 | Justin Crain is VP / Treasurer of American Concrete Co., Inc., which is owned by his parents, Dennis and Jane Crain. American Concrete is celebrating its forty-first anniversary and Justin is the third generation to be involved with the family company. American Concrete was awarded the 2011 Pittsburg Area Chamber of Commerce Small Business of the year award. Justin is currently the President of the Kansas Ready Mix Concrete Association and is currently on the board of directors for the Mount Carmel Foundation as well as the Pittsburg Public Library Foundation. Please visit our website, www.americanconcreteco.net, or call 620.231.1520. |
Concrete
2012-04-20 09:57:01
Avoiding cracks
Q: I am a DIY kind of guy and looking at pouring my own patio. I did a slab for my shed last summer and got a bunch of fine cracks on the surface. How do I avoid that for this project?
A: From the information you provided, I believe the cracks to which you are referring are plastic shrinkage cracking. These cracks appear 1 to 3 feet apart, relatively shallow, and generally do not enter the perimeter of the slab. While unsightly, these surface cracks do not impart the strength or durability of the concrete.
These cracks form on fresh concrete while still in the plastic stage and caused when the rate of evaporation is faster than the rate of bleed water, causing tensile force in the surface layers.
Some of the contributing factors may include:
• Wind velocity in excess of 5 mph
• Low humidity
• High ambient and/or concrete temperatures
It is important to remember that anything that delays the setting increases the possibility of plastic shrinkage cracking, like cool temperatures, cool subgrades and high water content.
To avoid the problem, pour concrete under ideal conditions and minimize anything that would contribute to their formation. You might consider putting up temporary windbreaks to reduce wind velocity over the surface of the concrete or sun shades to control surface temperature. If it is very hot, consider using a fog spray to control evaporation. Other important things to consider would be having enough manpower on hand to finish the task promptly, and the proper materials available. Finally, consider your choice of materials. Consider using synthetic fibers which help resist cracking. The synthetic fibers do not replace rebar, but do replace wire mesh, as the fibers are mixed evenly through the concrete.
