Bond Performance of Sanded Surface and
Threaded Smooth Gfrp Bars
Antony Kodsy1* and George Morcous2*
1Graduate Research Assistant, University of Nebraska, USA
2Professor of Construction Engineering, University of Nebraska, USA
Submission: June 11, 2019; Published: June 28, 2019
*Corresponding Author: Antony Kodsy, Graduate Research Assistant, University of Nebraska, 1110 S. 67th Street, Omaha, NE, USA George Morcous, Professor of Construction Engineering, University of Nebraska, 1110 S. 67th Street, Omaha, NE, USA
How to cite this article: Antony K, George M. Bond Performance of Sanded Surface and Threaded Smooth Gfrp Bars. Civil Eng Res J. 2019; 8(4):555743. DOI: 10.19080/CERJ.2019.08.555743
This article presents the findings of experimental testing done at the University of Nebraska structures lab on bond performance of three types of GFRP bars. Specimens with 0.43in. (11mm) and 0.55 in. (14mm) diameter with sanded surface, in addition to specimens with 0.43in. (11mm) diameter threaded surface were tested by pullout. Threaded surface bars bond strength was 2.5 and 1.8 times the 0.43in. (11mm) and 14mm (0.55in.) diameter sanded surface bars respectively. Measured bond strength results were compared against the required ACI 440.1R-15 development length. The ACI requirement of the sanded surface bars was approximately double the measured required development length, and 2.6 times the measured required development length for the threaded specimens. Threaded surface bars bond strength was also compared against previous results for helical and ribbed bars and exceeded them. This lead to the conclusion that threading the GFRP bar without the need to roughen its surface by sanding can be an efficient technique in increasing the bond strength. In addition, the use of GFRP nuts as an accessory can have several applications like bar splices or end anchors and can be advantageous to structural engineers in reinforcement detailing and reducing development length.
Steel Reinforcement corrosion has been reported to be a major contributor in the deterioration of concrete structures. The use of FRP bars as reinforcement in concrete structures has proven to provide an effective solution for this durability issue . The physical and mechanical properties and surface shape of GFRP bars are significantly different from steel bars. This difference cause GFRP bars to have lower bonding properties with concrete than steel bars causing bond performance to be a major concern when designing GFRP reinforced concrete elements . Several pullout tests have been done to evaluate bond strength of FRP bars which is a major factor in estimating development length [3-5]. The ACI 440.1R-15  defines development length as the embedment in concrete required to transfer the force in the bar through bond (equation 10.1a). The used GFRP bars in this study had a guaranteed tensile strength (mean tensile strength of test specimens minus three times standard deviation  of 121 ksi (834 MPa). This value falls within the range of the ACI 440.1R-15 GFRP bar tensile strength values of 70 to 230 ksi (483 to 690 MPa).
In this study three different types of GFRP bar surfaces were considered with a total of 16 specimens. Specimens included six 0.43in. (11mm) diameter plain bars (sanded surface), and six specimens of 0.43in. (11mm) diameter threaded GFRP
bars (smooth surface), in addition to four specimens of 0.55in.
(14mm) diameter plain GFRP bars (sanded surface). Specimens
were tested according to ACI 440.3R-12 test method for bond
strength of FRP bars by pullout testing .
Concrete cubes with a side length of 8 inches (20mm) were
cast around GFRP bars with a bonded length of 5 times bar
diameter. In order to create the bonded length, hollow polyvinyl
chloride (PVC) tubes were placed at the top of the block to
de-bond the required portion of the contact length. Average
concrete compressive strength was 10ksi (69MPa) at the time
of testing. Specimens were mounted on top of 120kip (533kN)
Universal Testing Machine as shown in Figure 1. A load cell and
two linear variable differential transforms (LVDTs) were used
on top of the machine to measure applied load and slippage at
bar free end. Load was applied by loading one end and pulling
the bar at a rate of 0.05 in./min. (1.25mm/min.). For the sanded
surface bars, metal sleeves were glued to the loaded end of the
bar to allow for the machine clamps to grip on the bar. While for
the threaded bars, GFRP nuts were used as a compatible product
with the bar and load was applied to the bar by bearing on the
nut as shown in Figure 2.
The ACI 440.3R-12  recommends that loading shall
be continued until rupture of the FRP bar or slippage of at
least 0.1in. (2.5mm) occurs. Bond strength is then calculated
according to the maximum recorded force divided by the
bonded area of contact. Table 1 summarizes bond strength
and failure modes for the tested specimens. Figure 3 shows
load-slip plot for the sanded 0.433in. (11mm) diameter bar
(#11) specimens, the results of one specimen were removed
from the data as it was considered an outlier. Figure 4 shows
load-slip plot for the threaded 0.433in. (11mm) diameter bar
(#T11) specimens; three specimens were tested at first using
one nut which could not cause any significant slippage and the
failure mode occurred at the threads, these results were not
considered. For the remaining three specimens, two nuts were
used and could not cause a slippage failure also. However, using
two nuts provided about double the bond strength of using one
nut. Only the results of using two nuts were considered. Figure
5 shows load-slip plot for the sanded 0.55in. (14mm) diameter
bars (#14), all four specimens’ results were considered. Figure 6
shows a comparison of the average bond strength for the tested
Guaranteed bond strength results were compared against
the required ACI 440.1R-15  development length using
equation (10.3a). The parameter affecting development length
are the transfer force (taken as the guaranteed tensile strength
times bar area), concrete compressive strength, bar location
modification factor (taken by 1), and the concrete cover to the
center of the bar. The ACI requirement for the sanded surface
bars was approximately double the measured development
length, and 2.6 times the measured development length for the
Resulting bond strength was compared against Katz et al.
 where sanded FRP bars were tested after cyclic loading. The
unloaded specimens had an average bond strength of 1.98ksi
for sanded specimens with a helical fiber wrapped around the
surface to enhance bond strength. Threaded specimens bond
strength in this study resulted in about 1.37 times that value.
Also bond strength was compared against Masmoudi et al. 2011
 where a commercial GFRP bar product with ribbed surface
was tested for long-term bond performance under temperature
ranging from 200 °C to 800 °C. Bond strength under 200 °C
temperature was highest. Threaded surface bars bond strength
in this study was about 1.35 times the 0.31in. (8mm) diameter
bars under 200C, and 1.6 times the 0.63in. (16mm) diameter
bars under 200 °C. This comparison shows the effectiveness
of the considered threads in this study in increasing the bond
strength compared to other threaded surfaces.
1. For 0.433in. (11mm) and 0.55in. (14mm) diameter
sanded GFRP bars, bond strengths were 1,173 (8.1) and
1,479 (10.2) psi (MPa) respectively which are comparable to
other sanded surfaces.
2. For the 0.433in. (11mm) diameter threaded GFRP bars,
a significantly small slippage was measured as the specimens
failed at the threads and/or nuts. Using two nuts yielded
approximately 2.3 times the bond strength measured for
sanded 0.433in. (11mm) diameter specimens.
3. Threaded bar surface is an efficient technique to
increase bond strength compared to other bar roughening
technique. And the use of the nut as an accessory can have
several applications and advantages.
Masmoudi R, Masmoudi A, Ouezdou M, Daoud A (2011) Long-term bond performance of GFRP bars in concrete under temperature ranging from 200 C to 80 0C. Construction and Building Materials 25: 486-493.
ACI Committee 440 (2012) Guide Test Methods for Fiber-Reinforced Polymer (FRP) Composites for Reinforcing or Strengthening Concrete and Masonry Structures (ACI 440.3R-12), American Concrete Institute, Detroit.