At first glance, concrete obviously seems like a very dense material. In reality however, it absorbs water, behaving more like a sponge. This can easily be observed on a summer day. Pour water on a sidewalk, and you can actually watch the water penetrate and be absorbed by the surface of the concrete. This isn’t a bad sign; quality concrete should contain air voids, and should be somewhat absorbent—but it does underline the importance of caring for your concrete properly to avoid problems when it comes to ice and snow.
In regions where freeze-thaw cycles can be severe—such as here in the Northeast—concrete surfaces may show flaking and chipping damage known as spalling, or scaling, where the thin top layer pops off the concrete, revealing the underlying aggregate material. Preventing spalling and scaling begins with the proper installation and finishing of the concrete.
Concrete for sidewalks should have a minimum compressive strength of 4,000 pounds per square-inch (with higher strength and/or reinforcement for driveways), and should have proper air entrainment as well. Air entrainment refers to tiny voids left in the material to allow for the expansion and contraction of the freeze-thaw process. Six percent air entrainment is common in this region of the country. Non-air entrained concrete will not have these voids, and as a result can spall or scale much more easily.
In addition to the use of strong, air-entrained concrete, concrete installation should require a level, stable, and well-compacted sub-base with good drainage under the slab. An allowance for thermal movement, contraction and subsequent expansion can be made by providing joints of adequate spacing and depth. Sawed or tooled contraction joints should be a minimum of one-fourth the depth of the slab. Grades in the immediate area of the slabs should be set such that surface water is carried away from the slab and the concrete is not continually saturated with moisture.
The placement and finishing of concrete is critical to its longevity and durability. The top surface of the concrete can be severely weakened by poor workmanship, and surface failures such as spalling can be traced to workmanship deficiencies. Sometimes workers add water to concrete at the jobsite, using it as a finishing aid or improperly “floating” the mix. The cement is the element of concrete that binds the various materials together, and diluting the amount of cement at the surface of the concrete weakens the material.