While state transportation engineers try to determine exactly what caused the thin-bonded Syn-crete overlay on a busy stretch of I-15 to disintegrate less than three months after it was laid, questions are mounting about whether Utah Transportation commissioners, who approved the $1.5 million project - and an additional $900,000 on Friday to rip it up - had the necessary details to make an informed decision.
A Deseret News investigation has revealed there were at least five potentially critical factors that differed from the Syn-crete test section at Snowville, Box Elder County - the test upon which commissioners based their decision - and the I-15 project that made an "apples to apples" comparison difficult:-Utah-produced Kennecott slag, rather than Idaho-purchased silica sand, was used as an aggregate in the Syn-crete laid on I-15, apparently without the knowledge of Utah Department of Transportation Director Gene Findlay or Commission Chairman Sam Taylor. Both were informed of the difference only two weeks ago - nearly four months after funding for the project was approved. Taylor said he would have been very skeptical about funding the project, which was unanimously approved by commissioners, had he known of the difference.
UDOT's chief maintenance engineer told Taylor that "the iron content in the slag - which silica sand doesn't have - may have been a contributing factor (in Syn-Crete's failure to bond on I-15). Now we don't know that, and we won't know until we run tests on it."
-Though UDOT engineers and officials with Hodson Chemical Construction Corp., the maker of Syn-crete, agree that slag is an acceptable substitute aggregate, the temperature of the slag is a critical factor that apparently wasn't addressed before the commission.
The Kennecott slag added to the I-15 Syn-crete was heated to 140 degrees, well above the 75 to 85 degree temperature of aggregate used in previous tests. In fact, according to a copy of an in-house evaluation written by Hodson Chemical on Oct. 23, "the high temperature of the aggregate was not detected by Hodson or UDOT's project supervisor until the latter part of the (I-15 resurfacing) project. This created a deviation in both the initial hydration of the early high-strength cement being used and the polymerization of the chemicals being used."
As a result, the document says, "graded, washed concrete sand should be considered for use in future placements. This is more readily available, easier to handle and lower in cost than crushed slag."
-UDOT Engineer Mike Roshek told the Deseret News that test sections containing the slag-aggregate Syn-crete were put down on U.S. 40 only six weeks before funding was approved in July for the I-15 experiment. Firmly-bonded core samples were taken two weeks later, but the section had not been subjected to extreme winter weather conditions or snowplowing like the three-year old Snowville test section. Minutes of a July 7 commission meeting show the commissioners were aware of this.
-The six-week old U.S. 40 test section west of the Salt Lake Airport was never subjected to significant traffic. UDOT spokesman Kim Morris confirmed that "it's an old section of road not used for anything other than local access. There is minimal traffic on it." And Roshek estimates that the Snowville test section experiences "maybe 10 percent" of the traffic that travels I-15 from 59th South to 33rd South.
-Both Syn-crete test sections were applied on a relatively small stretches of roadway, compared to the full-scale, three-lane I-15 project, and the 30 foot by 60 foot section in Snowville was applied by hand with a trowel. UDOT engineers agree that quality control with such application is superior to that available with a large-scale application.
The U.S. 40 test section was installed with a commercial "screed box" developed by a local contractor specifically for the project. The six-week old section was firmly bonded, as evidenced by core samples provided to commissioners. But according to commission minutes, the section was only four feet wide by 60 paces long - compared with the three full traffic lanes that were paved on I-15.