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Concrete Deterioration and Performance of Structures

Everyone feels happy and proud if the appearance of the concrete structure is good at the completion. However good looking concrete may not be sound in its strength and durability.

Long term load and exposure effects, reveal their performance characteristics in the form of their condition. In due course, environmental exposure offers opportunity for fair observations and for evolving guidelines regarding the problematic areas, which have not done well and where distress levels are high.

Somehow present knowledge is adequate, both for the designers as well as for the contractors, to foresee the impact of several construction provisions and in-built lapses, manifesting in the years to come. Also enough awareness is not observed about the adequacy of the provisions and construction stage.

As a result, the problems developed in the structures and their technical assessment and subsequent awareness for precautionary measures, becomes the only available data to improve our performance and equip us with required information data.

Assessment of Concrete Structures Damage & Preparation of Report

Interviews and Statements for Concrete Structure Damage

It is a common practice to invite all personnel connected with the project, to make a written statement to the Investigating Committee. Connected construction workers also would be important participants to make statements. Also, a suitable Press insertion is necessary about the Committee’s formation and to invite the Public to come forward with written statements or to make a statement before the committee.

At times, the response to the Press insertion is adequate, but mostly it is not so. Those connected with the project and all known witnesses are, therefore, officially invited through separate letters to appear before the committee. Often they respond to the letters, very seldom come prepared with a written statement.

The Committee then opts to ask them questions related to their part in the project. The statements made are then recorded in their presence. Each one is given a fair chance and also explained at length about the scope of the committee’s investigations, and why their co-operation is necessary.

All the same, it becomes necessary at times, to explain them their obligations (in any suitable capacity) to share their views and known matters with the committee, which will be for the long-term good of the profession, and help to avoid such problems repeating in future.

Each one who appears before the committee has very few things to share, and a few things to hide. He is not very sure, what he should state and what he should not. Someone would have lot of irrelevant information to state, because of several personal reasons. The best way is to make them talk, generate respectful confidence in them and after hearing, only the relevant concerned extracts are recorded as their statements in their presence.

The entire typed copy of their statement is read before them, explained, if required, and their confirmatory signature obtained. Each one is given a copy of the statement he has made for his records.

Committee may ask several questions to extract information. It is a developed skill with the investigating team as how to make a person talk, and find out how much is the truth. Several contradictory observations and statements do come out during these interviews. It is a matter of judgment, based on experience, to find the truth based on likely behavior of the structure and events in the industry.

But at times, non-technical observers, residents in the area, passersby, do come with startling and revealing observations which are most logical. It is observed that sometimes even technical personnel connected with the project, come out with divergent observations and sequence of events. This is an interesting contradiction to deal with.

It is therefore, necessary to focus the interview questions, particularly concentrating on the events, which took place few days before the collapse. Each one has to be asked as to ‘What could be the cause of collapse in his opinion?

Each most of the time, would either refrain from commenting or, if he comes forward to do so, would reveal has own reasoning for the problem. These constitute interesting asset of statements and some information may be forthcoming, either confirming the hypotheses about collapse or otherwise.

It is to be noted that, by the time interviews and statements are recorded, the committee is at an advanced stage of investigation. Generally at this stage, they have a few emerging conclusions towards the sources of the collapse. In this context, these statements become important.

At best, the information obtained from statements should be used to guide the committee; it is not to be used as a substitute for facts.

Evaluation & Reporting of Concrete Structure Damage

This is the most crucial and important part of the entire process of investigation. Evaluation and reporting, require technical skill as well as ability to communicate the results, in the manner to serve the intended purpose and to understood by all concerned.

The investigator has to analyze the entire lot of information, arrive at a hypothesis and evaluate his observations and findings in order to establish most logically reasonable conclusions.

Earlier investigation reports of a similar type of collapse, if available, would be an asset to start with. Use of simple language is a distinct merit of the report. It is to be useful not only to specialists, but also to a large number of public, to understand easily.

Detailed technical and backup information arising out of investigations is generally placed in the index or later part, as an annexure to the body of the main report. This information is generally meant for technical people.

The report must clearly bring out facts as observed, and conclusions as derived from these facts. Reliability and limitations, if any, should be discussed lucidly without bias.

Before the report is released, opinion from legal expert may be considered advisable in order to avoid controversies at a later date. Also, some of the observations can be toned down. It is a common pitfall that some pointed statements about the critical lapses would stress the point home, but would lead to displeasure of important block of professional community or group of people.

Some observations to the same effect can be made in a different language, with different tone and presentation. This would serve the intended purpose of the investigator without hurting anyone. However, this is more difficult to implement, than said in words.

It is an important observation that the role of investigator arises out of his superior position, expertise and knowledge. This role should not make him lose his balance and take the liberty to cover his observations, in the form of personal opined criticism in the report.

At this stage, perusal of some Press statements becomes necessary, since further interviews with Press reference, are not ruled out. Judicious control on press statements is necessary. Use of simple language, conveying straightforward meaning and clarity of facts in the report, is required when dealing with reporters. Latter is more experienced than the investigator, as far as placing the news and facts is concerned.

When the reporter talks to investigator, the role is changed. By his method of putting questions, the reporter may make the investigator say something out of context compared to the statement made in the investigation report. If such a thing happens, it leads to embarrassment to the entire team and nullify the efforts.

The results of the investigation are frequently compiled in a compact engineering document. The report covers various aspects, elements and data collected, and brings out the concept of reasoning to conclusions.

Analyzing the entire data generated, evidence collected and documents processed, in a unique specialization. This is an involved process of evaluation to assess the truth. Several pages of documents are to be weeded out, before a few relevant emerge as the gospel truth, on which the failure hypothesis is developed. All compatible events leading to failure investigations, are serialized.

The observed phenomenon in failure and the physical data observed, need correct matching. Leading guidelines are plenty, at times missing, at times distorted, irreverent, seemingly less important therefore overlooked, later on realized but lost — all need tireless and strict disciplined scrutiny. Then only wolfhounds and tested arguments and hypotheses would emerge, and one may stand the test of severe scrutiny by other experts. With plenty of uncertainty, one has to be certain of the most difficult reasoning.

The investigator must be aware that his report is likely to be subjected to rigorous review process. Therefore, all the issues of disagreement with Committee members need to be elaborated with pros and cons, and balanced reasoning reported.

The main body of the report, has to clearly bring out the mode and sequence of failure, and also clearly establish the location of failure, where the collapse mechanism first triggered. Then with effective reasoning, each successive step in the sequence of failure and how the stresses controlling the structure in sequential behavior exceeded their ultimate load carrying capacities should be brought out.

In many investigations, it becomes difficult to clearly establish this observation. In such cases, report must bring out the emergent limitations on the findings. However, ‘most probable’ reason and location of trigger and subsequent technical reasoning are generally established with experience, resources and facilities available to the investigator.

Often failure is initiated locally, but collapse occurs under resulting instability. Redundant structures are often found to have remarkable ability to overcome initial gross overstressing, by shedding load through other not well-defined paths.

When total failure occurs, the investigator is expected to indicate in his report that the resistance of every potential load transfer path or failure mode, has been exceeded by the initial effect.

Based on investigation results, broad causes of Structural Concrete Damages are

  • Inadequate design.
  • Foundation failure.
  • Subsidence
  • Inadequate material and deficient constructions, failures of codal provisions and accepted engineering practices, to appreciate with reasonable accuracy certain load combinations in initial or changed conditions during distress, or to establish limits of structural resistance of elements or structure together.
  • Imposition of extreme or abnormal loads that the structure was not intended to resist.
  • Combination of the above.

When dealing with strength issues at this stage. Other reserve strength of the structure has a substantial distress dispersal capacity, beyond what is defined as ultimate strength. It becomes necessary to establish even that this reserve also was exhausted.

At times, it is interesting to observe that one can easily prove why the structure failed, but would find it difficult to prove why it did not fail much earlier. There lies the adequate justification of unexplored strength in a three-dimensional structure.

It is a common experience that many times, members of the investigation committee do not agree on failure hypothesis. They have emphatic disagreement on the probable mode and reason of collapse.

But it must be resolved with detailed debate amongst members across the table. Before any agreement is ruled out as contributing factor, lot of background based on experience and evidence collected, needs scrutiny and application.

Report of Assessment of Concrete Structure Damage

The presentation of report covers documenting all the relevant information. Starting from front cover of the bound report, the impact of the report begins. Hence it should be presented in a pleasant manner, starting with index with usual sequence.

Presentation of Report

Normal report is presented in A4 size or foolscap size. It is either bound, or submitted with folder, or with coil bound compilation, neatly printed or specially typed. The front cover should be printed separately and inside matter may be type written and reproduced, photographic copy or Xerox, so as to have uniformly and clarity in the entire presentation. Extra copies of photographs are needed for inclusion in the report. Negatives may be kept ready. Description for information covered in photograph, needs proper thought to indicate details covered therein.

The report should normally contain:

  • Front cover — it would have name and authority initiating the investigation, at top. Title of the report is stated at the center, and the names of the committee members with required qualifications etc., are inserted at bottom.
  • Preamble — a brief history of events, including history of the building and collapse. If it is already existing and used, necessary usage and life at collapse etc. should be stated
  • Index.
  • Letter of appointment of committee and the terms of reference.
  • Acceptance letter for the above. This would normally include the names of the members selected on the investigation committee. This also states the names of the Chairman or Team Leader, and the name of the Technical Secretary of the Committee.
  • Forwarding letter by the Chairman of the investigation committee. This letter anomaly contains following information.
  • Forwarding letter by the Chairman of the investigation committee. This letter normally contains following information.
  • Brief note about letter of invitation and acceptance.
  • Brief note on procedure, investigation committee followed.
  • Important findings of the committee, based on investigations.
  • Salient observations of important technical data and its effects on the collapse.
  • Recommendations of the committee.
  • Brief summary of various observations made during the investigations, which have bearing on failure hypothesis, is stated chronologically. All the relevant references enclosed later on, are also to be stated.
  • The review of collected data. This covers manner and method in which data was collected by the Committee. Various technical investigations carried out, along with their need, and interpretation of test results. The contribution of this information to failure hypothesis, is clearly brought out if important.
  • Collapse Analysis — Here various factor affecting the serviceability, stability of structure, distress developments are identified, with specific reference to observed and collected data during investigation.
  • Recommendations — They include specific suggestions pertaining to specific investigation. If the structure is to be reconstructed completely or in parts, that is accordingly covered. Special precautions of stability of the entire structure, either remaining part or adjoining portion, need to be clearly brought out. Lapses including design shortcomings etc. of severe nature, which caused damage to the structure, observed during investigations, ARE clearly brought out.
  • Laboratory Tests — Relevant reproductions of various tests are compiled and presented in this part.
  • Photographs — Normally the photographs of the old existing structures before the damage, are presented in the beginning. It may not be easy to get such records. Lot of follow up and hunting is required to get them. Then present photographs of what happened, how did it fail etc. They include coverage on what was found during investigations and various test conducted etc.
  • Statements — Reproductions of all the statements – these may form second of the report. Also laboratory tests etc. , may be included in the second part
  • References — if any.
  • Acknowledgements.

This is considered a normal format of the report for presentations, with usual terms of reference. Based on the scope of work covered in terms of reference, suitable additions in the report presentation, can be made.

Overview of Failure Assessment of Concrete Structures

Failure investigations is one of the most demanding aspects of engineering undertakings. The Principal Investigator or the Team Leader, must have wide range of skills. Clear well defined objective and limitations of Terms of reference, if understood beforehand, will bring meaningful results. The success of investigations often depend on getting information before it is lost, forgotten or destroyed. The well-defined need of objectivity and the documentation, control all the phases of work.

Thus, it would be observed that the investigations of structural failures, need training and qualities that are distinct from other engineers involved in structural consultancy or contract business. It is not likely that such training could be provided in the universities, and the adequate literature giving insights in investigation techniques could be forthcoming.

It is also to be noted that engineering failures do not occur with a frequency, and they have such a diverge nature of involvement, that it is not possible to set up ‘ready-to-go’ or push-button system for the team to investigate. Most of the time, therefore, existing engineering organizations and adhoc panels, are set up with limited terms of reference.

This has led to severe restrictions on the need to develop well co-ordinates teams, panels or firms, to undertake investigations and share the developed skills. The real experience of the investigator does not restrict him to think in terms of what is required to specify as peer code, or how much is a factor of safety against failure, but encourages him to probe into the cause of a failure. This is a different kind of Engineering exposure. Very few designers can think in terms of failure, while others just cannot.

Regardless of all the limitations and restraints, failure investigation is an exciting challenge to those who want to acquire this difficult but unique experience.

Some of the observations about investigations may be of interest:

  • They are generally entrusted to academicians from various reasons. They, however, already have their heads full with their commitment in the Universities, and their involvement in field is restricted — this is only a general observation. They may not have good awareness and exposure to the complete gamut of prevailing practices in the industry and entire methodology adopted routinely in design and construction.
  • It may be difficult for them to find out several missing links, when nothing appears to be in order, without solid experience background. It needs hard work, diligent missionary efforts, and has to emerge as sound to outcome. However, it is to be appreciated that they make themselves available to undertake the investigations, even knowing full well that it is a thankless and demanding task. Also our future hope lies in training more students, right at university level. To undertake failure investigations. Then only can we achieve desired dissipation of rare knowledge arising out of failure investigations.
  • Collapse or failure investigations; like any other investigation lead to generalization of some kind of useful information, data and guidelines to reconsider prevailing practices in design, construction and usage parameters. It is a common observation that changes do take place in established practices, as a result of investigation of failures of structures. In fact the entire emphasis is on developing the art of investigations and knowing the reasons for failures, and more so in predicting them in advance, so that we can reverse the usual trend of occurrence of accidents. Every investigations normally leads to formulating new safety rules, thus ensuring safe and serviceable structure. However, the transit time creates confusion and mixed reaction, thereby delaying the appropriate implementation.
  • By identifying several prevailing failure parameters, one can not only avoid some particular mistakes in design and execution of structures that could lead to an accident, but can also develop insight into the gray areas in the overall process of design and construction and thereby further reduce susceptibility to accident.
  • The art of identifying the areas having proneness to failure, needs lot of experience and insight. Its beauty lies in being able to reasonably assess the type and mode of likely distress and to take corrective action at the design stage itself.
  • This is an age of information. Lot of information is available, but it is scattered and spread all over. The skill lies in its proper collection, assessment and utilization.
  • It is hoped that a reasonable insight would have been created in the minds of the readers about what goes on in investigation. Additional information is not covered, considering that it would be unwieldy. Each stage and step in investigation, involve collection of as lot of practical data and its appreciation and manifestations.

Read More: Concrete Damages, Defects and Prevention