Schmidt Rebound Hammer Test

Chapter 3 ~ Objective Measurements (Building Integrity)Hitachi Tower

Building Integrity

There are 2 tests for the testing of Building Integrity, they are the Schmidt Rebound Hammer test and the moisture meter test.

1) Schmidt Rebound Hammer Test

The Schmidt Rebound Hammer is a form of non-destructive test to determine the surface hardness of concrete. It is the most widely accepted instrument for measuring the surface hardness of concrete. A fixed mass of steel is charged with kinetic energy through a spring system by gradually pressing the plunger against the surface to be tested. The steel mass is released and impinges on the plunger, which remains in contact with concrete.

After this impact, the mass rebounds and the magnitude of the rebound is a measure of the hardness of the surface, indicated by the rider on a linear scale graduated in empirical rebound numbers. When the hammer is used to testa non-vertical surface, the rebound heading is corrected because of the change in the impact energy. We need to use the Cube Compressive Strength in psi plotted against the Rebound Number graph to get the actual strength of the concrete surface that is being tested. The empirical relationships between rebound numbers and strength have been established and in general, the higher the rebound number, the greater is the strength of the concrete.

During the testing of the hammer on the wall or concrete surface, several points on the surface should be taken and the average reading to be recorded. This is because different points have different hardness due to the differences in aggregate and matrix. Another point to note is that when the hammer is used in testing, the strength of the situ concrete in the test procedure and environmental conditions should be similar to those employed during calibration. The readings on five different parts of the wall are taken at a horizontal position (or 0 degree) and tabulated below:-

The test hammer is a useful tool for assessing the strength of concrete in structures. It is an inexpensive mechanical device and is easy to use. The technique can also be applied to assess the uniformity of concrete, for example, to locate the possible existence of an area of unsatisfactory concrete in a wall. By using the test hammer, it is possible that all parts on the largest structure can be tested in a very short time and to determine the deviations in the concrete quality over the different parts of a structure. But the limitations of the method is that for a given concrete, the rebound number can vary because of differences in hardness’ between aggregate and matrix and the possible variation in aggregate mineralogy. Thus, several readings are taken and the average value used.

REBOUND HAMMER TEST
Location / Values (N/mm2)
Columns / Avg. / 1 / 2 / 3 / 4 / 5
Raw condition : 1 / 51.60 / 50.00 / 48.00 / 52.00 / 56.00 / 52.00
Raw condition : 2 / 40.60 / 21.00 / 58.00 / 54.00 / 22.00 / 48.00
4th floor / 45.80 / 50.00 / 49.00 / 30.00 / 48.00 / 52.00
17th floor / 51.60 / 58.00 / 52.00 / 50.00 / 50.00 / 48.00
23rd floor / 44.80 / 50.00 / 52.00 / 52.00 / 20.00 / 50.00
34th floor / 48.60 / 48.00 / 52.00 / 56.00 / 50.00 / 37.00
36th floor / 46.80 / 53.00 / 29.00 / 46.00 / 55.00 / 51.00
Location / Values (N/mm2)
Walls / Avg. / 1 / 2 / 3 / 4 / 5
Raw condition : 1 / 31.80 / 30.00 / 27.00 / 37.00 / 30.00 / 35.00
Raw condition : 2 / 31.20 / 41.00 / 25.00 / 32.00 / 30.00 / 28.00
4th floor / 34.80 / 29.00 / 35.00 / 39.00 / 30.00 / 41.00
17th floor / 34.40 / 35.00 / 32.00 / 29.00 / 39.00 / 37.00
23rd floor / 30.80 / 18.00 / 36.00 / 34.00 / 32.00 / 34.00
34th floor / 33.00 / 20.00 / 35.00 / 32.00 / 38.00 / 40.00
36th floor / 32.80 / 29.00 / 32.00 / 38.00 / 30.00 / 35.00

Analysis

It can be seen from the results that the surface concrete strength of the columns are generally higher than that of the walls. This is perhaps due to the fact that a higher grade of concrete is used for the columns, as the columns are load-bearing structures whilst the walls being non load-bearing structures are constructed using a lower grade of concrete.

The results for the walls generally vary more than that of the columns. This could be due to the fact that the walls cover a larger area and thus there would be greater variations in surface concrete hardness.

2) Moisture Meter Test

1)Moisture Meter

• To measure the moisture content on materials, like wood, brick and concrete.
• Indications: Blue – determine % of water ( Highest – 28%; mainly for timber)

Black – determine temperature (not used)

• For brickwall or concrete wall, use green indicator.
• Range: 0 – 10 => no moisture; okay

10 – 15 => some moisture; not serious

16 – 100 => considerable amount of moisture

above 100 => serious

2)Digital Moisture Meter

• Green => okay
• Amber
• Red => Dangerous

The digital Moisture Meter is used to measure the amount of moisture in the building structural members. It gives a reading to 2 decimal places.

Care must be taken when using this instrument as it has two sharp points that are to be inserted into the object that measurements are to be taken.

These two points must be inserted forcefully into the objects or else a negative reading will be registered.

In addition, one must not move but need to keep one’s hands still while holding the instrument so as to avoid inaccurate and fluctuating readings. The instrument switches off automatically after 10 seconds to conserve the battery energy. Thus one has to read the reading fast and cannot afford to lose time caused by shaky hand movement.

MOISTURE CONTENT TEST
Location / Values
Walls / Avg. / 1 / 2 / 3 / 4 / 5
Raw condition : 1 / 9.28 / 8.60 / 8.90 / 10.10 / 9.50 / 9.30
Raw condition : 2 / 9.66 / 10.10 / 9.50 / 9.80 / 8.90 / 10.00
4th floor : 1 / 9.10 / 8.60 / 8.70 / 9.40 / 9.30 / 9.50
4th floor : 2 / 10.04 / 9.20 / 11.10 / 8.90 / 12.00 / 9.00
4th floor : 3 / 9.58 / 8.60 / 10.20 / 9.00 / 11.10 / 9.00
17th floor : 1 / 9.94 / 10.70 / 9.80 / 10.40 / 8.50 / 10.30
17th floor : 2 / 10.46 / 11.00 / 8.50 / 12.50 / 11.30 / 9.00
17th floor : 3 / 10.70 / 12.20 / 9.70 / 12.30 / 9.80 / 9.50
23rd floor / 9.76 / 9.10 / 8.90 / 10.50 / 9.60 / 10.70
34th floor / 9.38 / 9.20 / 9.40 / 8.90 / 10.40 / 9.00
36th floor : 1 / 8.90 / 8.70 / 9.20 / 9.10 / 8.40 / 9.10
36th floor : 2 / 11.32 / 11.60 / 12.80 / 13.10 / 10.10 / 9.00
36th floor : 3 / 9.12 / 5.40 / 8.90 / 11.20 / 10.10 / 10.00

Analysis

The moisture contents for most of the locations that we have tested are between 9.00 to 10.00. This indicates that there is very little moisture and they fall within acceptable ranges.

However, there are 4 locations (namely, 4th floor 2, 17th floor 2 and 3, 36th floor 2), which registered moisture content that are higher than 10.00, with 36th floor 2 hitting the highest with a value of 11.32. For these locations, there is some moisture but it is not serious. It is still considered acceptable.

17th floor 1 registered a value of 9.94 which indicates that 17th floor generally contains more moisture than the rest of the locations.

On the other hand, 36th floor 1 and 2 gave readings only 8.90 and 9.12, yet 36th floor 3 gave a reading of a high 11.32. The difference in readings of 36th floor is considerable and this indicates that the high moisture content of 36th floor 3 may be a localized one, unlike that of 17th floor.