A reader's Concrete and Concreting question: Hand mixed concrete?

Matt S   from   Australia   had this Hand mixed concrete question.

Hi Bill A question regarding pouring concrete in forms. If I were pouring a column at 2.4m tall by 400mm square is it possible to do this by mixing batches continually in a cement mixer say a 3cu ft model?
Other than being very tiring and ensuring batch consistency, would there be a problem because the first layer would be stiffening earlier than the next, or would this help as it would reduce the pressure slightly at the bottom of the form?
I also wondered if this column is being poured on top of a footing which obviously has previously cured, what preparation is required to the top face of the footing to ensure a good bond between it and the poured concrete of the column?

Bill's reply

Regarding mixing and pouring by hand. Should be no problem provided that you can keep up a steady rate of pour.

As a three man team we could do say 21 off 225 x 225 x 2700 high house columns in a morning, (you are talking about doing slightly less than four) at a time. You will need a hand though to keep up a reasonable speed.

We used to vibrate the concrete by belting the steel forms with a hammer and tamping the inside with a lump of timber. A small electric vibrator (Hire) would be excellent for this though.

I would say treat the form ply with some sort of form oil. Clean the forms really well after each use. What happens is that by dropping it in by bucket, some of the mix sticks to the sides of the form and on a slow pour it could start to go off. When the forms are stripped, this stuff that has partly dried on the form can be a bit flaky.

It is not really a structural defect, just cosmetic so bear it in mind.

Regarding the join between separate day's pours.

Strictly speaking, on an engineer inspected job, joints in walls, walls to footing, or the bottom top of columns, should be scabbled to remove the laitance.

That is, the watery, sandy scum that rises to the top with the vibration of the mix, should be chipped away and removed, to expose solid sharp clean concrete. This gives a good bond for the new concrete.

Normally, with residential jobs, providing that good quality concrete is used, then nobody bothers doing this. Sometimes in really obvious cases, like heavy rainfall while pouring or, in the case of mixing it on site, you used a too wet mix then it is advisable to scabble the joint surface.

The scabbling is done on small jobs with usually a hammer drill with a chipping chisel.

Larger jobs mostly use needle scalers, air scabblers etc.