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ICEAS-1707

Influence of Fly Ash on Self-healing Performance in Cracked Concrete

Mohamed Zakaria*

Muroran Institute of Technology, Muroran 050-8585, Japan

Faculty of Engineering, Aswan University, Aswan 81542, Egypt

[email protected]

Na Seung Hyun

Muroran Institute of Technology, Muroran 050-8585, Japan

[email protected]

Yukio Hama

Muroran Institute of Technology, Muroran 050-8585, Japan

[email protected]

Abstract

This paper investigates the self-healing ability in cracked concrete specimens, with or without

air entraining agents, incorporating by-products fly ash that is a kind of coal ash. Concrete

specimens are cracked by accelerated environmental action that is repeating cycles of

low/high temperatures or freezing/thawing process. The experiments examined the change of

relative dynamic modulus of elasticity, compression strength and carbonation coefficient as

index of self-healing properties in concrete specimens before cracking, after cracking, after

healing due to curing in water. The results revealed that incorporating fly ash in concrete

showed a positive effect on compressive strength, and it continues to increase even after 28

days due to its pozzolanic reaction and attributed to densification of concrete matrix.

Moreover, it was observed that self-healing effect is related to curing age before deterioration,

when curing age increases, for long-term curing age case, the healing ability decreases due to

decrease of un-reacted cement and fly ash in investigated concrete samples. Finally, it is

recommended to add air entraining agent in fly ash concrete specimens to increase the air

content and consequently the frost resistance of concrete.

Keyword: Concrete, Self-healing ability, Fly ash, Compressive strength, Carbonation

coefficient, Micro-cracks, Mineral admixture

1. Introduction

Concrete material has been widely used for a long time in civil engineering and construction

field all over the world for its wide application, as many benefits such as low cost, extremely

strong compression, etc. It is quite needed to maintain the structural performance of concrete

structures, such as serviceability and durability, in order to prolong their service life.

However, the deterioration of concrete is inevitable since there are many deterioration

mechanisms, such as carbonation, alkali-aggregate expansion reaction, freeze-thaw expansion,

salt scaling by deicing salts, autogenous and drying shrinkage, surface attack on exposure to

ground waters containing surface ions, sea water attack, and corrosion that caused by salts,

can harm adversely the performance of concrete structures due to its exposure to severe

weather conditions [1, 2]. This adverse harm can be increased greatly in case of cracked

concrete which has internal or external micro cracks, due to the rapid mass transport through

those micro cracks induced in mortar or concrete. Due to the deterioration more cracks occur

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into the concrete, leading to the reduction of service life time of concrete structure. Further,

when cracks are introduced into concrete, the cost and amount of labor required for diagnosis

and repair work should be considered in order to restore its original performance. Therefore,

to solve this problem, it is necessary to have smart technique to fill the micro-cracks to

extend the service life of the concrete structures. Mineral admixtures as by-products, which

are fly ash, ground granulate blast furnace slag and silica fume, etc, have been widely used to

improve the mechanical properties of concrete. However, the effect of such materials on selfhealing

ability have not been fully explored since the effect of environmental conditions as

deterioration mechanisms is complicated and can result in variable widths and locations of

micro-cracks inside the concrete matrix.

To date, research on self-healing ability has been widely performed all over the world. The

ability is unique and promising solution to recover the damaged concrete that caused by

various deteriorations such as carbonation, autonomous and drying shrinkage and frost

damage and so on. In 2007, JCI Technical Committee (JCI-TC075B 2007) was established

and the task of this committee was to investigate the self-healing ability in cementitious

materials as shown in Fig. 1.

Many researches have been carried out about self-healing ability in concrete through many

techniques [1, 4, 5, 6, 7]. However, those methods have many disadvantages such as cost

efficiency, difficulty of casting and limited amount of healing agent. Fly ash and ground

granulate blast furnace slag are a promising solution, due to the fact that the materials have

good pozzolanic and latent hydraulic activities in comparison with normal cement, thus,

improved workability, long-term strength, reduced alkali silica reactivity, lower porosity and

moreover reduce CO2 emissions considering it as eco-friendly solution to the environment.

Pipat et al. [4] have investigated the self-healing ability in cracked fly ash blended cement

systems for autogenous and drying shrinkage after 28 days, they pointed out that when

cement was replaced by fly ash, the compressive strength decreased. This problem may be

overcome if fine aggregate instead of cement is replaced by fly ash. The fly ash reacts with

calcium hydroxide and water then produces C-S-H gel, which called pozzolanic reaction. The

amount of cement reduction will vary depending on the reactivity of the pozzolan.

Engineered healing

Activated

repairing

Autogenous

healing

Natural

healing

Autonomic

healing

Fig. 1 Definition of self-healing [3]

The objective of this paper is to investigate experimentally the self-healing performance of

cracked concrete incorporating fly ash (FA) with or without air-entraining agent. Concrete

specimens are cracked by accelerated environmental action that is repeating cycles of

low/high temperatures or freezing/thawing process. The experiments discussed how the

change of relative dynamic modulus of elasticity, compression strength and carbonation

coefficient as index of self-healing properties in concrete specimens before cracking, after

cracking, after healing due to curing in water can be. The obtained findings are useful

information for concrete material field.

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2. Experimental Work

2.1 Experimental materials

Chemical and physical properties of used cement, fly ash, fine aggregate and coarse

aggregate are given in Tables 1, 2 and 3. Ordinary Portland cement (OPC) was used in

preparation of concrete specimens. All cylindrical concrete specimens (100Ø×200 mm) were

casted using water to cement ratio of 0.51 and sand to coarse aggregate ratio around 0.46 to

reach acceptable degree of workability and slump values around 180 mm, as shown in Table

4 for the mix proportions and fresh properties of examined mixtures. Four mixtures, which

included two types of normal concrete (N, F) without air-entraining agent and two types of

air-entrained concrete (NA, FA), were tested to evaluate the self-healing ability of concrete

samples w/o fly ash and w/o air-entraining agent. In Table 4, N and NA refer to OPC

concrete samples without adding fly ash, while F and FA refer to fly ash concrete samples

incorporating fly ash with 15% replacement ratio of fine aggregate by volume. All of the

investigated concrete samples were mixed in accordance with JIS A-1138 and were designed

to ensure the required slump and air contents. The slump was set 180±20 mm. After casting,

concrete samples were sealed and cured in laboratory for 1 day, then, placed in water

container at 20±3 degree until required test ages.

2.2 Test Methods

Compression tests of examined fly ash concrete samples were carried out according to JIS A-

1108 at age of 7, 28 and 91 days to know how fly ash can contribute into the development of

the strength.

Table 1 Characteristics of used cement

Cement

type

Density

(g/cm3)

Blain

(cm2/g)

Chemical composition (%) Mineral composition (%)

SiO2 Al2O3 Fe2O3 CaO MgO SO3 C3S C2S C3A C4AF

N 3.16 3250 21.5 5.4 2.9 64.3 1.9 1.8 52 23 10 9

Table 2 Properties of fly ash

Fly

Ash

Density

(g/cm3)

Blain

(cm2/g)

Chemical composition (%)

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3 TiO2 MnO

FA 2.11 3900 73.1 16.9 3.0 1.6 1.2 0.3 1.0 0.2 0.7 0.0

Table 3 Physical properties of used aggregate materials

Types of aggregates

Surface dried density

(g/cm3)

Absolute dried

density (g/cm3)

Water absorption

ratio (%)

Coarse

grain ratio

(%)

Fine aggregate 2.65 2.64 0.42 2.66

Coarse aggregate 2.67 2.62 1.93 6.64

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Table 4 Mix proportions of concrete samples and fresh characteristic

Note: s/a=fine aggregate/coarse aggregate, Ad1) = high performance water reducing agent, Ad2) = airentraining

agent and fly ash air entraining agent added only to FA mixture. W/C = water-to-cement

ratio

For self-healing evaluation, after curing concrete samples at 28 days, the relative dynamic

modulus of elasticity (RDM) was measured (recorded as initial value of RDM). Then, in

order to introduce the micro cracks in the fly ash concrete sample, the freeze and thaw test

was performed until 30 cycles of freezing/thawing in accordance with JIS A-1127. After

deterioration of concrete samples, curing conditions of concrete samples were considered in

water at 40􀉗 for 2 weeks as one cycle of deterioration/healing in order to investigate the

self-healing potential in designed fly ash concrete samples. Further, accelerated carbonation

test (phenolphthalein method) was performed to calculate the carbonation depth and

coefficient before/after deterioration of fly ash concrete occurred and after curing-based selfhealing

of fly ash concrete samples, in accordance with JIS A-1153.

3. Results and Discussion

3.1 Compressive Strength

Figure 2 shows the results of compressive strength for four mixtures of concrete samples at

the ages of 3, 7, 28, 91 and 365 days. The compressive strength for N and NA concrete

samples showed a significant increase in compressive strength until 7 days of curing age, and

then exhibited a slight increase at 28 days of curing age. While, the compressive strength of

concrete samples (F and FA) produced with fly ash as replacement of fine aggregate

continued to increase significantly after 28 days, particularly increase of compressive strength

for F sample increase was remarkable. Therefore, in the comparison to normal concrete

without fly ash (N and NA), fly ash concrete samples (F and FA) exhibited greater

compressive strength, this increase in compressive strength can be related to different pore

structure in concrete samples. Hence, the fly ash replacement of fine aggregate can improve

the compressive strength due to its pozzolanic effect and attributed to densification of

concrete matrix.

Mixture

W/C

(%)

s/a

(%)

Unit weight􃸦kg/m3􃸧

Slump

(mm)

Air

content

Water Cement (%)

Fly

ash

Fine

aggregate

Coarse

aggregate

Ad1) Ad2)

N

51.1

47 151 296 􃸫 955 1084 3.85 􃸫 165 4.0

NA 46 151 296 􃸫 880 1038 3.26 0.067 175 5.5

F 48 151 296 44.4 930 1065 3.85 􃸫 180 2.6

FA 46 151 296 44.4 836 1038 3.26 0.2893) 190 5.1

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0

10

20

30

40

50

60

70

0 100 200 300 400

Compressive strength(MPa)

Curing age (days)

N NA

F FA

Fig. 2 Compressive strength development with curing ages until 365 days

3.2 Frost Resistance

Figure 3 presented the results of freeze/thaw test for concrete samples with different curing

ages at 28 and 365 days according to the ASTM C 666 standards. The relative dynamic

modulus of elasticity is a non-damaged test method to evaluate the deteriorated concrete. For

example, RDM reached 60% at 42 cycles in case of F fly ash concrete sample cured at 28

days, as shown in Fig. 3. However, in the case of FA concrete sample RDM was similar to

NA concrete sample, which is the case of concrete with air-entraining agents. For the case of

curing at 365 days, RDM values of all concrete samples were rapidly decreased before 30

cycles. In addition, the variation of RDM for N, NA and FA concrete samples showed a

similar trend.

3.3 Self-healing Ability in AE Fly Ash Concrete

The results of relative dynamic modulus of elasticity for fly ash concrete samples which

include 15 % by volume fly ash replacement ratio w/o air entrained agent, F and FA samples

respectively, and normal concrete samples w/o air entrained agent, N and NA samples,

respectively, at different curing ages, considering the three studies cases which are No

cracking case (initial value), deterioration case (micro-cracking) and water-based-healing

case at 40􀉗 for 4 weeks in water, are shown in Fig. 4. In the case of 28 days, after

deterioration, it can be seen from the figure that most of the investigated concrete samples

decreased until 80-90 of RDM. Then, after healing case, they were healed almost until No

cracking case (initial value) in all concrete samples. It can be revealed that most of the

investigated concrete samples were partially recovered after healing case to initial values of

RDM for the case of 28 days initial curing. While samples with initial curing of 365 days did

not show the same behavior due to the fact that self-healing ability can be decreased with

increasing initial curing ages due to decrease of un-reacted cement and fly ash in concrete

samples.

Relative dynamic modulus of elasticity(%)

Number of cycles

Fig. 4 Self-healing ability based on change of RDM in concrete samples with different curing

age (a) initial water curing at 28 days (b) initial water curing at 365 days

Figure 5 shows the results of accelerated carbonation test for concrete samples after two

initial water curing conditions (before deterioration), including four cases which consist of

No cracking case, after deterioration case, after healing case and repetition of deterioration

and healing case. The carbonation coefficient in all investigated concrete samples was

calculated by measured carbonation depth value until 26 weeks.

It can be confirmed from the figure that in both two curing ages at 28 and 91 days a

significant decrease of carbonation coefficient for F and FA concrete samples obtained,

which involve 15 % by mass of fly ash replacement ratio, implying that incorporating fly ash

into fine aggregate exhibits high self-healing performance after curing in water at 40􀉗 for 4

weeks, and hence can delaying carbonation process after deterioration.

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No cracking after deterioration after healing repetition of deterioration and healing

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