PSI - Issue 70

R. Mohanraj et al. / Procedia Structural Integrity 70 (2025) 82–88

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UPV is a commonly used technique to evaluate the internal quality of concrete and mortar by measuring the velocity of ultrasonic waves passing through the material. Higher pulse velocity generally indicates a more compact and denser material, while lower pulse velocity may indicate the presence of cracks or voids. This test was performed on each of the cured mortar cubes to assess the effect of different SiO 2 concentrations on the internal structure and density of the mortar. The Ultrasonic Pulse Velocity (UPV) test was conducted to assess the internal quality and homogeneity of the mortar samples. This non-destructive test measured the velocity of ultrasonic waves passing through the material, helping to identify voids, cracks, and variations in density (Fig. 2). Higher velocity readings generally indicate a denser and more compact structure with fewer internal flaws. The test is particularly useful in assessing durability, as well as predicting long-term performance and structural integrity of cementitious Materials For the procedure, mortar cubes were placed between transmitting and receiving transducers connected to a UPV apparatus. The time taken for the ultrasonic waves to traverse through the sample was recorded, and velocity was calculated using the formula V = L/T, where V represents pulse velocity in meters per second, L is the distance between transducers in meters, and T is the recorded time in seconds. The standard classification for material quality based on UPV values is as follows: values above 4500 m/s indicate excellent quality, between 3500 and 4500 m/s signify good quality, 3000 to 3500 m/s suggest medium quality, and values below 3000 m/s indicate poor quality.

Fig.2. (a) UPVT Apparatus (b)UPV Test on Beam

3.2. Rebound Hammer Test The rebound hammer test, a non-destructive technique for evaluating the surface hardness and estimating the compressive strength of concrete, was conducted on mortar cubes incorporating nano- SiO₂. Calibration of the rebound hammer using a standard anvil ensured precision before testing. Each cube surface was cleaned and lightly ground if needed to remove irregularities, with test locations marked to avoid edges and ensure uniformity. During the test, the hammer was held perpendicularly to the surface, applying consistent pressure until impact, and rebound numbers were recorded, with multiple readings taken per sample. Following IS 13311-2:1992, rebound values were averaged and categorized to estimate strength: values above 40 indicated very strong concrete, 30 – 40 strong, 20 – 30 medium, and below 20 weak. Results revealed that mortar with 3% nano- SiO₂ achieved the highest rebound numbers, indicating optimal enhancement of surface hardness. However, beyond 4%, readings became inconsistent, likely due to densification that altered impact energy absorption. Nano- SiO₂ significantly influenced hydration by enhancing C -S H formation and refining the interfacial transition zone (ITZ), improving strength and bonding. Excessive refinement, however, could concentrate stress and distort rebound results. The test findings aligned with UPV results, supporting improved material quality with increased nano- SiO₂ dosage, though each test highlighted different characteristics — surface hardness versus internal compactness. The principle of the rebound hammer test lies in the correlation between surface hardness and rebound distance, which reflects compressive strength. While offering a quick, in-situ strength