Comparison of Compressive Strength of Hardened Concrete Using Schmidt Rebound Hammer and Conventional Testing Method

Abstract

Testing of concrete plays a crucial role in controlling the quality of cement in concrete works. Methodical testing of the raw materials, the fresh concrete and the hardened concrete is an indistinguishable part of any quality control program for concrete that helps to attain higher efficiency and prominent assurance of the performance of the concrete in respect to both strength and durability. The following work proposes a study showing variation in the magnitudes of the compressive strengths as obtained from NDT using Schmidt rebound hammer and DT using CTM (compressive testing machine). Concrete cubes of 150 mm × 150 mm × 150 mm size were collected with a concrete mix of grade 25 and 30 N/mm². 66 cubes were casted and used for the study of variation obtained in the values of compressive strength using NDT and DT, which was determined to be ranging from 0 to 15% with respect to DT for most of the cube samples. The result obtained proposes that using Schmidt rebound hammer as a non-destructive test to check the compressive strength of the hardened concrete is reliable.

1 Introduction

It would be difficult for us to imagine ourselves without concrete. Concrete is a visco-elastic moldable liquid consisting cement, sand, coarse aggregate, water with admixtures. It is fundamental building component for present day society. Each significant development venture utilizes concrete in some frame. Its wide use is governed due to its strength, durability, rigidity, low maintenance, resistance against water and fire, workability, economy, etc. With the use of concrete in infrastructural activities comes the parameter of safety that must be kept in mind and hence determining compressive strength is paramount which is done in order to judge concrete quality. Often, structures of concrete should be checked for its strength once the concrete has hardened for determining if the structure is serviceable. Various DT and NDT methods for determining strength of concrete have been suggested. The analysis of the concrete strength in existing structures by destructive method is costly, troublesome in some case and might not be possible in some cases. Hence, it is not always appropriate since they affect the physical characteristics of concrete whereas Non- destructive tests are reliable and time saving without undergoing any damage to the structure. The two tests that have been performed during the following work are compression test by compression testing machine and Schmidt rebound hammer test. DT investigates the mechanism of failure of material to determine its mechanical properties viz. yield and compressive strength. Non-Destructive techniques investigate properties without getting into the core of structure. The principle involved for Rebound Hammer Test is: The elastic rebound depends on surface hardness on which it strikes. When the plunger of Rebound Hammer is punched against the surface of hardened concrete, spring controlled mass rebounds and degree of such rebound depends upon hardness of the surface, thus rebound is related to the concrete’s strength. Value of rebound is obtained from the graduated scale and is assigned as rebound number or index. The strength of the concrete can then be read straight from graph that is present on the hammer body (Fig. 1).

Fig. 1

Schmidt rebound hammer. Source https://1.bp.blogspot.com/-6wa2pOhP47o/Xpx0Tc_LtUI/AAAAAAAAAXo/NAY8qT6hepQvN0V_vMraQdbZLxQwKBe5ACLcBGAsYHQ/s1600/rebound%2Bhammer.jpg

2 Literature Review

A series of connected works of national and global journals have been contemplated. Out of these, portions of the significant works are mentioned below:

• 1. R. Balamuralikrishnan (2017) have attempted to show that the percentage variation in the average compressive strength from NDT 25–32 N/mm² having w/c ratio ranging from 0.35 to 0.50 and destructive testing 30–36.24 N/mm² of same water-cement ratio varies from 11.50 to 16.70% with respect to destructive testing. The variation of the values of not more than 16.5% is the evidence that NDT by rebound hammer shows reliability in monitoring the health of structural elements. The study also suggested that the increasing rebound number represents the higher compressive strength and the results are affected by factors such as smoothness of surface, moisture condition of the concrete and the type of cement used.

• 2. Malek and Kaouther (2014) presented the calculations of compressive strength and modulus of elasticity determined from nondestructive and destructive tests. The investigation supported the use of NDT due to its ease of operation and economic advantages. The different results of the testing were conducted using compression testing machine and rebound hammer test. The compressive strength determined by destructive test (compression test) and Non-destructive test (rebound hammer test) at different ages of the concrete (7 and 14 days) showed that the resistances obtained by the compression test were higher than those obtained by the rebound hammer test and decreases considerably at the age of 28 days. The authors suggested that the rebound hammer test can be used to evaluate the compressive strength of old concrete and not young concrete.

• 3. Ali-Benyahia et al. (2017) have attempted to optimize the methodology of the calibration of NDT model on site. Combination of NDT techniques has been promoted in many studies but its efficiency remains controversial. The conclusions were centered around the enhancement of the significance and the viability of the NDT techniques in such operational circumstances.

• 4. Kumavat et al. (2014) performed a case study on condition assessment of concrete with NDT. A reduction of 5% in the minimum average compressive strength was found. It was presumed that the present strategies for ultrasonic testing of concrete require direct contact between the surface of concrete and the transducers. Since the contact is not always perfect, the air trapped in between may cause variable errors in the measurements.

• 5. Saleem et al. (2012) focused on non-destructive evaluation of a five-storied concrete frame structure to assess the existing condition. For this purpose, load tests and core tests were performed on four floors from basement to first floor. Test results showed that the structure has adequate strength for future use although it was unprotected against severe environmental conditions for several years. Study further confirms that a combination of tests, instead of performing just one type of test, provides more suitable results to confidently accept or reject the structure as a whole or its component for future use.

• 6. Shankar and Joshi (2014) compared the actual strength of a concrete by destructive test (DT) method and that by NDT method using Schmidt Hammer (SH) (or rebound hammer) and ultrasonic pulse velocity (UPV) as NDTs and test by compression testing machine (CTM) as DT. In this study, separate comparisons have been done for two NDTs and a procedure to follow while estimating strength of concrete by NDT has been recommended.

3 Methodology

66 Samples (Fig. 2) of concrete cubes were casted with the mix design of M25 and M30. The experimental procedure included the following steps to study the variation of compressive strength by NDT and DT. These steps include:(As per IS 516: 1979)

Fig. 2

66 cubes laid in order on ground

• 66 Cubes have been casted.

• All the specimens were cured before testing.

• Nomenclature is done for each specimen using a unique number with 10 points marked on each cube to facilitate Rebound hammer test and make identical testing for all cubes (Fig. 3).

  Fig. 3

  

  Detailing of 10 points on each cube with a unique number

• Specimens were then put at the center of compression testing machine and loaded to about 25.0% of their ultimate compressive strength.

• Before commencing the rebound hammer test, the apparatus should be tested against the test anvil to get reliable results.

• Ten readings were then taken to estimate the average rebound number using ten points marked on each cube.

• After reading the rebound number, the applied load was increased at a rate of approximately 140 kg/sq cc/min until failure and then cube.

• The compressive strength of each specimen was calculated (Fig. 4).

  Fig. 4

  

  The cube specimen placed in the testing machine

• The values obtained from both the tests were compared and the variation is then obtained in the values of NDT with respect to DT values.

4 Result

66 cubes were experimented by Rebound Hammer and Compression Testing machine and the variation in compressive test results has been shown in Table 1 followed by graphical comparison (Fig. 5).

Table 1 Variation between NDT and DT test results

Fig. 5

Variation in compressive strength obtained from NDT and DT

5 Conclusion

This experimental study suggests that the variation in the compressive strength obtained by Non- destructive test with respect to the Destructive test vary with a value ranging from 0 to 15% for most of the cubes samples.

Destructive and Non-destructive tests were performed on 33 cube samples of M25 grade and the average variation in values of strength using NDT with respect to DT is observed as 11.09% whereas in case of M 30 grade concrete, the average variation for 33 cube samples is 11.90%.

The result obtained suggests that the use of Non- destructive methods is reliable and can be used to estimate the compressive strength of hardened concrete. It can therefore reduce the number of cores taken from the structures to estimate the strength.