Why Concrete Cracks: Common Causes Explained
Concrete may be regarded as being indestructible, and to a large extent, it is. It can withstand extremely high compression, it is one of the most lasting building materials and it has been utilized in buildings that have been standing for centuries. But it cracks. Over time, almost every concrete surface will develop cracks, and when people notice that a new driveway or patio is developing crack lines they didn’t expect, it can feel like something is wrong.
Sometimes it did. In some instances, cracking is a predictable consequence of forces that concrete cannot withstand without appropriate design and installation. The reason behind a crack will provide you with an understanding of whether it is normal behavior, something you can correct, or something that is a matter of immediate concern. It also provides you with information about what you need to do to ensure that it doesn’t happen again after the repair.
For more than 37 years, Planit Dirt Excavation and Concrete has been performing concrete work in Jacksonville, Little Rock, Bryant, Sherwood, Conway, Cabot, Benton, and the surrounding central Arkansas areas.
Following are the most basic and common causes.
Shrinkage During Curing
This is where a significant number of concrete cracks start and it is one of the less understood properties of concrete.
The drying process of concrete is different from paint. It hardens by a chemical reaction called hydration, which involves water reacting with cement particles to create the crystalline structure of concrete, responsible for its strength. During the course of that reaction, concrete shrinks. Not dramatically, but enough to cause internal tension if the slab is unable to move freely, or if it dries unevenly.
The surface of a new slab may dry out faster than the interior on a hot, dry or windy day in central Arkansas. The surface is trying to shrink and the interior is still plastic. That difference creates surface cracks, which can manifest in a fine network pattern, known as crazing or map cracking, that are visible in hours or days after pouring.
The standard solution is control joints, the lines cut or formed into a concrete slab at regular intervals that create intentional weak points where shrinkage cracks are designed to occur. A crack that forms along a control joint is invisible because it happens where the concrete was already separated. A crack that forms between control joints, or on a slab with no joints at all, is visible and often difficult to repair cleanly.
Spacing control joints correctly for the slab thickness and dimensions is part of the design process. A slab poured without adequate joint spacing will crack on its own schedule in locations that have nothing to do with where repair is straightforward.
Subbase Failure and Soil Movement
If shrinkage cracking is about what happens above the ground, subbase failure is about what happens beneath it, and it tends to produce more serious damage.
Concrete slabs don’t float independently. They sit on the ground and whatever the ground does, the concrete responds to. If the ground under a slab settles unevenly, washes away or compacts under the load, the slab will not be supported in that area, and the unsupported sections will crack from their own weight or from the load they are carrying.
The underlying variable for most subbase cracking in central Arkansas is the type of clay soil. Clay swells with water and shrinks when it is dry. That cycle happens seasonally on a driveway slab that’s resting on unmodified clay; it may happen more than once in a year, during wet and dry periods. When the base preparation is not taken into account for the effect of clay, the movement is directly passed to the upper surface.
Voids beneath slabs develop for different reasons. Over time, water erosion will tend to wash fine material under the edge of slabs, especially if not managed properly at installation. Tree roots under a slab decompose over the years and the concrete fills the void until the unsupported span exceeds the load capacity. Localised settlement occurs when heavy vehicles are driving over an inadequately compacted area.
All of these produce cracks with vertical displacement, where one side of the crack sits higher or lower than the other. That offset is the signal that the ground has moved, not just that the concrete shrunk. A repair of the surface area without curing the underlying problem will result in a repaired crack, which will reopen in the same position.
Overloading
Concrete is strong under compression, that is, weight pressing down on it. It has much less capacity to sustain tension, being pulled apart or bent across a span. If a slab is carrying more than it was designed for, it fails in tension and cracks.
A house driveway designed for car parking is quite different when large delivery trucks or loaded trailers routinely park at the house. A standard thickness patio may not be enough when a hot tub or large outdoor kitchen is installed on top of it.
Failure by overloading doesn’t always happen at once, either. It will typically manifest itself as slow cracking that will start and progress over the course of several months until the accumulated load becomes too much for the slab. Any changes in use of a surface since it was poured will need to be taken into consideration when assessing the reason for the cracks appearing.
Freeze-Thaw Cycling
While central Arkansas doesn’t experience the same sustained winter cold as northern states, hard freezes do occur and when they occur coupled with moisture, they can cause concrete to deteriorate as well. Water that has penetrated concrete through cracks or deteriorated sealant expands when it freezes. That expansion generates internal pressure, which helps to enlarge the cracks and cause new ones to appear at weak points.
As temperatures increase and the water melts, it shrinks, making the concrete a little worse. Each cycle compounds the damage. If moisture is already in the slab, an overnight freeze followed by a thaw the next morning is sufficient to advance cracking.
Repairing cracks, not letting water in and keeping surfaces properly sealed break this cycle before it can form.
Mix Problems and Poor Curing
Not all cracks are evidence of soil movement or overloading. Some begin in the concrete itself, or in the mixing or curing processes. Water is one of the most common causes of cracking that can occur early in a concrete mix. Fresh concrete is easier to work with when it has water; this is why it is tempting on a hot day when the mix is stiffening rapidly. However, for every additional gallon beyond the mix design, the final strength decreases and the shrinkage during curing increases.
Weaker concrete that shrinks more cracks sooner. Curing conditions matter just as much. Concrete that loses moisture too quickly before the hydration reaction has built adequate strength develops surface cracking fast. On a hot, dry, windy Arkansas day, the surface of a slab can set much faster than the interior, creating differential conditions that crack the surface within hours of the pour.
Proper curing means keeping the surface moist for an adequate period, not just until it is hard enough to walk on.
What This Means for Repair and Prevention
Understanding why a specific crack formed changes what the right response is.
If shrinkage cracks have been stabilized and are not sufficiently large to permit water to enter the cracks, they may be sealed and monitored without more extensive intervention. Cracks from subbase failure need the cause addressed underground before a surface repair makes any lasting sense. Cracks from overloading need either a change in how the surface is used or a replacement pour designed for the actual load it needs to carry.
Looking at the pattern, location, and whether any displacement exists between the two sides of a crack tells most of the story about what caused it. Planit Dirt assesses the full picture before recommending a repair approach, because the visible crack is the result and the cause is almost always somewhere underneath it or in the conditions surrounding the original pour.
Call James at (501) 258-5935 or email planitdirt@yahoo.com for a free assessment across Jacksonville, Little Rock, Bryant, North Little Rock, Sherwood, Conway, Benton, Cabot, Alexander, and Maumelle.