Fine Aggregate Test : Alkali-Silica reactivity test

[sxchauhan]

The pre-pour test results per ASTM C 1260 is coming as 0.14%. The specimen prepared was without flyash. This value is beyond 0.10%, which is the acceptable limit. The concrete pour is coming up in less than a month. The actual concrete mix will have flyash in it. Is there a way 0.14% value could be bought-off? Are there other tests which could be carried out to test the reactivity? FYI-the same testing agency gave a value of 0.03% few months back for the same set of fine aggregate. The difference in results perplex me. What is the best way out?

Info on concrete mix:
w/c ratio = 0.31
HRWRA will be used
Cement Type I/II will alkali < 0.6%
Coarse aggregate passed the alkali-silicate reactivity test.
Flyash is more than 15% as compared to cement content.

Appreciate your input on this.


Thank you,
Sushil, PE
Nebraska

 

[Ron]

ASTM C1260 is a test that has quite a few meticulous control parameters to get consistent results. It would appear that you have only tested two sets of specimens, getting disparate results. While your second result is in the nebulous range of "maybe/maybe not" for reactivity, I would suggest that you should run a series of 3 tests, using a single sampling of aggregate, but mixing and testing the specimens separately. This will highlight test variations if those are at issue.

You should also consider ASTM C289 to test the aggregate only. That test is a bit simpler.

Next, you can subject the specimens from your second test under ASTM C1260 to a petrographic examination to see if the result was caused by reactivity or some test anomaly.

Lastly, when running tests on construction materials, particularly natural ones such as aggregate, you can expect variations in the test results, so do additional testing to verify that such results are indicative of the material, not just test procedure anomalies. I typically do not run any test of criticality in less than a 3-test set.

 

[sxchauhan]

Ron,

Thanks for your response. We have slow-reacting consituents in fine aggregate like quartzite. This simply excludes ASTM C 289 testing. The initial petrographic report did ask for additional tests for aggregate reactivity.

The current fine aggregate reactivity results report does have a line inserted below the charts stating "All specimens showed surface evidence of what appeared to be signs of reactivity. These were in the form of small, white, circular, raised areas."

Definitely we could go for one more set of testing using the same fine aggregate.

Time is kind of critical as the material needs to be procurred and brought to the site. All this involves hell lot of paperwork in nuclear industry. We are seriously thinking of going for ASTM C 1567 testing (aggregate+flyash).

Any more suggestions from yourside would be highly appreciated.

Thanks,
Sushil,PE
Nebraska

 

[Ron]

Adding flyash will reduce the amount of alkali available for reaction. That's about the easiest mitigation you can do at this point. Be prepared for slower strength gain in the concrete, though.