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TiltonBB · 08-07-2021, 02:04 PM

A rule of thumb is: For every 20 running feet concrete will shrink about 1/4 inch as it drys and sets. That is why concrete has control joints, to tell it where to crack so that you get a straight line.

Newer concrete should be air entrained. That is microscopic air bubbles chemically induced into the concrete to give it help with expansion and contraction. Many concrete companies use a product like Daravair manufactured by WR Grace to enhance freeze/thaw protection. It is put into the sand before it goes into the truck. Concrete placed outdoors will typically have 3% to 5% air entrained. When it is being placed air entrained concrete will appear to be wetter than it really is so it is easier to work with.

Concrete placed during the winter in New England will arrive heated in the truck to help start the curing process. That is usually done by making the concrete at the plant using hot water as it is loaded into the truck.

Also, regarding cracking, many basement floor contractors will tie the rebar in the floor to the rebar in the walls. That is wrong. It will prevent any contraction in the slab and most certainly cause it to crack.

08-07-2021, 02:04 PM	#7
TiltonBB Senior Member Join Date: Feb 2005 Location: Gilford, NH and Florida Posts: 2,897 Thanks: 644 Thanked 2,153 Times in 900 Posts	A rule of thumb is: For every 20 running feet concrete will shrink about 1/4 inch as it drys and sets. That is why concrete has control joints, to tell it where to crack so that you get a straight line. Newer concrete should be air entrained. That is microscopic air bubbles chemically induced into the concrete to give it help with expansion and contraction. Many concrete companies use a product like Daravair manufactured by WR Grace to enhance freeze/thaw protection. It is put into the sand before it goes into the truck. Concrete placed outdoors will typically have 3% to 5% air entrained. When it is being placed air entrained concrete will appear to be wetter than it really is so it is easier to work with. Concrete placed during the winter in New England will arrive heated in the truck to help start the curing process. That is usually done by making the concrete at the plant using hot water as it is loaded into the truck. Also, regarding cracking, many basement floor contractors will tie the rebar in the floor to the rebar in the walls. That is wrong. It will prevent any contraction in the slab and most certainly cause it to crack.