Concrete can make a great patio, driveway, or retaining wall. However, precautions are necessary to avoid unsightly cracks. In this article, we will discuss construction techniques to avoid concrete cracks.
Plastic Shrinkage Cracks
Wet concrete will shrink as it starts to dry or cure. This is due to evaporation of the water in the concrete. The cracking that forms during the curing process is call plastic shrinkage cracks. Plastic shrinkage cracks are typically caused by too much water in the concrete mix. Adding more water to the mix makes the fresh concrete more workable and easier to install. However, the downside is additional water weakens the concrete and and causes more shrinkage, i.e. more cracking. The soupier the concrete, the more cracks that will form and the weaker the concrete will be.
To avoid this type of cracking, use a drier concrete mix. This will make the concrete harder to place, but using less water increases the strength of the concrete. A good contractor will know to avoid adding excess water to the concrete mix. A good rule of thumb: the weight of water should be no more than half the weight of cement in a concrete mix. A gallon of water weighs 8.33 lbs. So, for every gallon of water, there should be at least 16.7 lbs of cement. Don’t forget your aggregate!
Crazing Cracks
Concrete will crack along the surface if the concrete dries too fast or gets too hot during the curing process. The ideal temperature for concrete to cure is between 50 F and 85 F. The curing process creates heat, so just because the air temperature is below 85 F does not mean the concrete will be. Wind and direct sunlight will dry out the concrete. Moisture is required for the concrete to cure and develop strength. As noted above, this does not mean adding additional water to the concrete mix.
Crazing cracks are mitigated by keeping the concrete wet and cool. This can be accomplished by lightly spraying or misting the surface of the concrete with water. Cover the surface of the concrete with a tarp to trap in the moisture. Alternatively, place wet burlap over top of the concrete until it cures. It takes up to 7 days for the initial curing process, so plan to keep the surface of the concrete wet and cool for a full 7 days to reduce cracking.
Lack of Control Joints
Even if the guidelines above are followed, concrete will still crack. Concrete typically shrinks by 1/2-in across 100-ft during curing. Control joints are grooves placed in the concrete to force a crack in a that location. Control joints are spaced in feet 2-3 times the thickness of the concrete in inches. So a 4-in thick patio requires control joints placed every 8-12 ft. A 8-in thick concrete retaining wall requires control joints every 16-24 ft. Control joints must be placed to a depth of 25% of the concrete thickness. So a 4-in patio requires 1-in deep control joints. Place control joints in abrupt changes in geometry, such as a bend or corner either in the concrete or in the object it is abutting to.
For a patio or other slab, use a groover* to form the joints on the concrete surface within an hour of placing the concrete. For a vertical face, such as on a retaining wall, use a concrete saw to sawcut the necessary groove immediately after removing the formwork. Remove formwork 24-48 hours after placing concrete. If you do not have a concrete saw, place a rod in the formwork to create the control joint. Use a form release agent* on the rod (and formwork) to prevent it from sticking to the concrete.
Settlement Cracks
If the ground underneath the wall or slab shifts or settles, the concrete will crack. Therefore, it is important to ensure you have a properly compacted foundation under a retaining wall. A patio or driveway requires a properly compacted subbase. A well draining aggregate helps prevent water buildup under a concrete structure. This prevents movement from frost heave and freeze thaw cycles.
Other Causes of Cracking
There are a few other less common causes of concrete cracking:
- Tree roots will slowly crack concrete.
- Water can washout the foundation material or subbase. Properly diverting surface water can prevent this.
- The ground under the concrete structure can shift.
- Differential settlement can occur. An example is when a concrete structure is partially bearing on a rocky surface while the rest bears on soft soil. As the soft soil compresses over time, the leading to part of the structure settling while the portion on the rock does not.
- Whenever a concrete structure abuts another object, such as a foundation, retaining wall, or house, always install expansion joint* material between the existing structure and the fresh concrete. Check out this post for additional details and a lesson learned.
With all concrete cracks, it is important to monitor them. Cracks that progressively increase in size should be dealt with immediately. In the next article, we will discuss when and how to repair cracks in concrete.
Do you have questions or additional advice? Share them in the comments!
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