Sir, In our project work rcc cube for 7 & 28 days are fail.What is the solution for checking by rebound hammer test or other. Please suggest.
Cube failed at 7 and 28 days test what should do next
- Thread starter ashok
- Start date
Take cored sample and subject to compression test. Rebound hammer is not acceptable. Rebound hammer may help to locate the specific location where the strength is relatively low compare to other location in the same structural element.
Steps to be taken at cube failures:
1. First of all immediately support the structure with props till the lab tests are completed.
2. Best option would be Core cut compression test, however, if you want then you can first go for the UPV/Rebound Hammer which will help you in ascertaining the actual strength, which if you are not satisfied then you can opt for the Core cut option.
3. Rebound Hammer, CAPO/Pullout, Windsor probe and ultrasonic pulse velocity tests give indirect evidence of concrete quality hence its recommended to go for core sampling and testing as its a direct assessment.
4.The general rule adopted for fixing the core size, besides the H/D ratio, is the nominal size of stone aggregate and the dia should be not less than 3 times the maximum size of stone aggregate. For a diameter of core less than 3 times the size of the stone aggregate, an increased number of cores have to be tested.
5. Prior to corecut, one has to observe for any steel which could get cut in the process of obtaining the core sample. so its advisable to get it done from a renowned Laboratory.
6. If after all the tests the structure is showing sign of failure then you can for structural strengthening methods which is a different subject all together wherein one can go for grouting, fiber-wrapping methods etc.
1. First of all immediately support the structure with props till the lab tests are completed.
2. Best option would be Core cut compression test, however, if you want then you can first go for the UPV/Rebound Hammer which will help you in ascertaining the actual strength, which if you are not satisfied then you can opt for the Core cut option.
3. Rebound Hammer, CAPO/Pullout, Windsor probe and ultrasonic pulse velocity tests give indirect evidence of concrete quality hence its recommended to go for core sampling and testing as its a direct assessment.
4.The general rule adopted for fixing the core size, besides the H/D ratio, is the nominal size of stone aggregate and the dia should be not less than 3 times the maximum size of stone aggregate. For a diameter of core less than 3 times the size of the stone aggregate, an increased number of cores have to be tested.
5. Prior to corecut, one has to observe for any steel which could get cut in the process of obtaining the core sample. so its advisable to get it done from a renowned Laboratory.
6. If after all the tests the structure is showing sign of failure then you can for structural strengthening methods which is a different subject all together wherein one can go for grouting, fiber-wrapping methods etc.
Dnyan Deshmukh
Staff member
It would be better if you provide more details about
What is grade of concrete
Date of concrete
Climatic condition
Concrete Mix Design Recipe
Cube test reports
Location of concrete like slab or column and at what floor, how many floors further will be constructed on top of it.
Above are the basic details required to analyze it.
If you provide those then, i would be in position to suggest you something which may become relevant to your case.
What is grade of concrete
Date of concrete
Climatic condition
Concrete Mix Design Recipe
Cube test reports
Location of concrete like slab or column and at what floor, how many floors further will be constructed on top of it.
Above are the basic details required to analyze it.
If you provide those then, i would be in position to suggest you something which may become relevant to your case.
You will have to carry out non destructive tests,such as rebound hammer, and if the client argues that the rebound hammer will show only the surface hardness, you go ahead to do pulse velocity or coring.But note that in coring,there is no universally accepted means of interpreting the results.This could be controversial.
Kindly go for NDT tests to check on site concrete status. If NDT also fails, with structural consultants opinion, need to break concrete and redo it.
shafi12344
Apprentice
All the below NDT tests are applicable but if the strength test is failed coring is the better option to know the insitu compressive strength.
rishabh saraf
Newbie
rameshchandrapatel
Active Member
Professional here don't have time. Please try to learn basic English.
Dear Friend,
Am not proficient in Hindi request you to use the GOOGLE TRANSLATOR online
sameer_shaid
Apprentice
Sir by taking some example like grade of concrete date of concrete u plz give some solution
BruneiShiva
Junior Member
Dear Mr Ashok
It is very common issue/problem happening in most of the construction projects because of ignorance and or lack of QC works knowledge @ each site on part of Supervisors/Engineers !
As a practicing Civil Engineer, please note that you can not simply say or comment RCC Cubes fail for 7 & 28days without providing any results or details......!
The compression strength of tested concrete Cubes may be lower than the design strength as specified in your contracts/specs...
Before seeking advise for remedial solutions, you need to provide & share complete details so that professionals in this forum can suggest appropriate recommendations...!
Shiva
Brunei
7th day cube test failed
Cube tests are considered as one of the prime and basic gauge to measure the strength of concrete after the pour. Though there are several methods and tests to ensure that the mix poured meets all required parameters, the strength of the concrete is judged primarily by cube test only. Nowadays, for all major works, concrete is batched centrally at a batching plant, and delivered to site by transit mixers. In such situations, site engineers are usually left with no options other than counting the batching report received along with the concrete as reference, on mix proportions.
Today is the 7th day after the concreting… the cube tests err ailed.
The first thing we need to understands is that the compressive strength test on 7th day is only an indicator; just to give a quicker idea about the strength of concrete and also, what may be in store for us after 28th day test. According to IS 456:2016, the compressive strength on 28th day is the sole criterion for accepting or rejecting the concrete. Failure of a cube test at 7th day doesn’t mean that the cubes should fail at its 28th day tests also, though it happens in most cases.
There might be various reasons for a cube test failure apart from the inadequate strength of concrete mix. There are situations, where the cube test results show non conformity even though the concrete is having sufficient strength.
Let us investigate the possible causes for a cube test failure.
Wrong mix poured: The mix poured is not as per the approved mix design for that particular strength.
Wrong mix design: The mix design does not give the required strength.
Improper sample collection: If the sample is not collected by a skilled worker, and is not supervised by a technical person, there are possibilities that the collected sample is not truly representing the fresh concrete supplied at site. The samples should be collected in accordance with IS 1199 provisions.
Improper cube preparation: The cubes to be prepared as per the guidelines given in IS 516. Compressive strength of the cube might be affected by several errors during cube making, such as,
Improper alignment of mould – resulting in distorted cube shape.
Improper / inadequate tamping- leaving the cube porous.
Improper filling of the moulds – creating weak zones of mortar without aggregates.
Inadequate curing of the cube samples: The cubes to be cured properly as per the guidelines given in IS 516. Once prepared, the cubes to be covered with wet gunny bags for a period of 24hrs from the time when water is first added to the concrete, and then removed from moulds and kept under clean water until the time of test. At all time the temperature should be in the range of 24 to 300C.
Testing wrong samples: The identification numbering system should be unique so that exactly the intended cubes are retrieved and tested at every time.
Errors during testing: like placing of moulds over the CTM, cleaning the surface, seating the cube properly, rate of load application etc.
Errors with the testing apparatus: This could include errors with display, loading or hydraulic systems or a manual error in reading and interpreting the displayed value.
Unauthorized w/c correction of the mix: unlawful addition of water to the mix without consulting the structural consultant.
Change in quality of materials: The quality of materials used for preparing the concrete may be different from those used while preparing the trial mix.
Batching plant error: error in preparation of the mix in the batching plant, due to any mechanical / software error, which went unnoticed.
Now, what are the actions to be taken after the cube failure at 7th day?
As we have already discussed, the 7th day test is a warning bell. We have 21 more days to finally accept or reject the member.
The first and foremost action to be taken on receiving a failed cube test report is to suspend further activities on the member/members in question.
Secondly, before continuing concreting activity on any member, recheck the mix design and ensure that the mix design is accurate. Also, reconfirm that it is as suggested by the structural consultant for that particular strength, if this is the first batch mixed with that particular mix design.
The above two steps are critical to ensure that the error, if at all happened, is not extending to other members or parts of the structure.
Now, each of the above possibilities shall be investigated in detail to find a possible cause for the non conformity. When we closely observe the possibilities listed above, we can identify two major chances after a cube failure.
1. The concrete poured is good - the cube failed due to some other reason.
2. The concrete poured is not good - hence the cube failed.
Trial mixes shall be taken with the same mix design, well supervised to eliminate all workmanship errors, and tested after 7th day at actual site conditions in the same testing machines and also at an external laboratory to ensure that the mix design is conforming to the strength requirement. Also, the structural consultant and architect should be alerted to the situation.
Further actions can be taken only after the cube test on the 28th day.
Next case. The cube test on 28th day also failed
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 option, however, does not harm the structure. But the option four….well…it is better to be optimistic in such situations.
When you get reports indicating that the 28th day cube tests are 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 as 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 2016 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 24hrs 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 24hrs after the removal of LL is less than 75% of the deflection, the test is repeated after 72hrs. 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, work ability, influence on other members and services etc. A detailed study considering all these factors is done before finalizing on a methodology to move forward.
Option 5 involves redesigning the members in and around the affected area in such a way that the load in excess to the load carrying capacity of the member under question is distributed to other members safely. Changes are made to any or all of the parameters like cross section area, percentage of reinforcement, concrete mix etc to meet the requirement. This could also include providing additional beams, columns and / or slabs for reducing effective spans and reducing load on the influence area. Architectural and services layout is also revised if required on the affected floors by relocating heavy equipments or assembly area away from the affected location.
Option 6 involves strengthening the weak member / members to take the original designed load. This is more convenient than redesign since in this method modification is mostly limited to the members that are weak, and hence is easy to keep a track on the process. However, the adaptability of this method depends on other factors also. There are various options to strengthen a weak concrete member, like,
Providing steel casing:
A steel casing is provided around the structure, especially columns, to make it a composite member to improve its structural performance.
Providing steel stiffeners:
Steel stiffeners are provided on the sides of the member to increase its load carrying capacity. The stiffeners are either bolted or epoxy bonded to the concrete member.
Providing Fiber Reinforced Polymer (FRP) sheet bonding:
A FRP sheet (basically with glass fiber - GFRP or carbon fiber- CFRP) with lesser thickness is wrapped around the surface of the concrete member and bonded with epoxy adhesive, so that the structural behavior is improved. This can also be provided in strips of suitable width, limiting to the area affected in case of slabs.
Providing Near Surface Mounted FRP reinforcement:
In this, FRP laminates (rigid plates) are bonded using epoxy polymer adhesive in the saw cutting made along the cover of concrete members. Once inserted, the laminated would be flush to the surface of the concrete member.
Sprayed concrete / shot-creting:
Reinforcement is drilled around the periphery of the member and shot-creting is done on the surface of the member to increase its cross section to modify the structural performance.
Attaching pre-tensioned cables to the concrete members :
This method is not so commonly used. This involves inducing the effect of pre-stressed member into the existing member.
Concrete Jacketing:
Jacketing involves covering the structural member at any or all sides with skillfully placed reinforced concrete. The reinforcement is welded to the existing reinforcement to impart proper bonding of jacket to the existing member. Jacketing increases the cross section area of the member and thereby improving its structural performance.
Option 7 would seem to be the easiest one, but there are several impediments with this method, like disposal of debris, practical difficulty in demolition without damaging other members, safety of the workmen, practical difficulty in shuttering and casting new member in the original position, quality control etc are a few hurdles to mention with this option.
shafi12344
Apprentice
Just perform core test As per ACI 85% of the design strength is enough to accept the structure .
Marimuthu S
Newbie
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