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1
GRANCRETE FOR FLEXURAL STRENGTHENING OF
CONCRETE STRUCTURES
Adolfo J. Obregon-Salinas, Sami H. Rizkalla, and Paul Zia
North Carolina State University, Raleigh, NC, USA
Synopsis: This paper presents an evaluation of the use of a new innovative cementitious
material, commercially known as Grancrete PCW, as an alternative to epoxy for FRP
strengthening systems used for reinforced concrete (RC) structures. Grancrete is an
environmentally friendly material that develops high early bond strength and possesses
an excellent resistance to fire. The study includes an experimental program to evaluate
the behavior of seventeen RC slabs strengthened by using different types of fibers. The
load carrying capacity, ductility, and mode of failure of the strengthened specimens were
evaluated and the results were compared to control specimens. Results of the
experimental program showed that Grancrete PCW paste could be used as an alternative
bonding material.
Keywords: cementitious material, externally bonded, fiber-reinforced Grancrete, fiber-
reinforced polymer, flexural strengthening, FRG, FRP, Grancrete.
2
Adolfo J. Obregon-Salinas
Graduate Research Assistant at the Constructed Facilities Laboratory, NCSU, Raleigh,
NC. Received his BS from North Carolina State University in 2008 and works toward his
MS in the Structural Engineering and Mechanics program at North Carolina State
University.
Sami H. Rizkalla
FACI, is Distinguished Professor of Civil Engineering and Construction at North
Carolina State University and the Director of the Constructed Facilities Laboratory. He
is a member and former chair of ACI Committees 440, Fiber Reinforced Polymer
Reinforcement, member of the Joint ASI-ASCE Committees 423, Prestressed Concrete,
and member of the ACI Committee 550, Precast Concrete Structures.
Paul Zia
ACI Honorary Member, is a Distinguished University Professor Emeritus at North
Carolina State University. He is an ACI Past President and is currently a member of the
Concrete Research Council, ACI Committee 363, High-Strength Concrete, Joint ACI-
ASCE Committees 423, Prestressed Concrete, and ACI 445, Shear and Torsion
INTRODUCTION
The need to strengthen and rehabilitate existing concrete and steel structures has
been one of the driving forces for the increased use of fiber reinforced polymers (FRP) in
structural strengthening applications. One of the problems with the use of FRP in the
strengthening of concrete structures is the possible degradation of the systems under
elevated temperatures. When FRP strengthening systems are subjected to a combination
of high temperatures and sustained loads, the resin polymer matrix can soften and
consequently loose its ability to transfer stresses from the concrete to the fibers (ACI
440.2R-08). Research conducted at Lulea University of Technology in Sweden has
proposed the use of cementitious materials as alternatives to epoxy resins in order to
address the fire resistance issue of FRP strengthening systems (Taljsten and Blanksvard
2007). This paper summarizes the research findings of an experimental program
undertaken to study the behavior of externally bonded FRP strengthening systems using
Grancrete PCW paste as an adhesive material. Grancrete is a new cementitious material
containing ashes and fibers. When mixed with water, this material forms a binding agent
that is rapid-setting and has high early strength, enhanced durability, and excellent fire
resistance. This experimental program consists of large-scale reinforced concrete (RC)
slabs strengthened to increase the flexural resistance using various types of fibers
including glass, carbon, and basalt.
3
RESEARCH SIGNIFICANCE
This paper introduces a new innovative strengthening system for reinforced concrete
structures using a cementitious material to replace epoxy as adhesive. The proposed
system is evaluated using seventeen full-scale reinforced concrete slabs. The research
investigated the effects of type of fibers and the application method for the proposed
system. The load carrying capacity, ductility and mode of failure of the tested specimens
were used to evaluate the system and the behavior was compared to similar specimens
strengthened with epoxy as adhesive.
EXPERIMENTAL PROGRAM
The experimental program consists of seventeen RC slabs of 11 ft x 2 ft x 6 in. (3.35
m x 0.61 m x 0.15 m). The slabs were cast in three sets. For each of the three casts, one
slab without strengthening was used as a control specimen. The internal steel
reinforcement consisted of five #4 reinforcing bars evenly distributed throughout the
width of the slabs. A single 50 mm strain gauge was attached to one of the longitudinal
rebar in order to determine the yielding point of the steel reinforcement during testing.
The material properties of the construction materials are given in Table 1.
Table 1 – Properties of construction materials
After a minimum of 28 days of curing, the tension side of the slabs was roughened
by coal slag media blasting before being strengthened with the respective strengthening
systems as shown in Table 2. In this experimental program, four parameters were studied:
1. Type of Fibers – Five different types of fibers were used including vinyl coated glass
fibers (small ¼ in. grid), vinyl coated glass fibers (large 1 in. grid), basalt fiber grid,
uniaxial carbon strand sheet (multiple narrow strips containing 10 fiber strands each), and
carbon fiber grid (C-grid).
2. Adhesive Material – Two types of adhesive materials were used in this study;
Grancrete PCW paste and epoxy resin.
3. Fiber Reinforcement Ratio – Selected slabs were strengthened using multiple layers of
fiber reinforcement and compared to those strengthened with a single layer of
reinforcement.
Construction Material Number of slabs Compressive Strength at 28 days
Concrete from Cast #1 8 6,260 lbf (27.85 kN)
Concrete from Cast #2 8 3,180 lbf (14.15 kN)
Concrete from Cast #3 8 4,160 lbf (18.50 kN)
A706 Steel Rebar 5 bars (#4)
#13 (soft metric
conversion) Fy = 68.4 ksi (472.3 MPa)
4
4. Application Method – Selected slabs were strengthened using an overhead spray
application and compared to a cast application to simulate strengthening of the positive
and negative moment zones in a typical slab.
Table 2 – Experimental program – Slab specimens
STRENGTHENING PROCEDURE
Two different application procedures were used for selected slabs to apply Grancrete
as the adhesive material for the strengthening systems. In the first procedure, the
Grancrete was applied by an overhead spray system, which is convenient for
strengthening positive moment regions in typical slabs. The procedure involved
supporting the 11 ft slab high above the ground where the Grancrete adhesive material
was sprayed overhead using a continuous mixer and pump system as shown in Figure 1.
The application of the fiber layer is shown in Figure 2. The second procedure involved
the casting of the strengthening system on the top of the slab, which is convenient for the
strengthening negative moment zones in typical slabs. The procedure required building a
½ in. (12.7 mm) wooden form around the slab to cast the Grancrete as shown in Figure 3.
Both the overhead spray application and the cast application of the fiber/Grancrete (FRG)
strengthening systems used the same sequence of application and the same continuous
mixer and pump system. The FRG strengthening systems were applied using one layer
of Grancrete PCW paste directly on the prepared concrete substrata followed by the
laying of the fiber reinforcement. The fibers were pressed into the underlying Grancrete
to ensure full contact between the fibers and the adhesive material. Finally, the top layer
Cast Type of Fiber Adhesive Material Layers of
Fiber
Application
Method Specimen ID
1
NONE NONE NONE NONE Control – 1
Glass Fibers (small ¼” grid)
Grancrete PCW Paste 1 Layer
Cast GS-G-1-C
Sprayed GS-G-1-S
Epoxy -------- GS-E-1-X
Basalt Grid Grancrete PCW Paste
1 Layer
Cast B-G-1-C
Sprayed B-G-1-S
Epoxy -------- B-E-1-X
2
NONE NONE NONE NONE Control – 2
Glass Fibers
(Large 1” grid)
Grancrete PCW Paste 1 Layer
Cast GL-G-1-C
Epoxy -------- GL-E-1-X
C-Grid Grancrete PCW Paste
1 Layer Cast CG-G-1-C
Epoxy -------- CG-E-1-X
Carbon Strand
Sheet
Grancrete PCW Paste 1 Layer
Cast CS-G-1-C
Epoxy -------- CS-E-1-X
3
NONE NONE NONE NONE Control – 3
Basalt Grid Grancrete PCW Paste 1 Layer
Cast B-G-1-C
2 Layers B-G-2-C
Total number of slabs 17
5
of Grancrete PCW paste was applied before the underlying layer was fully set. In order
to compare the behavior, the application of epoxy as the adhesive material was also cast
as shown in Figure 4. Properties of the fibers used for the different strengthening systems
are given in Table 3.
Figure 1 – Spray of Grancrete PCW. Figure 2 – Fibers being applied.
Figure 3 – Cast method of application. Figure 4 – Epoxy being applied.
Table 3 – Properties of strengthening materials.
The length of the strengthening systems used for each slab was 9 ft (2.74 m) and the
adhesive materials were extended an extra 3 in. (76.2 mm) beyond the fibers in order to
fully cover the edges. The strengthening system was stopped short of the bearing plate
Strengthening
Material
Arragement of
Fibers
Spacing of
Strands, o/c
Tensile Force
per Strand
Number of Strands
per Layer
Glass Fibers
(small ¼” grid) Bidirectional grid .25 in. (6.35 mm) 170 lbf (.76 kN) 90
Basalt Grid Bidirectional grid 1 in. (25.4 mm) 375 lbf (1.67 kN) 23
Glass Fibers
(Large 1” grid) Bidirectional grid 1 in. (25.4 mm) 495 lbf (2.20 kN) 23
C-Grid Bidirectional grid 1.25 in. (32 mm) 1,035 lbf (4.60 kN) 18
Carbon Strand
Sheet
Unidirectional
strands .064 in. (1.6 mm) 500 lbf (2.22 kN)
100 (10 strips of
10 strands each)
Grancrete PCW
Paste
Compressive Strength 5,300 psi – 6,150 psi (36.5 MPa – 42.4 MPa)
at Day of Testing
6
area to avoid any possible mechanical anchoring from the supports. The carbon strand
sheet was split into individual strips in order to ensure full confinement by the Grancrete
around the fibers. The number of strips used was selected based on preliminary material
testing of similar materials. Preliminary work also indicated that 75% of the compressive
strength of the Grancrete was developed in the first 24 hrs of air curing. The
strengthening systems using Grancrete PCW paste as the bonding material were allowed
to cure for a minimum of 14 days before testing. Based on the data provided by the
manufacturer, the curing time of the epoxy material is 72 hrs. The strengthening systems
using epoxy as the bonding material were allowed to cure for a minimum of 7 days
before testing.
TEST SETUP
The flexural response of the slab specimens was evaluated using a four-point loading
configuration. A schematic of the setup is shown in Figure 5 and the typical test setup
used is shown in Figure 6. Pin and roller supports were used with a 10 ft (3.04 m) span.
The two point loads were set 2 ft (.61 m) apart and distributed over the entire width of the
slab. A 50 kip (222.4 kN) MTS hydraulic actuator was used to load the specimens using
a displacement-controlled mode. In order to measure the midspan displacement of the
specimens, two 12 in. (305 mm) string potentiometers were installed at the midspan. The
two string pots were selected in order to capture any possible torsion or twisting caused
by variability in the construction of the slabs and/or supports. Two 8 in. (200 mm) PI
gauges were used to measure the strain at the top and bottom of the slab at midspan.
Figure 5 – Typical test setup for experimental program.
7
Figure 6 – Typical test setup for experimental program.
TEST RESULTS AND DISCUSSIONS
The criteria used to compare the flexural performance of the various strengthening
systems were the maximum measured load carrying capacity and the deflection at failure.
Figure 7 shows the results for the flexural response of the specimens tested from cast #1.
Table 4 shows the numerical values of the test results for the specimens tested from cast
#1. Note that the maximum displacement was recorded at the time of failure and does
not necessarily correspond to the displacement at the maximum applied load. All of the
strengthening systems showed an increase in flexural capacity in comparison to the
control specimen. Test results also indicated a significant increase in ductility by using
the proposed FRG systems regardless of the application procedure. Specimens B-G-1-S
and GS-G-1-S, were strengthened using an overhead spray method, while specimens B-
G-1-C and GS-G-1-C were strengthened using the cast procedure. Test results indicate
that the performance of the spray systems is similar to the behavior of the systems
strengthened using the cast procedure. The sprayed basalt FRG specimen had a lower
capacity than the cast basalt FRG specimen. In contrast, the sprayed GS-FRG specimen
had a higher capacity and ductility than the cast GS-FRG specimen. Test results indicate
that good composite action can be achieved with the spray method of application,
however it should be noted that the spray application method is sensitive to the type of
fibers used. It can be observed from Figure 7 that all of the specimens that use Grancrete
PCW paste experience a drop in load before failure. This behavior may be due to slip of
the fibers with respect to the Grancrete paste or intermediate cracking (IC) induced
debonding through the FRG strengthening layer at midspan within the constant moment
zone as shown in Figure 8. No cracks were observed at the development length of the
strengthening system, which provided adequate anchorage as evidenced by the ability of
the strengthened slab to maintain the load with significant ductility before failure. This
mechanism was similar to failure mechanisms observed by Burgoyne (1993), Charour
(2005), and Choi (2008) for the end-anchored partially-bonded FRP strengthening
systems. Post-test inspection of the specimens in the maximum moment zone revealed
further evidence of the proposed mechanism. It should also be noted that at the midspan
of the FRG strengthened specimens, the fibers and the outer layer of Grancrete was
completely delaminated at failure as shown in Figure 9.
8
Table 4 – Numerical results of specimens from cast #1.
Figure 7 –Results for specimens from cast #1.
Also worth noting is that the GS-E-1-X specimen, which uses epoxy as the bonding
material, experienced a brittle failure, with comparatively less deflection at failure and
without visual or auditory degradation of the system prior to failure. The cracking
pattern of the specimen strengthened with FRG systems was distributed uniformly along
the length of the slab and propagated from the tension side of the slab to the compression
zone as expected for typical ductile failure of RC structures. These specimens produced
audible sounds of the fibers rupturing near failure. In contrast, for the specimen GS-E-1-
X with epoxy, the cracks did not propagate into the strengthening system and no signs of
fibers rupturing were observed prior to the sudden and loud rupturing of the fibers.
Specimen ID Max Load Max Displacement% Increase of
Capacity
% Increase of
Ductility
Control -1 13,490 lbf (60.0 kN) 3.7 in (94 mm) 0.0 0.0
B-G-1-S 14,760 lbf (65.7 kN) 3.9 in (99 mm) 9.4 5.4
GS-G-1-S 16,230 lbf (72.2 kN) 6.0 in (152 mm) 20.3 62.2
B-G-1-C 16,090 lbf (71.6 kN) 6.7 in (170 mm) 19.3 81.1
GS-G-1-C 15,540 lbf (69.1 kN) 4.1 in (104 mm) 15.2 10.8
B-E-1-X 16,220 lbf (72.1 kN) 3.9 in (99 mm) 20.2 5.4
GS-E-1-X 17,960 lbf (79.9 kN) 3.3 in (84 mm) 33.2 -10.8
0 20 40 60 80 100 120 140 160 180
0
10
20
30
40
50
60
70
80
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 1 2 3 4 5 6 7
Midspan Displacement (mm)
Ap
plied
Loa
d -
2P
(K
N)
Ap
plied
Lo
ad
-2
P (
lbf)
Midspan Displacement (in)
GS-G-1-C
2P
9
Figure 8 – Midspan of GS-G-1-C, showing splitting/delaminating
cracks through strengthening layer.
Figure 9 – Tension side of GS-G-1-S, showing delamination of fiber layer
in maximum moment area
Figure 10 shows the test results of the specimens from cast #2. Table 5 presents the
results numerically. The specimens from cast #2 showed a significant increase of the
flexural capacity in comparison to the control specimen. All of the strengthened systems
from cast #2 failed due to rupturing of the fibers. No debonding or delamination of the
fiber layer was observed for the systems from cast #2. As expected, an increase of the
strength led to a significant reduction of the ductility.
Also worth noting is that the specimens that used Grancrete as the adhesive material
had distributed flexural cracks that propagated throughout the surface of the tension side.
Audible sounds of the fibers rupturing could be heard prior to failure for these FRG
strengthened slabs. In contrast, the specimens CS-E-1-X and CG-E-1-X, which used
epoxy as adhesive material, had no propagation of flexural cracks on the strengthening
system. Also, as failure approached no signs of fibers rupturing were observed prior to
the sudden rupture of the fibers. In terms of the increase of the load carrying capacity,
both the FRG and epoxy strengthen carbon strand slabs behaved similarly. The
difference in the flexural capacity between the slab using FRG and the slab using epoxy
is less than 7% and the difference in ductility between the two systems is less than 8%.
The two specimens strengthened with c-grid behaved similarly regardless of the adhesive
material used.
10
Figure 10 – Results for specimens from cast #2.
Table 5 – Numerical results of specimens from cast #2.
Figure 11 shows the flexural responses of the specimens from cast #3. Table 6
shows the numerical values of the test results. Specimen B-G-2-C, which has two layers
of basalt reinforcement, behaved as expected. Its load carrying capacity increased while
its ductility decreased in comparison to the control specimen. In contrast, specimen B-G-
1-C exhibited an increase in load carrying capacity and also an increase in ductility when
compared to the control specimen. This behavior may be due to slip in the fibers in the
maximum moment zone causing a shift in the bonding mechanism from that of a fully
bonded system to an end-anchored partially-bonded system. It is important to note that
this failure mechanism is sensitive to the amount of fiber reinforcement used, as shown
by specimen B-G-2-C where no slip was observed from the flexural response (Figure 11)
before rupture of the basalt fibers. The cracking pattern of the strengthened specimens
0 20 40 60 80 100 120 140 160 180
0
10
20
30
40
50
60
70
80
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 1 2 3 4 5 6 7
Midspan Displacement (mm)
Ap
plied
Load
-2P
(K
N)
Ap
plied
Lo
ad
-2
P (
lbf)
Midspan Displacement (in)
2P
GL-G-1-C
CG-E-1-X
Slab ID Max Load Max Displacement% Increase of
Capacity
% Increase of
Ductility
Control - 2 12,660 lbf (56.3 kN) 4.2 in (107 mm) 0.0 0.0
GL-G-1-C 14,020 lbf (62.4 kN) 2.6 in (66 mm) 10.7 -38.1
CG-G-1-C 15,000 lbf (66.7 kN) 2.2 in (56 mm) 18.5 -47.6
CS-G-1-C 18,680 lbf (83.1 kN) 2.1 in (53 mm) 47.6 -50.0
GL-E-1-X 14,881 lbf (66.2 kN) 2.2 in (56 mm) 17.5 -47.6
CG-E-1-X 14,987 lbf (66.7 kN) 2.4 in (61 mm) 18.4 -42.9
CS-E-1-X 19,690 lbf (87.6 kN) 1.8 in (46 mm) 55.5 -57.1
11
from cast #3 was distributed uniformly along the length of the slab and propagated from
the tension side of the slab to the compression zone as expected for typical ductile failure
of RC structures.
Table 6 – Numerical results of specimens from cast #3
Figure 11 – Results for specimens from cast #3.
CONCLUSIONS
This paper summarizes the results of an experimental program undertaken to study
the behavior and effectiveness of a new FRP/Grancrete strengthening system for
reinforced concrete structures. The behavior of the FRG strengthened slabs, using two
different application methods and different fibers, was compared to control specimens
without strengthening and specimens strengthened with epoxy bonded systems. The
following conclusions can be made based on the limited specimens reported in this paper:
Slab ID Max Load Max Displacement% Increase of
Capacity
% Increase of
Ductility
Control - 3 13,098 lbf (58.3 kN) 5.18 in (131.57 mm) 0.0 0.0
B-G-1-C 13,335 lbf (59.3 kN) 6.68 in (169.67 mm) 1.8 28.9
B-G-2-C 14,968 lbf (66.6 kN) 3.99 in (101.35 mm) 14.3 -22.9
0 20 40 60 80 100 120 140 160 180
0
10
20
30
40
50
60
70
80
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 1 2 3 4 5 6 7
Midspan Displacement (mm)
Ap
plied
Lo
ad
-2
P (
KN
)
Ap
plied
Lo
ad
-2
P (
lbf)
Midspan Displacement (in)
2P
Control - 3
12
1. Grancrete PCW paste can be used as adhesive for fiber strengthening systems to
increase the flexural capacity of reinforced concrete members.
2. FRG systems can be installed by using spray or cast method respectively, for the
positive moment or negative moment region of slabs and beams.
3. By using the strengthening system, an increase in the load carrying capacity is often
accompanied by a reduction in the ductility as usually is the case.
4. The proposed FRG system has the advantage of a high resistance to fire in
comparison with any other currently available FRP strengthening system.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the support provided by Grancrete, Inc., the
Constructed Facilities Laboratory (CFL) at North Carolina State University, and the
National Science Foundation (NSF) Industry/University Cooperative Research Center
(I/UCRC) for the Integration of Composites into Infrastructure (CICI) with award #
0934182.
REFERENCES
ACI Committee 440, 2008, “Guide for the Design and Construction of Externally Bonded
FRP Systems for Strengthening Concrete Structures (ACI 440.2R-08),” American
Concrete Institute, Famington Hills, MI, 4 pp.
Burgoyne, C. J., 1993, “Should FRP be bonded to Concrete?,” Cambridge CB2 1PZ, UK.
Chahrour, A. and Soudki, K., 2005, “Flexural Response of Reinforced Concrete Beams
Strengthened with End-Anchored Partially Bonded Carbon Fiber-Reinforced Polymer
Strips,” Journal of Composites for Construction, American Society of Civil Engineers, pp.
170-177.
Choi, H. T.; West, J. S.; and Soudki K. A., 2008, “Analysis of the Flexural Behavior of
Partially Bonded FRP Strengthened Concrete Beams,” Journal of Composites for
Construction, American Society of Civil Engineers, pp. 375-386.
Taljsten, B. and Blanksvard, T., 2007, “Mineral-Based Bonding of Carbon FRP to
Strengthen Concrete Structures,” Journal of Composites for Construction, American
Society of Civil Engineers, pp. 120-128.
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