Real-time sensing technology can help determine concrete strength and construction schedules

Cutting risk

Fast-paced construction schedules often expose concrete pavement to substantial loading conditions, even at its early age, which can cause significant reductions in the life span of roads and bridges. Besides the obvious waste of resources related to opening a road too early, there is also the increased life cycle costs and risk to workers safety that need to be considered. It would seem simple to conclude that fresh concrete pours in roadways and bridges should be allowed to fully cure before being opened for use. However, closed roads are rarely a welcome site; they cause issues for citizens, governments, commerce, and road construction companies.  

In effort to assuage these effects, the Purdue research team developed a reliable mix-independent test method to determine concrete strength development. Foregoing the need for laboratory tests and expensive time-consuming equipment is what drove the advancement to a reliable sensing method to monitor concrete strength development using piezoelectric sensors coupled with the electromechanical impedance (EMI) method. The working principle is to send vibration waves to the concrete and then to measure the resistance of the concrete to the vibration wave over time. The stiffness of concrete can be monitored in real-time by this method. In contrast to the maturity sensor, the EMI method directly measures the stiffness of concrete rather than correlating the temperature and curing time. As such, it is not dependent on concrete composition and is free from calibrations to a specific concrete mix. The research team determined this through a systematic investigation of the EMI sensing technology using extensive laboratory experiments with different mixes. They concluded that this sensing method is independent of the water-to-cement ratio, incorporation of supplementary cementitious materials (SCMs), and different types of cement.

A new method

By working with Indiana DOT engineers, the research team implemented its sensing technology in three interstate projects: I-70 West, I-74 (Batesville), and I-465 near Indianapolis. Among the field tests, the I-70 and I-465 projects were for concrete patching, and I-74 was a full-depth concrete paving project. For each job, almost 100 sensors were embedded into the concrete pavement to monitor real-time strength development from hour one through three subsequent days. (While the sensors are presently designed to be left in place, further iterations will allow for them to only be handled at installation.) In the concrete patching jobs, they started from the preparation of the 6-ft x 12-ft hole with a depth of 1 in. The connecting steel rods (dowel bar) were stuck in the longitudinal sides (12 in.) of the hole every 1 in., depending on the site condition. After the hole preparation, the piezoelectric sensors were firmly tied on the steel rods. The wires were extended outside the hole and temperately fixed on the ground for measurement. Later, the concrete was delivered through the ready-mix concrete mixer from plant to site and directly cast in the hole. The workers performed the standard patching work sequence. They vibrated the concrete to compact it. Then, the concrete was screeded using a roller tool. The work was then finished and broomed. Curing compound was sprayed on the top of the concrete at hour three and covered with wet burlap and plastic sheeting at hour five to maintain the humidity and temperature of the concrete (depending on the site conditions). For the full-depth concrete paving job (I-74), the sensors were bonded on the dowel bar baskets. The concrete paving machine later paved on top of the baskets. Real-time EMI monitoring was conducted from hour one through three days.

Initial EMI testing when compared to standard mechanical testing found a few differences between the methods. Standard cylinder samples were prepared alongside the EMI testing for mechanical testing to be performed on day 1 and 3 for comparison with the EMI testing. The results indicated that the one-day compressive strength of concrete from the EMI sensing method is higher than in cylinder samples. This has been determined to be due to the exothermic hydration reactions of the concrete. Simply put, large slabs have more heat of hydration than that of a cylinder sample. Another difference was found to be the water evaporation speed. Field concrete pavement samples were higher than the cylinder samples. This can be attributed to the water being better retained by the cylinder molds, thus reducing the exposed surface area affecting the degree of hydration. Therefore, the third-day EMI sensing results are a bit lower than mechanical test results. Indications are that the EMI sensing results can better reflect the real conditions of concrete pavement.

The EMI sensing technology provides instantaneous and accurate information of concrete strength to field engineers. This information directly helps them to determine optimal traffic opening times after concrete pours. Due to the widespread interest in this technology, the Federal Highway Administration (FHWA) is working with Purdue’s team to sponsor a nationwide pooled fund study to implement this technology in other states and federal agencies. Several states, including California, Texas, Missouri, and Kansas, are planning to participate alongside Indiana. 

This will give Purdue’s research team the opportunity to further refine their procedures and methods. They are looking forward to working on the signal processing software to eliminate small variations and further improve the consistency of this testing method. Plans are in place to compensate for the curing temperatures and the addition of the sensors being built with Wi-Fi or Bluetooth communications to further streamline the process. Overall EMI sensing is designed to give field engineers highly accurate results with minimum exposure to dangerous jobsite conditions. 

About The Author: Lu is an ACPA Scholar and associate professor at the Lyles School of Civil Engineering, Purdue University. Su is a graduate research assistant at the Lyles School of Civil Engineering, Purdue University.