Task 1: BRC Pavements TRL Ltd In Association with KACE and Intech - - PDF document

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Development of Local Resource Based Standards in Cambodia

SEACAP 019 TRL Ltd

In Association with

KACE and Intech Associates

Task 1: BRC Pavements

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Task 1: BRC Pavements



The historical context



The principles of bamboo reinforcement for pavements



Theoretical considerations



Practical evidence - Puok market road and results from Vietnam



Conclusions

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Literature search

 1140 items on bamboo reinforced concrete (BRC)  67 on BRC pavements, most about the Puok trials in

Cambodia

 Only 10 key papers  Most comprehensive study by Glenn in 1950 which

has been much quoted

 Glenn’s results confirmed and extended by others  Not much that is really new since then

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Glen’s results for BRC beams

 The load at which the initial cracking of the

concrete occurs does not depend on whether bamboo is present or not.

 The tensile strength of bamboo is high so the

ultimate load occurs in the bending test long after the concrete has cracked.

 The ultimate load of a bamboo reinforced

concrete beam can be 4 – 5 times higher than that of an unreinforced beam

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Work in Thailand (1975 -1977)

 Design and construction of a trial BRC

pavement at AIT

 But only two sections, 3m x 3m square  A design and construction report but no

performance report

 Just an interesting student exercise? The

thesis was almost entirely theoretical ?

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Work at Chiang Mai University 1985+

 With no reinforcing, a concrete slab is

expected to suffer shrinkage cracks if it is longer than about 5.0m, depending

• n quality of concrete and other factors

 Adding bamboo reinforcement seemed

to increase this to about 6.0m but the research was not comprehensive.

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Adoption of BRCP in Thailand

 Adopted bamboo reinforcement for use in

LVRRs but did not adopt the 6.0m slab.

 The stated reason was to eliminate the need for

reconstructing the existing road or track.

 The concrete was laid on a relatively

unprepared ground and therefore the quality and uniformity of the support was not guaranteed.

 We could find no evidence that it worked  Bamboo was quite quickly replaced by steel for

these roads.

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Why ?

There are 3 three reasons cited for using bamboo reinforcement in concrete pavements.

• To prevent cracking under traffic load by providing

most of the tensile strength required in the concrete

• slab. (e.g. Thailand LVRRs)
• To prevent the cracking that normally occurs when a

large slab of concrete cures and shrinks.

• To minimise the width of any cracks (that form in the

concrete, for whatever reason) and to hold the slab together as an entity for as long as possible

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Load induced cracking - 1

 Concrete has almost no tensile strength  Therefore significant tensile stresses applied to the

slab will cause it to crack.

 Reinforcement can only comprise a small fraction of

the cross section of the slab,

 Hence high tensile strength in the reinforcement is

essential if it is to carry the total load. Bamboo has this high strength.

 For the reinforcement to work in this way and to carry

a large proportion of the stress generated by the traffic load, its elastic modulus must be much greater than that of the concrete.

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Load induced cracking - 2

 If the elastic modulus is too low, as the strain

increases, the concrete will crack long before the stress in the reinforcement is high enough.

 The elastic modulus of bamboo is much lower

than that of concrete and so bamboo can never fulfil this function.

 Theoretic calculations confirm this.

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Shrinkage cracking - 1

 Initially the elastic modulus of concrete (and its

strength) will be very low.

 As the concrete cures, both its elastic modulus and its

tensile strength increase. The concrete begins to bond with the bamboo and shrinkage stresses also begin to develop in the concrete matrix.

 At the point when the concrete would normally crack,

the modulus and strength of the concrete compared with bamboo might be such that true reinforcement

• ccurs and cracking is prevented.

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Shrinkage cracking - 2

 With no reinforcing, worldwide experience tells

us that a concrete slab is only likely to suffer shrinkage cracks if it is longer than about 5.0m.

 The curing process is too complicated to

calculate this accurately but it can be tested by experiment (Chiang Mai University).

 The experiments showed that bamboo is not

effective (probably because its modulus is too low)

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Minimise crack width

 If cracks form (e.g. poor concrete, heavily overloaded

vehicles) it is important to minimise their width to maintain as much interlock across the crack and to minimise the amount of water that could enter and cause weakening of the underlying layers.

 Reinforcement with a material of high elastic modulus

means that the load can be borne by the reinforcement with

• nly a small amount of associated strain, hence the cracks

will only open a small amount.

 Bamboo has a relatively low elastic modulus hence cracks

will open significantly.

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Theoretical evidence

Mechanical Property Range of values (MN/m2) Typical value (MN/m2) Typical value (psi) Tensile strength1 75 – 250 130 18,850 Poison’s ratio 0.25 – 0.41 0.32 0.32 Modulus of elasticity 10,000 – 28,000 18,000 2.6x106

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Theoretical evidence

Concrete h = 150mm Concrete h = 100mm Concrete h = 40mm

A B

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Theoretical evidence

Tensile stress in the concrete (MPa Model A Model B Subgrade modulus (MPa) Tensile stress in bamboo MPa Bottom Bottom 50 0.47 2.01 1.97 100 0.40 1.74 1.70 200 0.33 1.47 1.44 300 0.29 1.31 1.28

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Conclusion

 BRCP are not very promising but worth

trying

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Recent experiences

 BRCP constructed at Puok market with ILO

• dowelled joints
• no real ‘control’ section (i.e. with no reinforcement)

 As part of the SEACAP programme in South East

Asia, trials of bamboo and steel reinforced concrete pavements have been built in Vietnam.

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Practical evidence from Puok market

 The concrete is of good quality  No cracking has occurred in the pavement

despite a period of heavy loading

 The reinforcement has deteriorated badly  Conclusion ?

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Practical evidence from Vietnam

 Out of 236 bamboo reinforced slabs, 27 have cracked

(11%)

 Out of 201 unreinforced slabs, 12 have cracked (6%).  These slabs need to be studied in more detail before

final conclusions can be drawn (e.g the width of the cracks needs to be assessed)

 But the evidence indicates that bamboo reinforcement

is having no significant effect

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Additional considerations

 Variability in the basic properties of bamboo

• Species of bamboo
• Age of the bamboo culm
• Moisture content
• Pre-treatment (i.e. how the bamboo is stored and weathered)

 Interaction with water – volume change  Bonding with concrete – treatment, anchors  The response of bamboo to repetitive loads is relatively

unknown - there is little or no information about the fatigue behaviour of bamboo reinforced beams

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Rigid pavement design

 Although the tensile strength of concrete is low, it is

usually strong enough under traffic loads to resist cracking in a road pavement.

 This is because a road pavement is designed to be

uniformly supported by the sub-base underneath and therefore the stresses induced by vehicle loads are normally lower than the critical tensile strength.

 Also, concrete is brittle. This means that as long as a

critical stress is not exceeded, the concrete should not fail through fatigue and should therefore have a very long life.

 The key to success is the uniform support.

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Rigid pavement design

 For relatively low traffic (< 8.0 mesa) no

reinforcement is recommended

 Slabs of 150mm minimum thickness

used