Effects of NSM CFRP Bars in Shear Strengthening of Concrete Members

1
Effects of NSM CFRP Bars in Shear Strengthening
of Concrete Members AKM Anwarul Islam, Ph.D.,
P.E., Assistant Professor, Youngstown State
University, Civil/Environmental Chemical
Engineering, One University Plaza, Youngstown, OH
44555, E-mail aaislam_at_ysu.edu.
Figures and Tables

• Abstract
• Investigated shear strength contribution of near
  surface mounted (NSM) carbon fiber reinforced
  polymer (CFRP) bars in concrete members.
• Four beams were casted three beams were
  strengthened in shear with CFRP bars using NSM
  technique. Strain gages were attached at critical
  points of steel reinforcement, concrete and CFRP
  bars.
• Beams were loaded until failure, and strains were
  recorded using data acquisition system.
• Data analyses revealed that
• NSM technique is very effective with no
  occurrence of delamination, debonding or
  fracture.
• A new formula has been proposed to calculate
  nominal shear strength provided by NSM CFRP bars.

• Test Results and Discussions
• Shear steel almost reached its ultimate strain
  before failure, whereas effective strains in the
  CFRP bars are within a range of 30 to 35 of its
  ultimate strain before shear failure of the
  beams.
• All four beams failed in shear, as seen in Fig.
  E.
• As a result of using the NSM technique for
  strengthening concrete beams in shear, there is a
  significant increase in shear strength of the
  concrete members, which ranges from 17 to almost
  25 of its original strength according to the
  data shown in Table 2.
• The load versus CFRP strain relationship for the
  three model beams is shown Fig. F. The maximum
  failure loads for Beam 2, 3, and 4 were recorded
  as 454, 427 and 436 kN, as shown in Table 2.
• The ratio of effective strain to ultimate strain
  of CFRP bars in Beams 2, 3 and 4 are 32.8, 35.3
  and 30.6, respectively, as shown in Table 2. On
  average at failure, almost one-third of the
  ultimate strain becomes effective in the CFRP
  bars attached with the concrete beams following
  the NSM technique. Therefore, the following
  formula in Eq. 1 is being proposed to calculate
  the nominal shear strength provided by NSM CFRP
  bars used in shear strengthening of concrete
  members.
• Vf (Affyfd)/(3s) 1
• where Af cross-sectional area of FRP bars, fyf
  tensile yield strength of FRP bars, d effective
  depth, s spacing of FRP bars. Comparison of
  effective and proposed shear strengths is shown
  in Table 3.

B. Plan and cross-section of model beam.
A. Beam dimensions and loading configuration.
C. Finished beam with strain gages attached.

• Introduction and Background
• FRP systems have been used in the United States
  for more than 20 years. Researchers have found
  flexural strengthening with FRP materials very
  effective. Not enough data are available on FRP
  shear strengthening using NSM technique. The
  proposed method and formula are expected to
  provide important guidance in developing design
  criteria.
• Model Beam
• Beam 1 was control beam without CFRP bars
  attached. Beam 2, 3, and 4 were model beams
  strengthened with CFRP bars using NSM.
• Model beam design and loading configurations are
  shown in Figs. A and B. Beams were 7 ft long with
  cross-section of 10X12, and 46 bars at bottom
  and 26 bars at top.
• Figure B also shows NSM technique of attaching
  CFRP bars to the beams. Concrete strength was
  7200 psi, which was determined by taking average
  compressive strength of 3 cylinders tested in the
  lab.
• Figure C shows finished beams with strain gages
  attached in the CFRP bars, steel bars and
  concrete at critical points. Beams were designed
  in such a way that it fails in shear before it
  fails in flexure. Table 1 shows mechanical
  properties of No. 10 ASLAN CFRP bars.
• Figure D shows beam 1 under four-point bending
  testing setup. Figure D shows all four beams
  after failure.

F. Load versus vertical strain in CFRP bars.
D. Beam under four-point bending.

• Conclusions and Recommendations
• As a result of strengthening the concrete beams
  in shear using NSM CFRP bars, increase of shear
  strength has been found in the range of 17 to
  25. The average gain in shear strength was
  calculated to be more than 20, which is
  significant in strengthening of shear-deficient
  concrete members.
• No delamination, debonding or fracture of FRP
  materials was observed.
• Although the proposed formula is not supported
  by a large number of test samples, it is expected
  to provide important guidelines for the future
  researchers to refine the equation, if necessary.

E. Beams after failure.
Table 1 Mechanical properties of No. 10 CFRP
bars
G. Load versus concrete strain.
Table 3 Comparison of effective and proposed
shear strength provided by CFRP bars
Table 2 Analysis of Experimental data
References (Partial List) ACI Committee 440
(2002). Guide for the Design and Construction of
Externally Bonded FRP Systems for Strengthening
Concrete Structures, American Concrete
Institute, Farmington Hills, Michigan.   ACI
Committee 318-05 (2005). Building Code
Requirements for Structural Concrete (ACI 318-05)
and Commentary (ACI 318R-05), American Concrete
Institute, Farmington Hills, Michigan.