Impact and Shock Loads-BERA VIOUR AND ANALYSIS OF BAMBOO REINFORCED

CONCRETE BEAMS UNDER FLEXURAL LOADING

----------------------------------------------------------.-----,I: F. Falade and T.A.I. Akeju

Dept of civil Engineering, University of Lagos, Akoka- Lagos, Nigeriaffalade@hotmail.com

BSTRACT

In this study, the behaviour of bamboo reinforced concrete beams was examined under a thirdpoint loading. Each beam was simply supported over an effective span of 600mm. The resultsshowed that the behaviour of the beams was governed by the strengths, moduli of elasticity andstress-strain relationship of its components (reinforcement and concrete). Bamboo reinforcedbeams exhibited enhanced cracking and failure strengths when compared to the equivalent inplain concrete. There was increase in first-cracking and post-cracking strengths for beams withincrease in bamboo content and curing age. A comparison of stress-strain distribution in thebeam at different percentages of reinforcement indicates an optimum of 5.20% of cross-sectional area of the beam for bamboo in bamboo reinforced concrete beams.

Keywords: Behaviour, Analysis, Bamboo, Reinforced Beams, Cracked, Uncracked, Post-cracked, Stress-strain.

DR F.Falade is currently a lecturer attached to the University of Lagos,Akoka- Lagos, Nigeria

DR. T.A.I. AKEJU, is currently a lecturer attached to the University ofLagos, Akoka- Lagos, NigeriaI

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The main constituents of reinforced concrete structures are plain concrete and steel Jdrs. 1iconcrete is essentially to carry compressive stress while the steel is to transmit tensile stresst:The high price of steel reinforcement has made it necessary to search for alternative structurmaterial to reduce the consumption of steel. The current research on bamboo as reinforcemein structural concrete elements is one of such schemes. The strength properties and low comade bamboo a material ideally suited for economical structures where its use is feasiblSharma (1) reported that there were about 75 genera and McClure (2) showed that there Wel1250 species of bamboo throughout the world. Each species has widely differircharacteristics that affect its usefulness as a construction material. While some species Lil

suitable as structural material others are not. It has been reported by Falade and Akeju (3) thoBambusa Vulgaris, a species of bamboo, constitutes 80% of bamboo population in Nigeria aithat its strength is sufficient in concrete for use in low-cost housing as an alternative materialsteel bar where limited load carrying capacity is required. The study further revealed that trstress-strain relationship of bamboo varied with the size of splints. For smaller splint sizethree stress levels were identified (comprising linear and non-linear portions), whereas larcsized splints showed one stress level (linear relationship throughout the loading period). T~use of bamboo as reinforcement in concrete is well known in Asia, Australia and to a limitEextent in Africa. Cox and Geymayer (4) had reported the use of bamboo as reinforcement flone-way and two-way slabs. They observed that the formation of the yield patterns is similar Ithat of steel reinforced slab. Glenn (5) reported that bamboo reinforcement considerabincreased the load capacity of beam at ultimate failure up to an optimum value compared to aunreinforced member. The load increased to about 4-5 times greater than that of plain concrelof equal dimensions. Masani et al (6) indicated that the load capacity of bamboo reinforcebeam was increased by using bamboo splint dowels as diagonal tension reinforcement alorthe sections of the beams where the vertical shear was generally high. Kankam (7) hereported that mature bamboo culm could be used safely as substitute for steel in structurmembers if adequately treated. The stress-strain relationship of bamboo reinforced concretedirect tension and compression had been reported by Pakotiprapha et al (8).

The objectives of this study are to (i) investigate the behaviour of bamboo reinforced concrelbeams under flexural loading (ii) investigate the behaviour of bamboo in concrete (iii) examirthe stress-strain relationship of the bamboo - concrete composite and the components I

uncracked, cracked and post-cracked stages (iv) evaluate the optimum bamboo contentbeams.

Materials

The cement used was Ordinary Portland cement whose properties conform to BS12 (9). T~sharp sand was obtained from Ogun River. The particles passing ASTM sieve No.4 (apertu'4.75mm) but retained on sieve No. 220 (aperture 0.063mm) were used. The particle size ran~of the coarse aggregate (granite chips) was 10-20mm. The bamboo culms were air-dried in trlaboratory to 12% moisture content. They were cut into the required splint size (8 x 10700mm) using a sawing machine 'Rapid'. The reinforcement volume fractions considered al0%, 0.67%, 1.34%,2.01 % and 2.68% of the gross cross sectional area of the beam.

Preparation and Testing of the Specimens

150 x 150 x 750mm beams were cast. The proportions of 1:1.86:4.34 (cement: sharp ssngranite chips) by weight were used with water-cement ratio of 0.55. When the constituents he

Impact and Shock Loads

Results and Discussion

Results

50

40

z 40:.::\J 350

'" 30...J

::.: 25creL..

20uti 15L..

u::'10

5

00

-+-0-..- O.67(unc)-+.- 2.01

--- 0.67(cot)-*-1.34-0- 2.68

7 '14 21 28 35 42 49 56 63 70 77 84Curing Age (Days)

FI G 2: Variation of First Crack Loads with Curing Age forDifferent Reinforcement Volume Fraction

Figures 2 and 3 show the variations of the first crack and failure loads with curing age fordifferent reinforcement volume fractions respectively.

sO~~--~--~--~~--~--__--~--~~--~--__--o 7 lol. 21 za 3S oil ol.'3 ffi 5) 70 77 (}t.

Curing Age (Days)FI G 3: Variation of Failure Loads with Curing Age for Different

Reinforcement Volume Fractions

SJ.IS

40

~35.....,"lil:no~255~.•..::: ISII:

10

-+- 0 --- O.67(cot)-.i;- O.67(unc) -7f- 1.34~2.01 ~2.68~

~~ ~. ------ ---~--=~

.-- ~-------------------------=------==-==-==~ -==It=-1 1

Tables 1 and 2 show the length of the fibres of the specimen obtained from the internodal an~,nodal specimens for both splints that were embedded in concrete and those that were nembedded in concrete.

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Impact and Shock Loads--

Table 1: Fibre lengths in bamboo splints

specimenInternode Node

No Length Width Thickness Length Width Thickness(mm) (mm) (mm) (rnrn) (mm) (mm) ,

1 1.456 0.0119 0.0107 1.380 0.0110 0.0109-1---_.

2 1.460 0.0114 0.0107 1.390 0.0115 0.0108 ,3 1.485 0.0110 0.0106 1.380 0.0107 0.0103 --l4 1.504 0.0110

--;

0.0106 1.333 0.0107 0.0104 I5 1.589 0.0110 0.0107 1.333 0.0110 0.01066 1.456 0.0110 0.0106 1.390 0.0110 0.01047 1.456 0.01190 0.0107 1.330 0.0110 0.01078 1.618 0.0110 0.0107 1.380 0.0110 0.01059 1.456 0.0110 0.0107 1.330 0.0110 0.010410 1.589 0.0110 0.0107 1.380 0.0110 0.0103

Average 1.508 0.0113 0.0107 1.363 0.0110 0.0105

Table 2: Fibre lengths in bamboo splints retrieved from concrete matrix after curing for 90 days

SpecimenInternode Node

No Length Width Thickness Length Width Thickness(mm) (mm) (mm) (mm) (mm) (mm)

1 1.905 0.0120 0.0108 1.504 0.0110 0.01082 1.994 0.0122 0.0107 1.618 0.0115 0.01083 1.934 0.0120 0.0106 1.608 0.0107 0.01034 1.954 0.0121 0.0108 1.475 0.0107 0.01045 1.976 0.0120 0.0106 1.589 0.0110 0.01076 1.945 0.0120 0.0106 1.608 0.0108 0.01037 1.994 0.0121 0.0107 1.618 0.0110 0.01088 1.905 0.0120 0.0106 1.589 0.0110 0.01059 1.980 0.0120 0.0106 1.618 0.0110 0.010610 1.965 0.0120 0.0105 1.608 0.0110 0.0105

Average 1.955 0.01204 0.01065 1.5835 0.01097 0.01057

Discussion

The strength of bamboo reinforced concrete beams varied with percentages of bamboo splintsin concrete. At the initial stage of loading, both concrete and bamboo resisted the applied loadbut subsequently microcracks appeared on the loaded beams. Observation made on the testspecimens showed that as the external load on the beam was gradually increased, differentstress-strain behaviours of the composite were observed to be different at uncracked, crackedand post-cracked stages.

The behaviour of the beam at each phase of loading after the commencement of cracks isexamined below

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r Impact and Shock Loads-~---------------------------------- -------

Behaviour of Bamboo Reinforced Concrete Beams

The first crack load corresponds to the load at which microcracks commenced on the be8'i1The load increases with increase in reinforcement content and the curing age. A comparison !Jf

first crack with failure load showed that bamboo imparted post-cracking strengths to beams.For example, in Fig 1 at 0.67% reinforcement volume fraction and 7-day curing, the first crac-load is -17.10kN while the ultimate load for the same age and percentage of reinforcement is25.60kN. This represents an increase of 49.71% above the first crack load. At 14-day for thesame reinforcement content, the increase is 73.84% while the increases are 84.31 (;/0 and58.82% for 28-day and 90-day curing ages respectively. The same trend was observed v.}hbeams containing other reinforcement volume fractions. The first crack load increased with agGup to 28 days but between 28 and 90 days the percentage decreased. The decrease in firstcrack load can be attributed to deterioration observed in bamboo splints embedded in concrete.Figs 2 and 3 show that the strength values obtained for the beams containing the coated splintswere lower than the corresponding values for the beams containing the uncoated splints. Butthe strength of beams containing coated splints increased up to 90 days while the strength otbeam containing uncoated splints decreased at the same curing age. Liese (12) reported thattreatability of bamboo differs according to species, age and moisture content of the culm,treatment method and type of preservative. Recommendation on the appropriate treatment forBambusa Vulgaris, a species of bamboo being used in this investigation, must await the resultsof an on-going investigation on the treatability of bamboo and its effects on the strengthcharacteristics of bamboo reinforced concrete.

Stress-strain Relationship

Experimental investigation into the composite action of concrete and bamboo splints has shownthat linear strain in concrete and cracks in tension zones have a significant effect on the stress-strain behaviour of reinforced concrete members.

During the tests three stages were identified with the behaviour of bamboo reinforced concretebeams.

(i) Uncracked Stage

This stage begins from zero leading and continues unti! cracking begins in the tension zone ofthe concrete. The diagram of tl i8 normal stresses in the tension and compression zones istriangular (Fig. 4)

A ~I

Nn t .

~----~-./\ 1\

v·····'1 / TenS!1/

II'

6bs

277-22 -

SEe. A-r\.

FIG 4. Variation of stress and strain in uncracked beam with suuare section.,

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Impact and Shock Loads:-=--

During this stage the stress in the concrete is below the ultimate tensile strength and both theconcrete and the bamboo are subjected to tensile stress. The stress in concrete and bamboo islow,strain is mostly elastic in nature, the relationship between the stress and strain is linear.

v + Vc = 1

(1)

(2)

Vc = 1 - V

Since both bamboo and concrete materials are in elastic state, strain

(3)

(4)

(5)

(6)

where,

am = Strength of bamboo reinforced composite

V = Volume of bamboo

Vc = Volume of Concrete

Eb = Young modulus of bamboo

Ec = Young modules of Concrete

s = Strain in bamboo = Strain in Concrete

(ii) Cracked Stage

!1 _

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impact and Shock Loads

This stage starts after cracking has occurred in the tension zone. During this stage, the tensstress at the cracks is taken by the bamboo splints and the concrete above the cracks and bethe concrete and the bamboo are subjected to tensile stress between the cracks.

B ~I

II

B ~I SEe: B-B

FIG 5.Variation of stress and strain in cracked beam with square section

At this stage, the stresses cease to be linear (Fig. 5), the relationship between the strength (the composite and its components is

O'er = V E, C et + (1 - V) E, e et (7)

where

O'er = Tensile strength of bamboo reinforced concrete matrix

c et = Tensile strain in concrete matrix

(iii) Post-cracked Stage

When the section has cracked, concrete matrix ceases to contribute to the tensile strength 01the member while bamboo splints carries the applied tensile load. Fig. 6 presents the stressesin both bamboo and concrete.

!---l--==.= -

SEC C - C

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In1pact and Shock Loads~

FIG 6.Variation of stress of stress and strain in beam at post- cracked stage

Thestress in bamboo reaches its ultimate tensile strength while that in compression zonereachesthe ultimate compressive strength. The ultimate tensile strength of the composite isgivenas:

Omu = (8)

where

Omu = Ultimate tensile strength

Cbta= Maximum strain in bamboo in the cracked concrete.

Thetensile strength of the unreinforced section is given by:

Omu = Ec C ctu (9)

where,

C ctu = Ultimate strain in concrete

Comparing equations 7 and 9, it can be seen that the incorporation of bamboo splints inconcretewill enhance the tensile strength of bamboo reinforced concrete at cracking if,

Ocr - Omu V (Eb - Ec) C ctu > 0 (10)

Themodulus of elasticity of bamboo is low (15833 - 23843N/mm2). The value is in the same

Grderus that of concrete, therefore the modular ratio Eb can be approximately taken as unity.Ec

Thisshows that tne incorporation of bamboo splints would not significantly improve the tensilecracking strength of bamboo reinforced beam. However, because the minimum number ofb~rnboo splints in concrete was two and the number increased with the percentage ofreinforcement, increase in tensile cracking strength was observed in this study. When equations8 and 9 are compared, assuming that E, and E, are nearly equal, it can be deduced that theIncorporation of bamboo splints enhances the ultimate strength of the bamboo reinforcedmember if

VEb C bta - Ec C ctu > 0

BehaViour of Bamboo in Concrete

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Impact and Shock Loads

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Anatomy

Observation on the anatomical structure of bamboo shows that its outer epidermis consists,'single-layered cell. Outer ground tissue comprises parenchyma cells, which are thick wail~(sclerified) and accompanied on the inner side by somewhat interrupted layer of the thick wallefibres of appreciable smaller diameter. Fibrous layer forms a zone of about 6-9 layers.sclerenchyma tissue vascular bundles scattered throughout the inner ground tissue. I~:vascular bundles consist of small and large bundles, the outmost being smaller than If,reminder and abutting directly on inner sclerenchyma ring.

Table 1 shows the length of bamboo fibres that were not embedded in concrete while table". (Indicates the fibre lengths in bamboo splints that were retrieved from concrete matrix afte'curing for 90 days.

Measurements revealed variations in the fibre lengths. At the internodes, the length varied frorr1.46-1.62mm with an average length of 1.51 mm, the average width was about 0.012mm with afl

average thickness of 0.011 mm. In the internodes the cells were axially oriented with overlap. Athe node the fibre length were shorter and without overlap. They varied from 1.33-1.39mm. Theaverage width was 0.011 mm and an approximate thickness of about 0.011. The fibres tapereatowards the ends and the lengths varied within the internodes and nodes. The nodes provide~transversal interconnections.

The fibre cell walls of the samples that were taken from splints retrieved from cured beams werebroken as against the initial smooth cell walls before embedding the splints in concrete. Thi~indicates direct weakening of the fibres by water in the pore of the composite and therefore theobserved reduction in strength in bamboo reinforced beams after 28 - day curing in water. Lea(13) reported that surface coatings with bituminous materials will not prevent the penetration ctwater indefinitely but they can postpone the commencement of attack. It is to be noted that theconsistent curing of bamboo reinforced concrete beams in water does not depict a practicaisituation, a better approach would be to observe the behaviour of a prototype structure unosapplied load.

Optimum Bamboo Content in Beam

The maximum percentage of bamboo considered in this experimental work is 2.68% of thegross sectional area of the beam. In order to determine the optimum percentage of bamboo, anassumption was made that the splints were fully bonded to concrete matrix, that is, equal strainin bamboo and matrix.

x = 0.72 X fbS x Abs (Falade, 14)0.4 feu b

Where,

x = depth of compression zone

fbs = stress in bamboo

Abs = Area of bamboo splints

feu = Characteristic strength of concrete

4-

Impact and Shock Loads---b = width of beam

For compatibility, strain in bamboo = strain in concrete matrix, the depth of compression ZO! I'·

equals.

X= d2

d = 0.72 x j~s X Ab52 0.4 feu b

A = 0.4 feu bdbs 2xO.72xj~s

Substituting for the known values in the above equation shows that the optimum value of thearea of bamboo splints in concrete beam is

Abs = O.052bd = 5.2%bd

This shows that the maximum percentage of bamboo in concrete for bamboo to yield beforeconcrete failure is 5.2% of the gross cross-sectional area of the beam.

CONCLUSION

From the foregoing, the following conclusions are made:

(1) The incorporation of a bamboo splints in concrete would not significantly improve thetensile cracking strength of concrete because of its low modulus of elasticity. However,the minimum number of splints in a beam was two therefore there was increase incracking strength with increase in percentage of bamboo content.

(2) There was increase in post cracking strength of the bamboo reinforced concrete beamswith increases in bamboo content and age.

(3) The consistent curing of bamboo reinforced concrete beams resulted in directweakening of splints fibres thus reducing the load carrying capacity of the beams.

(4) The optimum bamboo content in bamboo reinforced concrete beam is 5.20% of its crosssectional areas

REFERENCES

Seam - 5 4-

Impact and Shock Loads

the minimum number of splints in a beam was two therefore there was increase incracking strength with increase in percentage of bamboo content.

(2) There was increase in post cracking strength of the bamboo reinforced cone: ,_te uear..,with increases in bamboo content and age,

(3) The consistent curing of bamboo reinforced concrete beams resulted in directweakening of splints fibres thus reducing the load carrying capacity of the beams.

(4) The optimum bamboo content in bamboo reinforced concrete beam is 5.20% of its no'sectional areas

REFERENCES

1. Sharma, Y.M.L. (1980) 'Bamboo in Asia - Pacific Region. Bamboo Research in Asia,Proceedings of a Workshop, Singapore, May, 1980,99-120.

2. Mc Clure, FA. (1965), 'The Bamboo' Harvard University Press, Carnbrioqe, Mass,U,S.A.

3. Falade, F. A. and Akeju, T.A.1. 'The potential of bamboo as construction material. Proc.SEAM 4, pp. 373 - 387

4. Cox, F.B. and Geymayer, H.G. 'Bamboo Reinforced Concrete Paper C-70, US ArmyEngineer Waterways Experiment Station, C.E. Vicksbury, Mississippi, 15p (1970).

5. Glenn, H,E" Bamboo Reinforcement in Portland Cement Concrete, Clemson AgriculturalCollege, South Carolina 171p (1950).

6. Masani, N,J.; Djamani, B.C. and Sigh, B,; Studies on Bamboo Concrete CompositeConstruction' Constroller of Publication, Delhi 39p (1977).

7. Kankam, J.A. (1987) 'Bamboo as Reinforcement in Concrete: Proc SEAM, 1, pp. 373-387.

8. Pakotiprapha, B.; R.P. and Lee, S.L. Behaviour of a bamboo fibre-cement PasteComposite. Journal Ferro-cement 13, pp. 235-248 (1983)

9. British Standard BS 12, Portland Cement (Ordinary and Rapid Hardening), Part 2 BritishStandard Institution, London (1971).

10. British Standard BS 1881. Method of Making and Curing Test 3pecimens: Part 3, BritishStandard Institution, London (1971),

11. British Standard BS 1881: Methods of Testing for Strength: Part 4 British StandardInstitution, London (1971).

12. Liese, W. 'Preservation of Bamboo' Bamboo Research in Asia, Proc. of Workshop heldin Singapore, pp. 165 - 172 (1980).

13. F.M. Lea, 'The Chemistry of Cement and Concrete: Third Edition, Edward ArnoldPublishers Ltd., Glasgow, Britain. (1970).

14. Falade F. A. (1998) 'Utilization of Bamboo as Reinforcement in Concrete for Low-cost

Housing' Ph.D. Thesis. Dept. Civil Engineering, University of Lagos. Nigeria.

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