The I-15 CORE project team does its homework before placing concrete.
A concrete curing/tinning machine on the I-15 CORE project.
At a month past the half-way point, workers on the Utah County I-15 Corridor Expansion project have placed over 1.7 million square yards of concrete. “Since concrete pavement is smooth, requires less maintenance and resists potholes, it’s often a good choice on high-volume roads,” explains John Butterfield, UDOT Materials/Pavement Engineer for the project. But regardless of pavement type, “the main thing that drives pavement design is traffic.” I-15 CORE pavement is built for longevity and strength.
During the bidding process, UDOT asked for 30-year pavement. Provo River Constructors included a 40-year pavement design as a value-added feature in their winning proposal. The entire pavement section, bottom to top, consists of four layers: granular borrow, drainable granular borrow, asphalt base and 12 inches of Portland Cement Concrete Pavement on top. Together, the layers in the pavement achieve a structural value that is predicted from the road’s expected traffic volume.
Making sure the top layer of concrete meets final acceptance — and lasts that expected 40 years — starts with an understanding of how the specific concrete mix design will cure. For that important task, the I-15 CORE project team uses maturity meters — electronic sensors embedded in concrete and handheld readers. Using maturity meters has become standard practice in the building construction industry and is common in road construction.
Do the math
Before concrete is placed on the I-15 CORE project, “there is some homework that’s required,” says Butterfield, who explains the process. Because the compressive strength of a specific concrete mix design has a relationship to time and temperature, a maturity curve that shows that relationship can be charted. When inserted into core cylinders, maturity meters can be connected to a reader to access information on time and temperature, making the process very precise.
Data is collected by casting concrete cylinder samples and tracking the time and temperature of the concrete as it cures and gains strength. Then, the cylinders are broken at pre-set intervals to determine compressive strength, and that data is recorded and correlated to the time and temperature data.
Progressing work or allowing traffic on the pavement
Once data is collected and plotted, the “homework” maturity curve becomes an appropriate basis to measure the strength of in-place concrete. Workers assess concrete strength simply by taking a quick electronic reading in the field from data loggers embedded in the pavement. When the correct concrete strength is reached, work can progress or traffic can be allowed on the pavement.
For final acceptance, traditional cast cylinders made from the same batch as in-place concrete are still broken to measure 28 day compressive strength. However, for determining interim strength for the purpose of progressing subsequent work, maturity meters are a more efficient, precise and less expensive method than casting and breaking several additional cylinders.
“One of the greatest benefits of maturity meters is that they provide the strength of the actual ‘in-place’ concrete,” Butterfield says. “We no longer have to break cylinders, either lab-cured or field-cured, and speculate how closely they correlate with the strength of the actual structure.”