Influence of Nano-Materials on Mechanical and Permeation Characteristics of High Strength Concrete

K V S Gopala Krishna Sastry ,M Srinivasula Reddy ,A Ravitheja

(1.Civil Engineering,G Pulla Reddy Engineering College,Kurnool,AP,India;2.SVR Engineering College,Nandyal,Andhra Pradesh,India)

Abstract: Efforts have been made to evaluate the influences of the addition of nanoparticles on the strength,durability and mineralogical changes of high strength concrete (HSC).Therefore,mixes were prepared for conventional concrete mix (CCM) of M80 grade.Further,various mixes were prepared by replacing cementitious materials initially with 1% Nano-CaCO3 (NC),2% NC,3% NC in the CCM,and then 1% NC and Nano-SiO2 (NS) NS,2% NC and NS,3% NC and NS (NC and NS were in equal proportion) in the CCM.The developed concretes were then evaluated for mechanical properties,permeation characteristics,and mineralogical studies.From the studies,it is found that the concrete at 2% NCS possesses superior mechanical and superior permeation characteristics of all the mixes.A clear variation in the mineralogical structure with the addition of nanoparticles has been observed.

Key words: high strength concrete;nano silicon dioxide;nano calcium carbonate;mechanical properties;permeation properties

1 Introduction

Nanomaterials have acquired greater attention because of their filling ability,quantum,surface,and interface effects.However,the usage of nanomaterials in the construction industry is still at a fundamental research level and therefore its practical usage in the construction industry is very limited.Some of the research studies revealed that nanomaterials addition to the concrete improves the mechanical,electrochemical,permeation characteristics,and corrosion resistance properties.These,in turn,increase the long-term performance of the concrete,which is a very vital requirement in the construction industry.

The nano-materials addition to the conventional concrete helps in reducing the permeability of concrete.The reduced permeability of the concrete can result in improved durability,resistance to salts and other harmful elements.The term“Nanotechnology”was first coined by Taniguchi,a Japanese Researcher,in 1974.But,it was later popularized in 1986 by scientist K.Eric Drexler.

As for as the concrete industry is concerned,when the sizes of the material used in the concrete are reduced from micro level to nano level,significant changes in the chemical reactivity and mechanical properties would occur.The main reason behind this could be the increase in the surface area,which will aid in increasing the reactivity,and also acts as a filler material thereby refining the interfacial transition zone (ITZ) and reducing microvoids and porosity thereby densifying the concrete matrix.The durability of the structure can be enhanced by the photocatalytic response and self-cleaning reactions possessed by nano-particles.

Majority of the studies primarily focused on the application of Nano-SiO,Nano-AlO,Nano silica fume in the concrete development as silica and alumina are the two major chemicals involved in the hydration process of cement.Very few studies were reported on the application of Nano-CaCOand the combination of Nano-CaCOand Nano-SiOin concrete production.Keeping this in mind,an effort has been made in this study to develop high strength concrete by replacing cementitious materials with Nano-CaCOin various proportions

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,1%,2%,3% by weight,designated as NC1%,NC2%,NC3%,respectively,and with Nano-CaCOand Nano-SiO(in equal proportion) in various proportions

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1%,2%,and 3% by weight of cement,designated as NCS1%,NCS2%,NCS3%,respectively.

2 Experimental

2.1 Materials

Different materials used in this academic work to yield HSC of grade M80 are OPC 53 grade cement(confirming to IS 12269-1987),silica fume,course,and fine aggregates.Natural river sand and crushed granite stones were used as fine and coarse aggregates respectively,and their physical properties are depicted in Table 1.Superplasticizer of type Glenium B-233(Modified poly-carboxylate ether-based high range water reducing admixture) bearing specific gravity 1.08 is used in the present study.Nano-SiOused in the study was obtained from Aastra Chemicals,Chennai and Nano-CaCOfrom Nanoshell,Punjab,India.The properties of nano-materials used in this study are shown in Table 2.

2.2 Mix proportioning

M80 grade concrete has been designed,using ACI 211-4R-2008standard and the mix proportions were shown in Table 3.Cementitious materials comprising of 10% Silica fume and 90% OPC has been employed and water to binder ratio of 0.24 was maintained in all mixes.Superplasticizer at 1.1% by the weight of the cementitious material is added to compensate for the workability loss due to reduction in water content.The nano-particles ratio adopted in this study is 1%,2%,and 3% by the weight of the cementitious material.When the nano-materials nano-calcium carbonate and nano-silicon dioxide are used together they were added in concrete in equal proportions

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,50% NC and 50%NS.

2.3 Sample preparation

In the present study,HSC is produced both with and without adding nanoparticles and by maintaining low water to cementitious materials ratio of 0.24 in all the mixes.The sequence of the material mixing and the time of mixing plays an influential role in imparting strength to the concrete developed.Therefore,machine mixing with a special procedure is as follows;

Step 1:Cement,silica fume and Sand were mixed in the dry condition in a pan mixer for 2 minutes and then coarse aggregate was added and the mixing is continued for another 1 minute.

Step 2:Superplasticizer was mixed with 30%nano-particles in required quantity and stirred in a hand-held mixer until the nanoparticles are well dispersed.

Step 3:Super plasticizer and nanomaterials that were mixed in water were added to concrete simultaneously.After adding all the materials,the mixture was mixed until a slurry type uniform mix was obtained.

Step 4:The mix was then poured into the moulds of required specimen sizes and vibrated on a vibration table for 2 minutes to ensure adequate compaction.The specimens were then surface smoothened and covered with a wet cloth.

Step 5:After 24 hours from the time of casting,specimens were remoulded and cured in water until the testing.

2.4 Testing methods

2.4.1 Compression test

To determine the compressive strength of concrete,cube specimens of 100 mm size were prepared and the tests were conducted according to IS 516-1959standard in compression testing machine(CTM) having a maximum capacity of 2 000 kN and with 0.1 kN accuracy.

2.4.2 Flexural strength test

The flexural strength test was performed using auniversal testing machine (UTM) on specimens of size 500 mm × 100 mm × 100 mm using third-point loading method,as per IS 516-1959standard.

Table 3 Mix proportions of HSC/(kg/m)

2.4.3 Splitting tensile strength test

Splitting tensile strength test was conducted on cylindrical specimens of diameter 100 mm and height 200 mm using CTM as per IS 5816-1999standard.

2.4.4 Water absorption test

Water absorption test was executed as per ASTM C642-13standard on the concrete specimens of 100 mm × 100 mm × 100 mm size.

2.4.5 Rapid chloride permeability test

2.4.6 Volume of permeable voids

The permeability of concrete depends on the inter-connectivity of the pores,which in turn affects the strength and durability properties.Therefore,tests were conducted to determine the volume of permeable voids,in addition to the water absorption test,to evaluate the percentage of connected voids.The entire procedure has been followed as ASTM C 642-13standard.

2.4.7 Water sorptivity

The rate of water absorption is a test method to find the susceptibility of an unsaturated concrete to the penetration of water when only one surface is vulnerable to water and the dissimilar sides are sealed.This test was executed on concrete specimens of 100 mm diameter and 50 mm thickness as ASTM C 1585-13standard.Prior to the testing,the samples were conditioned according to ASTM C 1585-13.

2.4.8 XRD analysis

X-ray powder diffraction (XRD) is an analytical technique often used to obtain information regarding the structure of crystalline materials.XRD analysis in the present study is performed on the powdered concrete sample,which is oven-dried at 100 ± 5 ℃prior to testing to remove moisture.The sample to be tested is pulverized and placed in the sample holder and the excess powder on the sample holder is wiped out by a razor blade or glass slide and make sure that the powder surface is in level with the sample holder surface.The prepared sample is then placed in the X-ray diffraction model ‘X’ Pert Powder XRD set-up (voltage of 40 kV and a current of 40 mA,2 Theta ranging from 6° to 70° with a step size of 0.02 at a time interval of 0.5 sec for each step).

3 Results and discussion

3.1 Mechanical properties

Mechanical properties of the mixes were studied by conducting compressive strength test,splitting tensile strength test and flexural strength tests.Table 4 depicts the 7 and 28-day mechanical strength properties of the control mix and mixes developed by adding nano-CaCOand nano-silica in various percentages.

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3.2 Effect of nanoparticles on compressive strength

The 7 and 28-day compressive strength data for various mixes was plotted as shown in Fig.1.From the figure,it can be observed that with the mixes developed by the addition of 1%,2%,and 3% Nano-CaCOand 1%,2%,and 3% Nano-CaCO+Nano-silica,there is a minimal increase in the 7-day compressive strength and there is no clear indication of the optimum nanoparticles content.This minimum increase in the strength could be due to the pozzolanic nature of these nanoparticles which do not participate readily in the formation of primary reaction products.But,reaction with the C-H product to form secondary reaction products,could be the reason for a good increase in the 28-day compressive strength.It has been found that there is an improvement in compressive strength when compared to conventional concrete and is found to be about 13% for 1% NC and 19% for 2% NCS due to the inhibition of nanoparticles.The enhancement of compressive strength at inhibition of 1% NC and 2% NCS nanoparticles might be due to the action of nano particles as nuclei for cement phases,which enhances the hydration of cement because of their high reactivity.These nanoparticles by acting as nano reinforcement promote the densification of the microstructure and the interfacial transition zone,and thereby help in decreasing the porosity of the concrete.It is to be noticed that addition of a higher amount of nano particles must be accompanied by adding suitable superplasticizer dosage in the mix in order to make sure that the concrete specimens won’t be affected by the cracking and self-desiccation.The decrease in the strength,with further addition of nanoparticles,could be due to the formation of a weak zone in concrete that might be due to non-uniform dispersing of nanoparticles when the content is high.

Fig.1 Compressive strength of various mixes

3.3 Effect of nanoparticles on splitting tensile strength

Fig.2 Splitting tensile strength of various mixes

Fig.2 gives the 7 and 28-day splitting tensile strength data for various mixes developed.From the figure it can be observed that there is an increment in the splitting tensile strength with the addition of Nano-CaCOalone and combination of Nano-CaCOand nano-silica.But,the maximum splitting tensile strength was observed at 1% NC,which has shown a 10%increase over conventional concrete,and 2% NCS,which has shown a 11% increase over conventional concrete.A similar trend as that of compressive strength has been observed with the splitting tensile strength when the nanomaterials were added to the mix.This indicates that nanoparticles are playing the same role that they played in influencing compressive strength.

3.4 Effect of nanoparticles on flexural strength

Fig.3 depicts the 7 day and 28-day flexural strength of conventional concrete and concretes developed by adding NC and NCS in various proportions.The results showed a 12% increase in the flexural strength when NC of 1% was added in the mix and with further addition beyond 1% showed a decreasing trend in strength.While the concrete showed the highest strength when NCS of 2% was added and was a 17% increase over the flexural strength of conventional concrete.The flexural strength initially increased with an increase in the nanoparticles content and then decreases in flexural strength with a further addition of nanoparticles.It could be due to the same factors that are influencing compressive and splitting tensile strengths.

Fig.3 Flexural strength of various mixes

3.5 Durability tests on HSC

Various durability tests were conducted on all the concrete mixes and their effects are discussed

in detail.Table 5 presents the durability test results of the conventional concrete mix and the concrete mixes made by incorporating nanoparticles.

3.6 RCPT Test

Chloride permeability is an important property that describes the quality of concrete and low chloride permeation is an indication of a good structural concrete.In general,because of its micro filling ability the nano particles largely reduce the chloride ion penetration.In the present study,the chloride ion diffusion is assessed by following the procedure in ASTM C1202 standard.To assess the influence of the NC and NCS on the chloride diffusion of the concrete,RCPT is conducted on the conventional concrete specimens and concrete specimens with nanoparticles.The test results are depicted in Fig.4.

Fig.4 Rapid chloride permeation levels of various concrete mixes

From the figure,it can be noticed that the chloride permeability is reduced with the addition of nanoparticles in all the mixes,over conventional concrete,and the least chloride ion permeability is observed at a dosage of 1% NC and 2% NCS.The total charge passed in 1% NC concrete specimens is about 62% lower and that of in 2% NCS concrete specimen.Expectedly,the total charge passed decreased with the addition of nanoparticles.As per ASTM code,the qualitative representation of conventional concrete falls under the category ‘low’,whereas all the mixes made with nanoparticles falls under the category ‘very low’.

3.7 Water absorption and permeable voids

The initial and final water absorption of both conventional concrete and concretes incorporated with nanoparticles are presented in Table 5.From the table,it can be observed that the initial water absorption

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,30 minutes water absorption,of all the concretes,is lower than the limit specified for“excellent”concrete quality as per the specifications of CEB -1989.It can also be observed that,with the incorporation of nanoparticles,the water absorption both initial and final is reduced when compared to the conventional concrete.Out of all the mixes,water absorption was noticed the minimum at 2% dosage for both NC and NCS incorporated mixes.Even though,the same trend as that of strength properties is not observed with the water absorption results.This indicates that there is no direct relationship between water absorption and strength properties.In addition to water absorption,permeable voids of all the mixes are measured and are depicted in Fig.5.From the figure,it can be observed that the volume of permeable voids was higher when water absorption happens to be more.Though there are numerous factors affecting the water absorption,the trend obtained in Table 5 shows that the volume of permeable voids has a strong influence on the absorption.

Fig.5 Volume of permeable voids in various concrete mixes

3.8 Sorptivity

Fig.6 Sorptivity of the HSC mixes with different nano particles content

The results obtained in sorptivity tests give a gauge of the concrete’s capacity to absorb water or liquid by capillary action.From the results depicted in Fig.6,it is clear that with the addition of nanoparticles in the concrete the sorptivity is lower than that in conventional concrete in all the concretes with nanoparticles.The filling ability of the nanoparticles might be responsible for the decrease in the capillary connectivity,which affects the sorptivity value,in the nanoparticles incorporated concretes.The sorptivity test results indicate that the rate of water absorption is reduced considerably at 1% NC when compared to the conventional concrete.The merest rate of water absorption for NCS can be noticed at 1% nanoparticles addition.

3.9 XRD Analysis

XRD analysis has been carried out to determine the mineralogical phases from the conventional concrete to nanoparticles incorporated concretes.Fig.7,Fig.8 and Fig.9 shows the XRD analysis of conventional concrete,concrete with 1% NC and concrete with 2% NCS,respectively.

Fig.7 X-ray diffractogram of conventional concrete

Fig.8 X-ray diffractogram of NC1% concrete

As a product of cement hydration,CH reacts with the nanoparticles and forms C-S-H resulting in improved mechanical properties of HSC.Even the NC1% and NCS2% concrete's X-ray diffractogram showed Quartz,Calcite,Silica and Calcium hydroxide as predominant minerals.But,the intensity of the calcite and silica is increased with the addition of nanoparticles as compared to conventional concrete.This could be one of the major factors for enhanced strength and improved permeation.The intensity of Quartz,which is largely inert,is decreased and that of silica,calcite peaks broadened with the addition of nano CaCO.Similar XRD patterns can be noticed with the addition of nano CaCO+nano SiO,where Quartz intensity is decreased and that of silica,calcite is increased.The increase in the formation of silica and calcite minerals could be resulting in the strength enhancement over conventional concrete.Amorphous silica at 2-theta angle of 26.640 with increasing count from 3 000 to 6 000 for 1% NC is also responsible for strength increase in NC.Silica in amorphous state with strong peak 2-theta angle of 26.640 is shifted to 26.650 with intensity count decreasing from 4 500 to 2 000 for NCS over conventional concrete.Calcium hydroxide having its strong peak located at 2-theta angle of 34.102 0.Increase in the calcium hydroxide having a polymeric structure could also be responsible for the strength enhancement of 2% NCS concretes.

Fig.9 X-ray diffractogram of NCS2% concrete

3.10 SEM Analysis

SEM analysis is done for conventional concrete,concrete with 1% NC,and 2% NCS (Figs.10-12).Fig.10 shows the SEM image of conventional concrete which shows porous structure.Fig.11 shows the SEM image of 1% NC in which calcium hydroxide plates are seen and also less pores are observed when compared to the conventional concrete.Fig.12 shows the SEM image of 2% NCS in which very large amount of C-S-H gel formation in the form of foil like shape resulting in improvement of strength.This is due to the combination of nano SiOand CH produced from nano CaCO.This micro size C-S-H gel filled the pores there by making concrete more dense.Through the SEM analysis results,it can be finally concluded that the addition of nanoparticles at optimum dosages lead to increase in strength and durability.

Fig.10 SEM image of CC

Fig.11 SEM image of 1% NC

Fig.12 SEM image of 2% NCS

4 Conclusions

The optimum dose of Nano CaCOis 1% and Nano CaCO+Nano SiOare 2%.

The compressive,split tensile and flexural strengths were heightened by 13%,10% and 11%respectively for 1% NC incorporation when compared to conventional concrete.

The compressive,flexure and split tensile strengths were increased by 19%,17% and 11% respectively at 2% NCS.

The RCPT values were the least at an optimum dosage of 1% for NC and 2% for NCS.

Water absorption,sorptivity and volume of permeable voids got the minimum results at 1% NC and 2% NCS mixes.

HSC with NC and NS in equal proportions yield better results than with NC alone.HSC with 2% of NCS (1% NC and 1% NS) has optimum strength and durability.

Concrete with NCS2% and NC1% has more reactive silica components than the conventional concrete.