The excessive use of Normal Weight Concrete (NWC), lead to exploitation
of gravel and crushed rock at the expense of the ecological balance of
our environment. The increasing volume of concrete used in plain and reinforced
concrete works led us to manufacture of alternate Light Weight Aggregates
(LWA) that could be used in place of normal weight aggregates (NWA). In
most of the developed countries, the use of Light Weight Concrete (LWC)
in different forms, such as Light Weight Aggregate Concrete (LWAC), no-fines
concrete and aerated concrete enabled faster building rates than with
more traditional material. The most obvious characteristic of LWC is its
density as it reduces the dead load and lower haulages and handling costs.
Also LWC has relatively low thermal conductivity, as smaller walls built
using LWC can give thermal insulation four times greater than normal clay
In the industrially advanced countries, increased knowledge of materials
technology and of structural performance through research and experience
has been reflected in the modern design procedures. And hence the large-scale
development of new types of LWA is more rapid. However, in many developing
and underdeveloped countries in Asia and Africa, manufactured LWA are
not available and this may be attributed to many reasons, such as general
lack of understanding on production technique of LWA, towering production
cost, non-availability of raw materials and resources.
Manufactured 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. However, researches in Asia and
Africa on the use of organic natural aggregate in the form of palm kernel
shells (PKS) (1988; Basri et al., 1999; Mannan and Ganapathy, 2004;
Ata et al., 2006) are on the rise. One of the reasons for the 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.
The compressive strength depends on factors such as water, sand, aggregate
contents and density. Okafor (1988) has found that the failure of PKS
concrete is generally governed by the strength of PKS and this has been
agreed by other researchers. However, the smooth and convex surfaces of
PKS produce poorly compacted concrete and these result in bond failure
between PKS and cement matrix. 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 (Neville,
1996). 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.
As mentioned, most of the studies in the past on the PKS concrete produced
concrete of strength of about 25 MPa. And one of the reasons for such
low strength of PKS concrete has been the weaker bond between the PKS
and the cement matrix. Thus, there is a need for improvement of this weak
zone between the PKS and the cement matrix. This study focuses on this
objective of improving the aggregate-cement interface by adding 10% of
SF as cementitious material. In addition, the stiffness of the matrix
also plays an important role in the development of strength. Hence the
fine aggregate content has been altered to study its effect on the compressive
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. Both of these cementitious materials
were based on the weight of the cement. 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 strength development under different types
of curing has been analysed and compared. The influence of sand and coarse
aggregate (PKS) contents on workability and compressive strength has also
been studied and reported. The highest fresh density of PKS concrete was
found about 1940 kg m-3 and the as cured densities were found
in the range of 1802 to 1971 kg m-3.
MATERIALS AND METHODS
Cement and cementitious materials: Ordinary Portland cement conforming
to MS 522; Part-1:2003 with specific gravity and surface area of 3.10
and 335 m2 kg-1, respectively was used for all mixes.
The residue on 45 and 90 μm 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. The chemical compositions of cement, FA and SF
are given in Table 1. The superplasticizer (SP), Rheobuild
1000 M with specific gravity of 1.21 was used at different percentages.
The percentages of the FA, SF and the SP on the basis of weight of cement
used in different mixes are shown in the Table 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. Figure 1 shows the different
particle sizes of the PKS.
|| Chemical composition of cement, fly ash and silica
|| Mix proportions of PKSFC and PKSC
|| Palm Kernel Shells (PKS)
|| Particle size distributions of sand and PKS
The particle size distribution of fine and
coarse aggregates is shown in Fig. 2. PKS used as coarse
aggregates were obtained from local crude palm oil producing mill. Since
PKS are waste materials, the shells 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
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 min to 1 h is mandatory. The absorption during this period of pre-soaking
was determined and found to be in the range of 10 to 12%.
|| Curing environment
Particles with size
less than 3.35 mm were removed and not used in mixes due to large relative surface
area and high absorption.
Mix design and concrete mixtures: 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. Varying water to binder ratio (w/b), sand to cement ratio (s/c)
and aggregate to cement ratio (a/c) were used for designing mixes. A total
of six concrete mixes incorporating cementitious materials and varying
w/b and s/c ratios were prepared as shown in Table 2.
The mix series with varying w/b and s/c ratios were labelled as PKSC-W
and PKSC-S, respectively. Two mixes containing no cementitious materials
were also prepared for comparison and labelled as PKSC-PW and PKSC-PS.
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 of water with superplasticizer was then added; on
complete mixing, the remaining portion of materials were added in appropriate
order. The specimens were cast in 100 mm cube moulds and covered with
plastic sheeting in the uncontrolled laboratory condition for 24 h and
then demoulded. The cement content for mixes containing cementitious materials
varied in the range of 440 to 530 kg m-3.
Testing procedures: 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 standards. The samples were cured as shown
in Table 3. The specimens cured under P-1 type of curing
were wrapped immediately after demoulding using two layers of cling film.
However for P-2 type cured specimens, the samples on demoulding were cured
in water for 6 days and then wrapped in cling film. Both P-1 and P-2 type
cured specimens were stored in sealed condition at 23 °C and relative
humidity of 65 ± 5% until day of testing. The densities of all
specimens were measured before testing and the samples that were not cured
in water have been soaked in water before testing. The compressive strengths
based on British Standards were measured at 1,7,14, 28, 56 and 90 days.
RESULTS AND DISCUSSION
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 to 15 mm. The
relative density in saturated surface dry condition determined was found
to be 1.37. The loose and compacted densities were found as 568 and 620
kg m-3, respectively. The natural moisture content and 24 h
water absorption of PKS were found in the range of 8 to 15 and 25%, respectively.
The pre-soaking of PKS for a period of about 45 to 60 min. increased the
total moisture content. This however could be beneficial in the hydration
process for specimens cured under controlled environment. The above results
are close to the findings of the past researches.
Density: The measured fresh, as cured and oven dry densities as
of 28 day are given in Table 4. The fresh densities
of PKSC ranged between 1802 and 1940 kg m-3. It has been found
that oven dry densities were about 220 to 260 kg m-3 lower
than water cured densities. The highest density of 1971 kg m-3
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 density limit for LWC of
2000 kg m-3 and hence mixes containing s/c ratio higher than
1.6 was not considered. The densities of specimen cured under controlled
environment at the age of 28 days were found in the range between 1802
and 1946 kg m-3, a decrease of about two to five percent compared
with specimens cured in water. This reduction in density was due to moisture
loss during hydration and this trend increases with age.
Workability: Figure 3 and Table
4 show workability tests and their results respectively. The poor
workability of mixes PKSC: W1 and W2 was primarily due to lower w/b ratio
and higher PKS content. Higher PKS content combined with irregular and
angular shapes of PKS resulted in poor workability. This might be due
to friction between angular surfaces of PKS particles and lower fines
content. However for mixes PKSC: S1- S3 with w/b ratio of 0.35 and a/c
ratio of 0.8, the workability ranges from medium to high. Thus, a reduction
in PKS content and a subsequent increase in fine aggregate content increases
workability. Similar findings were reported by Okafor (1988). However
for mix, PKSC: S3 with a/c of 0.8 and s/c ratio of 1.6, medium workability
of about 50 mm was obtained, indicating higher fine aggregate content
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 containing cementitious materials, a slump of 105 mm was
found. The silica fume added as additional cementitious material has produced
cohesive mix and this resulted in lower slump values.
|| Workability tests
|| Properties of palm kernel shell concrete
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 (Neville,
Slump test tends to underestimate workability of lightweight aggregate
concrete (Clarke, 1993) and therefore 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. However
for mixes with lower sand and higher PKS content, lower flow values were
found. Though only 5% of FA 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 (Neville, 1996). The
addition of SP has also increased workability and the use of SP is mandatory
due to the inclusion of SF. A slightly higher SP content as mentioned
in Table 2 has been used for the mix PKSC: W1 due to
lower w/b ratio.
Influence of w/b, s/c and a/c ratios on compressive strength:
Figure 4 and 5 show progress of compressive
strength of PKSC for a period of 90 days in different curing environments.
It was observed that the compressive strength depends on factors such
as w/b, a/c and s/c ratios. As with normal weight concrete, lower the
w/b ratio, higher the compressive strength. The highest 28 day compressive
strength of about 30 MPa, amongst PKSC-W series was found for mix PKSC-W1
with w/b ratio of 0.30 (Fig. 4a). The mixes with w/b
ratio of 0.32 and 0.35, respectively produced lesser strength of about
9 and 12% compared to mix with w/b ratio of 0.3 on 28 day strength (Fig.
4c, d). Figure 5a-d show the
effect of higher fine aggregate and lower PKS contents on 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 m-3 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 for both concretes with w/c and s/c ratio
as variables. The 28 day strength of 36 MPa obtained for the mix PKSC:
SC3 is the highest and if compared with the previous findings (Mannan
and Ganapathy, 2004), it is nearly 45% higher. Similarly another comparison
with that of Ata et al. (2006) shows that it is nearly 2 ½
times higher. Thus, it is evident from the test results that the addition
of silica fume and an increase in sand content had influenced the compressive
strength in the PKSC.
Due to higher PKS content combined with lower fine aggregate content,
the failure of PKS in PKSC: W1-W3 occurred earlier. However for 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. Mixes of PKSC: S1-S3 series with
higher fine aggregate and lower PKS content show that increase of compressive
strength between 14 and 40% as compared to that of PKSC: W3.
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.
Influence of silica fume on compressive strength: The addition
of SF has influenced the compressive strength as seen from comparison
of Fig. 4 and 5. The 28 day compressive
strengths of PKSC-W3 and PKSC-PW show that an increase of about 10 to
15% for mix containing SF for same w/b ratio. However, for mixes PKSC:
W1 and W2 with low w/b ratio, the increase in 28 day compressive strength
was found in the range between 18 and 35% compared to mix, PKSC:PW that
contained no SF. A similar trend of increase in compressive strength was
noticed for PKSC: S series mixes. 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 (Neville,
1996). The infilling of the voids in the shells by very fine SF particles
may have increased the bond between PKS and cement matrix. 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 two to seven
percent on 28 day compressive strength, though not significant, indicates
that hydration continues at slower rate. Thus the addition of five percent
of FA hasn`t significantly improved hydration at later stages.
Effect of curing environment on compressive strength: It has been
found that controlled environment played a role in hydration and hence
strength gain. For both P-1 and P-2 types of curing, relative humidity
and temperature of about 65% and 23 ± 3 °C, respectively were
maintained. As seen from the Fig. 4 and 5,
specimens cured under P-1 cured condition show lower strength than the
water cured and P-2 cured specimens. During the first 28 days, the difference
in strength between P-1 cured and water cured specimens was found in the
range of two to four percent on water cured specimen except for PKSC:
S1 that had a difference of about 7%. This may be due to poor hydration
in P-1 cured samples as these samples have been sealed immediately after
demoulding. However, it can be shown from the results that hydration process
continued in specimens cured in P-1 condition indicating supply of water
for hydration from pre-soaked PKS. The gain of strength between 28 and
90 day period for water cured was about two to six percent, while P-1
cured recorded a gain in the range of 2-4%.
Specimen cured under P-2 curing condition showed negligible difference
in strength gain compared to water cured specimens and in some cases slightly
higher strength was recorded than water cured specimen. Here the gain
in strength can be attributed to water curing for 6 days on demoulding
as between 80 to 90% of strength is achieved during this period. Thus
P-2 cured specimens matched the strength as that of water cured specimens.
The constant temperature maintained coupled with continued unhindered
hydration of P-2 cured specimen may have contributed to slight higher
strength achieved than water cured specimens. The strength gain of P-2
type cured specimens was found in the range of two to 6% between 28 and
Comparison between PKSC with and without cementitious materials:
The cement contents used in the mixes were in the range of 440 to 530
kg m-3 for mixes containing cementitious materials. However,
the mixes PKSC: PW and PKSC: PS, containing no cementitious materials,
had cement content of about 550 and 570 kg m-3, respectively.
Thus a comparison of PKSC: W series containing cementitious materials
with PKSC: PW that had no cementitious materials has shown an increase
in strength between 12 to 35% for specimens cured in three curing environments.
Similar comparison between PKSC: S series and PKSC: PS showed that an
increase of about 10 to 35% between 28 and 90 days.
The fresh density of mix containing the highest sand to cement ratio
of 1.6 was found about 1940 kg m-3 and this is within the density
limit of 2000 kg m-3 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.
The addition of silica fume, higher PKS content and lower fine aggregate
content results in poor workability; the angular and irregular surfaces
of PKS increases the friction between aggregate particles; however increase
in sand content and subsequent decrease in PKS content yielded medium
to high slump; the use of superplasticizer is mandatory due to lower w/b
ratio and high sand content.
The fine aggregate and PKS contents have influence on the compressive
strength. However the addition of 5% of fly ash hasn`t contributed to
hydration at later stages.
The mixes containing silica fume produced higher strength in the range
of 12 to 35% than the mix without silica fume in different types of curing
The specimens cured under P-1 curing environment have shown a reduction
in strength between 2 and 4% as compared to water cured specimens. However
P-2 cured specimens had achieved almost equal strength as that of water
The addition of ten percent of silica fume as cementitious material to
PKS concrete is recommended where the target strength of PKSC is about
It is recommended to use lower sand to cement ratio than the ratio of
1.6 to obtain PKSC of density below 1900 kg m-3.
This project is funded by the Ministry of Science, Technology and Innovation
under the Science Fund No. 13-02-03-3053.