MECHANICAL AND ELECTRICAL PROPERTIES OF GRAPHENE

NANOPLATELET REINFORCED CEMENTITIOUS COMPOSITES

Radhika Pavgi

University of Virginia

Dr. Osman Ozbulut

Faculty Advisor

Sherif Daghash

Graduate Student Mentor

Word Count: 3028 words text + 13 tables/figures x 250 words (each) = 6278 words

August 1st 2015

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ABSTRACT In recent years graphene nanoplatelets have been gaining attention as a potential material for

reinforcement of cementitious composites, due to their ability to increase the durability of the

composites and enhance the electrical damage self-sensing ability of the composites.

This experiment investigated the effect of amount of graphene nanoplatelets on the mechanical

and electrical properties of mortar. GNP was added to mortar in 0.1%, 0.3% and 0.5% by weight

cement and the compressive strength, flexural strength and the electrical resistivity of the mortar

was measured. The compressive strength of the mortar did not significantly increase as a result

of the GNP. Similarly, the flexural strength of the mortar did not show a significant increase or

decrease as a result of the GNP, likely due to the difficulty in dispersion of the GNP throughout

the mortar mix. The change in electrical resistivity of the mortar was negligible, leading to the

conclusion that higher amounts of GNP are required to observe a change in the resistivity.

Keywords: Graphene nanoplatelets, mortar, cementitious composites

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INTRODUCTION

In recent years, there has been increasing research conducted on the incorporation of nano-scale

smart materials in cementitious composites. These smart materials are alternative materials such

as nanoparticles and nanofibers which enhance certain properties of the cementitious composite,

such as durability and strength by decreasing permeability and inhibiting nano-cracks in a

material. This reinforcement ultimately increases the lifespan of the material, making it a

sustainable choice for use in infrastructure (1).

Of these nanomaterials, carbon structure nanoparticles have been gaining attention as a

possible material used to reinforce cementitous composites. Carbon nanoparticles, when

dispersed in a composite, not only increase the mechanical properties of the material, but act as

conductors, enhancing the electrical properties of the material. As the material undergoes aging

or damage, the electrical properties change respectively, allowing the material to have damage

self-sensing and self-monitoring abilities (2).

Past research has been conducted into reinforcing cementitious composites with carbon

nanotubes, 1-dimensional cylindrical carbon based nanoparticles. However entanglement and

difficulties with dispersion of carbon nanotubes have led to increased research into using

graphene nanoplatelets (GNP), a 2-dimensional carbon based nano-material (3). Compared to

carbon nanotubes, graphene nanoplatelets are easier to completely disperse in a composite and

are lower cost (4). Additionally, the high surface area of GNP has been shown to act as sites for

the growth of calcium silicate hydrate crystals, optimizing the hydration process and increasing

the strength of the composite (5). However, further research is needed in order to fully

understand the effect of reinforcing cementitious composites with GNP.

This experiment aims to analyze the mechanical and electrical properties of GNP

reinforcement on cementitious composites. This report will review past research associated with

GNP in cementitious composites, and outline the procedure and results of this experiment,

including materials preparation, testing and results.

LITERATURE REVIEW In recent years, several studies have begun to research the properties of graphene nanoplatelet

reinforced cementitious composites. Of these several have focused on the mechanical properties,

durability and permeability of the composites, electrical applications and the proper dispersion of

the graphene nanoplatelets. Of these, many studies have focused on both the use of graphene

nanoplatelets (GNP) and graphene oxide (GO) a functionalized form of GNPs.

Studies have shown that the functional oxygen groups attached to the graphene oxide

crystal help in the dispersion of the nanoplatelets. A study by Pan et al. tested the effect of

graphene oxide (GO), or functionalized graphene on the strength of Portland cement paste. The

study concluded that adding 0.05 wt% GO increased the compressive strength by at least 15%

and the flexural strength by at least 41%. It showed that GO served as a barrier to crack

propagation, stopping cracks at the nanoscale (6). Another experiment published by the

American Society of Civil Engineers concluded that the introduction of 0.03% graphene oxide

by weight cement reduced the number of pores in the material and increase the strength by over

40% (7).

A study conducted by Lv et al. studied the effect of graphene oxide on the strength of

mortar mixes, and concluded that the addition of 0.03% GO by weight cement increased the

tensile strength of the mortar by 78.6%, the flexural strength by 60.7% and the compressive

strength my 38.9% (5). Similarly, research completed by Babak et al. in 2014 found that adding

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up to 1.5% by weight cement of graphene oxide resulted in a 48% increase in tensile strength of

mortar (9). Another experiment published in 2014 tested the effect of both graphene particles and

graphene oxide particles on the frost resistance and corrosion resistance of mortar. The study

tested GNP mixed with water reducing surfactants and GO without any water reducers. The test

concluded that size M graphene oxide particles were the most effective at decreasing the

permeability and increasing the freeze thaw resistance of the mortar (10).

However, when using non-functionalized graphene nanoplatelets, dispersion of the GNP

is a key step to ensuring a homogenous mixture. A research project conducted in 2014

investigated the effect of using a water reducing admixture as a dispersion agent in a

cementitious composite. The study concluded that a water reducing admixture to GNP ratio of

1.5 was necessary to disperse the graphene, along with sonication of the GNP mixture (11).

Another study compared three methods of dispersion of the graphene nanoplatelets, namely

ultrasonication, acid etching the GNP and surfactant coating were used to determine the most

effective form of dispersion. Surfactant coating, using sodium deoxycholate proved to be the best

method used to disperse and suspend the graphene nanoplatelets (12).

Studies conducted in 2014 have studied the electrical properties of graphene nanoplatelet

reinforced mortar, with specific focus on the damage self-sensing properties of the composites.

An experiment by Le et al. added GNP at 5%, 10%, 15%, and 20% by mass cement, created

notches in the center of the mortar prisms and then measured the resistivity across the notch. The

study found a relationship between the resistivity of the material and the depth of the crack to

develop an expression for the amount of damage as a function of resistivity (13). Another study

conducted in 2014, focused on measuring the piezoresistive strain of mortar specimens that were

reinforced with graphene nanoplatelets. The conductive mortar was tested using the electrical

potential method to determine a mathematical analogy for the damage of the material and the

resistivity of the material (2). Studies such as these demonstrate the potential and ability for

graphene nanoplatelets to be used in applications for damage self-sensing and self-health

monitoring.

EXPERIMENTAL TESTING

Materials and Specimen Preparation

To conduct the experiment, four separate batches of mortar containing various levels of graphene

nanoplatelets were mixed. The test matrix which lists the percent of GNP by weight of cement in

each batch is described below. (Table 1)

TABLE 1 Test Matrix for Mortars Reinforced with GNPs

Batch Specimen GNP Type

GNP % by

weight of

cement

GNP Size

(Average surface

area)

1 1-10 None -- --

2 11-20 Grade C 0.1 300 m2/g

3 21-30 Grade C 0.3 300 m2/g

4 31-40 Grade C 0.5 300 m2/g

Each batch of mortar was mixed as per ASTM C109, maintaining a water to cement ratio of

0.485 (14). Type I/II cement, water and natural silica sand were used in the mix design. The

sand was oven dried, had and absorption of 5.1%, and was sieved through a U.S. Standard No.8

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sieve resulting in a maximum aggregate size of 2.36 mm. Type C Graphene with a surface area

of 300 m2/g, obtained from XG Sciences, was used in preparation of the GNP reinforced mortar.

In order to aid in dispersion of the GNP and to increase the workability of the GNP reinforced

mortar, a surfactant, sodium deoxycholate was stirred into the mixing water by hand for 5

minutes. The measured GNP was then added to the mixture and the solution was ultrasonicated

for 1 hour in a bath ultrasonicator. The solution was then mixed with the sand and cement and

cast into six 50.8mm x 50.8 mm cubes and four 40mm x 40mm x 160 mm prisms, and tamped as

per ASTM standards. The mix design for the four batches is listed below and includes the

corrections for absorption of the sand and 10% loss of materials during mixing (Table 2).

TABLE 2 Mix Design for Mortar Specimens

Batch:

GNP

Type

GNP %

by weight

cement

GNP

(g) Surfactant (g) Cement (g) Water (g)

Fine

Aggregate

(g)

1: None -- -- -- 1364.00 854.05 3751.00

2: Grade C 0.1% 1.36 9.55 1364.00 854.05 3751.00

3: Grade C 0.3% 4.09 28.64 1364.00 854.05 3751.00

4: Grade C 0.5% 6.82 47.74 1364.00 854.05 3751.00

The cubes and prisms were demolded after 48 hours are air cured for 28 days in preparation for

testing. The planned tests for each batch of mortar are listed below, including compressive

strength tests for the cubes, flexural strength tests for 3 prisms and electrical resistivity

measurements for one prism (Table 3). During the curing process, flexural test specimens were

painted with white paint, and speckled with black permanent markers on the surfaces in order to

prepare for digital image correlation (DIC) analysis upon testing.

TABLE 3 Planned Tests for One Batch of Mortar

Specimen Size Test

1-6 50.8 mm x 50.8 mm x 50.8

mm (2 in x 2 in x 2 in) Compressive Strength

7-9 40 x 40 x 160 mm (1.57 in

x 1.57 in x 6.30 in) Flexural Strength

10 40 x 40 x 160 mm (1.57 in

x 1.57 in x 6.30 in) Electrical Resistance

Test Set-Up

The compression test setup followed ASTM C109 for the compression testing of mortar cubes. A

pedestal was used to allow for the cubes to be compressed in the machine. The specimens were

loaded at a rate of 25 psi/s and had a preload of 1,500 lbf. The machine was set to automatically

stop when the load had reached 50% of the peak load of the specimen, resulting in the test

concluding when the specimen was cracked until it could not handle additional loading.

The flexural test followed ASTM C348 standards for flexural test of mortar prisms. The

test consisted of using 3-point bending to load the prism and determine the flexural strength of

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the mortar (15). Digital image correlation (DIC) cameras were set up to measure the mid-span

deflection of the beam as it was loaded, as shown in Figure 1. The DIC cameras captured a

picture of the prism, every 10 frames per second, and used the movement of the speckles on the

prism in the pictures to determine the deformation as a result of loading. The DIC data pictures

were post-processed in software which collected the speckle data of the prisms and used that to

calculate the vertical and horizontal deformation of the beam.

FIGURE 1 Setup of the Flexural strength test with DIC camera on right

Along with this, laser analysis was used to measure the mid-span displacement of the beam. A

laser was set up with the beam running vertically through the prism and the bottom of the

support. A piece of reflective tape was attached to the middle tension zone of the prism and the

middle top of the support, as shown in Figure 2. As the prism was loaded, the laser measured the

change in distance between the bottom of the beam and the top of the support, and determined

the mid-span deflection with loading. The prisms were loaded at a force-control rate of 419

N/min and were loaded to failure.

FIGURE 2 Image of laser collected the mid-span deflection data for flexural test prism

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The electrical resistivity of the prism was measured using a Wenner four probe apparatus which

measured the resistivity across the prism. The Wenner probe allowed for two separate methods

of measurement. The first form of measurement was using the apparatus which had four probes

fixed at 38 mm apart from each other, as shown in Figure 3. The second form included using a

device that had four adjustable probes which were adjusted to be 40 mm apart from each other,

shown in Figure 4. In all measurements, the surface of the specimen was made moist with water

and the measurements were taken at the center of the prisms. The different methods were

compared to ensure accuracy of the resistivity measurements.

FIGURE 3 Wenner Probe for resistivity FIGURE 4 Wenner adjustable probe

RESULTS

Compressive Strength Results

The 28 days compressive strength of the mortar prisms was measured using a compressive test

machine. The results from the test are listed in the table below, and contain three compressive

strength values for each batch of mortar (Table 4). The three trials from each batch were

averaged and graphed in Figure 5 below.

TABLE 4 Compressive Strength Test Results

Amount Graphene by

weight cement (%)

Compressive Strength

(MPa)

Average Compressive Strength

(MPa)

0

37.1

35.2 34.3

34.1

0.1

36.5

33.6 31.3

33.1

0.3

21.3

21.7 22.2

21.7

0.5

32.5

32.2 31.3

32.7

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FIGURE 5 Graph of Average Compressive Strength of Mortar

The compressive strength of the 0.1% and 0.5% GNP does not show a significant increase or

decrease in strength compared to the control mortar mix. Difficulties in dispersion of the GNP

through the mixing water and clumping of the GNP are possible reasons for this lack of change.

The drop in strength of the 0.3% GNP mortar is likely due to the fact that the mortar was cast on

a different day than the rest of the batches which resulted in different mixing and curing

conditions and ultimately lower strength.

Flexural Test Results

The flexural test results included the peak flexural load data collected from the loading machine

and both the digital image correlation (DIC) data and the laser deflection data.

The peak flexural strength of the beam was calculated using the following equation:

𝜎 =3𝐹𝐿

2𝑏𝑑2 (1)

Where F is the loaf applied, L is the span of the prism at loading, b is the width of the prism and

d is the depth of the prism. The values obtained for the flexural strength are located in the table

below (Table 5).

TABLE 5 Flexural Strength of the Prisms Under Loading

Total Max. Load (N) Flexural Strength (MPa) Flexural Strength (MPa)

0

1997.992972 6.393140451

5.802304233 1719.504542 5.502038392

1722.534581 5.511733856

0.1

1957.087038 6.262250408

5.429152894 1361.467763 4.356399022

1771.623998 5.668809252

0.3

1511.407216 4.836172471

4.857650812 1559.271637 4.989328149

1483.680116 4.747451817

0.5

1398.414959 4.474621965

4.941379361 1734.641084 5.550472015

1499.803808 4.799044102

0.0

10.0

20.0

30.0

40.0

0 0.1 0.2 0.3 0.4 0.5 0.6

Co

mp

ress

ive

Str

en

gth

(M

Pa)

% Graphene Nanoplatelets by Weight Cement

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The figure below shows the average peak flexural strength of the mortar prisms as a function of

the percentage GNP by weight cement (Figure 6).

FIGURE 6 Peak Flexural Strength of the Prisms

The flexural strength of the beam shows a slight decrease in strength as a result of the addition of

GNP to the mix. The possible reason for this decrease is that the addition of GNP resulted in the

clumping of the GNP, forming irregularities in the mortar and ultimately decreasing the strength

of the GNP.

The mid-span deflection of the prism found using the DIC data and the laser was plotted

as a function of the stress on the prism. The lowest peak flexural stress was plotted for each

batch of mortar, as shown in Figure 7 below.

FIGURE 7 Displacement vs Flexural Stress for Worst Case Strength of Prisms

3

3.5

4

4.5

5

5.5

6

0 0.1 0.2 0.3 0.4 0.5 0.6

Fle

xu

ral

Str

eng

th (

MP

a)

% GNP by Weight Cement

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From the figure of the displacement vs. flexural stress, it can be seen that the peak flexural stress

does not significantly increase as a result of adding GNP. However, the stiffness of the prisms,

shown by the slopes of the graphs is seen to increase with the addition of 0.1% GNP. However,

the addition of 0.3% GNP results in the stiffness decreasing and the 0.5% GNP returning to the

stiffness of the control batch.

Electrical Resistivity

The electrical resistivity of the prisms was measured using a Wenner four probe apparatus. The

figure below shows the values of the resistivity measured using both the fixed 38 mm probe on a

damp and moist surface, and the adjustable probe on a moist surface (Figure 8).

FIGURE 8 Electrical Resistivity Measurements of the Prisms

The electrical resistivity measurements of the prisms showed no increase or decrease in value as

a result of GNP addition. With this information, it is possible to conclude that more GNP

addition is required to notice a change in the resistivity of the mortar.

CONCLUSION The primary objective of this project was to observe the effect of graphene nanoplatelet

reinforcement on the compressive strength, flexural strength, stiffness, and electrical resistivity

of mortar cubes and prisms. The GNP reinforcement at 0.1%, 0.3% and 0.5%, does not

significantly increase or decrease the compressive and flexural strength of mortar. The GNP

reinforcement increases the stiffness of the mortar at 0.1%, however the stiffness returns to that

of the control, as the amount of GNP increases to 0.5%. Higher percentages of GNP by weight of

cement are required to observe a change in resistivity. This leads to future work which may

include using higher amounts of GNP to observe a change in resistivity and ultimately

developing a relationship between resistivity and damage in material. In addition to this, analysis

of the permeability of the mortar will allow for an assessment of the durability of the material.

0

10

20

30

40

50

60

70

80

90

0 0.1 0.3 0.5

Re

sist

ivit

y (k

cm

)

% GNP by weight cement

Wenner 38mm--Damp/Dry

Wenner 38mm--Damp/Moist

Adjustable Probe--Damp/Moist

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ACKNOWLEDGMENTS

This project was completed with resources provided by the Mid-Atlantic Transportation

sustainability University Transportation Center (MATS UTC), the Virginia Center for

Transportation Innovation and Research (VCTIR), and the University of Virginia Department of

Civil and Environmental Engineering.

A note of thanks to Dr. Osman Ozbulut, graduate students Sherif Daghash, Muhammad Sherif,

and Evelina Khakimova, Dr. Emily Parkany, and Dr. Andrei Ramniceanu for their guidance and

assistance in completing this project.

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