I have just had the pleasure of being drawn into a fairly detailed technical discussion in one of my forums that discuss issues concerning Civil Engineering and Construction Management, when one of the members asked a seemingly innocuous question as to what needs to be done when he faces a failed Cube Test.
Cube Tests for the uninitiated, is basically the casting of a few standard sized cubes of concrete using the same mix as is used in the building concrete pours, and testing them for compressive strength in the lab. Thus, if a cube fails to achieve the required strength in 7 days, the concrete members cast with that mix are considered suspect.
I am firmly of the opinion that one needs to ensure a 100% Quality Control Regime in our sites, and also feel that in India, we seem to be happy to accept failure and shoddy work as something inevitable, akin to perhaps being a part of ones’ karma – an attitude I personally abhor. My recommendation would naturally be – to establish a Zero Tolerance Policy towards Controllable Errors, and asking the contractor to demolish the faulty section and re-cast. Thus, when I suggested this, I was quite shocked to find almost every one of the other respondents attempting to minimize the impact on the contractor, saying that one must only demolish as a last resort, and have suggested various (expensive) methods of testing and corrective measures lasting from 28 to 75 days to retain a flaw that should not have been permitted in the first place. I feel that our Engineers sadly, are sometimes too over confident in their views, and are incapable of a balanced view to perceive the problem from the clients’ perspective too. Perhaps an expectation that they also consider the ethics of the issue while giving decisions is wildly unjustified? Should it be so?
Speaking of Civil Engineering Construction and Project Management in India, I’m sorry to say that we have not imbibed the true Quality Philosophy that we ought to have since independence and we are quite happy to condone and even defend poor quality work, perhaps overlooking the fact that our actions may have repercussions on the safety, utility and life of the building, and also that it is ethically wrong to pass sub-standard work never seems to occur to many us.
In my experience of having worked abroad in South East and Far East Asia, and thus being able to compare the knowledge level and skill level of the Chinese Engineer and Workman and Indian Engineers and Workmen, I must say that the differences are quite stark. Most of the Indian Engineers are brilliant (and cocky!), full of theoretical knowledge and are mostly looking to find short-cuts rather than doing the job right. Of course, one cannot generalize, but Indians unfortunately seem to need a very high degree of constant supervision and review, sometimes just to ensure that they are seeing the project from the correct perspective, while Chinese Engineers (or Koreans and Japanese) are respectful, studious and measured in their approach. It is a pleasure doing business with them.
Similarly, a Chinese workman I once interacted with, really managed to impress me. It was for a Retail Store Fit-Out project in Jakarta, and considering the fact that I had to handover the project to Operations in 48 hours, I had asked him to complete a task (plastering) expeditiously, not bothering too much about the finish as it was not going to be visible above the false ceiling. He was visibly shocked at my suggestion, and rejected it outright saying that his conscience would never permit him to do a shoddy job, and that he would ‘lose face’ if another worker discovered his slip-shod work in future. How many Indian Workmen will have the same work ethic? Your guess!
(What follows is a verbatim reproduction of the discussions. It really makes for interesting reading. Please note that all the persons mentioned herein are actual persons and their comments have been reproduced without specific permission, but as this is also an academic discussion, I do hope none of them will object to such usage. Further, some portions have been edited to improve clarity and remove typos and grammatical errors).
What does one need to do if a cube test fails to achieve required design strength after casting a slab or any other R.C.C. work?
Added by: Ashutosh Sonawane on: 17 Feb, 2013 | Views: 118 | Comments: 27
Showing 27 recent comments
R. Sundara Raman: Best possible solution is to take 3 core samples and test its compressive strength, but this can be done only after 28 days of casting the element. Posted: 10 days ago
Dileep Kumar: Check the concrete strength after 28 days. If the strength is within allowable factor of safety of the design criteria we can continue with further construction. We have to redesign the structure. If the results are poor, we have to break and recast. Posted: 9 days ago
Narendra Kumar Mishra: I do agree with Mr. Dileep Kumar. The final strength is to be ascertained by core tests only. Posted: 9 days ago
V K Gupta: One can also conduct nondestructive testing to ascertain approximate strength. Load tests can also be done before acceptance of structure. However, payment terms would be governed by agreement. Posted: 9 days ago
Rajveer Singh: Conduct NDT by Ultrasound Pulse Velocity Test after that, a final decision can be taken by the designer. Posted: 9 days ago
Satendra O. Saxena: While Testing of Cubes, if trend is towards failure, some cubes minimum 3 or more should not be tested for 28 days strength. They may be tested for 56 or 75 days if further construction can be held up. Alternatively, Cores taken from structure may be tested for acceptance. Best Criteria for acceptance is to conduct Load test for Design Load, and if satisfactory accept or reject. Posted: 8 days ago
Hemanth Sharma: Technicalities apart, if the cube test fails, one must demolish and re-cast. Ethically, this is the best approach as it is a question of safety of the occupants. Further judging from the cost of delay in handing over projects these days, it may also work out cheaper than delaying the project by 28 days + testing time + decision making process. Posted: 8 days ago
Narendra Kumar Mishra: Demolishing the structure must be last option. But we should go for load test of the structure or other suitable tests to confirm the failure. Merely cube failure should not be the reason to demolish the structure. Posted: 8 days ago
Hemanth Sharma: Mr. Mishra, I would generally agree with you, however in this case, the concrete is most probably batch mixed in-situ and is not RMC, hence the veracity of the cube is also sometimes in doubt. I used to jokingly ask in my sites whether the cubes have been cast with a representative sample or a 'special Cube Mix', as the mason has the tendency to reserve his best concrete for casting his cubes. Thus, if the cube itself has failed, the chances are that the concrete cast was of even lower strength. Demolition of one pour of batch mix would be just a few Cmtr of concrete and its removal in its green state is much easier than when it has reached virtual strength. Projects are run with very stiff penalties these days and to wait for 30 odd days for testing, during which one cannot pour on that section of the structure, and then be asked to demolish if the strength test fails is too large a risk to take. Thus, demolition of one days' worth of pour is an option of minimal risks. Posted: 8 days ago
Narendra Kumar Mishra: Hemanth Ji, you are correct but my point is - we test the cube after 7days and within this period, concrete attains 60-70 % of its strength and further lifts are been cast. So it becomes difficult to dismantle the particular pour without affecting preceding / succeeding lifts. In all, it depends upon actual situation at site and decision regarding dismantle / retain should be taken accordingly. Posted: 8 days ago
Satendra O. Saxena: Responding to comment of Mr. Hemanth. Failure of concrete is evident only after testing of Cubes normally after 7 days, at this stage dismantling of structure may not be feasible unless the strength is very low. Normally it may be preferable to conduct Load test after 28 days to verify whether structure / component is stable. Posted: 7 days ago
Uday Joshi: It is also possible to wrap more reinforcement and use epoxy concrete to cover it and strengthen the member, after proper review of design. Dismantling is wasteful in time, material and money. Why waste if safety is taken care of?
Pethanna: Additional strutting / strengthening measures with structural beams can be handy if the site condition permits so. If the failure is due to some poor concreting / compaction, cement / epoxy grouting will be good enough. Posted: 5 days ago
Arvind Savant: Checking of the concrete strength at every required number of days as suggested by IS code is important, as we know that normally concrete attains only 60-70% of its strength in 7 days result. We also know that later the duration after its cast the slower is the speed of gaining its strength by concrete. If the strength is within allowable factor of safety of the design criteria (± permissible allowance as per IS code) we can continue with further construction. But, if the results are failing even after 30 days, we shall switch over to conduct core tests of concrete strength / load test etc. We can also conduct NDT (Non Destructive Test) by Ultrasound Pulse Velocity Test, even after the final tests have failed. It is also possible to wrap more reinforcement and use epoxy concrete (as mentioned above) to cover up its lost strength and to improve upon its ability on its behavior as per the concrete member (for ex. slab - a horizontal member has completely different load behavior when compared with column - a vertical member). Then a combined decision needs to be taken on mutual & proper peer review over design requirements and its re-execution feasibilities. Dismantling shall be kept as the last option which brings in a lot of impact over time & cost. Posted: 3 days ago
Rohit Mallaiya: Conduct NDT test eg. Rebound Hammer Test, also the same concrete mix design trial can be redone and conduct ACT Test, check result parameters, workmanship failure or design failure. Posted: 1 day ago
Naresh Chandra Arya: I fully agree with Mr. Hemanth Sharma. Posted: 1 day ago
Buchhi Sundararami Reddy Gangadhara: I believe nowadays the minimum accepted strength of RCC is M25, but to my knowledge, 20 years earlier M15 was accepted for all structures, I believe that structures are stable for average of 50 years (this is my knowledge and supervision), so correlating strengths with that day to present strengths, we need to ensure the pass of the structures of that load taking capacity, still if you are not convinced and if client is having too much money, then you can go for other tests as stated by various readers here. (or) if the results are well below M15, Then it should be dismantled there is no secondary opinion. Posted: 10 hours ago
Hemanth Sharma: I'm really happy to see the animated discussion taking place in response to the question. I would like to post my final views for the consideration of the forum. Everyone seems to be viewing this problem only from their perspective and answering while wearing their Structural Consultants' cap / Contractors' cap. I think we should view the problem primarily from the perspective of the end-user or client organization, and our considerations then would be quite different from the designers’ perspective.
1. When a contractor is carrying out batch mixing and not using RMC, the concrete manufacture is normally done by volumetric mixing. Thus, if he can go wrong in mixing in a ratio of 1:1.5:3 or some other specified by the consultant, it is a poor comment on either his capability or integrity. Thus, first and foremost, we need to have a Zero Tolerance Policy towards such a shortcoming in a contractor and he needs to reverse the damage caused to the building, by demolition of the pour relating to the failed 7-day cube test. Please note that I mean no disrespect to anyone, but I feel that we in India do not place a sufficient focus on quality and the 'First Time Right’ paradigm, and that needs to be urgently addressed.
I see comments justifying the failed test, and advocating post-facto corrective action. This is okay only in theory, as the building is not just a structure that has to satisfy basic design considerations, but is a living space, and hence has many more roles to play than that one, which simply satisfies the basic test of strength. Can one assure me that a failed pour that still satisfies the factor of safety will continue to behave in the same manner as a pure concrete structure would for the life of the building? For instance, will the slab be as impervious to humidity as sound concrete would and prevent the corrosion of the reinforcement for the expected life of the building? Or, will we have an assurance that we can safely suspend by anchor-fastening all the services loads like AHU’s without any failure, which can have catastrophic, life threatening consequences? Or, can we all assure the occupants (or even the designers) that during a future alteration (introduction of a staircase / escalator) the structure will remain stable and continue to meet design parameters? Surely not?
2. I would like repeat here, that as I mentioned before, as Engineers, we all have an ethical responsibility that far outweighs our technical ones - to build the best structure that we are capable of building using every ounce of our current knowledge to benefit the client. Giving him a building while knowing it has sub-standard elements is not what is expected of us as professionals, as the client does not pay a reduced rate for the volume of failed concrete poured, does he? Can we repair a clients’ professional status when a portion of the building fails, or repay the loss of life or property? If we do not want to take chances that endanger the client in any manner, we have no option but to demolish and rebuild. It is also unfair to expect the client to bear the costs of all the high tech tests that are being recommended, is it not?
3. I do however agree that corrective action can be taken if there are compaction problems or other construction errors with the guidance of the consultant, but quality failure of concrete established on day 7 should be sufficient reason to commence demolition. If the faulty pour has occurred in the slab, then the subsequent work will be minimal and may typically consist of a few column starters and lifts by day 7, and will not mean large-scale demolition. I do not therefore subscribe to the view that one needs to wait for 21 or 28 days before taking a view, as by then, the progress would be too far advanced to consider demolition.
4. A comment also suggested additional reinforcement and epoxy treatment, which may be fine in the realm of design, but such corrective action will alter the surface of the slab, add additional thickness, render it unacceptable to have as part of an exposed ceiling plan, cause problems in laying of under-deck conduits / raceways due to level difference and cause the surface to be unfit for laying of certain types of flooring above, and certain type of paint finishes below. Do we as Engineers have the right to place such constraints on the client or his tenants? Do they not deserve a 100% compliant building considering the money they have spent?
5. Finally, I would like to make a point that all of us seem to be so sure of our knowledge or specialties that we perhaps tend to forget that Civil Engineering is not an exact science by any stretch of the imagination. There is still so much we do not know about the behavior of materials, especially when they age. Modern Concrete itself has been studied for just over a 100 years, and its properties with advancing age are still being determined and defined as we go along. Thus, I would like to advocate a certain amount of circumspection and a spirit of inquiry, rather than over confidence in our current knowledge. Let us know that our knowledge is yet very far from complete and hence recommending the retention of a known flaw is both unethical and unprofessional on our part. Perhaps, only God knows how many unknown flaws one has to live with in a building, that is built with our current incomplete knowledge of our materials, attention to quality and quantum of supervision, so why add a flaw willfully?! Posted: 1 hour ago