Dear Tim,
After having a few of my drive spall off some 3 years old. I have drove
around town and to see just how bad the problem is. Talked to the ready mix
companies and been on the internet.
What I have noticed is that all 3 Ready mix companies and just about all
the concrete contractors have issues with spalling concrete even the City
has its own crew pour concrete and they are not immune from this problem.
Also all ages of concrete are affected.
I noticed that some driveways were deteriating worse than the side walk.
Yet both were doing it after talking to the supplier they said that the
sidewalk was machine poured and required a slump of 1-2 as it formed and
poured at the same time. This is why many concrete Hiways have less
spalling problems.
The contractors that the least of problems used a cureing sealer at
finishing time.
All the suppliers snd most of the contractors all have agreed that concrete
requires maintence.
Do not let snow or ice sit on concrete this allows many freeze thaw cycles
poping top of concrete.
Sealing of concrete every fall will help protect concrete.
Driveways that dont get sun to dry out concrete will see more problems than
south or west facing driveways. Also flater driveways will have more
problems due to water staying on them longer.
Driveways should be rinsed off to get road deicers off of driveway.
My conclusion is that concrete will be poured all year long so talk to the
contactor to make sure that He goes the extra mile to protect the concrete
and uses cureing sealer. Then the customer needs to do their part to keep
the driveway shoveled and seal in fall of every year.
Thanks
Mike
Deicing Salts And Concrete
Summary: Rock salt can be damaging to concrete driveways, walkways and patios if the concrete installation was not originally done correctly. Concrete's compressive strength, air entrainment, placement and finishing are all important to concrete's ability to resist freeze / thaw cycles and salt.
DEAR TIM: We just moved into our new house. Our builder has advised us not to use rock salt on our concrete sidewalks and driveway for snow and ice removal. He told us that it will damage the concrete. I don't believe him, because our city uses it on our streets every winter. Does my builder know what he is talking about? S. D.
DEAR S. D.: Your builder's advice is accurate. He is well informed on this subject. Rock salt can be one of concrete's worst enemies. Avoid using it on your concrete sidewalks, driveways, and patios if they have not been installed correctly.
Concrete is a magnificent material. As a paving material, it can be one of the longest lasting surfaces that you can use. However, its useful life can be significantly reduced if you do not recognize its weaknesses.
Concrete has high strength when it is compressed, or 'squeezed'. However, it is extremely weak when it is subjected to tension, or 'pulled'. Rock salt can take advantage of this weakness.
Believe it or not, while concrete appears to be a very dense material, it is in fact quite like a blotter. It can and does absorb water. You can actually see this happen on a hot summer day. Sprinkle some water on your sidewalk or driveway and look very closely. You can actually see the water penetrate the surface of the concrete.
When you spread rock salt on your concrete to melt snow and ice, the salt dissolves the snow and makes a salt water mush. The melting action of the salt allows water to enter the concrete. If the temperature then drops and the water freezes, the growing ice crystals can blast apart the concrete.
Salt is also hygroscopic. It attracts water. It can cause concrete to become more saturated with water than it would otherwise. The presence of this extra water in freezing conditions can spell trouble. The volume of water increases by 9 percent when it freezes within the concrete matrix. The pressure of the growing ice crystals can cause the surface of the concrete to fail. It usually spalls off.
Freshly poured concrete is most susceptible to damage. Concrete placed in the late fall needs at least 30 days of drying time. This young concrete is still highly saturated with water. The water within the concrete can freeze and cause the surface to pop off. However, if enough cement was in the initial mixture and this cement was not diluted by the addition of water, the concrete will be able to resist the damaging forces of the freezing water.
Always order concrete that will attain a minimum compressive strength of 4,000 pounds per square inch. Be sure that is air entrained as well. These two things are a good defense against salt attack. Concrete which is ordered, placed, finished, and cured properly can resist years of contact with rock salt. Concrete surface failures such as spalling can almost always be traced to workmanship errors.
The placement and finishing of the concrete is critical as well. The upper surface of concrete can be severely weakened by poor workmanship. Sometimes workers add water to concrete at the jobsite or use it as a finishing aid. These practices dilute the amount of cement at the surface of the concrete. The cement is the ingredient in concrete that holds everything together. To resist the freeze/thaw action of water, you need to have strong concrete at or near the surface.
There is an alternative to using rock salt. You can use sand. The sand will not melt the snow and ice, but it will provide you with traction.
You can also treat your concrete with clear coatings that minimize or eliminate the possibility of water being absorbed by your concrete. Some of these clear coatings contain silanes and siloxanes. These ingredients allow the clear coatings to breathe. Avoid using products that contain silicone or paraffin. These can produce a surface film. A surface film does not allow the concrete to breathe. Concrete soaks up water from the soil. This water passes through the concrete and eventually evaporates. However, if you trap this water at the surface with film forming sealants, you may cause spalling. Be careful!
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Comments:
Mike 10 Apr 2008, 18:45
AsktheBuilder 13 Apr 2008, 09:13
Mike,
You should read all of my columns on Deicing Salts and Concrete. There are many. Bottom Line: You can install concrete and NEVER have to seal it, and this concrete will NOT spall. Want to see some that is 20-years-old? Come to my house. It gets salt on it each winter, I rinse it off, it goes through countless freeze-thaw cycles, etc. My driveway apron looks like the day it was installed. Read all of my columns about Concrete Installation if you want to discover how I would install concrete at your house. View all comments |
