Seasonal Construction Tip

2011 Spring Fly Ash Shortage

According to recent communications between the Washington Aggregates & Concrete Association (WACA) and the Washington State Department of Transportation a ‘short term interruption and shortage‘ of fly ash is anticipated through July 2011.  The cause of the shortage is related to a probable shut-down at the coal-fired Centralia Big Hanaford Power Plant.

Fly Ash is produced as a by-product of coal-fired power production.  As commonly occurs, the Big Hanaford Plant halts power production when an abundant Cascade Mountains snowpack results in excess generation of hydro-electric power.  Currently, the regional snowpack is between 134 to 235 percent of normal (http://www.wrcc.dri.edu/snotelanom/basinswen.html).

The region faced a similar shortage in the spring of 2006.  At that time the Washington State Department of Transportation issued approval for a 1:1 substitution of slag for fly ash.  The DOT has yet to weigh in on the current issue, but the WACA has requested that the 1:1 substitution be re-instated.

Engineers of record should be prudent in reviewing their concrete batch tickets over the following months.  MTC recently encountered a concrete pour where, unbeknownst to the project design team, a slag substitution had been made and delivered to the jobsite.

 Some things to consider when approving this substitution are as follows:

A.  Slag has a higher specific density than fly ash, hence a heavier in-place weight.

B.  Slag will provide aggregate silicate reactivity (ASR) mitigation, but will need to be verified to meet project specifications and requirements.

C.  Concrete strength will not be adversely affected, and continued sampling and compressive strength testing should be conducted.