Kristin Dispenza, Contributing Author
Technologies being introduced to the concrete paving industry seem endless. Some affect equipment, such as internet-of-things (IoT) sensors that inform maintenance. Some affect concrete mixes, incorporating physical components that increase durability and strength. Still others are software-specific, enhancing project management and other tasks. While road owners and concrete paving companies are experimenting with a range of these, several technologies focus specifically on improving the performance and long-term durability—not to mention jobsite efficiency—of concrete pavement.
Digital sensors are perhaps the most noticeable technology being used on concrete paving projects. Sensors offer testing options that are in-situ, thereby providing a more accurate representation of actual conditions than off-site test methods. They work in real time, shortening project timelines, and they are non-destructive.
Since new concrete pavement can only be opened to traffic when it has achieved adequate strength, a major category of digital sensors assesses concrete’s strength development. Many of these sensors rely on the use of the maturity method—an old methodology that is being made new again.
Identifying Optimal Opening Strength
The conventional way to estimate strength is to create test beams or cylinders using a project’s mix design, transport those specimens to a laboratory setting and break them using compression at sequential intervals of time, recording the pressure required.
Laboratory testing not only takes time, but also creates opportunities for inaccuracies. The samples may be improperly handled during casting or transport, for example, thereby weakening and breaking at a lower strength that they otherwise would. The tests also are time-consuming to complete.
Furthermore, because the concrete in a beam or cylinder is subject to a different heat of hydration than the concrete in the mass of the pavement, and experiences different moisture conditions during curing, test specimens do not necessarily reflect in-situ strength. This means laboratory results do not perfectly represent the strength of the pavement itself.
To reduce reliance on this process, the industry is taking a new look at the maturity method, a decades-old, standardized technique that predicts the strength of in-situ concrete at early ages (up to 14 days). The maturity method, described in ASTM C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method, correlates concrete strength with hydration temperature history.
Maturity curves are developed in a lab for a specific project, but that curve can then be used for on-site assessment of the strength gain of in-place concrete. Some states, notably Iowa, have used the maturity method for decades. This has been a valuable proof-of-concept, bearing out the reliability of the method.
AASHTO recently approved “The Standard Method of Test for Estimating the Early Opening Strength of Concrete Pavements by Maturity Tests,” reducing the need for departments of transportation and other state highway agencies to gather extensive data and perform parameter testing on their own.
The new AASHTO standard uses the same theory as early versions of the maturity method, but makes straightforward recommendations for opening time, laying out standard numbers that can replace field testing.
“The new AASHTO standard provides a far more automated and reliable method to determine opening strength. It offers a boost to efficiency and speed. Old methods require cylinders every day, then you have to wait for the break. So even if that’s only a difference of eight hours, opening eight hours earlier would be a big improvement for drivers” said Armen Amirkhanian, P.E., Ph.D., associate professor of Civil, Construction and Environmental Engineering Department at the University of Alabama.
Sensors Prove Valuable
Sensors that complement the maturity method have seen extensive use on pavement projects, and Milestone Contractors has been using sensors on their jobsites with good results.
“For the brands of maturity sensors we’ve used so far, data correlates well with traditional methods,” said Steve Friess, vice president of concrete operations at Milestone Contractors. “You can build curves for all of your mixes. Data from embedded sensors in concrete is sent directly to an app, and once the output reaches the ‘magic number,’ you know it is safe to open a pavement to traffic. In Indiana, where beam testing is the norm, we’ve seen good correlation between sensor results and beam data.”
For sensors that tie to maturity curves, the calibration process involves making a batch of cylinders, completing a cycle of break testing and temperature testing, using the data points to plot a maturity curve, and then validating the curve with a couple of additional cylinders on a jobsite pour. For any change in the mixture, a new calibration should be created.
Beyond the benefit of decreasing cylinder/beam production, Erica Flukinger, digital director for Heidelberg Materials North America, said enhanced data may reduce the need for inspectors or third-party testing laboratories to conduct slump tests and air tests.
“And the continuous monitoring ensures workability and minimizes early-age distresses,” Flukinger said.
Some of the newest sensing systems to come to market eliminate reliance on cylinder/beam breaks and the maturity curve by converting mechanical forces to electrical charges that can be taken as readings. For the purposes of concrete construction, this means converting the sensor’s outputs to stiffness and strength measurements.
Piezoelectric crystals can generate a waveform that penetrates the concrete and measures impedance, according to Joseph Turek, chief operating officer and president of Wavelogix.
“The sensor identifies the concrete’s resonance frequency—and resonance frequency is proportional to compressive strength,” Turek said. “By using this method to determine compressive strength, we have a sensor whose results are independent of moisture, mix design and heat.”
Sensors using piezoelectric technology accurately predict 28- and 56-day strengths as early as five days, so decisions can be made on stripping formwork or opening to traffic. Because they don’t contain a battery, these water- and pressure-proof sensing systems also continue to offer reliable strength readings into the future.
“We are using sensors like these to provide greater quantities of data than has been possible with physical specimens, because the sensors stay in the concrete and will continue testing for years, if you have access,” said Friess.
Friess believes sensor-based technologies will only continue to grow in use.
“A next step for adoption will be for more agency specifications to allow the use of sensors,” he said.
Curing
Just as real-time, in-situ information is proving invaluable for assessing concrete’s strength development, using sensor-based technology to test conditions at the time of curing compound placement can provide great value.
Proper curing of concrete requires sufficient moisture retention to allow continuous hydration, so membrane-forming curing compounds are used on fresh concrete pavement to help minimize evaporation. To date, however, the evaluation of curing compound effectiveness has been difficult.
“It’s been common to rely on visual or other physical cues during application, such as judging the amount of compound and thickness of the layer based on its color,” said John T. Kevern, senior researcher for Cement and Concrete Decarbonization, Building Technologies and Science Center, National Renewable Energy Laboratory (NREL).
Kevern is developing an embedded electrical resistance approach to measure the drying behavior of fresh concrete and quantify the effectiveness of curing. Conceptually, the technique grew out of similar testing that had been done at the National Concrete Pavement Technology Center (CP Tech Center). Kevern further developed the concept to create a continuous resistivity probe that measures how resistance changes with time, application rate and thickness, and other variables.
So far, field testing has shown that the embedded resistance technique can define the evaporation rate for a concrete mixture across environmental conditions. Construction teams are then able to tell when moisture loss extends beyond ideal parameters and remediate in real time.
For example, if it is drying too quickly, there is the option to re-spray the surface with curing compound; if, as is more rarely the case, it is drying too slowly, crews can lessen the amount of compound being applied, saving money.
Kevern’s technique is still in the research and development stage, but the goal is to develop an app-based interface for the device. The NREL team expects to provide sample devices across the industry next year.
“Most recently, we wrapped up testing in the state of Wisconsin, using our resistance-measuring device to assess the effectiveness of various curing practices and timing,” said Kevern. “We believe there is great potential to improve concrete pavement’s volume stability, long-term strength development, and ultimate durability.”
New Kid on the Block
Whether integrated into software packages or hardware devices, artificial intelligence (AI) has arrived in the concrete paving industry. For example, predictive AI can run simulations and compare concrete mixes for performance and carbon content.
“We have AI packages, fueled by sensor and mix data, that help us design mixes that minimize carbon emissions. The AI takes thousands of data points, performs data analysis and offers us ways to lower carbon,” said Friess. “This is the wave of the future.”
Some AI applications still have room for growth, Friess cautioned.
“AI assumes consistency of materials,” he said. “But in the real world, to offer an example, sand grains are different sizes, and moisture collects in piles of sand, which then gets incorporated into a concrete mix. This makes for variability in actual mixes versus theoretical ones.”
Pat Strader, senior vice president, customer experience, at Cemex, said that AI can analyze vast amounts of data to determine the optimal mix of materials for concrete.
“This includes the proportions of cementitious materials, water, aggregates and additives,” Strader said. “Beyond optimizing design mixes, AI can enhance life-cycle assessments, predict the strength gain of concrete over time and contribute to material recycling by analyzing the properties of various recycled materials and determining how they can be incorporated into new concrete mixes without compromising quality.”
Technology Adoption
Considering the array of technologies in development or on the market, it can be difficult for design and construction teams to wade through the options. Often, technology adoption is facilitated when existing vendors or materials suppliers integrate digital solutions into their existing offerings.
Knife River Corporation's North Central Region pilots a Transportation Management System (TMS) for its ready-mix operation, and it worked with cement supplier GCC on early interfacing. With each GCC shipment, information from the bill of lading is automatically sent to the TMS, allowing Knife River to access delivery tickets electronically and bypass the traditional barcode scanning process.
"We see technology integration and information digitization as essential to the future of our industry, especially in a fast paced, multi-plant operation," said Jeremy Quinn, vice president, Knife River Corporation, Ready Mix Concrete Operations North Central Region. "Automating ticket management and delivery tracking not only improves accuracy, but also speeds up logistics."
Strader said that Cemex has embraced and created various technologies to simplify and expedite project processes. Along those same lines, Flukinger said that any technologies that combine information with secure and easy-to-connect functionality can unlock a new world.
“When you look across the planning and supply chain, too much time and resources are dedicated to manually moving and manipulating data,” said Flukinger. “As the ease of access grows, so does the historical information that can then be used to predict cost overruns, resource availability, complicated integration plans, material selections, equipment risks and environmental impact.
“Could this mean we are close to solving many long-standing frustrations within our industry and so we can recoup lost time spent with problems that have plagued us for decades? We believe we are firmly heading in that direction.” RB
Kristin Dispenza is an account manager at Advancing Organizational Excellence.
