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What causes cracks in a driveway?
Wednesday, September 19, 2018
Everyone notices cracks in their driveway. Weeds pop up and anthill mounds spill out of the crevices. Most driveway cracks are small and considered non-structural, but cracks with wider openings and heaved edges can be trip hazards and should be repaired.
A driveway or walkway can crack for several reasons:
•• Tree roots can push up under a driveway and cause heaving, but they can also cause cracking and a downward movement of the slab from excessive moisture some trees remove from the soil under the area.
•• Soil erosion is the slow washing away of soil on a sloped site due to rainwater flow from a higher level down to a lower elevation. It is often more pronounced near creeks and rivers because water is also moving under the soil toward the lower levels. The photo below shows where is a flow of water under the driveway has started to create a void. See our blog post What is the difference between soil subsidence, heave, creep, and settlement? for more on this subject.
•• Excessive weight placed on a driveway slab, especially at outside corners, will cause settlement and cracking—like in the photo at the top of this page. An example would be a concrete or septic pumping truck with one set of wheels on the slab and the other set on the ground.
•• Sinkholes can also cause driveway damage. While rare, they do occur in our area and most of Florida. Not all sinkholes are as dramatic as the ones in the news that swallow up a house overnight. Many start out as a small depression in the soil that grows over time. See our blog post ”What are the warning signs of a sinkhole?" to learn more.
•• Clay soil has an elasticity that causes it to shrink during dry spells, then swell during seasons with heavy rains. The swell/shrink cycle causes the ground under a driveway built over a layer of clay soil to heave up and down in seasons that are extra wet or dry. A hilly band of clay soil that geologists have labeled the “Hawthorn Formation” runs through the middle of Alachua County, roughly north-south along I-75 and east-west along Newberry Road.
•• Poor site preparation can cause driveway cracking in the first couple of years after construction, typically due to inadequate soil compaction before placing the concrete.
Concrete is strong in compression because the small and large aggregate stones in the mix can carry large loads. However it is weak in tension (bending). To offset this weakness, steel is added for strength. In the case of a driveway it would be a light gauge steel mesh embedded in the slab at the time the concrete is placed. Glass, steel or plastic fibers can be added to the concrete mix for added tensile strength. A concrete slab shrinks slightly during the curing process in the first couple of months after it is placed, and most driveways and walkways are scored to encourage the hairline shrinkage cracks to happen in the notched lines, where they are less noticeable.
These conditions can have the same effect on asphalt driveways and walkways, except asphalt also tends to crumble at edges as it deteriorates.
There are many products designed for home owners to repair small driveway cracks—from special caulks to hydraulic cement—but sometimes the cracks become more of an eyesore after filling them if the work is sloppy or the surface appearance of the fill material is significantly different than the surrounding concrete. Larger cracks that are trip hazards can be ground down even with the lower side of the crack by a concrete service. We consider an abrupt change in the surface of more than a half-inch to be a trip hazard. Driveways that have severe cracking will need the damaged area cut out and replaced.
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