PAVER manual agrees: coring and concrete petrographic analysis is the only definitive method to confirm ASR. A photo of the surface cannot settle the cause.

Is map cracking structural

Crazing from improper finishing and curing does not affect the structural integrity of the concrete, according to the FAA ASR handbook. A structural crack is a different distress. The PAVER manual says that low severity longitudinal, transverse and diagonal cracks are usually warping or friction related. Medium or high severity cracks are usually working cracks and are major structural distresses.

ASR is different. The FAA study behind its handbook confirmed that ASR distress can negatively affect the long-term performance of airfield pavements. The cause therefore matters before anyone calls the cracking harmless.

Why it matters on an airfield

AC 150/5380-6C says crazing may lead to scaling of the surface. Scaling is the loss of surface mortar. In the PAVER manual, the FOD potential of scaling grows with severity. At high severity, routine sweeping no longer avoids FOD problems. The manual also says scaling continues until the affected depth is removed or used up.

How it is rated

PAVER rates ASR by the width of the cracks and by the signs of damage:

• Low. Cracks at the surface are tight, mostly 1 mm or less, with little or no sign of movement.
• Medium. A pattern of wider cracks, mostly 1 mm or wider, that tighter cracks may subdivide. Some fragments and popouts may occur.
• High. Loose or missing concrete fragments with high FOD potential, or a slab surface that is badly degraded and needs immediate repair.

PAVER shrinkage cracks (73) are a different distress. They are hairline cracks only a few feet long that do not extend across the entire slab.

What to do about it

Table 6-2 of AC 150/5380-6C is the quick guide for repair of rigid pavement surface problems. It has no entry for crazing or map cracking. The actions follow from the cause:

• Crazing. Record it and watch for scaling, because AC 150/5380-6C says crazing may lead to scaling. The PAVER manual counts low-severity scaling only if more scaling will occur within 2 to 3 years.
• Suspected ASR. Take cores for petrographic analysis. The FAA handbook advises cores from different parts of the pavement, because ASR can be very localized. It also says that the severity of the cracks at the surface may not match the damage inside.
• Confirmed ASR in existing concrete. The FAA handbook says lithium is effective in controlling further expansion, and its effect depends on the depth of penetration. For airfield pavements it names topical application (ponding or spraying), vacuum permeation and pressure injection as the most practical methods. It does not recommend pressure injection if ASR has badly damaged the structure.
• New concrete. The handbook lists non-reactive aggregate, low-alkali cement, limits on the alkali content of the mix and supplementary cementing materials as preventive measures.

In an airfield inspection

An inspector records where the cracking is, how much of the slab it covers, and whether staining, popouts, scaling or slab movement are present. Those signs decide if the rating is scaling or ASR. When ASR is suspected, a core sample is taken for petrographic analysis.