Part 1: Avoid Cracks in Concrete
Slabs-On-Grade

By Harvey Haynes
Consulting Concrete Engineer

Part 1 of 2 :: Types of Cracks and Their Cause

By Harvey Haynes
Consulting Concrete Engineer

Part 1 of 2 :: Types of Cracks and Their Cause

Heave
Expansive clay soils swell or heave with an increase in moisture content. Cracks in concrete slabs caused by heave can be identified by vertical offsets, cracks running parallel to an exterior wall, or cracks exhibiting an X-shaped pattern in a small room. It is difficult to distinguish cracks caused by heave from those caused by settlement.

Settlement
Slabs that have been overloaded or have weak soil support will settle due to soil consolidation. The strength of the slab greatly depends on the strength of the soil support. A slab four inches thick can be quite strong on firm soil, but may crack easily on weak soil. Cracks due to settlement can appear as half-circle cracks at the edge of slabs, cracks raveling along the edges, slabs breaking into small pieces (sections of one or two feet per side), or diagonal cracks across corners of slabs.

Seasonal Temperature Change
If concrete is cast in the summer, it can experience a temperature decrease of 100° F by the middle of winter. This decrease in temperature can cause a slab 100 feet long to contract about 3/4 of an inch. This contraction movement may crack the slab. Contraction joints are placed in slabs to encourage the cracks to occur at the joint locations. If the slab is cast in winter, then the concrete could experience a 100° F increase in temperature by the summer. In this case, the slab would expand about 3/4 of an inch. Expansion joints are required to allow the slab to expand, or the slab may buckle.

Daily Temperature Change
Daily temperature change causes a varying temperature through the thickness of the slab. The sun heats the top surface, which causes the concrete near the top to expand, and the slab develops a hump shape where the middle is higher than the edges. If the top surface is colder than the bottom, the slab will have a curled shape where the edges are higher than the middle.

Heat of Hydration
Heat of hydration is heat internally generated by the concrete during the chemical process of cement hydration. Hydration is the mechanism by which cement sets and then gains strength.

During the first night after concrete is cast, a common crack that occurs is caused by the combination of heat of hydration and warm ambient temperatures on the day the concrete is cast. Typically concrete is cast in the morning, and by the afternoon a hot sun raises the temperature of the concrete, especially near the top surface. Internally, concrete generates a considerable amount of heat due to heat of hydration. By sunset of the first day, the concrete can be quite warm, easily 120° F. Cool evening temperatures initially reduce the temperature at the top surface, and this cooler concrete contracts. At this young age the concrete is quite weak, and the contraction movement can crack the concrete. By the next morning, the slab is found cracked. For this reason, contraction joints (also called control joints) must be installed the same day that the concrete is cast.

Crazing Cracks
These cracks appear at the very top surface layer of the slab where a thin layer of cement paste has lost water too rapidly and cracked. These cracks are very fine and shallow.

Plastic Shrinkage Cracks
Plastic shrinkage cracks develop when too much water evaporates while the concrete is fresh, or plastic in consistency. These cracks have a distinct form. They are quite wide at the surface, their depth into the slab is usually limited to about one to two inches, they range in length from about six inches to five feet, they usually develop parallel to one another, and they don't run to the edges of the slab.

Drying Shrinkage Cracks
These are the typical shrinkage cracks, which develop after the concrete is hard. They can appear randomly across slabs or have a uniform pattern. These cracks also extend from re-entry corners.

If a cube four inches on a side were to dry over several months, each side would decrease in width by about 0.002 inch. The entire cube will decrease in volume. When cubes are laid end to end for a distance of 20 feet, the change in length would be about 1/8 inch. In a slab this gap would be the crack width.

Why does the cube get smaller? Technically, there are two contributing reasons why moisture loss causes shrinkage, and the explanations relate to two different void sizes within the concrete. Concrete of residential home quality contains about 20 percent void volume. Part of this volume is microscopic pores called capillary voids, which were created by the original mixing water. Smaller voids, called gel voids, exist in the concrete within the hydrated cement particles. When water evaporates from the capillary voids, capillary forces develop which place the water in tension. Therefore the solids are placed in compression, and shrinkage occurs. When water within the gel voids evaporates, the hydrated cement particles become smaller and additional shrinkage occurs.

If ambient conditions are at 100 percent relative humidity a concrete slab will not shrink. At 40 percent relative humidity, and many months of drying time, a slab 100 feet long can shrink up to 3/4 inch. Let rain soak the slab for a couple of days and the concrete will swell such that a permanent shortening of 1/8 to 1/4 inch exists.

Uniform drying of concrete does not occur in slabs-on-grade because only one face is exposed to evaporation. A moisture gradient exists through the slab thickness where the top is drier than the bottom, so shrinkage will be greater at the top. This condition results in the slab having a cupped or curled shape. This shape can be explained by picturing the 4-inch cube, where the top of the cube is dry and shrinks by 0.002 inch, while the bottom is moist and does not shrink. The cube would have a wedged shape. Place these cubes tightly together, end-to-end, for a distance of 20 feet and the slab will have a curled shape.

It is difficult to observe the slight curvature of curling, but measurements have been made on highway pavement. For a slab 15 feet in length, measurements in the morning showed curling where the edges of the slab picked up about 1/8 inch. In the afternoon, the slab essentially was flat. The effect of the sun caused the top surface to expand, which countered the shrinkage, and the slab leveled out. When night returned, the slab cooled and shrinkage again created the curled shape of the slab.

There are situations where curling is restrained, such as a slab joined to a perimeter footing. When curling is restrained, the stresses on the top surface of the slab are greater than if the slab was free to curl, so cracks occur sooner. Because curling cannot be prevented, drying shrinkage cracks in concrete slabs can only be minimized, not prevented.