Will concrete that is poured at two different times stick to each other??

[bigcountry]

I was curious about something i saw on someone elses thread. I believe it was the Oklahoma 20x30...he dug a footer and had rebar sticking out of the top for the posts. I was wondering if this same practice would work for posts rather than a footer? The intention is to dig 42" down with a 14" auger. Fill in with concrete and 3 pieces of rebar vertically inside. The ground is fairly level. After the concrete had set up and maybe a week later i would like to pour the actual smaller posts that would hold the simpson brackets for the 6x12 beam.

[hpinson]

I have the same question. I just did this.  Poured an 8" sonotube half full (for a grape arbor) and ran out of concrete. I came back 1 day later and finished up. I wetted the earlier surface. I have no idea if the two pours will bond.

[rwanders]

Generally---yes. If you know you are going to have a "cold joint" as you described, leave the surface rough---the best technique is to make a key way at the joining surface by using a length of 2x2 or 2x4 to form a key way when you do the first pour--remove the 2x before you do the second pour. Of course, if there is rebar connecting the pours, so much the better. For larger areas or slab pours you can also buy various brands of liquid bonding agents that also will help.

[Don_P]

Rwanders knows more about this than me. My understanding of concrete is that in the curing process the concrete is forming crystalline "needles" that knit it together. If there is a cold joint this cannot occur between the two sections of the pour, so you are relying on a mechanical connection between the two pours... the roughened surface, keys, or reinforcing steel. Bonding agents are simply glues trying to enhance that mechanical bond.

[Squirl]

I would make an intuitive guess that there wouldn't be a problem since you are pouring it over top and most of the force will be downward.  I look at it from the viewpoint that many walls are made from brick and mortar and the mortar "sticks" to the bricks and other mortar.  Along those same lines I have seen concrete block basements with just mortar or surface bonding cement and both are processes involving cement over a long period of time.  Also slip form concrete walls.  Also as you pointed out footings and walls must be poured separately in my jurisdiction.  All of these methods are well accepted construction designs and involve joining two separate pours of cement based material after curing.

IIRC, I saw an episode of this old house were a basement was done out of just large pre-formed concrete walls, tilted up and bolted together.  So there was no "wet" concrete bonding at all.

[bigcountry]

Many thanks to all for your input...I will be starting this project April 2nd by cutting trees down to clear the lot. I will begin posting pictures soon thereafter. As will the many more posts for questions I am sure.

[glenn kangiser]

As mentioned above, it is more of a mechanical joint after the first day or maybe less so rebar and rough is good.

Bonding agents started out as Elmers glue in the old days but are now enhanced in some cases.

It is great for build up and repairs for added strength and assurance of sticking.

[archimedes]

Is it ok to pour a footer in two separate pours,  with rebar spanning the cold joint?