Category Supplementary Cementing Materials

Alkali-Silica Reaction

Hasparyk et al. [27] investigated the expansion of mortar bars made with different levels of cement replacement with rice husk ash (RHA). Two types of reactive aggregates (quartzite and basalt) were used. Percentages of RHA were 0, 4, 8, 12, and 15%. Tests were conducted as per ASTM C 1260. Expansion at 16 and

Table 5.42 Expansion of mortar bars containing RHA [27]

Replacement (%)

Expansion (%)

Quartzite

Basalt

16 days

30 days

16 days

30 days

0

0.28

0.53

0.84

1.10

4

0.16

0.36

0.76

0.99

8

0.21

0.45

0.32

0.81

12

0.09

0.25

0.08

0.33

15

0.06

0.15

0.04

0.25

30 days are given in Table 5.42. It can be seen from these results that inclusion of RHA was very effective in controlling the expansion of mortar bars at the age of 16 and 30 days.

[1]

Khatib and Hibbert [42] studied the effect of GGBS on the flexural strength of concrete. Portland cement (PC) was partially replaced with 0-80% GGBS. Control mix had a proportion of 1 (PC): 2 (fine aggregate): 4 (coarse aggregate). In the other three mixes, cement was partially replaced with 40, 60 and 80% GGBS (by mass). Figure 3.17 shows 90-day flexural strength values for mixes containing 0, 40, 60, and 80% GGBS. The flexural strength of concrete containing 60% GGBS was noticeably higher than the control, whereas a slight decrease at 40% and marked decrease at 80% replacement were observed.

Guo et al. [29] investigated the flexural fatigue performance of concretes with 50 and 80% proportions of ground granulated blast-furnace slag by mass of total cementitious materials. Control concrete had 460 kg/m3 of cement, aggregate – binder ratios and water-binder ratios were 2.4 and 0.35, respectively. The flexure fatigue tests were carried at cyclic loading frequency of 10 Hz. Six nominal stress levels (0.90, 0.85, 0.80, 0.75, 0.70 and 0.65) were chosen. Flexural strength of concrete was 7.65, 7.14 and 5.87 MPa with 0, 50, and 80% GGBS content. The flexural fatigue life of concretes is given in Table 3.17. It is clear that the fatigue life of 50% GGBS concrete is the longest and that of 0% GGBS concrete is the shortest with stress level of 0.80 or more. However, when stress level was lower than 0.80, the fatigue life of 80% GGBS concrete was the longest among three

[2] non-steady-state chloride diffusivities of mortar pastes increased with aggre­gate volume in samples with only Portland cement as binder; (3) the capillary porosity of the mortar paste increased with aggregate volume, indicating that the ITZ had a higher overall porosity than the bulk paste; and (4) the use of 10% metakaolin as a partial replacement for Portland cement reduced non-steady-state chloride diffusivities by approximately one-order of magnitude relative to those samples with only Portland cement as binder. Mortar-paste diffusivities did not vary significantly with variations in aggregate content. Relative to control samples, metakaolin reduced mortar-paste capillary porosities although the latter still increased with increasing aggregate volumes. The lack of variation in mortar-paste diffusivity and rising capillary porosity with increasing aggregate content supports the hypothesis that the use of metakaolin increases permeation path tortuosity and inhibits percolation.

Courard et al. [21] investigated the effect of metakaolin on the chloride diffu­sion rates of mortars. Metakaolin percentages as partial replacement of cement were 0, 10, 15, and 20%. One mixture with natural kaolin was also made. Mixture proportion of mortar was 3 parts of sand, 1 part of cement, with w/c ratio of 0.5. Diffusion rates of Cl – and Na+ ions into cement mortars were monitored using two-compartment diffusion cells. At periodic intervals, chloride concentration was measured by titration from a 20 cm3 sample of the solution. The evolution of chloride diffusion was measured for 314 days for cement, metakaolin and kaolin (Table 4.30). The breakthrough time was calculated from the intercept of the concentration versus time date. Breakthrough time represents the time necessary for the initiation of Cl – ion transfer through the sample. It gives a view of the porous skeleton. Mortar with 20% MK gave the best results as even after 1 year, no diffusion was observed. An increase from 10 to 15% metakaolin content seemed to induce an increase of 150% of occurrence time and a decrease of 170% for diffusion coefficient. Kaolin had no effect and seemed on the contrary to accelerate the phenomenon of diffusion in comparison with the reference mix.

[3] Ambroise, J., Maximilien, S., Pera, J.: Properties of metakaolin blended cements. Adv. Cem. Based Mater. 1(4), 161-168 (1994)

Deicing Salt Scaling Resistance

Zhang and Malhotra [55] studied the deicing salt-scaling resistance of concretes made with 10% RHA and 10% silica fume (SF). Scaling resistance results are given in Table 5.41. The visual evaluation of test slabs showed that the perfor­mance of the RHA concrete was similar to that of the control concrete but mar­ginally better than SF concrete. For both control and RHA concrete, no coarse aggregate was visible after 50 cycles whereas for SF concrete some coarse

Fig. 5.34 Expansion of mortars mixed with BRHA from an electricity generating power plant, water-to-binder ratio of 0.65 [12]

Table 5.41 Test results of deicing salt scaling [55]

Mix No.

Type of concrete

W/C

ratio

Compressive strength (MPa)

Visual

rating

Total scaling residue (kg/m3)

CO-D

Control

0.40

36.5

2

0.3

R10-D

10% RHA

0.40

45.5

2

0.6

SF10-D

10% SF

0.40

42.8

3

0.8

aggregates were visible. All the three concretes had a total mass of scaling residue of equivalent to or less than 0.8 kg/m3 after 50 cycles in the presence of deicing salts. The control, RHA, and silica fume concretes showed excellent performance in the freezing and thawing test. The RHA concrete had a durability factor of 98.3 and very small changes in length, mass, pulse velocity, and resonant frequency after 300 cycles of freezing and thawing.

Nehdi et al. [38] investigated the effect of RHA and SF on the deicing scaling resistance of concrete. Concrete mixtures were made with 10% RHA and 10% SF as partial replacement of cement. Tests were conducted up to 50 cycles as per ASTM C672 [4]. Results showed that RHA concrete specimens (visual rating 2) performed similar to the control concrete mixture (visual rating 2) but little better than SF concrete mixtures (visual rating 3). These results were in agreement with those reported by Zhang and Malhotra [55].

Sulfate Resistance

Sakr [45] studied the effect of RHA on the sulfate resistance of heavyweight concrete. The aggregates used were special natural heavy weight mineral ores, mainly ilmenite and baryte. They were used as the fine and coarse aggregates for the heavy weight concrete. Gravel and sand were used as the coarse and fine aggregate for the gravel concrete. Ilmenite, barite and gravel concrete mixtures were made with 0 and 15% RHA. For the sulfate resistance test, 100 mm cubes were immersed in a 5% MgSO4 solution for different periods (1, 3, and 6 months) and the loss in compressive strength due to sulfate attack was determined. It was observed that (1) reductions in compressive strength of the gravel, baryte, and ilmenite control mixtures was 8.5, 7.0, and 8.0%, respectively, after immersion in 5% MgSO4 for 28 days, while the reductions after 90 days of immersion were 16, 14, and 14%, respectively; (2) reduction percentages of the compressive strength of gravel, baryte and ilmenite concrete incorporating 15% of RHA when immersed in a sulfate solution for 28 days were 5, 6, and 6%, respectively; and (3) decrease
rate of the effect of sulfate ions on compressive strength was generally increased as the immersion time in the sulfate solution increased.

Chatveera and Lertwattanaruk [12] investigated the effect of black rice husk ashes (BRHAs), generated from an electricity generating power plant and a rice mill on the sulfate resistance of mortar. The ashes were ground and used as a partial replacement of cement. Replacement levels were 0, 10, 30, and 50% by weight of binder. The water-to-binder ratios were 0.55 and 0.65. For sulfate resistance, mortar specimens were subjected to 5% sodium sulfate (Na2SO4) and magnesium sulfate (MgSO4) solutions. The length change due to sulfate attacks was performed in accordance with ASTM C1012 [6] by using 25 x 25 x 285-mm bars. Sand-to-binder materials ratio of 2.75 by weight was used. Expansions of the mortars were measure up to 26 weeks. Expansion of mortars immersed in 5% Na2SO4 and MgSO4 solutions are shown in Figs. 5.31, 5.32, 5.33 and 5.34. It was found that the expansion rates are low at the beginning, and increases substantially after 4 weeks of curing.

As shown in Fig. 5.31, the expansion of the OPC mortar was higher than the mortars mixed with BRHA from a rice mill. Increase in BRHHA content tends to reduce the expansion due to the decreased amount of Ca(OH)2 for reacting with sulfate ion to produce gypsum and ettringite. For the aspect of sulfate resistance, the mortars mixed with 10% replacement of BRHA (10MRHA-0.55) yielded the expansion more than that of SPC mortar, whereas the expansions of the mortars with 30 and 50% replacement of BHRA were lower than that of the SPC mortar.

In the case of MgSO4 attack, after the mortars being immersed in MgSO4 solution, the reaction produces brucite (Mg(OH)2) and gypsum depositing on the surface of mortar. This increases the accessibility of sulfate ion to attack the mortar matrix. As a result, the structure of C-S-H is prone to be changed into M-S-H, which does not have binding capacity. Overall, the expansion of mortar under MgSO4 attack is lower than that under Na2SO4 attack due to the different mech­anisms. In Na2SO4 solutions, the deterioration is the result of expansion associated with ettringite formation. In MgSO4 solutions, a main cause of attack is decom­position of C-S-H. When increasing the water-to-binder ratio to 0.65 (as shown in

Fig. 5.31 Expansion of mortars mixed with BRHA from a rice mill, water-to-binder ratio of 0.55 [12]

Fig. 5.32 Expansion of mortars mixed with BRHA from a rice mill, water-to-binder ratio of 0.65 [12]

Fig. 5.32), the expansion of mortar increases due to the higher porosity of mortar structure (Neville 1995), allowing more sulfate ion penetration. For the expansions of mortars mixed with BRHA from an electricity generating plant as shown in Figs. 5.33 and 5.34, it was found that the results are similar to those mixed with BRHA from a rice mill.

Freezing and Thawing Resistance

Zhang and Malhotra [55] studied the freezing and thawing resistance of concretes made with 10% RHA and 10% silica fume (SF) as per ASTM C666 Procedure A [3]. Freezing/thawing results are given in Table 5.40. The control, RHA, and silica fume concretes showed excellent performance in the freezing and thawing test. The RHA concrete had a durability factor of 98.3 and very small changes in length, mass, pulse velocity, and resonant frequency after 300 cycles of freezing and thawing.

Carbonation

Cizer et al. [17] studied the carbonation of calcium hydrxide and calcium silicate binders with rice husk ash. Several cement mortar compositions blended with RHA and lime were prepared. Reference cement mortar (Cref) was prepared in 1:3 cement/sand ratio by weight. In RHA-cement mortars, cement was replaced with 30, 50, and 70% RHA and were designated as (RHA-C.3-7), (RHA-C.5-5) and (RHA-C.7-3), respectively. Two types of ternary blended mortars were prepared with RHA, cement and lime. The ratio of the cement was kept 10% for both mixtures. While one (RHA-C-L.7-1-2) was composed of 70% RHA and 20% lime, the other (RHA-C-L.5-1-4) contained 50% RHA and 40% lime. Standard mortar beams (40 x 40 x 160 mm) were cast and cast and carbonation was studied up to 120 days. Carbonation depth of the RHA-cement mortars increased as the content of the cement decreased (Fig. 5.29). Hardening of RHA-C.3-7 was mostly gov­erned by the hydration reaction as carbonation was relatively lower than the other blended mortars. RHA-C.7-3 containing the lowest cement content (30%-wt.) reached a full carbonation depth at the age of 60 days while carbonation of the rest still continued.

Gastaldini et al. [24] examined the role of chemical activators on the carbon – ation of concrete containing 20% of rice husk ash as partial replacement of cement.

Water/binder ratios used were 0.35, 0.50 and 0.65 and 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. Specimens of size 100 x 100 mm were cured for 28 days. Their top surface was sealed and they underwent a preconditioning cycle as required by RILEM TC 116-PCD [40]. They were then placed in a controlled atmosphere chamber with 5% CO2, at 23 ± 1°C and RH 65 ± 1%. The carbon dioxide penetration depth was measured at different exposure times, 4, 8 and 12 weeks by means of the phenolphthalein test carried out on the transversely split section of the cylinders specimens using the RILEM CPC-18 [41] method. Figure 5.30 shows the changes in these coefficients for the w/b ratios. For all the mixtures, Kc increased with the increase in w/b ratio because of the increase in concrete porosity and the lower concentration of cement. For the same w/b ratio (0.35, 0.50 and 0.65), the lowest carbonation coefficients were seen in the mixture with RHA and 1% K2SO4 showed. The values obtained were lower than those in the reference concrete.

20

18

Г’7 days

16

П 60 days В 120 days

14 –

12

10

8

6

4

2

0

cL

43

fi

.2

с

о

-O

o3

U

Corrosion Resistance

Saraswathy and Song [46] investigated the influence of RHA on the corrosion resistance of concrete made with 0, 5, 10, 15, 20, 25, and 30% RHA as partial replacement of cement. Proportion of control (with out RHA) mix was 1:1.5:3 with w/c ratio of 0.53. Corrosion performance was evaluated by impressed voltage test and open circuit potential measurements. After 28 days of curing, the specimens were subjected to impressed voltage test. In this technique, the concrete specimens were immersed in 5% NaCl solution and embedded steel in concrete was made anode with respect to an external stainless steel electrode serving as cathode by applying a constant positive potential of 12 V to the system from a DC source. The variation of current was recorded with time. For each specimen, the time taken for

RHA (%)

Time to cracking (h)

0

42

5

72

10

74

15

No cracking even after 144 h of exposure

20

No cracking even after 144 h of exposure

25

No cracking even after 144 h of exposure

30

No cracking even after 144 h of exposure

Table 5.39 Impressed voltage test results of OPC and various percentages of rice husk ash replaced concrete [46]

initial crack and the corresponding maximum anodic current flow was recorded. For open circuit potential test, specimens of size 100 x 100 x 100 mm were cast. 12-mm diameter rebar of 120-mm length were embedded at a cover of 25 mm from one side of the specimen. After casting, the specimens were subjected to water curing for 28 days. After 28 days of curing, cubes were taken out and dried for 24 h and subjected to alternate wetting and drying in 3% NaCl solution. One cycle consisted of 7 days immersion in 3% NaCl solution and 7 days drying in open atmosphere. The tests were continued over a period of 200 days. Open circuit potential measurements were monitored with reference to saturated calomel electrode (SCE) periodically with time as per ASTM C876 [5]. Impressed voltage results are given in Table 5.39. It can be observed from the results that there was no cracking in concretes made with 15, 20, 25 and 30% rise husk even after 144 h of exposure. Whereas in ordinary Portland cement concrete, the specimen was cracked even after 42 h of exposure in 5% NaCl solution. The concrete specimens containing 5 and 10% rise husk also failed within 72 and 74 h of exposure. This indicated that the replacement of rice husk ash refined the pores and thereby the permeability and corrosion gets reduced.

Open circuit potential measurements values (OCP) as per ASTM C876 were lesser than -275 mV versus saturated calomel electrode (SCE), was considered to be passive in condition. They concluded that all the rice husk ash replaced con­cretes had shown less negative potential than -275 mV even up to 100 days of exposure indicating the passive condition of the rebars. Beyond 100 days of exposure, all the systems showed a more negative potential than -275 mV versus SCE irrespective of the replacement ratio showing the active condition of rebars.

Chindaprasirt and Rukzon [16] investigated the accelerated corrosion with impressed voltage (ACTIV) of mortars made with blends of ordinary Portland and ground rice husk ash (RHA). Cement was replaced with 0, 10, 20 and 40% RHA. Sand-to-binder ratio of 2.75 by weight, water to binder ratio (w/b) of 0.5 was used. Mortar prisms of dimensions 40 x 40 x 160 mm in length with embedded steel of 10-mm diameter and 160 mm in length were used. The steel was protected such that it protruded from the top surface of the prism by 15 mm; thus, providing sufficient mortar cover of 15 mm. At the age of 28 days, the prisms were subjected to the accelerated corrosion test with impressed voltage (ACTIV) using a 5% NaCl solution and a constant voltage of 12 V dc The condition of prism was
continuously monitored and the time of initiation of first crack was recorded. This was used as a measurement of the specimen’s relative resistance against chloride attack and reinforcement corrosion. The results of the time of first crack of mortar subjected to ACTIV are shown in Fig. 5.28. The time of first crack of OPC mortar was lowest at 89 h. The time to initial crack of mortars is found to increase with the incorporation RHA. RHA was found to be very effective in increasing the time of first crack. The time of first crack of 10RHA, 20RHA and 40RHA mortars were 167, 168 and 166 h, respectively,

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.

Electrical Resistivity and Conductivity

Gastaldini et al. [25] studied the electrical resistivity and conductivity in concrete mixes made with 10, 20, and 30% RHA as partial replacement of cement. It was determined using the four-electrode method. Specimens of size 100 x 100 x 170 mm were tested at the age of 91 days (Table 5.33). It was observed that (1) with the increase in RHA content from 10 to 20%, electrical resistivity increased substantially. For a content of 30% RHA, the increase in electrical resistivity when compared with the mix with 20% RHA amounted to 71, 47 and 47% for w/b ratios of 0.35, 0.50 and 0.65, respectively. The mix with 30% RHA was the one that

Table 5.33 Apparent electrical resistivity of concrete mixtures containing RHA [25]

Mixture

w/b

Electrical resistivity (X m)

Electrical conductivity (1/X cm)

Ref

0.35

316

2.834

0.50

154

2.504

0.65

128

2.101

10 RHA

0.35

446

2.525

0.50

291

2.294

0.65

229

2.076

20 RHA

0.35

813

2.371

0.50

569

2.137

0.65

440

1.929

30 RHA

0.35

1,395

2.204

0.50

837

1.933

0.65

646

1.987

showed the highest electrical resistivity for the three w/b ratios: 1,395, 836.8 and

646.2 X m, which corresponds to increases of 340, 442 and 404% for the w/b ratios of 0.35, 0.50 and 0.65 when compared with the reference sample; (2) for the reference mixture (REF) and mixes 10 RHA, 20 RHA and 30 RHA, conductivity values ranged from 2.101/X cm to 2.834/X cm, from 2.076X cm to 2.525/X cm, from 1.929/X cm to 2.371/X cm and from 1.837/X cm to 2.204/X cm, respec­tively, when the w/b ratio changed from 0.65 to 0.35.

Drying Shrinkage

Zhang and Malhotra [55] studied the drying shrinkage strain of concretes made with 10% RHA and 10% silica fume (SF). Figure 5.21 shows the drying shrinkage strain of concretes after 7 days of initial curing in lime-saturated water. Results indicated that RHA concrete had a drying shrinkage of 638 x 10-6 after 448 days, which was similar to the strains for the control and silica fume concretes.

Sensale et al. [48] studied the effect of partial replacements of Portland cement with rice-husk ash (RHA) on the autogenous shrinkage of cement paste. Pastes with water/binder ratio 0.30 and substitutions of 5 and 10% cement by RHA were used. Two sources of ash were considered; a residual RHA (RRHA) from the common rice paddy milling industries in Uruguay and a homogeneous ash pro­duced by controlled incineration from the United States (CRHA). RRHA had SiO2 content of 87.2% whereas CRHA had SiO2 content of 88%. Autogenous defor­mation was measured up to 28 days (Fig. 5.22). It was observed that RHA decreased the autogenous deformation. With no RHA, a considerable autogenous deformation (600 im/m) was obtained during 4 weeks of sealed hardening. Replacement of 5 and 10% of Portland cement by RHA led to a successive reduction of this autogenous deformation. 10% RHA decreased the autogenous deformation by 250 цш/m after 4 weeks.

Fig. 5.21 Drying shrinkage 800

of RHA and SF concretes

[55]

600

c

b

СЙ

400

и

c

200

д

Fig. 5.22 Autogenous deformation versus age of Portland and RHA cement pastes [48]

Habeeb and Fayyadh [26] studied the shrinkage of concrete mixtures made with RHA. Cement was replaced with three grades of RHA (F1, F2 and F3 i. e. 180, 270 and 360 min of grinding, respectively). Fineness of F1, F2 and F3 RHA were 27.4, 29.1, and 30.4 m2/g. Drying shrinkage was investigated at the age of 7, 14, 28, 42, 56, 90, 180 days under water curing for initial 7 days, and then samples were left in the air (Fig. 5.23). The results showed that average particle size of RHA had a significant effect on the drying shrinkage, the 20F3 concrete mixture exhibited higher shrinkage than the control. 20F2 concrete was comparable, while the shrinkage for 20F1 was lower compared to the control. The reduction in the RHA particle size increased the pozzolanic activity and contributed to the pore refine­ment of the RHA concrete matrix. Some authors have concluded that that con­cretes incorporating pore refinement additives will usually show higher shrinkage and creep [37]. On the other hand, others showed that using pozzolanic materials as cement replacement will reduce the shrinkage [13, 55]. These contradictory results about shrinkage are probably due to interpretational differences based on

deferent concepts, definitions and measuring techniques [44]. And that may also be because the deferent characteristics and degree of reactivity of the pozzolanic materials used.

Tensile Strength and Modulus of Elasticity

Zhang and Malhotra [55] reported the mechanical properties of concrete made with 10% RHA and 10% silica fume (SF). Tests were conducted at the age of 28 days, and results are given in Table 5.30. These results indicated that splitting tensile strength, flexural strength and modulus of elasticity of control and concrete incorporating RHA and SF were comparable.

Tashima et al. [50] studied the effect of RHA on the splitting tensile strength and modulus of elasticity of concrete. Cement was replaced with 0, 5, and 10% RHA. RHA had SiO2 content of 92.99% and blain specific surface of 16,196 cm2/g. Results of splitting tensile strength and elastic modulus are given in Table 5.31. It was observed that (1) addition of RHA did not had significant effect on the ten­sile strength, however at 5% RHA, there was marginal increase in strength; and (2) elastic modulus decreased with the increase in RHA content.

Cizer et al. [17] determined the flexural strength of cement mortars up the age of 120 days. Several cement mortar compositions blended with RHA were pre­pared. Reference cement mortar (Cref) was prepared in 1:3 cement/sand ratio by weight. In RHA-cement mortars, cement was replaced with 30, 50, and 70% RHA and were designated as (RHA-C.3-7), (RHA-C.5-5) and (RHA-C.7-3), respec­tively. Standard mortar beams (40 x 40 x 160 mm) were made and flexural strength results are shown in Fig. 5.16. Flexural strength of the reference cement

Table 5.30 Mechanical properties of concrete mixtures Zhang and Malhotra [55]

Mix

RHA

(%)

SF

(%)

W/Cm

ratio

Splitting tensile strength (MPa)

Flexural strength (MPa)

Modulus of elasticity (GPa)

1

0

0

0.40

2.7

6.3

29.6

2

10

0.40

3.5

6.8

29.6

3

10

0.40

2.8

7.0

31.1

7 days

28 days

91 days

7 days

28 days

91 days

Control (0% RHA)

4.85

5.37

5.41

30.08

40.85

45.04

5% RHA

4.94

5.79

5.9

40.72

40.76

41.84

10% RHA

4.82

5.78

5.4

40.23

40.21

40.03

Mixture Splitting tensile strength (MPa) Modulus of elasticity (GPA)

mortar and RHA-cement mortars increased gradually upon hardening. Increase in the flexural strength of the RHA-C.3-7 mortar between 7 and 28 days was more than that of RHA-C.5-5 and RHA-C.7-3 mortars having lower cement content. Flexural strength of the RHA-C.3-7 mortar did not change between 28 and 60 days but afterwards it reached a higher value at 120 days.

Ahmadi et al. [10] studied the flexural strength and modulus of elasticity of self­compacting concrete made with 10 and 20% RHA as partial replacement of cement. Results were compared with ordinary concrete. Flexural strength and modulus of elasticity results up to the age of 180 days are shown in Figs. 5.17 and 5.18. It was observed that (1) flexural strength of SCC mixes were 12 to 20% higher than normal concrete. Mixes containing rice husk ash indicated lower compressive strength until 60 days rather than samples with no replacement and by increasing the rate of pozzolanic reactions of rice husk ash in the matrix, strength of composite mixes goes up. Also the mixes containing 20% rice husk ash achieved the highest flexural strength in all cases. Moreover, by increasing the amount of replacement, flexural strength increased. This increase was greater in normal concrete than SCC; (2) with aging and hardening of concrete mixes, the module of elasticity of concrete mixes increased. Normal concrete mixes showed higher module of elasticity around 9 to 17% more than of SCC ones. Also by increasing the amount of rice husk ash in the matrix, module of elasticity of all mixes reduced.

Ganesan et al. [23] measured the 28-day splitting tensile strength of concrete mixtures containing 0, 5, 10, 15, 20, 25, 30, and 35% RHA as partial replacement

of cement. Control concrete mixture was made with 383 kg of cement, 575 kg of sand, 1,150 kg of coarse aggregate per cubic meter with water-binder ratio of 0.53. Splitting tensile strength of control concrete was 4.5 MPa. Splitting tensile strength marginally increased with RHA content up to 20% and then decreased marginally at 25 and 30% RHA, even then, value of splitting tensile strength was equal to of OPC concrete (4.5 MPa). Splitting tensile strength of concrete with 5, 10, 15, and 20 RHA was 4.7, 4.8, 4.9 and 5 MPa, respectively.

Ramezanianpour et al. [43] studied the influence of RHA on the splitting tensile strength and modulus of elasticity of concrete. Concrete mixtures were made with 0, 7, 10 and 15% RHA as partial replacement of cement. Results of splitting tensile strength and modulus of elasticity are shown in Figs. 5.19 and 5.20, respectively. It was concluded that (1) concrete containing RHA achieved greater splitting tensile strength than that the control concrete at all ages. It is clear that with the increase in RHA content, strength increased up to 20%. For instance, at 90 days 15% RHA concrete had splitting tensile strength of 5.62 MPa compared with 4.58 MPa for the control concrete; and (2) with the increase in RHA content modulus of elas­ticity increased at 28 and 90 days. At 90 days, mixture containing 15% of RHA

Fig. 5.18 Modulus of elasticity [10]

Fig. 5.19 Splitting tensile strength of control and RHA mixtures [43]

Fig. 5.20 Modulus of 34

elasticity (GPa) of control and RHA mixtures [43]

03

Сц

32

‘о

сл

30

4-і

О

Si

28

О

s

26

showed 7% increase in modulus concrete.

Habeeb and Fayyadh [26] studied the effect of RHA on the flexural strength, splitting tensile strength, and modulus of elasticity of concretes. Cement was replaced with three grades of RHA having fineness 27.4, 29.1, and 30.4 m2/g. Results of mechanical properties are given in Table 5.32. It was observed that (1) flexural strength values were in the range of 4.5-6.1 MPa. Addition of RHA to concrete exhibited an increase in the flexural strength and higher strength was obtained for the finer RHA mixture due to the increased pozzolanic reaction and the packing ability of the RHA fine particles; (2) splitting tensile strength values were in the range of 2.6-3.9 MPa. Tensile property was enhanced by adding RHA to the mixture. The coarse RHA mixture (20F1) showed the least improvement; and (3) modulus of elasticity of concrete mixtures was in the range of 29.6-32.9 GPa. Addition of RHA exhibited marginal increase in the elastic properties of concrete; the highest value was recorded for (20F3) mixture due to the increased reactivity of the RHA.