Enhancing the Durability Properties of Concrete with Multiwalled Carbon Nanotubes

have been used as an additive in cement and the op-timum dosage of MWCNTs are identified. ABSTRACT: This paper describes the study of durability aspects of concrete incorporated with different dos-ages of (0 % - 0.40 %) of Multiwalled Carbon Nanotubes (MWCNTs). Each specimen was subjected to Acid attack test, Chloride attack test, Sulphate attack test and water permeability test for a duration of 28, 56 and 90 days respectively. It has been observed from the experiments that the specimen of 0.25% Multiwalled Carbon Nanotubes shows an extraordinary performance among the other specimens. It was also observed that this mix performed much better than the control specimen.


INTRODUCTION
Multi-walled carbon nanotubes (MWCNTs) are known for their high tensile strength and elastic modulus which makes them one among the stiffest materials [1]. Though carbon nanotubes are known to possess high strength properties they are weak in shear between the adjacent shells and tubes [2]. It has been observed that application of high energy electron radiation increases the strength of Multiwalled carbon nanotubes to around 60 Gpa. The deposition parameter and nature of synthesis [3] plays a vital role in the dimensions and properties of carbon nanotubes (CNTs). Various methods such as arc discharge [4], Laser ablation [5] and chemical vapour deposition (CVD) [6] are used in the manufacture of CNTs of which CVD is the most economical method for mass production of CNTs [7][8]. However this method produces carbon nanotubes with a non-negligible amount of defects, less strength and these carbon nanotubes are very difficult to disperse [9]. On the positive side, these defects help the MWCNTs to play a key role in cement reinforcement application.
Defects free CNTs obtained by graphitization process achieved by high treatment at high temperature in a vacuum, in spite having good mechanical properties [10], are incapable of forming proper adhesion with a matrix which is called sliding [11]. Prevention of the satisfactory load transfer can be arranged by nanotubes bundle additionally. A previous study indicates that the carbon nanotubes cement matrix gives a steeper increase in strength characteristics of engineered cement composites [12]. The MWCNTs contains mechanical strength and other fibers which are most beneficial reinforcing material. But the prevailing main challenge remains in case of geopolymer properties of the surface [13][14]. In order to magnify mechanical properties corresponding to strength and stiffness, numerous researchers are practicing carbon nanotubes to mix with cement paste and mortars for the last few years [15][16][17][18][19][20][21][22][23][24][25].
In the present study multiwalled carbon nanotubes have been used as an additive in cement and the optimum dosage of MWCNTs are identified.

Materials
In this study, 53 grade ordinary Portland cement was used. The standard requirements, physical properties and chemical compositions of Ordinary Portland Cement are shown in Table 1. Natural locally available fine aggregate and coarse aggregate was used and their physical properties of used aggregates are shown in Table 2. Table 3 shows the characteristic of superplasticizer, the properties of MWCNTs are shown in Table 4. Table 5 indicates the approach of the research; nine achievable mixes were distinguished with various ratios of MWCNTs for this experimental work.

Dispersion of MWCNTs
Investigation on the dispersion of carbon nanotubes (CNTs) and its influence on the mechanical properties of the cement matrix have been carried out by Sobolkine et. al. In the present study, ultrasonicator was used to disperse the multiwall carbon nanotubes (MWCNTs) for about 60 minutes per sample. Firstly the MWCNTs should be dispersed in water by using ultrasonicator for about 60 minutes. Cement, fine aggregate and coarse aggregate were mixed in a dry condition in a pan mixer. To produce fresh concrete, prepared MWCNTs solution was added to the well-mixed dry materials and mixed for about 5 minutes. Finally, super plasticizer was added to achieve a workable mix. Prior to casting, the inner sides of cube mould were layered with lubricating oil to prevent adhesion with the concrete specimens. The moulds were filled in three layers, with each layer being well compacted. The casted specimens were kept open at the room temperature of 30±20 o c and relative humidity of 65± 5 o c. Mix design details for the MWCNTs concrete is given in Table 6.
2.4 Testing All the hardened concrete cube specimens of size 150 mm ×150 mm ×150 mm were subjected to acid attack, Sulphate attack, and Chloride attack by immersing in Sulphuric acid, Sodium sulphate and Chloride for 28, 56 and 90 days. For Sulphuric acid attack, the ph of the Acidic solution was checked every week and maintained at a value of 1 by adding the required amount of the concentrated acid solution. The acidic solution was completely replaced every 30 days. Similarly, for Sulphate and Chloride attack, the solution replacement was done. The change in weight and compressive strength of specimens were tested at an interval of 28, 56 and 90 days of immersion. Testing for compressive strength was conducted on cube specimen, using a 2000 KN compression testing machine as per Indian standard specification [26]. The density of the hardened concrete before and after 28, 56 and 90 days of immersion were found out for all the hardened concrete specimens.   Incorporation of CNTs in concrete has been observed to enhance the physical and mechanical properties of concrete [35][36][37]. The introduction of CNTs in concrete has resulted in the growth of microcrystals in pores and voids of C-S-H gel in the micro level, thereby reducing the cement paste po-rosity in nano level and compaction of interporous structure resulting in increased compressive strength.

Sulphate test
To determine the resistance of concrete to sulphate attack test is carried out [32]. Where in the percentage loss of compressive strength or variation in compressive strength of the specimens subjected to sulphate test was found with respect to the specimens which were not subjected to sulphate test [33]. The concrete cubes of 150mm×150mm×150mm of size after being cured for a period of 28 days is im-mersed in water containing 5 % of (Na2so4) by weight of water. After an immersion period of 90 days, the specimen is wiped and cleaned of grit from the surface of the cubes. These cubes are then subjected to compressive strength testing following the procedure if IA-516-1959. This procedure of accelerated testing to find the compressive strength for assuring the sulphate resistance of concrete [34].   11.87 %, 11.42 %, 11.03 %, 9.89 %, 11.99 %, 12.23 % and 13.2% respectively. Studies [35] have shown that increasing the quantity of CNTs in concrete does not result in a higher strength.

Chloride test
Chloride test is used to find out the weight loss and strength loss of concrete specimens subjected to chloride attack. Concrete cubes of size 150 mm ×150 mm ×150 mm were immersed in water containing 5 % of NaCl [36] by weight of water, after a curing period of 28 days. Periodic measurement of weight and compressive strength of the test specimens were obtained for 25, 56 and 90 days respectively. The loss in the weight and strength of the test specimens in comparison with the specimens not subjected to chloride test gives the measurement of chloride attack on concrete.

Weight loss due to chloride attack
In this study all the specimens were subjected to 5 % of chloride solution. After 28 days of immersion the

Water permeability test
In this test, the depth of penetration of water in concrete under pressure can be measured. The procedure is to immerse concrete cube specimens inside water under a pressure of 0.5 Mpa for a period of 72 hours, after which the specimen is split into two halves, and to measure the average depth of wet pro-file inside the sample due to penetration of water. A high value of depth corresponds to higher permeability and thereby measuring less durability of concrete [37]. Water permeability test results are given in Fig-7. Test results show the less penetration depth in MWCNTs-0.25 for all the ages and higher penetration depth shows in MWCNTs-0.40 for all the ages.

CONCLUSIONS
Based on the results and discussion on durability properties of concrete influenced by Multiwalled Carbon Nanotubes (MWCNTs), the following conclusions were made: Based on the results and discussions on the durability characteristics of concrete along with MWCNT it can be concluded that the addition of MWCNT enhances the behavior and durability aspects of concrete considerably until the certain limit of additions. Through the mechanical behavior and durability tests, it was identified that the concrete increases its strength and resistance when 0.25% of MWCNT addition is made. The results suggest that the specimens with 0.25% of addition resist the penetration of chloride, sulphate, acid, and water. The specimens consisting of more than the stipulated value losses its integrity and strength, this will have an effect on the longevity of MWCNT admixed concrete. The addition of MWCNT should be limited to 0.25%, after which the concrete losses its strength. From the results, it is concluded that the addition of MWCNT increases the performance of concrete. Thus 0.25% is the best-preferred usage to manufacture a concrete of high durability.