Bonding Construction Joints 2

[PaddyViet]

The site I'm on is casting pad foundations and sub-columns in two stages.

So we cast the pad foundation, leave a few days to harden, chip the joint area back a bit for a nice solid interlock, put the sub-col formwork up and cast away.

For the majority of the work done so far, no bonding agent was used. I'm aware of sika grout but for bungalows it's a bit of an overkill.
The contractor has suddenly decided that adding a few mm of cement powder to the bottom of each formwork before pouring will give a stronger bond. As far as I can see, it'll only fill up the chipping and rob the new concrete of water.

The senior site engineer is happy to go with a small amount of VERY watery mix of cement powder and water. I think I can see the use, it provides extra water to saturate the old concrete, the cement powder might help gum up any cracks from chipping and the two will mix well with the poured concrete.
I'm just not convinced the benefits are worth the faff. are there any good sources for a site newbie like myself?

 

[Ron]

"Neat cement" (a mixture of cement and water) has long been used as a "bonding agent"...before the commercial bonding agents came about. It works well and as long as it isn't too thick, will serve its purpose.

 

[Zambo]

I would just get them to soak well before casting the concrete. As you mention this will stop an existing dry concrete surface drawing the water out of the new concrete. I wouldn't advise the use of additional cement as any possible advantage would be outweighed by the chance that it could be left to dry before casting the new concrete (perhaps a tea break or some interuption) causing a debonding rather than a bonding effect.

 

[rscassar]

I have never done anything special at the column-footing cold joint aside from providing a slightly roughen surface at the joint. Saturating the surface as suggested is a good idea but I don't get to concerned about it on-site when inspecting unless the bond is absolutley critical.

 

[PaddyViet]

That was the assumption I was working under.

As it is, the one set of cols they cast this way had the worst set of honeycombing so far, with two or three needing deep chipping and repairing at the join.

The contractor has quietly gone back to the old approach of chip, clean and cast.

 

[AAEC]

When I was a newbie on the jobsite, I saw contractor poured lean concrete first at the joint (probably about 1" thinck or less) interface between the footing and combination of retaining/containment walls. Then the contractor sub-sequently poured the regular concrete on top. I checked the drawings and sure enough it was engineered this way. There seems to be no problem when they took out the forms. The joints look good.

 

[JedClampett]

AAEC, did you mean lean concrete or grout? Lean concrete (50 to 150 lb. of cement per yard) would not seem appropriate, but we've used normal sand cement grout (~4000 psi) in the bottom two inches of the pour in the past.

 

[RHTPE]

Ron is on target as far as the "neat" cement. Generally though, column-to-footing joints never seem to get more involved than simply cleaning all loose material.

Regarding the honeycombing - that's a concrete placing problem that should be addressed.



Ralph
Structures Consulting
Northeast USA