# Evaluation of Early Age Concrete Pavement Strength by Combined Nondestructive Tests
## 1 Introduction
Transmitters in an ultrasonic device emit waves at a specified frequency that travel through the concrete pavement to be received again at the surface [5; 15; 16]. For concrete, the frequency is typically between 20 and 150 kHz, often 50 kHz. Results from ultrasonic testing provide the time it takes for a wave to travel a set distance through the concrete, which is used to determine the wave velocity. The received signal can be analyzed to estimate differences in stiffness, material properties, or defects [15]. Wave velocity is theoretically related to the elastic modulus, Poisson's ratio, and the density, which encourages a direct and accurate relationship to strength [17]. Traditional ultrasonic devices require acoustic coupling between transducers and the concrete surface that must be manually installed. Modern ultrasonic devices can be applied without coupling agents or surface preparation and have short data collection times. This allows for more freedom in scanning so variability within the pavement can be identified, critical stress locations can be monitored, or potential defects can be flagged [16]. Additionally, ultrasonic tests are unaffected by moisture or constraint conditions, unlike other nondestructive techniques, and the wave velocity-strength relationship is less sensitive to changes in the concrete mixture [17; 18]. The mixed material properties that come with a heterogenous material such as concrete can affect wavelength differently and increase result variability [4]. This is addressed by using linear array systems that have numerous transducers aligned in multiple channels, so many signal time histories are used to compute wave velocity. This adds redundancies and reduces measurement variability. Linear array devices increase the repeatability of ultrasonic testing over a traditional single transducer and receiver arrangement. However, when used on small specimens, the edge conditions affect wave propagation, creating discrepancies when relating laboratory specimens to in situ pavements of the same concrete mixture and age. Both maturity and ultrasonic testing have been used in strength estimations, but ultrasonic testing has been found to be more accurate than maturity at early ages [18; 19; 20]. The improved accuracy, in combination with the mobility and minimal set-up, make ultrasonic testing preferable for early age field use. This would remove the need for numerous permanent sensors along the pavement, as required by maturity testing. Maturity and ultrasonic testing have been suggested for combined use in the past [5]. One study proposed ultrasonic testing with pressure waves to check if maturity was over-estimating strength [21]. Another used an older form of ultrasonic testing, seismic modulus, for laboratory correlations and field use to estimate strength with success [7]. In recent years, ultrasonic testing has been upgraded and has become a popular research topic for combined use with the rebound hammer test [22; 23; 24; 25; 26]. This is useful but, as with ultrasonic testing on its own, it only offers a strength estimation at the time of testing. This study aims to revisit the successes of past research with modern ultrasonic technology in combination with maturity, and increase the strength development information available for present day construction. Accurate temperature modeling allows maturity to predict future strengths, but no such model exists for ultrasonic wave velocity. Combining the flexible field use of ultrasonic testing and the predicted strength development from the maturity-strength relationship provides a complete view of the concrete pavement strength. The purpose of this paper is to combine multiple nondestructive testing procedures to increase the efficiency of concrete strength estimations and predicted development in the field. Both nondestructive tests explored in this study provide reliable strength estimations but have certain limitations that can be improved by combining procedures.