What is Shrinkage Cracking in Concrete Pavement?

A shrinkage crack is a hairline crack that forms in a concrete slab while the concrete sets and cures.

Shrinkage is a loss of volume in the concrete. If the slab cannot move freely, the shrinkage builds up stress. A crack forms when that stress is higher than the tensile strength of the concrete. FHWA describes three kinds of shrinkage in a concrete pavement: plastic, drying and autogenous.

What a shrinkage crack looks like

The PAVER distress manual and FAA AC 150/5380-6C give the same description. A shrinkage crack is a hairline crack that is usually only a few feet long. It does not extend across the entire slab and usually does not go through the depth of the slab. Typically it reaches no more than 1/4 inch (6 mm) from the surface, and it may be in the finished surface paste only.

Why concrete shrinks and why it cracks

The shrinkage occurs mainly in the cement paste, and the aggregate acts as a restraint. The slab also rests on a base that resists its movement. FHWA says that this restraint at the slab and base interface builds up stress. Concrete is weak in tension, so a crack forms to relieve the stress when it is higher than the tensile strength.

A jointed concrete pavement uses this on purpose. FHWA says that joints control where the cracking occurs. Most of the cracking takes place in the early-age period, as the slab resists volume change. FAA AC 150/5320-6G says contraction joints give controlled cracking when the pavement contracts because the moisture content or the temperature drops.

Plastic, drying and autogenous shrinkage

Plastic shrinkage

FHWA describes plastic shrinkage cracks as short, irregular cracks on the fresh surface of the concrete. They are from a few centimetres to just under 1 m long, and from a few centimetres to 0.6 m apart. The cracks form when the evaporation rate is higher than the bleeding rate. The fresh concrete is not strong enough to resist the capillary stress in the paste.

The weather sets the evaporation rate. FHWA lists high concrete temperature, low relative humidity and high wind as the conditions that suit plastic shrinkage cracks. A dry base can also pull water out of the slab. A mix that bleeds little can dry out even at a normal evaporation rate. Low-slump paving concrete with a low water content and some cement replacement is an example. A mix that sets slowly stays plastic for longer, so it has more time to dry. FHWA (2004) says there is no way to predict with certainty when plastic shrinkage cracks will form.

FHWA states that plastic cracking does not necessarily harm the intrinsic quality of the concrete. If crews cure the concrete well afterwards, the defect is more cosmetic.

Drying shrinkage

Drying shrinkage starts after the concrete sets. Some of the water that the concrete loses to the air is irreversible, and it leaves a permanent shrinkage. FHWA lists the factors that set its size. They are the water-cement ratio, the cement type, the cement content, the admixtures, the type and amount of aggregate, and the climate. AC 150/5320-6G warns that a high cement content may increase the risk of cracking from drying shrinkage.

Designers expect drying shrinkage and plan for it with joint cutting. If the concrete dries heavily in the first days, cracks can form at a closer spacing than the contraction joints (FHWA).

Autogenous shrinkage

Autogenous shrinkage is a volume reduction of the cement paste from self-desiccation. In self-desiccation, physical and chemical changes in the cement during hydration dry the capillary pores. FHWA says it is greater in high-strength concrete with a low water-cement ratio and smaller at a high ratio. It does not depend on moisture transport, temperature change or external restraint.

Two measures reduce it. One is to keep water available during hydration, by water curing or by internal curing. In internal curing, pre-wetted lightweight aggregate releases water inside the concrete. The other is a shrinkage-reducing admixture.

How to tell plastic from drying shrinkage

Engineers make this decision on cores in a laboratory. FHWA gives these signs of a plastic shrinkage crack:

• It starts at the exposed surface and is widest there. It then tapers with depth, often to nearly nothing.
• The tension distorts the sides of the crack. Small bridges of paste may span the crack, and the two sides do not fit back together.
• Mortar that finishing worked into the top of the crack shows that the crack formed early.
• The inside face of the crack has tiny globules of cement paste. A fresh break of the same concrete has tiny angular planes.

A crack that goes around the aggregate particles is not enough proof of plastic cracking. Bleed channels can also look like plastic shrinkage cracks. A bleed channel shows signs of a waterway.

How to tell a shrinkage crack from a load crack

A shrinkage crack is not a structural distress. A crack that divides a slab into two or three pieces is a different distress. AC 150/5380-6C says that repeated loads and shrinkage stresses together usually cause it. The PAVER manual rates short hairline cracks as shrinkage cracks. For reinforced concrete, it adds a rule. A crack less than 1/8 inch (3 mm) wide, with no spalling or faulting, counts as shrinkage cracking.

A slab that intersecting cracks break into four or more pieces is a shattered slab. The PAVER manual says to ignore shrinkage cracks when it counts these pieces. A network of fine cracks in the top of the slab is map cracking. A pattern of cracks that runs parallel to a joint is D-cracking.

PAVER also gives a time clue. Plastic shrinkage cracking can occur on the day of construction and is apparent within the first year. Cracking from alkali-silica reaction is generally not seen in the first few years.

Shrinkage cracks in asphalt

This page covers concrete pavement. AC 150/5380-6C also uses the word for asphalt. It says that longitudinal and transverse cracks in flexible pavement may result from shrinkage or contraction of the hot mix asphalt surface. Oxidation and age hardening of the asphalt cause the shrinkage, and thermal fluctuations cause the contraction. This type of cracking is not load associated.

In a pavement condition survey

The PAVER manual, which implements the airfield Pavement Condition Index (PCI) method, lists shrinkage cracks as distress 73. It defines no severity levels. An inspector records only that shrinkage cracks exist. A slab with one or more shrinkage cracks counts as one slab with shrinkage cracks.

An FAA study tested small drone data at six airports. It recommends a 1.5 mm per pixel colour orthophoto and a 6.0 mm per pixel elevation model to detect and rate airfield distresses. For shrinkage cracks, it lists 2.5 mm per pixel as the minimum orthophoto resolution, and it says that detection is not always possible. The cracks can often be sub-millimeter in size.

Control during construction

Three measures control shrinkage cracking in a new slab. They are a good mix, protection of the fresh surface, and joints that crews cut at the right time.

Protect the fresh surface

FHWA says that curing starts when the surface starts to dry. At that point evaporation is higher than bleeding and the surface loses its wet sheen. The two common methods of initial curing are fogging and evaporation reducers. Fogging uses special fog nozzles to raise the humidity right above the slab. It is the most effective way to cut evaporation. FHWA also gives these options: moist cure at once and for at least 24 hours, wind barriers and sunshades.

FHWA gives ACI 308 limits for the evaporation rate. Cracks can occur at 0.5 kg/m2/h, and above 1.0 kg/m2/h precautions are necessary. A later FHWA brief says that engineers estimate the rate with the Menzel nomograph. It gives 0.20 lb/ft2/h (0.98 kg/m2/h) as a common critical value, and it notes that people question whether the value suits modern concrete. FHWA curing guidance says that paving concrete bleeds less than these limits assume, so the limits may not be conservative enough.

AC 150/5370-10H, Item P-501, sets these rules for new concrete pavement:

• Do not add water to the slab surface during finishing. The RPR may approve fog sprays or other finishing aids against plastic shrinkage cracking.
• Cure all exposed surfaces for a 7-day cure period, starting when finishing is complete and the bleed water is gone.
• Saw the joints as soon as the concrete is hard enough to cut without damage, and before uncontrolled shrinkage cracking occurs.

Cut the joints on time

FHWA says that late sawing of joints allows uncontrolled cracking. This can lead to poor load transfer and spalling. The concrete must first be strong enough to carry the saw and its operator, and that limits how early sawing can start.

AC 150/5320-6G sets the recommended maximum joint spacing from the panel thickness. The panel thickness controls the spacing, not the other way round. For a pavement without a stabilised base:

The AC gives a second table for a stabilised base. There the spacing runs from 12.5 ft for 8 to 10 inch panels to 20 ft above 16 inches. A spacing of more than 20 ft needs a technical analysis in the engineer’s report. The AC also says that smaller panels generally perform better in the long term.

Mix design

FHWA says that internal curing with lightweight aggregate reduces early-age shrinkage and the plastic shrinkage cracking that goes with it. It also says that polypropylene fibres can delay plastic shrinkage cracking.

What a shrinkage crack means on an airfield

A shrinkage crack is a surface defect that does not carry a severity rating in the PAVER method. The risk is that it is mistaken for a load crack, or the reverse. A hairline crack of a few feet is usually cosmetic. A crack that crosses the slab is not a shrinkage crack, and it needs a structural check. The PAVER manual says that a low-severity longitudinal, transverse or diagonal crack is usually warping or friction related. It says that a medium or high-severity crack is usually a working crack and a major structural distress.

Sealing a crack keeps surface moisture out of the pavement structure (AC 150/5380-6C). AC 150/5380-6C points to AC 150/5370-10, Items P-604 and P-605, for joint and crack sealants.

Repair

For a pavement in service, AC 150/5380-6C says shrinkage cracks are cosmetic and typically need no repair. A sealant keeps surface moisture out of the pavement structure.

For a new slab, P-501 covers cracks up to one-third of the slab depth. Clean the crack, then apply high molecular weight methacrylate or inject epoxy resin. Treat a deeper crack as a full-depth crack. P-501 does not allow this repair when the overall condition of the pavement makes an acceptable and durable result unlikely.