How to minimize cracking on ready mix concrete surfaces?
Updated: Mar 8, 2019
Let's face it, there isn't one contractor out there that has never experienced a surface crack in their ready mix concrete project. It's inevitable and unavoidable. There is no way anyone can produce crack-free concrete, but there are ways to minimize the visible cracking on the concrete surfaces. That is what we're here to inform you of today.
How to minimize surface cracking on your ready mix concrete project?
Subgrade & Prepping
1. Make sure when prepping the area for the concrete all soft spots & topsoil are removed, make sure your soil or granular fill is well compacted by rolling, vibrating or tamping.
2. A smooth & level subgrade will help prevent cracking, but ensure there is a slope in the grade for proper drainage.
3. If you're installing ready mix concrete outside in the winter make sure you remove any snow or ice prior to placing the concrete & never place concrete on a frozen subgrade.
4. Brace your formwork so it can withstand the pressure of the concrete without moving.
5. You should know that if you use vapor retarders directly underneath a concrete slab this will increase bleeding, which in turn will increase the potential for cracking. If you do use a vapor retarder make sure you cover it with 3 to 4 inches of a compacted granular fill. This will help reduce the bleeding of the concrete.
6. If you are placing concrete when the threat of severe drying conditions may exist, dampen the formwork, any reinforcements you may have & the subgrade immediately prior to placing the ready mix concrete.
7. Concrete Slump. Generally speaking your concrete mix should not be over 5 inches in slump. Try using water reducing mixtures instead of adding excessive water to the mix, if you need a higher slump. You want to avoid excessive bleeding & or segregation as this will cause cracking.
8. Prior to placing the concrete, avoid retempering the slump.
9. If placing concrete outdoors, make sure you specify that you want an air-entrained mix because it will allow the water to migrate into the entrained air bubbles to release pressure when freezing.
Finishing Concrete Surfaces
10. When performing initial screeding on a concrete slab, it must be followed by bull floating (do not confuse bull floating with floating).
11. If water is present on the surface or if the concrete hasn't completed finished bleeding, DO NOT perform any finishing operations.
12. Avoid over-working or over-finishing the surface.
13. If you are looking for better traction on exterior surfaces use a broom finish.
14. Avoid rapid drying between finishing operations. If this occurs you can use wind breaks, fog sprays & cover the concrete with wet burlap or polyethylene sheets.
Curing & Setting of Ready Mix Concrete
15. Spraying the surface with liquid membrane curing compound or covering it with damp burlap to keep it moist for at least 3 days while curing can help ensure durable crack-resistant concrete. Also, giving the surface a 2nd application of the curing compound the next day is a good quality assurance step.
Joints In A Ready Mix Concrete Slab
16. Contraction Joints/Control Joints: No matter what, your concrete will crack. As the old saying goes; there are two types of concrete, concrete that's cracked and concrete that hasn't cracked yet. What you need to do is control how and where concrete will crack. Contraction joints are placed in predetermined locations in the concrete's surface to avoid random cracking. By inserting the contraction joints you can control the cracks and have the cracks form along the straight contraction joint you inserted instead of cracking in an unsightly manor all over. In this way, the normal cracking that occurs will occur where you want it to in your joints. See picture below.
17. You can use concrete grooving tools or a early entry dry cut saw to cut contraction joints into fresh concrete, or a concrete saw to cut joints into hardened concrete. The best time to cut contraction joints is between 6 & 18 hours after finishing the concrete. You never want to cut contraction joints in the concrete after 24 hours.
17. The contraction joints should be 25% of the depth of the slab. Example, if your slab is 4" thick, the joint should be 1" deep.
18. The contraction joints should be no more than 2-3 times the thickness of the slab. Example, if your slab is 4" thick the spacing of the contraction joints should be between 8-12 feet apart.
19. Do not place contraction joints any further than 15 feet apart.
20. If restriction to freedom of vertical or horizontal movement is expected insertion of isolation joints are recommended to help prevent cracking. Some examples where you would need isolation joints would be when floors meet walls, columns, footings, drainage inlets, manholes, & lighting structures.
21. Unlike contraction & control joints isolation joints are full depth joints that are constructed by inserting a barrier of some type to prevent bond between the slab and the other structure.
Covering of Reinforcement Steel in Concrete
22. Covering reinforcement steel at least 2 inches in thickness will help keep salt & moisture from contacting the steel & will prevent cracks in reinforced concrete by eliminating the expansion of rust on the reinforcing steel.
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1. Concrete in Practice, CIP4 "What, Why & How? Cracking Concrete Surfaces"
2. "Control of Cracking in Concrete Structures", ACI224R, American Concrete Institute, Farmington Hills, MI.
3. "Guide for Concrete Floor & Slab Construction", ACI302.1R, American Concrete Institute, Farmington Hills, MI.
4. "Concrete Slab Surface Defects: Causes, Prevention, Repair" ISI77, Portland Cement Association, Skokie, IL.
5. Grant T. Halvorson, "Troubleshooting Concrete Cracking During Construction, Concrete Construction", October 1993.
6. "Cracks in Concrete: Causes, Prevention, Repair", A collection of articles from Concrete Construction Magazine, June 1973.
7. American Concrete Pavement Association website: http://metiebar.acpa.org/Concrete_Pavement/Technical/FATQ/Design/Expansion_Joints.asp
8. PCA America's Cement Manufacturers website: https://www.cement.org/learn/concrete-technology/concrete-construction/contraction-control-joints-in-concrete-flatwork