What is Slab Warping in Concrete Pavement?

Slab warping is the bending of a concrete pavement slab that a moisture difference between its top and bottom surfaces causes. Curling is the same bending from a temperature difference.

Warping and curling

Two effects bend a concrete slab out of flat. Warping comes from a moisture difference through the slab thickness. Curling comes from a temperature difference.

AC 150/5320-6G keeps them apart. It lists “temperature curling or moisture warping” as causes of loss of support. It also says that changes in temperature and moisture content cause volume changes and warping of panels. Some FAA texts use “warping” for both effects. A real slab usually shows a mix of the two.

Which way the slab bends

A warm top and a cool bottom bend the slab corners down. This is the normal daytime state. A cool top and a warm bottom bend the corners up, mostly at night. The largest upward curl is just before sunrise. The largest downward curl is around noon or in the early afternoon. Cloud cover and climate change these times.

A moisture difference works the same way. A wetter top bends the corners down and a drier top bends them up. The top surface is only partly saturated and the bottom is usually fully saturated. So upward warping is usually the more obvious.

Concrete recovers its shape when the temperature effect goes away. A slab can also keep curvature from the time it set. In an FHWA study most sites curled up, some curled down, and some changed direction.

Why it matters on an airfield

A lifted corner or edge can leave a void under the slab. Deflection testing assumes that the panel is in full contact with the foundation. A curled slab therefore affects the test result. AC 150/5320-6G lists curling and warping among the causes of deflection basin anomalies.

Curling stress is one cause of corner breaks. AC 150/5380-6C says that load repetition combined with loss of support and curling stresses usually causes them. A corner break is a crack through the full slab thickness. It meets both joints at a distance of half the slab length or less from the corner. A corner spall meets the joint at an angle, so it is a different distress.

PAVER also says that low severity cracks that divide a slab are usually warping or friction related. They are not major structural distresses.

Curling also affects ride quality. In the FHWA study the daily change in curl changed the Half-car Roughness Index by up to 0.63 m/km. The average was about 0.16 m/km.

How design limits it

Contraction joints decrease the stresses that warping and curling cause. Shorter joint spacing generally gives better long-term performance.

Table 3-7 of AC 150/5320-6G gives the recommended maximum spacing. On an unstabilized base it is 3.8 m for panels 152 mm thick or less, and 6.1 m for panels thicker than 229 mm. On a stabilized base it is 3.8 m for panels 203 to 254 mm thick, and 6.1 m above 406 mm.

A stabilized base gives higher warping and curling stresses than an unstabilized base. On federally funded projects a greater spacing needs an analysis. The analysis must show that the panel size is not more than 5 times the radius of relative stiffness.

What an inspection can see

A surface survey sees the results of loss of support, such as corner breaks and cracks. It does not see the void. AC 150/5320-6G covers loss of support in its analysis of deflection data (Appendix C).