Cement slurry as bonding agent

[ajk1]

The contractor wants to use a cement slurry as a bonding agent, to bond a new concrete where the top concrete has been removed to repair corrosion to top slab and beam bars, rather than using a proprietary bonding agent.

I have done this very successfully about 10 years ago and it gave good bond strengths when tested by the CSA bond pull test. I think at that time that I had the contractor proportion the slurry for a w/c = 0.40 and work it in to the concrete surface with a stiff bristled broom. But now I am not sure how to proportion the cement and water to get a w/c=0.40 becasue I am not sure if there is a "bulking" factor for the cement that has to be included in the calculation.

The contractor want to mix the slurry in 5 U.S. gallon pail. I figure that equals 0.67 cubic feet. If portland cement weighs 100 pounds per cubic foot, then to get w/c=0.40, my calculation is as follows:

w / c = 0.40
c/100 + w/62.4 = 0.67

where w = weight of water, pounds
c = weight of cement, pounds

Solving these 2 equations gives w = 2.75 pounds and c = 6.87 pounds.

But is that right? Is there a cement bulking factor to be included, and if so how do I include it?

 

[Ron]

ajk1....cement weighs about 197 lbs/cf. No bulking factor needed.

A w/c of 0.40 is probably too high for such a bonding paste. Remember...it needs to be a paste, not a slurry.

I would do a trial batch by consistency, not by w/c. Wet it a little extra if needed to keep if from drying out.

You are exactly right about the application procedure.

 

[ajk1]

Ron - thanks very much for the help.

 

[SkiisAndBikes]

ajk1, I have also successfully bond tested concrete repair patches where a sand/cement slurry was used ... that is except for the one time the contractor forgot to add the cement to the slurry!!! What an unbelievably good bond breaker. At least is was relatively easy to jackhammer the newly poured concrete repair patches out and repour again (also had a contractor use only Part A of the 2 part Sika HiMod bonding agent ... very gooey but no bond).

Back to the sand cement slurry. CSA A23.1 clause 7.6.4.2.2 provides an accepted procedure for sand cement slurries. Summarized as follows:
- proper surface prep
- prewet surface for at least 1 hour (remove any standing water for SSD)
- 1:1 sand:cement ratio with enough water for a flowable consistency
- w/c ratio similar to that of concrete being placed but in no case more than 0.45. I agree with Ron that consistency is key, I liken it to a thick cream.
- there are additional recommendations on maximum coarseness of sand and a definition of flowable consistency linked to a CSA test method.

In my experience, timing of the application is important, as well as scrubbing it into the surface. I preferred it when the slurry was scrubbed into the surface with a boat brush.