MECHANICAL BEHAVIOR TO SHEAR OF REINFORCED CONCRETE BEAMS REINFORCED WITH CARBON FIBERS BONDED WITH EPOXY ADHESIVE COMPORTAMENTO MECÂNICO AO CISALHAMENTO DE VIGAS DE CONCRETO REFORÇADAS COM FIBRAS DE CARBONO COLADAS COM ADESIVO EPÓXI

In Brazil, the concrete structure is widely used in civil construction. In this field, there is a common need to reinforce the structural elements, either by increasing the load, for calculation errors, flaws in the execution or modifications of use conditions. This experimental work aims to compare two reinforcement techniques with carbon fiber blanket on reinforced concrete beams submitted to shear stress. The tests will compare the results of reinforced concrete samples without any reinforcement, with shear reinforcement using carbon fiber blanket with techniques and materials popularly spread on the market. In this case using the structural adhesive S&P 220, and with shear reinforcement using carbon fiber blanket glued with structural adhesive NVT 201E.


INTRODUCTION
Although its use is very popular, concrete is not exempt from any factors that could compromise its useful life. The unexpected increase of loads in projects may impair the structural capacity for transferring efforts. Unfavorable environmental conditions and the aging of the structure can also contribute to the degradation of its use in service. Through the application of an external reinforcement, it is possible to restore the capacity to support loads of concrete structural elements (MUKHTAR;FAYSAL, 2018).
Expansion projects that are not foreseen in buildings also change the requests in the structural system already executed, that must be analyzed making new considerations regarding the actions and efforts acting. In certain cases, design errors occur, as well as, the execution of the reinforced concrete structure, being feasible to perform the structural reinforcement in the affected elements instead of the demolition and reconstruction of the structure already built. (BEBER, 2003).
Therefore, the need of civil construction technicians to know several techniques for reinforcements available at market today and their procedures, costs, advantages and deadlines. A relatively current option is the use of Fiber Reinforced Polymer (PRF). In relation to other more traditional materials, the choice of PRF is based on attractive factors such as high mechanical strength and stiffness, durability in chemically aggressive environments, low weight, ease and speed of installation (FRIGIONE; LETTIERI, 2018).

OBJECTIVE
Due to the increased use of carbon fiber as an intervention in civil construction, this experimental work will discuss the application of the use of structural adhesive NVT 201E (adhesive developed at the IPRJ's Adhesion and Adherence Laboratory used to repair pipes and sheet metal) to shear reinforcement and will address the execution method, advantages and disadvantages, so that one can finally compare to the conventional technique widespread in Brazil.
In this work, will discuss important concepts for the application of reinforcement on concrete structures using external pasted of laminated composite materials. Materials, test procedures and shear reinforcement techniques will be presented, and finally the results will be discussed based on a statistical analysis to compare the influence of the adhesives NVT 201E and S&P 220 on the shear strength in concrete substrate.

Laminated Composite Material
Composites consist of a thermoplastic matrix or a matrix with thermal curing (thermosets) and continuous fibers arranged randomly or in defined directions. The strength of the matrix is less than that of fibers. These fibers must withstand loads in order to obtain a greater gain in the strength of the composite as a whole. However, there is a drawback that should be pointed out regarding the sudden rupture of the fibers (MACHADO, 2010).
The definition of laminate according to Reddy and Miravete (1995) refers to a set of layers, called sheets, stacked so that the desired mechanical and physical properties are achieved in the finished material. These layers can contain fibers aligned in a unidirectional or multidirectional way and the way they are ordered is responsible for giving different properties to each type of laminate. There is also the possibility that a laminate can be manufactured with layers composed by fibers of different materials.
The matrix of composites with fibers has several functions. At principle, it is responsible for the union of the fibers that make up the composite, acting as the means by which external requests are transmitted and distributed to the fibers (HOLLAWAY, 1993;CALLISTER, 1997).
The growth in interest to the use of structural reinforcement systems with fiber-reinforced composites can be attributed to some factors, like its quick application. Although the costs of fibers and resins used in composites are relatively high if compared to traditional materials, like steel and concrete, the cost of labor and equipment to run this type of system are usually lower (AMERICAN CONCRETE INSTITUTE, 2008).

Carbon Fiber
Carbon fibers are characterized by a combination of low weight, high strength and high rigidity. Its high elasticity modulus and, by a way, high strength, depend on the degree of fibers orientation, that is, the parallelism between the axis of the fibers (SCHWARTZ, 1984). The use of this material has been increasing in civil construction, because its constitution leads to having low weight, resistance to corrosion, and good resistance when compared to other types of reinforcements, essential characteristics for reinforced concrete structures.
The production of these fibers requires exposure to air from the base fibers, followed by high temperature processing (in order of 1000º C to 1500º C) of organic polymer fibers. How bigger temperature of this process, most bigger the elasticity modulus of resulting material, ranging from 100GPa to 300GPa (MACHADO, 2010).

Epoxy Resin
The resins have the function of involving and agglutinating the fibers to ensure an adequate transfer of tensions between them and to protect them from environmental aggressions and wear. Its properties must also lead to a perfect adhesion between the concrete and reinforcement material interfaces and provide adequate durability and applicability to the composite (CARNEIRO, 2008). Epoxy based resins are more expensive than resins based on vinylester or polyester. However, it has better characteristics such as greater resistance to the attack of bad weather and has a longer useful life.
Regarding structural adhesives, those that cure at relatively low temperatures, such as epoxy resins, has been widespread in civil construction applications, mainly in relation to the repair or increase of load by bonding Fiber Reinforced Polymers (FRP). After mixing the resin with the agent and its cure, a period of time must be waited until the cure is complete so that maximum bond strength is achieved, enough for the transfer of loads to take place properly (MICHELS et al., 2016).
Structural reinforcements with composite materials, mainly CFRP, have become popular in Brazil due to their excellent properties for this purpose.
However, premature failure of the reinforcement has been observed, characterized by its detachment from the concrete substrate. This kind of failure is extremely undesirable because it occurs without warning and anticipates the collapse of the reinforced beam, making it impossible to take full advantage of the CFRP's tensile-resistant properties (FERRARI, 2002).

Adhesives
In the experiment, bonding was carried out using two types of structural adhesives on the concrete substrate, the S&P 220® adhesive provided by S&P Reinforcements Clever Solutions, and the NVT Pipe Fix® adhesive provided by

METHODOLOGY
For the elaboration of this article, some procedures were made that will be described in more detail. Some steps can be mentioned, such as concrete dosing, sizing of bodies test, the dimensioning of the laminated composite reinforcement, reinforcement application's, the axial compression test of the concrete and mainly the 4-point bending test for reinforced concrete. Which will be used as a basis for a statistical analysis of comparison between the use of two structural adhesives.

Reinforced Concrete
The

Laminated Composite Material
The laminated composite materials used to reinforce the beams of the

Reinforcement Application to the Concrete Substrate
For the experiment, reinforcements were made on 3 beams using the structural adhesive NVT 201E, on 3 beams using structural adhesive S&P 220 and 3 other beams remained without reinforcement, so they could be used as a

Axial Compression Test
Cylindrical test bodies with a diameter of 10 cm and a height of 20 cm were tested, as stipulated in ABNT NBR 5738, all 28 days old. The test was performed on June 27, 2019 and the types of rupture were mostly sheared and tapered. The test load was applied continuously, without shocks and at a speed of 0.5 MPa / s, according to ABNT NBR 5739; figure 4 shows the Solotest® hydraulic press. The dosage of these test bodies was dimensioned to have 30 MPa at 28 days and was the same as that of the beams that were reinforced by shear, including from the same batch. Table 1 presents tests results.

Figure 4:
Hydraulic press used for tests.

Shear Strength Test
11 composed beams of rectangular section were made and formed with positive armor of steel ½" CA-50, as outlined in Figure 2. For the tests, the structural adhesive NVT 201E composed of carbon fibers was used in 3 of these beams (reinforcing shear), the S&P 220 adhesive composed of carbon fibers was used in 3 of these beams (reinforcing shear) and 5 beams without shear reinforcement.
For the statistical determination of the comparison of the influence of the shear reinforcement with carbon fibers and with the NVT 201E and S&P 220, the beam was dimensioned so that the break would happen outside the middle third, where the shear has maximum value. Support was provided by an IPRJ laboratory technician, so that the support points are spaced 120 mm apart, as shown in figure 5.  The reinforcement was applied following the manufacturers technical recommendations and the mixture was done manually. The structural adhesive NVT Pipe Fix ® had a mix composition by weight of 1.9: 1.0 and the adhesive S&P 220® had a mix composition by volume of 4.0: 1.0. The lamination was done directly on the beams, and the curing time of 24 hours was waited with the reinforcement already applied on the beams as specified by the manufacturers.
In the 4-point bending test the central third of the beam is composed of pure bending, whereas in the external thirds there is flexion-shear, as shown in figure 7. The beams were designed with the same parameters for bending, and without shear reinforcement, therefore, the reference beams (which will not be reinforced with laminated composite material) will resist shearing only with the concrete portion and the longitudinal reinforcement portion. Statistical analysis will be made regarding the influence of reinforcements from the breaking load of the beams. Bending tests of 4 points were carried out on 5 reference beams (without shear reinforcement), on 3 shear reinforced beams using NVT 201E and on 3 shear reinforced beams using S&P 220. Breaking loads are shown in Table 2. The reference test bodies showed an average shear strength of 3.28 tf.
Those reinforced with the NVT 201E adhesive showed an average resistance of 5.22 tf and those reinforced with the S&P 220 adhesive showed an average resistance of 4.49 tf. The beams that used the structural adhesive NVT 201E obtained resistance 59.15% higher than the reference beams and 16.26% higher in relation to beams reinforced with structural adhesive S&P 220. The S&P 220 is the adhesive qualified for use on concrete substrates and widespread in the market. Figure 8 illustrates the test and some results of the test performed at LEFI, such as the arrangement of the test bodies and the way of breaking the reference samples and the reinforced ones.

CONCLUSIONS
Beams reinforced with structural adhesive NVT 201E obtained results superior to those obtained in beams reinforced with adhesive S&P 220, which is already established in the market and qualified by regulatory agencies as structural adhesive for use on concrete substrate. In addition to greater resistance in the 4-point bending test, the beams reinforced with NVT 201E showed a lower detachment rate of the reinforcement at the moment of rupture than those reinforced with the adhesive used for comparison. These results favorable to NVT 201E suggest that the adhesive can also be used to repairs in