BLAST FURNACE SLAG AS COARSE CONCRETE AGGREGATE - COMMENTS

[fne]

Contractor has submitted a concrete mix design using a blast furnace slag as course aggregate. This is permitted in ASTM C33, but our company has no experience in using it. Any comments, information would be appreciated. Accept it? Reject it? Reasons? Break tests show proper concrete strength. Quick research shows some absorptive qualities that may influence shrinkage cracking. How hard to control this?

 

[AnimusVox]

Our firm allows (and often requires) the usage of blast furnace slag or fly ash (never to be combined, always one or the other) as a substitute for some of the cementitous content in concrete, usually 15-25% of the total cement makeup. However we do not allow such additives to be added to exposed interior slab applications, I'm told for finishibility difficulties.

I have no experience using it as the course aggregate though. I would verify first that the combined course + fine aggregates pass workability and courseness requirements, as well as all of the applicable ASTM and other requirements. If finishibility is a concern, I would defer to someone that has used it on a project.

 

[Gumpmaster]

I've never had this submitted, but I guess I wouldn't have a problem with it depending on the following:

1) aggregate durability and soundness (including abrasion resistance)
2) Deleterious substances. These would probably be different than natural aggregates, but I'm sure there's impurities that could be in the slag.
3) Densities, gradations, etc...
4) Shrinkage. This is highly dependent on aggregate source. You can do a shrinkage test. A high shrinkage aggregate will cause a lot of cracking.

I'd ask for test results from the respective ASTM standards for each of those. I think I'd also ask for 3 or 4 pours that had already been in place for a couple of years with similar exposures so I could check out how they hold up.

All that may be too much to ask for from the concrete producer and, if so, it would probably just be best to go with a proven aggregate. I guess it also depends on the risk level of the pour. Probably not much risk if it's a sidewalk.

 

[Ron]

If truly blast furnace slag and not electric furnace slag, you're probably OK; however, require submittal of the following tests at least:

1. Aggregate gradation to comply with a standard ASTM C33 designation
2. Sulphate soundness tests
3. Aggregate reactivity tests

Unless the aggregate source has a traceable record of service for this aggregate, require the testing.

 

[racookpe1978]

If in an exposed outside concrete slab or wall, does the blast furnace slag have any rusting problems?