Special Cementitious Bonding Material For FRP Strengthening Systems

Global Thinking In Structural Engineering: Recent Achievements IABSE CONFERENCE, SHARM EL SHEIKH 2012

Special Cementitious Bonding Material For FRP Strengthening Systems

Judy SOLIMAN Graduate Student Faculty of Eng. Ain Shams University [email protected]

Tarek HASSAN Associate Professor Faculty of Eng. Ain Shams University [email protected]

[email protected]

Amr ABDELRAHMAN Professor Faculty of Eng. Ain Shams University [email protected]

Osama HAMDY Professor Faculty of Eng. Ain Shams University [email protected]

Sami RIZKALLA Distinguished Professor Civil, Construction and Env. Eng. Dept. NCSU, USA [email protected]

Abstract

This paper presents the feasibility of using a novel patented material, commercially known as Grancrete, as a bonding adhesive for FRP strengthening applications. The material can be used as special cementitious materials for new construction, repair of existing structural elements, sprayable coating for protection of structures from severe surrounding environmental conditions and for fire resistance. The experimental program undertaken in the current study consisted of three phases. The first phase was designed to obtain an optimum W/G ratio to be used in the mix design of Grancrete using compressions cubes. The second phase was designed to evaluate the bond strength of Grancrete with different strengthening systems including Basalt, Steel reinforced Polymers, etc. Different tests were conducted on concrete slabs measuring 600x600 mm to examine the bond characteristics of different strengthening schemes. The third phase focuses on real applications of Grancrete in flexural members. A total of 32 T-Section concrete beams were constructed and tested at the Structural Laboratory at Ain Shams University. The beams were strengthened using different FRP externally bonded systems. The main objective of the study was to investigate and evaluate the effectiveness of the proposed fiber reinforced Grancrete (FRG) strengthening system. The research includes different types of fibers bonded to the concrete substrate with Grancrete paste as an adhesive. The main variables included the type and configuration of the strengthening scheme, Grancrete thickness, and presence of U-wraps.

Keywords: beam, concrete, debonding, intermediate crack debonding, FRP, Grancrete,

strengthening.

1. Introduction

In recent years, repair and retrofit of existing structures have been among the most important challenges in civil engineering. Epoxy has been proven to have excellent bond characteristics which are sufficient to transfer stresses between the fibers and the substrata, such as concrete and steel. Despite the effectiveness of FRP strengthening systems, one of the major limitations on the use of epoxy in structural strengthening applications is the possibility of the complete loss of the strengthening system in the case of a fire. When FRP strengthening systems are subjected to a combination of high temperatures and sustained loads, the resin polymer matrix could soften and consequently lose its ability to transfer stresses from the concrete to the fibers [1,2]. Mineral based composites (MBC) strengthening systems consist of FRPs and a cementitious bonding agent which form a repair or strengthening system that is more compatible with the concrete substrata. MBC strengthening systems are considered environmentally friendly and can be applied on moist surfaces. Preliminary work by Taljsten and Blansksvard (2007) on reinforced concrete (RC) slabs strengthened with various MBC strengthening systems showed that similar results can be obtained when compared to slabs strengthened with epoxy-based FRP strengthening systems and to slabs with increased steel reinforcement.

mailto:[email protected]

https://www.researchgate.net/publication/234077423_Premature_failures_in_plate_bonded_strengthened_RC_beams_with_an_emphasis_on_premature_shear_A_review?el=1_x_8&enrichId=rgreq-0f81661795dd21683886e2ad563ea27a-XXX&enrichSource=Y292ZXJQYWdlOzI2MjkwNTg1NjtBUzoxODIyMTU4MjQ2NTAyNDdAMTQyMDQ1NTA0NjkxMg==


After a decade of development and successful demonstrations, Grancrete stands poised to bring its exciting family of new materials to be used. Grancrete is a novel patented material co-developed by Jim Paul of Casa Grande, LLC and Arun Wagh of Argonne National Laboratories. Grancrete is based on “Ceramicrete”, a material developed by Argonne National Laboratories in 1996 for the encasement of nuclear waste. Grancrete was developed and patented as a pumpable ceramic concrete for spraying on a Styrofoam base in the construction of inexpensive housing [3]. Grancrete is a new cementitious material that originates from the mining industry and is considered to be nontoxic, non-hazardous and environmentally friendly. When mixed with water, this material forms a binding agent that is rapid-setting, develops high early bond strength, has enhanced durability, and above all is a fire resistant material; tolerates until 2200

oF. Grancrete can be used as

a material to build something new, repair something worn or damaged, reinforce something under stress, or protect something at risk, including the environment, Grancrete's high performance. The proposed fiber reinforced Grancrete (FRG) strengthening system would have excellent fire and heat resistance in comparison to the current FRP strengthening systems.

2. Experimental Program

2.1 Phase I- Water/Grancrete Ratio

The Grancrete paste was selected from three different pastes of Grancrete, different in Water/Grancrete ratio (W/G) as follows:

1. W/G = 0.25

2. W/G = 0.22

3. W/G = 0.20

Using a wooden form of dimensions; 50X50X50 mm, three Grancrete cubes were casted from each paste and tested in compression as shown in Fig. 1.

Based on the results of the Compression Test as shown in Table 1, the Grancrete paste W/G ratio was selected.

Table 1: Compression Test Results of Grancrete Cubes

W/G WEIGHT

OF CUBE

LOAD AT

FAILURE

GRANCRETE

STRENGTH

RATIO GRM KN MPA

0.25 250 50 20

0.25 250 52 20.8

0.25 250 54 21.6

0.22 250 100 40

0.22 250 103 41.2

0.22 250 105 42

0.20 250 144 57.6

0.20 250 149 59.6

0.20 250 160 64

The Grancrete paste with W/G= 0.25 had a low compressive strength, lower than the compressive strength of the concrete that will be used for the test specimens, which was planned to be 35 MPa.

As for the Grancrete paste with W/G=0.2, the mixing procedure was hard for the available facilities to provide good mixing for it, which led to the selection of the mix with W/G= 0.22.

The modulus of elasticity of the Grancrete paste was determined according to the following equation, [4]:

and was found to be 10120 MPa.

Fig. 1: Compression test of Grancrete cubes.


2.2 Phase II- Pull-out test

Plain concrete slabs were casted with dimensions 600X600X150 mm using wooden forms. Each slab was divided into 4 parts as shown in Fig. 2 to provide a wide space for testing different parameters including:

1. Grancrete layer thickness,

2. Surface preparation technique;

- Bond of Grancrete on a smooth concrete surface,

- Bond of Grancrete on a rough concrete surface,

3. Bond between Grancrete and fibers on concrete prepared

surface;

- Bond of Basalt Fibers with Grancrete,

- Bond of SRP sheets with Grancrete.

Three different Grancrete thicknesses of mix: W/G=0.22 and W/G=0.2 were applied as follows:

5 mm. Grancrete layer thickness on a concrete smooth surface. 10 mm. Grancrete layer thickness divided into two layers each of 5mm. thickness to test the effect

of the cold joint, applied on a concrete smooth surface. 15 mm. Grancrete layer thickness divided into two layers each of 7.5mm. thickness to test the

effect of the cold joint, applied on a concrete smooth surface. 15 mm. Grancrete layer thickness divided into two layers each of 7.5mm. thickness without

finishing the first Grancrete layer applied on a concrete rough surface.

The proposed FRG systems included basalt fibers and SRP sheets. The strengthening systems were applied using one layer of Grancrete paste, placing the fibers, and then followed by a second layer of Grancrete paste (Cover layer) as shown in Fig. 3. The specimens were cored more than 50 mm deep into the substrata using the core drill machine. Steel disks measuring 50 mm diameter and 20 mm thick were bonded to the surface of each individual core specimens using epoxy. Four individual Pull-off tests were performed for each part of slab specimens as shown in Figure 4. The test setup used for the Pull-off tests consists of a loading apparatus located on a tripod.

According to the Pull-out Test results, the SRP sheets were modified to insure the continuity of

Grancrete layer through its thickness as shown in Figs 4-a, b and c.

Fig. 3: Application of Grancrete layer

Fig. 2: Plain concrete slabs for pull-out test

Fig. 4-b: first modification

of SRP sheet

Fig. 4-a: SRP sheet preventing the

continuity of Grancrete layer

Fig. 4-c: second modification

of SRP sheet


2.3 Phase III- Externally Bonded FRP Test Beams

The experimental program was extended to study the flexural behaviour of concrete beams strengthened in flexure using externally bonded fiber reinforced Grancrete systems. Thirty two simply supported T-beams with a span of 3.0 m were constructed and tested at the Structural Laboratory of Ain Shams University. Details of the test specimens are given in Table 2. Specimens cross section is shown in Fig. 5.

Fig. 5: Steel cage and typical cross section for test specimens

The top reinforcement consisted of two 10 mm diameter steel bars. All beams were constructed

with a depth of 505mm as the bottom concrete cover was to be removed for strengthening. Twenty

nine beams were reinforced with two 12 mm diameter steel bars as bottom reinforcement. Two

beams were reinforced with two 16 mm diameter bars as bottom reinforcement and another beam

with three 16 mm diameter bars as bottom reinforcement. Three different types of strengthening

schemes were used. Five beams were strengthened using Basalt Polymers sheets. Fourteen beams

were strengthened using Steel Reinforced Polymers (SRP), eleven beams were strengthened using

Carbon Strands which is a new material used for the first time in Egypt. Two beams were tested as

control specimens. With the maximum moment occurring at the mid-span section of the beam,

failure could be due to either debonding or rupture of the externally bonded sheets. The specimens

were adequately designed to avoid concrete crushing and premature failure due to shear.

2.3.1 Material Properties

The mechanical properties of all materials used in the current study are given in Table 3.

Strengthening sheets are illustrated in Fig. 6.

Fig. 6: Different strengthening sheets used in the current study

2.3.2 Test setup and instrumentations

All beams were tested using a four point bending configuration to develop a constant moment

region. The span of the beams was kept constant at 3000 mm. The test setup allowed a constant

moment region of 1000 mm and two shear spans of 1000 mm each. The beams were supported on

a roller support at one end and a hinged support at the other. One hydraulic jack of 400 kN capacity

was used to apply the load on top of a rigid steel beam that equally distributes the load at both load

Basalt fibers SRP sheets Carbon strands


points. Test results are used in the following sections to develop an analytical approach to predict

flexural and delamination-type failures.

Table 2: Specimens Details

Beam

Tension

Steel

Fiber Grancrete Layer

U-WRAPS

outside

loading area

Type

Area

Number

of Layers

Thickness Total

Thickness

Adhesive

Layer Number

mm2 mm

2 mm mm mm

BO1 226.28 N/A N/A N/A N/A N/A N/A N/A

BO2 226.28 N/A N/A N/A N/A N/A N/A N/A

B11 226.28 SRP 123.21 1 1.32 20 13 N/A

B12 226.28 SRP 123.21 1 1.32 20 10 N/A

B13 226.28 SRP 123.21 1 1.32 20 7 N/A

B21 226.28 SRP 82.14 1 1.32 20 7 N/A

B22 226.28 SRP 82.14 1 1.32 30 10 N/A

B23 226.28 SRP 82.14 1 1.32 40 13 N/A

B24 226.28 SRP 82.14 1 1.32 50 15 N/A

B31 226.28 SRP 82.14 1 1.32 20 7 2

B32 226.28 SRP 82.14 2 2.64 30 11 2

B33 226.28 SRP 82.14 2 2.64 30 11 2

B34 226.28 SRP 82.14 2 2.64 40 10 2

B35 226.28 SRP 82.14 2 2.64 30 12 3

B41 402.28 SRP 82.14 1 1.32 20 10 2

B42 603.43 SRP 82.14 1 1.32 20 10 2

B1 226.28 BASALT 46.96 1 0.602 20 12 N/A

B2 226.28 BASALT 46.96 1 0.602 20 13 N/A

B3 226.28 BASALT 46.96 2 1.204 20 12 N/A

B4 226.28 BASALT 46.96 3 1.806 30 10 N/A

B5 226.28 BASALT 46.96 3 1.806 30 10 N/A

C1 226.28 CARBON 10.03 1 0.572 20 7 N/A

C2 226.28 CARBON 10.03 1 0.572 30 10 N/A

C3 226.28 CARBON 10.03 1 0.572 40 12 N/A

C4 226.28 CARBON 10.03 2 1.144 20 7 N/A

C5 226.28 CARBON 10.03 2 1.144 30 10 N/A

C6 226.28 CARBON 10.03 2 1.144 20 7 2

C7 226.28 CARBON 10.03 3 1.716 30 12 2

C8 226.28 CARBON 10.03 2 1.144 20 9 3

C9 226.28 CARBON 10.03 2 0.572 20 8 N/A

C10 226.28 CARBON 10.03 2 0.572 20 7 N/A

C11 402.28 CARBON 10.03 2 0.572 20 11 N/A

Table 3: Mechanical properties of materials

Material Strength Modulus of Elasticity

Ultimate strain of

fibers

Type MPa GPa

Longitudinal Steel Yield strength 400

200 Ultimate strength 690

Concrete Compressive strength 36.5 28.5

Grancrete Compressive strength 41

10.120 Tensile strength 2.34

Basalt Grid Tensile strength 926 39.6 0.0233

SRP [5] Tensile strength 840 75 0.0112

Carbon Strands Tensile strength 2060 118 0.0175


0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100

B3

B4

Average Control

Mid-span Deflection (mm)

Lao

d (

kN

)

B4B3

3. Test Results and Discussions

Table 4: Strengthening details of test specimens

Gro

up

Bea

m

ID Longitudinal

Reinforcement

Strengthening

Sheet

Bonded

Length Number of

Layers

Grancrete

Layer

Thickness

U-warps for one side Bonded Length

Type Width # of U-

Wraps

Grancrete

Thickness

mm mm mm mm

Control X2 2ϕ12 N/A N/A N/A N/A N/A N/A N/A N/A

BA

SA

LT

B1 2ϕ12 Basalt 200 1 20 N/A N/A N/A N/A




B4* 2ϕ12 Basalt 600 3 30 N/A N/A N/A N/A

S1 B11 2ϕ12 SRP-1 400 1 20 N/A N/A N/A N/A

B12 2ϕ12 SRP-1 600 1 20 N/A N/A N/A N/A

B13 2ϕ12 SRP-1 800 1 20 N/A N/A N/A N/A

S2

B21 2ϕ12 SRP-2 800 1 20 N/A N/A N/A N/A

B22 2ϕ12 SRP-2 800 1 30 N/A N/A N/A N/A

B23 2ϕ12 SRP-2 800 1 40 N/A N/A N/A N/A

B24 2ϕ12 SRP-2 800 1 50 N/A N/A N/A N/A

S3

B31 2ϕ12 SRP-2 800 1 20 SRP-2 150 2 20

B32 2ϕ12 SRP-2 800 2 30 SRP-2 150 2 20

B33 2ϕ12 SRP-2 800 2 30 SRP-3 150 2 20

B34 2ϕ12 SRP-2 800 2 40 SRP-2 150 2 30

B35 2ϕ12 SRP-2 800 2 40 SRP-2 120 3 20

S4 B41 2 ϕ16 SRP-2 800 1 20 SRP-2 150 2 20

B42 3 ϕ16 SRP-2 800 1 20 SRP-2 150 2 20

CS

1 C1 2ϕ12 Carbon Strands 800 1 20 N/A N/A N/A N/A

C2 2ϕ12 Carbon Strands 800 1 30 N/A N/A N/A N/A


CS2 C4 2ϕ12 Carbon Strands 800 2 20 N/A N/A N/A N/A


CS

3 C6 2ϕ12 Carbon Strands 800 2 20 Basalt 150 2 20

C7 2ϕ12 Carbon Strands 800 2 20 CFRP 150 2 20

C8 2ϕ12 Carbon Strands 800 3 30 CFRP 120 3 20

CS

4 C9 2ϕ12 Carbon Strands 800 1 20 N/A N/A N/A N/A


C11 2 ϕ16 Carbon Strands 800 1 20 N/A N/A N/A N/A

3.1 Basalt Grid Sheets

Specimens strengthened using Basalt Grid sheets varied in strengthening layer bonded length, Grancrete layer thickness and number of sheet layers as illustrated in Table 4. Load-Deflection curves of Basalt-strengthened beams in comparison with the control beam are illustrated in Fig. 7. Classical flexural failure of the control beam is shown in Fig. 8 and a typical rupture of the Basalt-strengthened beams is shown in Fig. 9. Test results indicated that the pre-and-post-cracking stiffness were identical for all beams regardless of the number of Basalt sheet layers and the Grancrete layer thickness.

This is attributed to the low elastic modulus of the Basalt sheets. Therefore, its contribution to the overall stiffness of the beams was negligible. Failure of all tested beams was due to rupture of

Fig. 7: Typical Load-Deflection behavior of beams

strengthened with Basalt grid sheets


0

50

100

150

200

250

300

0 10 20 30 40

B0

B21

B31

Mid-span deflection (mm.)

Lo

ad (k

N)

B21 B31

B0

Basalt sheets. Doubling the number of layers of Basalt sheets increased the ultimate load carrying capacity by only 6 percent. The increase in the Grancrete layer thickness led to a decrease in the deformability of the beam, which almost eliminates the effect of the bonded sheet.

It should be noted that the increase of the bonded length from 200 to 400 mm arrested the crack width. No sudden increase in the crack width was observed in the beam with 400 mm bonded length unlike the beam with 200 mm bonded length. In general, no significant increase in the load carrying capacity was observed for the beams strengthened using Basalt grid sheets.

3.2 Steel Reinforced Polymers SRP

The use of SRP sheets with Grancrete paste

is a promising strengthening procedure.

Four major parameters guided this

technique; the first is the allowance of good

penetration of the Grancrete paste through

the SRP sheet to ensure a composite action;

the second is the bonded length of the SRP

sheet, the third is the adhesive layer

thickness (first layer of Grancrete between

the sheet and concrete surface), which

highly affects the plate end debonding

phenomenon; and the last and most

important is the use of U-wraps.

The SRP sheets were modified to allow

Grancrete paste continuity and to ensure best use

of the full capacity of the sheets. Four SRP groups

were tested as shown in Table 4. The first Group

S1 was tested using the SRP sheet modified as

SRP-1. Due to the poor continuity of Grancrete

paste through the SRP sheet thickness, failure was

controlled by sheet delamination at the SRP sheet

level, as shown in Fig. 11.

Fig. 8: Failure of control beam Fig. 9: Typical failure of beams

strengthened with basalt grid sheets

Fig. 11: Sheet delamination for

SRP-1 at sheet level

Fig. 10: Typical Load-Deflection behavior of

beams strengthened with SRP sheets


The second group S2 was strengthened using SRP-2 and using the same strengthening properties. Failure was due to delamination of the sheets at the concrete interface as shown in Fig. 12. In this group, different Grancrete layer thicknesses were used. It was observed that increasing the Grancrete layer thickness of the strengthening layer slightly led to the increase in the load carrying capacity of the beam. The plate end debonding failure was divided into three debonding modes; first was due to peeling of the adhesive material, Grancrete;

second was due to internal combined normal and shear stresses in the Grancrete level and the third was due to internal combined shear and normal

stresses at the concrete level. Special emphasis for plate-end debonding failure modes is discussed in a companion paper [5].

The third group was strengthened with SRP-2 sheets and using U-wraps outside the loading area. For this group of beams, failure was due to intermediate crack debonding (ICD) as shown in Fig. 13 and rupture of the sheets as shown in Fig. 14. Increasing the number of U-wraps provided better crack propagation, higher load carrying capacity and allowed full use of sheet strength as illustrated in Table 5.

3.3 Carbon Strands

Carbon strands sheet is a promising material to be used with Grancrete in the fiber reinforced Grancrete FRG systems. The use of carbon strands sheets led to an increase in the load carrying capacity of the beam and a decrease in the beam deflection as shown in Fig.15. A typical Load-Deflection curve showing the beams strengthened with carbon strands in comparison to the control beam B0 highlights the significant improvement in the overall behavior and the ultimate load carrying capacity of the beams externally strengthened with carbon strands sheets.

Fig. 16 shows an Intermediate crack debonding of Beam C6 where no debonding cracks where observed prior to failure.

Fig. 13: Intermediate crack debonding for

beams with U-wraps

Fig. 12: Sheet delamination for

SRP-2 at concrete level

Fig. 14: SRP sheet rupture for

beams with U-wraps

https://www.researchgate.net/publication/263213787_Prediction_of_Plate_End_Debonding_and_Flexural_Failures_for_Fiber-Reinforced_Grancrete_Systems?el=1_x_8&enrichId=rgreq-0f81661795dd21683886e2ad563ea27a-XXX&enrichSource=Y292ZXJQYWdlOzI2MjkwNTg1NjtBUzoxODIyMTU4MjQ2NTAyNDdAMTQyMDQ1NTA0NjkxMg==


0

50

100

150

200

250

300

0 10 20 30 40

B0 C1

C2 C4

C5

Mid-span Deflection (mm.)

Lo

ad (

kN

)

B0

C4

C2

C1

C5

Table 5: Ultimate capacities and failure modes of strengthened beams

Basalt Grid sheets SRP sheets Carbon Strands sheets

Beam ID Failure Load

Failure Mode


Failure Mode


Failure Mode

kN kN kN B1 165 SR B11 142.5 EPD C1 175 EPD B2 170 SR B12 160 EPD C2 180 SR B3 180 SR B13 200 EPD C3 170 EPD B4 165 SR B21 180 EPD C4 210 EPD B5 165 SR B22 195 EPD C5 210 EPD

B23 200 EPD C6 200 SR B24 170 EPD C7 260 ICD B31 190 EPD C8 220 ICD B32 250 SR C9 205 SR B33 250 ICD C10 170 EPD B34 230 ICD C11 320 SR B35 310 ICD B41 310 SR B42 440 ICD

EPD: End Plate Debonding; SR: sheet rupture; ICD intermediate crack debonding

4. Conclusions

Based on the findings of the current study, the following conclusions could be drawn:

1. Fiber Reinforced Grancrete (FRG) systems showed promising performance and great potential in strengthening reinforced concrete structures.

2. Grancrete paste showed good adhesion characteristics with different FRP systems considered in the current study. The material is promising as a substitute for epoxy adhesives especially for fire-resistant applications.

3. Water/Grancrete ratio is the main parameter affecting the compressive strength of the Grancrete paste. The tensile strength as well as the elastic modulus, which are main parameters affecting the bond behaviour are highly dependent on the W/G ratio.

4. Basalt grid sheets showed no increase in the load carrying capacity or stiffness of the strengthened beams.

5. Shear anchorage using U-wraps is an essential parameter for FRG systems. A good distribution of U-wraps allowed full utilization of the strength of the sheets. The U-wraps

Fig. 15: Typical Load-Deflection behavior of

beams strengthened with Carbon strands sheets Fig. 16: Intermediate crack debonding failure

for Beam C6 strengthened with Carbon strands

sheets


prevented plate end debonding, delayed Intermediate crack debonding failure and increased the ultimate load carrying capacity of the beams.

6. SRP sheets are ideal for use with Grancrete paste. A good Grancrete paste continuity through the sheets shall be guaranteed to provide a complete composite action and to ensure full utilization of the strength of the materials.

5. References

[1]. ACI 440.2R-08, 2008. “Guide for the Design and Construction of Externally Bonded FRP

Systems for Strengthening Concrete Structures”, American Concrete Institute, Farmington

Hills, Michigan, USA.

[2]. Jumaat, M. Z; Rahman , M. A.; Alam, M.A.; and Rahman, M. M., 2011. ”Premature

Failures in Plate Bonded Strengthened RC Beams with an Emphasis on Premature Shear: A

Review”. International Journal of the Physical Sciences, Vol. 6, No. 2, pp. 156-168.

[3]. Obregon-Salinas, Javier, Adolfo, 2010, “Use of Grancrete as Adhesive for Strengthening

Reinforced Concrete Structures”, MD thesis, North Carolina State University, Raleigh,

U.S., 233 pp.

[4]. Montesdeoca Solorzano, O. F., 2008, “Basic Characteristics of Grancrete HFR”. Raleigh,

NC: Master's thesis, North Carolina State University.

[5]. Soliman, J., Hassan, T., Abdelrahman, A., Hamdy, O., and Rizkalla, S. 2012, “Prediction of

Plate End Debonding and Flexural Failures for Fiber-Reinforced Grancrete Systems”,

Accepted for publication in IABSE Conference, Cairo, Egypt, May 7-9, 2012.







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Special Cementitious Bonding Material For FRP Strengthening Systems