Shear Deformability of Reinforced Concrete Beams Strengthened with the FRCM System

Research measuring the residual bearing capacity of reinforced concrete elements has been developed in response to growing tendencies in cost minimization during reconstruction. This is particularly true of industrial structures, where reinforced concrete components are frequently utilized and whose application varies depending on market demands. Therefore, it is necessary to modify existing buildings and structures in order to reduce expenses while implementing new jobs.

In the article [1], the necessity of evaluating the technical state of building structures and figuring out the remaining bearing capacity of damaged flexural reinforcing concrete elements is taken into consideration. It is emphasized how difficult it is to calculate damaged components as a result of numerous flaws and damage and how composite reinforced concrete is. The article [2] provides a case study of the cost-effectiveness of various combinations of concrete class, rebar type, and reinforcing ratio in terms of the dependability of prestressed reinforced concrete structures according to the shear bearing capacity.

A non-destructive method of acoustic emission [4] may be used to examine such structures in non-laboratory conditions. Another example is the combination of a traditional reinforcing frame and dispersed reinforcement of concrete with steel fiber with the variation of the volumetric distribution of steel fiber [3]. In the work [5], the fundamental rules for calculating the flexural elements made of fiber concrete and reinforced concrete in relation to bending moments are discussed.

The article [6] discusses the significance of researching how damage affects the bearing capacity of reinforcing concrete parts in the building sector. The example study in the paper [7] illustrates how the unpredictability of the vibration load impacts bearing capacity and deformability. The analysis entails assessing structural damage brought on by machine operation as well as measuring the actual vibration level.

The article [8] presents the findings of an experimental study of reinforced concrete beams damaged in the compression zone. The main focus of attention is on how concrete degradation affects the beams’ capacity to support themselves.

The impact of varying reinforcement damage on bent parts of rectangular sections is a topic covered in the article [9]. In these studies, reinforced concrete beams with tensile reinforcement damage are experimentally examined to determine the effects of introducing an initial load. The distinctive qualities of the reinforcement’s construction must also be considered when analyzing the consequences of corrosion; in work [10], this effect is taken into account when using thermally enhanced reinforcement. The examination into T-shaped reinforced concrete beam damage is described in [11]. The shear span, the ratio between the thickness of the overhang and the working height of the beam section, the reinforcement ratio for shear reinforcement, and the degree of pre-stress in the tensile rebar were all taken into consideration. In accordance with the physics of reinforced concrete, it is advisable to think of the grid as a continuous reinforced concrete beam [12]. Consider the work in [13], which illustrates the use of FRCM reinforced with a composite system, as a method for reinforcing weak beams. Although prestressed bending sections also have a similar vital importance as discussed in the article [14], the issue of high-quality suitable work of diverse materials is particularly crucial for reinforcements built of composite materials. Due to the fact that reinforced concrete is a composite material, it is crucial to take into account both its material properties and crack resistance, especially when reinforcement is utilized. The relationship between crack resistance and load type was looked at in [15].

The purpose of this paper is to research parameters of the strength and deformability of reinforced concrete beams without transverse reinforcement, strengthened by the FRCM system under different loading levels.

To realize the aim, four experimental samples were designed and manufactured, with cross-sectional dimensions of 200x100 mm and a length of 2100 mm. The working tensile reinforcement is adopted class A400C Ø18 mm, compressed reinforcement - A400C Ø10 mm. There is no transverse reinforcement in the zone of action of the shear force. The estimated beam span is 1900 mm.

Beams are labeled, according to the following type: BС - control beam or BSC - beam strengthened with composite material; the first digit is the serial number, the second digit is the test sample number, and the third digit is the tested support area. For example, BC 1.2–2 means that the second support area of the second beam from the first series was tested. The index 0…0.5 means the level at which strengthening was performed, taken from the obtained destructive one, from control beams data.

The beams are designed in such a way that even after strengthening the support area, the failure occurs due to the shear force. None of the samples is destroyed by the bending moment. During the research, each sample was tested twice - each support area separately [13].

According to the research program, reinforced concrete beams were reinforced by gluing P.B.O. fabrics in the form of vertical strips with a width of 70 mm, for the possibility of observing the concrete strains in the support area. Samples BSC 1.1-0 were reinforced without initial load; beams BSC 1.2-0.3 and BSC 1.3-0.5 were strengthened at the level of the initial load equal to 0.3 and 0.5 from the destructive one determined by experimental testing of control samples. The criterion for the loss bearing capacity was adopted similar to that for unreinforced samples: the exhaustion of the bearing capacity on the shear was equated to the physical destruction of the compressed concrete zone above the top of the diagonal crack. The destruction of a reinforced concrete beam on the shear strengthened with a composite system occurred in the following sequence:

When the load is further increased, the ends of the fabric are completely peeled off and its anchoring is disturbed.

Exhaustion of the bearing capacity occurred at the moment of exfoliation of the concrete compressed zone, together with a sharp elongation of the fabric tape and damage to the protective layer of the FRCM system in the area of propagation of the diagonal crack (Fig. 1).

The bearing capacity for the action of the transverse force was: for the sample BSC 1.1 – V_Ed = 137.5 kN, for the beam BSC 1.2-0.3; V_Ed = 120 kN and for BOD 1.3-0.5 - V_Ed = 110 kN.

At the same time, the nature of the failure has changed for the strengthened samples: the beam loses its bearing capacity more plastically, there is no fallout of concrete particles and no visible plastic deformation of the reinforcing frame. The deformation distribution, which is shown on the isopoles (Fig. 2), indicates the distribution of tensile forces over a larger area of the support area.

For the sample strengthened without initial load, stress concentration occurred at the half height of the support area, as in the unstrengthened sample. Therefore, this is not characteristic of the strengthened samples under the action of the load. This deformations distribution is caused by a more effective inclusion of the tape in the work, during strengthening without the action of the load. For other samples, strengthening was performed in the presence of significant tensile deformations in the element, which led to a change in the distribution of forces in the section.

Concrete tensile deformations, together with the opening width of diagonal cracks, are similar in nature to those of the control samples but reach significantly higher values (Fig. 3).

For the beam BSC 1.3-0.5, strengthening was performed already after the opening of the diagonal crack, as evidenced by the rapid increase in deformations without increasing the load. Tensile deformations reach their maximum values for the beam BSC 1.1-0 and decrease by the decrease in the bearing capacity on the shear.

The deformations of the strengthening system were measured in the longitudinal direction - the direction of placement of working fibers. The strain graph of the fibers of the reinforcement tape is shown in Fig. 4.

The maximum deformations reach, is 57% of the ultimate elongation, for the BSC 1.1-0 beam. This is a very high indicator of the use of reinforcing tape. With a change in the load level, the maximum deformations of the reinforcing tape also change and amount to 26% for the beam BSC 1.2-0.3 and 43% for the beam BSC 1.3-0.5. Strains of the strengthening element of the beam BSC 1.2-0.3 showed the lowest values, which is associated with the strengthened at the onset of the ultimate tensile deformations of concrete, and the inclusion of the strengthened element in the operation before the opening of the diagonal crack. During the exhaustion of the bearing capacity on the shear, the strengthening fabric received significant deformations, which led to the loss of its initial length, but the fabric rupture was not observed.

According to [16], the use of strengthening tape is recommended to be designed at the level of 40% of its ultimate elongation strain, based on which we can conclude - according to experimental data, this type of strengthening is also an effective way of using high physical and mechanical characteristics of the composite material when reinforced beams on the shear.

Based on the above, the following conclusions can be drawn: