2. 2
UNIVERSITY OF NORTHUMBRIA AT NEWCASTLE
SCHOOL OF THE BUILT ENVIRONMENT
The Effects of Adding Waste Wood Chippings and Silica Fume to the Flexural Strength
Capacity of Concrete
A DISSERTATION SUBMITTED TO THE SCHOOOL OF THE BUILT
ENVIRONMENT IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR
THE DEGREE OF
BSc Architectural Technology
By 06012242
APRIL 2010

3. 3
Abstract
This piece of research investigates the effects of adding waste wood chippings form the
timber industry, and silica fume, to the flexural strength capacity of concrete. Three groups of
concrete beams were made, group 1 consisting of plain concrete, group 2 had wood chippings
added to the mix and group 3 was a hybrid of wood chippings and silica fume. After
compiling a suitable methodology and carrying out a flexural test programme, results showed
that additions of wood and silica fume reduce flexural strength, however wood and silica
additions are shown to increase maximum deflection before failure of the concrete beams.
The results were encouraging towards the research and production of more sustainable
concrete technologies.
i

4. 4
Table of Contents
Abstract i
1.0 Proposal 5
1.1 Hypothesis 5
1.2 Research Aims 5
1.3 Objectives 6
2.0 Literature Review 7
2.1 An overview of sustainable concrete 7
2.1.1 Fly Ash Concrete 7
2.1.2 Wood Chippings in Concrete 8
2.2 Flexural Testing of Fly Ash and Wood Chip Concrete 8
2.3 Mineral Based Composites 9
2.4 Optimum Ratios 9
2.5 Conclusion 9
3.0 Methodology 10
3.1 Structural Elements and the Need to Perform Flexural Testing 10
3.2 Production of Beams 10
3.2.1 Materials 10
3.2.2 Mix Design 11
3.2.3 Quantities 11
3.2.4 Slump Testing 11
3.2.5 Pouring and Compacting the Concrete 12
3.3 Curing 12
3.4 Methods of Performing Flexural Tests 12
3.4.1 Loading Rate 13
3.5 Calculating Flexural Strength 13
4.0 Results and Analysis 14
4.1 Results of Plain Concrete Beam Flexural Testing Programme 14
4.2 Results from using Wood Chippings as Addition to the Concrete Beams 15
4.3 Results from using Wood Chippings and Silica Fume as Addition
to the Concrete Beams 16
4.4 Overview of Flexural Strength Results 17
4.5 Analysis of Results 17
4.6 Determining the Density Characteristics of the Concrete Mixes 18
4.7 Drying Curve 20
5.0 Conclusions 23
5.1 Research Limitations 23
6.0 Recommendations 25
7.0 References 26
8.0 Appendices 28

5. 5
1.0 Proposal
There has been extensive research into trying to increase the sustainability of concrete whilst
retaining high levels of flexural strength.
The core of this research seems to be investigating the use of mineral based composites
(MBCs). Research conducted by Blanksvard (2009), highlights the drawbacks of using
epoxies, and concludes that in compressive strength tests that MBCs contribute to increasing
load bearing capacity for strengthened concrete considerably.
Further research into MBCs reveals that there is actually however, conflicting results to the
levels of strength improvement in concrete. One such study tested flexural strength as oppose
to compressive and concluded that the ‘stress at the cut off point is considerably smaller for
the MBC system compared to that of the epoxy based carbon fibre reinforced polymer
(CFRP) system’ (Johansson, 2005).
Increased research into mineral bonding agents (MBAs) brings to attention that the use of fly
ash in concrete is very common practice, as well as other recycled agents such as crushed
glass and silica fume. A research paper on the effects of fly ash in concrete concluded that
each tonne of cement that can be replaced by fly ash reduces CO² emissions by 1 tonne also
(Bjork, 1999). Another piece of research also uses fly ash as well as mixing in waste wood
ash, replacing 25-35% of cement used. This was found to significantly improve the
performance of concrete in both compression and flexural testing (Naik, 2002).
A research publication that tested the effects of untreated sawdust as an additive to polymer
concrete, concluded wood waste did improve flexural strength and fracture toughness when
applied as reinforcement to polymer concrete, (Reis, 2006). However, this is thought not to
be the same case for compression, as research by Becchio (2009), concluded that when wood
waste was used as an aggregate, compression strength tests indicate a reduced performance in
mechanical properties such as crushing resistance and bulk density.
Research by Coatanlem (2005), and concluded that by saturating wood chippings with
sodium silicate, improves significantly the compressive strength of concrete. However, it also
concluded that without the sodium silicate, the compressive strength was reduced. This is
thought to be as the sodium silicate improves the bond between the wood – cement interface.
This research conducted tests on compression alone, failing to expand the investigation
beyond this parameter. Therefore the behaviour of this mix design on flexural strength has to
be investigated.
As no tests have currently been done on flexural performance and with conflicting research
conclusions, a further investigation may be proposed to research into the effects of using
untreated wood chippings as reinforcement, with the additive of silica fume to improve the
bond interface, thus filling this knowledge gap.
1.1 Hypothesis
It is suggested that if waste wood chippings and silica fume is added as reinforcement to a
concrete beam, then the flexural strength of concrete beams will be improved.
1.2 Research Aims
To test to see if the use of waste wood chippings from the timber industry and the addition of
silica fume, can improve flexural performance of concrete.

6. 6
1.3 Objectives
1) To test 10 concrete beams with no reinforcement
2) To test 10 concrete beams with waste wood chipping added as reinforcement
3) To test 10 concrete beams with waste wood chipping and silica fume added as
reinforcement
4) To record results appropriately
5) To compare and analyse the 3 groups of test results
6) To evaluate the effectiveness of adding waste wood chippings and silica fume to
concrete mixes.
The following chapter will consider existing research into sustainable concrete, looking at the
additions of by-products to concrete to improve sustainability as well as maintaining adequate
levels of strength.

7. 7
2.0 Literature Review
This chapter intends to review existing pieces of research into the production of sustainable
concrete. The need for sustainable concrete and the search for suitable by-products, with
specific reference to mineral based composites, will be addressed within this chapter.
Extensive research has been carried out into trying to increase the sustainability of concrete,
Li-hua (2009), Naik (2002), Becchio (2009), Turgut (2007), whilst retaining high levels of
both compressive and flexural strength. The following section will outline research that has
been conducted into improving sustainability of concrete whilst trying to retain high
compressive strength values.
2.1 An Overview of Sustainable Concrete.
Research carried out by Canbaz et al. (2004) used waste glass, as a course aggregate within
concrete. The glass used was between 4-16mm and used in a mix proportion of up to 60%.
Testing of this determined that it actually had a slight reduction in compressive strength. A
study was conducted by Shayan (2002) looks at glass ground in to a fine powder and added to
the mix, replacing 30% of the cement used. The results from this test found that this did
provide a significant increase to the concrete’s compressive strength. In addition to this,
Soroushian et al, (2003) found that the addition of recycled plastics also has a positive effect
of concrete compressive strengths. Galetakis (2004) also conducted compression tests using
limestone dust as an addition with Portland cement mix, while the dust/cement ratio was
found to be the key factor, results showed that compressive strengths can be increased.
The studies discussed above show that many materials have been used as aggregates in
concrete. However, whilst many different pieces of research have been carried out using
different waste materials as additions to the concrete mix, the most extensive amounts with
the most positive results, come from testing carried out using ash or wood chipping additions.
Focusing on compression tests, the next section reviews studies that have used ash as an
addition to the concrete mix.
2.1.1 Fly Ash Concrete
Very early research was carried out by Miller et al. (1975) into the use of fly ash, as well as
other reclaimed materials, as alternative aggregates. The research found that fly ash offered
the most improvement to concretes compressive strength. The most recent and in depth piece
of research conducted undertaken by Naik (2002) using fly ash alone, tests were conducted
using between 25-35% of fly ash blended with the concrete mix. After a 28 day curing period
it was concluded that the compressive strength of the concrete was significantly increased.
Research previous to this was also carried out by Carette (1993) using between 55-60% fly
ash in a concrete mix, concluded that after 91 days curing that the concretes compressive
strength was up to 50 N/mm². However, another study conducted by Bjork (1999) found that
by using a water/cement ratio of 0.3, can have as much as 60% of the cement replaced with
fly ash and can produce a 55 N/mm² a compressive strength concrete after 28 days. A study
by Li-hua (2009) confirmed that the addition of fly ash content between 10-55% will improve
compressive strengths as well as improving workability and durability, however more than
55% content will reduce compressive values after 90 days, as the compressive strength of fly
itself is less that concrete.
The above research shows that the addition of fly ash to concrete can significantly improve
compressive strength; this is because fly ash is comprised of the non-combustible mineral
portion of coal consumed in a coal fuelled power plant. The particles are glassy and spherical

8. shaped that are typically finer than cement particles. The fly ash undergoes a ‘pozzolanic
reaction’ with the lime created by the hydration (chemical reaction) of cement and water, to
create the same binder (calcium silicate hydrate) as cement. The pozzolanic reaction that
takes place is the fly ash changing into a siliceous material that reacts with the lime, forming
a compound that is cementitious at normal temperatures. It is this pozzolanic reaction that
will be looked at in section 2.3, when silica is looked at as a mineral based composite.
After looking at the effects of fly ash, as previously mentioned, the addition of waste wood
also has had extensive research carried out on its effect to compressive strengths on concrete.
Section 2.1.2 will give an overview to these.
8
2.1.2 Wood Chippings in Concrete
Research by Becchio (2009) tried to increase sustainability of concrete by replacing natural
aggregates in the mix, with waste wood chippings. Different mix proportions were of
cement/admixture/water was made and cubes formed. Their compressive strength was tested
after a 28 day curing period. The testing concluded that in all cases the addition of wood
waste decreased the compressive strength of the concrete, the decrease in strength was found
to be in proportion to the drop in density as wood in less dense than aggregate. A previous
study to this was performed by Turgut (2007) who investigated the use of high levels of
waste sawdust to concrete mixes, and the results obtained were enough to satisfy international
standards. Although this test does not increase the compressive strength, it shows that equal
strengths can be achieved by using sustainable methods. A much earlier piece of research was
carried out by Al Rim et. al. (2000) also reused wood waste as an aggregate for concrete,
although the research was aimed at the thermal properties of this, it did conclude that the
addition of the wood waste reduced the compressive strength.
The studies and tests in the previous paragraph show contradicting results for the addition of
wood wastes to the concrete mixes, this could be due to a number of factors. However, all of
the previous research looked at, measured concrete under compression testing. The next
section will focus on tests that have been carried out to increase the flexural strength of
concrete, and see if the same materials have the same effects.
2.2 Flexural Testing of Fly Ash and Wood Chip Concrete
Coatanlem (2002) conducted research using waste wood chippings as fibres for concrete
reinforcement. The wood chippings were between 0.5-10.0mm and the beams were
40x40x160mm. The optimum wood/cement ratio was 3.6, and the optimum water cement
ratio was 0.75. After 28 days, the beams were tested and it was found that the flexural
strength of all the samples was less than 10MPa; this is a significant reduction to that of
normal Portland cement, recording 45MPa. Research by Reis (2006) tested plain epoxy
polymer concrete reinforced with wood waste. Mixes were made with varying proportions of
cement and waste wood. Flexural strengths were then tested and compared to that of plain
epoxy polymer concrete. The results obtained showed that flexural strength increased by
6.03%.
The comparison between the two above studies by Coatanlem (2002) and Reis (2006) show
that wood chippings alone will not increase the flexural strength of concrete, but adding
wood waste to an epoxy polymer concrete will improve the strength. However, the use of
epoxy concrete poses disadvantages such as hazardous working environments for the
workers, as well as being sensitive to moisture and thermal conditions, (Tliljsten et al. 2006).

9. 9
The next section will discuss research into Mineral based Composites; these are designed to
be a more sustainable, replacement to epoxy adhesives.
2.3 Mineral Based Composites
Blanksvard (2009) has carried out extensive research into the use of Mineral Based
Composites (MBCs) as an addition to carbon fibre reinforced concrete. The first part of his
research uses MBCs as a strengthening system to repair concrete. However flexural tests
were also conducted within this research to expand the data collection. This research
concluded that the uses of minerals, such as silica fume, improve the bond between the
cement, to the reinforcement fibres. The reaction that improves the bond is the same as
discussed in section 2.1.1, as the use of minerals like silica fume undergoes a pozzolanic
reaction. This result would explain the difference in the two previously discussed tests carried
out by Coatanlem (2002) and Reis (2006), as Reis used the wood chippings in epoxy
concrete, so the epoxy would have created a stronger bond between the cement and the wood
chippings. Blanksvard also concluded that for flexural strength to be improved, optimum
ratios of water/cement and also polymer/cement ratios need to be found. These are addressed
in the following section.
2.4 Optimum Ratios
Research conducted by Schulze (1999) show that the flexural strength of concrete can be
increased substantially by altering the water/cement ratio, within a modified mortar. This
confirmed earlier research by Beton (1990). Research into polymer/cement ratios was
conducted by Pascal et al. (2004) and Van Gemert et al. (2005) who found that increases in
flexural strength were found significant when polymer/cement ratios were between 10% and
15% weight content. Ratios higher than this are shown to result in a decrease in the strength
properties. The research by Pascal et al. (2004) and Van Gemert et al. (2005) confirmed the
earlier findings from Rao (2003) and Huang Cheng-Yi et. al. (1985), whose work addressed
the performance of silica fume within concrete. Both of these researchers tested silica fume
levels between 0% and 30% content by weight within a mix and found that optimum levels
for increasing flexural strength lie between 15% and 22% by weight.
2.5 Conclusion
From the research presented, it has been shown that the flexural strength of concrete can
actually decrease by replacing aggregates and fibres with only recycled materials such as
wood, ash and glass. It has also been shown that in many cases the addition of epoxies and
Mineral Based Composites can then go on to increase the flexural strength of concrete as the
bond between the recycled fibres and the cementitious material is strengthened. Further
research into these bonds concluded that water/cement and polymer/cement ratios are of
significance to the flexural strength. The use of Mineral Based Composites, such as silica
fume, increase sustainability of concrete as it is a by product of producing silicon metal.
Silica fume also increases workability, compared to an epoxy resin. The tests looked at above
use silica fume with polymer fibres, and no tests were found that adopted silica fume as an
additive to concrete reinforced with waste wood chippings acting as a fibre addition. The
study by Reis (2006) did use wood chippings, but with epoxy concrete.
The intention of this work is to address the knowledge gap in the research, as presented
within this chapter, by exploring the use of silica fume as an epoxy replacement to concrete
reinforced with waste wood chippings. Chapter 3.0 identifies the methodology used to
explore this work.

10. 10
3.0 Methodology
The previous chapter indentified an apparent lack of data on sustainable methods to improve
the flexural strength of concrete, with specific reference to the use of silica fume and waste
wood chippings. This chapter will justify the use of a flexural test programme and determine
the materials used in the production of the beam test programme in address.
3.1 Structural Elements and the Need to Perform Flexural Testing
From research presented in the literature review, it is shown that tests have been carried out
on improving both compressive and flexural strengths of concrete. However the research
presented does not focus on increasing the sustainability of concrete. In order to collect data
for the improvement of flexural strength of a concrete beam then a test was required. Whilst
other forms of data collection, such as comparing case studies, may have given an indication
of what is likely to happen to the flexural strength, actual first hand data could not be
obtained without testing.
There are also a number of different practical uses for flexural strength improvement,
as concrete is widely used as a material for structural elements such as beams and other
framing members. It is important that when adopted for this type of application, it can
withstand loads without failing. Beams spanning across any length within a building will be
under flexural loading, whether it be carrying dead or live loads.
With a huge pressure on the modern day construction industry to increase
sustainability and reduce carbon emissions, it is important that methods for meeting
sustainability criteria are explored without compromising strength. The implications of this
are that higher strength and sustainability within concrete will offer greater opportunities for
architects, technologists, contractors and clients to want to use concrete. As well as this, with
higher flexural strengths, larger spans can be reached allowing architects to design more
diverse buildings. The next chapter will look at the how the beams to be tested were
produced.
3.2 Production of Beams
As discussed within the literature review, beams will be tested with the addition of waste
wood chippings and silica fume. The following sections will look at beam design, including
acquiring the materials, ratios of materials used and the quantities of beams made.
3.2.1 Materials
The wood chippings that were added to the mix came from a local timber merchant, and were
ungraded, untreated and from a variety of trees. The decision to use chippings like these was
to add to the sustainability of the beams, as costs are incurred to sort and grade the wood, as
well as treat it. The idea was that the wood chippings were used as ‘raw’ waste from the
timber industry. The silica fume was obtained from an international chemical company who
specialize in making environmentally friendly metals and materials. The silica fume used was
undensified meaning that the grains are slightly larger than those of densified silica, also the
undensified silica fume is easier to work with and offers no hazardous working conditions
with regard to skin contact and inhalation. The silica fume fully conformed to the American
Society for testing Materials code C1240 for the Standard Specification for Silica Fume Used
in Cementitious Mixtures (ASTM C1240 -05). The cement used was ordinary Portland
cement which conforms to BS 12, this was the same as all of the tests presented within the
literature review and is the most readily available to use. The aggregates, sand and water
were all readily available from the universities laboratory.

11. 11
3.2.2 Mix Design
As presented in the literature review Schulze (1999) and Rao (2003) showed that optimum
ratios of water/cement and silica fume/cement can have major influences on the flexural
strength of concrete. The optimum silica fume amount was found to be between 15-22% of
the mix, however later research by Van Gemert et al. (2005) suggested 15% of a silica
addition was the maximum before a decrease in flexural strength would appear. For that
reason a 15% addition was added to the mix.
The ratio of wood chippings to the mix was chosen at cement/wood ratio of 3. This
was due to recent research by Becchio (2009) suggesting this was the optimum ratio. Earlier
research by Coatanlem (2002) as presented in the literature review suggested 3.6, however
this was also rounded down to 3 for the testing. This research also concluded that the
optimum water/cement ratio was 0.75 so in order to maintain validity the same ratio was
used.
The ratio of mix between cement/sand/gravel was 3/6/10 in accordance to BS 8500.
For all beams made, the cement sand and gravel were mixed first, and then the correct
amount of water added accordingly. Mixing was followed by the addition of the wood
chippings to the batches that included both wood and silica fume. The silica fume was added
last and mixed in full accordance with the manufacturer’s guidelines for mixing (See
Appendix 1).
3.2.3 Quantities
With the above ratios used, this section will look at exactly how much of each material was
added to the mix. 30 beams were made in total, 10 beams in a control group with plain
concrete. 10 beams with wood chipping alone and the final 10 beams were a hybrid of silica
fume and wood chipping. The decision to use 10 beams within each group was made by
looking at research in the literature review, where research carried out by Shayan (2002),
Galetakis (2004), Naik (2002) and Becchio (2009) all used between 6 and 10 specimens for
each group. By using 10, it is also easier to identify any anomalous results and calculate
levels of significant difference between groups. Table 1 below shows the quantities for all
materials used.
All amounts are based on batching 10 beams per group and include a 10% waste addition.
Cement Sand Gravel Wood Silica Fume Water
Plain group 21.56kg 38.81kg 58.00kg 0.00kg 0.00kg 16.17kg
Wood alone 21.56kg 38.81kg 58.00kg 21.56kg 0.00kg 16.17kg
Hybrid 21.56kg 38.81kg 58.00kg 21.56kg 3.25kg 16.17kg
Table 1.
3.2.4 Slump Testing
A slump test measures the workability of the concrete, showing how easy the concrete is to
place handle and compact. Slump tests must measure within a set range or tolerance from the
target slump. A slump test was not carried out within this experiment as the mix would give
different slump values due to the wood and silica additions and no target slump are values are
specified for this type of mix. As well as this, all the quantities of materials were carefully
measured and mixed in the exact same way; there were also time constraints within the
laboratories making it difficult to complete a slump test. For producing plain concrete without
limitations, a slump test must always be carried out.

12. 12
3.2.5 Pouring and Compacting the Concrete.
All beams dimensions were the same at 100mm x 100mm x 500mm; this size was chose as it
was the same dimensions as flexural testing carried out by Reis (2006). Pouring the concrete
into the moulds must be carried out in a precise method. Using a trowel, the beams moulds
were filled half way then compacted using steel hand tamp. Compaction takes place in order
to reduced air content within the concrete, improving its performance, there should be a
minimum of 100 compactions before pouring the remaining half into the mould, before a
further 100 compactions are made (www.brmca.org.uk/Placing, 2009). The beam should be
finished with a float to remove excess mix from the edges of the mould, this makes it easier
to remove the beams from the moulds after curing. This process was repeated for each beam.
3.3 Curing
The curing period for the beams was 28 days; this was the same as all of the tests presented in
the literature review. 28 days is also the advised time given by Portland cement
(http://www.concrete.net.au, 2004). During the curing period, the mass of each beam was
obtained by weighing the beams every 2 days in order to monitor their change in mass over
the 28 day period. This gave the opportunity to look at the beams’ densities and compare
differences between the groups.
3.4 Methods of Performing Flexural Tests.
The two main methods of testing flexural capacity of concrete are known as either the ‘single
point’ load test or the ‘third point’ load test. Figure 1 below shows diagrammatically the two
tests..
A B
Figure 1. Flexure testing diagrams (Taken from www.nrmca.org/aboutconcrete, 2005)
Figure 1A, shows the third point test as it applies the loads across two points at one third
proportions of the beam. Figure 1B shows the single point test where the entire load is
applied to the centre of the beam.
With a brittle material such as concrete, the single point test is more common, although this
test only provides data for flexural strength only, not stiffness
(http://www.instron.co.uk/application /flexure, 2010). Research was also carried out into the
difference between single point and third point tests, concluding that after testing 4 different
types of polymers, that between the two tests no significant difference could be found in the
results for modulus of rupture, (Chitchumnong et al. 1989). Figure 2 below shows the
difference in between the two types of flexural test, displayed using a bending moment
diagram.

13. 13
A B
Figure 2. Bending Moment Diagrams (Taken from www.nrmca.org/aboutconcrete,
2005)
The areas shaded in red are the areas of the beam to which load will be exerted. Figure 2A
shows how the stress is applied constantly over a large area. With this method the position of
the first crack is unforced and can appear anywhere within the shaded region. However this
means that the crack will follow the weakest path through the concrete beam that it can find.
Figure 2B shows that the stress is applied to a smaller area than the third point test. The
position of the first crack in the single point test is forced at a single point meaning that the
crack cannot just follow the weakest path.
With these pieces of research presented, a single point test was used, as use of the third point
loading machine was limited and also was unable to be linked to a computer programme
measuring the applied loads and deflection. All testing carried out conformed to BS EN
12390-5:2009, which specifies a method for testing the flexural strength of specimens of
hardened concrete.
3.4.1 Loading Rate
The loading rate applied was 1.1mm/minute, this is the rate proposed in research by
Coatanlem (2002). There is no set rate at which the load can be applied, however for tests
similar to this, it is recommended between 0.5 and 1.5 mm/minute
(http://www.astm.org/Standards, 2010).
3.5 Calculating Flexural Strength
Flexural strength, considered as the strength under normal stresses, was determined by
applying the following equation known from the strength of materials:
R = 3P L Where,
2bd² R = Modulus, measured in N/mm².
P = Correct load indicated, Newtons.
L = Span length between supports, mm.
b = Width of beam, mm.
d = Depth of beam, mm.
(www.astm.org, 2009)

14. First crack
0 0.15 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35 1.50
14
4.0 Results and Analysis
In this chapter the results for the flexural tests of the three groups are presented. Each groups
individual beams maximum load, deflection and flexural strength have been placed in a table
of results along with a graph representing the relationship between load and deflection. The
results for the plain concrete beams are shown in Section 4.1.
4.1 Results of Plain Concrete Beam Flexural Testing Programme.
After completing the flexural test programme for the plain concrete beams, the results were
recorded and are displayed below in Table 2.
Max. Load (N) Max. Deflection (mm) Flexural Strength (N/mm²)
Plain 1 10792.82 2.60 4.86
Plain 2 11400.82 1.87 5.13
Plain 3 13835.71 1.77 6.22
Plain 4 11370.79 2.28 5.11
Plain 5 13106.59 0.98 5.89
Plain 6 11398.64 0.55 5.12
Plain 7 10749.95 1.60 4.83
Mean Value 11807.90 1.66 5.31
Table 2. The table shows the maximum load, deflection and flexural strength for plain
concrete beams.
The maximum loads for the 7 plain beams were averaged and plotted against the average
maximum deflection in the graph shown in Figure 3 below.
14000
12000
10000
8000
6000
4000
2000
0
Load applied (N)
Deflection mm
Deflection (mm)
Figure 3. Graph showing the relationship between load and deflection for plain concrete
beams.

15. First crack
0 1.50 3.00 4.50 6.00 Axi7s. 5T0it le 9.00 10.50 12.00 13.50 15.00
15
4.2 Results from using Wood Chippings as Addition to the Concrete Beams.
The results for wood chippings alone are presented within Table 3 below.
Max. Load (N) Max. Deflection (mm) Flexural Strength (N/mm²)
Wood 1 6057.04 22.00 2.73
Wood 2 8660.77 12.15 3.89
Wood 3 10137.44 10.73 4.56
Wood 4 5758.72 20.19 2.59
Wood 5 6883.53 15.14 3.09
Wood 6 7428.96 12.47 3.45
Wood 7 6575.69 9.81 2.96
Wood 8 8187.26 14.69 3.68
Mean Value 7461.18 14.65 3.36
Table 3. Results from the flexural testing programme for using wood chippings alone as an
additive to concrete beams.
As was done for the plain concrete group beams, the maximum loads for the 8 beams were
averaged and plotted against the average maximum deflection in the graph shown in Figure 4
below.
8000
7000
6000
5000
4000
3000
2000
1000
0
Load applied (N)
Deflection (mm)
Figure 4. Graph showing the relationship between load and deflection for concrete beams
with wood chipping added

16. 4.3 Results from using Wood Chippings and Silica Fume as Addition to the Concrete Beams.
First crack
0 1.50 3.00 4.50 6.00 7.50 9.A00x i s T 1it0le.50 12.00 13.50 15.00 16.5 18.00
16
The results recorded from the flexural testing of wood chippings and silica fume within
concrete beams are presented within Table 4 below.
Max. Load (N) Max. Deflection (mm) Flexural Strength (N/mm²)
Hybrid 1 4115.64 22.36 1.85
Hybrid 2 4454.58 23.44 2.01
Hybrid 3 3665.14 19.99 1.65
Hybrid 4 5561.08 17.44 2.50
Hybrid 5 4217.87 19.35 1.89
Hybrid 6 6531.98 12.41 2.93
Hybrid 7 5463.34 14.89 2.74
Hybrid 8 6100.41 19.52 2.89
Hybrid 9 7310.51 19.36 3.29
Mean Value 5268.95 18.75 2.41
Table 4. Results from the flexural testing programme for using wood chippings and silica
fume as an additive to concrete beams.
The maximum loads for the 9 hybrid beams were averaged and plotted against the average
maximum deflection in the graph shown in Figure 5 below.
6000
5000
4000
3000
2000
1000
0
Load applied (N)
Deflection (mm)
Figure 5. Graph showing the relationship between load and deflection for concrete beams
with both silica fume and wood chippings added

17. 17
4.4 Overview of Flexural Strength Results.
The table below gives an overview comparison between the 3 groups, comparing average
maximum loads, maximum deflection and flexural strength.
Group Average Max load
(N)
Average Max
Deflection (mm)
Average Flexural
strength (N/mm²)
Plain 11807.90 1.66 5.31
Wood alone 7461.18 14.65 3.36
Hybrid 5268.95 18.75 2.41
Table 5. Showing the average results for the 3 groups maximum load, maximum deflection
and flexural strength.
When comparing the wood alone to plain concrete, the maximum load achieved is decreased
by 36.8%, however the average maximum deflection reached before complete shear is
increased by 782.5%. When comparing the hybrid concrete group average maximum load to
the average for plain concrete maximum load, the maximum load before first crack is 56.5%
lower. The position of first crack is shown in Figures 3, 4 and 5. However, the maximum
deflection recorded for the hybrid concrete beams is 1029.5% greater than that of the plain
concrete beams. The flexural strength of the hybrid beams decreased by 55.6% when
compared to the plain beams. The flexural strength of the beams with wood chippings alone
decreased by 37.7% when compared to the plain group.
4.5 Analysis of Results.
In order to explain these results, a more detailed look at what is happening to the beams must
be taken. As presented within the literature review, Section 2.3, silica fume undergoes a
pozzolanic reaction, improving the bonds between the cementitious materials to the wood
reinforcement. The improvement of the bonds between the wood and the concrete suggests
this is why maximum deflection values are increased even between the wood chipping
beams, and the hybrid beams, as the bonds are stronger and pull out is resisted further. The
term ‘pull out’ refers to the wood reinforcement being de-bonded from the concrete due to
the load applied, and removed without the wood breaking. Plate 1 below shows a photograph
of a beam that exhibits pull out after it had failed and had been removed from the flexural
testing machine.
Plate 1. Photograph of a failed beam showing pull
out of the wood reinforcement
Large areas are left where the wood
chipping reinforcement has been de-bonded
and pulled out from the
concrete beam.

18. Here, a length of wood chipping is shown
to have been pulled out from the concrete
beam
It can be seen here that wood chippings
within the beam are spanning the crack and
acting as reinforcement to the beam.
18
Plate 2, shows a concrete beam where it possible to see the wood chippings within acting a
reinforcement to the beam.
Plate 2. Shows the wood waste within the
beam acting as reinforcement.
Whilst the addition of silica fume increased the maximum deflection, it also reduced the
maximum load the beams can withstand as well as reducing the flexural strength. The basic
theory of concrete flexural strength is that flexural strength of concrete decreases as density
decreases, (Rossignolo, 2002. Park, 1975). This may potentially explain the reason the plain
concrete groups flexural strength is higher, as the density of the wood chippings present in
the other two concrete group beams being a lot less than that of concrete, therefore reducing
the density of the beam. The next section will consider the density of the beams as a reason
for reduced flexural strength.
4.6 Determining the Density Characteristics of the Concrete Mixes.
After the beams were taken out of the moulds for testing they were weighed, and from this
the density could be calculated. Figure 6 below shows the densities of the 3 groups.
2500
2400
2300
2200
2100
2000
1900
1800
Plain Wood alone Wood & Silica fume
Density (kg/m³)
Density kg/m³
Figure 6. Bar chart showing the density of the 3 concrete groups after 28 days.

19. Comparing the plain beam densities, to that of the wood chipping group it can be shown that
a 9.30% reduction in density is achieved. The hybrid beams have a 6.60% reduction in
density compared to the wood alone, and a 15.30% reduction compared to the plain group.
Table 6 below shows averages across the 3 groups for maximum load, maximum deflection,
flexural strength and also density.
19
Group Average Max
load, (N)
Average Max
Deflection,(mm)
Average Flexural
strength (N/mm²)
Density (kg/m³)
Plain 11807.90 1.66 5.31 2380
Wood 7461.18 14.65 3.36 2160
Hybrid 5268.95 18.75 2.41 2040
Table 6. Shows averages for maximum loads, deflections, flexural strength and density.
By presenting data from Table 6 in graphical form, such as Figures 7 and 8. It is possible to
see that density and load are proportional, as well as density and flexural strength, as the lines
on both graphs are straight.
2500
2400
2300
2200
2100
2000
1900
1800
Wood
Hybrid
2 3 4 5
Density (kg/m³)
Flexural strength (N/mm²)
Plain
Figure 7. Graph showing the relationship between density and flexural strength for the three
groups tested.

20. 20
2500
2400
2300
2200
2100
2000
1900
1800
Hybrid
Wood
Plain
5115
5480
5845
6211
6576
6941
7306
7672
8037
8402
8767
9133
9498
9863
10228
10594
10959
11324
11689
Density (kg/m³)
Maximum load, (N)
Figure 8. Graph showing the relationship between density and maximum load for the three
groups tested.
This means that by adding wood chippings to the mix, the density is reduced and this in turn
reduces the flexural strength of the concrete. However by adding silica fume, the bonds at the
interface between the concrete and the wood are improved, thus further deflection is
measured before failure.
Whilst the data above gives reason to why the flexural strength is the reduced with the
addition of wood chippings, the hybrid group’s average maximum load and flexural strength
decreases further than that of the wood chippings alone, this is an unexpected result as the
same weight of wood chippings was use for both of the groups. One reason for this could be
that although the same weight of wood chipping was used, the volume of the wood chippings
may have increased; this would explain the drop in density therefore the decrease in flexural
strength. However, other reasons may be established by looking at the rate of drying for all
beams. The next section will consider the drying curves for the beams.
4.7 Drying Curves.
This section will look at the drying rates for the 3 groups and analyse factors that influence
the final density of the beams. Figure 8 below shows the rate at which each group of beams
dried out over 28 days.

21. 21
3000
2500
2000
1500
1000
500
0
Density kg/m³
Days
Plain
Series1
Wood
Series2
Series3
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28
Days
Hybrid
Figure 8. Graph showing the drying curves for the 3 groups over a 28 day drying period
From the graph it is possible to see that the hybrid beams dry out and lose density the more
rapidly over the 28 day drying process. Research by Kadri (2009) suggests that the presence
of the silica fume in concrete will accelerate exothermic reactions during the hydration
process. Exothermic reactions begin to occur when the cement first comes into contact with
water in the mix and heat is produced. The main reaction that occurs is between the aluminate
within the Portland cement, and water. This reaction forms an aluminate-rich gel; the gel will
then react with sulphates within the solution to form what is known as ettringite
(http://www.understanding-cement.com/hydration.html, 2010). Ettringite is a crystal like
structure and forms in microscopic needles such as shown in Plate 3.
In the photograph microscopic needles
of white ettringite can be seen forming
in tiny air voids within concrete.
Plate 3. Photograph showing the microscopic needles
of ettringite. (Taken from http://www.cement.org/tech,
2010)

22. The silica fume within the concrete was added to improve the bond between the concrete by
drawing in the ettringite needles to the woods surface in accordance to the findings of section
2.1.1 of the literature review. However, this has meant that the spaces within the concrete
where the ettringite crystals were before being drawn to the wood chippings are now tiny air
voids (http://www.cement.org/tech, 2010). With these air voids present in the concrete, it is
suggested that the concrete beams are able to dry out faster, become less dense and will have
less flexural strength due to this. The hybrid beams have exhibited all of these properties,
being less dense than both the other groups, and having a smaller flexural strength value than
the plain concrete and the concrete with only wood chipping added.
From this chapter it is possible to see clear differences between the plain, wood chipping
alone and hybrid mixes. The results have been analysed and contextualised against our
current knowledge of concrete materials behaviour and structure. From doing this it can be
suggested that the introduction of silica fume to a mix can improve the deflection reached
before complete failure occurs. It can also be suggested that silica fume and wood chippings
decrease the flexural performance of concrete beams due to the decrease in density.
22
Chapter 5 will conclude the research and reflect upon the hypothesis, research aims and
objectives of the flexural testing programme.

23. 23
5.0 Conclusions
In this study the effects of adding waste wood chippings and silica fume to concrete were
investigated and analyzed. The aims of the study were see if the use of waste wood chippings
from the timber industry and the addition of silica fume, would improve flexural performance
of concrete, or not. The main objectives of the study were (1) to test 10 concrete beams with
no reinforcement, (2) test 10 concrete beams with waste wood chipping added as
reinforcement (3) test 10 concrete beams with waste wood chipping and silica fume added as
reinforcement, (4) to record the results appropriately, (5) to compare and analyse the 3 groups
of test results and (6) to evaluate the effectiveness of adding waste wood chippings and silica
fume to concrete mixes. By conducting testing in accordance with the method presented in
Chapter 3, results of the flexural testing programme were obtained and analysed in Chapter 4.
From analysing the results of the flexural testing programme carried out, the following
conclusions were derived.
1 The results of the flexural testing programme indicate that the addition of wood
chippings and silica fume does not improve the flexural strength of concrete, the
values for both wood chipping and silica fume groups were lower than that of
plain concrete.
2 The addition of wood chipping alone improves maximum deflection, compared to
plain concrete.
3 The addition of wood chippings and silica fume improves maximum deflection by
the greatest value.
4 Due to the addition of silica fume, ettringite crystals are drawn towards the wood,
leaving air holes, reducing density and strength even further than wood chippings
alone.
The hypothesis in Section 1.1, suggested that if waste wood chippings and silica fume is
added as reinforcement to a concrete beam, then the flexural strength of concrete beams will
be improved. From the conclusions drawn, this can be rejected. The aims of the investigation
stated in section 1.2 have been met, and the objectives stated in Section 1.3 have also been
achieved.
Although the hypothesis for this investigation was rejected, this study shows feasibility for
producing a more sustainable concrete. There are many different reasons for the results
obtained from this investigation; there are also many opportunities for further research and
testing to be carried out within this area. Section 6.0 will suggest recommendations for
further studies.
5.1 Research Limitations
The data gather from the flexural testing programme made for interesting analysis. By
collecting data from previous similar studies in the literature review and compiling the
methodology, it is apparent that many variations could be made within this research, with
reference to mix design and testing methodology.
It was extremely difficult to acquire the silica fume, as it was necessary for it to be shipped
from Holland, in only very small amounts. With more accessibility to silica fume, further
testing could have been conducted using different amounts of silica fume.
Another major limitation was the time available to test concrete within the laboratories, as
compression testing on concrete cubes would have provided a further insight into the effects

24. of adding waste wood and silica fume to concrete. If more time was available, a further 10
beams would have been tested using silica fume as the only addition to the concrete mix, to
allow for comparisons to the concrete beams with wood chipping alone and the hybrid
concrete beams. It would also be useful to measure the flexural strengths of the 3 groups of
concrete after a 60 day curing period, as this is suggested to increase flexural strengths further
(Naik, 2002).
Whilst this chapter has concluded the findings from this research and assessed the limitations.
The next chapter will consider recommendations for future research.
24

25. 25
6.0 Recommendations for Future Research
The replacement of cement by materials that induce a pozzolanic reaction, such as fly ash
Naik (2002), or silica fume, can have advantages for both sustainability and to the strength of
concrete. Although this investigation rejected the hypothesis that suggested that if waste
wood chippings and silica fume is added as reinforcement to a concrete beam, then the
flexural strength of concrete beams will be improved, it is still important to drive forward
sustainability issues by means of reducing carbon emissions, reducing materials used and
costs of production. There are many different variations to the methodology of the flexural
testing programme undertaken within this study, with reference to the mix design. This
chapter will address these variations in order to recommend and benefit future studies within
this area.
By varying the content ratios between the water, silica fume and waste wood chippings is
suggested to give different results for both flexural strength tests and compressive strength
tests. (Schulze, 1999, Beton, 1990., Pascal et al., 2004. and Van Gemert et al. 2005)
With reference to the mixing method of the batches, it is suggested that pre soaking the wood
chippings to draw cementitious particles into the wood will improve the bond between the
wood and the concrete further than if no pre soaking takes place, (Reis, 2006). As well as
varying the method of adding the wood, the method of which the silica fume was batched
could also be altered, for this research the silica was batched in accordance to the
manufacturer and supplier, however there are different suggestions to mixing which
potentially increase the strength of concrete, (Blanksvard, 2009).
The above recommendations provide platforms for future research to improve upon current
knowledge of concrete behaviour, and can potentially lead to more sustainable methods of
making concrete. The recommendations made, are based upon researchers conducting tests
without the limitations this paper has identified in Section 5.1.

26. 26
7.0 References.
http://www.astm.org/Standards (2010) (Accessed 23 March 2010)
Becchio, C. (2009) Improving environmental sustainability of concrete products,
investigation of MWC thermal and mechanical properties. Published doctorate. Turin
polytechnic, Italy.
Bjork, F. (1999) ‘Concrete technology and sustainable development’ Vancouver symposium
on concrete technology and sustainable development. December 1999. p.5.
Blanksvard, T. (2009) Strengthening of concrete structures by the use of mineral based
composites. Published doctoral thesis. Lulea University of technology.
http://www.brmca.org.uk/downloads/Mpa_Placing_PrintR (2009) (Accessed 24 March 2010)
Carette, G., (1993) “Mechanical Properties of Concrete Incorporating High Volumes of Fly
Ash From Sources in the U.S” November 1, 1993.
http://www.cement.org/tech, (2010) (Accessed 26 March 2010)
Chitchumnong P, Brooks S.C., Stafford G.D. (1989), ‘Comparison of three- and four-point
flexural strength testing of denture-base polymers’ Dental Materials Pp2-5
Coatanlem, P. (2005) Lightweight wood chipping concrete durability. Published masters.
Department of civil engineering. Rennes, France.
Galetakis M. and Raka S., (2004) ‘Utilization of limestone dust for artificial stone
production: an experimental approach’ Minerals Engineering
Gemert V, D., Czarnecki, L., Maultzsch, M., Schorn, H., Beeldens, A., Lukowski, P and
Knapen, E. (2005) Cement concrete and concrete–polymer composites: Two merging worlds:
A report from 11th ICPIC Congress in Berlin, 2004. Cement and Concrete Composites
Wendehorst, R. (1992) Baustoffkunde. Weinheim, Germany.
Huang Cheng-yi and R.F. Feldman. (1985) Influence of silica fume on the micro structural
development in cement mortars. Science direct [online]. Available at:
https://www.sciencedirect.com (Accessed: 8 January 2010)
http://www.instron.co.uk/wa/application /flexure (2010) (Accessed 22 March 2010)
Johansson, J. (2005) ‘End peeling of mineral based CFRP strengthened concrete structures- a
parametric study.’ Proceedings of the international symposium on bond behaviour of FRP in
structures. BBFF 2005.
Kadri, E., Duval, R., (2009) ‘Hydration heat kinetics of concrete with silica fume
Construction’. Building Materials. Volume 23, Issue 11, November 2009.
Miller, R. H., and Collins, R. J., (1975) “Waste Materials as Potential Replacements for
Highway Aggregates,” NCHRP Report No. 166.
Naik, T.R. (2002) ‘Greener concrete using recycled materials’ Concrete international. July
2002.

27. 27
http://www.nrmca.org/aboutconcrete (2005) (Accessed 22 March 2010)
Pascal, S., Alliche, A and Pilvin, Ph. (2004) ‘Mechanical behaviour of polymer modified
mortars’ Materials Science and Engineering. Elsevier B.V.
Rao, G. (2003) Investigations on the performance of silica fume-incorporated cement pastes
and mortars. Swetswise [online]. Available at: https://www.swetswise.com (Accessed: 8
January 2010)
Reis, J.M.L. (2006) ‘Fracture and flexural characterisation of polymer concrete reinforced
with wood waste’ Brazilian congress for engineering and science materials. November 19.
Pp 2871-2878
Rim, A. Ledhem, O. Douzane, (1999) R.M. Dheilly and M. Queneudec, ‘Influence of the
proportion of wood on the thermal and mechanical performances of clay–cement–wood
composites’, Cement and Concrete Composites.
Rossignolo J.A, Agnesini M.V.C. (2002) ‘Mechanical properties of polymer-modified
lightweight aggregate concrete’. Cement and Concrete Research. Volume 32, Issue 3.
Schulze, J. (1999) ‘Influence of water-cement ration and cement content on the properties of
polymer-modified mortars’ Cement and Concrete Research.
Shao Y., Lefort T., Moras S. and D. Rodriguez, (2000) ‘Studies on concrete containing
ground waste glass’ Cement and Concrete Research. Volume 30, Issue 1
Shayan A., (2004) ‘Value-added utilization of waste glass in concrete’, Cement and Concrete
Research. Volume 34, Issue 1
Soroushian P., Plasencia, J and S. Ravanbakhsh (2003) ‘Assessment of reinforcing effect of
recycled plastic and paper in concrete’ ACI Materials Journal.
Topcu I.B. and Canbaz M., (2004) ‘Properties of concrete containing waste glass’ Cement
and Concrete Research. Volume 21, Issue 7
Tliljsten et al. (2006) ‘Strengthening of Concrete Beams in Shear with Mineral Based
Composite Laboratory Tests and Theory’ Third International Conference on FRP
Composites in Civil Engineering
Turgut P., (2007), ‘Cement composites with limestone dust and different grades of wood
sawdust’, Building and Environment

28. 28
8.0 Appendix 1.