In-Place Test methods to estimate concrete strength has been extremely popular among engineers and inspectors involved in the field of structure inspection, evaluation and condition assessment. Various non-destructive methods have been developed by different researchers over the past few decades to assess in-place strength of concrete. The rebound hammer, developed by Ernst Schmidt, provides an inexpensive and quick method for nondestructive testing and evaluation of the hardness of concrete. In this article, we will review the rebound hammer, how it works, and how rebound number is determined. Moreover, we will discuss how to estimate concrete strength using rebound hammer.
The test has widely been used, since its introduction in 1948. The main reason behind its popularity is its simplicity, and convenience of use for field applications. Since its introduction, it has extensively been studied by several researchers around the globe, and have found its way into different standards and guidelines such as the ASTM C 805 “Standard Test Method for Rebound Number of Hardened Concrete”, and EN13791:2003 “Evaluation the Compressive Strength in Concrete Structures Using the In-situ Test Methods”, and ACI 228.1R “In-Place Methods to Estimate Concrete Strength”.
What is Rebound Hammer (Schmidt Hammer) ?
The rebound hammer is a nondestructive testing apparatus, whereby the rebound of the spring driven mass is measured after its impact with concrete surface. The output of the rebound hammer is referred to as rebound number and are correlated with surface hardness of concrete. The internal mechanism of a typical Schmidt Hammer is illustrated in Fig. 1. The plunger is pushed against the concrete, perpendicular to the surface. As the hammer body is pushed towards the concrete, the force of the causes the latch to release, and make an impact on the concrete. At this point, the hammer impacts the shoulder of the plunger rod & rebounds. During the rebound the slide indicator is moved from the hammer mass, and the rebound distance is recorded. The hammer can be placed in many configurations; upwards, downwards, horizontal vertical & at any intermediate angle. Commercial devices in the market use different means to display the rebound number after each strike. Older generation devices displays the rebound number through a mechanical slider. Nowadays, most modern hammers use electrical or even wireless means of displaying the results on data loggers, or smart devices such as a tablet.
How To Use The Rebound Hammer ?
Rebound Hammer is a very simple apparatus to work with; however, in order to collect reliable, and repeatable readings, certain conditions should be met ahead of testing.
Preparing Test Surface
Inspectors should pay attention to the following considerations prior to conducting a rebound hammer test:
The testing surface must be at least 150 mm diameter.
Ground concrete surface (until its flat) if it is heavily textured or contains loose mortar. Note: It is essential to know that the results from prepared surface are generally not comparable to those obtained from unprepared surfaces.
If there is any free moisture or water on the concrete surface, it must be removed prior to testing.
Do not test frozen concrete. Note: Concrete should only be tested after it has thawed, since frozen concrete tends to have high rebound numbers.
Avoid direct testing over steel reinforcement when cover thickness is less than 20 mm. It is recommended to use a rebar locator to avoid testing on shallow depth reinforcement.
Rebound Hammer Direction
In order to perform the rebound hammer test, the inspector should start by holding the instrument firmly, and ensuring that the plunger is perpendicular to the test surface. It is also important to record the orientation of the instrument with respect to horizontal to the nearest 45 degree increment. If the instrument is pointing upwards, a positive angle should be used & if the instrument points downward a negative angle must be recorded.
Once the device is lined up, and the angle has been recorded, the inspector then gradually pushes the instrument toward the test surface until the hammer impacts. After impact, maintain pressure on the instrument and, depress the side button of the instrument to lock the plunger in its retracted position. The rebound number is then recorded to the nearest whole number.
In order to ensure accuracy, 10 readings should be taken from each test area. In addition, the distances between impact points should be at least 25 mm, and the distance between impact points and edges of the member should be at least 50 mm. It is also important to note that if the impact crushes or breaks through a near surface air void the reading must be disregarded and another reading should be taken.
As previously mentioned, due to different effects of gravity on the rebound as the test angle is changed, the rebound number will be different for the same concrete and will require separate calibration or correction charts.
How to Calculate Rebound Number
The following example shows how to rebound number is calculated and reported. Let’s assume that the following 10 readings are obtained from one test location:
36 | 34 | 36 | 34 | 37 | 35 | 36 | 36 | 43 | 35
The average of these 10 readings is calculated:
Average 1 = (36+34+36+34+37+35+36+36+43+35)/10 = 36.2 🡪 36
Now, discard the readings that differ from the average of 10 readings by more than 6 units and determine the average of the remaining readings.
Upper limit: 36+6 = 42
Lower limit: 36-6 = 30
By looking into the original readings, we can see that 43 (larger than upper limit 42) should be discarded from the list, and the average of the remaining readings calculated and reported as the average rebound number:
Average Modified = (36+34+36+34+37+35+36+36+35)/9 = 35.4 🡪 35
Note: If more than 2 readings differ from the average by 6 units, discard the entire set of readings and determine rebound numbers at 10 new locations within the test area.
Note: Therefore, the range of ten readings should not exceed 12.
Applications of Rebound Hammer
The main application of a rebound hammer is to measure the hardness of concrete using the rebound principle. However, researchers have tried to use the method to assess different properties of concrete. Among them, the following applications are widespread:
Assess the in-place uniformity of concrete
Delineate regions in a structure of poor quality or deteriorated concrete
As an in-place method to Estimate Concrete Strength
Among these applications, using rebound hammer to estimate strength of concrete has remained the most controversial use. Researchers are divided when it comes to evaluating concrete strength by the use of rebound hammer.
Estimate Concrete Strength Using Rebound Hammer (In-Place Method)
In this section, we will review how to Estimate Concrete Strength Using Rebound Hammer. Initially, this was made through certain calibration curves provided by manufacturers (see Fig below)
However, further research has showed little apparent theoretical relationship between the strength of concrete and rebound number (Malhotra and Carino, 2004). However, it is shown that if a relationship between concrete strength and rebound number for a given concrete is established, the rebound number might be used for in-place evaluation of strength.
How To Establish Relationship Between Concrete Strength and Rebound Number?
Different methods have been proposed to establish a relationship between rebound number and compressive strength. The following describes two most widely used approach to establishing relationship between concrete strength and rebound number.
In order to establish a relationship between rebound number and concrete strength, Inspectors should take a minimum of 2 replicate cores, from 6 or more locations (12 concrete cores in total) with different rebound numbers. According to the ASTM C805 standard, test locations should be selected such that a wide range of rebound numbers in the structure is obtained. Collect, prepare, and test cores in accordance with Test Method C42/C42M. The strength relationship will be applicable for the same orientation as used If the rebound number if affected by the orientation of the instrument during testing, the strength relationship is applicable for the same orientation as used to obtain the correlation date.
The EN 13791 requires only 9 cores (taken from 9 different locations) in order to establish the relationship between strength and rebound number. Papworth et al. (2015) have reviewed how to use the rebound number ro estimate the compressive strength of concrete.
Note: According to the ASTM C 805, Locations where strengths are to be estimated using the developed correlation shall have similar surface texture and shall have been exposed to similar conditions as the locations.
Click here if you would like to learn about other non-destructive methods for on-site evaluation of concrete.
What Affects Rebound Hammer Readings
Although the rebound hammer provides a quick, inexpensive means of checking the uniformity of concrete, the results can be affected by the following parameters:
Smoothness of test surface
Size, shape, and rigidity of the specimens
Age of test specimens
Surface and internal moisture conditions of the concrete
Type of coarse aggregate
Type of cement
Type of mold
Carbonation of the concrete surface
The rebound hammer developed by Schmidt provides an inexpensive and quick method for nondestructive testing of concrete.
When using the rebound hammer, the limitations of the test method should be recognized and taken into account.
It is extremely important to note that the hammer must not be regarded as a substitute for standard compression tests but rather as a method for determining the uniformity of concrete in the structures, comparing one concrete against another, and reducing the number of core samples.
ASTM C805 / C805M-18, Standard Test Method for Rebound Number of Hardened Concrete, ASTM International, West Conshohocken, PA, 2018, www.astm.org (DOI: 10.1520/C0805_C0805M-18)
ACI 228.1R-03 In-Place Methods to Estimate Concrete Strength, American Concrete Institute Committee 228.1R
BS EN 13791:2007 Assessment of in-situ compressive strength in structures and pre-cast concrete components
M. Malhotra, V & Carino, Nicholas. (2004). CRC Handbook on Nondestructive Testing of Concrete. CRC Press Inc..
Papworth, Frank & Corbett, David & Barnes, Reuben & Wyche, Joseph & Dyson, Jonathon. (2015). In-situ Concrete Strength Assessment based on Ultrasonic (UPV), Rebound, Cores and the SONREB Method. 10.1201/b18972-37.