# Evaluation of Early Age Concrete Pavement Strength by Combined Nondestructive Tests
## 4 Discussion  
This dataset agrees with previous research that the shear wave velocity correlates better to strength than maturity [20]. Other studies have observed inconsistencies in the accuracy of strength estimations using maturity with over-estimations [9], under-estimations [35], or both, depending on the concrete mixture and maturity-strength relationship used [19; 36]. In this study, the strength estimated using maturity had an average difference from compressive and flexural strengths of 8.5% and 7.4%, respectively. Ultrasonic testing is more reliable in the field as the strength estimated using shear wave velocity had an average difference from the compressive and flexural strengths of 5.7% and 1.9%, respectively. This may appear as a minimal increase in accuracy, but when considering early age concrete, accuracy is extremely important. Early age concrete changes quickly, gaining substantial strength and stiffness in a relatively short amount of time. It is important to accurately know concrete strength to properly schedule construction procedures such as joint sawing and opening to traffic. Incorrect strengths can accelerate or delay procedures that when mistimed, can reduce the integrity of the pavement.  
Another advantage of ultrasonic testing is the opportunity to minimize or eliminate the need for permanent equipment installation while increasing the monitoring area. This can be achieved by using minimal temperature sensors to obtain the mid-slab concrete temperature, or by using a temperature model such as EICM to predict the pavement temperature without any permanent installation.  
Ultrasonic testing uses a single measurement of shear wave velocity at a specific time to estimate strength, while maturity can use temperature models to predict changes in  
Figure 5: Using different temperature to determine the maturity–strength relationship after a strength of 20.7 MPa.  
maturity and provide predictions on strength development. The relationships established in this study allow strengths estimated from ultrasonic testing to adjust the predicted strength development. By using this method repeatedly, the predicted strength gain rate can be continuously adjusted to match the more accurate strength estimate obtained from shear wave velocity. This method provides a comprehensive understanding of strength development as it evolves in the pavement.  
Figure 6 shows how the strength gain rate was adjusted for the test section on days 3 and 5. The original line shows the strength gain rate from the initial pour according to the maturity-strength relationship. It aligns well with the day 1 field strength estimated by the shear wave velocity. However, on day 3, when the field strength was again determined, the prediction was overestimated. The strength gain rate can be adjusted down to predict the strength gain from that point. Again, on day 5, it was found that the prediction underestimated the strength in the field. The strength gain rate can then be adjusted up where it continued to represent the actual field strength well.  
When considering a specific strength, the final outcomes from this procedure include the shear wave velocity to watch for in the field and the predicted strength development moving forward in construction. This collaboration of nondestructive tests increases the available concrete strength information for immediate field use and for future construction scheduling.