SHEAR BEHAVIOR OF REINFORCED CONCRETE BEAMS STRENGTHENED BY CFRP STRIPS

In this research, twelve normal reinforced concrete beams are used with dimensions of 110Χ10Χ20 cm, the compressive strength for all specimens is 30 MPa. Deformed bars 2Ø12mm with Ø6mm were used for longitudinal reinforcement, while for transverse reinforcement deformed bars with 5cm, 10cm and 15cm spacing were used. The specimens were tied by different shapes of CFRP strips (tied, inclined and X-shaped). All specimens were tested with two points load by a hydraulic machine to determine the modes of failure, maximum load and deflection. Specimens without CFRP strips were also tested and compared with other specimens. Specimens with the X-shaped strips shows 70% increment in Pu and decrement in maximum deflection of 39%. The addition of CFRP strips as a tying material helps in improving the strength and bearing strength of concrete.


INTRODUCTION
Reinforced Concrete (RC) henceforth is one of the popular materials used in buildings all around the world. Structures like buildings and bridges uses, as their chief construction material, reinforced concrete. Some or parts of such structures are not fulfilling their structural functions because of some defects on the concrete caused by the poor construction practices, corrosion damage, fire damage, accidental damage, or deterioration caused by environmental action. Some reinforced concrete structures need to be improved as the design and construction faults and load increment or damage induced to the structural members by a seismic or other action. In addition, increasing in traffic volume may result in urgent need for bridges improvement. The replacing of deficient structures requires enormous investments and this would not be a desirable option. Therefore, strengthening becomes more suitable way to improve load-carrying capacity of concrete and prolonging their service life. Despite the effectiveness of other techniques that are widely accepted, engineers succeed in developing a new, better and most promising technique. This technique depends on using carbon fiber reinforced polymer (CFRP) which is more beneficial and gained popularity worldwide. FRP is listed as one of the fruitful technique which is currently used by structural engineers as a new and promising material in construction industry [1]. Khalifa and Nanni (2002) [2] state that the shear strength of beams is increased by CFRP composite in the experimental investigations which are conducted on twelve full-scale reinforced concrete where simply supported beams are designed to fail in shear. Buyukozturk et. al. (2003) [3] report that the failure pattern that raises concerns is the sudden brittle manner where the CFRP plate deboned prior to ultimate failure. Therefore, such failure pattern needs further critical and close examination.
In this study, a comparison of CFRP wrapped beams with the conventional reinforced concrete beam is focused upon.

Carbon fiber reinforced polymer (CFRP)
Carbon Fiber Reinforced Polymer (CFRP) materials have become popular materials for reinforced concrete (RC) structures strengthening in flexural and shear. Several advantages of using CFRP are found. Some of these are the externally bonded material over the conventional steel, high strength to weight ratio, outstanding durability in a variety of environment, ease and speed of installation, flexibility in application techniques, electromagnetically neutral, outstanding fatigue property and low thermal conductivity. CFRP plates or strips can be bonded to the reinforced concrete members' exterior by using wet lay-up procedures with an epoxy resin/adhesive. CFRP strips or plates are either bonded to tension faces of flexural elements for increasing their bending capacity, or bonded to their side faces for increasing the capacity of shear. Existing reinforced concrete (RC) beams Strengthening is required for numerous reasons for example; heavier loads design, and restoration of the capacity due to deterioration or due to erosion design and construction [4]. Carbon fiber reinforced polymer is a widespread technique used for shear strengthening and it has been applied to many structural elements like beams, columns, and slabs etc. This research focuses upon CFRP consisting of flexible strips where carbon fiber reinforced polymer is strong and light fiber reinforced plastic contains carbon fibers. This can be used where ever we require high strength and rigidity. Furthermore, these CFRP strips are bonded by using epoxy adhesive [5].

Materials
In this study, the performance and structural behavior of the specimens have been checked by using the materials: A. Cement The ordinary Portland cement OPC type I is used for entire work which is from Bazyan, Al-Sulaimaniya, Iraq.

B. Fine aggregates
The river sand particles are used as the fine aggregates with fineness modulus (2.6) and specific gravity 2.63. The fine aggregates has a maximum size less than (5mm) and this conforms to the British Standards (BS 882-1992) [6] as shown in Table 1 and Figure 1    using epoxy adhesive. Figure 3 below shows the type of epoxy named Sikadur 530 that is used in the study. The mix proportion of Epoxy resin (1:4) and time of lefting the Epoxy resin to dry on the specimen is 7 days according to the manufacturer. F. Steel Reinforcement In this study the longitudinal steel is deformed bars 2Ø12mm for tension stresses and 2Ø4mm for compression stresses and for bonding with transverse reinforcement which is deformed bars Ø6mm for different spacing 5cm, 10cm and 15cm as shown in Figure 4 below:

MIX DESIGN
Normal concrete beams of (110*10*20) cm has been made using confident proportions of cement, fine aggregate, coarse aggregate and water. In the program of testing, the compressive strength of concrete is kept constant which it is 30 MPa. The mix proportion is (1:2:3) by weight for cement, sand and gravel respectively with w/c ratio of (0.35). All specimens are casted in wooded mold as shown in Figure 5 and totally vibrated on a vibrating table to reduce the air content. The vibration time to reach full compaction is certain upon by the end of air bubbles passage from concrete fresh state. The specimens are then cast into three layers, in which 25-30 seconds are required for compaction per layer and left for curing for about 28 days. The beam is tested by a hydraulic universal machine of (3000 kN) capacity. This machine compressed the beam by two points load as shown in Figure 6. The beams are classified into four groups according to the shape of CFRP strips as it will explain later in Table  3 then wrapping with CFRP strips of 5 cm width of strips with 10 cm spacing between strips as shown in Figure 7. After that, the specimens left for 7 days for adhesion with epoxy according to the manufacturer.

TEST RESULTS
As mentioned before, the main aim of this research is to study the shear behavior of 12 beams are designed with various shapes of CFRP strips such as ( ties, inclined and X-shape) and compared them with the reference beam which does not have CFRP strips as shown below:

Ultimate loads and cracking loads
The ultimate load is the maximum load of the member which reaches to failure, while Cracking load is defined as the load which is the first visible surface cracks on the surfaces of the member. Table 4 and Figures (8-11) show the values of these loads. From Table 4  10cm-X-shaped CFRP Beam with X-shaped CFRP strips and 10 cm spacing between transverse reinforcement (ties reinforcement)

5cm-X-shaped CFRP
Beam with X-shaped CFRP strips and 5 cm spacing between transverse reinforcement (ties reinforcement)   of X-shaped strips technique. Also, the average increment of Pu is 14%, and 11% for decreasing the spacing between ties reinforcement by 5 cm. the Figures (8-11) shows these results.

Load-Deflection Curves
The load-deflection curves in this work are taken at center of all the tested beams.   15cm-X-shaped CFRP 100 5.5 10cm-X-shaped CFRP 111.5 4.7 5cm-X-shaped CFRP 122 4.22   (12-14) show the load-deflection curves of specimens. As shown in Table 5, the average decrement of maximum deflection for specimens was 14%, 22% and 39% for ties, inclined and the X-shaped strips respectively. Also, the average decrement of maximum deflection is 10% and 8% for decreasing the spacing between ties reinforcement by 5cm. The Figures from 12 to 14 show the maximum deflection values.

Crack patterns and modes of failure
There are two modes of failure the first one is flexural failure, this failure mode occurs when the loads on the beam exceed its flexural capacity. The shear strength of the beam should be greater than its flexural strength otherwise the shear failure would occur before flexural failure. The second one is the shear failure, this failure occurs when the beam has shear resistance lower than flexural strength and the shear force exceeds the shear capacity of different materials of the beam. This type of failure is sudden and provides no warning i.e. brittle failure. This mode of failure is common in beams with low or no web reinforcement. Beams, examined here, are designed with deformed bars of Ф5@(5, 10 and 15) cm spacing as ties reinforcement. These beams fail by punching or flexure according to the reinforcement and the applied load. Steel reinforcement    tied well together to ensure that no bond failure between steel bars and surrounding concrete can take place. Table 6 and Figure 15 show the maximum cracks for each beam. Also, Figures (16-19) show the modes of failure for each beam. From the Figures above, the all three beams without wrapping CFRP strips performed modes of diagonal shear failure only, and the width of cracks of beam failure which has 15cm spacing is bigger and clearly shown than 10 and 5cm. When the beams are wrapped with ties CFRP strips, the failure varies according to the spacing between transverse reinforcement as shown in Figures. In 15 cm spacing, the failure is diagonal shear with appeared small cracks of flexural failure. While the spacing is 10 cm, the beam tends to fail shear and flexural but with thinner width cracks. Finally in 5 cm spacing between ties reinforcement, the beam shows better performance of failure with diagonal shear failure only with very thin crack width. When the beam wrapped with inclined CFRP strips, as shown in figures above, the beam tends to control the cracks generally with shear failure only especially at 15 cm and 10 cm spacing. While the beam with 5 cm spacing, the failure is flexural only and the beam control the shear failure so, it is better than the beams with 10 cm and 15 cm spacing between ties reinforcement. As a result, the beams with inclined wrapped CFRP strips tend to show performance of failure better than tied strips because they are tied against the shear failure.
The Figures above show that the excellent behavior of beam against failure is the X-shaped CFRP wrapping, it could control the patterns and crack width of shear and flexural failure for the beams to minimum. The beam has the higher values of strength and the failure is only at edges with very thin crack width as the spacing between transverse reinforcement decreased.

CONCLUSION
Based on the experimental results of all 12 beams subjected to two-point loads, the following conclusions are