Influence of Sand Content and Silica Fume on Mechanical Properties ...
ICCBT2008
Influence of Sand Content and Silica Fume on Mechanical Properties of Palm Kernel Shell Concrete U.Johnson Alengaram*, SEGi College Malaysia, MALAYSIA M. Z. Jumaat, University of Malaya, Kuala Lumpur, MALAYSIA H. Mahmud, University of Malaya, Kuala Lumpur, MALAYSIA
ABSTRACT This paper reports the results of an experimental investigation carried out on the use of palm kernel shell as lightweight aggregate to produce grade 35 lightweight concrete. The cementitious materials included 10% of silica fume as additional cementitious materials and 5 % fly ash as cement replacement materials. The variable, sand to cement ratio (s/c) was varied between 1.0 and 1.6. All mixes have been superplasticized. It has been found that the increase in sand content has positive influence on the mechanical properties of concrete. The saturated density and compressive strength of the concrete were found in the range of 1850 – 1960 kg/m3and 28-38 MPa. The other mechanical properties such as flexural and splitting tensile strengths were found in the range of 2.76 – 4.76 MPa and 1.9 – 2.61MPa, respectively over a period of 90 day. Other notable improvement was found in the modulus of elasticity. The static modulus of elasticity was found in the range of 8 – 11 GPa. The addition of silica fume resulted in cohesive mix and showed slumps in the range of 65-105 mm. The sand content is likely to increases both density and compressive strength. However an increase of s/c ratio beyond 1.6 is likely to make the concrete density above 2000 kg/m3.When the sand content was increased from 1.0 to 1.6, the increase in 28-day compressive strength was found about 24% for density increase of about 4%. Keywords: Palm kernel shell, silica fume, fly ash, mechanical properties
*Correspondence Author: U.Johnson Alengaram, Tel: +60361452777 Fax: +60361452888. E-mail: [email protected]
ICCBT 2008 - A - (23) - pp251-262
Influence of Sand Content on Mechanical Properties of Palm Kernel Shell Concrete
1.
INTRODUCTION
Manufactured lightweight aggregates (LWA) have been used to produce structural concrete in developed countries for many years. The use of synthetic lightweight aggregates from natural raw materials like clay, slate, shale etc., and from industrial by-products such as fly ash and slag ash hasn’t been fully explored in developing and underdeveloped countries in Asia and Africa. However researches in these regions on the use of organic natural aggregate in the form of palm kernel shells (PKS) are on the rise. One of the reasons for use of such natural organic materials is the availability of such industrial by-products as waste materials. Malaysia is the second largest producer of palm oil and in that process it produces millions of tonnes of PKS as waste material. 1.1 Significance of Research The past researches on using PKS as LWA produced compressive strength of PKS concrete in the range of 15 MPa to 28 MPa. The compressive strength depends on factors such as water, sand, aggregate contents and density. It has been found that the failure of PKS concrete is generally governed by the strength of PKS. However the smooth and convex surfaces of PKS produce poorly compacted concrete and these result in bond failure between PKS and cement matrix. In order to achieve PKS concrete over grade 30, the bond between mortar and PKS has to be improved. One of the ways to improve the bond is to check the influence of sand content as mechanical properties, in general, is governed by density of concrete. Also, Silica fume (SF) has been used to produce high strength concrete and SF particles are 100 times smaller than cement particles. The extremely very fine SF particles have the ability to be located in the very close proximity of the aggregate particles. Thus the zone between aggregate and cement paste interface, which is called zone of weakness, could be strengthened by the use of SF. However the study on properties of concrete containing PKS as coarse aggregates incorporating SF as cementitious material hasn’t been carried out. In this study, 10% of SF on weight of cement has been used as additional cementitious material. In addition, 5% of class- F fly ash (FA) was also used as cement replacement material. The effect of SF and FA as cementitious materials on workability and compressive strength up to the age of 90 days has been studied. The objective of this study was to investigate the influence of sand content and silica fume on the mechanical properties of PKS concrete. Also the dynamic modulus of elasticity was determined to establish a relationship between static and dynamic modulus.. The influence of sand content on workability and compressive strength has been studied and reported.
2.
MATERIALS USED IN THE INVESTIGATION
2.1 Cement and Cementitious Materials Ordinary Portland cement conforming to MS 522; Part-1:2003 with specific gravity and surface area of 3.10 and 335m2/kg respectively was used for all mixes. The residue on 45μm and 90μm were respectively 6.8% and 0.6%. Class - F fly ash obtained from Lafarge Malayan Cement with SiO2 content about 65% and relative density of 2.10 was used. SF in densified form with specific gravity of 2.10 was used as additional cementitious material for all mixes.
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The chemical compositions of cement, FA and SF are given in Table 1. The percentage of FA and SF on weight of cement used for different mixes is shown in Table 2. About 1% of superplasticizer (SP) on cement weight was used for all mixes. Table -1 Chemical composition of cement, fly ash and silica fume (%) Materials Cement Fly ash Silica fume
SiO2 19.8 64.6 94.6
Al2O3 5.10 20.9 0.14
Fe2O3 3.10 4.00 0.11
CaO 63.4 1.00 0.01
Oxide composition MgO SO3 K2O 2.50 2.40 1.00 0.66 0.30 1.20 0.01 0.01 0.62
Na2O 0.19 0.32 0.01
LOI 1.80 5.10 4.10
TiO2 1.10 0.01
P2O5 0.07 0.22
2.2 Fine and Coarse Aggregates Mining sand was used as fine aggregates with particle density of 2.7. It was dried and sieved to a particle size range between 0.15 and 2.36 mm. The particle size distribution of fine aggregates is shown in Fig.1. PKS, as seen in Fig. 1 (c) used as coarse aggregates were obtained from local crude palm oil producing mill. Since PKS are waste materials, these are normally stockpiled in open fields, thus subject to varying climatic conditions. As Malaysia is a tropical country with unpredictable rainfall throughout the year, the shells are bound to absorb moisture during such storage conditions; also during sunny days, the surface moisture may be dried out leaving some moisture inside the pores of PKS. Hence the water absorption characteristics of PKS were determined.
3.
EXPERIMENTAL WORKS
3.1 Preparation of PKS as Coarse Aggregate Preparation of PKS was done first by drying, sieving and washing the aggregates with detergents in order to remove dust, oil and mud particles that adhered to the surfaces of PKS. After washing, the particles were again dried under roof and then stockpiled. Due to high water absorption of PKS (about 25%), pre-soaking of aggregates for about 45 minutes to 1 hour is mandatory. The absorption during this period of pre-soaking was determined and found to be in the range of 10 to 12 %. Particles with size less than 3.35 mm were removed and not used in mixes due to large relative surface area and high absorption. 3.2 Mix design, Concrete Mixtures and Testing The mix design was done based on relative densities of materials, 5 % FA as cement replacement, 10 % SF as additional cementitious material and proportion of the constituent materials. A total of three concrete mixes incorporating cementitious materials and varying s/c ratio were prepared as indicated in Table 2. The water to binder ratio (w/c) and aggregate to cement ratio (a/c) were kept constant for all mixes at 0.35 and 0.8 respectively for all mixes. The variable sand to cement (s/c) ratio was varied between 1.0 and 1.6. One mix without any additional cementitious materials and similar mix proportions as that of PKSC-S1 was also prepared for comparison purposes. All the materials were weight batched. The mixing of materials was done in the following order: Firstly one-half of PKS and sand were mixed in the mixer. This was followed by addition of one-half of cement, fly ash and silica fume; part ICCBT 2008 - A - (23) - pp251-262
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of water with superplasticizer was then added; on complete mixing, the remaining portion of materials were added in appropriate order. The specimens, 100 mm cube, 150Φ x 300mm height cylinders and 100 x 100 x 500 mm prism moulds were cast and covered with plastic sheeting in the uncontrolled laboratory condition for 24 hours and then demoulded. The cement content for mixes PKSC-S1 – S3 was between 470- 510 kg/m3, while for mix, PKSCPS it was about 600 kg/m3. The fresh, as cured and oven dry densities of PKS concrete were measured. Workability tests by slump and flow measurements were done in accordance with BS EN. The compressive strengths were measured at 1,7,14, 28, 56 and 90 days. The splitting tensile, flexural strengths and modulus of elasticity were measured at 28, 56 and 90 days.
4.
DISCUSSION ON FRESH CONCRETE PROPERTIES
4.1 Properties of PKS The thicknesses of shells were in the range of 1.7 to 2.6 mm and the sizes of shells vary between 2 to15 mm and the particle size distribution is shown in Fig.1. The relative density in saturated surface dry condition determined was found to be 1.27. The loose and compacted densities were found as 568 and 620 kg/m3 respectively. The natural moisture content and 24 hour water absorption of PKS were found in the range of 8 to15% and 25% respectively. The pre-soaking of PKS for a period of about 45 to 60 minutes increased the total moisture content.
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P alm kernel s hell
100
Percentagefiner
80
60
40
20
0 0.1
1
10
100
S i e ve S i z e (m m )
Figure 1(a). Particle Size Distribution of Sand and PKS
Figure. 1(b). Palm Kernels
Figure 1(c). Palm kernel Shell (PKS)
Figure.1 Particle Size Distribution of Sand and Palm Kernel Shells and natural Palm Kernel Shell 4.2 Density The measured fresh, as cured and oven dry densities as of 28 day are given in Table 3.The fresh densities of PKSC ranged between 1852 and 1940 kg/m3. It has been found that oven dry densities were about 220 to 260 kg/m3 lower than water cured densities. The highest density of 1971 kg/m3 was reported for mix containing s/c ratio of 1.6. Increase in sand content beyond s/c ratio of 1.6 might have resulted in higher density than the limit for LWC of 2000 kg/m3 and hence mixes containing s/c ratio higher than 1.6 was not considered. 4.3 Workability Table 2 also shows the measured slump and flow values for the mixes. The mixes PKSC: S1S3 with constant w/b ratio of 0.35 and a/c ratio of 0.8, showed medium to high workability. Though s/c ratio of 1.0 and 1.2 showed high workability, further increase in sand content requires more water or SP to get high workability. Thus for mix, PKSC: S3 with s/c ratio of 1.6, medium workability of about 75 mm was obtained, indicating higher fines content reduces workability. ICCBT 2008 - A - (23) - pp251-262
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The mix PKSC: PS containing no cementitious materials produced very high workability with slump value of 160 mm. However for mix PKSC: S1 of similar mix proportion as that of PKSC-PC, a slump of 105 mm was found. Thus silica fume added as additional cementitious material has produced cohesive mix in PKSC-S1 and this resulted in lower slump values. This could be related to the effect of SF, as it increases the cohesiveness of the mix due to its fineness and filling the gap between particles of cement. Table-2 Mix proportion and properties of PKSC
Mix PKSC-S1 PKSC-S2 PKSC-S3 PKSC-PS
Water /binder ratio
Fly ash (%)
Silica fume (%)
Sand / cement ratio
Fresh Density (kg/m3)
Saturated Density (28 Days) (kg/m3)
Oven Dry Density (kg/m3)
Slump (mm)
Flow (mm)
0.35 0.35 0.35 0.35
5 5 5 0
10 10 10 0
1.0 1.2 1.6 1.0
1852 1912 1940 1895
1893 1940 1971 1915
1639 1705 1715 1694
105 103 75 160
220-370 200-330 200-290 -
Workability
Slump test tends to underestimate workability of lightweight aggregate concrete and therefore the flow values using flow table test were measured. Higher flow table values in the range of 220-370 mm were recorded for mixes having higher sand and lower PKS contents. Though only 5% fly ash was added, its contribution to workability can’t be ignored as spherical shape of FA reduces friction forces between aggregate particles and increases the workability. The addition of SP has also increased workability and the use of SP is mandatory due to the inclusion of SF. 5.
DISCUSSION ON HARDENDED CONCRETE PROPERTIES
5.1 Compressive Strength 5.1.1 Influence of Sand Content on Compressive Strength Figure 2 shows the progress of compressive strength of PKSC for a period of 90 days. Generally compressive strength depends on factors such as density, w/b, a/c and s/c ratios. As with normal weight concrete, lower the w/b ratio, higher the compressive strength. Generally higher density concrete produces higher strengths. For mix PKSC: S3, the s/c ratio was maintained at 1.6 and this had resulted in the highest density of approximately 1970 kg/m3 and the highest 28 day strength of about 36 MPa was achieved. It was evident during test that the breaking of PKS took place before final failure, thus indicating failure of PKS than mortar. Hence it can be concluded that the failure of PKS governed the strength. In the mixes PKSC: S1-S3, the strength gain due to higher fine aggregate and lower PKS contents was evident as good bond between PKS and cement matrix enabled the concrete to sustain higher load. The presence of high volume of pores in PKS which was evident because of high water absorption of about 25 % may weaken the particle strength and stiffness. Though the pores may weaken the compressive strength and elastic modulus properties of PKSC, these pores may help in development of good bond by the suction of the paste into the pores of PKS. However further investigation is required to study this effect. 256
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The comparison of 28-day compressive strength shows the difference between PKSC-S1 and S2 shows that PKSC-S2 has an increase of about 17% of strength compared to PKSC-S1. Similarly, PKSC-S3 also has an increase of about 24% compared to PKSC-S1. Thus the influence of sand content on compressive strength is evident. It can also be shown that the increase in compressive strength between PKSC-S2 and S3 is not high as compared to PKSCS1 and it can be concluded that s/c ratio of 1.2 could be ideal mix as far density is concerned. The density of mix PKSC-S3 is almost close to 2000kg/m3 and hence it is likely that a small increase in any constituent material may cause further increase in the density and thereby go beyond the limit of 2000kg/m3 set for lightweight concrete.
Fig. 2. Progress of Compressive Strength of PSKC
Fig 3. Relationship between Modulus of Rupture and Compressive Strength
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5.1.2 Influence of Silica Fume on Compressive Strength The addition of SF has influenced the compressive strength. High early strength of about 40 to 50% and 80 to 90% in one day and 7 day respectively on 28 day strength was observed for mixes containing SF. This may be attributed to fineness of SF and reaction between silicon dioxide and calcium hydroxide. The infilling of the voids in the shells by very fine SF particles may have increased the bond between PKS and cement matrix. Thus SF plays major role in early strength development, allowing aggregates better to participate in stress transfer. However, as mentioned earlier further research is required to study the effect of SF in the pores of PKS. Thus for all PKSC specimens containing SF, the failure was predominantly due to failure of PKS that was evident during test. The development of strength beyond the period between 28 and 90 days has been in the range of 2 to 7 % on 28 day compressive strength, though not significant, indicates that hydration continues at slower rate. Thus the effect of 5% fly ash on hydration within 90 days couldn’t be ascertained. Table 3 Tensile Strengths of PKSC Mix
Splitting tensile strength (MPa)
Modulus of rupture (MPa)
PKSC: S1
28-day 1.90
56-day 1.95
90-day 2.00
28-day 2.76
56-day 2.81
90-day 2.84
PKSC: S2
2.00
2.11
2.21
3.22
3.25
3.30
PKSC: S3
2.35
2.56
2.61
4.10
4.45
4.56
PKSC: PS
1.98
2.01
2.10
2.79
2.98
3.17
5.2 Modulus of Rupture and Splitting Tensile Strengths of PKSC The flexural and splitting tensile strengths of PKSC are shown in Table 3. It can be seen from the results that these strengths also follow a similar trend as that of compressive strengths. As the sand content is increased the flexural and splitting tensile strengths increased further. The highest flexural and splitting tensile strengths were obtained for PKSC-S3 that contained the highest s/c ratio of 1.6. The ratio of splitting tensile and flexural was found between 60-70%. The increase in these strengths after 28 days was found between 2 and 8% over a period of 90 days. The mix PKSC-PS that contained no cementitious materials produced comparable strengths as that of mix PKSC-S3, designed with similar mix proportion. However, PKSC-PS had higher cement content of about 600 kg/m3 and hence likely to cause shrinkage cracks. A relationship between flexural strength and cube compressive strengths yields the following: fcfc = 0.3(fcuc)2/3 Where fcfc and fcuc are respectively flexural and cube compressive strengths in MPa. 5.3 Static and Dynamic Moduli of Elasticity Table 4 shows both static and dynamic moduli of elasticity of all mixes. The past research works by few researchers on PKS concretes had shown static modulus of elasticity or Evalues in the range of between 7 and 8 GPa. The lower E-values of PKS concrete is generally attributed to poor stiffness of PKS and its lower particle density. Thus, the lower E-values tend to produce larger deflections and hence the E-values have to be increased. The highest E258
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values of about 11 GPa was obtained for mix PKSC-S1 that contained higher sand content and density. Thus an increase of about 13-40 % on E-value compared to the previous results is significant improvement and hence this mix may produce lesser deflection if used as structural concrete. The increase in E-value between S1 and S3 mixes was found about 27% and a slight increase in sand content as a positive effect on E-values. However, the mix PKSC-PS that contained no cementitious materials produced the lowest E-values. This may be attributed to the addition of SF to other mixes that produced good bond between aggregate and matrix that enable to sustain higher strains in PKSC-S1 – S3. Even a comparison between mixes PKSC-S3 and PKSC-PS with similar mix proportions shows that PKSC-S3 has about 20% higher E-values. Table 4 Elastic Properties of PKSC Static modulus of elasticity (kN/mm2) 28-day 56-day 90-day 8.57 8.59 9.01 10.01 10.21 10.25 10.90 11.51 11.87 7.08 7.91 7.98
Mix PKSC: S1 PKSC: S2 PKSC: S3 PKSC: PS
Dynamic modulus of elasticity (kN/mm2) 28-day 56-day 90-day 13.20 13.76 14.02 15.47 15.73 15.75 16.69 20.01 20.15 14.35 14.65 15.27
As dynamic modulus test is non-destructive test and easy to perform using prisms, the values can be used to ascertain the E-values of PKSC, similar to normal weight concrete (NWC). A relationship between the static modulus and dynamic modulus is shown in Fig. 4. Thus it can be seen the equation predicts the static modulus of elasticity for PKSC, if the dynamic values are known. The relationship for static modulus can be written as Es = 1.2 (Ed) 0.76, where Es and Ed are static and dynamic moduli of elasticity in GPa.
Static modulus of elasticity (Es) GPa
13 Es = 1.2 Ed0.76 R = 0.96 12
11
10
9
8 12
15 18 Dynamic modulus of elasticity (Ed) GPa
21
Fig 4. Relationship between Static and Dynamic Modulus of Elasticity
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Influence of Sand Content on Mechanical Properties of Palm Kernel Shell Concrete
In addition to the above relationship a solid and reliable equation can be established between cube compressive strengths and static modulus of elasticity similar to that of NWC. The following relationship can be established for static modulus of elasticity and compressive strength: Es = 0.2fcuc1.1 where Es and fcuc are static modulus and compressive strengths in GPa and MPa respectively.
12
Modulus of Elasticity (GPa)
Es = 0.2 fcuc1.1 R = 0.99 11
10
9
8 28
32 36 Compressive strength (MPa)
40
Fig 5. Relationship between Modulus of Elasticity and Compressive Strength
6.
CONCLUSIONS 1) The fresh density of mix containing the highest sand to cement ratio of 1.6 was found about 1940 kg/m3 and this is within the density limit of 2000kg/m3 for lightweight concrete and 28 day compressive strength of about 36 MPa was achieved. Thus using PKS as coarse aggregate lightweight concrete of grade 35 could be produced. 2) The use of superplasticizer is mandatory due to lower w/b ratio and high sand content. The mixes yielded medium to high slump and the mix with the highest sand content produced lower slump and flow values. The addition of silica fume produced cohesive mix. However, the mix PKSC-PS that contained no SF produced very high workability. 3) The fresh and saturated densities of PKSC-S1 – S3 varied between of PKSC varied between 1852 – 1971 kg/m3, while the oven dry densities were about 15% lower than the saturated densities. The highest density of PKSC-SC3 shows that the concrete could be categorised under lightweight. An increase in sand content beyond s/c of 1.6 may result in density of more than 2000 kg/m3, thus making it normal weight concrete.
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4) The increase in compressive strength between PKSC-S1 and PKSC-S3 was found about 24% and the corresponding value between PKSC-S1 and PKSC-S2 was 17%. 5) The increase in s/c ratio from 1.0 to 1.6 resulted in slight increase in density between 2 – 5 %. However, the increase in the compressive strength was found to vary between 16 and 28%. The highest 28-day compressive strength of about 38 MPa was obtained for PKSC-S3 with s/c of 1.6. Thus grade 35 PKS concrete can be produced with cement content of about 470 kg/m3 and addition of 10% silica fume. 6) The positive influence of higher sand content is seen in all the mechanical properties and the tensile strengths via, splitting and flexural tensile strengths were found in the range of 1.90 - 2.61 and 2.76 - 4.56 MPa, respectively. The ratio between splitting tensile and flexural strength was found in the range of 60 – 70%. 7) The E-values of PKSC-S1 – S3 produced values in the range of about 9 – 12 GPa. This shows an increase of about 13 - 40% compared to previous research works. Thus an increase in sand content and addition of silica fume produced higher modulus of elasticity and this may results in lower deflections if used as structural concrete. 8) Using the dynamic modulus test, a simple relationship between static and dynamic moduli of elasticity can be established. 9) Relationship between compressive strength and static modulus of elasticity shows that the static modulus can be found using the relationship Es= 0.2(fcuc) 1.1.
REFERENCES [1]. Abdullah, A.A.A., 1984. Basic Strength Properties of Lightweight Concrete Using Agricultural Wastes as Aggregates, Proceedings of International Conference on Low-cost Housing for Developing Countries, Roorkee, India. [2]. ASTM, 2003. ASTM C 29/C29M. Standard Test Method for Bulk Density and Voids in Aggregate. Annual Book of ASTM Standards. [3]. ASTM, 2003. ASTM C 127. Standard Test Method for Density, Relative Density and Absorption of Coarse Aggregate. Annual Book of ASTM Standards. [4]. ASTM, 2003. ASTM C 136. Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates. Annual Book of ASTM Standards. [5]. ASTM, 2003. ASTM C 138. Standard Test Method for Density, Yield and Air Content. Annual Book of ASTM Standards. [6]. Ata, O, Olanipekun E.A, Oluola K.O. 2006. A comparative study of concrete properties using coconut shell and palm kernel shell as coarse aggregates. Building and Environment 41 (3), 297301. [7]. Basri H.B, Mannan M.A, Zain M.F.M., 1999. Concrete using oil palm shells as aggregate. Cement and concrete Research 29 (4), 619-622. [8]. BSI, 2000. BS EN 12350-2. Testing fresh concrete-Part-2: Slump test, British Standards Institution, London. [9]. BSI, 2000. BS EN 12350-5. Testing fresh concrete-Part-5: Flow table test, British Standards Institution, London. [10]. BSI, 2002. BS EN 12390-3. Testing hardened concrete-Part-3: Compressive strength of test specimens, British Standards Institution, London. ICCBT 2008 - A - (23) - pp251-262
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Influence of Sand Content on Mechanical Properties of Palm Kernel Shell Concrete [11]. Clarke, J.L., 1993. Structural lightweight aggregate concrete. Chapman Hall, Glasgow. [12]. Okafor, F.O., 1988. Palm kernel shell as aggregate for concrete. Cement and Concrete Research 18 (6), 901-910. [13]. Mannan, M.A., Ganapathy, C., 2004. Concrete from an agricultural waste-oil palm shell (OPS). Building and Environment 39 (4), 441-448. [14]. Neville A.M. 1996. Properties of concrete. Longman Group Limited, London.
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Influence of Sand Content and Silica Fume on Mechanical Properties of Palm Kernel Shell Concrete U.Johnson Alengaram*, SEGi College Malaysia, MALAYSIA M. Z. Jumaat, University of Malaya, Kuala Lumpur, MALAYSIA H. Mahmud, University of Malaya, Kuala Lumpur, MALAYSIA
ABSTRACT This paper reports the results of an experimental investigation carried out on the use of palm kernel shell as lightweight aggregate to produce grade 35 lightweight concrete. The cementitious materials included 10% of silica fume as additional cementitious materials and 5 % fly ash as cement replacement materials. The variable, sand to cement ratio (s/c) was varied between 1.0 and 1.6. All mixes have been superplasticized. It has been found that the increase in sand content has positive influence on the mechanical properties of concrete. The saturated density and compressive strength of the concrete were found in the range of 1850 – 1960 kg/m3and 28-38 MPa. The other mechanical properties such as flexural and splitting tensile strengths were found in the range of 2.76 – 4.76 MPa and 1.9 – 2.61MPa, respectively over a period of 90 day. Other notable improvement was found in the modulus of elasticity. The static modulus of elasticity was found in the range of 8 – 11 GPa. The addition of silica fume resulted in cohesive mix and showed slumps in the range of 65-105 mm. The sand content is likely to increases both density and compressive strength. However an increase of s/c ratio beyond 1.6 is likely to make the concrete density above 2000 kg/m3.When the sand content was increased from 1.0 to 1.6, the increase in 28-day compressive strength was found about 24% for density increase of about 4%. Keywords: Palm kernel shell, silica fume, fly ash, mechanical properties
*Correspondence Author: U.Johnson Alengaram, Tel: +60361452777 Fax: +60361452888. E-mail: [email protected]
ICCBT 2008 - A - (23) - pp251-262
Influence of Sand Content on Mechanical Properties of Palm Kernel Shell Concrete
1.
INTRODUCTION
Manufactured lightweight aggregates (LWA) have been used to produce structural concrete in developed countries for many years. The use of synthetic lightweight aggregates from natural raw materials like clay, slate, shale etc., and from industrial by-products such as fly ash and slag ash hasn’t been fully explored in developing and underdeveloped countries in Asia and Africa. However researches in these regions on the use of organic natural aggregate in the form of palm kernel shells (PKS) are on the rise. One of the reasons for use of such natural organic materials is the availability of such industrial by-products as waste materials. Malaysia is the second largest producer of palm oil and in that process it produces millions of tonnes of PKS as waste material. 1.1 Significance of Research The past researches on using PKS as LWA produced compressive strength of PKS concrete in the range of 15 MPa to 28 MPa. The compressive strength depends on factors such as water, sand, aggregate contents and density. It has been found that the failure of PKS concrete is generally governed by the strength of PKS. However the smooth and convex surfaces of PKS produce poorly compacted concrete and these result in bond failure between PKS and cement matrix. In order to achieve PKS concrete over grade 30, the bond between mortar and PKS has to be improved. One of the ways to improve the bond is to check the influence of sand content as mechanical properties, in general, is governed by density of concrete. Also, Silica fume (SF) has been used to produce high strength concrete and SF particles are 100 times smaller than cement particles. The extremely very fine SF particles have the ability to be located in the very close proximity of the aggregate particles. Thus the zone between aggregate and cement paste interface, which is called zone of weakness, could be strengthened by the use of SF. However the study on properties of concrete containing PKS as coarse aggregates incorporating SF as cementitious material hasn’t been carried out. In this study, 10% of SF on weight of cement has been used as additional cementitious material. In addition, 5% of class- F fly ash (FA) was also used as cement replacement material. The effect of SF and FA as cementitious materials on workability and compressive strength up to the age of 90 days has been studied. The objective of this study was to investigate the influence of sand content and silica fume on the mechanical properties of PKS concrete. Also the dynamic modulus of elasticity was determined to establish a relationship between static and dynamic modulus.. The influence of sand content on workability and compressive strength has been studied and reported.
2.
MATERIALS USED IN THE INVESTIGATION
2.1 Cement and Cementitious Materials Ordinary Portland cement conforming to MS 522; Part-1:2003 with specific gravity and surface area of 3.10 and 335m2/kg respectively was used for all mixes. The residue on 45μm and 90μm were respectively 6.8% and 0.6%. Class - F fly ash obtained from Lafarge Malayan Cement with SiO2 content about 65% and relative density of 2.10 was used. SF in densified form with specific gravity of 2.10 was used as additional cementitious material for all mixes.
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The chemical compositions of cement, FA and SF are given in Table 1. The percentage of FA and SF on weight of cement used for different mixes is shown in Table 2. About 1% of superplasticizer (SP) on cement weight was used for all mixes. Table -1 Chemical composition of cement, fly ash and silica fume (%) Materials Cement Fly ash Silica fume
SiO2 19.8 64.6 94.6
Al2O3 5.10 20.9 0.14
Fe2O3 3.10 4.00 0.11
CaO 63.4 1.00 0.01
Oxide composition MgO SO3 K2O 2.50 2.40 1.00 0.66 0.30 1.20 0.01 0.01 0.62
Na2O 0.19 0.32 0.01
LOI 1.80 5.10 4.10
TiO2 1.10 0.01
P2O5 0.07 0.22
2.2 Fine and Coarse Aggregates Mining sand was used as fine aggregates with particle density of 2.7. It was dried and sieved to a particle size range between 0.15 and 2.36 mm. The particle size distribution of fine aggregates is shown in Fig.1. PKS, as seen in Fig. 1 (c) used as coarse aggregates were obtained from local crude palm oil producing mill. Since PKS are waste materials, these are normally stockpiled in open fields, thus subject to varying climatic conditions. As Malaysia is a tropical country with unpredictable rainfall throughout the year, the shells are bound to absorb moisture during such storage conditions; also during sunny days, the surface moisture may be dried out leaving some moisture inside the pores of PKS. Hence the water absorption characteristics of PKS were determined.
3.
EXPERIMENTAL WORKS
3.1 Preparation of PKS as Coarse Aggregate Preparation of PKS was done first by drying, sieving and washing the aggregates with detergents in order to remove dust, oil and mud particles that adhered to the surfaces of PKS. After washing, the particles were again dried under roof and then stockpiled. Due to high water absorption of PKS (about 25%), pre-soaking of aggregates for about 45 minutes to 1 hour is mandatory. The absorption during this period of pre-soaking was determined and found to be in the range of 10 to 12 %. Particles with size less than 3.35 mm were removed and not used in mixes due to large relative surface area and high absorption. 3.2 Mix design, Concrete Mixtures and Testing The mix design was done based on relative densities of materials, 5 % FA as cement replacement, 10 % SF as additional cementitious material and proportion of the constituent materials. A total of three concrete mixes incorporating cementitious materials and varying s/c ratio were prepared as indicated in Table 2. The water to binder ratio (w/c) and aggregate to cement ratio (a/c) were kept constant for all mixes at 0.35 and 0.8 respectively for all mixes. The variable sand to cement (s/c) ratio was varied between 1.0 and 1.6. One mix without any additional cementitious materials and similar mix proportions as that of PKSC-S1 was also prepared for comparison purposes. All the materials were weight batched. The mixing of materials was done in the following order: Firstly one-half of PKS and sand were mixed in the mixer. This was followed by addition of one-half of cement, fly ash and silica fume; part ICCBT 2008 - A - (23) - pp251-262
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Influence of Sand Content on Mechanical Properties of Palm Kernel Shell Concrete
of water with superplasticizer was then added; on complete mixing, the remaining portion of materials were added in appropriate order. The specimens, 100 mm cube, 150Φ x 300mm height cylinders and 100 x 100 x 500 mm prism moulds were cast and covered with plastic sheeting in the uncontrolled laboratory condition for 24 hours and then demoulded. The cement content for mixes PKSC-S1 – S3 was between 470- 510 kg/m3, while for mix, PKSCPS it was about 600 kg/m3. The fresh, as cured and oven dry densities of PKS concrete were measured. Workability tests by slump and flow measurements were done in accordance with BS EN. The compressive strengths were measured at 1,7,14, 28, 56 and 90 days. The splitting tensile, flexural strengths and modulus of elasticity were measured at 28, 56 and 90 days.
4.
DISCUSSION ON FRESH CONCRETE PROPERTIES
4.1 Properties of PKS The thicknesses of shells were in the range of 1.7 to 2.6 mm and the sizes of shells vary between 2 to15 mm and the particle size distribution is shown in Fig.1. The relative density in saturated surface dry condition determined was found to be 1.27. The loose and compacted densities were found as 568 and 620 kg/m3 respectively. The natural moisture content and 24 hour water absorption of PKS were found in the range of 8 to15% and 25% respectively. The pre-soaking of PKS for a period of about 45 to 60 minutes increased the total moisture content.
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U.Johnson Alengaram et. al. Mining s and
P alm kernel s hell
100
Percentagefiner
80
60
40
20
0 0.1
1
10
100
S i e ve S i z e (m m )
Figure 1(a). Particle Size Distribution of Sand and PKS
Figure. 1(b). Palm Kernels
Figure 1(c). Palm kernel Shell (PKS)
Figure.1 Particle Size Distribution of Sand and Palm Kernel Shells and natural Palm Kernel Shell 4.2 Density The measured fresh, as cured and oven dry densities as of 28 day are given in Table 3.The fresh densities of PKSC ranged between 1852 and 1940 kg/m3. It has been found that oven dry densities were about 220 to 260 kg/m3 lower than water cured densities. The highest density of 1971 kg/m3 was reported for mix containing s/c ratio of 1.6. Increase in sand content beyond s/c ratio of 1.6 might have resulted in higher density than the limit for LWC of 2000 kg/m3 and hence mixes containing s/c ratio higher than 1.6 was not considered. 4.3 Workability Table 2 also shows the measured slump and flow values for the mixes. The mixes PKSC: S1S3 with constant w/b ratio of 0.35 and a/c ratio of 0.8, showed medium to high workability. Though s/c ratio of 1.0 and 1.2 showed high workability, further increase in sand content requires more water or SP to get high workability. Thus for mix, PKSC: S3 with s/c ratio of 1.6, medium workability of about 75 mm was obtained, indicating higher fines content reduces workability. ICCBT 2008 - A - (23) - pp251-262
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The mix PKSC: PS containing no cementitious materials produced very high workability with slump value of 160 mm. However for mix PKSC: S1 of similar mix proportion as that of PKSC-PC, a slump of 105 mm was found. Thus silica fume added as additional cementitious material has produced cohesive mix in PKSC-S1 and this resulted in lower slump values. This could be related to the effect of SF, as it increases the cohesiveness of the mix due to its fineness and filling the gap between particles of cement. Table-2 Mix proportion and properties of PKSC
Mix PKSC-S1 PKSC-S2 PKSC-S3 PKSC-PS
Water /binder ratio
Fly ash (%)
Silica fume (%)
Sand / cement ratio
Fresh Density (kg/m3)
Saturated Density (28 Days) (kg/m3)
Oven Dry Density (kg/m3)
Slump (mm)
Flow (mm)
0.35 0.35 0.35 0.35
5 5 5 0
10 10 10 0
1.0 1.2 1.6 1.0
1852 1912 1940 1895
1893 1940 1971 1915
1639 1705 1715 1694
105 103 75 160
220-370 200-330 200-290 -
Workability
Slump test tends to underestimate workability of lightweight aggregate concrete and therefore the flow values using flow table test were measured. Higher flow table values in the range of 220-370 mm were recorded for mixes having higher sand and lower PKS contents. Though only 5% fly ash was added, its contribution to workability can’t be ignored as spherical shape of FA reduces friction forces between aggregate particles and increases the workability. The addition of SP has also increased workability and the use of SP is mandatory due to the inclusion of SF. 5.
DISCUSSION ON HARDENDED CONCRETE PROPERTIES
5.1 Compressive Strength 5.1.1 Influence of Sand Content on Compressive Strength Figure 2 shows the progress of compressive strength of PKSC for a period of 90 days. Generally compressive strength depends on factors such as density, w/b, a/c and s/c ratios. As with normal weight concrete, lower the w/b ratio, higher the compressive strength. Generally higher density concrete produces higher strengths. For mix PKSC: S3, the s/c ratio was maintained at 1.6 and this had resulted in the highest density of approximately 1970 kg/m3 and the highest 28 day strength of about 36 MPa was achieved. It was evident during test that the breaking of PKS took place before final failure, thus indicating failure of PKS than mortar. Hence it can be concluded that the failure of PKS governed the strength. In the mixes PKSC: S1-S3, the strength gain due to higher fine aggregate and lower PKS contents was evident as good bond between PKS and cement matrix enabled the concrete to sustain higher load. The presence of high volume of pores in PKS which was evident because of high water absorption of about 25 % may weaken the particle strength and stiffness. Though the pores may weaken the compressive strength and elastic modulus properties of PKSC, these pores may help in development of good bond by the suction of the paste into the pores of PKS. However further investigation is required to study this effect. 256
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The comparison of 28-day compressive strength shows the difference between PKSC-S1 and S2 shows that PKSC-S2 has an increase of about 17% of strength compared to PKSC-S1. Similarly, PKSC-S3 also has an increase of about 24% compared to PKSC-S1. Thus the influence of sand content on compressive strength is evident. It can also be shown that the increase in compressive strength between PKSC-S2 and S3 is not high as compared to PKSCS1 and it can be concluded that s/c ratio of 1.2 could be ideal mix as far density is concerned. The density of mix PKSC-S3 is almost close to 2000kg/m3 and hence it is likely that a small increase in any constituent material may cause further increase in the density and thereby go beyond the limit of 2000kg/m3 set for lightweight concrete.
Fig. 2. Progress of Compressive Strength of PSKC
Fig 3. Relationship between Modulus of Rupture and Compressive Strength
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5.1.2 Influence of Silica Fume on Compressive Strength The addition of SF has influenced the compressive strength. High early strength of about 40 to 50% and 80 to 90% in one day and 7 day respectively on 28 day strength was observed for mixes containing SF. This may be attributed to fineness of SF and reaction between silicon dioxide and calcium hydroxide. The infilling of the voids in the shells by very fine SF particles may have increased the bond between PKS and cement matrix. Thus SF plays major role in early strength development, allowing aggregates better to participate in stress transfer. However, as mentioned earlier further research is required to study the effect of SF in the pores of PKS. Thus for all PKSC specimens containing SF, the failure was predominantly due to failure of PKS that was evident during test. The development of strength beyond the period between 28 and 90 days has been in the range of 2 to 7 % on 28 day compressive strength, though not significant, indicates that hydration continues at slower rate. Thus the effect of 5% fly ash on hydration within 90 days couldn’t be ascertained. Table 3 Tensile Strengths of PKSC Mix
Splitting tensile strength (MPa)
Modulus of rupture (MPa)
PKSC: S1
28-day 1.90
56-day 1.95
90-day 2.00
28-day 2.76
56-day 2.81
90-day 2.84
PKSC: S2
2.00
2.11
2.21
3.22
3.25
3.30
PKSC: S3
2.35
2.56
2.61
4.10
4.45
4.56
PKSC: PS
1.98
2.01
2.10
2.79
2.98
3.17
5.2 Modulus of Rupture and Splitting Tensile Strengths of PKSC The flexural and splitting tensile strengths of PKSC are shown in Table 3. It can be seen from the results that these strengths also follow a similar trend as that of compressive strengths. As the sand content is increased the flexural and splitting tensile strengths increased further. The highest flexural and splitting tensile strengths were obtained for PKSC-S3 that contained the highest s/c ratio of 1.6. The ratio of splitting tensile and flexural was found between 60-70%. The increase in these strengths after 28 days was found between 2 and 8% over a period of 90 days. The mix PKSC-PS that contained no cementitious materials produced comparable strengths as that of mix PKSC-S3, designed with similar mix proportion. However, PKSC-PS had higher cement content of about 600 kg/m3 and hence likely to cause shrinkage cracks. A relationship between flexural strength and cube compressive strengths yields the following: fcfc = 0.3(fcuc)2/3 Where fcfc and fcuc are respectively flexural and cube compressive strengths in MPa. 5.3 Static and Dynamic Moduli of Elasticity Table 4 shows both static and dynamic moduli of elasticity of all mixes. The past research works by few researchers on PKS concretes had shown static modulus of elasticity or Evalues in the range of between 7 and 8 GPa. The lower E-values of PKS concrete is generally attributed to poor stiffness of PKS and its lower particle density. Thus, the lower E-values tend to produce larger deflections and hence the E-values have to be increased. The highest E258
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values of about 11 GPa was obtained for mix PKSC-S1 that contained higher sand content and density. Thus an increase of about 13-40 % on E-value compared to the previous results is significant improvement and hence this mix may produce lesser deflection if used as structural concrete. The increase in E-value between S1 and S3 mixes was found about 27% and a slight increase in sand content as a positive effect on E-values. However, the mix PKSC-PS that contained no cementitious materials produced the lowest E-values. This may be attributed to the addition of SF to other mixes that produced good bond between aggregate and matrix that enable to sustain higher strains in PKSC-S1 – S3. Even a comparison between mixes PKSC-S3 and PKSC-PS with similar mix proportions shows that PKSC-S3 has about 20% higher E-values. Table 4 Elastic Properties of PKSC Static modulus of elasticity (kN/mm2) 28-day 56-day 90-day 8.57 8.59 9.01 10.01 10.21 10.25 10.90 11.51 11.87 7.08 7.91 7.98
Mix PKSC: S1 PKSC: S2 PKSC: S3 PKSC: PS
Dynamic modulus of elasticity (kN/mm2) 28-day 56-day 90-day 13.20 13.76 14.02 15.47 15.73 15.75 16.69 20.01 20.15 14.35 14.65 15.27
As dynamic modulus test is non-destructive test and easy to perform using prisms, the values can be used to ascertain the E-values of PKSC, similar to normal weight concrete (NWC). A relationship between the static modulus and dynamic modulus is shown in Fig. 4. Thus it can be seen the equation predicts the static modulus of elasticity for PKSC, if the dynamic values are known. The relationship for static modulus can be written as Es = 1.2 (Ed) 0.76, where Es and Ed are static and dynamic moduli of elasticity in GPa.
Static modulus of elasticity (Es) GPa
13 Es = 1.2 Ed0.76 R = 0.96 12
11
10
9
8 12
15 18 Dynamic modulus of elasticity (Ed) GPa
21
Fig 4. Relationship between Static and Dynamic Modulus of Elasticity
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Influence of Sand Content on Mechanical Properties of Palm Kernel Shell Concrete
In addition to the above relationship a solid and reliable equation can be established between cube compressive strengths and static modulus of elasticity similar to that of NWC. The following relationship can be established for static modulus of elasticity and compressive strength: Es = 0.2fcuc1.1 where Es and fcuc are static modulus and compressive strengths in GPa and MPa respectively.
12
Modulus of Elasticity (GPa)
Es = 0.2 fcuc1.1 R = 0.99 11
10
9
8 28
32 36 Compressive strength (MPa)
40
Fig 5. Relationship between Modulus of Elasticity and Compressive Strength
6.
CONCLUSIONS 1) The fresh density of mix containing the highest sand to cement ratio of 1.6 was found about 1940 kg/m3 and this is within the density limit of 2000kg/m3 for lightweight concrete and 28 day compressive strength of about 36 MPa was achieved. Thus using PKS as coarse aggregate lightweight concrete of grade 35 could be produced. 2) The use of superplasticizer is mandatory due to lower w/b ratio and high sand content. The mixes yielded medium to high slump and the mix with the highest sand content produced lower slump and flow values. The addition of silica fume produced cohesive mix. However, the mix PKSC-PS that contained no SF produced very high workability. 3) The fresh and saturated densities of PKSC-S1 – S3 varied between of PKSC varied between 1852 – 1971 kg/m3, while the oven dry densities were about 15% lower than the saturated densities. The highest density of PKSC-SC3 shows that the concrete could be categorised under lightweight. An increase in sand content beyond s/c of 1.6 may result in density of more than 2000 kg/m3, thus making it normal weight concrete.
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4) The increase in compressive strength between PKSC-S1 and PKSC-S3 was found about 24% and the corresponding value between PKSC-S1 and PKSC-S2 was 17%. 5) The increase in s/c ratio from 1.0 to 1.6 resulted in slight increase in density between 2 – 5 %. However, the increase in the compressive strength was found to vary between 16 and 28%. The highest 28-day compressive strength of about 38 MPa was obtained for PKSC-S3 with s/c of 1.6. Thus grade 35 PKS concrete can be produced with cement content of about 470 kg/m3 and addition of 10% silica fume. 6) The positive influence of higher sand content is seen in all the mechanical properties and the tensile strengths via, splitting and flexural tensile strengths were found in the range of 1.90 - 2.61 and 2.76 - 4.56 MPa, respectively. The ratio between splitting tensile and flexural strength was found in the range of 60 – 70%. 7) The E-values of PKSC-S1 – S3 produced values in the range of about 9 – 12 GPa. This shows an increase of about 13 - 40% compared to previous research works. Thus an increase in sand content and addition of silica fume produced higher modulus of elasticity and this may results in lower deflections if used as structural concrete. 8) Using the dynamic modulus test, a simple relationship between static and dynamic moduli of elasticity can be established. 9) Relationship between compressive strength and static modulus of elasticity shows that the static modulus can be found using the relationship Es= 0.2(fcuc) 1.1.
REFERENCES [1]. Abdullah, A.A.A., 1984. Basic Strength Properties of Lightweight Concrete Using Agricultural Wastes as Aggregates, Proceedings of International Conference on Low-cost Housing for Developing Countries, Roorkee, India. [2]. ASTM, 2003. ASTM C 29/C29M. Standard Test Method for Bulk Density and Voids in Aggregate. Annual Book of ASTM Standards. [3]. ASTM, 2003. ASTM C 127. Standard Test Method for Density, Relative Density and Absorption of Coarse Aggregate. Annual Book of ASTM Standards. [4]. ASTM, 2003. ASTM C 136. Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates. Annual Book of ASTM Standards. [5]. ASTM, 2003. ASTM C 138. Standard Test Method for Density, Yield and Air Content. Annual Book of ASTM Standards. [6]. Ata, O, Olanipekun E.A, Oluola K.O. 2006. A comparative study of concrete properties using coconut shell and palm kernel shell as coarse aggregates. Building and Environment 41 (3), 297301. [7]. Basri H.B, Mannan M.A, Zain M.F.M., 1999. Concrete using oil palm shells as aggregate. Cement and concrete Research 29 (4), 619-622. [8]. BSI, 2000. BS EN 12350-2. Testing fresh concrete-Part-2: Slump test, British Standards Institution, London. [9]. BSI, 2000. BS EN 12350-5. Testing fresh concrete-Part-5: Flow table test, British Standards Institution, London. [10]. BSI, 2002. BS EN 12390-3. Testing hardened concrete-Part-3: Compressive strength of test specimens, British Standards Institution, London. ICCBT 2008 - A - (23) - pp251-262
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Influence of Sand Content on Mechanical Properties of Palm Kernel Shell Concrete [11]. Clarke, J.L., 1993. Structural lightweight aggregate concrete. Chapman Hall, Glasgow. [12]. Okafor, F.O., 1988. Palm kernel shell as aggregate for concrete. Cement and Concrete Research 18 (6), 901-910. [13]. Mannan, M.A., Ganapathy, C., 2004. Concrete from an agricultural waste-oil palm shell (OPS). Building and Environment 39 (4), 441-448. [14]. Neville A.M. 1996. Properties of concrete. Longman Group Limited, London.
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