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.

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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.

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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)

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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

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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

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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.

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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.

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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

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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.

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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

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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

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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.