Compressive strength is one of the most important characteristics of concrete, which can be understood by conducting tests on standard samples. Concrete is the most suitable, durable, and strong building material worldwide. Concrete has revolutionized the entire construction industry and made it possible to build simple to complex structures on land or sea. No structure can resist earthquakes, but we can control the effects of earthquakes through concrete. Precast concrete is an excellent building material. The performance of precast concrete walls under seismic loads largely depends on the quality of precast concrete and its design. High-quality materials and high production standards help to improve the way walls and fences react to earthquakes. A high-quality product can be produced because precast concrete is mixed and assembled on-site. Temperature and humidity can be maintained at optimal levels for the highest concrete strength.
Compressive strength is one of the most critical characteristics of hardened concrete. Evaluating reinforced concrete structural members involves determining concrete strength through tests on standard samples, both quantitatively and qualitatively. However, these samples may not represent the natural characteristics of concrete. Several methods have been devised to estimate the compressive strength of concrete in place. The non-destructive method of ultrasonic waves is used based on the prevailing mechanism. The test is done with the pundit device. The transmitting transducer is placed in contact with both faces of the concrete, and the vibrations are received by the receiving transducer after passing through the concrete. The compressive strength of concrete is in MPa, and the velocity of ultrasonic waves is in km/s.
In this research, the shear wave velocity of earthquakes in concrete has been investigated. The study considers the conditions of concrete processing, changes in the type and amount of materials from one unit to another, and the concrete transfer method. The structure's compaction level is not considered. It is necessary to create destructive stresses to determine the strength of concrete, and in this study, non-destructive tests were used. The relation of compressive strength can be determined using these tests, based on the structural parameters, by calibration curves. In this study, based on the non-destructive ultrasonic method, the velocity of waves was investigated to evaluate the strength of hardened concrete, and the shear velocity in concrete was calculated.
Five important structures in Qazvin city, built based on one concrete, were selected. The composition of concrete used in these structures was sand, gravel, and cement. Laboratory concrete was also selected from river-type sand, crushed sand of sandstone type, and type 2 cement. The dimensions of this concrete were considered to be 15 cm in all three dimensions. Ultrasonic pulse waves were investigated in two faces of concrete for 28 days in dry and wet conditions. The concrete sample is placed in water for 28 days. There is no practical significance in choosing a life of 28 days. The simple justification of this issue is that the cement strength process is slow, so the strength description should be based on concrete where a significant portion of cement hydration has occurred. After 28 days, the concrete sample is taken out of the water basin, and its weight is measured with the help of a scale. It is placed inside the concrete breaker jack, and the resistance is calculated.
The comparison of two dry and wet conditions shows that the percentage of moisture significantly affects the estimation of compressive strength. So, if the calibration is done on more cubic samples, the structure's resistance will be estimated as lower than the actual value. Also, based on a 95% confidence level, the error rates for concrete samples stored in wet conditions and laboratory environments were determined to be ±17% and ±18%, respectively. The average dispersion of ultrasonic wave velocity test results for concrete samples kept in wet and laboratory environments was obtained as 2.6% and 2.9%, respectively. This test showed that the above sample shows a lower compressive strength. Therefore, in dealing with an earthquake with Mw≥7, due to the shear wave velocity passing through this type of concrete, buildings made with this concrete will not show stability. Therefore, among the studied structures, the Bahonar Bridge needs serious retrofitting.