2.1 Characteristics of the historical building.

The building to be revitalized is called Casarão and is located at Brazil avenue number 310 in the city of Rio Branco, Acre state. Featuring 405.1 m² of built area, the Casarão is composed of basement, ground floor and upper floor, with a parking with 705.0 m² and a free area of 154.7 m² in front of the building. All stages of the building revitalization process were the Acre State Government responsibility and followed an executive chronogram previously established and detailed. The municipal director plan, by law 1611 of 27 October 2006, classified Casarão as a building located in ZPHC (Cultural Historic Preservation Zone), with occupancy rate of 70% (810.0 m²⁾, coefficient of utilization 6 (7800.0 m²⁾ and 10% of permeability rate (130.0 m²⁾. During the process of revitalization it was found the need for structural strengthening only for the underground stretch of the columns P21, P30 and P41 for new loadings, which did not occur with the other columns. Figure 2 (top floor wood molds) shows the original situation of the columns and the strengthening proposal.

Figure 2 Location and dimensions of columns P21, P30 and P41 before and after structural strengthening (right). 

In the reform of the building, period features have been retained and built-up areas that were not in the original architectural design have been removed. The services performed were the revitalization of the original wood walls, structural strengthening of the columns, replacement of the roof structure, the roof, the floorboards of the ground and upper floors, replacement of electric installations, logic, water and sewer pipes, fire-fighting system, creating a seating area with benches and bins, maintenance of three rooms upstairs and manufacture of masonry in the three floors. Figure 3 shows the appearance of the side and front facades of the revitalization project.

Figure 3 Final aspect designed for Casarão. 

2.2 The finding of the problem

To check the existing structural safety, inspection visits and monitoring of the construction site during the execution of the revitalization work were carried out, where the procedures, techniques and equipment used in operations were identified and the photographic record being held at all stages of the executive process in 2009 and 2010. Before performing any procedure on the structure, the technical team highlighted the importance of axial compression tests on concrete testimonies to enable identification of any eventual problems related to concrete strength, since the old concrete was originally made with ceramic pieces of bricks and roof tiles. The test results indicated that, in general, the structural concrete has low compressive strength and contained coarse aggregate larger than 50 mm, which contributed to the formation of voids inside of some columns. It notes that the use of coarse aggregates derived from reducing of ceramic elements such as bricks and tiles is still a common practice in the city of Rio Branco, where there is shortage of gravel and rolled pebble. Figure 4 shows the location of extraction of the concrete testimonies in a column and Table 1 shows the results for concrete compressive strength of concrete testimonies from columns P21, P30 and P41, which further analysis indicated the need for structural strengthening in the basement level columns.

Figure 4 Testimonies' extraction hole in a column. 

Besides the low compressive strength, it was also found that the reinforcements of some columns had advanced corrosion and severe weight loss, compromising bearing capacity to service loadings and even to permanent ones, once some bars were completely sectioned by corrosion. Figure 5 shows the appearance of the corroded reinforcement bars of the columns. It was concluded that the damage was characteristic of deterioration processes with systemic nature, i.e. related to the quality of the concrete used in the manufacturing of the structural system and especially the thickness of reinforcements coatings that showed values between 10 mm and 15 mm, considered inadequate to provide the required service life of a structure made of concrete containing such porous aggregates.

Figure 5 Columns' longitudinal reinforcement eroded (corrosion). 

2.3 Structural strengthening

The applied methodology for defining the structural strengthening technique considered the set of information on the conservation status of the entire structure in order to drive the strengthening designer engineer to draw up a more accurate diagnosis of the causes and consequences of the pathologies found, because, according to ^{Julio et al. (2003)}, the success of rehabilitation activities or intervention in the structure depends on a clear and precise definition about the problems encountered, including a diagnosis and prognostic evaluation of the acting causes. This preliminary study was instrumental in determining the level of intervention to be performed in the structure, which can range from simple repairs located to the need for demolition and reconstruction. For structural reinforcement was chosen the reinforced concrete jacketing technique, as the concrete can be cast into molds or projected, making it the most versatile material for the enhancement or restoration of reinforced concrete structures, and used in all kinds of structural elements and in different situations and conditions (^{Mehta & Monteiro, 2014}). The structural reinforcement was then indicated to the columns P21, P30, P41, in the basement floor lengths, with the main steps consist of superficial concrete scarification, positioning of additional reinforcement, installation of wooden molds and placing of concrete with compressive strength (f_ck) of 25 MPa.

2.3.2 Reinforcements

The longitudinal and transverse reinforcement (stirrups) before and after the structural strengthening of the columns were composed of steel bars with diameters, numbers and spacing presented in Table 2. The concrete cover for the columns was 25 mm and 15 mm at the short and long sides' dimensions, respectively. Figure 6 shows details of the cross sections of the columns before and after the structural strengthening with reinforced concrete jacketing, and Figure 7 shows details of the transverse and longitudinal bars. The columns P21, P30 and P41 were only strengthened in the basement floor length because these columns presented safety factors below the recommended by the Brazilian standard for reinforced concrete structures design for the future loadings. Figure 8 shows the placement of the reinforcements, where it is possible to see that, despite efforts to preserve as much original architecture, the smaller final dimensions of the columns, plus the 20 mm thick coating mortar layer, would be greater than the wall thickness.

Table 2 Columns' cross section dimensions and reinforcements. 

Figure 6 Columns' cross section before and after the structural strengthening. 

Figure 7 Original and additional reinforcement. 

Figure 8 Longitudinal reinforcement and stirrups of the structural strengthening. 

2.3.3 Concrete casting

So that there was greater adherence between the old and new concrete its surfaces of the columns were wet before the concrete casting. The concrete was carefully released (Figure 9) and the density was achieved with immersion vibrator with a head diameter of 20 mm, first in 50% of the volume of concrete cast in each column and then the remaining volume. After removal of the wooden mold the columns received a mortar coating layer to allow the finishing, or painting. Figure 10 shows the final appearance of the columns with smaller dimensions exceeding in 35 mm the wall thickness.

Figure 9 Concrete casting of the structural strengthening. 

Figure 10 Final aspect of the strengthened columns. 

3.1 Current columns' safety evaluation

The calculation methodology to check the columns' safety with and without structural strengthening followed the prescriptions of ^{NBR 6118 (ABNT, 2014)}. A simplified analysis which were determined and compared only the normal loading and resistant capacity for P21, P30 and P41 columns before the strengthening with its normal loading force multiplied by γ _u (Equation 1) and an additional increase coefficient (Equation 2). Since design normal resistant force was determined by Equation 3, Equation 4 was used to determine the concrete's design compressive strength (f_cd ) and Equation 5 enables the determination of the safety factor for each column.

Normal acting force (N_Sk ) was obtained from Altoqi Eberick 2002 software and the results found with the preliminary analysis for the safety factors of the columns P21, P30 and P41 before strengthening not meet the normative safety requirements, as shown in Table 4. Figure 11 shows the interaction envelopes of resistant forces of these columns, i.e. the consideration of acting and resistant characteristic bending moments without any coefficient of increase or decrease in these actions. In this case, it is observed that the bending moments are more important and destabilize the columns (^{Marí & Hellesland, 2005}), which did not occur because the current actions were lower than those considered in the structural design up to now. It is also possible to conclude with more refined analysis that the progress of corrosion on the flexural reinforcement could significantly reduce the resistant capacity of the columns to flexure and even if the maximum designed loads did not occur, small additions could cause the loss of stability of these columns (^{Tang & Yang 2011}), evidencing the need to strength these structural elements.

Table 4 Columns' safety coefficient before and after the structural strengthening 

Figure 11 Envelopes for columns P21, P30 e P41 before the structural strengthening. 

3.2 Strengthened columns' safety evaluation

Columns' design safety factors for P21, P30 and P41 strengthened columns was based on the prescriptions of ^{NBR 6118 (ABNT, 2014)} following the same criteria used for the original columns without strengthening. The results are presented through the interaction envelopes of Figure 12. The safety factors of each column were determined by the ratio between the length of the straight line from the origin to the outer envelope, passing through the acting loads point, and the distance from this point to the origin, and results are presented in Table 5.

Figure 12 Envelopes' for strengthened columns P21, P30 and P41.