For highway officials in Missouri and Wisconsin, three is turning out to be a magic number—specifically, cement composed of three key ingredients.
In both states, ternary mixes—containing portland cement, slag cement and fly ash—have performed exceptionally well in recent projects, and DOT engineers are now drafting new specifications allowing their wide use.
Standard paving specifications in both states had been using portland cement and fly ash. Engineers and contractors have long recognized that adding slag cement to the mix makes concrete easier to place, provides higher strengths and enhances long-term durability. But they have been leery about the challenges of handling, storing and proportioning three separate materials, especially for major repaving projects. Contractors have been concerned about the costs of adding extra silos or storage at their batch plants. DOT officials have worried about adding another variable to paving projects.
So why the change of heart? Because cement manufacturers in the region have recently ramped up production of portland-slag blended cements, which make ternary mixes a lot easier to produce.
Lafarge, which provided cement for the recent projects, has been expanding production of its MaxCem-brand portland-slag cement blend in many of its North American markets. The company has installed new blenders at several sites throughout the Midwest to meet the growing demand for blended cements.
With these blends, contractors who have been producing binary mixes can use the same methods and equipment to create a ternary mix. They can simply replace the portland in one silo with the portland-slag blend and continue using their second silo for the fly ash.
Choice of Cheeseland
Wisconsin’s first major project using a ternary mix involved one of the oldest highways in the country. Highway 29 is part of Wisconsin’s original numbered-highway system, dating back to 1918. Today it is the major east-west corridor across the northern part of the state. It also is the only state highway in Wisconsin’s Corridors 2020 Backbone Routes, a 1,550-mile network of multilane routes that form the core of the state’s highway system. Much of Highway 29 has been upgraded to interstate-highway specifications in recent years.
The Wisconsin Department of Transportation (WisDOT) specified the mix for a 28-mile stretch of four-lane pavement between Elk Mound and Chippewa Falls, a segment that links heavily traveled I-94 to the central and eastern part of the state.
The previous specification called for a 70-30 portland-fly ash mix for paving work. The contractor, Trierweiler Construction, used Lafarge’s MaxCem 80-20 portland-slag blend mixed with 10% fly ash. The resulting mix contained 72% portland cement, 18% slag cement and 10% fly ash.
The ternary blend made the paving job go more smoothly, according to Trierweiler. Slag cement contains glassy particles that reduce the amount of water required in the mix, which makes it easier to place. Crews were able to place as much as 11?2 miles of pavement in a single day. Consistency of the mix also was enhanced. In fact, the mix was so consistent that Trierweiler’s quality-control manager was able to spend much of his time on the finishing crew because fewer mix adjustments were required. The addition of slag cement to the mix also dramatically increased 28-day strength, allowing Trierweiler to maximize its strength bonus under the contract.
Momentum with MoDOT
The Missouri Department of Transportation used Lafarge’s 75-25 MaxCem blend in its pilot projects. The blend was combined with 15% Class C fly ash for a paving job on I-35 near Pattonsburg in Daviess County. A smaller project, Rte. 13 in Ray County, combined the 75-25 blend with 17% fly ash, because of the differences in the aggregates used on the two jobs. The contractors on the projects were APAC-Kansas, Overland Park, and Ideker Inc., St. Joseph.
On the I-35 job, crews placed approximately 10 miles of pavement in a continuous paving operation. Crews reported that the mix was especially good at holding the edges and was easier to place than the portland-fly ash mix. The slag cement also retarded set times, which helped prevent formation of cold joints on the job. Strength reached approximately 4,000 psi within four days, minimizing traffic disruption. The slag cement also increased 28-day strengths.
As a result of these projects, both highway departments are now developing standard specifications that will allow the use of ternary blends statewide.
