Rebound Hammer Test, also known as Schmidt Hammer Test, is a non-destructive test that is performed to find the compressive strength of concrete surface. There are some factors that affect the process and results of this test which are described here with title Factors Affecting Rebound Hammer Test and How to Avoid them.

Factors Affecting Rebound Hammer Test | How to Avoid

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The important factors that affect the Rebound Hammer Test are given below:

1. Type of cement
2. Type of aggregate
3. Surface and moisture condition of concrete
4. Type of curing and age of concrete
5. Carbonation on concrete surface
6. Position of Concrete Surface

6 Factors Affecting the Rebound Hammer Test

Since the Rebound Hammer Test is applied for concrete compression test so the affecting factors include the properties of cement and aggregates, from which concrete is made.

Now read about these factors in detail.

1. Type of Cement

The concrete made of high alumina cement has higher compressive strength compared to Ordinary Portland Cement (OPC). So the use of high alumna cement increases the rebound number compared to that of OPC.

The concrete made of super sulphated cement has 50% less compressive strength compared to OPC. So the use of super sulphate cement decreases the rebound number compared to that of OPC.

2. Type of Aggregate

The correlation between the compressive strength and rebound numbers of concrete varies with the different types of aggregates used. The use of normal aggregates like gravels and crushed rock aggregates gives normal correlations in the results. The concrete made by use of lightweight aggregates require special calibration to undergo the rebound hammer test.

3. Surface and Moisture Condition of Concrete

The Schmidt/Rebound hammer test gives best and suitable results only for close texture concrete. The test doesn't give good results for open texture concrete. Open texture concrete like typical of masonry blocks, high honeycombed concrete or no-fines concrete is not suitable for the Rebound Hammer Test.

All correlations assume full compaction of concrete, as the strength of partially compacted concrete doesn't bear unique relationship to the rebound numbers.

The compressive strength is overestimated by the test when testing on trowelled or floated surfaces compared to moulded surfaces.

When the wet concrete surfaces is tested under dry conditions then it will give a lower strength value than usual. This underestimation of the strength of concrete structure can be about 20% lower than in an equivalent dry concrete.

4. Type of Curing and Age of Concrete

The relation between the hardness and strength of concrete varies as a function of time. Variations in initial rate of hardening, subsequent curing and moisture exposure conditions of concrete surfaces also affect this relationship.

Separate calibration curves are required for different curing regimes but the effect of age can generally be ignored for concrete with an age between 3 days to 3 months.

5. Carbonation on Concrete Surface

The effect of carbonation on concrete surface is very significant in terms of rebound number. Carbonated concrete gives an overestimate of compressive strength value by the rebound hammer test. It is estimated to be 50% higher.

So the rebound hammer test have to be conducted  after removing the carbonated layer from concrete surface to obtain more accurate result.

6. Position of Concrete Surface

The vertical distance from the bottom of concrete placement is also affect the rebound hammer test. During compaction of concrete mix the concentration of aggregates is higher at the bottom of texture. So a higher rebound number is observed near the bottom of concrete placement.

How to Avoid the Factors Affecting Rebound Hammer Test

Keep these things in mind before hammering for better results.

1. Remove loose particles like dust, gravel and unnecessary fine aggregates.
2. If the surface is roughed then make it smooth and clean with a grinding wheel or stone.
3. Strike the rebound hammer at least 20 mm away from the edge or shape discontinuity.
4. Conduct the rebound hammer test horizontally on vertical surfaces or vertically upwards or downwards on horizontal surfaces at different points.
5. Take six readings around each point of the test. The average value of these readings after deleting outliers as per IS 8900 : 1978 taken as the rebound index for the corresponding point of observation.
6. Check the carbonation depth in cases where the age of concrete is more than six months.
7. Remove the carbonated layer from the surface.
8. Test the concrete surface with the rebound hammer on the non-carbonated concrete.

After doing this, the effects on the test can be reduced to great extent.