# Evaluation of Early Age Concrete Pavement Strength by Combined Nondestructive Tests
## 3 Results  
Maturity and ultrasonic testing are both reliable nondestructive methods individually but have unique advantages over each other. Combining these methodologies can use both tests to their highest ability and provide the user with thorough information on concrete strength development. This procedure will allow strength estimations from ultrasonic testing to utilize and adjust the predictive maturity-strength relationship, accurately providing both current and future field strength estimations. This proposed procedure is outlined below and is applicable using either imperial or metric units. The dataset summarized in Table 1 will be used to exemplify the procedure. _Step 1: Performing Laboratory Testing_  
As with separate maturity and ultrasonic testing procedures, preconstruction laboratory testing must be performed based on the project concrete mixture. Cylinder and beam specimens should be prepared for compressive and flexural strength testing. Several ages should be tested to establish a proper strength gain rate. The final day cylinder and beam specimens should be imbedded with thermocouples to monitor temperature for the entirety of the testing period. Each beam should be scanned with an ultrasonic device before flexural testing. _Step 2: Establishing Maturity-Strength and Wave Velocity-Strength Relationships_  
The maturity-flexural strength, maturity-compressive strength, and wave velocity-flexural strength relationships must be established using the procedures outlined in the previous sections. This will provide calibration coefficients for each of the three models. _Step 3: Determining the Wave Velocity-Compressive Strength Relationship_  
When using linear array ultrasonic devices, compressive strength specimens cannot be scanned for wave velocity. The following equation relates wave velocity to compressive strength through maturity:  
\[\mathrm{f^{\prime}_{c}}=\mathrm{f^{\prime}_{cu}}\times\exp\left(-\left(\frac{ \mathrm{c_{m}}}{\mathrm{a_{m}}}\right)^{\mathrm{d_{m}}}\left(-\mathrm{a_{s}SWV }-\mathrm{b_{s}}\right)^{\frac{\mathrm{d_{m}}}{\mathrm{b_{m}}}}\right). \tag{5}\]  
The required laboratory work was performed on the dataset to establish the model coefficients described in Steps 1-3 for Equations (2), (3), (4), and (5) as shown in Table 2. Visualizations of these models compared to destructive strength testing are shown in Figures 1 and 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Maturity–Strength Model** & \multicolumn{2}{c}{**SWV–Strength Model**} \\ \hline  & 249.9 & \(\mathrm{a_{s}}\) & 0.002 \\ \(\mathrm{b_{m}}\) & 0.94 & \(\mathrm{b_{s}}\) & \(-\)5.424 \\ \(\mathrm{c_{m}}\) & 490.9 & & \\ \(\mathrm{d_{m}}\) & 1.026 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Strength model coefficients. _Step 4: Choosing a Strength to Consider_  
The user must select a compressive or flexural strength (f\({}^{\prime}_{\text{c0}}\) or M\({}_{\text{r0}}\)) to be considered for an early age procedure. This can be any strength the user may want to confirm in the field. The example will use a compressive strength of 20.7 MPa (3000 psi). This is a common strength for opening to traffic that is typically measured with destructive testing. _Step 5: Calculating Relative SWV_  
The strength chosen in Step 4 must be converted to shear wave velocity. Equations (4) and (5) can be rearranged to find the wave velocity relative to the chosen flexural or compressive strength. Using Equation (7) and the respective coefficients from Table 2, the relative shear wave velocity at 20.7 MPa (3000 psi) can be calculated to be 2647 m/s.