I have always thought that navijack was sand and gravel, a component
of concrete. Yet, when I search google for navijack, I get ZERO hits.
I tried different spelling variations without any luck. Is navijack a
local Canadian term or is there some obscure spelling or is no one on
the internet interested in it?

The material that you're speaking of is "navvy jack," a term rarely
used except in Canada.

In the 19th Century, a Welsh immigrant, John Thomas -- whose nickname
was "Navvy Jack" -- gained fame for conveying a mixture of sand and
gravel to construction sites in western Canada. "Navvy" is a British
slang term for a laborer who works in excavation or construction.

"Navvy Jack is a combination of washed sand and aggregate. It is ideal
for making concrete and is much cheaper than using bags of concrete
mix."

Sunwood Building Centre Ltd: Bult Landscape Products
http://www.sunwoodbc.com/bulk.asp

"Navvy Jack
Sand and gravel blended in the correct portions to be used in concrete
mix. Usually between 4 and 6 parts of Navvy jack to 1 part cement."

Bulk Landscape Supplies
http://www.growell.bc.ca/price%20list/bulk%20landscape1a.pdf

"1868
John 'Navvy Jack' Thomas, who began the first ferry service across
Burrard Inlet, began extracting sand and gravel from the west side of
the Capilano River. This material was used in the making of concrete
and was transported in his five-ton sloop to construction sites in
Moodyville, Hastings and Gastown."

History of Vancouver: West Vancouver Timeline
http://wvma.net/history/

Google search strategy:

Google Web Search: "navvy jack" + "sand OR gravel"
://www.google.com/search?hl=en&ie=UTF-8&q=%22navvy+jack%22+sand+OR+gravel

I hope this is helpful. If anything is unclear or incomplete, please
request clarification; I'll be glad to offer further assistance before
you rate my answer.

Best regards,
pinkfreud

Good job pinkfreud. I especially appreciate the promptness.