|NDT.net - February 2000, Vol. 5 No. 02|
- International Symposium on NDT Contribution to|
the Infrastructure Safety Systems, 1999 NOV 22-26 Torres,
published by UFSM, Santa Maria, RS, Brazil
|TABLE OF CONTENTS|
Keywords: Durability of structures, Corrosion of reinforcement, Marine environment, Case study, Life cycle analysis, Numerical modelling.
The building was located in an urban area, 3 km distant from the sea, in a crossing of two streets of high traffic. It's structure has a garage pavement and 13 floors. It's composed by reinforced concrete elements (columns, beams and slabs), with a triangular form in the plant. The structure has a residential use, and didn't suffer any change by it's occupants.
Visual Inspections/ Carbonation Depth/Profile of Chloride Ions
Was made a preliminary and detailed inspections in the structure. Every structural element was analised, in a methodology described in some references [7,8], when this stage of work was responsable by the correct assessment of the damages. The presence and physical characteristics (eg.: width, length) of cracks, spalling, deflections and others forms of damage's evidence were the measurable parameters employed for to provide informations for to establish the structure deterioration degree. Measures of the carbonation depth in the columns located in garage and ground level were made, since they were in areas with highest concentration of carbon dioxide (CO2) of the environment. The analysis was made in four columns in the structure, changing the sample points in function of the building orientation. Concrete samples were removed from four points of the structure in three differents deepers for to predict the chloride profile. In each one was collected a concrete sample and sent to the laboratory for determination of chloride in mass (corresponding to the amount of element in the cement weight).
Deterioration Degree of Structure
Nowadays, every information about deterioration in reinforced concrete structures is exposed in qualitative forms, without any quantitative classification about the damage evolution and the right moment to conduct the rehabilitation works. A new methodology was developed in University of Brasília, Brazil, for maintenance of reinforced concrete structures , with the purpose to establish a quantitative classification for degradation degree to individual elements, family of elements (columns, slabs, beams) and the whole structure, taking in account the more expressive parameters for damage's definition, your evolution and the environmental influence in this process.
However, a very important consideration might be made: it's very difficult to make any kind of prognostic about the life cycle of structures. The big amount of factors that have a significant influence in many deterioration processes doesn't interact separatedly, but has a holistic (or sinergic) effect. This sinergism made difficult the reliability of all deterioration models proposed . So, this methodology is a starting point for to infer the possibility of damages classification based in simplificated procedures, in which is necessary more detailed analysis to validate completely the program.
The bulding is divided in families of structural elements, in order to identify groups that have similar structurals characteristics. After this, a matrix is made for each structural element, exposed to the possible damages to the family, with a damage consideration factor (Fp). This factor, varying from 1 to 10, shows the relative importance of a specific damage in a family, whereas the serviceability of the element.
The same damage can have different consideration factors, depending on the family in which they are inclosed, and the damage's consequences in each one. So, the next step is to establish the evolution of the damage, through the damage intensity factor (Fi). This factor considers some aspects related to funcionality, aesthectics and structural safety of element, with the environmental influences in function of exposure conditions and the protection systems adopted. The damage degree (D) for a structural element is tied to the damage consideration factor and the intensity of damage, as shown in Figure 1.
|Fig 1: Damage degree (D) versus damage intensity factor(Fi) |
The first part of the graphic corresponds to the damage's iniciation stage, starting at 0, in which there is no degradation evidence, until the point at (2,5;10). For the propagation stage, that corresponds at D > 10, the damage's effect is more evident, and can take to an unserviceably state. For Fp = 10, which represents the more unfavourable condition of element, the degree is:
|D = 4 Fi for Fi · 2||Eq. 1|
|D = 60 Fi - 140 for Fi · 3||Eq. 2|
Damages with consideration factors smaller than 10, the damage degree is obtained by the expressions:
|D = 0,4 Fi · Fp for Fi · 2||Eq. 3|
|D = (6 Fi - 14) Fp forFi · 3||Eq. 4|
The equations 1 and 3 try to model the initiation phase, take into consideration the intensity factor of damage. In accordance with Figure 1, when the damage is in initiation phase, the value of Fi is less than 2. The same analogy is made for the equations 2 and 4, what considers the propagation phase for to establish the damage degree. These parameters was adopted for to provide quantitatives boundaries, based in damage's evidence, for each stage of damage.
The next step is to calculate the element deterioration degree (Gde). For two different damages in the same element, the deterioration degree corresponds to the greatest damage, in order to avoid any simplification (as a medium value) that can take to mistakes in the expression of damage's reality. The next step is to calculate the deterioration of family elements, Gdf .
For the Gdf determination, is established that only the most expressive damages, the ones with Fi³ 2,5. This procedure was adopted to avoid that the value of damages with no great importance influences in the final result. Finally, the structure deterioration degree (Gd) is calculated, in funcion of the degradation of the elements family and of the factor of structural relevance(Fr).
Based in several detailed inspections on the structure, it was observed that there is a great amount of damages. At the garage level the Gerber beams have deterioration in an advanced stage, with cracks that vary from a few milimeters until 1,5 centimeters. These problems happen because of the expansion forces originated in a corrosive process, that leads to the crushing of the columns top by innadequate stress distribution.
Failures in waterproof systems in roof gutters were observed in contour beams. In Brazil, this type of protection system has a warranty time of nearly 5 years, when, over this period, the product must be replaced. This fault can lead to concrete leaching, with the ocurrence of stalactites of calcium carbonate in beams deep.
The most dangerous damage observed in structural elements happens in the columns, in different intensities. From the garage level until the top of columns the cracking of concrete cover is evident, with extensively variable width over the elements.
The insuficient concrete cover was observed in many points of the analysed structure. In any construction, mainly the ones located in saline environment, this quality parameter of construction must be reached, in order to maximize the physical and chemical protection at the reinforcement. Poor quality concrete, with varying resistence between 15 and 18 MPa, and insuficient concrete cover can lead to the occurence of corrosion of the reinforcement, decreasing the life service of the structure.
The improvement of CO2 level in the concrete elements didn't reach the reinforcement in any analysed point. In the garage pavement, where the concentration of CO2 is biggest than the environment one in function of the vehicle discharge, the carbonation depth is smaller than the concrete cover for all samples.
In all points the amount of chloride ions was biggest than the reccomended values (0,4% in cement weight). However, we doesn't have a clear tendency about the possible source of this element in structural members. When the chloride comes from only the environmental action (saline wind), the amount of Cl ions is bigger in the surface of the structural element, and decreases with the increase the depth of the same. However, in this specific case, the graphic configuration indicates possible contamination of construction materials used in construction process. The chloride ions were deposited over the sand and incorporated to the concrete mass. That occured during the long interruptions in the construction phase of building.
Other possible source of this element is the use of accelerators, that have one or more calcium chloride (CaCl2) forms as the active ingredient in it's chemical composition. One more source for this occurrence is a very common procedure in Brazil of washing the columns ceramic coating. This process is done using muriatic acid (a commercial form of chloridric acid), that has a big amount of chloride ions. That's done without any kind of care, because the users don't have any idea about the harmful effects in the durability of the structure.
|Fig 2: Penetration curves of chloride ions in structural elements|
Analysis of Structure Deterioration Degree
The structural elements inspected were the columns (in number of 18) and frontal beams (16) and Gerber beams (10) at the garage level, totalizing 44 elements. The most dangerous damages verified are due to the presence of a high amount of chloride and excessive cracking.
The deterioration degree (Gde) for a collumn was calculated, having been obtained the value of 123,2. So, observed that the structural element has a Gde greater than the critical level reccomended (>80), it's showed the immediate need of an intervention in this specific element. The next step is to determinate the structure deterioration degree, as showed in Table 1.
|Element family||Gdf||Fr||Gdf x Fr|
|Gd = 95,2|
|Table1: Structure deterioration degree (Gd)|
Thus, observed that the value obtained is high, compared with the ones showed at Table 2, it is necessary the immediate start of the recuperation tasks.
|Deterioration level||Gd||Necessaries measures|
|Low||0 - 15||acceptable level|
|Medium||15 - 40||periodic observation|
|High||40 - 60||detailed periodic observation|
|Critical||>60||intervention immediate for reestablish the funcionality|
|Table 2: Levels of structure deterioration |
The basis for the corrective actions explained above was developed take into account some works where the methodology proposed is applied. The expert opinion in this kind of work have a great importance for to establish these limits values. The model proposed is tested in many structures in Brazil and the results obtained showed a great correlation between the damages observed in practice and the numerical limits reached.
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