4. Results and Discussion
Among the five series as shown in
Table 3, it was observed that, at each age and at each w/c ratio, M4 (presented the highest compressive strength values and M5 (expanded clay) showed the lowest compressive strength values. Comparing the results of series M1 and M2, it was observed that when the maximum dimension of the coarse aggregate of gneiss was reduced, in general, there was a decrease in the compressive strength of the concrete. The coarse aggregate was changed from gneiss to trachyte (series M1 and M3) at the age of 3 days, and there was a clear decrease in strength. For other ages, this decrease did not always occur.
The ultrasonic pulse velocity (UPV) was obtained at ages of 3 days, 7 days, 14 days, 28 days and 90 days by averaging the test results of 4 specimens. Among the speed results obtained in the five series, at each age and at each w/c ratio, the lowest UPV was from the M5 series (expanded clay), with those from M2 (Dmax=9.5mm) being greater than those from M5 but smaller than those from series M1, M3, and M4.
The values of the compressive strength tests and the rebound hammer index obtained in the M1, M2 and M3 series are lower than those of the M4 series (CP V cement) and higher than those of the M5 series (lightweight aggregate).
Table 3 shows the range of values obtained for the different types of tests.
The type of cement and lightweight aggregate influences the compressive strength results, while the UPV is primarily influenced by the lightweight aggregate and the Dmax of the coarse aggregate.
Ultrasonic Pulse Velocity (UPV) is a widely accepted non-destructive method for evaluating the quality and integrity of concrete. Numerous studies have highlighted the significant influence of both cement type and aggregate characteristics on UPV readings.
This paper investigated the effect of different cement types on concrete’s UPV and compressive strength [
1,
48]. They found that mixes made with blended cements exhibited lower UPV values than those with ordinary Portland cement (OPC), mainly due to increased porosity and slower hydration rates. Similarly, [
49] [2] examined concretes with slag and fly ash cements and reported reduced UPV values when compared to OPC concretes, further linking lower UPV to higher internal porosity [
49].
Aggregate type is another critical factor influencing UPV. [
50] [3] demonstrated that concretes containing high-density aggregates such as crushed granite recorded higher UPV than those with lightweight aggregates like expanded shale, due to better wave transmission properties. This finding is consistent with the work of [
50] [4], who noted that the interaction between aggregate type and cement paste significantly modifies the UPV–strength relationship, particularly in mixes with higher aggregate angularity and stiffness [
51].
Moreover, Farooq and Siddiqui [
51] [5] developed predictive models for UPV based on variations in cement and aggregate types. Their results confirmed that material composition—especially cement type and aggregate gradation has a measurable impact on ultrasonic pulse velocity and, consequently, on the interpretation of concrete quality [
52].
In summary, the selection of cement and aggregate types plays a decisive role in influencing UPV results. These variations must be accounted for when using UPV to evaluate in-situ concrete strength or uniformity [
52].
Table 4 presents the relationship between compressive strength and Ultrasound Pulse Velocity (UPV).
In the series studied, it was found that in the correlation between compressive strength and ultrasonic wave propagation speed, the factors that have the most significant influence are:
a) the specific mass of the coarse aggregate, since the most significant differences are between lightweight and conventional concrete
b) the type of cement and the type of aggregate influence is the most significant parameter affecting the correlation between compressive strength and rebound hammer index. It can be seen that more considerable differences occur between the curves of the series made with conventional concrete and those of lightweight concrete.
Table 5 presents the relationship between compressive strength and the rebound hammer index (RH).In the concrete mixes studied, it was found that in the correlation between fc and RH, the factors that significantly influence are:
a) the specific mass of the coarse aggregate
b) the type of cement
The main objective of combining non-destructive testing methods is to increase the accuracy of the compression strength estimate. The coefficient of determination obtained in multiple correlation is higher than that of simple regression for the relationships between fc and the data obtained in the non-destructive test. Moreover, through the combination, the influence of some parameters on the evaluation of fc can be minimized. For the study of the combination of methods, series M1, M2, and M3 are grouped together, as in the simple correlations, which show the closest curves. Series M4 and M5 will be analyzed separately, as the type of cement and the lightweight aggregate were the factors that significantly influenced the correlations between fc and the magnitudes measured in the non-destructive tests.
Table 6 presents the relationship between compressive strength and rebound hammer index (RH) and UPV.
As shown in
Table 7,
Table 8 and
Table 9 and
Figure 3 The comparative analysis between our study and recent literature highlights that the predictive accuracy of compressive strength (fc) using Non-Destructive Testing (NDT) parameters such as Ultrasonic Pulse Velocity (UPV) and Rebound Hammer Index (RH) aligns well with current research trends.
Our study's average coefficients of determination (R²) for fc vs UPV (0.84) and fc vs RH (0.86) are slightly lower than the highest reported values (e.g., Gil-Minguet et al. 2021 with 0.94 and 0.88 respectively), but still demonstrate a strong predictive capability.
Notably, the combined model involving both UPV and RH yields a higher R² (0.91), consistent with the findings from Angiulli et al. (2024) who reported an R² of 0.96, suggesting that multi-parameter regression models improve accuracy.
Differences in R² values among studies may be attributed to variations in concrete mix design, testing conditions, and data sets. The close agreement between our results and recent studies confirms the robustness of the applied correlation models and supports the practical applicability of combined NDT methods for estimating concrete strength.
Future work could focus on expanding the data range and exploring machine learning techniques to further enhance predictive performance.
Influence of Mix Design Parameters on the Mathematical Correlation Models
The predictive correlation equations developed in this study reflect not only empirical trends but also the inherent physical and chemical interactions driven by variations in the concrete mix constituents. As detailed in
Table 2, all five mixtures (M1–M5) share controlled ranges for cement content (277–450 kg/m³), water content (180 L), and water-to-cement ratio (0.40–0.65), but they differ markedly in aggregate type, maximum aggregate size, and cement type factors that significantly influence the resulting mechanical properties and their correlation with indirect energy (RH) and volume (V).
Models M1–M3, for example, utilize normal weight aggregates - Gneiss and Trachyte with Dmax values of 9.5 mm and 19 mm. These dense, angular natural aggregates enhance interparticle friction and packing density, thereby increasing compressive and flexural strengths, which is reflected in the steeper gradients and nonlinear behavior of the fc response surfaces. In contrast, Model M5 employs lightweight expanded clay aggregate, characterized by high porosity and lower specific gravity, which reduces the overall mechanical stiffness of the composite. This results in a significantly different response surface shape, with a lower fc range and smoother curvature, indicating a more gradual sensitivity to changes in RH and V.
Furthermore, while M1–M3 and M5 utilize ASTM Type IV low-heat cement, mixture M4 incorporates ASTM Type III high early-strength cement. The latter has a higher C₃A and C₃S content, which accelerates hydration reactions, leading to faster strength gain and altered pore structure at early stages. This change in cement chemistry introduces a shift in the response surface of M4, particularly in how quickly Fc increases with increasing RH—suggesting stronger coupling between energy absorption and microstructural densification.
From a modeling standpoint, these compositional differences manifest in the mathematical coefficients and curvature of the surface. The presence of pozzolanic reactions, variances in aggregate-matrix interfacial transition zones (ITZ), and early-age stiffness evolution all contribute to the differences observed between the models. Consequently, these equations not only curve fits but also encapsulate the complex material-specific interactions inherent in each mixture. Therefore, applying the derived models to other mixes requires careful consideration of material similarity, especially in terms of aggregate type, binder chemistry, and hydration kinetics.