Keywords: reinforced concrete, fibre reinforced polymer, carbon FRP, steel, bond, ultimate moment, failure mode, deflection.

This paper presents the details of the flexural testing of concrete beams reinforced with steel and carbon fibre reinforced polymer (CFRP) tension bars. Results of duplicate beams with each type of reinforcement are discussed. Each of these beams was 2 m long. The cross section was 120 x 200 mm. A 20 mm clear cover was used for these beams. Steel reinforced beams were used as control specimens. The beams were designed according to ACI code [1,2]. Steel reinforced beams were under-reinforced beams whereas FRP reinforced beams were designed over-reinforced. Two 10 mm diameter high strength deformed bars were used on the tension face in steel reinforced beams. FRP beams were reinforced with 2-9.5 mm diameter CFRP tension rods. Top reinforcement consisted of 2-8 mm diameter high strength deformed steel rebars for all beams. Shear reinforcement comprised of closed rings of 6 mm diameter plain bars, which were spaced at 100 mm centre in both type of beams.

All of these steel bars were tested in the laboratory to determine their mechanical properties. Results of tensile tests on CFRP bars were provided by the manufacturer. Cubes of 100 mm size were tested at 28 days and on the day of testing of beams to find out compressive strength of concrete. Equivalent cylindrical strength of concrete after 28 days came out to be 43.31 MPa [3], which was determined from the average strength of four cubes.

The deformation and slippage of bars was measured with the help of strain gauges, which were bonded to one of the bars in each beam at various locations. Linear variable differential transducers (LVDTs) were used to measure the midspan deflection of the beam. These LVDTs were positioned on both sides of the longitudinal centre of the beam. The slippage of the CFRP bars was also measured at the bar end with the help of horizontal LVDTs. A continuous record of load, deflection, slip and strain was taken with the help of computer aided data acquisition systems.

The beams were tested as simply supported over a span length of 1750 mm under four-point load. The loads were 400 mm apart giving a shear span of 675 mm. The load was applied with the help of a stiff spreader beam in small increments of 2.5 kN and 5 kN for the steel and the CFRP reinforced beams, respectively. The load was applied with a hydraulic jack and was controlled manually. The applied load was measured by a 200 kN load cell. All beams were tested to failure. For all tests the load was removed after the applied load dropped substantially below the ultimate load. The time taken for each beam for complete failure was almost same (about 1 hour).

The plot of strain results showed a good and consistent bond of the CFRP rods with concrete similar to steel bars. The provided development length of the CFRP bars was 790 mm, which was slightly smaller than the length of 860 mm recommended by the ACI code [2]. However, so signs of any premature bond failure were found. The results of slip of bars as measured by horizontal LVDTs also supported this observation. The CFRP bars developed nearly 80 to 90 % of their tensile strength.

Yielding of steel bars caused failure in steel reinforced beams. The beam reinforced with the CFRP rods failed by concrete crushing. Both of these modes were intended in their design based on code equations [1,2]. The CFRP reinforced beams carried twice as much load as steel reinforced beams. The failure load on the steel beams was accurately predicted by the code [1]. The ACI code [2], however, under-estimated the capacity of CFRP reinforced beams. The possible reason is the increase in the concrete strain beyond assumed value of 0.003. CFRP reinforced beams were less stiff than steel reinforced beams. However, the deflection of CFRP reinforced beams was found satisfactory at service load corresponding to the theoretical capacity of beams. The load carrying capacity of the CFRP reinforced beams dropped gradually after the crushing of concrete, which is an indication of a ductile failure mode and a kind of energy dissipation mechanism.

1

American Concrete Institute, "Building code requirements for structural concrete, ACI 318-95", Detroit, Michigan, 1995.

2

American Concrete Institute, "Guide for the design and construction of concrete reinforced with FRP Bars, ACI 440.1R-01", Detroit, Michigan, 2001.

3

A.M. Neville, "Properties of concrete", Longman Scientific & Technical, England, 1981.

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