Cracks! At the sight of a crack in concrete, most of us panic. Is my house falling apart? Does the foundation need replacement? Lots of questions and few answers. This article is intended to provide you with some basic reasons as to why concrete cracks and answer some of your questions.
Why Does Concrete Crack?
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Most cracks occur as a result of shrinkage of concrete. Shrinkage is simply a reduction in the volume of concrete as it hardens. If this reduction in volume were unrestricted, then a crack would not occur. However, in reality, ground friction and a number of things such as structural connections inhibit free shrinkage and thus cause cracks.
How much shrinkage is normal? A 100-foot-long regular-weight concrete slab normally would shrink by about 3/4 inch. In other words, you should expect cracks totaling in widths up to 3/4 inch in every 100 feet of concrete. Lightweight concrete shrinks more. It is important to note that concrete does crack and that this is normal. What is not normal is an unsightly and excessive amount of cracks.
Not every crack threatens the structural safety of a building. In fact, in many instances, cracks are merely cosmetic in nature. These cracks are typically seen in flat work such as walkways and curbs.
Typical causes of these cracks are:
- Poor workmanship
- Inappropriate joint detailing
- Higher shrinkage of concrete
Sometimes such nonstructural cracks in driveways and sidewalks become more than just an eyesore. Tree roots and impact from vehicles can cause raveling as well as vertical and horizontal offsets at the cracks. When these offsets become trip hazards, repairs are necessary.
A majority of structural cracks occur as a result of the following conditions:
- Design deficiency
- Construction deficiency
- Settlement or heaving of soil
- Reinforcement corrosion
Sometimes structural cracks manifest themselves with some side effects. Doors and windows do not open and close easily. Floors feel uneven. Vinyl flooring tears as a result of crack movement. Stucco begins to show new cracks and even interior corners may develop new cracks. Longitudinal cracks can develop along the length of the foundation as a result of corrosion of reinforcement.
What Do I Do Next?
When in doubt, consult a professional concrete engineer to investigate the cause of distress and evaluate the condition of the concrete.
As a homeowner, you could assist such a professional by keeping a periodic log of:
- Crack progression
- Crack widths
- Landscape irrigation schedule, especially big tree removal dates
Removal of a big tree next to a house can cause sudden changes in the soil moisture level and cause soil movement.
Investigation and Repair
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As in medicine, a good diagnosis is a must for a successful cure. Investigating the cause of the distress in concrete is necessary in order to recommend an appropriate repair material.
Also as in medicine, a variety of repair materials are available and each has a specific use. For example, some cracks are caused by movement in the structure. Repairing such cracks with epoxy could cause reoccurrence of a crack in the future at a nearby location. Use of an appropriate material and technique can not only produce savings on the cost of repair, but also can ensure a durable repair.
In summary, concrete does crack; however, not every crack is a structural threat to a building. The cause of a crack should be investigated prior to performing a repair.
Author Profile: Ashok Kakade, P.E., is a consulting concrete engineer with Concrete Science located in Castro Valley, California. He has more than 15 years of experience in construction, investigation, and repair of concrete structures.