Construction

The rebound hammer test is a non-destructive test for the strength of concrete for field testing of concrete.

As per the Indian code IS: 13311(2)-1992, the rebound hammer test has the following objectives:

1. To determine the compressive strength of the concrete by relating the rebound index and the compressive strength
2. To assess the uniformity of the concrete
3. To assess the quality of the concrete based on the standard specifications
4. To relate one concrete element with other in terms of quality

The instrument can be used on both horizontal and vertical surfaces. The main point to be noted is that the graph had to be read appropriately. These days, the hammer is equipped with a digital scale to make life easier.

The IS: 13311(2)-1992 elaborates the procedure in a more detailed manner, making it easier for even beginners to understand the concept.

The point of impact should be at least 20 mm away from any edge or shape discontinuity. For taking a measurement, the rebound hammer should be held at right angles to the surface of the concrete member.

The apparatus is placed on the spot where the test is desired. The hammer is allowed to drop freely. After it strikes the concrete surface it rebounds back. The raise of the rebound is recorded. This test is repeated at different locations and the rebound value is recorded.

The compressive value of concrete is obtained by doing a cube test of the test cube of the batch of concrete on which the rebound test is conducted. The value of rebound and compressive strength of concrete is compared and the value of compression is adjudged from the values of compression test and rebound values.

Around each point of observation, six readings of rebound indices are taken 2nd average of these readings after deleting outliers as per IS 8900: 1978 becomes the rebound index for the point of observation. ( This outlaying is a calculation based method, can be discussed on one’s interest).

Now, we will calculate comp. Strength from rebound number. There are three positions of taking reading

1. Vertical Downward:- Weight of Hammer is increasing stress on spring of hammer
2. Vertical Upward:- Weight of hammer is decreasing the stress on spring of hammer
3. Horizontally :- Weight of hammer have no effect on spring.

This is why, there are three graphs in Rebound Number-Compressive Strength graph of Rebound Hammer.

You will have to calculate the strength from the graph as per position of Rebound Hammer.

For Example,

For Vertical Upward position of Hammer

If 15 Rebound number readings are

48,42,43,43,44,44,45,45,45,45,46,47,48,50

, as per IS:8900, Clause 5.1.1(TESTS FOR TWO OR MORE OUTLIERS , AT LEAST ONE OUTLIER AT EACH END), 42 and 50 are Outlaying values.

Removing those, Average Rebound number is 45.27

From the graph( fro Vertical Upward) , Compressive Strength Will be 40.54 Mpa

(FYI : PPC is environment friendly as it consists fly ash in some specific percentages by parts, which is a waste product of burnt coal, usually collected from thermal power plants)

OPC (m53) attains strength in a shorter time, while PPC (m43) takes longer.
OPC therefore dries more quickly, but produces more heat during chemical reaction during which it gains strength, as compared to PPC
Faster is the drying process, more is the risk of cracking of cement, and therefore it need more care and more amount of water for curing in a lesser time.
PPC on the other hand can attain strength equal to OPC, but takes a longer time.

For a building slab, PPC is more widely used since it requires less care.
The strength depends on design mix (the water cement and aggregate ratio) more than type of cement.

so essentially, If you need construction to take place faster and have means for good curing, you may use OPC.
But if you need better quality and strength in less effort, use PPC.

It still depends on the structure’s design requirements and a structural designer’s advice is to be followed.

Hope this helps, but do consult an engineer before using any type of cement.
Building construction is a responsible job and needs to be done safely and one must adhere to engineer’s specifications.