No tags yet.
When we consider concreting at site and their respective cube tests, there are four possible combinations as listed below.
1. Concrete is strong; cube is also strong.
2. Concrete is weak; cube is also weak.
3. Concrete is strong; but cube is weak.
4. Concrete is weak; but cube is strong.
We normally consider the first two possibilities while analyzing cube test results, the third and fourth possibilities are generally being neglected.
The third possibility, however, does not harm the structure.
But possibility number four….well…it is better to be optimistic in such situations. When you get reports indicating that the 28th day cube tests failed, remember that it is something serious. It cannot be treated just like in the case of 7th day test failure. As we have discussed already, 28th day cube tests are considered the deciding criterion for acceptance of the concrete. Before going deep into our scenario of cube failure, let us quickly see how the cube results are generally interpreted as per IS code. According to IS 456 2000 Cl 16.1, two conditions are to be met for accepting a concrete after cube test. For concrete of grade M20 and above, the average strength of a group of four consecutive test results should not be less than the greater of fck+4 N/mm2 or fck+0.825σ where σ is the standard deviation established. Individual cube strength should not be less than fck-4 N/mm2. Let us analyze this with an example.
The following are the 28th day cube test results of four M40 concrete samples: 45, 43, 42 and 47 N/mm2. The average strength of these samples is 44.25 N/mm2 . Now, the first condition is that this average should not be less than the greater of fck+4 N/mm2 or fck+0.825σ.
Let us assume a standard deviation (σ) of 5 N/mm2, as per table 8 of IS 456 for M40 concrete. Thus the minimum strength should be the greater of 40 + 4 = 44N/mm2 or 40 + 0.825 x 5 = 44.125 N/mm2 , ie 44.125 N/mm2 . The first condition is met. The second condition is that no individual test result should have values less than fck–4 N/mm2 .
In our case it should not have values less than 36N/mm2. We meet the second condition too. So the concrete sample is acceptable.
Now, let us go back to our problem of failure of cubes on 28th day test. If the cubes do not have the required minimum compressive strength, and thereby failed, the following actions are to be taken urgently.
1. Stop all activities on the member under question.All further construction on the members under question need to be suspended immediately to prevent further damage to the structure.
2. Test the compressive strength of the structural member under question.This step is to clarify whether the member itself is weak, or just the cube. There are different methods for testing this like core testing and Non–Destructive tests including rebound hammer test, ultrasonic pulse velocity test, pullout test, probe penetration tests, maturity test etc. These tests will be explained in another post. If it is found that the member under question is not having the required compressive strength, the stages mentioned below to be considered.
3. Test the load carrying capacity of the member.Load tests also can be carried out on the members to decide on their load carrying capacity. This can be normally done on flexural members by applying a load equal to DL+1.25LL for a period of 24hrs. The deflection is noticed at the end of 24 hrs and then the LL part is removed.If the maximum deflection is less than 40l2/D mm, where ‘l’ is the effective span in meter and ‘D’ is the overall depth in mm, the member is safe in flexural load.If the deflection is more than 40l2/D mm, the recovery of deflection after the removal of LL is analyzed. If the recovery within 24 hrs after the removal of LL is less than 75% of the deflection, the test is repeated after 72 hrs. If the recovery in this case also is less than 80%, the structure is considered as unacceptable.
4. Determine the maximum load the member can take at the present condition.Once we realize that the member is not strong enough to carry the designed load, the next process is to calculate the maximum load that can be safely transferred to the member. This can be obtained normally by studying the compressive strength of the cubes tested earlier, and the compressive strength of the cores taken from the member. The quality of concrete obtained by other N-D tests also needs to be studied before reaching a final value. Though not advisable, anyhow at this stage, we also need to consider the factor of safety assumed for the member by the structural consultant. Perhaps the assumed load combination during the design may not be the likely one at the given / changed scenario, and in such case analysis may be reviewed with a more likely load combination.
5. Redesign of members so as to distribute the excess load to other members in a safe way. AND / OR
6. Strengthen the member under question using various strengthening methods.OR
7. Removing the member under question and casting a new member with adequate strength.
Choice / combination between options 5,6 and 7 depends on various factors including severity of the case, time, cost, functionality, technical feasibility, aesthetics, workability, influence on other members and services etc.