Introduction
The purpose of this article is to provide calibration curves for measuring the compressive strength of existing concrete structures using the "Twist-off" test. With the simple, inexpensive, and accurate "Twist-off" method, the compressive strength of many structures can be estimated in the shortest possible time and, if necessary, they can be reinforced before a crisis occurs so that less damage is caused to the structures after the crisis and loss of life and property can be prevented. Many laboratory studies have been conducted using the "Twist-off" test. In previous studies on concrete with different treatments, a correlation coefficient of about 0.94 was obtained between the results of the "Twist-off" test and the strength of concrete. In another study that used the "Twist-off" test to measure the compressive strength of cement mortars, it was concluded that the above test has a good accuracy. Also, the relationship between the modulus of rupture of pozzolanic concrete beams and the results of the "Twist-off" test also indicates the high accuracy of the above test for measuring various characteristics of pozzolanic concrete.
Methodology
In the in-situ "Twist-off" test, a steel cylinder with a radius of 25 mm is glued to the concrete surface and a torsional moment is applied to it. The equipment and supplies used in this test are very simple and inexpensive. The damage to the concrete in this test is very superficial and insignificant.
Results and discussion
Using power statistical analysis, it is observed that the correlation coefficient is above 95%, which indicates the high accuracy of the in-situ "Twist-off" test for evaluating the compressive strength of concretes with different strength classes. Therefore, in order to measure the in-situ strength of concretes, using the "Twist-off" method, the torsional moment obtained from the above test can be used to convert it into concrete strength and by using the power calibration curve. Concrete water tanks and concrete retaining walls have adequate compressive strength and therefore do not require reinforcement. However, the concrete bridges tested in this study and the concrete foundation do not have adequate compressive strength and a solution must be devised to reinforce them so that they can still be used after an earthquake. Before this study, standard cores had to be removed from the concrete to measure the compressive strength of existing concrete structures or elements and then tested in the laboratory. This would cause destructive damage to the structure. Or, the in-situ "pull-off" test had to be used, which requires an imported and very expensive device.
Conclusion
In this study, to measure the in-situ compressive strength of concrete structures, a new and in-situ "Twist-off" test was introduced and initially the results were compared with the results of the standard test. Then, laboratory samples and real concrete structures were evaluated using the above test. The results are as follows:
1- According to the country's crisis management law, cities must become resilient to upcoming crises. One of the issues of resilience is the need to know the compressive strength of existing concrete structures of different ages.
2- Given the accuracy of the new and in-situ "Twist-off" test, the compressive strength of existing structures can be measured with high accuracy without the need to destroy concrete.
3- By comparing the in-situ "Twist-off" test and the standard laboratory compressive strength test, it was determined that there is a power relationship with a correlation coefficient of over 95 percent between the results, which indicates the accuracy of the "torsion" test in assessing the in-situ compressive strength of concrete.
4- By performing a "twist" test on a number of existing concrete structures in Tehran, it was determined that a number of them need to be reinforced so that they can be used after the earthquake. Therefore, it is necessary to evaluate their compressive strength using the above test without destroying the concrete of the structures.
5- Using SEM photography, it was observed that the concrete samples under water curing had integrity and no large porosity was observed in them. However, insufficient curing caused the formation of extensive networks of fine pores in the microstructure of the concrete.
Funding
This work is based upon research funded by Iran National Science foundation (INSF) under project No. 4030409.
Authors’ Contribution
Authors contributed equally to the conceptualization and writing of the article. All of the authors approved thecontent of the manuscript and agreed on all aspects of the work declaration of competing interest none.
Conflict of Interest
Authors declared no conflict of interest.
Acknowledgments
We are grateful to all the scientific consultants of this paper.