Durabiliy Properties of Concrete Containing RHA

5.6.1 Permeability

Zhang and Malhotra [55] studied the chloride-ion penetration resistance of con­cretes made with 10% RHA and 10% silica fume (SF). Details of the concrete mixture along with the chloride-ion penetration test conducted as per ASTM C1202 [7] are given in Table 5.34. It can be seen that use of RHA and SF has drastically reduced the chloride-ion penetration at both the ages. These values were less than 1,000. As per ASTM C1202, when charge passed through concrete is less than 1,000 C, the concrete has very high resistance to chloride-ion penetration.

Nehdi et al. [38] made a comparative study of the rapid chloride permeability of concrete mixtures made with (1) 0% rice husk ash; (2) Egyptian rice husks; EG – RHA (A), EG-RHA (B) and EG-RHA (C); (3) a raw rice husk ash (RAW-US-RHA) and a high quality RHA (US-RHA) from USA, produced using fluidized bed technology; and (4) silica fume (SF). Three different percentages (7.5, 10, and 12.5%) of Egyptian rice husk ashes, SF, and two percentages (7.5 and 10%) of raw US rice husk ashes were used. Chloride permeability results are shown in Fig. 5.24. They concluded that (1) non-ground RHA did not significantly change the rapid chloride penetrability classification of concrete; (2) finely ground RHA reduced the rapid chloride penetrability of concrete from a moderate rating to low or very low ratings depending on the type and addition level of RHA. Such reductions are comparable to those achieved by SF.

Coutinho [21] investigated the rapid chloride permeability of concrete made with RHA (10, 15, and 20%) as partial replacement of cement and when using controlled permeability formwork (CPF). Controlled permeability formwork

Table 5.34 Test results of resistance of concrete to chloride-ion penetration [55]

Mix no.

Type of concrete

W/Cm

Unit

weight (kg/m3)

Compressive strength (MPa)

Chloride-ion resistance (C)

28 days

90 days

CO-D

Control

0.40

2,320

36.5

3,175

1,875

R10-D

10% RHA

0.40

2,340

45.5

875

525

SF10-D

10% SF

0.40

2,310

42.8

410

360

Replacement Replacement Replacement

Fig. 5.24 Rapid chloride permeability at 28 days for various concrete mixtures made with rice husk ashes from Egypt and US [38]

(CPF) is the technique developed for directly improving the concrete surface zone. This technique reduces the near-surface water/binder ratio and reduces the sen­sitivity of concrete to poor site curing. CPF consists of using a textile liner on the usual formwork, allowing air bubbles and surplus water to drain out but retaining the cement particles and so enabling the water-cement ratio of the outer layer to become very low and the concrete to hydrate to a very dense surface skin as the filter makes enough water available at the right time to activate optimum hydra­tion. So CPF enhances durability by providing an outer concrete layer which is richer in cement particles, with a lower water/binder ratio, less porous and so much less permeable than when ordinary formwork is used. Resistance to chloride penetration was assessed with the AASHTO T277-83 test method up to the age of 100 days (Table 5.35). It is evident from these results that inclusion of RHA significantly reduced the charge passed. Further more, when CPF was used, it greatly reduced the permeability of concrete mixtures.

Sensale [47] studied the air-permeability of concretes made with two sources of rice-husk ash; a residual RHA from rice paddy milling industry in Uruguay and

Table 5.35 Rapid chloride permeability results [21]

Type of mixture

Average charge passed (C)

Control

2349.3

Control + CPF

1916.3

10% RHA

435.0

10% RHA + CPF

384.7

15% RHA

322.0

15% RHA + CPF

245.0

20% RHA

260.0

20% RHA + CPF

202.0

w/(c + RHA) RHA Permeability coefficient (m2)

Type %

0

1.08

X

10-16

UY

10

0.23

X

10-16

20

0.05

X

10-16

USA

10

0.08

X

10-16

20

0.03

X

10-16

0

28.20

X

10-16

UY

10

71.82

X

10-16

20

49.10

X

10-16

USA

10

26.36

X

10-16

20

14.20

X

10-16

another from USA. Two (10 and 20%) replacement percentages of cement by RHA, and two water/cementitious material ratios (0.32 and 0.50) were used. The percentage of reactive silica contained in the USA RHA was 98.5% and in the UY – RHA was 39.55%. Air-permeability for concrete was measured with the ‘‘Torrent permeability tester’’ method [51, 52]. Permeability results of RHA concretes are given in Table 5.36. It can be seen that (1) for a particular water-cementitious ratio, permeability of UY-RHA concrete was more than that USA-RHA concrete; (2) with the increase in water-cementitious ratio, permeability increased for both types of RHA. The results of air permeability revealed the significance of the filler and pozzolanic effect for the concretes with RHA. On the one hand, the results are consistent with the compressive strength development at 28 days for the USA RHA. On the other hand, in the concretes with UY RHA, lower air permeability was observed, which can be due to the fact that with residual RHA, the filler effect of the smaller particles in the mixture is higher than the pozzolanic effect.

Saraswathy and Song [46] studied the effect of rice husk ash (RHA) on the chloride permeability of concrete. Proportion of control (with out RHA) mix was 1:1.5:3 with w/c ratio of 0.53. Cement was replaced with 0, 5, 10, 15, 20, 25, and 30% RHA. 28-day rapid chloride permeability [8] results are given in Table 5.37. It was observed that replacement of rice husk ash drastically reduced Coulomb values. As the replacement level increased, the chloride penetration decreased.

Table 5.37 Chloride diffusivity of rice husk replaced concrete [46]

RHA (%)

Charge passed (C)

0

1,161

5

1,108

10

653

15

309

20

265

25

213

30

273

Table 5.38 Chloride-ion permeability of RHA concretes [24]

Mixture

w/b ratio

Total charge passed (C) 28 days

91 days

REF (control)

0.35

1,727

1,288

0.50

3,166

2,136

0.65

3,681

2,866

20 RHA

0.35

999

452

0.50

1,557

692

0.65

2,677

1,176

20 RHA 1% Na2SO4

0.35

933

515

0.50

1,393

630

0.65

2,004

760

20 RHA 1% K2SO4

0.35

820

326

0.50

1,312

552

0.65

2,242

818

20 RHA 1% Na2SiO3

0.35

704

342

0.50

914

578

0.65

1,470

732

As per ASTM C1202, RHA reduced the rapid chloride penetrability of concrete from a low to very low ratings from higher to lower replacement levels.

Gastaldini et al. [24] studied the influence of chemical activators on the chlo­ride-ion permeability of concrete made with 20% of rice husk ash as partial replacement of cement. Water/binder ratios were 0.35, 0.50 and 0.65 whereas binder/aggregate ratios were 1:3.75, 1:5.25 and 1:6.9. Potassium sulfate (K2SO4), sodium sulfate (Na2SO4) and sodium silicate (Na2SiO3) were used as chemical activators in concentrations of 1% by weight of cement. Results of chloride-ion penetration are given in Table 5.38. They concluded that (1) at 28 days, RHA concrete exhibited significant reduction in the total charge passed. This reduction amounted to 42, 51 and 27% for w/b 0.35, 0.50 and 0.65, respectively. At 91 days, the same w/b ratios showed reductions of 65, 68 and 59%; (2) concrete mixtures with activators showed lower total charge passed when compared with the mixture without activator. The mixtures activated with K2SO4 showed the best results. At 28 days, the mixture activated with Na2SiO3 showed the lowest charge passed. Overall, the best results at 91 days were seen in the sample activated with K2SO4; and (3) at 91 days, all mixtures with chemical activators showed very low charge passed (100-1,000 C), even for w/b ratios as high as 0.65 which can be rated as very low as per ASTM C1202.

Ganesan et al. [23] examined the influence of RHA on the chloride permeability of concrete. Cement was replaced with 0, 5, 10, 15, 20, 25, 30, and 35% RHA. Control concrete mixture was made with 383 kg of cement, 575 kg of sand, and 1,150 kg of coarse aggregate per cubic meter with water-binder ratio of 0.53. The rapid chloride permeability test results for RHA blended concrete specimens are shown in Fig. 5.25. It was observed that the chloride permeability reduced con­siderably by partial replacement of OPC with RHA up to 30%. The total charge

passed for 30% RHA blended concrete was considerably reduced (more than 70% reduction) both at 28 and 90 days. Since the total charge passed through the concrete depends on the electrical conductance, the lower unburnt carbon content (loss on ignition value 2.1%) present in RHA might have contributed to the sig­nificant reduction in the electrical charge passed. It is worth mentioning that the unburnt carbon particles may contribute to the conductivity of the medium and a reduction in the unburnt carbon content may be beneficial from the chloride permeability point of view.

Chindaprasirt et al. [15] determined the chloride penetration resistance of blended Portland cement mortar containing ground rice husk ash (RHA) at the age of 28 days. Ordinary Portland cement (OPC) was partially replaced with 20 and 40% RHA by weight of cementitious materials. RHA had silica content of 93.2%. The 100 x 50 mm cylinders were tested at the age of 28 days for rapid chloride penetration test (RCPT) in accordance with ASTM C1202. The results of the RCPT test are shown in Fig. 5.26. The charge passed was substantially reduced with incorporation of RHA as compared to 7,450 C of normal OPC mortar.

Fig. 5.27 Resistance to chloride ion penetration (Coulomb) at various ages of control and RHA mixtures [43]

The incorporation of 20 and 40% RHA reduced the charge passed to 750 and 200 C at 20 and 40% replacement levels.

Ramezanianpour et al. [43] investigated the influence of RHA on the chloride – ion penetration of concrete up the age of 90 days. Concrete mixtures were made with 0, 7, 10 and 15% RHA as partial replacement of cement. Concrete slices of size 100 x 50 mm were cut from 100 x 200 cylinders for RCPT test. Results of rapid chloride permeability of concrete are shown in Fig. 5.27. It was observed that RHA drastically enhanced resistance to chloride penetration compared to control concrete on average, around 4-5 times higher for the 15% RHA. At 7 days, the control concrete showed the highest value of 6,189 C while the charge passed through the 15% RHA concrete was 1,749 C.

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