Analysis of influence of concrete aggregate in evaluation of form removal time of concrete using ultrasonic pulse velocity methods

Abstract

Recently, due to the increase in high-rise and large-scale buildings, the use of ordinary aggregates has increased, and research is continuously being conducted on the use of lightweight aggregates to reduce weight. However, because concrete exhibits different compressive strength due to material influences, it is believed that there will be a difference in the form demoulding time for ordinary aggregate and lightweight aggregate in the early age. In order to evaluate the characteristics of concrete in a quantitative manner, an experiment was conducted to predict the time of form demolding through correlation by measuring compressive strength and ultrasonic pulse velocity (UPV). In this study, the target strengths of normal aggregate concrete (NC) and lightweight aggregate concrete (LC) were set at 30 MPa, 45 MPa, and 60 MPa. To reach the target strength of each test specimen, the W/B ratio was set to 41.0%, 33.0%, and 28.0%. In addition, in order to meet the same mixing conditions, the mixing was carried out by setting the fine aggregate excluding aggregate, W/B ratio, and cement amount to the same level. In the case of compressive strength, NC30 was 13.4% higher than LC30 at 24 h, and NC45 and LC45 showed similar compressive strength. NC60 showed 29.9% higher compressive strength than LC60, and it can be seen that NC generally shows higher compressive strength than LC. It can be seen that as the target intensity increases, the difference between NC and LC at the same target intensity increases. At 24 h, UPV showed 21.9% and 11.9% lower rates for NC30 and NC45 compared to NC60. Additionally, compared to LC60, LC30 and LC45 showed 22.3% and 12.2% lower UPV, and it is believed that the timing of form demolding can be evaluated by utilizing the fact that UPV varies depending on aggregate and W/B ratio. The prediction models for NC and LC based on the correlation between compressive strength and UPV showed a coefficient of determination (R2) of 0.90 or higher. Although the cement content significantly impacts the UPV of concrete, UPV may vary depending on the porosity and density of the aggregate, thus making it necessary to consider these factors for strength estimation.