Properties of bacterial rice husk ash concrete

Construction and Building Materials 121 (2016) 112–119

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Properties of bacterial rice husk ash concrete Rafat Siddique a, Karambir Singh a, Kunal b,1, Malkit Singh c,⇑, Valeria Corinaldesi d, Anita Rajor e a

Department of Civil Engineering, Thapar University, Patiala, Punjab, India School of Agricultural Biotechnology, Punjab Agricultural University, Ludhiana, Punjab, India c Punjab State Power Corporation Limited, Patiala, India d Department of Materials, Environmental Sciences and Urban Planning, Marche Polytechnic University, Ancona, Italy e School of Energy and Environment, Thapar University, Patiala, Punjab, India b

h i g h l i g h t s Calcite producing bacteria improved strength of RHA concrete. Water absorption, porosity and chloride permeability reduced with RHA and bacteria. Abrasion loss was minimum in RHA-bacterial concrete. SEM and XRD analysis indicated the formation of calcite in bacterial concrete.

a r t i c l e

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Article history: Received 7 March 2016 Received in revised form 16 May 2016 Accepted 26 May 2016

Keywords: Abrasion Bacteria Strength Porosity RCPT Rice husk ash Water absorption

a b s t r a c t Influence of bacteria on the properties of concrete made with rice husk ash (RHA) is presented in this paper. For this purpose, control concrete was designed to have 28-d strength of 32.8 MPa. In the control concrete, cement was partially replaced with (0%, 5%, 10%, 15% and 20% by weight) RHA. Then, bacterium Bacillus aerius (105 cells/mL) was mixed in water during making of concrete. Tests were performed for compressive strength, water absorption, porosity, chloride permeability and abrasion resistance up the age of 56 d for all concrete mixtures with and without bacteria. Results indicated that inclusion of bacteria in RHA-concrete enhanced its compressive strength at all ages. However, best performance was achieved with 10% RHA wherein 28-d compressive strength was 36.1 MPa, and with bacteria, it was 40.0 MPa. Inclusion of bacterium in RHA concrete reduced its water absorption, porosity, and permeability at all ages, due to calcite precipitation, which in turn improves these properties. SEM and XRD analysis exhibited the formation of ettringite in pores, calcium silicate hydrate (CSH) and calcite which made the concrete denser. Findings of this investigation indicated the use of RHA and bacterium enhances the durability properties of concrete. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Approximately, yearly concrete production is about 10 billion cubic meters [1]. Cement is a very important constituent of concrete, and approximately 4180 million tons of cement were produced in 2014 globally [2]. Production of one ton of cement releases approximately one ton of CO2 which makes up 7% of all CO2 emissions produced globally [3]. Hence, there is necessity to

⇑ Corresponding author. E-mail addresses: [email protected] (R. Siddique), [email protected] (K. Singh), [email protected] (Kunal), [email protected] (M. Singh), [email protected] (V. Corinaldesi), [email protected] (A. Rajor). 1 Participated in this work when worked in Department of Civil Engineering, Thapar University till July 2015. http://dx.doi.org/10.1016/j.conbuildmat.2016.05.146 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

use supplementary cementitious materials (SCMs) as partial replacement of cement in concrete. Utilization of SCMs reduces the consumption of Ordinary Portland cement, and thereby reduces the energy consumption and green house gas emissions associated with cement production. As per Food and Agriculture Organization (F.A.O) statistics, world production of rice has risen from about 150 million tons in 1960 to over 740 million tons in 2013. Paddy consists of about 72% rice, 5–8% bran, and 20–22% husk [4]. In 2014, global production of paddy was 741.3 million tons, and consequently resulting in 148 million tons of rice husk [5]. Rice husk when properly burnt in incinerators at temperature lower than 700 °C generates rice husk ash containing highest proportion of reactive amorphous silica [6,7]. Generally, each tone of husk produces about 0.18–0.20 tons of ash [8].

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RHA due to its fine size (3–10 lm) mainly serves as a microfiller, pozzolanic, and viscosity modifier in concrete. RHA behaves as a reactive pozzolanic material because of its extreme surface fineness and high silica content [9]. RHA reacts with calcium hydroxide and produces additional CSH gel. Micro-filling effect and pozzolanic activity of RHA results in refining the pore structure of the matrix and interfacial transition zone. Inclusion of up to 15% (95 lm RHA) and 20% (5 lm RHA) RHA improved the compressive strength of concrete [10]. RHA concrete exhibited excellent improvement (30.8%) in compressive strength with 10% replacement, and up to 20% of cement could be valuably replaced with RHA without adversely affecting the compressive strength [11]. Saraswathy and Song [12] observed that with increase in RHA content (0–30%), chloride penetration decreased. Similar results were also observed between 90 and 100 d; where maximum reduction of 81.4% in charge passed was exhibited by specimens having 0–10% RHA [13]. Concretes made with 10% RHA exhibited about 72% reduction in 28-d chloride permeability [14]. Chloride-ion permeability of RHA blended concrete decreased with increase in RHA content up to 30% [15]. Water absorption of concrete decreased with increase in RHA content [10,15]. At 90 d, binary concrete containing 10% RHA content had lower water absorption than the control concrete [16]. Several other studies also reported that RHA enhanced the strength and durability properties of concrete [17–20]. SCMs improves the strength and durability of concrete however, the micro-cracks remained the main cause of concrete durability [21,22]. Various available traditional repair systems are chemical based which are expensive and hazardous to environment and health [21]. For the last 10–15 years, the interaction between microorganisms (particularly bacteria) and concrete structures is gaining ground in research for improvement in the durability of concrete [23–29]. Several researchers have proposed bacterial induced calcite precipitation (BICP) as an alternative approach to self-healing of concrete cracks by incorporating dormant but viable spores of alkali-resistant urease producing bacteria that convert organic compounds to inorganic mineral precipitates i.e. calcite [30–35]. Ramakrishnan et al. [23,24] and Van Tittelboom et al. [27] found that calcite precipitation by Bacillus pasteurii and Bacillus sphaericus was effective in plugging the cracks of concrete. Apart from B. pasteurii and B. sphaericus, other bacillus species such as Bacillus pseudofirmus and Bacillus cohnii [31], Bacillus alkalinitrilicus [33], and other genera such as Shewanella species [26,27], Acinetobacter johnsonii [36,37], Pseudomonas aeruginosa [38], Myxococcus xanthus [39], Proteus mirabilis and Proteus vulgaris [40] have also been studied extensively for calcite production in concrete. Chahal et al. [28] observed that inclusion of up to 30% fly ash along with 105 cells/ml of S. pasteurii in concrete exhibited ‘‘very low” chloride permeability values (762 C). Achal et al. [41]

observed ‘‘low” chloride permeability (1000–2000 C) in concrete specimens containing S. pasteurii (Bp M-3) whereas control concrete specimens showed ‘‘moderate’’ chloride permeability. Inclusion of 0.33 mg/ml of bacterial cell wall (Bacillus subtilis) in saline solution significantly increased the 28-d compressive strength by 15.6% and decreased the porosity by 1.64% [42]. Several studies have been reported on the use of RHA as partial replacement to cement in the production of concrete [10,15,16] and use of calcite producing bacteria for remediation of concrete cracks [23,25,42] but no such work have been reported on the use of bacteria in concrete containing RHA as partial replacement to cement. The calcite producing bacterium has been used in this research work to study its effect on strength and permeation properties of concrete. The calcite produced by the bacteria in the concrete pores, densifies the matrix which results not only in improvement of compressive strength but also reduces the pore size, thereby, improving the permeation properties. Therefore, the present study was conducted to provide technical data about the strength and permeation properties of concrete containing RHA and calcite producing bacteria. 2. Materials and methods A bacterium containing urease enzyme was isolated from marble sludge suspended in sterile saline solution (0.85% NaCl), serially diluted and plated on urea agar medium (Himedia) having pH of 6.8. Bacterial isolate was selected after incubation at 37 °C on the basis of changing the color of the medium from orange to pink. The selected bacterial isolate was then screened for calcite (CaCO3) production, and grown in calcite broth medium (urea 20 g, sodium carbonate 2.12 g, ammonium chloride 10 g, nutrient broth 3 g, calcium acetate 25 g, and distilled water 1000 mL) with pH from 7.5 to 8.0. After incubation at 37 °C, X-ray diffraction (XRD; PANalytical X’Pro; using CuKa radiation (k = 1.5418 Å); for diffraction angles 2 theta ranged between 5° and 60°) was used to analyze the precipitates in broth for the calcite production by bacterium. The XRD peaks were marked, compared and identified from the Joint Committee on Powder Diffraction Standards (JCPDS) data file. The isolate was identified using 16S rRNA gene sequencing technique and the 16S rRNA sequence was submitted to GenBank-NCBI. The 16S rRNA gene sequencing study was performed at Council of Scientific and Industrial Research Institute of Microbial Technology (CSIR-IMTECH), Chandigarh, India. The 16S rRNA gene sequence of the strain AKKR5 was processed manually, analyzed at NCBI (National Centre for Biotechnology Information) server (http://www.ncbi.nlm.nih.gov) using BLAST tool and compared to the corresponding neighbor sequences from the GenBank-NCBI database. Ordinary Portland cement (OPC) having specific gravity, standard consistency, initial and final setting time as 3.10, 28%, 123 min and 270 min, respectively, was used as per Indian standard specification BIS 8112 [43]. Chemical analysis of OPC done by X-ray fluorescence (XRF) showed that cement was mainly composed of lime (CaO; 63.5%), silica (SiO2; 21.25%), alumina (Al2O3; 4.74%), iron oxide (Fe2O3; 4.3%) followed by sulfur trioxide (SO3), magnesium oxide (MgO), potassium oxide (K2O), sodium oxide (Na2O) and titanium oxide (TiO2). Natural sand (size 4.75 mm) and crushed stone (size 12.5 mm) were used as fine and coarse aggregate, respectively, and were tested for their suitability in concrete as per Indian Standard Specifications BIS: 383 [44]. Fineness modulus of fine aggregate was 2.58, whereas specific gravity and moisture content was 2.68 and 0.16%, respectively. Coarse aggregate had specific gravity of 2.7 and water absorption of 1.14%.

Table 1 Mix proportions. Mixture

Cement (kg/m3)

RHA (%)

RHA (kg/m3)

Sand (kg/m3)

Coarse aggregate (kg/m3)

W/C ratio

Water (kg/m3)

Bacteria content (cfu/ml)

Slump (mm)

R0 R5 R10 R15 R20 BR0 BR5 BR10 BR15 BR20

390.0 370.5 351.0 331.5 312.0 390.0 370.5 351.0 331.5 312.0

0 5 10 15 20 0 5 10 15 20

0 19.5 39.0 58.5 78.0 0 19.5 39.0 58.5 78.0

569.0 569.0 569.0 569.0 569.0 569.0 569.0 569.0 569.0 569.0

1164.0 1164.0 1164.0 1164.0 1164.0 1164.0 1164.0 1164.0 1164.0 1164.0

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

185.0 185.0 185.0 185.0 185.0 185.0 185.0 185.0 185.0 185.0

0 0 0 0 0 105 105 105 105 105

90 83 77 72 66 – – – – –

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Rice husk ash (RHA) was collected from KGR Agro Fusions Private Limited, Ludhiana, Punjab (India). Chemical analysis of RHA showed high content of amorphous silica (90.4%) consists of fine spherical particles along with small amounts of aluminum (1.7%), iron (1.6%), magnesium (0.7%), alkali oxides (3.2%) and trace elements. Control concrete mix was designed to have 28-d compressive strength of 32.8 MPa [45]. Then, cement was partially replaced with 0%, 5%, 10%, 15% and 20% RHA by weight of cement, and bacterial cells having concentration of 105cells/ml were added. Concrete mix proportions are given in Table 1. For compressive strength, water absorption and porosity, concrete cubes of size 150 mm were prepared whereas specimens of 100 50 mm size were used for rapid chloride permeability test and sorptivity. For abrasion test, specimens of size 70.6 70.6 mm with and without bacterial culture were used. The casting of specimens was in accordance with Indian Standard BIS: 516 [47]. Cubes and cylinders were cast and compacted on a vibration machine, and were allowed to remain in iron molds for first 24 h at room temperature (27 ± 2 °C). After de-molding, all specimens were cured in water for 7, 28 and 56 d. The compressive strength of cube specimens of size 150 150 150 mm was determined as per Indian standard BIS 516 [46] at 7, 28 and 56 d in triplicate. Water absorption and porosity of the cube specimens were determined as per ASTM C 642 [47] method. Rapid chloride ion penetration and abrasion resistance of concrete were determined according to ASTM C1202 [48] and BIS 1237 [49], respectively, at 7, 28 and 56 d in triplicate. The XRD spectrum of powdered concrete was taken and analyzed from 2 h = 5° to 60°. The peaks of different phases were identified using X’pert HighScore Plus software. Scanning electron microscopic (SEM; JEOL JSM 6510 LV, USA) analysis was performed by mounting small broken concrete specimens on brass stubs using carbon tape. The samples were coated with gold and then analyzed at 20 kV. The XRD and SEM analysis was done on concrete specimens after 28 d of curing.

3. Results and discussion 3.1. Isolation of calcite producing bacteria The selected bacterial strain was able to hydrolyse urea in urea agar medium due to presence of urease enzyme in it, which in turn increased the pH of the medium. The color change from yellow to pink of the urea agar medium confirmed the presence of enzyme. The bacterial isolate produced some precipitates in the liquid calcite broth medium. The precipitates were filtered, air dried, and then analyzed by XRD. X-ray diffraction analysis of the precipitate revealed formation of calcite from calcium acetate by bacterial isolate (Fig. 1). The 16S rRNA gene sequence (1477 bases) of bacterial strain was analyzed and found to be closely related (99% identical) to Bacillus aerius strain 24 K (AJ831843). Therefore, the isolated bacterial strain was identified and named as B. aerius strain AKKR5. 3.2. Compressive strength Compressive strength results of control concrete, RHA concrete and bacterial concrete mixtures are shown in Fig. 2. At early age, compressive strength of concrete mixtures containing RHA increased with increase in RHA content up to 15% as cement

Fig. 1. XRD analysis of precipitate in calcite broth containing calcium acetate.

Fig. 2. Compressive strength of RHA containing (a) control and (b) bacterial concrete at 7, 28 and 56 d.

replacement and concrete mixture R20 displayed lower compressive strength than the control concrete. However, at 28 and 56 d, RHA concrete mixtures showed higher compressive strength than the control concrete. The concrete mixture containing 10% RHA as replacement of cement displayed optimum increase in compressive strength at all the ages. The increase in compressive strength of concrete mixture R10 was by 8.7%, 10% and 13.4% with respect to control concrete at the ages of 7, 28 and 56 d. The increased strength of RHA concrete mixtures was due to fineness of RHA and reactive silica content that reacted with hydration products of cement and produced secondary calcium silica hydrate (CSH) gel [11,50]. Water absorption and permeable pore space test results also confirmed that concrete mixture R10 exhibited dense matrix and as a results, displayed higher strength. The addition of bacterial cells in control and RHA concrete mixtures resulted in increase in compressive strength. Comparison to control concrete, concrete mixture containing 10% RHA and bacterial cells displayed 10.2%, 11.8% and 14.7% higher compressive strength at 7, 28 and 56 d, respectively. Similarly, increase in strength of bacterial concrete mixture BR10 was 6.2%, 10.7% and 13% with respect to concrete mixture R10 at the age of 7, 28 and 56 d, respectively. Increase in strength in bacterial concrete was due to the formation of calcite within the pores of the cement sand matrix [25] with concentration of 105 cell/ml. On the contrary, Jonkers et al. [31] observed reduction in strength (up to 10%) due to incorporation of a high number of bacterial spores (6 108 cm3) after the age of 3, 7 and 28 d cured specimens. Bang et al. [51] found increase in compressive strength at later curing ages (28 d) compared to 7 d.

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Fig. 3. Water absorption of RHA containing (a) control and (b) bacterial concrete at 7, 28 and 56 d.

3.3. Water absorption and porosity Inclusion of RHA as replacement of cement in concrete resulted in decrease in water absorption and permeable pore space (Figs. 3 and 4). The minimum water absorption and porosity was observed in concrete mixture R10 at all the curing ages. Water absorption in concrete mixture R10 was 2.51%, 1.66% and 1.47% and in bacterial concrete mixture BR10 was 1.86%, 1.03% and 0.80%, at 7, 28 and 56 d, respectively (Fig. 3). With age, decrease in water absorption in RHA concrete mixtures was observed. Inclusion of fine particles of RHA causes segmentation of large pores and increases nucleation sites for precipitation of hydration products in cement paste [52]. Formation of additional CSH gel as a product of pozzolanic reaction between calcium hydroxide and silica filled the voids and increased the density of concrete [53]. Addition of bacteria further decreased the water absorption and porosity of bacterial concrete mixtures compared to control and RHA concrete mixtures. Lower values of porosity were also observed in bacterial RHA concrete mixture BR10 specimen at all ages compared to bacterial control concrete mixture BR0. The porosity values of BR10 at 7, 28 and 56 d was 3.32%, 2.0% and 1.52%, respectively (Fig. 4). Water absorption and porosity of concrete mixtures containing RHA and bacterial cells was reduced due to the filling of pores by calcite produced by bacteria [35,54].

3.4. Rapid chloride permeability test Rapid chloride penetration test results of control concrete, RHA concrete and bacterial concrete mixtures are given in Table 2. Use of RHA as cementitious material resulted in decreased total charge passed through concrete mixtures at all the ages. In terms of

Fig. 4. Porosity of RHA containing (a) control and (b) bacterial concrete at 7, 28 and 56 d.

Table 2 RCPT values (coulombs) of RHA concrete with and without bacteria. RHA %

7d

28 d

56 d

RHA concrete

RHA and bacterial concrete

RHA concrete


RHA concrete


0 5 10 15 20

3305 2502 1998 2263 2705

3032 2400 1693 1967 2559

2366 1714 1285 1478 1867

2075 1471 1036 1250 1633

1947 1431 1024 1201 1395

1650 1137 799 943 1216

chloride permeability, 10% replacement of cement by RHA in concrete was the optimum dosage. RCPT results confirm water absorption and porosity test results. Total charge passed decreased with increase in RHA content up to 10%, and thereafter it increased; but remained lower than that through control concrete. Total charge passed through concrete mixture R10 decreased by 60.5%, 54.3% and 52.6% with respect to control concrete (R0) at the age of 7, 28 and 56 d, respectively. Addition of bacterial cells in concrete mixtures resulted in further reduction in total charge passed through control and RHA concrete mixtures. Similarly, bacterial concrete specimen BR10 exhibited the minimum charge passed at all curing ages. Charge passed in bacterial concrete specimen BR10 decreased by 55.8%, 49.9% and 48.4% with respect to bacterial control concrete specimen BR0 at the age of 7, 28 and 56 d, respectively. The permeability range for the samples remained between ‘‘low” to ‘‘moderate” for RHA concrete mixture and between ‘‘very low” to ‘‘moderate” for bacteria RHA concrete mixture as per ASTM

116


Table 3 Average depth resistance values (mm) of RHA concrete with and without bacteria. RHA %

7d RHA concrete


RHA concrete


RHA concrete


0 5 10 15 20

0.980 0.895 0.831 0.886 0.950

0.931 0.845 0.752 0.811 0.877

0.720 0.622 0.585 0.633 0.678

0.640 0.556 0.501 0.542 0.599

0.615 0.537 0.491 0.523 0.556

0.533 0.447 0.400 0.415 0.480

6000

56 d 5000

Q

4000

3000

Q/CSH

C/CSH

P Q

Q

E

P

0

5500

10

20

30

40

50

60

Degree 2 theta

5000 4500

Fig. 6. X-ray diffraction of BR10 concrete at 28 d. Q

4000 3500

3.5. Abrasion

3000 2500

P

E

500

E

P Q E

Q Q

Q

P

Q/CSH

1000

Cb

Q

Q/CSH C/P

1500

CS C/CSH CSH

2000

Q/CSH

Intensity

E

Q/CSH C/P

6000

Q

E

CSH

P E

Cb

1000

Cb

2000

Q/CSH

Intensity

28 d

0 5

10

15

20

25

30

35

40

45

50

55

60

65

Degree 2 Theta Fig. 5. X-ray diffraction of R10 concrete at 28 d.

C1202 [48]. Decrease in chloride ion penetrability of concrete was due to calcite precipitation by bacteria [55]. Chahal et al. [28] also reported that the pore blockage by bacterial calcite deposition resulted in resistance towards the chloride permeation.

Abrasion resistance was evaluated by measuring the depth of wear. Reduction in depth of wear indicated enhanced abrasion resistance and vice versa. Thickness loss due to abrasion on control concrete, RHA concrete and bacterial concrete mixtures is shown in Table 3. RHA concrete and bacterial RHA concrete mixtures R10 and BR10 exhibited optimum resistance to abrasion. The abrasion resistance results are in concurrence with compressive strength results. Depth loss of 0.831, 0.585 and 0.491 mm at 7, 28 and 56 d, was recorded in concrete mixture R10 specimens whereas bacterial concrete mixture BR10 exhibited depth loss of 0.752, 0.501 and 0.400 mm at similar ages. Abrasion resistance of concrete is closely related to its compression strength [56]. It has been observed that inclusion of bacteria increased the abrasion resistance in all RHA-concrete specimens. The increase in abrasion resistance in bacterial concrete mix was probably due to deposition of calcite on the concrete surface and within the pores.

Fig. 7. SEM image of R0, R5, BR0 and BR5 concrete at 28 d.


3.6. XRD analysis XRD analysis of concrete samples with or without bacteria-RHA showed presence of calcium silicate hydrates (CSHs), calcite (C), portlandite (P), calcium silicate (CS) and ettringite (E) main phases (Figs. 5 and 6). Peaks for ettringite phase were observed at 2 theta angles 9.0185°, 15.9644°, 23.1374° and 35.1347° (BR10). The remnants of aggregate and sand in the form of quartz (Q) and cristobalite (Cb) were also observed in XRD examination. The base line deviation between 26° and 36° showed the formation amorphous material. No qualitative change in phase composition was evident from the XRD analysis of control and bacterial concrete samples. However, Dick et al. [57] observed that X-ray analysis of the samples with and without bacteria shows that there were some extra peaks in the XRD spectra of the bacteria treated samples, which are absent in the control samples. Calcite formation as confirmed by XRD analysis is considered responsible for lowering the permeability of the concrete specimens [23]. XRD analysis revealed that majority of carbonate deposits was present as calcite crystals along with other components such as quartz [58]. 3.7. SEM analysis Figs. 7 and 8 shows the scanning electron microscope (SEM) analysis of control and bacterial concrete containing 0%, 5%, 10%,

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15% and 20% RHA. SEM images show the formation of calcium silicate hydrate (CSH) and dense structure in R10 due to the hydration reaction in the concrete specimen. In control concrete specimen for R10 (Fig. 8), more CSH and dense structure was observed after 28 d of curing whereas in R0, R5, R15 and R20 concrete samples, a significant amount of portlandite (CH) was observed. Jumate and Manea [59] observed that after 28 d, CSH forms a mass that exhibits more density, more compactness and more continuity leading to increase in strength. Similar observations were made for bacterial concrete BR10 (Fig. 8) that showed CSH gel formation. Calcite (C) was also observed in all bacterial concrete samples responsible for improved strength and reduced pore size in bacterial concrete. More voids were found in BR0, BR5, BR15 and BR20 compared to BR10 which had a dense structure. The dense matrix in control and bacterial (10% RHA) showed higher compressive strength and lower water absorption and porosity due to the growth of calcite crystals within the pores of the cement–sand matrix [60]. 3.8. Relationship between compressive strength and porosity Fig. 9 depicts the relationship between compressive strength and porosity of bacterial concrete made with or without RHA as cement replacement, obtained from the present study. The linear equation expressing the relationships between compressive

Fig. 8. SEM image of R10, R15, R20, BR10, BR15 and BR20 concrete at 28 d.


Compressive strength (MPa)

118

50

pore structure. It is evident from the above equation that higher the chloride penetrability lower is the compressive strength.

45 40 35

4. Conclusions

30 25 20 15 10

y = -10.085x + 60.613 R² = 0.9767

5 0 0.00

1.00

2.00

3.00

4.00

5.00

Porosity (%) Fig. 9. Relation between compressive strength and porosity of RHA - bacterial concrete.

Compressive strength (MPa)

50.00 45.00

1. Bacterial cells addition in RHA concrete further improved its compressive strength and permeation properties. Optimum dosage of RHA as cement replacement in concrete was 10%. Increase in compressive strength by 9% and 11.8% at the age of 28 and 56 d was observed in bacterial concrete compared to control due to plugging of the pores inside the concrete matrix by bacterial induced calcite precipitation. 2. Addition of bacteria causes reduction in water absorption and porosity due to calcite precipitation which in turn increase the durability of concrete structures. 3. Inclusion of bacterial in RHA concrete resulted in reduction in the chloride ion penetration. 4. The abrasion loss was less in bacterial concrete mixes compared to control concrete mixes at all ages.

40.00 35.00

References

30.00 25.00 20.00 15.00

y = -0.0117x + 54.296 R² = 0.7611

10.00 5.00 0.00

0

500

1000

1500

2000

2500

3000

3500

Charge passed (coulombs) Fig. 10. Relation between compressive strength and RCPT of RHA - bacterial concrete.

strength (r) and the porosity (P) in percent, together with the coefficients of determination (R2) derived is given below:

r ¼ 10:08P þ 60:61 R2 ¼ 0:976 where r = Compressive strength in MPa P = Porosity of the specimen in percent A high value of coefficient of determination, (R2 = 0.976) indicates good relevance between the data points and regression curve. It is evident from the above equation that the higher is the porosity of the concrete, lower is the compressive strength. 3.9. Relationship between compressive strength and RCPT Fig. 10 shows the relationship between compressive strength and RCPT of bacterial RHA - concrete. The coefficients of determination (R2) derived from the linear equation expressing the relationships between compressive strength (r) and the total charge passed (C) in coulombs, is given below:

r ¼ 0:011C þ 54:29 R2 ¼ 0:761 where r = Compressive strength in N/mm2 C = Total charge passed through the specimen in coulombs The high value of coefficient of determination (R2 = 0.761) indicates good correlation between compressive strength and resistance to chloride ion penetration of concrete linked to its

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