First question is whether your specimens have structures of the type the standard was created to test.

ASTM standard c1674-11: "This test method is used to determine the mechanical properties in flexure of engineered ceramic components with multiple longitudinal hollow channels, commonly described as “honeycomb” channel architectures. The components generally have 30 % or more porosity and the cross-sectional dimensions of the honeycomb channels are on the order of 1 millimeter or greater." 

If not, and you choose to use the standard anyway, you should create and state your rational for using the test in connection with reporting any results. 

Second, I would highly recommend you use 4-point (not 3-point) bending. 3-point bend test results of ceramics are subject to more scatter because of the chance of material defects at the center point where stress is at a maximum. In 4-point bending the surface stress is uniform between the two interior loading points. The standard gives dimensions for 4-point bend specimens: 

"1.3.2 Test Method B—A 4-point- ¼ point bending test with a defined rectangular specimen geometry (13 mm × 25 mm × > 116 mm) and a 90 mm outer support span geometry suitable for cordierite and silicon carbide honeycombs with small cell sizes." 

Thanks Daniels , my sample has interconnected porosity or honeycomb structure. I will consider your suggestion of 4 point bend test.

Kindly can you send me the pdf of the document?

Hi Bilal, you can try unconfined compression test, not sure if there's a standard for HA though

Bilal Naqvi ..Good question… Please check references below.. hope it will be helpful for you…

Regards…

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4726111/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3654023/

http://ptcer.pl/pobierz/341

http://downloads.hindawi.com/journals/jnm/2012/213549.pdf