Scholarly article on topic 'An experimental investigation on the erosion resistance of concrete containing various PET particles percentages against sulfuric acid attack'

An experimental investigation on the erosion resistance of concrete containing various PET particles percentages against sulfuric acid attack Academic research paper on "Agriculture, forestry, and fisheries"

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{"Sulfuric acid attack" / "Polyethylene Terephthalate (PET)" / "Waste management" / "Ultrasonic test" / "Weight loss and crushing load variation"}

Abstract of research paper on Agriculture, forestry, and fisheries, author of scientific article — H. Janfeshan Araghi, I.M. Nikbin, S. Rahimi Reskati, E. Rahmani, H. Allahyari

Abstract PET (Polyethylene Terephthalate) is one of the most common plastics for various productions. The rapid increase in the use of these products causes increment in solid waste problems. Eventually, management and prevention of solid waste resulted by urban and industrial waste are dramatically important. On the other hand, deterioration of sewage purification structures, part of industrial structures and concrete sewer pipes attacked by sulfuric acid is an important issue of the world that has communicated with substantial financial need in all countries. Thus, not so many researches has been investigated the effect of sulfuric acid erosion on concrete containing PET particles, this study represents the laboratory investigation of different values of 0%, 5%, 10%, and 15% of PET particles as an alternative aggregate on erosion caused by sulfuric acid in concrete. All samples were cured during 28days, 3 samples from each mix design were tested, finally. In intervals of 15, 30, and 60days of immersion in 5% sulfuric acid, 3 specimens were taken out from sulfuric acid solution. Then, after drying these specimens and measuring their dimensions, crushing load, weight and ultrasonic wave velocity values were obtained. The results showed that by increasing in amount of PET particles as an alternative aggregates in concrete, there is less reduction in values of crushing load, weight loss and ultrasonic wave velocity. Also, it could be seen that ratios of crushing load reduction to weight loss for specimens attacked by 5% sulfuric acid solution were increased by increasing in percentage of PET particles, which illustrates that in a particular range of crushing load, less weight loss was occurred in concretes included more PET particles.

Academic research paper on topic "An experimental investigation on the erosion resistance of concrete containing various PET particles percentages against sulfuric acid attack"

ELSEVIER

Contents lists available at ScienceDirect

Construction and Building Materials

journal homepage: www.elsevier.com/locate/conbuildmat

An experimental investigation on the erosion resistance of concrete containing various PET particles percentages against sulfuric acid attack

H. Janfeshan Araghia, I.M. Nikbinb, S. Rahimi Reskatia'*, E. Rahmanic, H. Allahyaric

a Department of Civil Engineering, Gorgan Branch, Islamic Azad University, Gorgan, Iran b Department of Civil Engineering, Rasht Branch, Islamic Azad University Rasht, Iran c Department of Civil Engineering, Babol University of Technology, Iran

HIGHLIGHTS

• PET particles were used as an alternative aggregate against erosion caused by sulfuric acid in concrete.

• Concretes included PET particles more retain their load bearing capacity under sulfuric acid attack.

• Under sulfuric acid attack, changes in crushing load are nearly similar to ultrasonic wave velocities.

• Concretes included PET particles are able to utilize in environments under sulfuric acid attack.

ARTICLE INFO

ABSTRACT

Article history:

Received 14 September 2014

Received in revised form 18 December 2014

Accepted 22 December 2014

Available online 14 January 2015

Keywords: Sulfuric acid attack Polyethylene Terephthalate (PET) Waste management Ultrasonic test

Weight loss and crushing load variation

PET (Polyethylene Terephthalate) is one of the most common plastics for various productions. The rapid increase in the use of these products causes increment in solid waste problems. Eventually, management and prevention of solid waste resulted by urban and industrial waste are dramatically important. On the other hand, deterioration of sewage purification structures, part of industrial structures and concrete sewer pipes attacked by sulfuric acid is an important issue of the world that has communicated with substantial financial need in all countries. Thus, not so many researches has been investigated the effect of sulfuric acid erosion on concrete containing PET particles, this study represents the laboratory investigation of different values of 0%, 5%, 10%, and 15% of PET particles as an alternative aggregate on erosion caused by sulfuric acid in concrete. All samples were cured during 28 days, 3 samples from each mix design were tested, finally. In intervals of 15, 30, and 60 days of immersion in 5% sulfuric acid, 3 specimens were taken out from sulfuric acid solution. Then, after drying these specimens and measuring their dimensions, crushing load, weight and ultrasonic wave velocity values were obtained. The results showed that by increasing in amount of PET particles as an alternative aggregates in concrete, there is less reduction in values of crushing load, weight loss and ultrasonic wave velocity. Also, it could be seen that ratios of crushing load reduction to weight loss for specimens attacked by 5% sulfuric acid solution were increased by increasing in percentage of PET particles, which illustrates that in a particular range of crushing load, less weight loss was occurred in concretes included more PET particles. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://

creativecommons.org/licenses/by/4.0/).

1. Introduction

1.1. PET concrete

PET (Polyethylene Terephthalate) is a usual plastics which is used as a raw material for making products like blown bottle which is used to produce soft drinks, food containers, etc. In recent decade

* Corresponding author. Tel.: +98 9113781399. E-mail addresses: dr.h.janfeshan@gmail.com (H. Janfeshan Araghi), Nikbin@ iaurasht.ac.ir (I.M. Nikbin), saman.rahimi.r@gmail.com (S. Rahimi Reskati), Ebrahim. rahmani84@gmail.com (E. Rahmani), allahyari.h@gmail.com (H. Allahyari).

due to rapid increment in the use of PET bottles, the problem of solid waste is increased. Therefore, management and prevention of solid waste due to residential and industrial wastes has become very important [1]. In Beijing more than 150 tons of PET is wasted in a year [2]. This issue is known clearly that waste PET bottles need too long time to decompose in the nature [3]. Also destroying PET bottles improperly, causes lots of environmental problems in the nature. For example, burning PET as fuel, releases toxic gases in environment and contributes to produce acid rains which cause to concerns about air pollution and public Health. Therefore, investigations on proper and economical recycling methods in this field is significantly needed. Investigations on conversion of PET bottle

http://dx.doi.org/10.1016/j.conbuildmat.2014.12.037 0950-0618/® 2015 The Authors. Published by Elsevier Ltd.

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

wastes in structural industry due to its many environmental and economic benefits, have conducted globally.

Laboratory investigations indicate that use of PET as resin in production of polymer concrete, not only has positive environmental effects but also causes decrease in production cost of polymer concrete [1].

Recently significant growth in use of waste materials as substitute for proportion of concrete admixture materials has been done. Researchers used materials like sintered sludge pellets, rubber wastes, waste plastics, fiber glass waste materials, blast furnace slag, and granulated coal ash in concrete and investigated its effects on different properties of concrete [4].

According to this, usage of PET waste in concrete as an aggregate could be one of its extensive usage, has disposal of waste advantages and causes decrease in environmental damages due to less use of natural aggregate resources [4].

Generally, aggregate consists 65-80 percent of concrete proportion and has the main role in concrete behavior such as durability, dimensional stability and workability which could improve some of mechanical and durability properties of concrete due to low thermal conductivity, good abrasion behavior, high toughness and high heat capacity [5-7].

Akcaozoglu et al. [1] research has shown that use of PET in concrete due to lower specific gravity in comparison with ordinary aggregate causes decrease in concrete weight which is valuable advantage in design of structures.

Since, water absorption of mortars made by PET is very low, researchers suggest that due to its high resistance against erosion, it could be used in aggressive environments as a proportion of concrete [8]. Also, Won et al. and Sehaj et al. [9,10] showed that use of PET in concrete could increase ductility and decrease plastic shrinkage cracks which are related with concrete durability. Another investigations conducted by Akcaozoglu et al. [1] indicated that in concrete with PET particles, permeability and resistance against carbonation are improved.

However adding waste PET to concrete could have negative effects in quality of concrete such as decrease in compressive strength, tensile strength and modulus of elasticity due to low surface energy because of its weak mechanical bond [11-13]. Recent researches generally indicate that use of plastic wastes in concrete as substitute of aggregate could affect some of its properties.

1.2. Sulfuric acid erosion

In recent decade, durability of concrete in design of structures especially in infrastructures has become so important. Significant improvements in our knowledge about chemical processes of different concrete deteriorations have been achieved. Anyway our knowledge about interactions of this processes in final mechanical properties of concrete is limited yet.

Generally, exposing concrete in acid environments is the main debate about durability of concrete structures which affect performance, life time and maintenance cost of vital structural infrastructures. In fact, previous researches have illustrated that acids in ground water, chemical waste water or acids resulted by oxidation of sulfur compounds in backfill could attack concrete substructure members and influence their durability [14,15]. In addition, lots of concrete structures especially in industrial regions that often included sulfuric acid are exposed to acid rain erosion [14]. On the other hand, deterioration of sewer pipes and waste water purification structures attacked by sulfuric acid is global challenge which already have lots of economic necessities all around the world. Deterioration of sewer systems lead to serious problems such as reduction of ability in waste water transfer, pollutions of soil and ground water and excessive ground settlement cave-in [16]. Therefore, supplying high quality wastewater infra-

structure needs significant expenditure on concrete with expected quality. For example only in U.S, annual control and maintenance expenditures are even more than investing in construction of new wastewater structures [17].

Hewayde et al. [18] estimated that costs of maintenance and repair due to biogenic sulfuric acid attack on sewer systems are almost 100 billion U.S dollars. Therefore, motivation of researches in field of concrete sewage systems durability exposed to erosion is increasing.

Olmstead et al. [19] were first one who reported erosion in sewer systems. In 1920, hydrogen sulfide (HS) was known as erosion factor in Cairo sewer systems too. Initially it was believed that HS is produced by reduction of sulfate in sewage flow, and then is changed to sulfuric acid by breaking out to sewer atmosphere and oxidation [20].

In 1945 Parker [21] attributed this phenomena to chemical-microbial interactions in sewage systems and expressed that influence of sulfuric acid in concrete is more destructive than sulfate attack. Because in this situation there is dissolution effects consequent of hydrogen ions beside of sulfate ions attacks.

In fact, sulfuric acid reacts with calcium hydroxide (CH) existence in concrete and produce gypsum. Creation of gypsum in concrete causes volume to increase by factor of nearly 2, although some researchers mentioned that this reaction plays a secondary role in erosion procedure. Indeed, reaction between gypsum and calcium aluminate hydrate (C3A) and consequently creating ettringite, leads to much more deterioration. Volume of ettringite is greater than initial compounds (nearly 7 times) [22]. These voluminous compounds cause inner pressure in concrete that lead to formation of cracks [23]. While the erosion continues, cracked surface is become soft and white and the corroded concrete loses its mechanical strength that contributes to more cracking, spalling and finally leads to completely destruction [24-28]. The findings of the present investigations are not used in many of common constructions yet. Lack of enough researches on presenting a proper relation for designing a durable structure, expresses that mechanisms related to concrete deterioration are not widely accepted and understood yet.

Furthermore, erosion of water and wastewater infrastructures is on the top of dissensions during last decades. There are many opinions about designing and characteristic of a proper infrastructures to deal with corrosive environment during service life [29-34]. Some of these researches on the one hand illustrate improvement of concrete properties by use of various admixtures and innovative materials compounds and at last introduce a modern concrete, and on the other hand prevent growing sulfuric acid-producing bacteria [35,36].

So far, various researches have been done on effects of cement type, water to cement ratio (W/C) [14,37,38], supplementary cementitious materials [18,35,39,40], polymer materials and type of aggregates [16], to improve normal concrete and mortar resistance against acid attacks.

In spite of many accomplished researches, previous data on influence of various materials in normal concrete resistance against sulfuric acid attack are contradictory. As it was mentioned, nowadays use of concretes included PET particles around the world are increasing because of their economic and environmental useful aspects. Therefore, utilizing this kind of concrete in a more appropriate way in structural industry needs more researches concentrating on their properties and durability. In recent decades, concrete mechanical properties consisted of PET particles have been investigated in various points of view. Investigations on durability of this kind of concrete are the main factor in designing and constructing infrastructures. As regards, a few studies have been conducted in field of sulfuric acid effect on PET concretes erosion, it is likely that durability of PET concretes be different with normal

Fig. 1. Gradation curve of fine and coarse aggregates with ASTM C33 [41] standard limits.

concretes. Main motivation of this study is experimental investigation on effect of various amount of PET particles as substitution of aggregate on erosion resulted by sulfuric acid in concrete and efforts to relate such influences on physical and mechanical changes imparted by these admixtures. The relationship between concrete crushing load reduction and its weight loss because of sulfuric acid attack was recognized. Moderate decline in concrete weight loss due to the utilization of effective admixtures contribute to significant gains in structural integrity and substantial life cycle performance importance which maybe in interest to manufactures of sewer equipment and owners of water treatment infrastructures [18]. It is hope that this study would increase understanding of the deterioration mechanisms and equip the introduction of a performance base design approach. After exposing concrete specimens to 5% sulfuric acid solution (PH « 1), the behavior of concrete included various percentages of PET particles were investigated. Because of substitution of PET particles in mix proportions of this study as proportion of fine aggregate, it may modify the erosion development mechanisms in concrete. Therefore, effects of different PET values in concrete due to sulfuric acid attack was studied. The samples were investigated by measuring their weight loss, load bearing capacity and ultrasonic wave velocity.

2. Experimental program

2.1. Materials

Table 1

Chemical properties of cement.

Component Portland cement type II

ISIRI 389 Iranian standard Test result

SiO2 >20 21.90

Al2Û3 >6 4.86

Fe2O3 <6 3.30

CaO 62-66 63.32

MgO <5 1.15

SO3 <3 2.10

K2O 0.5-1 0.56

Na2O 0.2-0.4 0.36

Free CaO - 1.10

Blaine (cm2/gr) - 3050

Table 2

Physical and chemical characteristics of the fine (sand) and course (gravel)

aggregates.

Aggregate Gravel Sand

Specific gravity (g/cm3) 2.51 2.75

Unit weight (kg/m3) 1581.3 1728.9

Moisture content (%) 0.2 0.4

Moisture of saturated surface dry (%) 0.5 0.7

Fines modulus (FM) - 2.82

Sand equivalent value (SE) (%) - 80

2.1.1. Cement

The cement utilized in this study is Portland cement type II. Its density is 3.14 gr/cm3 and specific surface area (Blaine surface) is 3050 cm2/gr. The chemical composition is given in Table 1.

2.1.2. Aggregate

The specific gravity in saturated surface dry (SSD) states for coarse and fine aggregate were 2.51 gr/cm3 and 2.75 gr/cm3, respectively. The maximum size of coarse aggregates was 19 mm. The physical and chemical properties of aggregates are shown in Table 2, while the Fig. 1 illustrates the gradation results.

2.1.3. PET particles

PET is a polymer with flexural modulus of elasticity of 2.5 GPa and tensile modulus of elasticity of 2.9 GPa. It has high chemical resistance and maximum tensile strength of 60MPa. PET is a semi-crystalline polymer with melting point of 260 "C [42]. In this paper, PET particles are produced by grinding and utilized as fine aggregates in concrete as shown in Fig. 2. These particles were provided from waste PET bottles which were washed and grinded by means of industrial machines. PET particles had the maximum size of 7 mm and the estimated unit weight of 464 kg/ m3 with specific gravity of 1.11 gr/cm3. The sieve analysis results are shown in Table 3.

2.2. Concrete mix design

Percentages of concrete components were considered according to ACI-211-1-89 [43]. Table 4 give the concrete mixture designs. For mixing the substances together, first half of the sands and gravel were mixed together, after that the PET particles were added in the mixture so it could be mixed with the existing materials. Then, in order to saturate the aggregates, 20% of the existing water was added. After that, remained cement and sand were added in the mixture and finally the remained water was poured to the materials. It is important to note that by increasing in the number of PET particles, the materials mix duration is raised. All of the concrete's mechanical properties with substitution of 5,10,15 volumetric percentages of PET with sand are compared to control specimen with 0% of PET.

2.3. Procedure

All of samples were cured at 20 "C and 95% RH for 28 days and finally 3 samples of each mix design were tested. The remained samples were divided in two equal groups. One of them was kept in curing container and the other one was transferred to 5% sulfuric acid solution (PH » 1) for simulating sewer pipes environment in laboratory accelerated test. Sulfuric acid solution were refreshed every week and it was kept by adding control values of sulfuric acid in PH » 1. The purpose of samples

m r vr^i'i'i'i'im

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periods of 15, 30, and 60 days and rinsed three times with tap water to remove loose reaction products. Then, after drying at room temperature for 30 min, their dimensions were measured and crushing load, weight loss, and ultrasonic wave velocity tests were conducted. The percentage of weight loss at each date was utilized as an indicator for assessment concrete deterioration subjected to sulfuric acid attacks. For each samples, cumulative weight reduction (MLt) is calculated by Eq. (1):

mt - mj

Fig. 2. Type of PET particles.

Table 3

PET particles specification.

Sieve size

Percent remaining on the sieve

7 mm 4.75 mm 2.36 mm 1.18 mm 600 im 300 im 150 im <150 im

Unit weight (ASTM C29) (kg/m3) Specific gravity (g/cm3)

12.5 67.5 15 2.5 1.5 1 0

464.265 1.11

Table 4

Concrete mixture proportion.

Component Content (kg/m3 W/C = 0.54

0% PET 5% PET 10% PET 15% PET

Cement 379.6 379.6 379.6 379.6

Water 210.2 210.2 210.2 210.2

Gravel 976.1 976.1 976.1 976.1

Sand 745.9 708.6 671.3 634

PET - 10 20.1 30.1

division was ability of specimen's comparison and clear definition analysis of those samples in sulfuric acid solution. 3 samples of each mix design immersed in 5% sulfuric acid solution and 3 samples of each mixture in curing container extracted in

where Mt: weight at time t (kg), M^ initial weight before exposure to sulfuric acid (kg).

Weight loss is a simple and traditional measuring factor in acidic attack tests [16]. However, weight loss results could depend on sample size, cement type, and also it could be significantly under effect of reactions products characteristics and decomposed cement paste on specimens. So, in this paper, 10 x 10 x 10 cubic sample's crushing load bearing capacity is considered in order to the more effective measuring of concrete resistance against sulfuric acid attack. During measuring sample's compressive strength, there were lots of problems in measuring sample's dimensions after acidic attack because the dimensions could become irregular with exposed aggregates. To prevent this problem, the load bearing capacity of a cubic sample may be determined by the maximum load recorded in a compressive test [16]. Compressive load in sample's test before and after acidic attack according to Chang et al. [16] is mentioned as crushing load and this measuring method is called crushing load test. Also for measuring the reduction value of crushing load to reference value before subjecting to sulfuric acid was calculated like weight loss formula. Also, to measure ultrasonic wave velocity according to ASTM C597 [44], an ultrasonic non-destructive electronic machine (PUNDIT MODEL PC1012) with the accuracy of 0.1 is was used. A transducer with vibration frequency of 52 kHz with accuracy of ±1% for travel time and ±2% for distance was also utilized. Nine measurements were performed for three cubic samples of each design in various ages, and the minimum time among them was recorded.

3. Results and discussion

Relation between 28 days crushing loads of concrete samples and their weight losses during the immersion in 5% sulfuric acid solution have been shown in Fig. 3. As it is shown, the graph was divided to four quadrant with the center point intersection of 28 days crushing load and 60 days weight loss of control samples attacked by 5% sulfuric acid.

- Points on up and left quadrant of graph have higher crushing load and lower weight loss than control concrete (OBP).

- Points on down and left quadrant of graph have lower crushing load and weight loss than control concrete (OD).

- Points on up and right quadrant of graph have higher crushing load and higher weight loss than control concrete (OD).

- Points on down and right quadrant of graph have lower crushing load and higher weight loss (OWP).

Fig. 3. Relation between 60 days weight loss and 28 days crushing load. OBP, overall better performance; OWP, overall worse performance; and OD, object dependent.

t(day)

Fig. 4. Deterioration ratio of crushing load.

40 t(day)

Fig. 5. Relative changes in crushing loads after immersion in 5% sulfuric acid solution.

Moderate reductions in weight loss of concrete due to the use of effective admixtures imply substantial gains in structural integrity and significant life cycle performance improvements, which may be of interest to manufacturers of sewer facilities and owners of water treatment infrastructure [18].

With regard to the graph, points resulted by crushing load and weight loss tests of concrete samples included PET particles are located on down and left quadrant of graph (OD). This illustrates that by increasing in PET particles in concrete, although crushing load is slightly decreased, weight loss value of concretes with PET particles against sulfuric acid attack is decreased which indicates increase in their durability in aggressive environment. It is expected that crushing load could be increased with supplementary cementitious materials in concrete to achieve more benefits from this type of concrete. In the other words, if the purpose is to increase in concrete durability, supplying PET particles in concrete could be effective which it is likely that crushing load is slightly decreased. So, this type of concrete could be applicable

in cases that the goal is higher durability and have the least structural load capacity.

According to Fan et al. [45] to compare trend of decreasing in crushing load values of concrete specimens against sulfuric acid attack, Dcc ratio could be used. That is calculated by Eq. (2):

Dcc = ^ (2)

J cn,t

where Dcc: relative crushing load deterioration ratio for deteriorated concrete specimen. fcr: the crushing load of concrete exposed to the acid solution for t days. fcn t: the crushing load of control specimens cured in the water at the same age.

As it could be seen in Fig. 4, by increasing in immersion time in sulfuric acid solution, Dcc ratio decreases, although Dcc variations versus time have reduction trend by adding PET particles. In other words by increasing in PET particles in concrete samples, the lower crushing load reduction in samples attacked by sulfuric acid is

Fig. 6. Crushing load values and ultrasonic wave velocity versus immersion time in 5% sulfuric acid solution for different percentages of PET particles (a = 0% PET, b = 5% PET, c = 10% PET, d = 15% PET).

seen. The values of Dcc ratio reduction of samples included 0%, 5%, 10%, 15% of PET after 60 days immersion in 5% sulfuric acid solution are 59.3%, 55.7%, 54.6%, 48.2%, respectively. It is clear that load bearing capacity reduction percentage in concrete specimens with 15% of PET particles is minimum which indicates positive effects of PET particles on the concrete erosion.

Fig. 5 represents the relation between crushing load differences of concrete samples immersed in 5% sulfuric acid solution and their

values at 28 days age versus immersion time. As it could be seen, crushing load reduction trend was started from primary ages, but this trend had obvious difference in samples with 15% PET particles from the beginning and followed lower reduction trend. Specimens with 0%, 5%, and 10% of PET particles until less than 60 days immersion indicated nearly similar reduction trend. According to graph, it could be expressed that samples included 15% of PET particles significantly showed lower resistance reduction during sulfu-

Fig. 7. Values of crushing loads and ultrasonic wave velocity versus various PET percentages in different ages of immersion in 5% sulfuric acid solution (a = 0 day immersion, b = 15 days immersion, c = 30 days immersion, d = 60 days immersion).

ric acid immersion. Also, reduction trend of specimens with 5% and 10% PET particles after 50 days immersion was became constant while the control specimen was continued its reduction trend.

As it is seen in Fig. 6, graphs of crushing load and ultrasonic wave velocity versus immersion time in 5% sulfuric acid solution of specimens included 0%, 5%, 10%, 15% of PET particles were drawn. According to graphs, both crushing load and ultrasonic wave velocity reduction trend are nearly the same. This fact could

illustrates that by increasing in immersion time in 5% sulfuric acid solution and penetrating acid into the concrete, beside that concrete has less healthy core, erosion products are produced in concrete and because of having more porosity and less integrity, it contributes to ultrasonic wave velocity reduction. This reduction trend in specimens included 15% of PET particles attacked by 5% sulfuric acid was significantly reduced in crushing load and ultrasonic wave velocity. It may be said that this is because of better

Fig. 8. Weight changes in samples with 0%, 5%, 10%, and 15% of PET particles attacked by 5% sulfuric acid solution.

Fig. 9. Weight of samples versus 0,15, 30, 60 days immersion in 5% sulfuric acid solution.

capacity of concrete included PET particles in resistance against internal pressure caused by increasing in the volume of cement paste during the reaction to sulfuric acid due to its more porosity and ductility of PET particles. That is why, the concrete has lost its load bearing capacity, integrity and its dense structure relatively more is retained. Thus the decrease in the ultrasonic wave velocity and load bearing capacity reduction was minimum.

Fig. 7 shows values of crushing load and ultrasonic wave velocity versus various PET percentages in 0, 15, 30, 60 days of immersion in 5% sulfuric acid solution. As it seems at different ages of immersion in sulfuric acid, crushing load and ultrasonic wave velocity graphs have decreasing trend which shows that by increasing in amount of PET particles in concrete, crushing load and velocity of ultrasonic wave are decreased. Since the erosion at early ages is not considerable, decrement of crushing load and ultrasonic wave velocity is due to presence of PET particles, while at older ages the slope of graph slightly is increased. In the other words, resistance of PET particles against erosion in concretes included 15% of PET particles, causes increment of crushing load

and compensates decrement of crushing load due to addition of PET particles. By increasing in immersion age up to 60 days, the slope of graph increases slowly. It might be said that the reason of this increment is due to more resistance of concrete included PET particles in acidic environment. In this experiment, resistance of samples included 15% of PET particles seems to be maximum. According to increment in slop of ultrasonic wave velocity's line, it could be concluded that samples with more amount of PET particles are more integrated and have higher density during immersion in 5% sulfuric acid solution. It might be due to this reason that samples with 15% of PET particles have higher ultrasonic wave velocity with gradually increasing the immersion age.

Fig. 8 shows weight changes in samples included 0%, 5%, 10%, and 15% of PET particles attacked by 5% sulfuric acid solution. It is obvious that weight changes in samples with higher percentage of PET particles is decreased. So that, in samples included 0%, 5%, 10%, and 15% of PET particles, 13.45, 10.26, 8.98, and 6.57 percent decrement in weight was occurred, respectively in comparison with their weight after 28 days curing. As it was mentioned earlier,

Fig. 10. Relations between weight loss of concrete specimens and loss in their crushing load due to 60 days of immersion in 5% sulfuric acid solution (a = 0% PET, b = 5% PET, c = 10% PET, c = 15% PET).

samples included 15% of PET particles had lowest weight loss which again, its reason could be high resistance of PET particles against erosion.

Fig. 9 shows weight of samples attacked by 5% sulfuric acid solution versus immersion time. It is observed that samples included PET particles have lower weight in comparison with control concrete in early days, gradually the weight difference became minimum at the age of nearly 30 days of immersion and at the age of 60 days immersion, this difference is quiet reversed and samples included higher PET percentages have highest weight at the age of 60 days immersion in 5% sulfuric acid solution which shows their

higher resistance against sulfuric acid attack. It should be noted that lower weight of samples included PET particles in comparison with control concrete in early days is due to lower weight of PET particles in comparison ordinary aggregates.

Graphs shown in Fig. 10 show relations between weight loss of concrete specimens and loss in their crushing load due to 60 days of immersion in 5% sulfuric acid solution for samples included 0%, 5%, 10%, and 15% of PET particles. In each graph higher value of regression coefficient (R2), shows that the proposed relation is more proper. As it seems crushing load and weight are declined. Analysis of regression coefficients shown in graphs in Fig. 10, illus-

trate a linear equation for concretes included 0%, 5%, 10%, and 15% of PET particles with 0.98, 0.96, 0.91, and 0.95 values of R2, respectively.

Best fit line for samples included 0%, 5%, 10%, and 15% of PET particles shows that decrement of crushing loads are 4.02, 5.18, 5.52, and 5.79 times their weight loss, respectively.

It could be observed that this values were increased by increment of PET particles percentages in concrete immersed in sulfuric acid solution which shows that within a specified range of crushing load changes, lower weight loss in concrete included more amount of PET particles was occurred. In this experiment samples included 15% of PET particles have highest coefficient of equation (ratio of crashing load to weight loss) and it could be shown again that concretes included higher percentages of PET particles have higher resistance against erosion.

4. Conclusions

In this study the effect of sulfuric acid attack on concrete resistance included various PET percentages (0, 5, 10, 15) as substitute aggregate by crushing load, weight change and ultrasonic wave velocity tests was investigate. Which the results are listed below:

- It has been observed that weight loss values for samples included 0%, 5%, 10%, 15% of PET particles are 13.47%, 10.26%, 8.98%, 6.57%, respectively. So it could be concluded that samples with 15% of PET particles have lower weight losses and better resistance against sulfuric acid attack.

- Decrement in crushing loads for samples included 0%, 5%, 10%, and 15% of PET particles are 53.92%, 49.8%, 51.95%, 35.29%, respectively. So it could be mentioned that samples included 15% of PET particles more retain their load bearing capacity under sulfuric acid attack.

- Decreasing in ultrasonic wave velocities for samples included 0%, 5%, 10%, and 15% of PET particles are 32.56%, 22.65%, 32.75%, and 20.7%, respectively. It could be expressed that samples included 15% of PET particles, kept their integrity more and had higher density.

- It is observed that under sulfuric acid attack, Changes in crushing load are nearly similar to ultrasonic wave velocities.

- Coefficient of equation represented for relations between weight loss of concrete specimens and loss of their crushing load increased by increasing in PET percentages in samples immersed in 5% sulfuric acid solution. As a result, in a particular change in crushing load, lower weight loss in concrete included higher amount of PET particles was occurred which in this experiment samples included 15% of PET particles had highest coefficient.

- According to results, it could be concluded that concretes included PET particles are able to utilize in environments under sulfuric acid attack.

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