preventing concrete from cracking

[Chuck Wintle]

We wish to pour concrete inside a metal frame and there can be no cracks. What methods can be employed to prevent cracking. There is fiber that can be added but does it really work?

[Fred Belknap]

Keeping concrete from cracking is kind of like keeping wood from cracking. There is several factors involved. Concrete expands with heat and contracts with cold about 1/8" per degree per 100'. Concrete expand about the same as steel so that is one reason for using steel rebar in concrete. The fiber (glass) you mention helps to prevent cracks but leaves a kind of fuzzy surface. Air entrainment in concrete makes it much less prone to cracking, it is a chemical that lets tiny bubbles of air to become suspended in the concrete, think like fine foam. I think your best bet is to get the concrete company involved, they probably have a test lab to check the quality of the concrete. This is probably more info than you need but you didn't give many details. I use to be a concrete tech.

[Chuck Wintle]

air entrainment is not a possibility because maximum mass is necessary. Do the shrinkage-reducing admixtures really work to prevent shrinkage? basf and Grace each make their own brand.

[Chuck Wintle]

what would be some of the more practical suggestion to preventing concrete cracking? I know that slowing water evaporation is a big one...but the final product we need is a solid block of concrete inside a steel ibeam frame, and a second frame is to be added on top of this one, concrete filler also. possibly welding in rebar will work alos.

[Eddie Watkins]

I put a lot of extra fiber in my shop floor when I built it 11 years ago and, so far, no cracks. Fred is right, there will be lots of little fibers sticking up when you are done. I got a buffer and knocked off the little fibers after the concrete dried a couple of weeks. That solved the problem.

[Fred Belknap]

air entrainment is not a possibility because maximum mass is necessary. Do the shrinkage-reducing admixtures really work to prevent shrinkage? basf and Grace each make their own brand.

air entrainment is done by adding a chemical at the mixing plant. It is required on many jobs like roads and bridges. It's isn't an expensive additive. It is something the inspectors check for.

[Chuck Wintle]

Thanks Fred and Eddie for those words of wisdom!

chuck

[Steve Griffin]

When I had my shop slab poured, the concrete guy said it comes with two guarantees:

1)No one will steal it
2)It will crack.

-Steve

[Brian J. Williams]

To reduce the amount/frequency of cracks-

1. Keep the water-cement ratio to less than 0.45- the weight of water to the weight of cement in the mix. It will be a stiff mix, but adding water to increase workability will decrease the strength and increase the long-term shrinkage. You could add a watrer reducing agent (superplasticizer) to temporarily make the concrete more workable, but whatever you do DO NOT add more water - it only damages the durability and causes the concrete to crack as it cures. Screwing with the water-cement ratio is the number one cause of unstressed cracking of concrete.

2. You could add fibers to the mix to reduce cracking. Both steel and synthetic fibers have their applications- call your locaL supplier or go on line to get a phone number and ask them about your intended application.

3. Only use the water you need (see above) and mix in a power mixer.

BJW, PE, PG

[Scott T Smith]

To reduce the amount/frequency of cracks-

1. Keep the water-cement ratio to less than 0.45- the weight of water to the weight of cement in the mix. It will be a stiff mix, but adding water to increase workability will decrease the strength and increase the long-term shrinkage. You could add a watrer reducing agent (superplasticizer) to temporarily make the concrete more workable, but whatever you do DO NOT add more water - it only damages the durability and causes the concrete to crack as it cures. Screwing with the water-cement ratio is the number one cause of unstressed cracking of concrete.

2. You could add fibers to the mix to reduce cracking. Both steel and synthetic fibers have their applications- call your locaL supplier or go on line to get a phone number and ask them about your intended application.

3. Only use the water you need (see above) and mix in a power mixer.

BJW, PE, PG

Brian is absolutely correct. The number one cause of concrete cracking is too much water in the mix. I have used superplasticizer agents before with very, very good results.

Something else to do is to allow the concrete to cure slowly, and at around 50 degrees F.