Slag in Flatwork Conc.? 1

[Spudnik]

As an inspector in my city gov't, I noticed on a recent concrete flatwork pour that the supplier had added a product called, "Slag". While I understand their desire to use slag to replace hard-to-find Portland cement, and that flyash is no longer available here in the Pacific NW, the slag is definitely not included in our published specs for concrete flatwork mix, which is 6-sack Level I or Level II.

If we decide to allow slag to replace 1 to 1-1/2 sacks of cement per yard of 6-sack mix, what will be the result? While we're concerned with the long-term, I would like to also ascertain the immediate strength concerns, especially surface strength in flatwork. The product is imported, and apparently consists of finely ground oil-blast-furnace slag.

Does the addition of slag affect the percent of air entrained?

Thanks for all help. I am a first-time poster, so be gentle.

Rocky

 

[Ron]

I assume you are referring to Ground Blast Furnace Slag Cement (GBFS). This generally has no detrimental effect on the long term properties of concrete; however, the strength gain will likely be retarded, so you'll see a slower strength gain with GBFS cement. Most ready mix suppliers limit GBFS to 30 to 50 percent of the cementitious material. Some specifications (State Departments of Transportation for instance) will limit the amount of GBFS.

If your application has critical form removal and reshoring, be careful with GBFS because of the slower strength gain. Unless your ready mix supplier has a lot of data, the strength predictions that you are used to with conventional portland cement concrete will not work. For instance, you would expect that at 7 days with conventional pcc, you would get about 70 percent of its 28 day strength. With GBFS, you might get 45 or 50 percent, but it varies depending on the source and on the amount of GBFS in the mix.

 

[boffintech]

Plan on all of your 7 day strengths being lower than 70% of f'c.