1. Flexural Behaviour of Concrete Beams Reinforced with GFRP
rebars: Design philosophy
Davath Bharath Kumar
Department of Civil Engineering, VJIT, Hyderabad, India
2. Design philosophy of Structural Concrete Reinforced with
GFRP Bars (ACI 440)
• The mechanical behavior of FRP reinforcement differs from the behavior of conventional steel
reinforcement
• Accordingly, a change in the traditional design philosophy of concrete structures is needed for FRP
reinforcement.
• Fiber-reinforced polymer materials are anisotropic and are characterized by high tensile strength only
in the direction of the reinforcing fibers. This anisotropic behavior affects the shear strength and dowel
action of FRP bars as well as the bond performance.
• Furthermore, FRP materials do not yield; rather, they are elastic until failure.
• Design procedures should account for a lack of ductility in structural concrete members reinforced
with FRP bars. Initially, GFRP bars were considered a viable alternative to steel as reinforcement for
polymer concrete because their use eliminated the need to address the incompatibility of thermal
expansion characteristics between polymer concrete and steel.
3. Flexural design philosophy
• Steel-reinforced concrete sections are commonly designed to ensure tension-controlled
behavior exhibited by yielding of steel before the crushing of concrete.
• The yielding of the steel provides ductility and a warning of failure of the member.
• The nonductile behavior of FRP reinforcement necessitates a reconsideration of this
approach. If FRP reinforcement ruptures, failure of the member is sudden and
catastrophic, however, there would be limited warning of impending failure in the form of
extensive cracking and large deflection due to the significant elastic elongation that FRP
reinforcement experiences before rupture.
• In any case, the member would not exhibit ductility as is commonly observed for tension-
controlled concrete beams reinforced with steel reinforcing bars, in which the bars exhibit
plastic deformation prior to concrete crushing.
• Compression-controlled behavior is marginally more desirable for flexural members
reinforced with FRP bars.
• By experiencing concrete crushing prior to tensile rupture of the FRP reinforcement, a
flexural member does exhibit some inelastic behavior before failure
4. • In conclusion, both compression- and tension-controlled sections are acceptable in the
design of flexural members reinforced with FRP bars, provided that strength and
serviceability criteria are satisfied.
• To compensate for the lack of ductility, the member should possess a higher reserve of
strength. The margin of safety suggested by this guide against failure is therefore higher
than that used in traditional steel-reinforced concrete design.
• The use of high-strength concrete allows for better use of the high-strength properties of
FRP bars and can increase the stiffness of the cracked section, but the brittleness of high-
strength concrete, as compared with normal-strength concrete, can reduce the overall
deformability of the flexural member.
• For the tension-controlled GFRP-reinforced section, the concrete dimensions are larger
than for the other beams to attain the same design capacity. The contribution of FRP
reinforcement should be neglected when used as reinforcement in columns, in compression
members, or as compression reinforcement in flexural members.
• It is acceptable for FRP tension reinforcement to experience compression due to moment
reversals or changes in load pattern.
5. • Because the elasticity of FRP bars is linear up to failure and the strain at rupture is low,
the flexural failure mode of FRP-reinforced concrete (FRP-RC) beams is brittle rather
than ductile.
• For FRP-RC members, compression failure by concrete crushing, which provides various
warnings prior to failure, is a suitable failure mode. In other words, in contrast to the
common design practice for steel-reinforced concrete (steel-RC) beams, over-reinforced
designs are preferable to under-reinforced designs for FRP-RC beams.
• Moreover, becasue of the low stiffness of FRP bars, FRP-RC members exhibit larger
deflections and wider cracks than do steel-RC members. These factors limit the range of
application of FRPs.
7. Limitations of using GFRP bars
• Fiber-reinforced polymer reinforcement has a high tensile strength, significant
elongation, and exhibits linear stress-strain behavior to failure.
• The use of FRP reinforcement should be limited to structures that will
significantly benefit from other properties, such as the noncorrosive or
nonconductive behavior of its materials.
• Due to lack of experience in its use, FRP reinforcement is not recommended for
moment frames or zones where moment redistribution is required.
8. Design philosophy of Structural Concrete Reinforced with GFRP Bars (ACI 440)
There are two situations when a reinforced beam fails due to bending. One is when the steel reaches its yield stress ‘f'y’ and
the other is when the concrete reaches its maximum compressive stress ‘fc’’. If the reinforced concrete beam fails by steel
yielding then the failure is termed as ‘ductile’ which means that steel will stretch for a long period of time before it actually
breaks. If the concrete fails first then the failure is termed as ‘brittle’ because the concrete reaches the maximum strain first.
Balanced section is where the concrete reaches its maximum strain at the same time steel reaches its yielding stress.
1. Maximum allowable strain of 0.003 is adopted as safe limiting value in concrete.
2. Concrete stress of 0.85fc’ is uniformly distributed over an equivalent compressive zone, where fc’ = Specified
compressive strength of concrete in psi.
3. The tensile strain for the balanced section is fy/Es
9. a=depth of the equivalent rectangular stress block
ab=depth of the equivalent rectangular stress block for balanced section
c=depth of the neutral axis
cb=depth of the neutral axis for balanced section
f'y = yield stress of steel
fc’= maximum compressive stress
b=width of the section
d=effective depth of the section
As= Area of reinforcement provided
Asb = Area of reinforcement provided for balanced section
=reinforcement ratio
10. b = reinforcement ratio for balanced section
Total compressive force C = 0.85fc’*b*a
Total Tensile force T = As*fy
C = T
0.85fc’*b*a = As*fy
a = As*fy / (0.85fc’*b)= *d*fy / (0.85fc’)
reinforcement ratio = As / b*d
Moment of Resistance, Mn = 0.85fc’*b*a*(d – a/2)
or Mn = As*fy*(d – a/2)
= *b*d*fy [ d – (*d*fy/1.7fc’) ]
= *fc’ [ 1 – 0.59 ] bd2
= (*fy) / fc’
Nominal strength Mn = Kn b*d2
Kn = fc’[ 1 – 0.59 ]
Design strength Mu = Mn = Kn bd2 = Strength Reduction Factor
Recommended Value of Strength Reduction Factor is:
1) Beams in Flexure=0.90 and 2) Beams in Shear and Torsion =0.85
The design strength of a member refers to the nominal strength calculated in accordance with the requirements stipulated in th
multiplied by a Strength Reduction Factor , which is always less than 1.
Strength Reduction Factor is for:
1. To allow for the probability of understrength members due to variation in material strengths and dimensions
2. To allow for inaccuracies in the design equations
3. To reflect the degree of ductility and required reliability of the member under the load effects being considered.
4. To reflect the importance of the member in the structure
