Development of laboratory equipment to obtain powdered concrete samples to determine chlorides concentration for durability studies
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The determination of chloride concentration in reinforced concrete structures present in marine environment is an important tool in the context of their service life, allowing the development of models capable of estimating the beginning of the bars corrosive process and, at the same time, the reduction of the load bearing capacity of the reinforced concrete structure. In general, the methods for collection of powder concrete samples from real structures are based on the execution of holes with a drill or from the direct grinding of the concrete surface, however, there is the possibility of contamination of samples between the execution of successive continuity of the holes. Thus, another possible method deals with the extraction of concrete cores from the structures and subsequent sectioning in the laboratory. At this point, sectioning procedures not allowing the grinding of layers smaller than 0.5 cm, and still may cause material loss due the rotation speed of the cutting discs. In this way, this work presents an equipment developed, for academic purposes, with the objective to perform the grinding of concrete layers with thickness of 2 mm, at an affordable cost, to obtain powder concrete samples in order to determine chlorides. The results showed that the equipment meets technical and economic aspects successfully and can be used in several world laboratories.
KeywordsChlorides Corrosion Concrete Durability
It is consensus in the literature that the chlorides action in reinforced concrete structures, present in the marine environment, is the main responsible for the corrosion of the reinforcement of these structures, leading to degradation problems and involving significant financial resource regarding the maintenance and rehabilitation of these structures. In this way, studies related to the chlorides penetration in reinforced concrete structures present in the marine environment has a key role in the development of models that aim not only to estimate their service life but also to design maintenance and rehabilitation plans with the objective of preserving the integrity of the structure present in this aggressive environment [1, 2, 3, 4, 5, 6, 7].
With respect to corrosion dynamics of reinforcement in concrete structures, the alkaline solution present in the concrete pores provides a suitable environment to the formation of a passivating film, which covers the reinforcements inside the concrete, protecting them against corrosion. This film has as one of its characteristics remain stable in the alkaline environment of the concrete, however, the action of external agents, such as chlorides present in marine environment, end up destroying this film, giving conditions for the beginning of the corrosive process of the bars, with the consequent formation of corrosion products [4, 8, 9, 10].
These corrosion products are expansive and, as they are formed, they deposit on the periphery of the reinforcement, producing volumetric variations in relation to the metal consumed in the process. Such a mechanism generates stresses in the radial direction to the bars axis, which are not supported by the limited plastic deformation capacity presented by the concrete. Consequently, cracks are formed with the subsequent spalling of the cover layer, increasing the penetration of aggressive agents and accelerating the degradation process of the reinforced concrete structures . Besides the cracking and spalling of the cover layer, the reinforcement corrosion leads to structural damages, due to a reduction of the adhesion between the concrete and the reinforcement, impairing the monolithism between these elements, and a progressive reduction of the bars cross section as the corrosive process intensifies, leading to a progressive reduction of its mechanical properties and, at the same time, a decrease in the bearing capacity of the affected reinforced concrete structures [4, 9].
At this point, Schweitzer  points out that chloride-induced corrosion may, depending on the geometry and extent of the damage, cause structural failure, such as fragile fractures, due to a localized loss of material in the strength section of the reinforcement. In fact, Zhu and François  have shown that the damage caused by chloride-induced corrosion to the cross sections of the bars eventually produces axis eccentricities between corroded and non-corroded sections. Thus, the authors observed, by means of tension tests the greater the eccentricity, lower are the mechanical properties observed in the bars.
Balestra et al.  demonstrated the corrosion effects on the mechanical properties of naturally degraded reinforcements presented for decades buried in the soil. The results showed that chloride-induced corrosion can produce small perceptual variations in reinforcement mass, but significant reductions in the mechanical strength, especially, in ductility when subjected to tension. In this case, even corroded bars with a percentage variation of mass of less than 5% presented yield and ultimate strength and final elongation smaller than bars with twice the perceptual variation. This fact is related to the damages produced by the corrosion to the cross sections of the reinforcement, since this type of corrosion may lead to severe punctual reductions of cross section of the bars, even with small variations of mass.
Due to the exposed problem, and considering that chlorides are the main agents responsible for the degradation of reinforced concrete structures present in marine environment, from the perspective of the reinforcement corrosion, the chloride concentrations profiles are characterized as an important tool in the evaluation of the corrosive process of the reinforcement, contributing with relevant information about the real level of aggressiveness to which a structure is subject. At this point, the chloride concentration profiles are defined as the representation of the chloride concentration as the depth in the concrete, from its surface increases. Thus, quantitative information regarding the penetration of chlorides into reinforced concrete structures, especially real structures, are justified, and are the first step in order to develop measures to increase the service life of reinforced concrete structures present in the marine environment.
In this topic, in a more specific way, the chloride penetration models in reinforced concrete structures, Castro et al.  and Trocónis de Rincón et al.  point out that reliable models can only be developed through the knowledge obtained from chloride concentration profiles from real structures, degraded naturally taking into account the environmental parameters involved. Medeiros et al.  point out that although there are lines of research dealing with the subject, service life models still present unsatisfactory results, requiring data obtained from real structures for the improvement of such models.
The drilling method.
The direct surface grinding method.
The method of concrete core extraction.
Although this methods are normalized and adopted by several works found in literature, as observed, for example, in Medeiros et al. , Andrade et al.  and Otieno et al. , both drilling and surface grinding methods can present in a given sample, at a given depth, contamination due to the presence of residual powder present in the collection apparatus obtained from the previous grinding, which can lead to errors in the correct determination of the concentration of chlorides in the analysis in the laboratory and, concurrently with this, leading to errors in the construction of the chloride concentration profiles. In addition, in a general way, the depth of sample collection from these two methods are of the order of 0.5 cm depth per sample, and it is difficult to collect samples from shallower depths, which could refine the analysis, making the chloride concentration profiles contemplating a greater number of points for future numerical modeling. Thus, the extraction of concrete cores, from real structures, could be characterized as a better way to obtain concrete samples of powder.
In this way, through the exposed problem, this work presents the development of a laboratory equipment, at an affordable cost, for millimeter grinding of concrete cores, to obtain powder samples, which aim to determine the chloride concentration at depths of the order of 2 mm.
2 Experimental procedure
The Island is a rocky formation with approximately 37,000 m2 distant from 1.6 km of Pernambuco beach in the city of Guarujá, south coast of São Paulo State, Brazil. The history of the Island dates back to the 1950s when it was granted by the Navy of Brazil for scientific purposes to Engineer Fernando Lee. In this way, several reinforced concrete structures were executed in different marine aggressive zones in order to support the researches .
Low rotation of the equipment, not leading to a possible loss of material due to the rotation speed of the equipment (480 rpm).
The possibility of adjusting the glass saw depth penetration in the concrete cores by moving the base of the bench drill.
The low temperature due to the dry friction between the surface of the concrete core and the glass saw during the grinding process, avoiding possible losses of chlorides due to an increase in temperature.
The test concrete core is fixed by the Nylon support in the vise, receiving the plastic container for the collection of the material coming from the grinding.
After the set is raised until the surface of the concrete core exceeds the diamond end of the glass saw by 2 mm.
In the sequence, the drill is driven and the concrete core is manually moved against the diamond saw’s glass surface. In this way, as the contact occurs between the concrete core and the diamond end of the moving glass saw, the grinding of the concrete is possible, where the powder particles, resulting from this grinding, fall into the inner part of the plastic container.
After sweeping the entire surface of the concrete core, the ground material is collected for analysis using brushes, and then is stored, identified and sealed in plastic bags. Before continuing the grinding operations the container is cleaned with damp cloths and dried with brief air jets. After cleaning, it is again coupled to the concrete core and the process is repeated with a new depth.
After the ground material has been identified, the powder samples were analyzed by the X-ray fluorescence spectroscopy technique for the determination of chlorides concentration, as described rigorously in Balestra .
3 Results and discussion
It can be seen from Fig. 6 that, in fact, it is possible to grind the surface of the concrete cores with a thickness of millimeters and plastic container is capable of collecting a significant amount of material. In addition, the concrete core surface receives a complete sweep of its section, regardless of whether aggregates or only cement hydration products are present. It is also worth mentioning that the surface temperature of the specimen was determined immediately after grinding, with values lower than 28 °C for all test specimens, which prevents the loss of chlorides due to temperature effects due to the dry friction between the surface of the concrete core and the glass saw.
The construction of the chloride concentration profile was obtained successfully, as observed in Fig. 7, and it is possible to clearly verify the presence of a peak. This fact demonstrates that the grinding equipment developed was able to provide powder concrete samples that can be used for the determination of the chloride concentration. Besides it was possible grind concrete cores thickness in layers of 2 mm. In this way, it is possible to obtain a larger number of powder concrete samples to determine the chloride concentration, representing a greater number of points for the construction of a more accurate chloride concentration profile, allowing the development of more accurate models to the real concrete structures condition present in marine environment. This can be observed by the clearly peak of the profile in Fig. 7.
Another important point deals with the time of grinding operations and the cost of the equipment. At this point, the average grinding cycle time of each step was around 11 min, to obtain up to 35 g of powder sample. In addition, the overall cost of the equipment was approximately US $290.00 (data for the month of July 2017 in Brazil), taking into account the acquisition cost of the drill, the vise, the Nylon device, the plastic container with the sealing material and the glass saw. Such facts demonstrate that the equipment developed is capable of optimizing the sampling steps with a representative amount of sample was obtained, free from possible contamination, presenting an affordable cost. Besides, it is worth mentioning that the amount of powder sample obtained in each grinding step allows several tests to be performed for the same depth, allowing counter-proof tests in cases where it is necessary.
In terms of the equipment durability it is possible to emphasize that the main consumption material, after being assembled, is the glass saw, requiring a regular replacement. The cost of the glass saw is approximately US $25.00 (data referring to the month of July 2017 in Brazil), however, with a single saw it was possible to obtain up to 60 powder concrete for a concrete core with a nominal diameter equal to 75 mm. This reinforces that this grinding equipment is able to be used in several laboratories of different universities worldwide.
The equipment developed was able to successfully meet the demand for powder samples to determine chloride concentration, as evidenced in the profiles obtained.
The developed equipment was able to obtain samples of powdered concrete from layers with thickness of the order of 2 mm, allowing to refine the data and points presented in chloride concentration profiles for analysis of the durability and service life of real concrete structures present in marine environment.
The average milling time was less than 15 min and the overall cost of purchasing and assembling the equipment was less than US $300.00 (data for July 2017 in Brazil). In addition, the periodic maintenance cost of the equipment is estimated at US $25.00. These facts show that this is an affordable equipment, both for purchase and assembly of the parts, as well as in terms of maintenance, so this equipment developed can be used in the laboratories in various parts of the world.
The authors would like to thank Fernando Lee Foundation for the support during the works at Arvoredos Island.
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