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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

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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.

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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

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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.

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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.

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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

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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.

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).

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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.

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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.

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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.

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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..

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.

A B

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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)

A B

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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.

Figure 3. Graph showing the relationship between load and deflection for plain concrete

beams.

0

2000

4000

6000

8000

10000

12000

14000

Load

ap

plie

d (

N)

Deflection mm 0 0.15 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35 1.50

Deflection (mm)

First crack

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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.

Figure 4. Graph showing the relationship between load and deflection for concrete beams

with wood chipping added

0

1000

2000

3000

4000

5000

6000

7000

8000

Load

ap

plie

d (

N)

Axis Title0 1.50 3.00 4.50 6.00 7.50 9.00 10.50 12.00 13.50 15.00

Deflection (mm)

First crack

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4.3 Results from using Wood Chippings and Silica Fume as Addition to the Concrete Beams.

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.

Figure 5. Graph showing the relationship between load and deflection for concrete beams

with both silica fume and wood chippings added

0

1000

2000

3000

4000

5000

6000

Load

ap

plie

d (

N)

Axis Title0 1.50 3.00 4.50 6.00 7.50 9.00 10.50 12.00 13.50 15.00 16.5 18.00

Deflection (mm)

First crack

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

Plate 2, shows a concrete beam where it possible to see the wood chippings within acting a

reinforcement to the beam.

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.

Figure 6. Bar chart showing the density of the 3 concrete groups after 28 days.

1800

1900

2000

2100

2200

2300

2400

2500

Plain Wood alone Wood & Silica fume

Den

sity

(kg/m

³)

Density kg/m³

Plate 2. Shows the wood waste within the

beam acting as reinforcement.

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.

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.

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.

Figure 7. Graph showing the relationship between density and flexural strength for the three

groups tested.

1800

1900

2000

2100

2200

2300

2400

2500

2 3 4 5

De

nsi

ty (

kg/m

³)

Flexural strength (N/mm²)

Plain

Wood

Hybrid

20

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.

1800

1900

2000

2100

2200

2300

2400

2500

51

15

54

80

58

45

62

11

65

76

69

41

73

06

76

72

80

37

84

02

87

67

91

33

94

98

98

63

10

22

8

10

59

4

10

95

9

11

32

4

11

68

9

De

nsi

ty (

kg/m

³)

Maximum load, (N)

Hybrid Wood

Plain

21

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.

0

500

1000

1500

2000

2500

3000

Den

sity

kg/

m³

Days

Series1

Series2

Series3

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Days

Plain

Wood

Hybrid

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.

Chapter 5 will conclude the research and reflect upon the hypothesis, research aims and

objectives of the flexural testing programme.

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.

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

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8.0 Appendix 1.