Concrete question - DIY Chatroom

blue_can · 11-05-2009, 11:15 PM

I have a question for the concrete experts here.

Is it possible to pour concrete as two separate pours and have the second pour bond to the first pour as though it were a single pour. The second would be on top of the first.

The reason for this is that I’m pouring a small footing for a small (about 18” – 24” high) side wall. The original soil had a slope. I removed the slope for the footing trench but this means one side of the trench is higher than the other. I was planning to pour a step shaped footing with the lower step forming the base for the wall and the upper to rise up to the higher level on the other side to form a small retainer (reinforced with vertical rebar).

However, I was only planning to place a wood form on the lower side (lengthwise) and let the concrete fill upto the dirt on the other sides. So this poises a problem for making a form to pour the higher “step” ssince there will be no support under the form so I was wondering if I could let the lower part cure first and then use that as a support for the form for the upper step.

Bob Mariani · 11-06-2009, 07:35 AM

The two pours will not bond as one, nor be the same strength. If this must be done, then rebar needs to be used to tie the two sections together and a bonding agent used.

blue_can · 11-06-2009, 03:18 PM

Thanks for the reply Bob. I was curious on what they do for big projects involving concrete (say the construction of a dam). I guess it takes months to complete project like that rather than 1 hour. What techniques to they use to bond all the concrete together - is it the same as what you mentioned.

Daniel Holzman · 11-06-2009, 04:47 PM

Under certain circumstances, it is necessary to have a continuous concrete pour. This is known as a monolithic pour, and if it involves a lot of concrete (say a dam or a bridge abutment), a lot of engineering goes into the design of the pour. If it is not essential to have a continuous concrete structure, the pours are separated by a joint, and we simply accept that the concrete is not going to be continuous across the joint.

If the structure is steel reinforced, the steel is almost always continuous across the joint, so the joint carries moment and shear via the steel reinforcing. The concrete carries compression load only, so the fact that the concrete is not continuous is not normally a problem (since the steel reinforcing is continous).

For residential construction, the only purpose of the steel reinforcing is for crack control of the concrete, therefore joints in the concrete walls are not a serious problem, as long as the joint is filled with a waterproof material.

Bob Mariani · 11-06-2009, 05:05 PM

I used to work on silos. Once the concrete pour stops it does not end until the job is complete. No separate pours are done on commercial work as a rule.

Willie T · 11-06-2009, 07:04 PM

Here in Florida, almost all footings that require a "step" are poured by simply beginning your pours at the lower portion of the step. Then you finish off the rest of the lower stuff, and then move on to the upper portions. Using properly stiff concrete, the lower parts will have set enough to stay in place when the uppers are "gently" poured.

This is how stepped-down garage slabs are poured. Also stairs...... from the bottom, up. The concrete will bond to concrete poured 15 or 20 minutes earlier with no problem... just the weight helps in that.