Performance of Palm Kernel Shells as a Partial replacement for Coarse Aggregate in Asphalt Concrete


Peter Ndoke NDOKE


Department of Civil Engineering, Federal University of Technology, Minna




This paper looks at the potentials of palm kernel shells as coarse aggregates in road binder courses with emphasis on strength of the asphalt concrete as given by the Marshal Stability and flow values. It was observed that palm kernel shells can be used to replace coarse aggregate up to 30% before drastic reductions become noticeable. It is therefore recommended that for heavily trafficked roads, palm kernel shells up to 10% can be used for the replacement while even 100% replacement is possible for lightly trafficked roads in the rural settings.

Key words

Palm kernel shells, coarse aggregate, marshal stability




Palm kernel shells are not common materials in the construction industry. This is either because they are not available in very large quantities as sand or gravel, or because their use for such has not been encouraged. For some time now, the Nigerian government has been clamouring for the use of local materials in the construction industry to limit costs of construction. There has therefore been a greater call for the sourcing and development of alternative, non conventional local construction materials. Palm kernel shells are derived from the oil palm tree (elaeis guineensis), an economically valuable tree, and native to western Africa and widespread throughout the tropics [1]. In Nigeria, the oil palm tree generally grows in the rain forest region close to the coastal areas and adjacent to some inland waterways. [2] Wrote that palm kernel shells are used mostly as a source of fuel for domestic cooking in most areas where they occur. He stated further that the shells are often dumped as waste products of the oil palm industry. [3, 4] Stated that the two predominant varieties of palm fruits namely tenera and dura; produce about 1.5 million tones of palm kernel shells per annum in Nigeria. Palm kernel shells have been used as aggregates in light and dense concretes for structural and non-structural purposes [5, 6]. [7] Went further to show that the 28 day compressive strength of concrete with palm kernel shells as aggregate range between 0.3 and 20.5N/mm2 depending on the proportion in the mix.

Asphalt concrete is derived from a mixture of coarse and fine aggregates, stone dust, mineral fillers and binder, usually bitumen. The mix is done such that the finished product does not have too much bitumen which will eventually lead to a bleeding and frictionless surface, or too much coarse aggregate to lead to raveling of the surface. The asphalt and aggregate are usually mixed and heated at a central location. Asphalt concrete surfaces are fairly easy to construct and repair [1].



Materials and Methods


Sourcing of materials


The palm kernel shells were obtained from the local market and crushed to remove the seeds. Physical and mechanical properties like particle size distribution, were then determined in the laboratory. The mineral filler, stone dust, river sand and granite of size 5-16mm and 16-27mm were purchased from local suppliers. The bitumen was obtained from the oil refinery in Kaduna.


Sample Preparation

Specified proportions for the sample that is, 4% filler of size 0.075mm, 35% stone dust of maximum size 5mm, 6% river sand of maximum size 5mm also and 20% crushed stone of size 5-16mm and 35% crushed stone of size 16-27mm with 5% bitumen of penetration grade 60-70 were mixed together at 145˚C and compacted 2 times 50 blows to obtain cylindrical samples for the Marshall stability tests. Palm kernel shells were added at 10, 20, 30, 50 and 70% by weight of total coarse aggregate still maintaining the percentages so that the palm kernel acts as a replacement for the coarse aggregate and mixtures gotten. Five samples were prepared for each proportion of palm kernel shells. Marshall stability tests were carried out on the samples and the average of the results for each mix proportion determined.



Results and Discussions


The results presented below are a summary of the average values gotten for the palm kernel shells, bitumen and mixtures.


Physical properties of the palm kernel shells


Table 1 gives the physical properties of the palm kernel shells while the particle size distribution is shown in figure 1.


Table 1. Physical properties of the Palm kernel shells



Bulk density Mg/m3


Dry density Mg/m3


Void ratio


Porosity (%)


Water content (%)


Water absorption (%)


Specific gravity


Impact Value (%)



According to [8], the specific gravity of the palm kernel shells does not place it in the category of common rock groups whose gravities range from 2.62-3.00. However, it showed same porosity as granite, quartzite, far above limestone. The arrangement of the shells as they touched each other would have accounted for this. Dry density values place the palm kernel shells in the same category as lightweight aggregate especially pumice, scoria or vermiculate for natural and fly ash and clinker for processed aggregates.

Figure 1. Particle size distribution for palm kernel shell.


From the particle size distribution the palm kernel shells will act as stone dust, sand and coarse aggregate even as mineral fillers by virtue of the distribution. Proportions of materials retained on 5.00 mm sieve were weighed and used for the mixes. This was done to avoid altering the properties of the other components of the mix. The effect here is that palm kernel shells will replace mostly the coarse aggregate between range 5 and 16mm.



Properties of Binder


The binder used in the production of binder courses for roadway pavements is the petroleum bitumen of grade 60-70, whose properties were obtained in the laboratory thus, and shown in Table 2.


Table 2. Properties of the Petroleum Bitumen 60-70


Test Values

Specific gravity at 25˚C


Softening Point R and B (˚C)


Penetration at 25C - 0.1mm


Ductility at 25˚C  (cm/min)


Loss on heating after 5 hours at 163˚C in % by weight of mix


Solubility in Trichloroethane % by weight


Drop in penetration after heating (%)


Flash point (open cup) ˚C min


Ash % by weight



These all met standards specified in BS 598 for bitumen to be used in binder courses [9].


Properties of the Mix


Particle size distribution

Figure 2 shows a distribution of all the components of the mix as specified. These were carried out as specified in BS 812 [10].

Test values

Values for the Marshal Stability and flow are shown in Table 3.


Table 3. Strength Characteristics of  the Asphalt Concrete

Palm kernel ratio (%)

Impact value (%)

Density (kg/m3)

Water absorption (%)

Marshal Stability (kN)

Flow (mm)






































Figure 2. Particle size distribution for the constituent materials for asphalt concrete

It was observed that on the average as the coarse aggregates were reduced and palm kernel Shells added, the Marshall stability reduced while the flow increased. This is to be expected because a reduction in flow implies more strength for the mix. Marshall Stability value of 14.44kN was obtained using granite alone as coarse aggregate and 7.78kN using palm kernel shells alone as coarse aggregates. This shows a reduction in percentage of up to 47%.

According to [11] Marshall Stability range for road surfaces carrying between 1 and 6000 commercial vehicles per day should be between 2 and 10kN. The implication is that with the proper stress calculation and appropriate thicknesses of underlying layers, even the asphalt concrete with 100% palm kernel shells is suitable for lightly trafficked roads. Since the same amount of bitumen was used, the flow values are almost similar. The increase would have been caused by the weaker bonds created by the palm kernel shells and their high water retention capacity which can mix the bitumen to increase its water content, hence viscosity.

Although increases in flow values were noticed they still fell below the specified range of 5mm maximum.





From the results obtained, it can be concluded that:

1.      Palm kernel shells can be used as partial replacement for coarse aggregate up to 10% for heavily trafficked roads and 50% for light trafficked roads.

2.      For the very lightly trafficked roads in the rural communities palm kernel shells can be used as full replacement for the coarse aggregates. This will go a long way into reducing construction and maintenance costs of these roads.

3.      The quest by governments in developing countries, especially those in Africa south of the Sahara for use of locally available materials in infrastructure development will be met with the use of palm kernel shells as a road construction material.

4.      The economic power of the rural dwellers will be enhanced if they are encouraged to plant palm trees from which these shells could be gotten.





[1] Encarta (2005), Microsoft Encarta Reference Library, 2005.

[2] Omange G. N., Palm kernel shells as road building materials, Technical Transactions of the Nigerian Society of Engineers, 2001, 36(1), p. 17-25.

[3] Nigerian Institute for Oil palm Research (NIFOR), Highlights of Activities and Achievements, pp. 1-35, 1985.

[4] Nigerian Institute for Oil palm Research (NIFOR), Oil palm products and their Uses. Information Bulletin No 3, 1986.

[5] Uviasah M. O., Properties of Palm kernel shells and Fibre as aggregates for making concrete, Unpublished Bachelor of Engineering Thesis, Department of Civil Engineering, University of Benin, Benin, Nigeria, 1976.

[6] Akpe E., Characteristics Strength of Concrete using Palm kernel shells as light weight aggregate, Unpublished Technical Report, Department of Civil Engineering, EDSU, Ekpoma, 1997.

[7] Nuhu-Koko M. K., The use of Palm kernel shells as aggregates for concrete, Paper presented at 21st Annual Conference of Materials Testing, Control and Research, FMW, Lagos, Nigeria, pp. 20, 1990.

[8] Neville A. M., Properties of concrete, Pearson Education, Asia Pte ltd, Edingburg, Scotland, pp. 108-176, 1995.

[9] British Standards Institution (BS 598, 1985), Methods of determination of index properties of pitch-bitumen Binder, pp. 2-16, 1985

[10]. British Standards Institution (BS 812, 1975), Methods of determination of mechanical properties of aggregate, 1995, Part 3, p. 4-8.

[11] Oglesby C. H., Hicks R. G., Highway engineering, 4th Edition, John Wiley and Sons, New York, pp. 683-685, 1982.