Journal of Science & Technology Vol. (15) No.(1) 2010 JST 3
Flexural Behavior of Lightweight Ferrocement Sandwich Composite Beams Mahmoud Abo El Wafa + Kimio Fukuzawa ++
Abstract
The structural behavior of lightweight ferrocement (LWF) sandwich composite beams is investigated by conducting flexural tests on eight simply supported rectangular beams under three point loads. The main objective of the study is to investigate the comprehensive comparisons of eight beams represented in six lightweight ferrocement (LWF) beams and two reinforced concrete (RC) beams. The performance of the LWF and RC beams is investigated in terms of crack load, load-deflection curves, stiffness, energy absorption capacity, ductility index, ultimate flexural load-to-weight ratio, load-strain curves, crack patterns, number of cracks, average crack width, crack spacing and the failure mode. The method outlined by ACI Building Code is used to compute ultimate moment capacities. The test results reveal the remarkable enhancement in the flexural behaviour and potential application of lightweight ferrocement (LWF) sandwich composite beams to produce lightweight structural elements as compared to that of the reinforced concrete (RC) beams, which leads towards the industrialization of building system and meets with innovation and responsible application of concrete construction technology results in better efficiency of the composite.
Keywords: Lightweight ferrocement, sandwich composite, wire mesh ferrocement reinforcement, polystyrene foam block, cracks, ultimate moment capacities. 1. Introduction The development and construction of lightweight sandwich structural elements in building construction is a growing trend in the construction industry all over the world due to its high strength to weight ratio, reduced weight, and good thermal insulation characteristics [1,2]. They have been used in the aerospace industry for many years and more recently they are being used as load bearing members in naval structures [3]. Presently, they have gained attention as an effective structural form in the building and construction industries. Sandwich elements are structural members made up of two, stiff, strong skins separated by a lightweight core. The separation of the skins by the lightweight core increases the moment of inertia of the element with little increase in weight, meaning a highly efficient component for resisting bending [4]. The introduction of novel technique of lightweight sandwich composite element presents new possibilities in the design of sandwich construction. This can be done by encasing lightweight polystyrene foam blocks as core with high performance ferrocement wire mesh reinforcement as skin layers and then completely encapsulated in high flowable high strength cement grout mortar. Polystyrene foam block is the generic industry name for a white rigid material made by expanding polystyrene beads with steam and bonding the beads together under pressure in a lightweight block or shape mold to achieve closed cell foam that resists moisture. +++Visitor, Dept. of Civil Eng., Faculty of Science and Engineering, University of Science & Technology, Sana’a, Yemen, Lecturer, Dept. of Civil Eng., Faculty of Eng.-Aswan, South Valley University, Egypt E-mail: [email protected]. ++++++Professor, Department of Urban and Civil Engineering, Ibaraki University, Hitachi, Japan.
Journal of Science & Technology Vol. (15) No.(1) 2010 JST 4 The polystyrene foam block insulation is preferred by more architects, designers, builders, contractors, and building & homeowners than any other foam brand. It is valued because of its excellent water resistance, high insulation value and superior compressive strength. moreover, it is considered one of the most economical materials and can be cut easily into an unlimited variety of shapes, it is lightweight and easy to handle and stays true to shape and size without any gaps or holes. Ferrocement is one of the relatively new cementitious composite considered as a construction material. It is a type of thin walled reinforced concrete commonly consisting of cement mortar reinforced with closely spaced layers of continuous and relatively small wire mesh [5]. The closely spaced and uniformly distributed reinforcement in ferrocement transforms the otherwise brittle material into a superior ductile composite. Thus, ferrocement technology is becoming more attractive to housing construction, particularly for roofs, floors, slabs and walls because of its relatively low cost, durability and weather resistance [6,7]. Its versatility further increases its utility for producing prefabricated components required in housing. The fabrication technique of ferrocement is easy to learn and ferrocement structures, if properly built, are practically maintenance free. It has advantageous properties such as strength, toughness, water tightness, lightness, durability, fire resistance, and environmental stability which cannot be matched by any other thin construction material [6 9]. The ferrocement is the promising composite material for prefabrication and industrialization of the building industry [1,2]. Studies conducted on ferrocement as encasement for structural strengthening [10 13] have also shown great promise. Studies also conducted on the contribution of ferrocement wire mesh reinforcement, with various sizes and volume fractions in direct tension, have also shown great improvement on the service and ultimate tensile crack behavior of the composite plates after sufficient warning, as well as multi cracking stage elongated and a distinct failure stage [14,15]. Cement grout mortar is designed for critical use where high strength, non staining characteristics and positive expansion are required. It contains only natural aggregate and an expansive cementitious binder. It is extremely viscous, and when cured, appears similar in appearance to concrete. The cement grout mortar has numerous advantageous properties such as non staining natural aggregate for better appearance, non shrink performance for excellent bearing, flowable and self leveling, high strength, appearance of plain concrete, and does not contain any added chloride ions. A number of research work has been reported on the use of ferrocement as facing or skin over different lightweight materials for a variety of applications. Most recently, its application as core material in FRP AAC (Aerated autoclaved concrete blocks) sandwich panels has been reported [1,2]. However, the literature is silent and the authors are aware of no systematic investigation reported yet to produce a lightweight sandwich composite with the novel technique; by encasing lightweight polystyrene foam blocks as core with high performance ferrocement wire mesh reinforcement as skin layers and then completely encapsulating this in high flowable high strength cement grout mortar. Therefore, the main objective of this investigation is to explore the comprehensive comparisons of flexural behaviour of eight simply supported rectangular beams represented in six lightweight ferrocement (LWF) beams and two reinforced concrete (RC) beams under three point loads. The method outlined by ACI Building Code is used to compute ultimate moment capacities of LWF and RC beams. The results obtained using this method are compared with corresponding experimental results of the RC and LWF beams.
Journal of Science & Technology Vol. (15) No.(1) 2010 JST 5 2. Experimental Program The main objective of the study is to investigate the comprehensive comparisons of eight beams representative in six lightweight ferrocement (LWF) beams and two reinforced concrete (RC) beams. The tests of the LWF beam specimens used for the flexural behaviour study have the dimension of the cross section as 300 x 200 mm & 300 x 250 mm and compared with RC beams have the dimension of the cross section 300 x 200 mm. All beam specimens were loaded in flexure under a three point loading system, as simply supported beam with center point loads over the span length of 1800 mm up to failure. The details of RC and LWF beams are listed in tabular form in Table 1 and their setup configurations are shown in Figure 1.
Table (1): Details of RC and LWF beams
Web Beam Cross Section Tension Reinforcement Compression Unit Weight Beam Specimens Reinforcement b x d (mm) Area (mm 2) (%) Reinforcement (Density t/m 3) (Stirrups)
RC Beams RC 200 3D13 300 x 200 3D13 mm 398.2 mm 2 0.76 % 2D10 mm 8 Str. D8 mm 2.5
LWF 200 4 300 x 200 4 Layers 129.2 mm 2 0.25 % 3 Layers 3 Layers 1.06
LWF Beams LWF 200 5 300 x 200 5 Layers 161.5 mm 2 0.31 % 3 Layers 3 Layers 1.06
LWF 200 6 300 x 200 6 Layers 193.8 mm 2 0.38 % 3 Layers 3 Layers 1.06
RC Beams RC 200 4D13 300 x 200 4D13 mm 530.9 mm 2 1.01 % 2D10 mm 8 Str. D8 mm 2.5
LWF 250 4 300 x 250 4 Layers 129.2 mm 2 0.19 % 3 Layers 3 Layers 1.02
LWF Beams LWF 250 5 300 x 250 5 Layers 161.5 mm2 0.24 % 3 Layers 3 Layers 1.02
LWF 250 6 300 x 250 6 Layers 193.8 mm 2 0.29 % 3 Layers 3 Layers 1.02 P
2D10 D8 200 mm 3D13
50 mm 1700 mm 50 mm 300 mm 1800 mm Beam cross section RC Beams P 3 Layers 25 mm 3 Layers
25 mm 20 mm 275 mm 20 mm 250 mm 200 mm 3 150 mm Polystyrene 150 mm Layers Polystyrene 200 mm foam block 3 Layers foam block 25 mm 5 Layers 5 Layers 50 mm 50 mm 1700 mm 300 mm 1800 mm Beam cross section LWF Beams
Fig. (1): Setup configurations of RC and LWF Beams .
Journal of Science & Technology Vol. (15) No.(1) 2010 JST 6 Figure 2 shows the more clearly details of the arrangement of ferrocement wire mesh reinforcement layers of the cross section of (LWF 200 5) beam that introduce the novel technique of lightweight sandwich composite by encasing lightweight polystyrene foam blocks as core with high performance ferrocement wire mesh reinforcement as skin layers and then completely encapsulated in high flowable high strength cement grout mortar. The performance of the LWF and RC beams is investigated in terms of first crack load, load deflection curves, stiffness, energy absorption capacity, ductility index, ultimate flexural load to weight ratio, load strain curves, crack patterns, number of cracks, average crack width, crack spacing and the failure mode. 25 mm 3Layers 25 mm
25 mm
3Layers Polystyrene foam block 200 mm 187.5 250 x 150 mm Cement grout mortar
25mm
5 Layers 300mm
Fig. (2): Details of the cross-section of (LWF-200-555) .beam 2.1 Material Properties The materials used in casting the lightweight ferrocement (LWF) beams were cement grout material (free flowing powder with light gray color and containing well graded aggregates, special hydraulic cement, and special cementitious binder which were designed to be mixed with water directly), polystyrene foam blocks see Photo 1, and ferrocement stainless steel wire mesh reinforcement see Photo 2. The materials used in making the reinforced concrete (RC) beams were ordinary Portland cement, fine & coarse aggregate and steel bars. The characteristics of the ingredients and proportions of the materials (based on laboratory tests) used in casting all beam specimens are summarized in Table 2.
Photo (1): Polystyrene foam blocks. Photo (2): Ferrocement stainless steel wire mesh reinforcement.
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Table (2): Material characteristics & proportions.
Beam specimens Materials Properties
Portland cement Specific gravity 3.15
Fine aggregate (crushed sand stone) N. M.S 2.5 mm, Sp.gr. 2.63, F.M. 2.82 Reinforced concrete Coarse aggregate (crushed sand stone) N. M.S 20 mm, Sp.gr. 2.64, F.M. 6.74 (RC) beams
Tension reinforcement bars D = 13 mm, fy = 360 MPa, f u = 510 MPa, Es = 205 GPa
Concrete Mix Proportions 1:2.0:2.4, w/c = 0.45
Cement grout Specific gravity 2.14
Polystyrene foam blocks Block sizes used 150x250x275 mm & 200x250x275 mm Lightweight ferrocement (LWF) beams Ferrocement stainless steel wire mesh Wire diameter 0.9 mm, Mesh opening 5.45 mm, reinforcement fy = 550 MPa, fu = 870 MPa, Es = 200 GPa
Mortar Mix Proportions Water/Cement grout = 0.14 2.2 Fabrication and Casting The beam specimens were cast in steel molds consisting of a rigid base and two sides. In order to produce the LWF beam test specimens, ferrocement wire mesh reinforcement and cement grout mortar mixture were prepared. The following procedures were adopted to achieve a uniform layer in the LWF beam test specimens. First, 10 mm thick timber edges were fixed to the steel base and then ferrocement wire mesh reinforcement layers were placed with spacers to produce 5 mm cover. The high flowable high strength cement grout mortar was placed to encase the tension reinforcement of ferrocement layers. Then the polystyrene foam blocks were placed. A great deal of care was taken to ensure that the polystyrene foam blocks that had smaller density does not float up. Therefore, 10 mm thick timber strips were fixed on the polystyrene foam blocks. Finally, the top layers of ferrocement wire mesh reinforcement were placed and then encased by cement grout mortar. The molded specimen was further vibrated to obtain bonding between the layers. Photo 3 shows the steps of fabrication and casting of the LWF beam test specimens. The cylinder test specimens were prepared at the same time to get the hardened properties of cement grouted mortar and concrete that used in casting the LWF and RC beam specimens, respectively. The LWF and RC beam specimens were covered with wet clothes and plastic sheeting. The formwork was dismantled three days after pouring. Water was sprinkled twice a day to keep the specimens moist for 7 days and then left to cure under laboratory ambient conditions until the day of testing. 2.3 Testing Methodology The cylinder test specimens of sixteen cases, with three identical specimens of each case were conducted after 28 days of curing for cement grout mortar (φ50x100 mm) and concrete (φ100x200 mm) that used in casting the LWF and RC beam specimens, respectively, to obtain the hardened properties in terms of compressive strength, splitting tensile strength and Young’s modulus. The testing of LWF and RC beam specimens in flexure were also conducted after 28 days of curing. The three point flexural test was conducted according to the requirements of standards methods of testing structural beams in flexure [5,16]. The load was applied using the universal testing machine of capacity 200 kN in uniform increments of 5 kN up to failure.
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(3 1) (3 2)
(3 3) (3 4)
Photo (3): Fabrication and casting steps of the LWF beam test specimens.
The mid span deflection was continually recorded using a linear variable displacement transducer (LVDT) placed under the centre point load of the beam specimens, as well as, electrical strain gauges were attached in tension and compression side of beam specimens, as shown in Figure 3. At each load stage, the readings of deflection and strains were recorded automatically using a Data Logger connected to a computer. The crack pattern was also noted at each load stages. Cracks were marked on the surface of the beam specimens. The number of cracks, crack spacing, extent of the cracked zone over the length of the beam specimens and the ultimate load were all noted. General structural behavior of the beam specimens was carefully observed during the load application. The failure load is identified when excessive cracking occurs at the bottom, the applied load drops and the deflection increases.
P
Electrical strain gauge for Electrical strain gauge for strain in tension side strain in compression side
LVDT for mid span deflection 50 mm 1700 mm 50 mm 1800 mm
Fig. (3): Setup configurations for deflection and strain measurements of LWF and RC beams.
Journal of Science & Technology Vol. (15) No.(1) 2010 JST 9 3. Results and Discussion
3.1 Hardened Properties Table 3 presents the 28 days average values of sixteen cases, with three identical specimens of each case of hardened properties in terms of compressive strength, splitting tensile strength and Young’s modulus from the tests conducted on concrete and cement grout mortar which was used in casting the RC and LWF beam specimens, respectively.
Table (3): Hardened properties of concrete and mortar used in the RC and LWF beams.
Compressive Splitting Tensile Young’s Beam Specimens Strength Strength Modulus (MPa) (MPa) (GPa) RC Beams RC 200 3D13 38.0 3.9 28.0
LWF 200 4 78.6 4.3 31.0
LWF Beams LWF 200 5 79.3 4.3 31.2
LWF 200 6 76.2 4.4 30.6
RC Beams RC 200 4D13 36.0 3.9 27.4
LWF 250 4 79.5 4.3 31.8
LWF Beams LWF 250 5 75.1 4.4 30.2
LWF 250 6 76.4 4.3 30.9
3.2 Flexural Load–Deflection Behavior of LWF & RC Beams Figure 4 shows the comprehensive comparisons reqarding behaviour of load curves of the LWF & RC beams under central point flexural load. 100 100 RC 200 4D13 LWF 250 6 LWF 250 5 80 80 LWF 200 -6 LWF 250 4 RC 200 3D13 LWF 200 5 60 60 LWF 200 4
40 40 Load (kN) Load (kN) 20 20 LVDT 50 mm for measuring LVDT 50 mm for measuring mid span deflection mid span deflection 50 1700 mm 50 50 1700 mm 50 1800 mm 1800 mm 0 0 0 10 20 30 40 50 0 10 20 30 40 50 Deflection (mm) Deflection (mm)
Fig. (4): Load-deflection behavior of LWF and RC beams. Journal of Science & Technology Vol. (15) No.(1) 2010 JST 10
As can be seen from Figure 4, the load versus mid span deflection relationship indicates three specific regions; a linear region to yield (precracking stage), a transition region of continuous yield (multiple cracking stage), and a region of full plastic deformation until failure (post cracking stage). Based on the investigations, during the precracking stage, both materials (reinforcing system & mortar (concrete) matrix) made of LWF and RC beams are elastic, and the load– deflection curve is linear with the applied load. It is observed that up to first cracking, all beams have comparable pre cracking stiffness (initial stiffness) depending on reinforcing system as numbers of ferrocement layers in LWF beams and numbers of the steel bars in RC beams. However, each beam has cracked at a different load level. Subsequently, during the multiple cracking stages, the contribution of the cement grout mortar of LWF beams and concrete matrix of RC beams in tension zone may be neglected. During this stage a transfer of load from cement grout mortar to the wire mesh layers in LWF beams and from concrete matrix to the steel bars in RC beams takes place. The reinforcing system leads to redistribution of stresses beyond the cracks under additional flexural load, thus transferring the load back to the mortar (concrete) matrix, thereby producing new cracks. Afterward, the failure stage is indicated by intensification of existing cracks with the increase of flexural load. Here the load is carried mainly by the reinforcing system (ferrocement layers in LWF beams and steel bars in RC beams). The beam specimens undergo rapid increase in deflection, and cracks can extend deep through the depth of the beam specimens, reaching the top surface of the beam. Then, the failure load is identified when excessive cracking occurs at the bottom, the applied load drops and the deflection increases. Table 4 shows the comparison of flexural behavior results between the LWF and RC beam specimens based on the investigations of flexural load–deflection behavior of LWF & RC beams. The results show that the LWF beams exhibit better performance and have a more favorable advantage in achieving the improvements on pre cracking stiffness (initial stiffness), load carrying capacity, energy absorption capacity (area under load deflection curve), ductility index and a higher ultimate flexural load to weight ratio compared with RC beams. The test results revealed the remarkable enhancement in the structural behavior and potential application of lightweight ferrocement (LWF) beams to produce lightweight structural elements as compared to that of the reinforced concrete (RC) beams which lead towards the industrialization of building system and meets with innovation and responsible application of concrete construction technology results in better efficiency of the composite.
Table (4): Comparison of flexural behavior results between the RC and LWF beams.
Deflection at Deflection at Ultimate Flexural First Crack Load Ultimate Load Energy Absorption Ductility Index (P /P ) First Crack Ultimate (δ /δ ) Load to Weight Beam (P ) (P ) cr u cr u Capacity (δ /δ ) cr u (δ ) (δ ) u cr Ratio Speciemns Ratio cr u Ratio Measured Relative Measured Relative (%) Measured Relative Measured Relative (%) Measured Relative Relative Relative Measured Measured (kN) (%) (kN) (%) (mm) (%) (mm) (%) (kN.mm) (%) (%) (%) RC 200 3D13 25 100 65 100 38 0.6 100 21 100 2.9 932 100 35 100 26 100
LWF 200 4 15 60 49 75 31 0.9 159 44 213 2.1 1412 151 47 134 46 178
LWF 200 5 15 60 60 92 25 0.8 128 43 206 1.8 1601 172 56 161 57 218
LWF 200 6 20 80 72 111 28 0.7 119 38 186 1.8 1767 190 54 156 68 261
RC 200 4D13 30 100 91 100 33 0.3 100 8 100 3.0 502 100 33 100 36 100
LWF 250 4 25 83 80 88 31 0.9 354 48 575 1.9 2506 499 54 163 78 218
LWF 250 5 30 100 82 90 37 0.9 348 30 360 2.9 1674 333 34 104 80 223 LWF 250 6 30 100 87 96 34 0.7 276 25 305 2.7 1481 295 37 111 85 237
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3.3 Crack Characteristics of LWF & RC Beams Photo 4 shows the comprehensive comparisons behaviour of crack patterns of the LWF & RC beams under central point flexural load. It is observed that the development and propagation of crack patterns up to first cracking of all LWF & RC beams have comparable pre cracking stiffness and each beam has cracked at a different load level which initiated on the bottom surface of the LWF & RC beams and spread vertically upward. Further increase in flexural load increased the number of cracks and reached the load point. It is also noticed that all LWF beams have hairy cracks. The formation of multiple hairy cracks reflects the enhanced ductility of the LWF beams compared with RC beams.
RC 200 3D13 RC 200 4D13 91 91 91
LWF 250 4 LWF 200 4 35 35 49 49 49 49 49 35 35 35 35 49
LWF 200 5 LWF 250 5 60 60 60 60 60 60 60 60 50 50 50 50 50 50 50 50 50 50 60 60 50 50 50 50 50 50 50 40 50 40 40 40 40 40 40 40 40 40 40 40 50 30 40 40 40 40 30 30 40 40 40 30 30 20 30 30 30 30 30 40 30 20 20 30 15 20 LWF 250 6 87 LWF 200 6 70 70 70 70 70 60 60 70 60 60 60 60 60 60 60 50 60 50 50 50 50 50 50 50 50 50 50 50 50 60 40 50 50 40 50 40 55 40 40 40 50 40 40 40 40 40 40 40 30 40 Photo (4): Behavior of crack patterns of the LWF & RC beams. Table 5 illustrates the comprehensive comparisons behaviour of the crack characteristics of LWF & RC beams in terms of number of cracks, average crack spacing, average crack width and the failure mode. It is clearly seen from investigation of the results of Table 5 that the RC beams have a limited number of cracks that are distributed with larger crack width and spacing. While in the LWF beams, the cracks that developed are much finer, hairy cracks with smaller crack width and spacing compared with RC beams. The LWF beams referred to as high performance composite that developed multiple hair cracks under flexural load in contrast to a limited number of cracks of RC beams. Multiple hairs cracking provides a mean of energy dissipation and prevent catastrophic fracture failure at the structural level, thus contributing to structural safety and service behaviour. This advancement is very noticeable in the LWF beams that exhibit better performance and reveal the remarkable enhancement of the LWF beams as compared to that of the RC beams.
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Table (5): Crack characteristics of the RC and LWF beams.
Average Crack Average Crack Number of Beam Specimens Spacing Width Failure Mode Cracks (mm) (mm)
RC Beams RC 200 3D13 7 120 2.0 Flexural Tension
LWF 200 4 34 32 1.2 Flexural Tension
LWF Beams LWF 200 5 32 30 1.2 Flexural Tension
LWF 200 6 38 25 1.0 Flexural Tension
RC Beams RC 200 4D13 7 150 1.8 Flexural Tension
LWF 250 4 26 25 1.2 Flexural Tension
LWF Beams LWF 250 5 33 20 1.0 Flexural Tension
LWF 250 6 36 18 1.0 Flexural Tension
3.4 Load–Strain Behavior of LWF & RC Beams Figure 5 shows the comprehensive comparisons behaviour of load strain curves in the tension and compression zone of the LWF & RC beams under central point flexural load. The load strain relationship of the LWF & RC beams was linear up to first crack load. It is observed that the strains in tension and compression zone were in good linear trend up to first crack and have comparable strain behaviour depending on the reinforcing system as numbers of ferrocement layers in LWF beams and numbers of steel bars in RC beams. However, each beam has cracked at a different load level. Further, increase in flexural load, the strains in tension and compression zone of the LWF & RC beams after cracking were affected by the location of the cracks within the LWF & RC beams. The load strain curves indicated the advantage behavior of LWF beams as compared to that of the RC beams. 100 100 RC 200 4D13 Electrical strain gauge for measuring Electrical strain gauge for measuring LWF 250 6 strain in compression zone Electrical strain gauge for measuring 90 strain in compression zone Electrical strain gauge for measuring 90 strain in tension zone strain in tension zone LWF 250 4 50 1700 mm 50 80 50 1700 mm 50 80 1800 mm LWF 200 -6 1800 mm 70 70 60 60 50 50 LWF 200 -6 LWF 250 5 40 40 Load (kN) Load (kN) 30 LWF 200 5 30 LWF 250 4 LWF 200 4 RC 200 4D13 20 20 10 10 Strain in compression zone Strain in tension zone Strain in compression zone Strain in tension zone 0 0 5000 3000 1000 1000 3000 5000 5000 3000 1000 1000 3000 5000 Strain ( ε*10 6) Strain ( ε*10 6) Fig. (5): Load–strain behavior of LWF & RC beams.
Journal of Science & Technology Vol. (15) No.(1) 2010 JST 13 In fact the composite action (numbers of ferrocement layers in LWF beams) is considered as a major parameter along with the strength, when LWF beams are in consideration in regards to structural elements for design. This concluded the feasibility of the novel technique by encasing lightweight polystyrene foam blocks as the core with high performance ferrocement wire mesh reinforcement as skin layers and then completely encapsulated in high flowable high strength cement grout mortar to act as a composite to produce lightweight structural elements as compared to that of the reinforced concrete RC beams.
3.5 Prediction of Ultimate Moment Capacity by ACI Building Code The adoption of conventional reinforced concrete theory using the ACI Building Code (ACI Standard 318 05) [16] has been recommended by ACI Committee 549 (1988) [5] to compute ultimate moment capacities of the LWF & RC beams under bending. Figure 6 illustrates the details of actual and idealized sections used for the calculation of LWF beams. b b t t y Polystyrene foam y block d h d h bw bw y y t t
Tension reinforcement (As) Tension reinforcement (As) (Actual section) (Idealized section used for the calculation)
Fig. (6): Details of actual and idealized sections of LWF beams
b ' εc 0.85fc .c
c 1 Cc N.A β /2) .c
h d β (d
Ts εs Tension reinforcement (As) Strain dist. Stress dist. (Idealized section) (Stress and strain dist. at ultimate)
Fig. (7): Stress and strain distribution at ultimate in a ferrocement section under bending. Figure 7 shows the stress and strain distribution at ultimate in a ferrocement section under bending. The analysis takes into account the effective cross sectional area and position of the reinforcing layers with respect to the neutral axis. Subsequently, during the multiple cracking stages, the contribution of the cement grout mortar (concrete) in tension is neglected and the tension reinforcement is assumed to take the total tensile force.
Journal of Science & Technology Vol. (15) No.(1) 2010 JST 14 It can be determined the distance from the extreme compression fiber to the neutral axis c by tedious trial and error computations. Begin by assuming a value for c. If this estimated distance from the extreme compression fiber to the neutral axis is correct, then the summation of all compressive forces should equal the summation of all tensile forces. This is check on the accuracy of the assumed distance. If this condition is not met, another assumption must be made for the correct distance, the internal forces recalculated, and the accuracy rechecked. After a number of trials, it can be found the value of c for which equilibrium is satisfied, then, the ultimate moment capacities Mu are finally determined (ACI Committee 549, 1988) [5]. The ultimate moment capacities (Mu) of the LWF & RC sections can be expressed as:
' β c M = 85.0 f bβ c(d − 1 ) ……… (1) u c 1 2
ρ ⋅ f y c = ' d …..…. (2) 85.0 fc β1
Where :
' fc : is the compressive strength of the mortar (concrete),
f y : is the yield strength of the tension reinforcement,
ρ : is the reinforcement ratio (ρ = As bd ), c is the depth of neutral axis,
β1.c is the depth of the equivalent rectangular stress block and the value of the stress block depth factor β1 is [16]:
' 85.0 for18 < fc ≤ 28 MPa ' ' β1 = 85.0 − 05.0 ()145 fc − 4000 1000 for28 < fc ≤ 56 MPa …… (3) 65.0 ' forfc > 56 MPa Table 6 compares the predicted ultimate moment capacities using the method outlined by ACI Building Code with corresponding experimental results of the RC and LWF beams.
Table (6): Comparisons between experimental and predicted ultimate moment capacities.
Experimental Computed Exp./Computed Moment Capacity Moment Capacity Beam Specimens Moment Ratio (M u Exp. ) (M u Compute ) (M u Exp. /M u compute ) (kN.m) (kN.m) RC Beams RC 200 3D13 27.63 25.82 1.07
LWF 200 4 20.83 20.86 1.00 LWF Beams LWF 200 5 25.50 25.90 0.98 LWF 200 6 30.60 30.87 0.99 RC Beams RC 200 4D13 38.68 33.88 1.14 LWF 250 4 34.00 26.49 1.28 LWF Beams LWF 250 5 34.85 32.88 1.06 LWF 250 6 36.98 39.32 0.94
Journal of Science & Technology Vol. (15) No.(1) 2010 JST 15 On the average, it is seen that the formula of ACI Building Code [16], which recommended by ACI Committee 549 [5], provides satisfactory predictions of ultimate moment capacities with corresponding experimental results of the RC and LWF beams in flexure. This consistency between the experimental and predicted values may encourage research into further information on the structural properties of the LWF beams. 4. Conclusions From the results presented and discussed, the following conclusions can be drawn: 1. The LWF beams exhibit better performance and have a more favorable advantage in achieving the improvements on pre cracking stiffness, load carrying capacity, energy absorption capacity, ductility index and a higher ultimate flexural load to weight ratio compared with RC beams. 2. The LWF beams referred to as high performance composite, developed multiple hair cracks under flexural load in contrast to the limited number of cracks of the RC beams. 3. The formation of multiple hairy cracks reflects the enhanced ductility of the LWF beams compared with RC beams. 4. Multiple hairs cracking provides a mean of energy dissipation and prevent catastrophic fracture failure at the structural level, thus contributing to structural safety and service behavior. 5. The strains of the LWF & RC beams in tension and compression zone were in good linear trend up to first crack and have comparable strain behavior after cracking which was affected by the location of the cracks, depending on the reinforcing system of the LWF & RC beams. 6. The ACI Building Code 318 77 produced conservative predictions of ultimate moment capacities of LWF beams. 7. The LWF beams revealed the remarkable enhancement in the structural behavior and potential application of lightweight sandwich ferrocement polystyrene foam composite as compared to that of the RC beams. This leads towards the industrialization of building system and meets with innovation and responsible application of concrete construction technology which results in better efficiency of the composite.
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