Effects of rice straw ash and micro silica on mechanical properties of pavement quality concrete

Journal of Building Engineering 26 (2019) 100889

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Effects of rice straw ash and micro silica on mechanical properties of pavement quality concrete

T

Arunabh Pandey*, Brind Kumar Department of Civil Engineering, IIT (BHU), Varanasi, 221005, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Rice straw ash Microsilica Compressive strength Flexural strength Split tensile strength XRD SEM

This research deals with improvement in the mechanical strength of Pavement Quality Concrete (PQC) when admixed with Rice Straw Ash (RSA) and Microsilica (MS). Nine mixes were prepared by partially substituting Ordinary Portland Cement (OPC) by MS (2.5%, 5%, 7.5%, and 10%), RSA (10%) and RSA-MS composite (5%–5%, 5%–7.5%, 10%–5% and 10%–7.5%). Maximum improvement was found when OPC was partially replaced by 7.5% in the case of MS and 5%–7.5% in case of RSA-MS composite. All the mix showed increased strength, w.r.t the control mix. X-Ray powder diffraction (XRD) and Scanning Electron Microscope (SEM) techniques were employed for characterization of the selected samples. Power regression equations were established to predict split tensile and flexural strength from compressive strength. They were compared with universally accepted equations and were found to be more accurate for RSA and MS admixed concrete. Mix R1M3 (5% RSA, 7.5% MS) is recommended based on the findings.

1. Introduction

technologies are making them unprofitable. Rice straw can be converted into ash without using enhanced burning techniques. When rice straw is burnt, it produces ash which is highly pozzolanic and fulfils the necessities of ASTM C618-19 [15] Class N, F, and C pozzolan. The specific gravity and specific surface area of rice straw ash are 2.25 and 1.846 m2/g, respectively [5]. Micro silica is an indistinguishable (non-crystalline) polymorph of silicon dioxide, silica. It is an ultrafine powder gathered as a by-product of the silicon and ferrosilicon composite production and comprises spherical particles with an average particle diameter of 150 nm [12]. There has been less amount of work done to explore the possible results of using rice straw ash in PQC and even less number of these explorations tends to the use of natural RSA in PQC (RSA that is either not ground to fine particle sizes and/or that isn't produced by an enhanced burning technique). The present study is the continuation of the work in which analysis of rice straw ash was done by its chemical, mineralogical, thermal and structural properties [6]. Also, a preliminary study was done on the cement paste containing rice straw ash, microsilica, and their composite by finding out its normal consistency and setting times [7]. The compressive strength of the mortar cubes of various proportions was determined after 3, 7, 28, 60, 90 and 365 days of curing in water [16]. It was found out that the mortar cubes of mix M3 (7.5% microsilica) give the maximum strength. The principal purpose of this analysis is to develop the use of rice

The construction sector of India is growing at a rapid rate and cement is the most important material required in the construction sector. In fact, India is the largest producer of cement in the world behind China. The capacity of cement plants in India was around 502 million tonnes in 2018 [1]. The production of cement raises serious environmental concerns like emission of carbon dioxide gas (CO2) [2]. Emission of carbon dioxide gas causes the greenhouse effect. In fact, cement plants account for 5% of the global emission of CO2. Around 900 kg of CO2 is liberated into the atmosphere in production of 1000 kg of cement [3]. As the consumption of cement is increasing day by day, a reduction in CO2 emissions can be done by partially substituting OPC with mineral admixtures. Mineral admixtures such as rice straw ash [4–8] and micro silica [9–12] can be a feasible solution for partial replacement of OPC in PQC, thus reducing the emission of CO2. The resulting cementitious material will be cheaper, resulting in more affordable concrete for pavement construction. Rice straw is an agricultural by-product of rice. It is mainly produced in Asia where its yearly production is 95% of world production [13]. Rice straw production is highest among the agro-residues production in India [14]. Traditionally rice straw has numerous competing uses such as animal feed, fodder, fuel, roof thatching, packaging and composting. These uses, however, will soon diminish because advanced

*

Corresponding author. E-mail address: [email protected] (A. Pandey).

https://doi.org/10.1016/j.jobe.2019.100889 Received 23 February 2019; Received in revised form 1 June 2019; Accepted 19 July 2019 Available online 20 July 2019 2352-7102/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Building Engineering 26 (2019) 100889

A. Pandey and B. Kumar

with rice straw ash but was higher than that of the control mix. Beyond 10% replacement of OPC by rice straw ash, the compressive strength of the mortar began to decrease significantly and was much below than that of the control mix. The primary motive of this study was to mix microsilica in rice straw ash admixed concrete to increase its strength. It is clear that the microsilica particles are smaller in size and are highly pozzolanic; thus, they tend to enhance the characteristics of the hardened concrete significantly. Microsilica has already been utilized in numerous projects in India and worldwide [26]. Therefore mixtures were prepared by replacing OPC by weight with the composite of RSA and MS, as shown in Table 5.

straw ash (with or without microsilica) as the part replacement material of cement in M40 grade PQC. It is done by determining various strength (compressive, flexural and split tensile) of the M40 grade concrete, and the mix selection was made based on the previous study [16]. XRD of the selected powdered samples is also done to reveal the hydrated compounds. SEM analysis is done to analyse the microstructure of the hardened concrete. 2. Experimental investigations 2.1. Materials used The cement utilized was OPC of 43 grade in confirmation with IS: 8112-2013 [17]. The specific gravity of OPC as per IS 2720 (III) [18] and specific surface area of OPC by Blaine's Method [19] was found out. 920D-grade microsilica was used which was acquired from Elkem South Asia Pvt. Ltd. Rice straw was obtained from Agricultural Farm, BHU. It was burnt in the open environment without any usage of the enhanced burning technique. The ash was then collected carefully to avoid any unwanted particles like dust. It was then sieved through 0.09 mm sieve. The rice straw ash passing through 0.09 mm sieve was later used as the cementitious material. The specific gravity of rice straw ash and microsilica was found as per IS 2720 (III) [18] and specific surface area was found by BET test. Specific surface area, specific gravity and chemical composition of Grade 43 OPC, micro silica and rice straw ash are shown in Table 1 and Table 2 [7]. The mean grain size was found out by laser particle size analyzer and is shown in Table 1. Fig. 1 shows the graphical comparison between the chemical composition of grade 43 OPC, rice straw ash and micro silica. Potable water was used for mixing and curing as per IS 456:2000 [20]. Conplast SP430 superplasticizer was utilized to counterbalance the decrease in workability. The workability was reduced due to an increase in the overall surface area of the particles as OPC was partially replaced by smaller sized microsilica and rice straw ash particles. It was obtained from Fosroc Chemicals (India) Pvt Ltd. The datasheet about the characteristics of Conplast SP430 superplasticizer is given in Table 4. Natural coarse aggregates of a maximum nominal size of 20 mm and 10 mm [21] were obtained from Dalla quarry, Sonebhadra, Uttar Pradesh, India. Fine aggregates confirming to Zone II grading [21] were obtained from Chopan quarry, Sonebhadra, Uttar Pradesh, India. The specific gravity and water absorption of the aggregates were observed as per IS 2386 (III) [22] and are shown in Table 2. Crushing value and impact value of the coarse aggregates were found as per IS 2386 (IV) [23] and are shown in Table 3.

2.3. Mix design of M40 grade concrete The mix design was done as per IRC 44 [27] and IS 10262 [28]. The final concrete mix design was confirmed after several hit and trial mixes. The water-cementitious material ratio was kept fixed at 0.39. The reduction in workability due to the increase in surface area of the mix particles was compensated by using superplasticizer. The dosage of HRWR superplasticizer for each mix was determined by slump test [29] keeping the slump at 50 mm and is shown in Table 5. The quantity of materials per unit volume of concrete is shown in Table 6. The ratio between coarse aggregates and fine aggregates in the concrete was kept at 0.644 and 0.356, respectively. The ratio between 20 mm and 10 mm maximum nominal size coarse aggregates were maintained at 0.6 and 0.4 respectively. 2.4. Sample preparation Three M40 grade concrete cubes (size 15 cm), 3 concrete prism (50 cm × 10 cm x 10 cm) and 3 concrete cylinders (15 cm dia. & 30 cm height) of each mix were cast for each day of curing [30]. Following 24 h of casting, the units were removed from the mould and put in the curing tank for 3, 7, 28, 60, 90 and 365 days of curing in water. The curing water was restored every week, and its temperature was maintained at 27° ± 2 °C. Total no. of concrete cubes, cylinders and prism were 180 each. After completion of 3, 7, 28, 60, 90 and 365 days of curing in water, respective samples were taken outside curing tank and examined for their compressive and flexural strength as per IS 516 [31] whereas split tensile strength was determined as per IS 5816 [32]. XRD analysis was done on the powdered samples of the mix R0, R2, R1M2, R1M3, R2M2 and R2M3 after 28 days of curing in water. Scanning Electron Microscopic (SEM) pictures of the hardened concrete were obtained and investigated to analyse the impact of mineral admixtures on concrete.

2.2. Mixture proportion 3. Results and discussions Different mixtures were set up for M40 grade PQC containing distinctive amounts of rice straw ash and micro silica. Mixtures were prepared by replacing OPC by weight with micro silica at a consistent interval of 2.5%–10%, as shown in Table 5. Only mix R2 (10% rice straw ash) was selected in the present investigation because it was concluded in the previous study [16] that the compressive strength of the mortar rises to only 5% replacement of OPC with rice straw ash. The compressive strength of mortar at 10% replacement was less than the strength at the 5% replacement of OPC

3.1. Properties of materials It can be observed in Table 1 that the specific gravity of rice straw ash and microsilica are nearly equal but less than that of the cement. The mean grain size (Table 1) of the microsilica is much smaller than that of the cement and rice straw ash particles. In other terms, microsilica and rice straw ash particles were 28 times and five times finer than the OPC particles, which were as per reported by other researchers [12]. The specific surface area of MS was more than that of the RSA particles and the OPC particles (Table 1). Rice straw ash can be listed as class F pozzolan as it satisfies the criteria listed in ASTM C618-19 [15]. Microsilica and rice straw ash have a negligible amount of CaO while they have high SiO2 content of 91.3% and 79.82%, respectively (Table 2). It signifies the importance of microsilica and rice straw ash as pozzolans as they have very little or no cementitious value. SiO2 present in rice straw ash and microsilica reacts with water forming Orthosilicic Acid as shown in Eq. (1) which in turn

Table 1 Properties of OPC, RSA and microsilica. properties 2

specific surface area, m /g specific gravity mean grain size, microns

OPC

RSA

Microsilica

0.3 3.2 17

1.846 2.25 3.3

16.14 2.23 0.6

Abbreviations. OPC: Ordinary Portland Cement, RSA: Rice Straw Ash. 2

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Table 2 Chemical composition of OPC, RSA, and microsilica. material

OPC rice straw ash microsilica

Compound, % by weight CaO

SiO2

MgO

Al2O3

Fe2O3

P2O5

SO3

Na2O

SrO

Cl

TiO2

S

K2O

P

ZnO

42.16 0.370 0.171

26.34 79.82 91.3

14.79 7.54 1.29

6.13 1.13 0.616

3.7 0.245 1.47

2.03 3.75 –

1.73 – 3.13

1.33 0.501 0.082

0.45 – –

0.305 4.06 –

0.189 – –

– 1.16 –

– 1.07 0.653

– – 0.38

– – 0.244

Abbreviation. OPC: Ordinary Portland Cement.

reacts with calcium hydroxide Ca(OH)2 forming C–S–H gel in Eq. (2) which has excellent binding properties. As the amount of binder (C–S–H gel) increases, the number of sites where the adhesive is formed is widely distributed [26]. SiO2 + 2H2O → Si(OH)4

(1)

Ca(OH)2 + H4SiO4 → C–S–H

(2)

Table 3 Properties of aggregates. properties

specific gravity water absorption, % crushing value, % aggregate impact value

The RSA particles were finer than the OPC particle, but microsilica particles were ultrafine with a smoother surface [7]. Therefore, a mixture of rice straw ash and microsilica particles will lead to the filling of voids between OPC particles. It might cause an increase in the strength and decrease in the permeability due to the densification of the concrete matrix. On mixing OPC-RSA-MS, the surface area of the particles of the mix increases, causing more affection towards the water. Therefore, the dosage of HRWR also increases (Table 5) to compensate for the reduced workability.

fine aggregate

coarse aggregate 20 mm

10 mm

2.78 0.6 10.6 9.5

2.72 0.75 10.6 9.5

2.65 1.1 – –

Table 4 Characteristics of superplasticizer.

3.2. Effects of microsilica and rice straw ash on mechanical properties 3.2.1. Compressive strength The replacement of OPC by microsilica increased the strength of the M40 grade pavement quality concrete to a great extent as can be observed in Table 7. The substantial increase in surface area, because of the addition of microsilica in the concrete, causes a corresponding rise in internal surface forces, which indirectly increases the cohesiveness of the concrete [26]. Therefore, the mixtures containing microsilica had higher compressive strength because as a result of cohesiveness, no. of voids decreases. Mix M4 at 3 and 7 days of curing in water had less compressive strength than the control mix R0. It was the only mixture which had lower strength than the control mix at early days of curing in water as the % change in compressive strength w.r.t. to the control mix was negative (Table 8). It was because as the percentage of microsilica in the mix increases, the amount of SO3 also increases (Table 2). Due to an increase in the amount of SO3, alite and aluminate content decreases

Standard compliance

Complies with IS:9103:1999 [24] and BS:5075 Part 3 [25]

Specific gravity Chloride content Air entrainment Compatibility

1.220–1.225 at 30 °C Nil Roughly 1% extra air is entrained Compatible with all types of cement except high alumina cement

while belite content increases [33]. Therefore compressive strength reduces at early stages of curing due to the reduction in the amount of alite and increases in later stages of curing as the formation of belite takes place [33]. M3 (7.5% microsilica) had the highest compressive strength amongst all the mix. Mix R2 did show an increase in the strength, but the percentage increase w.r.t the control mix was less than 1%. Maximum strength gain in R2 over R0 was at 60 days of curing in water. Amongst the mixes containing a composite of microsilica and rice straw ash, R1M3 shows the maximum strength. In Tables 7 and 8 and Fig. 2, it can be seen that addition of microsilica increases the compressive strength, but the addition of rice straw ash decreases the compressive strength though it remains more than the compressive strength of R0. For instance, the compressive strength of R1M2 at 3

Fig. 1. Graph showing the comparison between the chemical composition of OPC, RSA, and MS. 3

Journal of Building Engineering 26 (2019) 100889

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Table 5 Mix proportions of cementitious materials & HRWR dosage. material

mix proportion, % by weight

OPC RSA MS HRWRa

R0 or control

R2

M1

M2

M3

M4

R1M2

R1M3

R2M2

R2M3

100 – – 0.4

90 10 – 1.5

97.5 – 2.5 0.9

95 – 5 1.05

92.5 – 7.5 1.2

90 – 10 1.3

90 5 5 1.7

87.5 5 7.5 2

85 10 5 2.4

82.5 10 7.5 2.5

Note: OPC - Ordinary Portland Cement; RSA - Rice Straw Ash; MS – Microsilica; HRWR - High Range Water Reducer. a HRWR dosage was in % by weight of cementitious material (OPC + RSA + MS). Table 6 Quantities per unit volume of concrete. material

fine aggregate

coarse aggregate (nominal size)

quantity, kg/m3

20 mm

10 mm

755.91

493.06

cementitious content

water

Fig. 2. Relationship between Various Mixes and their Compressive Strength. 663.87

406

158

and 365 days of curing in water respectively. With the increase in the ratio of rice straw ash in the mix, the amount of the unburnt carbon (due to uncontrolled burning) in the mix also increases thus leading to increased water affinity of the mix. It was one of the factors for reduction in the strength of concrete when the amount of rice straw ash was increased. The physical and chemical properties of cement are affected by Magnesium Oxide (MgO) content. According to Mazurok et al. (2017) [34], the strength of concrete decreases with increase in the MgO content of OPC because the hydration of cement in the presence of MgO leads to the formation of Magnesium Hydroxide (Mg(OH)2) which does not have adequate strength. According to Borhan and Al-Rawi (2016) [35], the formation of Mg(OH)2 causes internal stresses in the concrete, thus decreasing its strength. They concluded that the stresses were caused because of delayed expansion in concrete due to the formation of Mg(OH)2. Also, the optimum gypsum content of cement reduces with an increase in the amount of MgO content [35]. The amount of MgO in microsilica was less than rice straw ash (Table 2), and thus microsilica admixed concrete gave higher compressive strength as compared to the rice straw ash admixed concrete. The most suitable amount of MgO in the cement was found to be in the range of 2–3%. It was mainly due to the better fluidity effect of the MgO at this range [36]. The compressive strength of concrete cubes was dependent on the age of curing in water. Therefore, the mathematical relationship between them was described by logarithmic equations with a good coefficient of determination (Table 9) in the form of Eq. (3)

Table 7 Compressive strength of various mixtures. mix

R0 M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

compressive strength (MPa) at different curing ages (days) 3

7

28

60

90

365

27.55 28.09 28.80 29.15 26.31 27.91 28.54 27.64 27.74 27.59

37.00 38.00 38.80 39.20 32.80 37.80 38.70 37.70 37.74 37.20

48.77 53.02 54.08 55.01 51.07 50.96 53.61 48.99 49.22 48.88

52.00 55.01 55.93 56.62 54.08 53.38 55.46 53.04 53.21 52.52

52.93 55.70 56.50 57.31 55.12 54.08 56.16 53.97 54.03 53.04

53.04 56.16 57.20 57.66 55.93 55.23 56.62 54.08 54.60 53.56

days of curing in water was 27.91 MPa while the compressive strength of R2 was 27.59 MPa. It was observed that if 5% microsilica of mix R1M2 (5% rice straw ash, 5% microsilica) was replaced by 5% rice straw ash, there was a decrease in the compressive strength of the resulting mix R2 (10% rice straw ash) w.r.t mix R1M2 but remained higher than the control mix R0. Similar trends were also observed for 7, 28, 60, 90 and 365 days of curing in water. The percentage increase in the compressive strength of the mix R1M2 and mix R2 w.r.t. control mix R0 decreases from 1.31%, 2.16%, 4.49%, 2.66%, 2.17%, 4.14% to 0.15%, 0.53%, 0.23%, 1%, 0.21%, 0.98% (Table 8) at 3, 7, 28, 60, 90

(3)

f(x) = a+ b[ln (x)]

where, f(x) or y is the variable for compressive strength of various

Table 8 Percentage Change in Compressive Strength w.r.t. Control Mix (R0). mix

M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

% change in the compressive strength at different curing ages (days) 3

7

28

60

90

365

+1.95 +4.54 +5.81 −4.49 +1.31 +3.58 +0.33 +0.68 +0.15

+2.70 +4.86 +5.93 −11.4 +2.16 +4.59 +1.89 +1.99 +0.53

+8.71 +10.89 +12.79 +4.73 +4.49 +9.93 +0.46 +0.93 +0.23

+5.78 +7.56 +8.88 +4.00 +2.66 +6.66 +2.00 +2.32 +1.00

+5.24 +6.75 +8.28 +4.14 +2.17 +6.10 +1.96 +2.07 +0.21

+5.88 +7.84 +8.71 +5.45 +4.14 +6.75 +1.96 +2.94 +0.98

4

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Table 9 Prediction models for Compressive Strength of Concrete Cubes w.r.t. Days of Curing in water. mix

prediction models (x ≥0)

R2

R0 M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

y y y y y y y y y y

0.8461 0.8273 0.8274 0.8182 0.8570 0.8507 0.8241 0.8524 0.8604 0.8515

= = = = = = = = = =

5.5556 6.1309 6.1850 6.2359 6.8335 5.8780 6.1066 5.7450 5.8174 5.6392

ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x)

+ + + + + + + + + +

25.889 26.336 27.038 27.466 22.115 26.115 26.940 25.920 25.853 25.849

specimens, a = constant, x = duration of curing in water in days, and b = coefficient of variable x. The regression equation between the compressive strength of R2M3 and days of curing in water had the highest coefficient of determination (R2=0.8604). It implies that the compressive strength of R2M3 could be predicted more accurately as compared to other mixes from the days of curing in water.

Fig. 3. Relationship between various mixes and their flexural strength.

early days of curing in water and compressive strength at later days of curing in water. The mathematical relationship between flexural strength and the days of curing in water was described by logarithmic equations with a good coefficient of determination mentioned in Table 12 in the form of Eq. (3) where f(x) or y is the variable for flexural strength of various specimens, ‘x’ is days of curing in water, ‘b’ is the coefficient of variable ‘x’ and ‘a’ is a constant. The logarithmic equation between the flexural strength of R2M3 and days of curing in water had the highest coefficient of determination (R2=0.9065) which means that flexural strength of R2M3 could be predicted more accurately from the days of curing in water as compared to other specimens.

3.2.2. Flexural strength The effects of rice straw ash, microsilica and their composite on flexural strength of the admixed concrete are shown in Table 10 and Fig. 3. Similar to compressive strength, flexural strength also increases with an increase in the percentage of microsilica in the mixture. Similar to compressive strength, the flexural strength of the mix M4 (at 3 and 7 days of curing in water) was also less than that of the control mix R0. But at 28, 60, 90 and 365 days of curing, the flexural strength of M4 was more than that of the R0. It was because of the absence of alite, which is responsible for strength at the early days of curing and also because of the presence of belite, which is responsible for strength at later stages. Amongst the mixes containing a composite of mineral admixtures, R1M3 had the maximum flexural strength while M3 had the maximum flexural strength amongst all the specimens at each day of curing in water. The effect of mineral admixtures was more profound on the flexural strength at 3 and 7 days of curing in water while they affected mostly compressive strength at the late days of curing in water (28, 60, 90 and 365 days) as can be seen in Tables 8 and 11. For example, for mixture M1, the percentage increase in the compressive strength w.r.t R0 at 3 and 7 days of curing in water was 1.95% and 2.70% respectively while the percentage increase in flexural strength for the same was 10% and 8.33%. The percentage increase of compressive strength at 28, 60, 90 and 365 days of curing in water was 8.71%, 5.78%, 5.24%, and 5.88% respectively whereas the percentage increase in flexural strength for the same was 4.98%, 5.56%, 4.69%, and 5.43% respectively. Therefore it can be said that the mineral admixtures affect the flexural strength at

3.2.3. Split tensile strength The effects of rice straw ash, microsilica and their composite on the split tensile strength of the admixed concrete are shown in Table 13 and Fig. 4. Similar to compressive and flexural strength, split tensile strength also increases with an increase in the percentage of microsilica in the mixture. The split tensile strength of M4, unlike compressive and flexural strength, was higher than that of the control mix R0 at 3 and 7 days of curing in water. Thus it can be said that SO3 did not affect the split tensile strength of M4. Amongst the mixes containing a composite of mineral admixtures, R1M3 had the maximum split tensile strength while M3 had the maximum split tensile strength amongst all the specimens at each day of curing in water. Similar test results were found for compressive, flexural strength and split tensile strength of each specimen. However, it can be seen from Tables 8, 11 and 14 that mineral admixtures mostly affected split tensile strength as compared to compressive and flexural strength except at 7 days of curing in water. The mineral admixtures affected the flexural strength the most at 7 days of curing in water. For example, for mix M2, percentage rise in the compressive and flexural strength w.r.t R0 at 3, 7, 28, 60, 90 and 365 days of curing in water was 4.54, 4.86, 10.89, 7.56, 6.75, 7.84 and 25, 20.83, 11.03, 6.35, 6.25, 6.20 respectively. The percentage rise in split tensile strength for the same was 28.32, 16.5, 16.45, 16.75, 16.67, and 16 respectively. Therefore it can be said that the mineral admixtures mostly affected the flexural strength at 7 days of curing in water and split tensile strength at 3, 28, 60, 90 and 365 days of curing in water. The mathematical relationship between split tensile strength and days of curing in water was described by logarithmic equations with a good coefficient of determination mentioned in Table 15 in the form of Eq. (3) where f(x) or y is the variable for split tensile strength of various mixtures, ‘x’ is days of curing in water, ‘b’ is the coefficient of variable

Table 10 Flexural strength of various mix. mix

R0 M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

flexural strength (MPa) at different curing ages (days) 3

7

28

60

90

365

4 4.4 5 5.1 3.86 4.3 4.8 4.1 4.25 4.02

4.8 5.2 5.8 6 4.6 5.1 5.7 4.95 5 4.9

5.62 5.9 6.24 6.5 5.8 5.75 6.07 5.67 5.7 5.65

6.3 6.65 6.7 6.8 6.6 6.55 6.67 6.45 6.5 6.35

6.4 6.7 6.8 6.9 6.65 6.6 6.73 6.5 6.55 6.45

6.45 6.8 6.85 6.95 6.75 6.7 6.81 6.6 6.65 6.5

5

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Table 11 Percentage Change in Flexural Strength w.r.t. Control Mix (R0). % change in the flexural strength at different curing ages (days)

mix

M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

3

7

28

60

90

365

+10 +25 +27.5 −3.5 +7.5 +20 +2.5 +6.25 +0.5

+8.33 +20.83 +25.00 −4.17 +6.25 +18.75 +3.13 +4.17 +2.08

+4.98 +11.03 +15.66 +3.20 +2.31 +8.01 +0.89 +1.42 +0.53

+5.56 +6.35 +7.94 +4.76 +3.97 +5.87 +2.38 +3.17 +0.79

+4.69 +6.25 +7.81 +3.91 +3.12 +5.16 +1.56 +2.34 +0.78

+5.43 +6.20 +7.75 +4.65 +3.88 +5.58 +2.33 +3.10 +0.78

Table 12 Prediction Models for Flexural Strength of Concrete Prisms w.r.t. Days of Curing in water. mix

prediction models (x ≥0)

R2

R0 M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

y y y y y y y y y y

0.898 0.9033 0.8816 0.8493 0.9005 0.9038 0.8773 0.9012 0.9065 0.8941

= = = = = = = = = =

0.5442 0.5288 0.3889 0.3774 0.6604 0.5293 0.4194 0.5496 0.5343 0.5441

ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x)

+ + + + + + + + + +

3.7019 4.1023 4.8789 5.0624 3.4128 3.9923 4.6712 3.7999 3.9166 3.7525

Table 13 Split tensile strength of various mix. mix

split tensile strength (MPa) at different curing ages (days) 3

7

28

60

90

365

2.26 2.55 2.9 3.2 2.47 2.4 2.87 2.33 2.37 2.3

2.97 3.18 3.46 3.68 3.11 3.11 3.44 3.01 3.04 2.99

3.89 4.17 4.53 4.81 4.39 4.03 4.23 3.99 4 3.95

4.12 4.42 4.81 5.09 4.65 4.27 4.75 4.23 4.24 4.19

4.2 4.5 4.9 5.16 4.75 4.35 4.85 4.3 4.32 4.25

4.25 4.52 4.93 5.2 4.8 4.4 4.9 4.32 4.35 4.27

Fig. 4. Relationship between Various Mixes and their Split Tensile Strength. R0 M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

Table 14 Percentage Change in Split Tensile Strength w.r.t. Control Mix (R0). mix

M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

‘x’ and ‘a’ is a constant. The logarithmic equation between the split tensile strength of R1M3 and days of curing in water had the highest coefficient of determination (R2=0.9049) which means that split tensile strength of R1M3 could be predicted more accurately from the days of curing in water as compared to other specimens. 3.3. Relation between compressive, flexural and split tensile strength

% change in the split tensile strength at different curing ages (days) 3

7

28

60

90

365

12.83 28.32 41.59 9.29 6.19 26.99 3.10 4.87 1.77

7.07 16.50 23.91 4.71 4.71 15.82 1.35 2.36 0.67

7.20 16.45 23.65 12.85 3.60 8.74 2.57 2.83 1.54

7.28 16.75 23.54 12.86 3.64 15.29 2.67 2.91 1.70

7.14 16.67 22.86 13.10 3.57 15.48 2.38 2.86 1.19

6.35 16.00 22.35 12.94 3.53 15.29 1.65 2.35 0.47

Table 15 Prediction Models for Split Tensile Strength of Concrete Cylinders w.r.t. Days of Curing in water.

Fig. 5 shows the graph between the experimental compressive strength of each mix at each day of curing and their corresponding experimental flexural and split tensile strength. The coefficient of determination (R2) between the compressive-flexural strength and compressive-split tensile strength was 0.9052 and 0.9331, respectively. Therefore, it can be said that as compared to the flexural strength of prism, the split tensile strength of cylinder can be predicted more precisely from compressive strength of cube because of the higher coefficient of determination. 3.3.1. Relationship between compressive strength and flexural strength According to studies done by many researchers in the past [37–41], 6

mix

prediction models (x ≥0)

R2

R0 M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

y y y y y y y y y y

0.8593 0.8576 0.8684 0.8670 0.8627 0.8609 0.9049 0.8504 0.8572 0.8472

= = = = = = = = = =

0.4321 0.4381 0.4594 0.4594 0.5256 0.4347 0.4584 0.4384 0.4356 0.4326

ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x) ln(x)

+ + + + + + + + + +

2.1121 2.3662 2.657 2.9254 2.2001 2.2479 2.5787 2.1717 2.2049 2.1536

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A. Pandey and B. Kumar

Table 18 Empirical relationships between compressive and split tensile strength.

Table 16 Empirical Relationships between Compressive and Flexural strength [42]. country

empirical expression

equation

Canadian Code of Practice [43] BS-8110 EC-02 NZS-3101 ACI

Canada

fr = 0.60√f′c

(5)a

Britain Europe New Zealand United States of America India

fr = 0.60√f′c fr = 0.201fc fr = 0.60√f′c fr = 0.62√f′c

(6)a (7) (8)a (9)

fr = 0.7√fc

(10)

IS 456 : 2000

equation

Zain et al. [44] Neville [45] ACI 363R-92 [46] ACI 318-99 [47] CEB-FIB [48]

ft = f′c/(0.1f′c+7.11) ft = 0.23f′c0.67 ft = 0.59√f′c ft = 0.56√f′c ft = 0.30f′c0.67

(12) (13) (14) (15) (16)

3.3.2. Relationship between compressive strength and split tensile strength Table 18 shows the recommended empirical relationships between compressive strength and split tensile strength of concrete [44–48]. As shown in Fig. 5, the prediction model suggested by this study for compressive strength (fc) vs split tensile strength (ft) of M40 grade concrete is shown in Eq. (11).

ft =0.1367f 0.8734 c

(11)

Where, fc = concrete cube compressive strength at 28 days of curing in water in MPa

Note: fc = concrete cube compressive strength at 28 days of curing in water in MPa. f′c = concrete cylinder compressive strength at 28 days of curing in water in MPa = 0.8*fc. fr = concrete prism flexural strength at 28 days of curing in water in MPa. a Eq. (5), (6) and (8) have the same expression.

f′c = concrete cylinder compressive strength at 28 days of curing in water in MPa = 0.8*fc ft = concrete cylinder split tensile strength at 28 days of curing in water in MPa The experimental split tensile strength at 28 days of curing in water obtained in this study was compared to the theoretical split tensile strength obtained from Eq. (11) and Eqs. (12-16) listed in Table 18. From Table 19, it can be seen that the theoretical split tensile strength obtained from Eq. (11) was closer to the experimental split tensile strength. Eqs. (12-16) also gave nearly equal theoretical split tensile strength as compared to the experimental split tensile strength. The only exception was Eq. (13) given by Neville [45] which gave quite low theoretical split tensile strength as compared to the experimental split tensile strength. It can be concluded that the equations given in Table 18 accurately predict the split tensile strength of rice straw ash and microsilica admixed concrete with Eq. (13) being an exception.

the compressive strength of concrete cylinders (15 cm dia and 30 cm height) is nearly 0.8 times of compressive strength of concrete cubes (15 cm × 15 cm x 15 cm). Table 16 shows the recommended empirical relationships between compressive strength and flexural strength of concrete [42]. As shown in Fig. 5, the prediction model suggested by this study for compressive strength (fc) vs flexural strength (fr) of M40 grade concrete is given in Eq. (4).

fr = 0. 561f 0.6128 c

empirical expression

it can be seen that the theoretical flexural strength obtained from Eq. (4) was closer to the experimental flexural strength followed by Eq. (10) given by IS 456: 2000. The ratios between experimental and theoretical flexural strength given by all the other codes were either significantly higher (ratio > 1) or significantly lower (ratio < 1). However, equations given in Table 16 were based on the tests done on plain concrete. Therefore, it can be concluded that these equations (Eqs. 5–10) do not predict the flexural strength of rice straw ash and microsilica admixed concrete precisely.

Fig. 5. Comparison of long-term compressive strength to flexural/split tensile strength.

code

source

(4)

The experimental flexural strength at 28 days of curing in water obtained in this study was compared to the theoretical flexural strength obtained from Eq. (4) and Eqs. (5–10) listed in Table 16. From Table 17, Table 17 Comparison of flexural strength. mix

R0 M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

experimental strength, MPa

theoretical strength (MPa) from equations

ratio of experimental & theoretical values

fc

f′c

fr

(4)

(5)

(7)

(9)

(10)

fr/(4)

fr/(5)

fr/(7)

fr/(9)

fr/(10)

48.77 53.02 54.08 55.01 51.07 50.96 53.61 48.99 49.22 48.88

39.01 42.42 43.26 44.00 40.86 40.77 42.89 39.20 39.38 39.10

5.62 5.9 6.24 6.5 5.8 5.75 6.07 5.67 5.7 5.65

6.07 6.39 6.47 6.54 6.25 6.24 6.44 6.09 6.11 6.08

3.75 3.91 3.95 3.98 3.84 3.83 3.93 3.76 3.77 3.75

9.80 10.66 10.87 11.06 10.27 10.24 10.78 9.85 9.89 9.82

3.87 4.04 4.08 4.11 3.96 3.96 4.06 3.88 3.89 3.88

4.89 5.10 5.15 5.19 5.00 5.00 5.13 4.90 4.91 4.89

0.93 0.92 0.96 0.99 0.93 0.92 0.94 0.93 0.93 0.93

1.50 1.51 1.58 1.63 1.51 1.50 1.54 1.51 1.51 1.51

0.57 0.55 0.57 0.59 0.56 0.56 0.56 0.58 0.58 0.58

1.45 1.46 1.53 1.58 1.46 1.45 1.50 1.46 1.47 1.46

1.15 1.16 1.21 1.25 1.16 1.15 1.18 1.16 1.16 1.16

7

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Table 19 Comparison of split tensile strength. mix

R0 M1 M2 M3 M4 R1M2 R1M3 R2M2 R2M3 R2

experimental strength, MPa

theoretical strength (MPa) from equations

ratio of experimental & theoretical values

fc

f′c

ft

(11)

(12)

(13)

(14)

(15)

(16)

ft/(11)

ft/(12)

ft/(13)

ft/(14)

ft/(15)

ft/(16)

48.77 53.02 54.08 55.01 51.07 50.96 53.61 48.99 49.22 48.88

39.01 42.42 43.26 44.00 40.86 40.77 42.89 39.20 39.38 39.10

3.89 4.17 4.53 4.81 4.39 4.03 4.23 3.99 4 3.95

4.08 4.38 4.46 4.53 4.24 4.24 4.43 4.09 4.11 4.08

4.07 4.27 4.32 4.36 4.18 4.17 4.30 4.08 4.09 4.07

3.11 3.29 3.33 3.37 3.21 3.20 3.31 3.12 3.13 3.11

4.12 4.30 4.34 4.38 4.22 4.21 4.32 4.13 4.14 4.12

3.91 4.08 4.12 4.15 4.00 4.00 4.10 3.92 3.93 3.92

4.06 4.29 4.35 4.40 4.18 4.18 4.32 4.07 4.08 4.06

0.95 0.95 1.02 1.06 1.04 0.95 0.95 0.98 0.97 0.97

0.96 0.98 1.05 1.10 1.05 0.97 0.98 0.98 0.98 0.97

1.25 1.27 1.36 1.43 1.37 1.26 1.28 1.28 1.28 1.27

0.94 0.97 1.04 1.10 1.04 0.96 0.98 0.97 0.97 0.96

0.99 1.02 1.10 1.16 1.10 1.01 1.03 1.02 1.02 1.01

0.96 0.97 1.04 1.09 1.05 0.96 0.98 0.98 0.98 0.97

3.3.3. Relationship between flexural strength and split tensile strength It can be seen in Table 10 and 13 that flexural strength has higher values as compared to the split tensile strength. The mathematical relationship between flexural and split tensile strength of M40 grade pavement quality concrete haven't been given much emphasis by the researchers in the past. Thus a relation between the two was suggested as shown in Eq. (17) derived from experimental values given in Tables 10 and 13. The coefficient of determination (R2) between the two was 0.9445 and was higher than the coefficient of determination of Eq. (4) and Eq. (11). It means that the mathematical relation between flexuralsplit tensile strength will give a more precise result as compared to the mathematical relation between compressive-flexural strength and compressive-split tensile strength suggested in this study. Fig. 6 shows the change of flexural strength with the split tensile strength.

fr =2.295f t0.6923

(17)

where, fr = concrete prism flexural strength at 28 days of curing in water in MPa ft = concrete cylinder split tensile strength at 28 days of curing in water in MPa

Fig. 7. XRD patterns of all the samples.

approximately the same 2-theta. Fig. 8A shows the influence of Quartz (SiO2). The hydrated compounds (CaCO3, Ca(OH)2, C–S–H) were also present in a small amount. Fig. 8B shows the XRD pattern of R1M2 mix. Increase in the amount of MS and RSA increased the content of SiO2 in the mixture. The peak of calcium hydroxide Ca(OH)2 disappeared, which means that the reaction

3.4. Mineralogical analysis of concrete The XRD analysis of 6 samples (R0, R1M2, R1M3, R2M2, R2M3, and R2) after 28 days of curing in water is given in Fig. 7. It shows XRD patterns of all the 6 samples in a single graph. It is clear that the position of the highest peak in XRD of all the samples occurs at

Fig. 6. Comparison of long-term split tensile strength to flexural strength. 8

Journal of Building Engineering 26 (2019) 100889

A. Pandey and B. Kumar

Fig. 8. XRD Analysis of Sample (A) R0 or control mix (B) R1M2 (C) R1M3 (D) R2M2 (E) R2M3 (F) R2.

pattern of the mix R2M3. It shows that all the peaks of calcium carbonate and calcium hydroxide have disappeared. It may be because the addition of microsilica decreases the amount of Ca(OH)2 by converting it into C–S–H gel [11]; therefore, the amount of C–S–H has increased significantly w.r.t. to the other mixtures. Fig. 8F shows the XRD patterns of mix R2 (10% rice straw ash). It clearly shows the dominance of the hydrated product like C–S–H in the concrete matrix and a lesser amount of silica SiO2. It implies that mix R2 has attained nearly maximum strength at 28 days of curing in water as can be seen in Table 7. R2 attains more than 92% of its strength at 28 days of curing in water.

of SiO2 with Ca(OH)2 in the presence of water led to the formation of C–S–H gel in the concrete mix. Fig. 8C shows the XRD pattern of the mix R1M3. It shows the prominence of calcium compounds. The amount of SiO2 decreases because as more amount of microsilica is added as compared to the mix R1M2, the reaction of silica with Ca(OH)2 in the presence of water takes place leading to the formation of C–S–H gel which in turn increases the strength of the sample. The amount of calcite in the form of calcium carbonate also increases, which acts as the filler, thus contributing to the strength of the mix R1M3. Fig. 8D shows the XRD pattern of the mix R2M2, which shows the prominence of C–S–H gel in the concrete matrix. It implies that most of the silica has taken part in hydration reaction leading to the formation of C–S–H gel. It also shows the presence of a small amount of hydrated products like calcium carbonate and calcium hydroxide. Fig. 8E shows the XRD

3.5. SEM microanalysis The SEM images were obtained for samples R0, R1M2, R1M3, 9

Journal of Building Engineering 26 (2019) 100889

A. Pandey and B. Kumar

Fig. 9. SEM Images of Sample (A) R0 or control mix (B) R1M2 (C) R1M3 (D) R2M2 (E) R2M3 (F) R2.

number of voids (V) was very less. The formation of C–S–H can be seen, and the presence of ettringite was prominent. Fig. 9F shows the SEM image of R2 (10% rice straw ash). The ettringite formation was found in abundance, causing a decrease in strength. The strength of R2 was more than the control mix and less than R1M2, R1M3, R2M2, and R2M3. Fewer formations of C–S–H and voids (V) can also be seen.

R2M2, R2M3 and R2 at 28 days of curing in water. Fig. 9A shows the SEM image of the control mix in which abundant amount of voids (V), C–S–H and little amount of calcium hydroxide (C–H) can be found, and it can be validated by its XRD in Fig. 8A. The presence of voids leads to a decrease in the strength of the control mix. The average size of the particles was less than 1 μm. Fig. 9B shows the SEM image of mix R1M2 (5% rice straw ash, 5% microsilica). The prominent presence of SiO2 (S) can be seen in the image where S is circular. The voids present in the mix were very fine. C–S–H gel was also present in a small amount. The SEM image confirms the absence of Ca(OH)2 from the sample, as shown in Fig. 8B of XRD analysis. Fig. 9C shows the SEM image of R1M3. The prominent presence of C–S–H can be seen in the image. The amount of voids and Ca(OH)2 is fewer because due to carbonation some amount of Ca(OH)2 is converted to CaCO3 in the presence of CO2 from the atmosphere. Calcium Carbonate acts as a filler and fills the voids. The presence of calcium carbonate can be verified by the XRD of R1M3 in Fig. 8C. It was the main reason behind the strength of R1M3 being the maximum among the 6 samples tested for XRD and SEM. The formation of ettringite can also be seen. The average size of the particles was nearly 0.5 μm. Fig. 9D shows the SEM image of R2M2. The number of voids present was more than the other samples that is why R2M2 showed the least strength among the samples containing microsilica and rice straw ash composite. The small presence of C–S–H and Ca (OH)2 was also found. Fig. 9E shows the SEM image of R2M3. The

4. Conclusions As microsilica is a costly material, its inclusion in the concrete will increase the construction cost of the structure. Therefore a material was needed that is economical as well as it also imparts strength to the concrete. Rice straw ash has all the properties of a better pozzolan, and it is much cheaper than most of the mineral admixtures. The experiments done in this study led to the following conclusion: 1. The particles of microsilica and rice straw ash were 28 times & 5 times finer than the OPC particles. 2. R2 (10% rice straw ash) showed only marginal strength gain w.r.t the control mix. 3. Adding of microsilica improved the compressive, flexural and split tensile strength of the concrete significantly w.r.t. the control mix except for compressive and flexural strength of the mix M4 (10% microsilica) at 3 and 7 days of curing in water. M3 (7.5% 10

Journal of Building Engineering 26 (2019) 100889

A. Pandey and B. Kumar

4.

5. 6.

7.

8.

9.

microsilica) gives the maximum strength amongst all the mixtures tested in this study. There was an improvement in the strength of concrete when OPC was replaced with rice straw ash-microsilica composite. Amongst the mixes of rice straw ash-microsilica composite, R1M3 (5% rice straw ash-7.5% microsilica) gave the maximum compressive, flexural and tensile strength. The strength gain in microsilica admixed concrete was more as compared to rice straw ash admixed concrete. The relationships between the strengths of every mix and days of curing in water were defined by logarithmic equations with a high coefficient of determination (R2). The relationship between compressive-flexural strength and compressive-split tensile strength (fr = 0.561fc0.6128 and ft = 0.1367fc0.8734) was defined by power equations with high coefficient of determination (R2). These equations were compared with the equations given by other researchers and were found to be more accurate for rice straw ash and microsilica admixed concrete. Mineralogical analysis (XRD) and microstructural analysis (SEM) established the results obtained from the compressive, flexural and split tensile strength tests of the specimens. By findings from this study, mix R1M3 is recommended when strength is the more dominant factor as compared to the economy while R2M3 is recommended for vice-versa. An advantage of using the composite of rice straw ash-microsilica in concrete is that they give sufficient strength without compromising with the economy.

0000019. [13] H. Pathak, A. Jain, N. Bhatia, Crop Residues Management with Conservation Agriculture: Potential, Constraints and Policy Needs, Indian Agricultural Research Institute, New Delhi, 2012. [14] N.H. Ravindranath, H.I. Somashekar, M.S. Nagaraja, P. Sudha, G. Sangeetha, S.C. Bhattacharya, P. Abdul Salam, Assessment of sustainable non-plantation biomass resources potential for energy in India, Biomass Bioenergy 29 (3) (2005) 178–190, https://doi.org/10.1016/j.biombioe.2005.03.005 0961–9534. [15] ASTM C618-19, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, ASTM International, West Conshohocken, PA, 2019. [16] Arunabh Pandey, Brind Kumar, Investigating the performance of cement mortar containing Rice Straw Ash, Microsilica and their Composite by compressive strength, Int. J. Recent Technol. Eng. 7 (6) (2019) 2277–3878. [17] IS 8112, 43 Grade Ordinary Portland Cement - Specification, Bureau of Indian Standards, New Delhi, 1990. 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[47] ACI-Committee-318, Building Code Requirements for Structural Concrete (ACI 318–99) and Commentary (318R–99), Farmington Hills, Mich, 1999. [48] Evaluation of the Time-dependent Behaviour of Concrete” Bulletin d'Information No. 199, Comite European du Béton/Fédération Internationale de la Precontrainte, Lausanne, 1991.

strength of high-performance concrete, Cement Concr. Res. 32 (2002) 1251–1258 https://doi.org/10.1016/S0008-8846(02)00768-8. [45] A.M. Neville, Properties of Concrete, Fourth and Final Edition, Prentice Hall, United Kingdom, Pearson, 1995. [46] ACI-Committee-363, State of the Art Report on High-Strength Concrete (ACI 363R–92), Farmington Hills, Mich, 1992.

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