STRENGTH PROPERTIES OF GRADE 30 RICE HUSK ASH CONCRETE

Kartini, K.*, Universiti Teknologi MARA, Malaysia

Mahmud, H.B, University of Malaya, Malaysia Hamidah, M.S, Universiti Teknologi MARA, Malaysia

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31st Conference on OUR WORLD IN CONCRETE & STRUCTURES: 16 – 17 August 2006, Singapore

STRENGTH PROPERTIES OF GRADE 30 RICE HUSK ASH CONCRETE

Kartini, K.*, Universiti Teknologi MARA, Malaysia Mahmud, H.B, University of Malaya, Malaysia

Hamidah, M.S, Universiti Teknologi MARA, Malaysia

Abstract

With the increase growth of population in the country, the need to provide a decent and affordable accommodation to the people as part of the national economic strategy, i.e eradication of poverty and improving the living quality of the people is seriously looked into. However, in the building of houses (low-cost), the availability of building material which is cheap and abundant is of paramount importance. To overcome this problem, the development of new construction materials, which must be inexpensive and require very little energy to produce, must be researched. One such material is rice husk ash (RHA) which is a by-product of agriculture. Rice husk ash having pozzolanic properties would reduce the demand of Portland cement and thus it should reduce unit cost of concrete.

This paper presents a study on the strength properties of rice husk ash concrete of grade 30 with or without superplasticizer (Sp). Adding the optimum amount of RHA is important for achieving high strength, however, being cellular in nature, the use of RHA tends to increase the water requirement. Therefore, proper dosage of Sp needs to be incorporated. The results reveal the influence of RHA on the workability of concrete and its strength performance (compressive strength, flexural strength and tensile splitting) by replacing ordinary Portland cement (OPC) with different replacement levels of RHA. In this study, six series of concrete mixtures were prepared; in which three series comprise of control mix (OPC concrete) incorporating different proportions of RHA (20% and 30%). The water/binder ratio for these mixes were fixed at 0.63, 0.68 and 0.70 and designed to achieve slump in the range of 40 mm to 50 mm. The other three mixes were the OPC, 20% and 30% RHA incorporating Sp to achieve slump in the range of 100 mm to 150 mm, while maintaining the water to cementitious material ratio constant at 0.63. The results of this study show that the combined effects of incorporating Sp and partial replacement of OPC with RHA provided higher compressive strength compared to the one without Sp. The flexural strength of RHA concrete with or without Sp were lower, but the tensile strength without Sp at the later ages were better than OPC concrete. Keywords: Superplasticizer (Sp); Rice Husk Ash (RHA); Ordinary Portland Cement (OPC);

Workability; Compressive, Flexural and Tensile Splitting Strengths. 1 Introduction

Growth in human population over the last fifty years has doubled from 3 to 6 billion and it is expected to increase in years to come. It is estimated that by year 2050, more than 85 percent of the world’s population will live in urban areas. To serve the needs and changes, large amounts of materials are needed for the construction of houses, office buildings, roads and infrastructures required for decent living [1]. Concrete as a construction material has the largest production of all man made materials. The world wide consumption of this material is of the order of ten billion tonnes per year, next only to total consumption of water [2]. One of the constituent materials in making concrete is cement. Knowing that the raw material from earth sources will be scarce and the demand for cement is increasing as a result of economic and population growth, the effort to find alternative materials which must be of both inexpensive and require very little energy to produce has to be

undertaken. Thus, it is a current trend nowadays to use by-products or waste materials to partially replace cement in making concrete.

RHA has the potential as a cheap cementing material since rice husks are essentially waste material having high silica (SiO2) content, highly porous morphology, lightweight, angular and have a very high external surface area. Its absorbent and insulating properties are useful to many industrial applications, and the ash has been the subject of many research studies. It is estimated that the annual production of paddy rice husk globally was 600 million tonnes in 2002 [3] and with a husk to paddy ratio of 20% [4], and ash to husk ratio of 18% [5], therefore the total global ash production could be as high as 21.6 million tonnes per year and this figure is expected to increase. Consequently, the tremendous amount of cost could be saved by partially replacing OPC with RHA.

For countries where rice production is abundant, the use of RHA to partially substitute cement is attractive because of its reactivity. It is reported that the chemical properties of rice husks depends on the burning conditions and are similar to silica fume in which the SiO2 content is about 90% to 95%, 1% to 3% K2O, carbon content of 5.91% and less than 5% unburnt carbon [6]. The specific gravity of RHA is 2.05, lower than that of cement which is 3.15 and its specific surface area is about 20 m²/g to 50 m²/g [7]. In general, the average particle size of RHA is between 5 to 10 µm while that of cement is approximately 13 µm. The rice husk has a large dry volume due to its low bulk density (90-150 kg/m³) and possesses rough and abrasive surface that are highly resistant to natural degradation [6]. It is a very fine pozzolanic material which forms cementitious compound when reacts with lime and water. Table 1 shows typical chemical composition of RHA obtained from various references.

This paper reports on the study conducted on the performance of concrete with replacement of OPC with RHA at different percentages (20% and 30%) by weight of cement, and with or without Sp. The performance is assessed in term of engineering behavior due to compressive, flexural and tensile strengths.

Table 1: Chemical composition of RHA

Chemical composition (%) Present study

Present study

[7] [8] [9] [10]

OPC RHASilicon dioxide (SiO2) 21.38 96.70 90.7 93.15 88.73 87.2 Aluminium oxide (Al2O3) 5.60 1.01 0.4 0.21 2.03 0.15 Ferric oxide (Fe2O3) 3.36 0.05 0.4 0.21 1.99 0.16 Calcium oxide (CaO) 64.64 0.49 0.4 0.41 2.93 0.55 Magnesium oxide (MgO) 2.06 0.19 0.5 0.45 0.77 0.35 Sodium oxide (Na2O) 0.05 0.26 0.1 - 0.27 1.12 Potassium oxide (K2O) - 0.91 2.2 22.31 1.20 1.19 Phosphorous oxide (P2O5) - 0.01 1.5 - - 0.50 Titanium oxide (TiO2) - 0.16 0.3 - 0.31 0.01 Sulphur trioxide (SO3) 2.14 - 0.1 - 0.46 0.24 Loss on ignition 0.64 - 2.4 2.36 1.32 8.55

2 Experimental Programme

For determination of the performance of concrete in terms of its engineering properties due to compressive, flexural and tensile strength at 20% and 30% replacement of OPC by weight of RHA and with or without Sp, six (6) series of concrete specimens were cast, namely, OPC, RHA20, RHA30, OPCSp, RHA20Sp and RHA30Sp. In this study, the first three series comprise of control mix (OPC concrete) and concrete incorporating 20% and 30% RHA. The water/binder ratio for these mixes was varied at 0.63, 0.68 and 0.70 respectively in order to achieve a slump in the range of 40 mm to 50 mm. The other three mixes were the OPC, 20% and 30% RHA mixes, each mix having Sp of 0.4%, 1.50% and 2.08% respectively to achieve slump in the range of 100 mm to 150 mm. The water to cementitious material ratio of these mixes was maintained at 0.63. The optimum replacement level of RHA was determined from the results of the compressive strength test. Further casts were made to ascertain their flexural and tensile strengths. For this study only OPC, RHA20, OPCSp, and RHA30Sp were selected.

Materials Used

Fineness of RHA and OPC were determined by wet method in accordance to BS 3892: Part 1:1982 [11] as that retained on 45 μm sieve, and they were 21.87% and 6.12% respectively. As specified in BS 3892 and ASTM C430 [12], the prepared RHA conforms to grade A for the dry pulverized-fuel ash i.e 12.5% to 34% fineness. The chemical composition of RHA and OPC used in the present investigation is shown in Table 1. Figure 1 and Figure 2 show the elementary diffraction X-ray (EDAX), whereas Figure 3 and Figure 4 show XRD of OPC and RHA particles respectively.

Other materials used in the concrete mixture were granite coarse aggregate of 20 mm maximum

size and mining sand of 5 mm maximum size as fine aggregate. The fineness modules for the coarse and fine aggregates were 2.43 and 4.61 respectively. The Sp used is sulphonated naphthalene formaldehyde condensed polymer based admixture.

Mix Proportions

Figure 1: EDAX of Portland cement Figure 2 : EDAX of 100%RHA

Figure 3 : XRD of Portland Cement Figure 4 : XRD of RHA

The control OPC concrete was designed to achieve 30 N/mm2 using the DOE method [13]. Based on this, 325 kgm-3 of the cement content was adopted for all mixes. The water to cementitious material ratio (W/B) of the control mix was 0.63 with a slump of about 40 mm to 50 mm. Due to the absorptive cellular nature character of RHA particles and its high fineness, an increase in the amount of RHA content resulted in dry mixed concrete, therefore Sp was used to enhance the fluidity of the mixes. The Sp dosage in subsequent mixtures was tailored to achieve slump in the range of 100 mm to 150 mm. The range of mixes studied was limited to emphasize on the principal replacement additive parameter and hence all mixes had a coarse aggregate content of 940 kg m-3 and 900 kg m-3 of fine aggregate. The only variations in the mixes were the percentage content of RHA and the necessary adjusted W/B and Sp content. Table 2 summarises the mix proportions of all the concrete mixes, with and without Sp.

Table 2: Mixture Proportion of RHA concrete with and without Sp

Mixes W/B Sp (%)

Slump (mm)

Mass per unit volume of material (kg/m3)

Aggregate

OPC RHA Water Fine Coarse OPC 0.63 - 51 325 - 205 900 940 OPCRHA20 0.68 - 44 260 65 221 894 930 OPCRHA30 0.70 - 42 228 97 228 890 927 OPCSp 0.63 0.40 130 325 - 205 900 940 OPCRHA20Sp 0.63 1.00 120 260 65 205 900 940 OPCRHA30Sp 0.63 1.61 122 228 97 205 900 940

3 Results and Discussion Workability

As evident in Table 2, it is clearly seen that an increase in the percentage of RHA in the mix resulted in an increased amount of water and Sp to obtain the targeted workability of the concrete. The amount of W/B required in maintaining the workability of the mixes increases from 0.63 to 0.70 and the dosage of Sp varies from 0.4% to 1.61% in order to enhance the fluidity and consistency of the mix. The reasons being, concrete containing RHA require more water for a given consistency due to its adsorptive character of cellular RHA particles and its high fineness and thus, this increases its specific surface area. The addition of Sp will be adsorbed onto the cement particles, and imparts a strong negative charge, which helps to lower the surface tension of the surrounding water considerably and thus, greatly enhances the fluidity of the mixes. The results obtained are also in line with studies conducted by Nehdi et al.[14], Zhang and Malhotra [10] and Mehta [15,16]. Compressive Strength

The results of the compressive strength for the various mixes are given in Table 3 and presented graphically in Figure 3. It shows that the compressive strength of RHA concrete, with and without Sp, reduced as the percentage of RHA replacement increases and also the concrete with or without Sp did not significantly influence the later age strengths. This might be due to lower total cementitious content of the mixes. Results by Hwang and Wu [17] are in line with the present study, indicating that large amounts of RHA have an adverse effect and reduced the strength of concrete.

For RHA concrete specimens without Sp, it can also be seen that their compressive strengths are well above the target strength of 30 N/mm2 at 28 days, lower than the control concrete due to the higher water content of the mixes, in order to maintain similar workability. Nevertheless, strength of concrete with replacement of cement by up to 30% RHA manage to attain the target strength of 30 N/mm2.

For concrete with Sp, their compressive strength at the age of 28 days is higher than those

without Sp. The compressive strength of concrete with 20% RHA at age of 28 days recorded marginally higher value then the OPC concrete. However, if compared to OPCSp concrete, addition of RHA in superplasticized concrete would not be able to improve or enhance the compressive strength further. As can be seen in the Figure 5, it shows that the compressive strengths of OPCRHASp

concrete are lower at both 20% and 30% RHA replacements. This revealed that the increase in compressive strength is mainly due to the addition of Sp. Up to 20% replacement level of RHA, the addition of Sp did help further enhancement in compressive strength. In the case of OPCRHASp concrete, the amount of W/B ratio remained constant. However, the increased in Sp dosage due to increase in RHA content contributed to increased in strength.

Table 3: Compressive Strength of Control and RHA concrete, with and without Sp Mixes W/B Sp Compressive Strength (N/mm2)

(%) 1d 3d 7d 28d 90d 180d OPC 0.63 - 11.6 17.7 20.8 31.7 33.6 35.9 OPCRHA20 0.68 - 12.2 16.0 18.7 30.3 32.5 34.4 OPCRHA30 0.70 - 9.2 12.2 15.5 30.0 30.1 30.1 OPCSp 0.63 0.40 10.8 19.1 27.8 37.8 44.7 45.8 OPCRHA20Sp 0.63 1.00 12.8 18.6 22.8 32.0 37.6 37.6 OPCRHA30Sp 0.63 1.61 11.4 15.7 25.1 30.1 33.7 35.0

0

5

10

15

20

25

30

35

40

45

50

0 20 40 60 80 100 120 140 160 180

Days

Co

mp

ress

ive

Str

en

gth

(N/m

m2 )

OPC

RHA20

RHA30

OPCSp

RHA20Sp

RHA30Sp

Figure 5 : Compressive Strength of Control and RHA concrete with and without Sp

Flexural Strength

For the flexural strength test, only selected mixes were cast. Data on the flexural strength of the concrete specimens are shown in Table 4 and Figure 6. It shows that the flexural strength of concrete specimens incorporating 20% RHA decreases as compared to the OPC (control). This is attributed to higher W/B ratio of the former.

Table 4 : Flexural strength of Control and RHA concrete mixes

Mixes

Water/Binder (W/B)

Sp (%)

Flexural Strength (N/mm2)

7 days 28 days 90 days 180 days OPC 0.63 - 3.5 4.1 4.7 4.9 RHA20 0.68 - 2.4 3.8 4.0 4.2 OPCSp 0.63 0.40 3.2 4.1 4.6 5.0 RHA30Sp 0.63 1.61 2.8 3.6 3.7 3.9

The addition of Sp did not enhance significantly the flexural strength of RHA30Sp concrete

specimens compared to the control OPC and OPCSp concrete. RHA30Sp concrete exhibited slightly lower flexural strengths than the RHA20 concrete without Sp. The findings on the flexural strength obtained from this study were in line with the finding by Shimizu & Jorillo [18].

0

1

2

3

4

5

6

0 30 60 90 120 150 180Days

Fle

xura

l Str

engt

h (N

/mm

2 )

OPC

RHA20

OPCSp

RHA30Sp

Figure 6 : Flexural strength of RHA concrete specimens and OPC (control)

Tensile Splitting Strength

The tensile strength of the concrete specimens is shown in Table 5 and Figure 7. It shows that the tensile strength of concrete specimens incorporated with 20% RHA is lower at 7 days but similar at 90 and 180 days.

The addition of Sp in the RHA concrete mix does not enhance the tensile strength. RHA30Sp concrete exhibited lower strength than RHA20 at all ages. This result shows that higher RHA amounts will not increase the flexural and tensile strengths of Grade 30 concrete

The published data by Shimizu & Jorillo [18] and Cook et al. [19] on the tensile strength of RHA concrete showed that as the percentage of RHA replacement increases, the tensile strength reduces.

Table 5 : Tensile Splitting Strength for various RHA concrete mixes

Designation

Sp (%)

Water/Binder (W/B)

Tensile Splitting Strength (N/mm2)

7 days 28 days 90 days 180 days OPC(Control) 0.63 - 2.1 2.4 2.5 2.6 RHA20 0.68 - 1.4 2.0 2.6 2.6 OPCSp 0.63 0.40 1.8 2.3 2.4 2.7 RHA30Sp 0.63 1.61 1.2 1.7 2.1 2.1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 30 60 90 120 150 180Days

Ten

sile

Spl

itti

ng S

tren

gth

(N/m

m2 )

OPCRHA20OPCSpRHA30Sp

Figure 7 : Tensile splitting strength of Control and RHA concrete

4 Conclusions

From the result of the research work obtained, the following conclusions can be drawn:

1. Without Sp, RHA concrete attained lower compressive strength than that of the control due to the higher amount of water needed for similar workability. However, it can still attain strength of 30 N/mm2 at 28 days.

2. Up to 20% RHA content, strengths of RHA concrete is not much different than the control.

Further increase of RHA content decrease the compressive strength due to high water content required to maintain similar workability.

3. Addition of Sp to RHA mixes enhances the compressive strength of concrete and higher

replacement levels are possible. Replacing cement with RHA did not seem to increase further the compressive strength of superplasticised concrete.

4. Replacement of RHA does not improve the flexural and tensile strengths of concrete

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