Experimental studies for evaluation of the removal time of form on concrete using ultrasonic pulse velocity methods

In this study, the form removal time of concrete was evaluated using ultrasonic pulse velocity (UPV). The evaluation was performed based on the form removal strength specified in the American Concrete Institute (ACI), British Standard (BS), and Korean Construction Standards (KCS). Additionally, physical and mechanical properties of the concrete were evaluated from the early stages up to 28 days after casting. Specimens of both plain and lightweight concrete (LWC) were prepared for analysis. For the purpose of evaluating the concrete at various strengths, the water-to-cement (W/C) ratios were established at 0.41 and 0.28, aimed at achieving target compressive strengths of 30 MPa and 60 MPa for plain concrete, respectively. The measurement of mechanical properties was conducted through the assessment of penetration resistance, compressive strength, and ultrasonic pulse velocity (UPV). Models for predicting the strength based on regression analysis were proposed, taking into account the characteristics of the setting times (both initial and final). Additionally, the aggregate cross-section, matrix structure, and crystalline phases were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). In the results of the experiments, it was observed that Mortar60 reached both initial and final set quicker than Mortar30, a phenomenon attributed to the higher cement content. Initially, Plain and LWC specimens exhibited comparable trends in compressive strength and ultrasonic pulse velocity (UPV). However, when examining the compressive strength development from 1 to 28 days, a notable difference of approximately 33 % was observed between Plain60 and LWC60. Similarly, differences in UPV ranged from 10.12 % to 11.88 %. The logistic regression model was found to predict the form removal strength more accurately than the exponential model, as indicated by the regression analysis results. When these results were compared with previous strength prediction models, the difficulty in predicting form removal strength became evident, primarily due to the substantial errors present in these models. Analysis using scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed the presence of hydration products within the lightweight aggregate, which signifies the hydration reaction. These products seemed to enhance the performance of the interfacial transition zone (ITZ) between the aggregate and cement paste. Furthermore, Mortar60 was characterized by larger crystalline hydration products, a denser matrix structure, and higher XRD peaks in comparison to Mortar30.