- Email: [email protected]
Structural behaviour of babadua reinforced concrete beams
Construction and Building Materials 13 Ž1999. 187]193
Structural behaviour of babadua reinforced concrete beams Charles K. KankamU , Brigitte Odum-Ewuakye Department of Ci¨ il Engineering, Uni¨ ersity of Science and Technology, School of Engineering, Kumasi, Ghana Received 6 February 1998; accepted 6 April 1999
Abstract The strength and deformation characteristics of concrete beams reinforced with babadua bars ranging from 2.87 to 12.13% were examined from tests performed on the beams. The beams were tested to failure mostly under third-point loading. Collapse of the beams occurred mostly through either flexural failure of concrete in compression or diagonal tension failure. The experimental failure loads averaged, respectively, 6.40 and 2.62 times the theoretical flexural strength and theoretical shear strength of the unreinforced concrete section. Also, the experimental failure loads were only approximately 1.18 times the theoretical flexural strength of the reinforced concrete and 1.05 times the theoretical shear strength of the concrete sections taking into consideration the resistance of the tension reinforcement. Q 1999 Elsevier Science Ltd All rights reserved. Keywords: Babadua reinforcement; Flexural strength; Cracking
1. Introduction In most tropical countries, technologies for construction of houses depend to a large extent on imported building materials. Recently, attempts have been made in some of these countries by researchers to fully utilise locally available building materials and to improve their own traditional construction methods in order to achieve a better durability of the buildings. Studies have been carried out on natural reinforcing materials such as bamboo w1x, raffia palm w2x, jute w3x, wood w4x and palm stalk w5x. These studies have yielded good results, nevertheless, non-ferrous, natural material reinforcement still remains a dynamic field of further research. In line with this objective the present paper presents the results of study of the strength and deformation characteristics of concrete beams reinforced with babadua Žbotanical: Thalia geniculata. bars. Babadua is a tall straggling shrub which grows up to U
Corresponding author. Tel.: q233-51-60226; fax: q233-51-60226.
a height of 8 m and above. It is known to thrive in closed forests and swampy savannah lands of the tropical and sub-tropical world. It grows in West and Southern Africa, Asia and in the region stretching between Florida and Brazil w6x. It is also cultivated w7x. In rural communities of Ghana, babadua is used in thatching and its stems are tied into framework of houses before daubing with mud w8x.
2. Experimental program 2.1. Specimen details The test programme included eight large and 15 small beams. The large specimens had a total span of 1.8 m, a shear span of 580 mm and a cross-section bxh s 135 = 235 mm, while the small beams had the corresponding dimensions of 1.5 m, 430 mm and 100 = 180 mm, respectively ŽTable 1.. Beams B1]B4, B7 and B20 contained stirrups that were formed from approximately 8 mm diameter babadua bars and were spaced
0950-0618r99r$ - see front matter Q 1999 Elsevier Science Ltd All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 9 9 . 0 0 0 2 0 - 3
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
188 Table 1 Description of beams Beam No.
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22a B23a a
Crosssection Ž b = D.
Spanr overall depth Ž LrD .
100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 135 = 235 100 = 180 100 = 180 100 = 180 135 = 235 100 = 180 135 = 235 100 = 180 135 = 235 135 = 235 100 = 180 135 = 235 135 = 235 135 = 235
7.17 7.17 7.17 6.67 6.67 6.67 6.67 6.67 6.67 7.06 7.17 7.17 7.17 7.40 7.17 7.40 7.17 7.40 7.20 7.40 7.40 7.57 7.57
Tension reinforcement
Compression reinforcement
Area Ž Ab . Žmm2 .
100r Ab bD Ž%.
Area Ž AXb . Žmm2 .
100r X AbD Ž%.
517 930 1580 1270 1232 1750 1297 1373 1677 2699 1880 1939 2184 3574 1179 2740 1218 2011 2028 1760 2150 2319 2320
2.87 5.17 8.78 7.05 7.12 9.72 7.20 7.63 9.32 8.51 10.44 10.77 12.13 11.26 6.55 8.64 6.77 6.34 11.27 5.55 6.78 7.31 7.31
340 328 303 315 310 335 326 339 292 652 716 736 324 372 387 645 330 591 867 619 554 422 516
1.89 1.82 1.68 1.75 1.72 1.86 1.81 1.88 1.62 2.05 3.98 4.09 1.80 1.17 2.15 2.03 1.84 1.86 4.82 1.95 1.75 1.33 1.62
Cube strength ŽNrmm2 .
Modulus of rupture ŽNrmm2 .
12.2 28.9 22.7 23.5 23.5 23.5 20.2 20.2 20.2 23.9 19.9 19.9 23.1 22.7 19.3 19.0 22.0 22.0 27.6 21.4 24.4 20.0 20.0
1.5 2.1 1.9 2.0 2.0 2.0 1.7 1.7 1.7 2.0 1.6 1.6 2.0 1.8 1.6 1.6 1.8 1.8 2.1 1.8 1.8 1.7 1.7
Beams tested under central-point loading.
approximately 100 mm. The stirrups consisted of two horizontal and two vertical pieces with provision of small incisions at their ends. The two ends meeting at each of the four corners were tied together by means of copper wire. The rest of the beams had no shear reinforcement. The beams were reinforced with babadua bars ranging from 2.87 to 12.13% with a top and bottom clear concrete cover of approximately 12 mm. Ordinary Portland cement was used, and the aggregates consisted of river sand and crushed rock of 10 mm, maximum size. The properties of concrete and the reinforcing bars used are summarised in Table 2.
3. Test procedure Two beams were tested after 378 days, two after 100 days and the rest after 28 days. The beams were simply supported at their ends on steel beams that formed part of a rigid steel frame. Two-point symmetrical loading was applied to produce a constant moment in the mid-span region of the beams. A typical experimental set-up is shown in Fig. 1. Three beams were subjected to 10 load cycles, one to sustained loading for 30 days before collapse, and the rest to monotonic loading. The test procedure included crack monitoring and
Table 2 Details of concrete and babadua reinforcement Plain concrete Cement:sand:coarse aggregate 28th day cube strength 28th modulus of rupture Babadua reinforcing bar Ultimate tensile strength
Modulus of elasticity
1:1.5:3 28.3 Nrmm2 2.03 Nrmm2
1:2:4 22.3 Nrmm2 1.72 Nrmm2
Specimen with nodes Minimum: 75 Nrmm2 Maximum: 125 Nrmm2
Specimen without nodes 150 Nrmm2 200 Nrmm2 Minimum: 50 kNrmm2 Maximum: 100 kNrmm2
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
189
ranging from 8 to 20 mm. Its ultimate strength varied considerably with a minimum and maximum of 75 and 200 Nrmm2 , respectively. Fracture of babadua bars almost invariably occurred at the nodal points whenever present and, in such cases, the ultimate strength was found to be relatively low. The optimum moisture content was about 0.9%. In nodal-free specimens, the ultimate strength approached the maximum limit. 6.2. Load]deflection cur¨ es
Fig. 1. Typical experimental set-up.
central deflection measurements by means of dial gauges for all load increments.
4. Flexural theory The cracking moment M cr of a plain prismatic concrete section, based on its modulus of rupture is expressed as: Mcr s f t c bh2r6 where f t c denotes the modulus of rupture of the concrete, and b and h the width and overall depth of the beam, respectively. For a simply supported beam that is loaded at its third points, the ultimate flexural load Pult is given by:
Figs. 2 and 3 show typical load]deflection curves for the beams that were subjected to monotonic and cyclic loadings, respectively. Prior to cracking, the beams exhibited linear elastic characteristics, but as cracks initiated and subsequently developed, a relatively large domain of inelastic response generally developed. As cracking initiated a sharp increase in the observed deflections of the beams was due to the relatively low value of the modulus of elasticity of babadua Ž50]100 kNrmm2 . compared to that of steel Žapprox. 200 kNrmm2 .. 6.3. Cracking and failure loads The cracking, experimental failure, and theoretical flexural loads of the beams are given in Table 3. Theoretical failure loads estimated from fracture of tension bars were based on 100 Nrmm2 tensile strength of babadua with a partial factor of safety Žgm . of 3.
Pult s 6 MultrL where Mult denotes the ultimate moment of resistance of the beam and L the span of the beam.
5. Shear strength In accordance with the British Standard BS 8110:1985 method w9x the theoretical shear strength of the beams was calculated by considering: Ža. the concrete section alone; Žb. both concrete section and tension reinforcement; and Žc. the concrete section, the tension reinforcement, and the babadua stirrups.
6. Theoretical and experimental results 6.1. Material properties of the beams Details of the concrete and reinforcing babadua bars are given in Table 2. The tensile strength of babadua was obtained from tests performed on bars of diameter
Fig. 2. Load]deflection curves for babadua reinforced beams.
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
190
are given in Table 4, which also includes comparisons with the cracking and failure loads.
7. Discussion of test results 7.1. Modes of failure
Fig. 3. Load]deflection curve for beam B14 subjected to 10 cycles of loading prior to collapse.
6.4. Shear strength Details of the theoretical shear strength of the beams
In a simply-supported beam subjected to third-point loading the middle third of the span bears a constant maximum bending moment and zero shear force, while the remaining adjoining spans are subjected to maximum shear force and varying bending moment. Collapse of beams may occur either through flexural failure caused by the crushing of the concrete andror fracture of the tension bars, diagonal tension failure of the concrete, or shear-bond failure. The mode of failure, however, depends on the amounts of tension and shear reinforcement, the strength of the concrete, and the shear span-to-effective depth ratio. In the 23 beams tested to failure the tension reinforcement varied from 2.87 to 12.13% of the gross concrete section and the shear span-to-effective depth ratios ranged between 2.49 and 4.44. As expected collapse loads generally increased with the amount of
Table 3 First-crack and failure loads for beams Beam No.
Firstcrack load Ž Pcr .
Experimental failure load Ž Pult . ŽkN.
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 Average:
6.0 6.5 5.5 5.5 8.5 8.5 11.5 8.0 8.0 10.5 9.5 7.0 9.0 11.5 6.0 12.0 4.4 11.0 7.5 11.0 12.0 5.5 6.0
18 22 30 34 28 30 42 36 30 52 34 32 40 62 28 46 17 43 37 38 44 29 27
Theoretical flexural strength ŽkN.
Pcr
Pcr
Pult
Based on concrete section alone X Ž Pult .
Including babadua bars in tension Y Ž Pult .
Pult
Pult
Pult
Pult
3.77 5.27 4.77 5.39 5.39 5.39 4.59 4.59 4.59 8.98 4.02 4.02 5.02 7.71 4.02 6.86 4.52 7.72 5.23 7.71 7.71 4.65 4.65
9.4 16.4 27.8 23.6 23.0 32.7 24.8 26.2 32.1 49.8 30.7 31.4 36.5 63.1 19.8 47.0 20.5 34.5 35.4 30.3 37.1 26.1 26.1
0.33 0.30 0.18 0.16 0.30 0.28 0.27 0.22 0.27 0.20 0.28 0.22 0.23 0.19 0.21 0.26 0.26 0.26 0.21 0.29 0.28 0.19 0.22 0.24
1.59 1.23 1.15 1.02 1.58 1.58 2.51 1.74 1.74 1.17 2.36 1.74 1.79 1.49 1.49 1.75 0.97 1.42 1.43 1.43 1.56 1.18 1.29 1.53
1.91 1.34 1.08 1.44 1.22 0.92 1.69 1.37 0.93 1.04 1.11 1.02 1.10 0.98 1.41 0.98 0.83 1.25 1.02 1.25 1.17 1.11 1.03
4.77 4.17 6.29 6.31 5.19 5.57 9.15 7.84 6.54 5.79 8.46 7.96 7.97 8.04 6.97 6.71 3.76 5.57 7.07 4.93 5.64 6.24 5.81 6.40
Y
Pult X
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
191
Table 4 Shear strength of beams Beam No.
Shear spanr effective depth ratio
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22a B23a Average:
2.76 2.80 3.04 2.69 2.54 2.54 2.61 2.49 2.49 2.61 2.75 2.72 2.71 2.72 2.97 2.91 2.97 2.91 2.72 2.90 2.89 4.44 4.44
a
Theoretical shear strength ŽkN.
Pcr
Pult
Pult
Pult
Without babadua Ž Ps1 .
Including babadua in tension Ž Ps2 .
Including babadua in tension and stirrups Ž Ps3 .
Ps1
Ps1
Ps2
Ps3
10.92 10.76 10.10 10.40 11.02 11.03 10.71 11.26 11.26 20.05 10.96 11.07 11.09 20.12 10.13 18.81 10.13 18.81 10.58 18.89 18.94 18.52 18.52
26.52 28.90 25.41 26.43 28.04 28.05 26.02 27.35 27.36 50.99 26.62 26.88 27.89 50.59 24.60 45.67 25.19 46.77 28.12 46.41 46.53 44.98 44.98
28.44 39.09 34.98 36.28 ] ] 36.17 ] ] ] ] ] ] ] ] ] ] ] ] 59.66 ] ] ]
0.55 0.60 0.54 0.53 0.77 0.77 1.07 0.71 0.71 0.52 0.87 0.63 0.81 0.57 0.59 0.64 0.43 0.58 0.71 0.58 0.63 0.22 0.22
1.65 2.04 2.97 3.27 2.54 2.72 3.92 3.20 2.66 2.59 3.10 2.89 3.61 3.08 2.76 2.45 1.68 2.29 3.50 2.01 2.30 1.51 1.51 2.62
0.68 0.76 1.18 1.29 0.99 1.07 1.61 1.32 1.10 1.02 1.28 1.19 1.43 1.23 1.14 1.01 0.67 0.92 1.32 0.82 0.93 0.62 0.62 1.05
0.63 0.56 0.86 0.94 ] ] 1.16 ] ] ] ] ] ] ] ] ] ] ] ] 0.64 ] ] ] 0.80
Beams tested under central-point loading.
tension reinforcement. As babadua is markedly flexible and ductile, collapse of the beams, even through the crushing of concrete or diagonal tension failure of concrete, was significantly gradual when the compression zone was also reinforced. Unlike in steelreinforced concrete which is generally designed preferably as under-reinforced with a limit to the amount of reinforcement to avoid brittle failures, relatively higher percentages of babadua reinforcement can be utilised in concrete members and yet obtain sufficient warning prior to collapse of members. Although theoretical analyses had indicated that the tension reinforcement would fail first in most of the beams, collapse occurred mostly through crushing of the concrete in the compressive zone after excessive deflection of the beams or, diagonal tension failure of the concrete in the shear span. This was due partly to the fact that the theoretical estimate of the tensile strength of babadua was based on the nodal strength which could be as low as 75 Nrmm2 , while the internodal strength could be as high as 200 Nrmm2 . Inspection of the collapsed beams revealed that the nodes in the reinforcing bars did not always coincide with the crack positions where bar stresses would be maximum.
A crack pattern of this kind would lead to higher flexural strengths of beams than the theoretical predictions. 7.2. Cracking and failure loads From Table 3 the cracking loads Pcr averaged 0.24 X Žthe Pult Žthe experimental failure load. and 1.53 Pult theoretical flexural strength of the concrete section alone.. Within the range of 6.6]7.6 of span-to-depth ratio, the ratio of cracking load to experimental failure load varied slightly between 0.16 and 0.33, and was independent of the amount of reinforcement and the age of concrete. Tables 3 and 4 show that the experX Žthe theoretimental failure loads Pult averaged 6.4 Pult ical flexural strength of the concrete section alone., 2.62 Ps1 Žthe theoretical shear strength of the concrete section alone., 1.05 Ps2 Žthe theoretical shear strength of the concrete including the effect of the tension reinforcement . and 0.80 Ps3 Žthe theoretical shear strength including the effects of tension reinforcement and babadua stirrups.. This indicates that babadua reinforcement made significant contributions to both flexural capacity and shear resistance of the concrete
192
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
beams. Results from beams B15, B16 and B17, B18 which were tested after 100 and 378 days, respectively, did not indicate any sign of deterioration of the babadua bars with time. The contribution of the babadua stirrups to resistance against collapse of beams B1 to B4, B7 and B20 was slight, and diagonal tension failure occurred in beams B3 and B20. This might be attributed to the lack of rigidity in babadua stirrups when tied by means of copper wires to the main bars as could be found in steel stirrups Table 5 provides information on mode of failure and crack patterns in the beams. The presence of several short-spaced cracks within the constant moment span in most cases indicates good bond between babadua bars and their surrounding concrete, probably due to the bearing action of the nodal points.
monotonic load-deflection path. This means that there was no significant deterioration of strength within the limited load repetitions carried out in the study. 7.4. Effect of creep Figs. 4a and b show the effect of sustained load of magnitude approximately equal to half its theoretical failure load Ž37 kN. on beam B21 for 30 days. The failure load appeared to be least affected by creep as the load]deflection curve could be seen to pick up its original path as collapse approached. This could mean that within the limited duration of sustained loading employed in this study the rigidity of the beam was minimally affected.
8. Conclusions
7.3. Effect of cyclic loading Load-deflection curves of the beams under cyclic loading formed hysterisis loops ŽFig. 3.. After ten load cycles, the curve appeared to re-trace its original
On average the experimental failure loads of the beams were approximately 2.7 times the theoretical shear strength of the concrete section alone. Collapse of beams occurred through either diagonal tension
Table 5 Crack pattern and mode of failure Beam No.
Mode of failure
No. and type of cracks at failure
Max. crack width Žmm.
B1 B2 B3 B4 B5 B6
Flexural in tension bar Flexural in tension bar Diagonal tension Flexural in concrete crushing Flexural in concrete crushing Diagonal tension
4 3 3 3 4 3
B7 B8 B9 B10
Bond failure Diagonal tension Flexural in concrete crushing Diagonal tension
B11
Bond failure Žspalling of concrete over node. Flexural in concrete crushing Diagonal tension Diagonal tension
2 flexural cracks 4 flexural cracks 1 diagonal shear q 4 flexural cracks 1 diagonal shear q 3 flexural cracks 1 flexural-shear q 3 flexural cracks 2 flexural-shear q 3 flexural q 1 short diagonal cracks 1 flexural-shear q 4 flexural cracks 1 diagonal shear q 4 flexural cracks 2 flexural cracks 1 diagonal shear q 1 short flexural-shear q4 flexural cracks 2 short flexural-shear q 3 flexural cracks
B12 B13 B14 B15 B16 B17
Flexural in concrete crushing Žtest after 100 days. Flexural in concrete crushing Diagonal tension
B18 B19 B20 B21
Flexural in concrete crushing Diagonal tension Diagonal tension Flexural in concrete crushing
B22 B23
Shear-bond Diagonal tension
2 3 5 4 2.5
1 flexural-shear q 5 flexural cracks 1 diagonal shear q 4 flexural cracks 3 diagonal shear q 2 flexural-shear q3 flexural cracks 2 flexural cracks Žtest after 100 days.
2 3.5 4
3 flexural cracks Žtest after 100 days. 1 diagonal shear q 1 flexural cracks Žtest after 378 days. 2 flexural cracks Žtest after 378 days. 4 shear q 4 flexural cracks 1 flexural-shear q 3 flexural cracks 1 flexural-shear q 3 flexural cracks Žsubjected to sustained loading. 1 diagonal q 2 flexural-shear cracks 1 diagonal q 6 flexural-shear cracks
3.5 5
4.5
4 4 3 5 5
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
193
failure of the concrete in the shear span or flexural failure through crushing of concrete. Babadua stirrups provided additional strength to the beams. For a spanto-depth ratio of simply supported beams up to about 8.0, a short-term factor of safety of about 1.5 against cracking and one of approximately 6.4 against collapse were obtained from the results of the investigations. Long-term tests indicated no sign of deterioration of the babadua bars, and that the flexural strength and behaviour of babadua reinforced beams remained intact more than a year after. References
Fig. 4. Ža. Load]deflection curve for beam B21 under 20 kN sustained load for 30 days prior to collapse. Žb. Time]deflection curve for beam B21 under 20 kN sustained loading.
w1x Kankam JA, Ben-George M, Perry SH. Bamboo-reinforced beams subjected to third-point loading. ACI Struct J 1988;85Ž1.. w2x Kankam CK. Raffia palm-reinforced concrete beams. J Mater StructrMateriaux Construct ŽRILEM. 1997;30. w3x Manzur MA, Aziz MA. A study of jute fibre reinforced cement composites. Int J Cement Compos Lightweight Con 1982; 4:75]82. w4x Andonian P, Mai YM, Cotterell B. Strength and fracture properties of cellulose fibre reinforced cement composites. Int J Cem Compos Lightweight Con 1979;1:151]158. w5x Kankam CK. The influence of palm stalk fibre reinforcement on the shrinkage stresses in concrete. J Ferrocement 1994;24Ž3.. w6x Irvine FR. Woody plants in Ghana. Oxford University Press, 1961 w7x Lyman Benson. Plant classification. Published by D.C. Heath and Company, 1965. w8x Schreckenbach H, Abenkwa JGK. Construction Technology for a Tropical Developing Country. Published by German Agency for Technical Co-operation ŽGTZ. for the Department of Architecture, University of Science and Technology, Kumasi, Ghana, 1982. w9x British Standards Institution. Structural use of concrete. BS 8110:Part 1:1985. Code of Practice. London, 1985.
Structural behaviour of babadua reinforced concrete beams Charles K. KankamU , Brigitte Odum-Ewuakye Department of Ci¨ il Engineering, Uni¨ ersity of Science and Technology, School of Engineering, Kumasi, Ghana Received 6 February 1998; accepted 6 April 1999
Abstract The strength and deformation characteristics of concrete beams reinforced with babadua bars ranging from 2.87 to 12.13% were examined from tests performed on the beams. The beams were tested to failure mostly under third-point loading. Collapse of the beams occurred mostly through either flexural failure of concrete in compression or diagonal tension failure. The experimental failure loads averaged, respectively, 6.40 and 2.62 times the theoretical flexural strength and theoretical shear strength of the unreinforced concrete section. Also, the experimental failure loads were only approximately 1.18 times the theoretical flexural strength of the reinforced concrete and 1.05 times the theoretical shear strength of the concrete sections taking into consideration the resistance of the tension reinforcement. Q 1999 Elsevier Science Ltd All rights reserved. Keywords: Babadua reinforcement; Flexural strength; Cracking
1. Introduction In most tropical countries, technologies for construction of houses depend to a large extent on imported building materials. Recently, attempts have been made in some of these countries by researchers to fully utilise locally available building materials and to improve their own traditional construction methods in order to achieve a better durability of the buildings. Studies have been carried out on natural reinforcing materials such as bamboo w1x, raffia palm w2x, jute w3x, wood w4x and palm stalk w5x. These studies have yielded good results, nevertheless, non-ferrous, natural material reinforcement still remains a dynamic field of further research. In line with this objective the present paper presents the results of study of the strength and deformation characteristics of concrete beams reinforced with babadua Žbotanical: Thalia geniculata. bars. Babadua is a tall straggling shrub which grows up to U
Corresponding author. Tel.: q233-51-60226; fax: q233-51-60226.
a height of 8 m and above. It is known to thrive in closed forests and swampy savannah lands of the tropical and sub-tropical world. It grows in West and Southern Africa, Asia and in the region stretching between Florida and Brazil w6x. It is also cultivated w7x. In rural communities of Ghana, babadua is used in thatching and its stems are tied into framework of houses before daubing with mud w8x.
2. Experimental program 2.1. Specimen details The test programme included eight large and 15 small beams. The large specimens had a total span of 1.8 m, a shear span of 580 mm and a cross-section bxh s 135 = 235 mm, while the small beams had the corresponding dimensions of 1.5 m, 430 mm and 100 = 180 mm, respectively ŽTable 1.. Beams B1]B4, B7 and B20 contained stirrups that were formed from approximately 8 mm diameter babadua bars and were spaced
0950-0618r99r$ - see front matter Q 1999 Elsevier Science Ltd All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 9 9 . 0 0 0 2 0 - 3
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
188 Table 1 Description of beams Beam No.
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22a B23a a
Crosssection Ž b = D.
Spanr overall depth Ž LrD .
100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 100 = 180 135 = 235 100 = 180 100 = 180 100 = 180 135 = 235 100 = 180 135 = 235 100 = 180 135 = 235 135 = 235 100 = 180 135 = 235 135 = 235 135 = 235
7.17 7.17 7.17 6.67 6.67 6.67 6.67 6.67 6.67 7.06 7.17 7.17 7.17 7.40 7.17 7.40 7.17 7.40 7.20 7.40 7.40 7.57 7.57
Tension reinforcement
Compression reinforcement
Area Ž Ab . Žmm2 .
100r Ab bD Ž%.
Area Ž AXb . Žmm2 .
100r X AbD Ž%.
517 930 1580 1270 1232 1750 1297 1373 1677 2699 1880 1939 2184 3574 1179 2740 1218 2011 2028 1760 2150 2319 2320
2.87 5.17 8.78 7.05 7.12 9.72 7.20 7.63 9.32 8.51 10.44 10.77 12.13 11.26 6.55 8.64 6.77 6.34 11.27 5.55 6.78 7.31 7.31
340 328 303 315 310 335 326 339 292 652 716 736 324 372 387 645 330 591 867 619 554 422 516
1.89 1.82 1.68 1.75 1.72 1.86 1.81 1.88 1.62 2.05 3.98 4.09 1.80 1.17 2.15 2.03 1.84 1.86 4.82 1.95 1.75 1.33 1.62
Cube strength ŽNrmm2 .
Modulus of rupture ŽNrmm2 .
12.2 28.9 22.7 23.5 23.5 23.5 20.2 20.2 20.2 23.9 19.9 19.9 23.1 22.7 19.3 19.0 22.0 22.0 27.6 21.4 24.4 20.0 20.0
1.5 2.1 1.9 2.0 2.0 2.0 1.7 1.7 1.7 2.0 1.6 1.6 2.0 1.8 1.6 1.6 1.8 1.8 2.1 1.8 1.8 1.7 1.7
Beams tested under central-point loading.
approximately 100 mm. The stirrups consisted of two horizontal and two vertical pieces with provision of small incisions at their ends. The two ends meeting at each of the four corners were tied together by means of copper wire. The rest of the beams had no shear reinforcement. The beams were reinforced with babadua bars ranging from 2.87 to 12.13% with a top and bottom clear concrete cover of approximately 12 mm. Ordinary Portland cement was used, and the aggregates consisted of river sand and crushed rock of 10 mm, maximum size. The properties of concrete and the reinforcing bars used are summarised in Table 2.
3. Test procedure Two beams were tested after 378 days, two after 100 days and the rest after 28 days. The beams were simply supported at their ends on steel beams that formed part of a rigid steel frame. Two-point symmetrical loading was applied to produce a constant moment in the mid-span region of the beams. A typical experimental set-up is shown in Fig. 1. Three beams were subjected to 10 load cycles, one to sustained loading for 30 days before collapse, and the rest to monotonic loading. The test procedure included crack monitoring and
Table 2 Details of concrete and babadua reinforcement Plain concrete Cement:sand:coarse aggregate 28th day cube strength 28th modulus of rupture Babadua reinforcing bar Ultimate tensile strength
Modulus of elasticity
1:1.5:3 28.3 Nrmm2 2.03 Nrmm2
1:2:4 22.3 Nrmm2 1.72 Nrmm2
Specimen with nodes Minimum: 75 Nrmm2 Maximum: 125 Nrmm2
Specimen without nodes 150 Nrmm2 200 Nrmm2 Minimum: 50 kNrmm2 Maximum: 100 kNrmm2
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
189
ranging from 8 to 20 mm. Its ultimate strength varied considerably with a minimum and maximum of 75 and 200 Nrmm2 , respectively. Fracture of babadua bars almost invariably occurred at the nodal points whenever present and, in such cases, the ultimate strength was found to be relatively low. The optimum moisture content was about 0.9%. In nodal-free specimens, the ultimate strength approached the maximum limit. 6.2. Load]deflection cur¨ es
Fig. 1. Typical experimental set-up.
central deflection measurements by means of dial gauges for all load increments.
4. Flexural theory The cracking moment M cr of a plain prismatic concrete section, based on its modulus of rupture is expressed as: Mcr s f t c bh2r6 where f t c denotes the modulus of rupture of the concrete, and b and h the width and overall depth of the beam, respectively. For a simply supported beam that is loaded at its third points, the ultimate flexural load Pult is given by:
Figs. 2 and 3 show typical load]deflection curves for the beams that were subjected to monotonic and cyclic loadings, respectively. Prior to cracking, the beams exhibited linear elastic characteristics, but as cracks initiated and subsequently developed, a relatively large domain of inelastic response generally developed. As cracking initiated a sharp increase in the observed deflections of the beams was due to the relatively low value of the modulus of elasticity of babadua Ž50]100 kNrmm2 . compared to that of steel Žapprox. 200 kNrmm2 .. 6.3. Cracking and failure loads The cracking, experimental failure, and theoretical flexural loads of the beams are given in Table 3. Theoretical failure loads estimated from fracture of tension bars were based on 100 Nrmm2 tensile strength of babadua with a partial factor of safety Žgm . of 3.
Pult s 6 MultrL where Mult denotes the ultimate moment of resistance of the beam and L the span of the beam.
5. Shear strength In accordance with the British Standard BS 8110:1985 method w9x the theoretical shear strength of the beams was calculated by considering: Ža. the concrete section alone; Žb. both concrete section and tension reinforcement; and Žc. the concrete section, the tension reinforcement, and the babadua stirrups.
6. Theoretical and experimental results 6.1. Material properties of the beams Details of the concrete and reinforcing babadua bars are given in Table 2. The tensile strength of babadua was obtained from tests performed on bars of diameter
Fig. 2. Load]deflection curves for babadua reinforced beams.
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
190
are given in Table 4, which also includes comparisons with the cracking and failure loads.
7. Discussion of test results 7.1. Modes of failure
Fig. 3. Load]deflection curve for beam B14 subjected to 10 cycles of loading prior to collapse.
6.4. Shear strength Details of the theoretical shear strength of the beams
In a simply-supported beam subjected to third-point loading the middle third of the span bears a constant maximum bending moment and zero shear force, while the remaining adjoining spans are subjected to maximum shear force and varying bending moment. Collapse of beams may occur either through flexural failure caused by the crushing of the concrete andror fracture of the tension bars, diagonal tension failure of the concrete, or shear-bond failure. The mode of failure, however, depends on the amounts of tension and shear reinforcement, the strength of the concrete, and the shear span-to-effective depth ratio. In the 23 beams tested to failure the tension reinforcement varied from 2.87 to 12.13% of the gross concrete section and the shear span-to-effective depth ratios ranged between 2.49 and 4.44. As expected collapse loads generally increased with the amount of
Table 3 First-crack and failure loads for beams Beam No.
Firstcrack load Ž Pcr .
Experimental failure load Ž Pult . ŽkN.
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 B23 Average:
6.0 6.5 5.5 5.5 8.5 8.5 11.5 8.0 8.0 10.5 9.5 7.0 9.0 11.5 6.0 12.0 4.4 11.0 7.5 11.0 12.0 5.5 6.0
18 22 30 34 28 30 42 36 30 52 34 32 40 62 28 46 17 43 37 38 44 29 27
Theoretical flexural strength ŽkN.
Pcr
Pcr
Pult
Based on concrete section alone X Ž Pult .
Including babadua bars in tension Y Ž Pult .
Pult
Pult
Pult
Pult
3.77 5.27 4.77 5.39 5.39 5.39 4.59 4.59 4.59 8.98 4.02 4.02 5.02 7.71 4.02 6.86 4.52 7.72 5.23 7.71 7.71 4.65 4.65
9.4 16.4 27.8 23.6 23.0 32.7 24.8 26.2 32.1 49.8 30.7 31.4 36.5 63.1 19.8 47.0 20.5 34.5 35.4 30.3 37.1 26.1 26.1
0.33 0.30 0.18 0.16 0.30 0.28 0.27 0.22 0.27 0.20 0.28 0.22 0.23 0.19 0.21 0.26 0.26 0.26 0.21 0.29 0.28 0.19 0.22 0.24
1.59 1.23 1.15 1.02 1.58 1.58 2.51 1.74 1.74 1.17 2.36 1.74 1.79 1.49 1.49 1.75 0.97 1.42 1.43 1.43 1.56 1.18 1.29 1.53
1.91 1.34 1.08 1.44 1.22 0.92 1.69 1.37 0.93 1.04 1.11 1.02 1.10 0.98 1.41 0.98 0.83 1.25 1.02 1.25 1.17 1.11 1.03
4.77 4.17 6.29 6.31 5.19 5.57 9.15 7.84 6.54 5.79 8.46 7.96 7.97 8.04 6.97 6.71 3.76 5.57 7.07 4.93 5.64 6.24 5.81 6.40
Y
Pult X
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
191
Table 4 Shear strength of beams Beam No.
Shear spanr effective depth ratio
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22a B23a Average:
2.76 2.80 3.04 2.69 2.54 2.54 2.61 2.49 2.49 2.61 2.75 2.72 2.71 2.72 2.97 2.91 2.97 2.91 2.72 2.90 2.89 4.44 4.44
a
Theoretical shear strength ŽkN.
Pcr
Pult
Pult
Pult
Without babadua Ž Ps1 .
Including babadua in tension Ž Ps2 .
Including babadua in tension and stirrups Ž Ps3 .
Ps1
Ps1
Ps2
Ps3
10.92 10.76 10.10 10.40 11.02 11.03 10.71 11.26 11.26 20.05 10.96 11.07 11.09 20.12 10.13 18.81 10.13 18.81 10.58 18.89 18.94 18.52 18.52
26.52 28.90 25.41 26.43 28.04 28.05 26.02 27.35 27.36 50.99 26.62 26.88 27.89 50.59 24.60 45.67 25.19 46.77 28.12 46.41 46.53 44.98 44.98
28.44 39.09 34.98 36.28 ] ] 36.17 ] ] ] ] ] ] ] ] ] ] ] ] 59.66 ] ] ]
0.55 0.60 0.54 0.53 0.77 0.77 1.07 0.71 0.71 0.52 0.87 0.63 0.81 0.57 0.59 0.64 0.43 0.58 0.71 0.58 0.63 0.22 0.22
1.65 2.04 2.97 3.27 2.54 2.72 3.92 3.20 2.66 2.59 3.10 2.89 3.61 3.08 2.76 2.45 1.68 2.29 3.50 2.01 2.30 1.51 1.51 2.62
0.68 0.76 1.18 1.29 0.99 1.07 1.61 1.32 1.10 1.02 1.28 1.19 1.43 1.23 1.14 1.01 0.67 0.92 1.32 0.82 0.93 0.62 0.62 1.05
0.63 0.56 0.86 0.94 ] ] 1.16 ] ] ] ] ] ] ] ] ] ] ] ] 0.64 ] ] ] 0.80
Beams tested under central-point loading.
tension reinforcement. As babadua is markedly flexible and ductile, collapse of the beams, even through the crushing of concrete or diagonal tension failure of concrete, was significantly gradual when the compression zone was also reinforced. Unlike in steelreinforced concrete which is generally designed preferably as under-reinforced with a limit to the amount of reinforcement to avoid brittle failures, relatively higher percentages of babadua reinforcement can be utilised in concrete members and yet obtain sufficient warning prior to collapse of members. Although theoretical analyses had indicated that the tension reinforcement would fail first in most of the beams, collapse occurred mostly through crushing of the concrete in the compressive zone after excessive deflection of the beams or, diagonal tension failure of the concrete in the shear span. This was due partly to the fact that the theoretical estimate of the tensile strength of babadua was based on the nodal strength which could be as low as 75 Nrmm2 , while the internodal strength could be as high as 200 Nrmm2 . Inspection of the collapsed beams revealed that the nodes in the reinforcing bars did not always coincide with the crack positions where bar stresses would be maximum.
A crack pattern of this kind would lead to higher flexural strengths of beams than the theoretical predictions. 7.2. Cracking and failure loads From Table 3 the cracking loads Pcr averaged 0.24 X Žthe Pult Žthe experimental failure load. and 1.53 Pult theoretical flexural strength of the concrete section alone.. Within the range of 6.6]7.6 of span-to-depth ratio, the ratio of cracking load to experimental failure load varied slightly between 0.16 and 0.33, and was independent of the amount of reinforcement and the age of concrete. Tables 3 and 4 show that the experX Žthe theoretimental failure loads Pult averaged 6.4 Pult ical flexural strength of the concrete section alone., 2.62 Ps1 Žthe theoretical shear strength of the concrete section alone., 1.05 Ps2 Žthe theoretical shear strength of the concrete including the effect of the tension reinforcement . and 0.80 Ps3 Žthe theoretical shear strength including the effects of tension reinforcement and babadua stirrups.. This indicates that babadua reinforcement made significant contributions to both flexural capacity and shear resistance of the concrete
192
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
beams. Results from beams B15, B16 and B17, B18 which were tested after 100 and 378 days, respectively, did not indicate any sign of deterioration of the babadua bars with time. The contribution of the babadua stirrups to resistance against collapse of beams B1 to B4, B7 and B20 was slight, and diagonal tension failure occurred in beams B3 and B20. This might be attributed to the lack of rigidity in babadua stirrups when tied by means of copper wires to the main bars as could be found in steel stirrups Table 5 provides information on mode of failure and crack patterns in the beams. The presence of several short-spaced cracks within the constant moment span in most cases indicates good bond between babadua bars and their surrounding concrete, probably due to the bearing action of the nodal points.
monotonic load-deflection path. This means that there was no significant deterioration of strength within the limited load repetitions carried out in the study. 7.4. Effect of creep Figs. 4a and b show the effect of sustained load of magnitude approximately equal to half its theoretical failure load Ž37 kN. on beam B21 for 30 days. The failure load appeared to be least affected by creep as the load]deflection curve could be seen to pick up its original path as collapse approached. This could mean that within the limited duration of sustained loading employed in this study the rigidity of the beam was minimally affected.
8. Conclusions
7.3. Effect of cyclic loading Load-deflection curves of the beams under cyclic loading formed hysterisis loops ŽFig. 3.. After ten load cycles, the curve appeared to re-trace its original
On average the experimental failure loads of the beams were approximately 2.7 times the theoretical shear strength of the concrete section alone. Collapse of beams occurred through either diagonal tension
Table 5 Crack pattern and mode of failure Beam No.
Mode of failure
No. and type of cracks at failure
Max. crack width Žmm.
B1 B2 B3 B4 B5 B6
Flexural in tension bar Flexural in tension bar Diagonal tension Flexural in concrete crushing Flexural in concrete crushing Diagonal tension
4 3 3 3 4 3
B7 B8 B9 B10
Bond failure Diagonal tension Flexural in concrete crushing Diagonal tension
B11
Bond failure Žspalling of concrete over node. Flexural in concrete crushing Diagonal tension Diagonal tension
2 flexural cracks 4 flexural cracks 1 diagonal shear q 4 flexural cracks 1 diagonal shear q 3 flexural cracks 1 flexural-shear q 3 flexural cracks 2 flexural-shear q 3 flexural q 1 short diagonal cracks 1 flexural-shear q 4 flexural cracks 1 diagonal shear q 4 flexural cracks 2 flexural cracks 1 diagonal shear q 1 short flexural-shear q4 flexural cracks 2 short flexural-shear q 3 flexural cracks
B12 B13 B14 B15 B16 B17
Flexural in concrete crushing Žtest after 100 days. Flexural in concrete crushing Diagonal tension
B18 B19 B20 B21
Flexural in concrete crushing Diagonal tension Diagonal tension Flexural in concrete crushing
B22 B23
Shear-bond Diagonal tension
2 3 5 4 2.5
1 flexural-shear q 5 flexural cracks 1 diagonal shear q 4 flexural cracks 3 diagonal shear q 2 flexural-shear q3 flexural cracks 2 flexural cracks Žtest after 100 days.
2 3.5 4
3 flexural cracks Žtest after 100 days. 1 diagonal shear q 1 flexural cracks Žtest after 378 days. 2 flexural cracks Žtest after 378 days. 4 shear q 4 flexural cracks 1 flexural-shear q 3 flexural cracks 1 flexural-shear q 3 flexural cracks Žsubjected to sustained loading. 1 diagonal q 2 flexural-shear cracks 1 diagonal q 6 flexural-shear cracks
3.5 5
4.5
4 4 3 5 5
C.K. Kankam, B. Odum-Ewuakye r Construction and Building Materials 13 (1999) 187]193
193
failure of the concrete in the shear span or flexural failure through crushing of concrete. Babadua stirrups provided additional strength to the beams. For a spanto-depth ratio of simply supported beams up to about 8.0, a short-term factor of safety of about 1.5 against cracking and one of approximately 6.4 against collapse were obtained from the results of the investigations. Long-term tests indicated no sign of deterioration of the babadua bars, and that the flexural strength and behaviour of babadua reinforced beams remained intact more than a year after. References
Fig. 4. Ža. Load]deflection curve for beam B21 under 20 kN sustained load for 30 days prior to collapse. Žb. Time]deflection curve for beam B21 under 20 kN sustained loading.
w1x Kankam JA, Ben-George M, Perry SH. Bamboo-reinforced beams subjected to third-point loading. ACI Struct J 1988;85Ž1.. w2x Kankam CK. Raffia palm-reinforced concrete beams. J Mater StructrMateriaux Construct ŽRILEM. 1997;30. w3x Manzur MA, Aziz MA. A study of jute fibre reinforced cement composites. Int J Cement Compos Lightweight Con 1982; 4:75]82. w4x Andonian P, Mai YM, Cotterell B. Strength and fracture properties of cellulose fibre reinforced cement composites. Int J Cem Compos Lightweight Con 1979;1:151]158. w5x Kankam CK. The influence of palm stalk fibre reinforcement on the shrinkage stresses in concrete. J Ferrocement 1994;24Ž3.. w6x Irvine FR. Woody plants in Ghana. Oxford University Press, 1961 w7x Lyman Benson. Plant classification. Published by D.C. Heath and Company, 1965. w8x Schreckenbach H, Abenkwa JGK. Construction Technology for a Tropical Developing Country. Published by German Agency for Technical Co-operation ŽGTZ. for the Department of Architecture, University of Science and Technology, Kumasi, Ghana, 1982. w9x British Standards Institution. Structural use of concrete. BS 8110:Part 1:1985. Code of Practice. London, 1985.