Fracture toughness of RC beams on the shear, strengthening by FRCM system

. This research paper present the experimental study of reinforce concrete beams strengthened by FRCM system in the shear area. All samples are without transverse reinforcement at the shear distance. First beams tested as control sample, with shear distance a/d=2. Another three beams tested with strengthening by FRCM system. Variable parameter is the level of initial load, before strengthening. The initial load values were 0, 0.3, 0.5 from carrying capacity of control samples. All RC beams were designed to fail in shear, even strengthened samples. In the result of this research we determined that fracture toughness of RC beams increases with a decrease of span to the effective depth ratio. Results are comparable with the increasing of bearing capacity. The first inclined crack opend in mid-height cross section at the load 50 kN at an angle equal to 45 0 and it does not depend on the shear span. The maximum width of inclined cracks decrease with decreasing of shear span. The maximum width of inclined cracks decrease with decreasing of the shear span. The maximum values of width of inclined cracks are fixed at the value of 0.55 - 0.85 mm. Using the limit crack opening width as criteria of exhaustion of serviceability (SLS) we saw that about 16 - 29% of bearing capacity still remains before the danger of shear failure of the beam.


Introduction
In influence of time, environment and different factors most structures need repairing and strengthening. There are a lot of different type of strengthening. Some of them are jacketing with self-consolidating concrete [1], application of steel plates for and others.
Most popular materials nowadays are different composites called FRP (Fiber Reinforce polymer) or FRCM (Fiber Reinforce Cement Polymers) [2].
There are interest for using different polymer plates for shear strengthening of RC beams by many researchers [3,4]. There are some works with different computer modeling of shear crack [5].
The goal of this article is to experimentally determine shear strength and serviceability of reinforced concrete beams strengthened with FRCM. Main variable in the experiment was the initial load applied to the beam before strengthening.

The testing program
Four reinforced concrete beams were tested. All samples are without transverse reinforcement at the shear distance. First beams tested as control sample, with shear distance a/d=2. Another three beams tested with strengthening by FRCM system. Variable parameter is level of initial load, before strengthening. Initial load was 0, 0.3, 0.5 from carrying capacity of control samples. All RC beams were designed to fail in shear, even strengthened samples. For the research, a technique has been developed for testing each beam twice, which was described in our article [8].
Tested beams had 2100 mm length, 100 mm width, and 200 mm height of cross section. As the beam's tension reinforcement, A400C Ø18 mm rebar was chosen, A400C Ø10 mm -as compressed reinforcement. Transverse reinforcement -A240C Ø 6 mm rebar located in the area without shear force [9].
Concrete class of tested beams was C32/40. According to the testing program, three specimens were strengthened by reinforcing FRCM system with 70 mm wide strips ( Fig. 1).  Beams marking were adopted as follows: BO -beam ordinary, BSC -beam strengthened by composite material; the first digit -serial number, the second digit -sample number and the third digit -the section number. Index 0 -0.5 means the loading level under which the strengthening was performed and depends on shear strength (V Ed ) of non-strengthened control sample. For example, BSC 1.2.2-0.3 means that the tested sample from the first series of the second beam of the second section, with initial load 0.3·V Ed .

The overview of shear failure of beams
For the RC beam, the limit value of concrete compressed deformation above the inclined crack was taken as the criterion of shear strength (ULS) [Błąd! Nie można odnaleźć źródła odwołania.]. Achievement of limit deformations in transverse reinforcement can not be taken as a criterion for the exhaustion of bearing capacity, because there is a cut of the rebar, which leads to the onset of the yield strength before approaching the limit tensile deformations of the reinforcement.
The bearing capacity of reinforced concrete beams in shear was for BO 1.    Exhaustion of the bearing capacity in shear occurred at the time of the destruction of the compressed concrete zone with a sharp extension of the FRCM tape and damage to the protective layer of the FRCM system in the zone of inclined cracks.

Fracture toughness in testing samples
For detailed information about fracture toughness, we measured the width of the crack opening, depth of distribution and location. Width of crack were measured by microscope MBP-3, with accuracy 0.05 mm. Limit width of crack w max =0.4mm [7,10] opened on the level V Ed =80kN for sample BO 1.1. BSC 1.1-0 increased serviceability 1.38 times, BSC 1.2-0.3 -1.25 times, and BSC 1.3-0.5 -1.13 times. Rapidly decreas of serviceability for BSC 1.3-0.5 caused by strengthening after opening of the first inclined crack.
The boundary values of the crack opening width are fixed at 78 -84% of the load carrying capacity of the samples (Table 3).
Using the limit crack opening width as criteria of exhaustion of the serviceability (SLS) we determined that the beam still has 16 -22% reserve of bearing capacity, before the danger of shear failure of the beam. Bearing capacity reserve for all samples are in one range and are close to the value of control beam.  Maximum crack opening in FRCM strengthening system are 2 times less than crack width in the concrete for beam BSC 1.1-0. In this sample maximum level of strengthening, increased serviceability were achived, so the maximum width of inclined crack in FRCM system and concrete is at the shear distance. For another two beams, width of crack opening is on the same level, lower than for BSC 1.1-0.