Utilizing hammer tests and ultrasonic pulse velocity to ascertain the compressive strength of concrete

This study aimed to estimate the compressive strength of concrete in an existing building using non-destructive testing methods, specifically the hammer test and ultrasonic pulse velocity. The hammer test employed two types of tools, type N, which reads the rebound number (R), and another type that reads the rebound coefficient (Q) value. In the study, 40 column samples were tested, revealing concrete quality measurements of 30.5 MPa and 27.8 MPa obtained from hammer test type N and rebound coefficient (Q), respectively. Meanwhile, the ultrasonic pulse velocity reading was 3288 m/s. The data from the hammer test type N and rebound coefficient (Q) were correlated with the velocity of an ultrasonic pulse velocity, resulting in a linear regression formula of f’c = 0.0137x - 17.12 and f’c = 0.0099x - 2.1973, respectively. The coefficient of determination (R2) of hammer test type N and velocity of an ultrasonic pulse velocity was 0.64, while the coefficient of determination (R2) of hammer test Q value and velocity of an ultrasonic pulse velocity was 0.70. The research predicted that the model for determining the quality of concrete by correlating the hammer test Q value with the velocity of an ultrasonic pulse velocity was better than using hammer test type N.


Introduction
Concrete is a fundamental material in the construction industry and plays a crucial role in building infrastructure.High-quality concrete contributes to the durability and longevity of structures, reducing the need for frequent maintenance and reconstruction, which, in turn, helps in achieving infrastructure sustainability.To ensure the quality of a newly constructed building, it is important to monitor and evaluate the structural elements such as beams, columns, and floor plates [1] [2].This can be achieved through various tests on the concrete, including destructive and non-destructive tests [3][4][5][6][7].The destructive test, for instance, involves crushing concrete samples using a Compressive Strength Machine to determine the maximum compressive strength of the concrete.
Alternatively, there exists a non-destructive method to assess concrete strength, which involves evaluating the material without causing any damage.This approach is particularly useful for assessing concrete strength in completed structures where destructive testing is not possible.Examples of nondestructive tests include the Rebound Hammer Test and the Ultrasonic Pulse Velocity Test [8], [9][10].
Despite its benefits, the non-destructive test has some drawbacks, such as lower accuracy due to its reliance on estimates of the compressive strength of concrete [10][11].The Rebound Hammer Test, for instance, employs a steel hammer that strikes a steel launcher against the concrete surface using spring force.There are two types of Rebound Hammer Test available: the N type and the NR type, which differ based on their data recorder or recorder.The N type does not feature a data recorder, whereas the NR type does [1].In contrast, the Ultrasonic Pulse Velocity (UPV) method estimates concrete hardness by measuring the velocity of waves through the material, which is then used to determine its strength.This approach relies on the relationship between the velocity of waves through the concrete and its strength [2].

Methodology
The study began by defining the problem, outlining the research objectives, and acknowledging the research limitations.Relevant literature was reviewed to establish a theoretical framework.The locations of the columns to be tested were identified, and a map was created to facilitate the testing process.Non-destructive tests were then performed on 40 column samples taken from the BINUS Syahdan building (Figure 1) using the Hammer Test Type N, Hammer Test Q value, and Ultrasonic Pulse Velocity tests with the direct method.The results of the three tests were analyzed using regression comparisons.Based on the comparison results, recommendations were made regarding the preferred test tool.The findings of the recommended test tool were connected with previous research in the field.The analysis was concluded, and suggestions for future research were provided.The study utilized a quantitative method where experiments were conducted on columns of the BINUS Syahdan building as the sample.The columns had a size of 33.5 cm x 55 cm, and a total of 40 column points were tested in the building as seen in figure 2.
For this study, two types of Hammer Test were employed: the Hammer Test Type N and the Hammer Test value as seen in figure 3 and figure 4. Both tools were used to conduct tests on 40 columns of the BINUS Syahdan building in a consistent order.The testing process was carried out according to the guidelines set by SNI-03-4430-1997, which outlines the Concrete Hammer Test Method.The general process for the Hammer Test involves applying 10 strikes to a concrete column sample at a point that is at least 5 cm away, and then calculating the average of the results obtained.

Figure 3. Hammer test type N and distance between test points
The table 1 will show the results of the averaged rebound hammer test for each of the 40 columns tested in this study.The test was conducted 10 times at a predetermined point in each column using two different tools, the Hammer Test Type N and the Hammer Test Q value.The results of the tests were averaged to provide an estimate of the compressive strength of each column.The table 1 will likely include the column number, the tool used, the averaged rebound number or coefficient value, and the estimated compressive strength of the concrete in MPa.
The Hammer Test Q value and Hammer Test type N have slightly different calculation stages due to their features.The Hammer Test Q value is a digital hammer that can store data and directly convert the rebound value (Q) into MPa.The subsequent non-destructive examination is the Ultrasonic Pulse Velocity Test, which assesses the quality of concrete by measuring the speed of wave propagation within it.Nevertheless, the UPV device solely provides the RX (receive) travel time value in microseconds, which is the time taken for the wave to travel from the TX (transfer) to the RX (receive) in µs units.The test process uses the UPV tool as shown in the figure 5.

Results and discussion
The objective of the study is to estimate the compressive strength of concrete in an existing building using non-destructive testing methods, specifically the Hammer Test and Ultrasonic Pulse Velocity (UPV).The following is the analysis of the research are as follows.

Hammer test
In this investigation, two tools were utilized for conducting the Rebound Hammer Test, specifically the Hammer Test Type N and the Hammer Test Q value.Both tools were used to examine 40 columns of the BINUS Syahdan building in the same sequence, in compliance with the Concrete Hammer Test Method described in SNI-03-4430-1997.Essentially, this method involves applying 10 strikes with the hammer on the concrete column sample at a minimum distance of 2.5 cm from each other, and then averaging the obtained results.

Hammer test type N
Table 1 displays the estimated compressive strength, which is derived from the conversion of the curve between the rebound number and the impact position line of the tool against the sample.This conversion involves drawing a vertical line from the rebound number perpendicular to the position of the tool and then drawing a horizontal line in the direction of compressive strength.As an illustration, in Building J, the sample column J1 has an average rebound number of 37.2 and an estimated compressive strength of 31.2MPa, as depicted in Figure 6.The estimated compressive strength is calculated automatically by the Rebound Hammer Q value tool, based on the average Q value obtained from the test.The graph below illustrates the results of the Hammer Test Q value, generated by the tool and processed in the application:

Ultrasonic pulse velocity test
The next non-destructive test is the Ultrasonic Pulse Velocity Test which uses the value of wave propagation speed in concrete to determine the quality of the concrete.However, the device produced by Matest only issues the RX (receive) travel time value which receives the frequency from the TX (transfer) in units of µs.Table 3 shows the travel time data for each of the 40 columns with five tests using the direct method:   Generally, the correlation between these two factors suggests that a higher velocity of the column sample corresponds to higher estimates of compressive strength.Therefore, it can be concluded that the comparison between the estimated compressive strength and the velocity of propagation is linear or directly proportional.This is supported by the trend line in the graph, which indicates a rising trend with an R 2 value of 0.6476.

Correlation hammer test Q value and ultrasonic pulse velocity
The estimated compressive strength obtained from the Hammer Test Q value test can be correlated with the fast wave propagation observed in the UPV test, as illustrated in Figure 9.   9 displays an ascending trend line, indicating a strong positive correlation between compressive strength and wave propagation speed.The graph suggests that a higher MPa value corresponds to a higher wave propagation speed.The coefficient of determination (R 2 ) value of 0.7018 indicates a strong level of relationship between the two factors.

Conclusion
Based on the research that has been conducted, the conclusions of the research are as follows: a.The predicted compressive strength values resulting from the two tools, namely the Rebound Hammer Type N and Rebound Hammer Q value, are quite different, where the average existing concrete column samples are 30.5 MPa and 27.8 MPa with a difference of 9%.b.The regression linear formula for Hammer test Type N and Hammer Test Q value were f'c = 0.0137x -17.128 and f'c = 0.0099x -2.1973, respectively.c.The coefficient of determination (R 2 ) of Hammer Test Type N and velocity of an ultrasonic pulse is 0.6476.Whereas, the coefficient of determination (R 2 ) of Hammer Test Q value and velocity of an ultrasonic pulse is 0.7018.d.The research prediction model in determining quality of concrete from the correlation between Hammer Test Q value and velocity of an ultrasonic pulse is better than Hammer Test Type N.

Figure 2 .
Figure 2. Location of the column points tested

Figure 4 .
Figure 4. Hammer test Q value

Figure 5 .
Figure 5. UPV data collection and reading

Figure 7 .
Figure 7. Rebound numbers at 10 test points in 1 column

3. 3
Correlation hammer test type N and ultrasonic pulse velocityBy comparing the results of the Hammer Test Type N and Ultrasonic Pulse Velocity tests, a correlation can be established between the estimated compressive strength (MPa) of the concrete column sample and the velocity of propagation (m/s) as shown in Figure8.

Figure 8 .
Figure 8. Correlation hammer test type n and ultrasonic pulse velocity Figure 8 displays the relationship between the estimated compressive strength derived from the Hammer Test Type N and the velocity value obtained from the Ultrasonic Pulse Velocity results.Generally, the correlation between these two factors suggests that a higher velocity of the column sample corresponds to higher estimates of compressive strength.Therefore, it can be concluded that the comparison between the estimated compressive strength and the velocity of propagation is linear or directly proportional.This is supported by the trend line in the graph, which indicates a rising trend with an R 2 value of 0.6476.

Figure 9 .
Figure 9. Correlation hammer test Q value and ultrasonic pulse velocity

Figure
Figure9displays an ascending trend line, indicating a strong positive correlation between compressive strength and wave propagation speed.The graph suggests that a higher MPa value corresponds to a higher wave propagation speed.The coefficient of determination (R 2 ) value of 0.7018 indicates a strong level of relationship between the two factors.

Table 1 .
Compressive strength prediction data of column based on hammer test type N The calculation process of the Hammer Test Q value test differs slightly from that of the Hammer Test Type N, owing to its distinct features.The Hammer Test Q value is a digital hammer that can store data and directly convert the rebound coefficient (Q) into MPa.

Table 2 .
Compressive strength prediction data of column based on hammer test Q value

Table 3 .
Average of time