Why is structural diagnostic important?
As everything else developed by humans, reinforced concrete buildings are prone to mistakes that affect their safety, reliability, and ability to serve the intended purpose optimally. Most of such mistakes do not happen by design; but they do happen all the same. To counter the ensuing shortcomings and prevent catastrophes, structural diagnostics are essential. The common purpose of a diagnostic investigation is however to analyze and draw conclusions “about the cause of an observed behavior” (Krysander 16).
To offer a clear understanding of the nature of reinforced concrete buildings, it is worth noting that concrete is made from the combination of cement, gravel, sand, and water. Once the four identified components are mixed, a chemical reaction that binds them into a solid mass occurs. The use of steel to reinforce concrete building was discovered in the 1850s when one William Wilkinson proposed a patent for using steel rods for the reinforcement of concrete structures (Brown 129). In an ideal situation, Wilkinson hypothesized that concrete’s compressive behavior would protect the steel from chemical and physical attacks hence preserving its tensile properties and thus preserving its potential as a reinforcing component. Notably however, not all buildings conform to the ideal situation hypothesized in (Brown 129). Specifically, it is worth noting that existing reinforced concrete buildings may be compromised right from the construction phase. This is in addition to the fact that the integrity the reinforced concrete buildings depreciate overtime due to exposure to different environments.
To start with, it is worth noting that some structures have complex forensic engineering, rehabilitation and restoration challenges. To address such challenges, architects and other professionals in the construction industry rely on structural diagnostics as the main information-gathering tool. Based on information attained, the architects then customize solutions that address the individual challenges facing individual structures. As (Pepenar 1) notes for example, diagnostic investigation is a necessary tool in determining resistance, stability, and durability of concrete structures. Usually, diagnostic investigations enables the detection of damages, and this enables investigators to recommend intervention measures, which in turn serve to prevent human losses, material losses, undesirable ecological effects and technological accidents.
Establishing the origin or cause of structural shortcomings or failures is also another reason why structural diagnostics are necessary. Knowing that microscopic defects are likely to occur in constructions, civil engineers usually account for the same by using materials that yield the appropriate strength in order to ensure that the construction performs to its best possible design specifications. However, other damages that are beyond the civil engineers’ ability to account for during the design and construction phase usually occur. Such damages may occur from unanticipated environmental events such as seismic activity, material degradation due to exposure to environmental conditions, or overloading conditions that may weaken the materials used in the construction among other reasons.
Usually, diagnostic investigations consider several concepts in a targeted building. To start with, (Pepenar 2) notes that a building is “regarded and analyzed as a whole, taking into account its initial design as well as the evolution of its condition up to the moment of intervention.” As such, diagnostic investigations into a building cannot be partial since even localized defects may alter load distribution mechanisms that were initially envisaged by the building designers. Secondly, (Pepenar 2) notes that although the design activities and investigations may appear independent, they are mutually interconnected since making diagnosis requires the structural engineers to inspect initial designs against the buildings current form. Finally, (Pepenar 2) notes that corrosion is inevitable in reinforced concrete buildings. However, to gauge the extent of corrosion that a building is exposed to, investigators must establish the corrosive agents in every building (Langford and Broomfield 33). In principle, there are different types of corrosives; as such, the effects that the different corrosives have on reinforced concrete buildings vary. This raises the need for an investigator to identify factors contributing to corrosion in the targeted building correctly, if indeed the necessary preventative or corrective measures are to be identified.
After carrying out a diagnostic investigation, the investigator should ideally classify the inspected building in one of the following categories:
- No action needed since the building has met the “safety and serviceability requirements” having shown no distress signs (Pepenar 2).
- Repair and strengthening needed to meet the criteria initially set by the owner or user since the building is distressed or deficient
- Badly damaged building, which cannot be repaired or strengthened. In such a case, the building should be demolished
While it is acceptable that human error accounts for a significant number of structural inadequacies, sudden occurrences that are beyond the human being’s capacity to control also weaken reinforced concrete structures. Combined, human error and unforeseen circumstances lead to a blend of factors that include deficient designs, poor reinforcement, poor construction, corrosion of reinforcement, and structural systems that cannot resist the forces of natural hazards such as earthquakes or cyclones. With continued use, the foundation of reinforced concrete buildings may end up settling in a manner that may cause structural weaknesses, while extreme loading may cause distress on the structure.
(Krysander 12) sums up the importance of diagnostic investigation by observing that it not only increases safety, but also the reliability of reinforced concrete structures. Further, diagnostic investigation is necessary for protecting the environment and improving maintenance practices targeting existing reinforced concrete buildings. This is especially important since (Arya and Agarwal 2) have listed lack of maintenance as one of the main reasons why buildings deteriorate with time, some to the extent that they cannot be rehabilitated. With proper diagnostic investigations however, necessary interventions can be designed to counter defects that occur on the construction materials.
After the diagnostic investigation is complete, the investigator needs to record any observed or detected damages. He or she should also find what caused the damage or distress in addition to assessing the extent of the damage. Further, the investigator should estimate the overall strength of the building’s structural components (Arya & Agarwal 3). Out of the records created during the investigation, the investigator is then able to design a rehabilitation plan that strengthens and prevents further deterioration on existing reinforced concrete buildings. Ideally, the investigator should group buildings into one of the three categories as discussed elsewhere in this paper (i.e. no sign of distress , hence no action needed; building distressed hence repairs or reinforcement needed; and building badly distressed, hence the need to demolish it).
The table below contains incidents that could have been avoided if the necessary diagnostic investigation of reinforced concrete buildings had been carried out. Notably, most collapses occurred on illegally constructed additions that were made on the main buildings. Such include rooftop structures and balconies. As evident from the six story building that collapsed on 29th January 2010 in Mau Tau Wai Road, Hung Hom, such incident not only lead to injuries, but loss of human life as well.
The Scope
A diagnostic investigation on existing reinforced concrete building usually takes three core steps. They are: i) analyzing the technical documents of the building; ii) determining the building’s current state; and iii) investigating the building’s current state (Pepenar 2). As (Lo 129) notes however, the scope of the investigation can cover “the structural integrity, the external finishes, and the fire safety.”
Analyzing the technical documents of the building
From existing technical documents, the investigator is able to gather the building’s biography, which includes its description, design, and execution plan. Additionally, the technical documents contains the service life of the building as envisaged by the initial designer, and may also contain information about interventions carried out in the course of the building’s existence.
The technical documents contain data from the building’s owner (at the time of construction), and the design engineer. In the documents, the investigator should find the specifications of the materials used during construction, quality certificates issued by authorities, and the conclusion documents made by the owner, builder, and the design engineer.
The specification of the materials used is especially important to a diagnostic investigation since an investigator is able to relate the material’s susceptibility to defects such as corrosion. As (Grantham and Gray 4) note for example, concrete is made of several components, which include cement, water, and aggregate. Cement can be classified into ordinary Portland cement, sulfate resisting cement, and high alumina cement. The ordinary Portland cement has significant alkali content and hence the investigator ought to consider possible alkali reactions. Additionally, the investigator needs to consider whether those responsible for constructing the building cured the concrete adequately in order to avoid shrinkage cracking, or thermal cracking when exposed to varying temperatures. The sulfate resisting cement on the other hand contains chloride salts, and where used, the investigator ought to consider an increased risk of corrosion. On its part, the high Alumina cement is more prone to chemical attacks, and as (Grantham and Gray 4 ) note, construction experts are have in the past raised concern over its loss of strength and carbonation. As such, an investigator checking a building where the high alumina salt has been used ought to check whether the concrete is strong enough, and whether reinforcement corrosion has occurred.
In addition to the cement used for construction, (Reed, Schoonees and Salmond 30), suggest that water should also be a focus of the investigator’s scrutiny. Ideally, the original documents detailing the materials used in the construction should specify if the water used has any dissolved salts, lead or other minerals that may affect how the cement settles. By a visual observation of the concrete, an investigator may be able to determine whether too little or too much water was used in the construction. Usually, the telltale signs of inefficient use of water are evident where there is porous concrete or concrete that does not compact easily.
The quality of the aggregates used in reinforced concrete buildings should also capture the attention of the investigator. According to (Grantham and Gray 4-5), impurities such as potassium salts, chlorides, sodium, or sulfates affects the quality of the aggregates. Additionally, the investigator ought to consider the physical properties of the aggregates in order to establish their crushing value, and ascertain whether the aggregates are porous, expandable, or shrinkable.
Some of the other considerations that an investigator may want to consider when analyzing the technical documents of a building include the type and quality steel and the admixtures used in the construction or any subsequent repairs. Steel can be galvanized, stainless, ferritic, weathering, high yield, or mild steel. Notably however, “Steel is rarely responsible for the problems with reinforced concrete. Corrosion of the steel, where it is put, or whether it is put there at all…” should be the investigators point of focus (Grantham and Gray 5).
Admixtures rarely bring out problems in concrete reinforced buildings, but the investigator need to consider possibility of insufficient or overuse of the same when carrying out a diagnostic investigation. (Grantham and Gray 5) for example note that the incorrect use of polymeric bonding agents may end up presenting as bonding problems in the concrete reinforced building in future. Additionally, buildings where chlorides were used could be exposed to increased reinforcement corrosion.
Determining the building’s current state
According to (Pepenar 2), determining the current state of a building requires one to consider its exposure to aggressive environment from both within and externally. Additionally, a diagnostic investigator needs to consider the technological processes taking place in the building. Further, the investigator needs to consider the functions of those technical installations and their impact on the building. (Grantham and Gray 15) state that the initial survey is essential since the investigator is able to study the specific areas such as the ground or roof levels, and identify any problem areas.
Investigating the building’s current state
Investigating the building’s current state takes several approaches, which include “ the visual examination, the use of non-destructive methods, and the use of methods that require taking samples, but which do not endanger the durability, strength and stability” of the building (Pepenar 2).
Some of the physical features that the investigator may want to look out for include damp or wet surfaces, surface deposits, varying texture or color, past remedial spots, grout loss or honeycombing, hollow surfaces, rust staining or corroded steel, spalling, and cracks (Grantham and Gray 15).
To back up the visual evidence, investigators may take pictures of the structural damages or defects. Ideally, the pictures should be colored in order to provide the clearest details possible. The visual investigation also enables the investigator to verify whether the construction accurately followed the original drawings. If not, the investigator is able to identify any alterations made in the construction, without the proper indications being made in the original documents (Arya and Agarwal 7).
Non-destructive diagnostic investigation methods are on the other hand, used to establish the characteristics of certain construction elements (Pepenar 2). For example, an investigator may use such a method to verify the presence of cracks, segregations, or to determine if any metallic pieces are in the concrete, hence suggesting that corrosion is taking place.
Taking samples from a building is done on the condition that the sample taking does not “endanger the durability, resistance and stability” of the building (Pepenar 3). The samples are necessary in determining the extent of changes on the concrete and the type and extent of damage occasioned by the changes. Usually, samples are tested to determine their physico-mechanical, electrochemical and physico-chemical. Such tests determine the chemical characteristics in a sample, in addition to determining the humidity, alkalinity, and porosity of the concrete samples. Where the samples include steel reinforcement, the tests establish their physico-chemical and mechanical characteristics. As (Pepenar 3) notes, the state of corrosion in steel reinforcement depended on the nature of corrosion, the physical properties of the reinforcement, the reinforcement’s physic-mechanical characteristics, and the structure of the steel reinforcement.
The result of a diagnostic investigation is a verdict, which forms the framework for recommending or taking up intervention measures. In a case where temporary measures are needed for purposes of preventing potential or imminent structural failures, a preliminary examination is advisable. Such preliminary investigation is especially necessary in the aftermath of natural disasters since they help experts in structural engineering determine whether buildings are safe for human occupation (Arya and Agarwal 8). Here, the investigator conducts a visual analysis of the building and gauges it against the knowledge that he or she has pertaining to structural construction. The preliminary conclusions from such an exercise are however meant to provide short-term answers and the investigator should assume a more detailed investigation. Among the notable observations that an investigator may make include diagonal tension cracks, which may occur after soil liquefaction, landslides and even surface fault rupture. When such occurrences happen, differential settlement may occur in the reinforced concrete buildings hence explaining the diagonal tension cracks.
The detailed investigation includes measurements conducted on site and in laboratory settings. Such an investigation aims at pointing out the main damages or defects in the building and explaining their causes. Usually, defects can occur from corrosion, technological installations, or may even occur from other structural features such as the sewage system, the ventilation, or even the waterproofing installations.
Defects that an investigator may detect
Depending on the diagnostic tools that a structural investigator uses, he or she may detect outright defects such as cracks that appear on the building’s exterior, to more concealed defects such as interior voids and cavities. Even where cracks are apparent, the structural investigator may want to conduct a deeper investigation in order to find out their length and depth beyond what is apparent on the surface.
Concrete hardness, quality, and strength
Diagnostic investigation also included monitoring the changes that occur to concrete. According to (Neville 451-452), the structure of concrete changes overtimes, especially when the building comes into contact with water, sulfates and frost among other things. In the event of a fire engulfing a reinforced concrete building, estimating the concrete’s strength is especially essential in order to determine the viability of the building’s continual use.
Determining the surface hardness of an existing reinforced concrete building is identified as one of the ways that structural investigators can determine the hardness, quality, and strength of concrete. Among the widely accepted diagnostic investigation tool is the Schmidt rebound Hammer, which was developed in the 1940s by Ernst Schmidt. According to (Grantham and Gray 40), the rebound hammer test “is based on the principle that the rebound of an elastic mass depends on the hardness of the surface upon which it impinges…” To enhance accuracy, it is recommended that the investigator should take 12 readings in his or her area of interest, and then get an average of all the readings.
The structural investigator may also want to test the penetration resistance, maturity, internal fractures, and temperature-matched curing properties of the concrete. In addition to the Schmidt rebound hammer test that determines the surface hardness of the concrete, other methods used to determine the quality, durability or deterioration of the concrete include mechanical, electronic, radioactive, and microscopic tests. Using such, the investigator is able to determine factors such as the air content in the concrete, its expansion properties, its sulfate content, absorption levels, relative humidity, abrasion resistance, and permeability among others (Arya and Agarwal 9).
Corrosion rate
It is a certainty among construction professionals that steel corrodes when exposed to the right conditions. As such, an investigator needs to establish if indeed the subject building contains corroded steel, and if so, he or she must be able to estimate the extent of the corrosion. Some of the factors that affect or contribute towards corrosion include the cover thickness, the quality (diffusivity and permeability) of concrete, environmental conditions, cracks on the concrete, and the pH value and chloride content in the concrete cover.
(Grantham and Gray 56) identify the GECOR 6 device as the most accurate among other devices in measuring corrosion. To measure the concrete’s resistivity electrical tests are needed, while measuring the cover depth requires electromagnetic tests. Carbonation depth on the other hand requires either chemical or microscopic tests, while discovering the chloride concentration in the concrete requires the structural investigator to perform chemical or electrical tests (Arya and Agarwal 9).
The half-cell potential test is on the other hand recommended as suitable for detecting the most corroded zones, since the equipment used places value on each steel part that is examined. The lowest values represent the most corroded parts. To enhance the chances of accuracy, (Grantham and Gray 51) recommends that the half-cell potential test should not be used in isolation; rather, it should be used together with identified levels of chloride content and the amounts of carbonation detected in steel.
When a chemical analysis on the concrete is performed, (Arya and Agarwal 10) observe that the pH value and chloride content is checked. Ideally, concrete whose chloride content is low and the pH value greater than 11.5 is deemed to harbor no corroded steel. In cases where the chloride content is high and the pH value high, the steel therein is prone to corrosion and further tests would be required to establish the extent of such corrosion. In cases where the pH value is low and the chloride contents high, then the steel therein is more likely to be corroded. As such, further tests would be needed to verify the extent of the corrosion.
To determine if there are indeed any changes in the steel, the investigator should take samples from the target reinforced concrete building. The investigator should however uphold caution in order to obtain such specimens from locations that have comparably minimal load, and hence negligible stress levels. To ensure that no harm is caused to the building, the investigator should ensure that the site where the samples are taken is repaired according to the specifications issued by a structural designer (Arya and Agarwal 14).
Profiling soil at the site
When performing diagnostic investigation on existing reinforced concrete buildings, it is also necessary that the structural investigator profile the soil where the concrete building is located. Ideally, the soil profile helps the investigator determine whether the foundation laid is strong and deep enough to support the building and its weight when in use. Broadly, soil profiles are categorized as soft, stiff, rocky, soft rich, and very dense soil (Arya and Agarwal 15). Each of the identified categories requires different considerations when laying the foundation. As such, the diagnostic investigator needs to determine whether indeed the constructor considered the properties of the soil, and whether the foundation is suitable in the identified soil profile.
Conclusion
It is evident that diagnostic investigation of existing reinforced concrete buildings is essential if the necessary repairs are to be carried out, or if injuries and deaths that occur when buildings collapse are to be averted. Regardless of how strong, stable or well constructed a building is, structural weaknesses and defects are bound to occur as time passes on since the building is exposed to different conditions within its external and internal environments. As such, the importance of diagnostic investigations cannot be overemphasized especially as a means of designing timely interventions, or demolishing buildings that are unfit for human use.
As evident in this report, the scope of diagnostic investigation is wide, but the bulk of the work lies in determining the quality of the concrete. Determining whether the steel reinforcement is corroded is also an important part of the diagnostic investigation since corrosion in steel creates weak points in the reinforcement, which may compromise the integrity of the building, and most especially its ability to bear the load initially envisaged during construction. Other areas that the investigator needs to focus on include determining whether the constructors met the acceptable requirements when laying down the foundation. This necessitates profiling the soil where the construction is based and studying the technical construction documents. Overall, a diagnostic investigator needs to analyze and draw conclusions about the current form of an existing reinforced building. To accomplish such a goal however, he would need to know all the requisite processes and procedures essential for every part of the investigation.
Works Cited
Arya, Anand, and Ankush Agarwal. “Condition Assessment of Buildings for Repair and Upgrading.” GoI-UNDP Disaster, Risk Management Program (2006), 1-16. Web.
Grantham, Michael, and Mike Gray. “Diagnosis, Inspection, Testing and Repair of Reinforced Concrete Structures.” MG associates 2000. Web.
Brown, Joyce. “W.B Wilkinson (1819-1902) and his Place in the History of Reinforced Concrete.” Transactions of the Newcomen Society 39 (1967): 129-142.
Krysander, Mattias. “Design and Analysis of Diagnostic Systems Utilizing Structural Methods.” 2003. Web.
Langford, Peter, and John Broomfield, “Monitoring the Corrosion of Reinforcing Steel” Construction Repair 1.2 (1987): 32-36.
Lo, Siu Ming. “A Building Safety Inspection System for Fire Safety Issues in Existing Buildings,” Structural Survey 16.4 (1998): 209 – 217.
Neville, Adam M. Properties of Concrete. London: John Wiley & Sons, 1996.
Pepenar, Ioan. “Damage Evaluation of Reinforce Concrete Structures in Aggressive Environments: Investigation and Diagnosis.”NDTCE’09, Non-Destructive Testing in Civil Engineering, Nantes, France. 2009. Web.
Reed, Peter John, Kate Schoones, and Jeremy Salmond. Historic Concrete Structures in New Zealand. Overview, Maintenance, and management. Wellington, NZ: Science & Technical Publishing, Dept. of Conservation, 2008.