- 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

Abstract Introduction Structure Description Inspection Methodology Visual Inspections/ Carbonation Depth/Profile of Chloride Ions Deterioration Degree of Structure Results and discussion Visual Inspections Carbonation Depth Chloride ions Analysis of Structure Deterioration Degree Final Considerations References

Abstract

Nowadays the corrosion of reinforcement is the most harmful damage that occoured in reinforced concrete structures. The case study is one analysis form utilized for to verify the damages evolution when the structure is exposed in environment. In a residential building located at saline environment in Brazil was made measures of carbonation depth, chloride ions and to apply an new assessment method for to establish quantitatives boundaries for structures degradation. The results showed that the carbonation depth was smaller than concrete cover, but the amount of chloride ions in structural elements was biggest than the threshold value. The structure deterioration degree showed which the structure have a critic level of degradation, where is necessary the immediate intervention on structure for to restore the serviceability levels. So, this work show make real the possibility of to verify the damages evolution in a structure, using simple inspection procedures with a numeric analysis.

Keywords: Durability of structures, Corrosion of reinforcement, Marine environment, Case study, Life cycle analysis, Numerical modelling.

Introduction

Nowadays a great amount of damages that happens in reinforced concrete structures is observed. Many countries around the world are located in very agressives environments, like the Norway Coasts [1] and the Arabian Gulf [2]. In the United States resources at an order of millions of dollars were spent over the last years in inspections and rehabilitation programs, mainly in the railway structures [3]. So, many studies are conducted with the purpose of identifyng the degradation forms and providing informations to minimize this kind of occurence in new structures. In this way, the case studies are extremely important, having been done in high scale all around the world [4,5], when significant information is given about the construction and climatic effects in many degradations forms. In Brazil, a high temperated country in which the most important cities lay near the sea, analysis as this one were conducted by many authors, who did inspections in industrial instalations that presented a high degree of agressivity because of the chemical product emission and analysis in residential buildings that presented corrosion of reinforcement at advanced level [6]. So, this paper made an analysis about the degradation problems in a structure located in a saline environment and tried to make considerations about the source of damages as well as to quantify the structure deterioration degree [9].

Structure Description
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.

Inspection Methodology


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 [8], 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 [7]. 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 (F_p). 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 (F_i). 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) [11]

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 F_p = 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 F_i 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 (G_de). 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, G_df .
For the G_df determination, is established that only the most expressive damages, the ones with F_i³ 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 (G_d) is calculated, in funcion of the degradation of the elements family and of the factor of structural relevance(F_r).

Results and discussion

Visual Inspections
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.

Carbonation Depth
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.

Chloride ions
The results of laboratorial analysis of this element vary in a range of 0,455 at 1,526% in cement weight. This data was plotted in a graphic, as shown in Figure 2.

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 (G_de) for a collumn was calculated, having been obtained the value of 123,2. So, observed that the structural element has a G_de 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
Columns	109,48	5	547,4
Beams	67,12	5	335,63
Gerber beams	112,39	4	449,56
	Total	14	1332,59
			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 [8]

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.

Final Considerations

In Brazil a lack of information about the many degradations processes in reinforced concrete structures is observed. The sinergism existent among the saline environment, high temperatures, relative humidity and the poor quality control of concrete production process (workmanship, materials and procedures inadequate) can lead to damages in reinforced concrete structures [7].
So, a case study was conducted in a reinforced concrete structure in order to identify the sources, the occurrence forms and the factors that affect the most important damage in these structures: the reinforcement corrosion. The inspection works have a critical importance in a correct diagnostics of the problem, because it can define the efficiency of a rehabilitation task.
The visual analysis showed a holistic effect in many damages occurrences that lead to the increasing of the deteriorating level. Concrete leaching, high humidity and insufficient concrete cover were also problems that can also lead to the maximization of the corrosion process. The carbonation depth was smaller than the concrete cover. But many analysis points have been made for a greater reliability of data, although the same was collected in areas with a big concentration of CO₂ (garage pavement).
The amount of chloride ions in the structural elements was highest than the accepted limits proposed by many authors (0,4% by cement weight).
Despite the difficulty in order to determinate the service life of many kinds of structures, it is possible to estimate the deterioration level from single procedures, with a satisfactory degree of reliability. This approach has a fundamental importance because it establishes numerical limits for a correct evaluation of deterioration process, and indicates the best moment for the rehabilitation works [7,8].

References

1. MEHTA, P. K. (Ed.) Odd E. Gjorv Symposium on Concrete for Marine Structures. Proceedings. New Brunswick, 1996. 279 pp.
2. AL-AMOUNDI, O. S. Durability of Reinforced Concrete in Agressive Sabhka Environments. ACI Materials Journal, v. 92, nº 3, 1995. pp 236-245.
3. STEWART, M. G.; ROSOWSKY, D. B. Time-Dependent Reliability of Deteriorating Reinforced Concrete Bridge Decks. Structural Safety, 20, 1998. pp. 91-109.
4. SHALABY, H. M.; DAOUD, O. K. Case Studies of Deterioration on Coastal Concrete Structures in Two Oil Refineries in the Arabian Gulf Region. Cement and Concrete Research, v. 20, nº 6, 1990. pp. 975-985.
5. NOVOKSHCHENOV, V. Deterioration of Reinforced Concrete in the Marine Industrial Environment of the Arabian Gulf - A Case Study. Materials and Structures, v. 28, nº 181, ago, 1995. pp. 392-400.
6. CASCUDO, O.; REPETTE, W. Damages Induced for Chloride Ions in Reinforced Concrete Structures in Residencial buildings: a case study. 37^a Annual Meeting of Brazilian Institute of Concrete, Goiânia, Brazil (in Portuguese). Proceedings 1995. pp. 219-231.
7. ANDRADE, J. Durability of Reinforced Concrete Structures: Analysis of Damages In Northeast Region of Brazil. M.Sc. Dissertation (in Portuguese). Federal University of Rio Grande do Sul, Brazil, 1997. 148 pp.
8. CASTRO, E. K. Development of Methodology for Maintenance of Reinforced Concrete Structures. M.Sc. Dissertation (in Portuguese) University of Brasília, Brazil 1994. 185 pp.

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