FLEXURAL BEHAVIOUR OF FERROCEMENT WITH PVA FIBRE REINFORCED HIGH-STRENGTH MORTAR

Transcription

1 FLEXURAL BEHAVIOUR OF FERROCEMENT WITH PVA FIBRE REINFORCED HIGH-STRENGTH MORTAR Rasiah Sriravindrarajah and Micheal Alvaro, Centre for Built Infrastructure Research, University of Technology Sydney, Australia, Alvaro Bros Builders, Strathfield, Australia Abstract: Ferrocement is a composite material, consisting of mortar reinforced with continuous steel-wire mesh layers. The strength, stiffness, deflection and cracking characteristics of the ferrocement composite in flexure are affected by a number of factors, namely member thickness, span length, quantity and distribution of the wire-mesh reinforcement and mortar quality. This paper reports the results of an experimental investigation into the flexural behaviour of 100mm by 500mm by 19mm ferrocement slabs with four layers of welded wire mesh reinforcement embedded in a high-strength (28-day compressive strength of 80MPa) mortar. Uncoated PVA fibres, having either 6mm or 12mm in length, with a fixed volume fraction of 0.25% were incorporated in the mortar matrix. The results showed that ferrocement with high-strength mortar is a highly ductility composite material and the addition of PVA fibres in mortar had improved the failure load of ferrocement but significantly modified the flexural behaviour of the ferrocement by reducing the postfailure load ductility. Although the increased in the PVA fibre length from 6mm to 12mm had improved the failure load of the ferrocement, it had reduced the post-failure load ductility. INTRODUCTION Ferrocement composite consists of multiple layers of closely spaced either woven or welded steel wire-mesh reinforcement embedded in mortar. Research had shown that the ferrocement has significant crack resistance and suitable to produce thin structural elements [1-3]. It is widely accepted as a suitable construction material for the construction of water storage tanks, silos, bus shelters and sunshades due to easy fabrication, using locally available materials [4]. Similarly, steel fibre reinforced mortar showed improved resistance to cracking [5-6]. The quality of mortar matrix influences the behaviour of ferrocement through its tensile strength, stiffness and bond strength with steel. Traditionally, the compressive strength of mortar is about 30 to 40 MPa and with the availability of high-range water reducing admixtures, production of highly workable high-strength mortar of 100MPa or more is feasible. However, due to reduced ductility of mortar with the increase in its strength, the use of ductile fibres in mortar matrix is considered as a solution to produce high-strength ferrocement with high ductility and energy absorption capacity. Polyvinyl alcohol (PVA) fibre is considered as a suitable fibre in cementitious composites, in place of hazardous asbestos fibres, due to its high aspect ratio, high tensile strength and chemical durability [7]. However, its performance in PVA fibre-reinforced composites (volume fractions of 0.4% and 1.2%), with respect to strength, energy absorption and 111

2 cracking, was found to be not significant when compared with steel fibre reinforced composite [8]. However, research had shown that a high performance ductile fibre composite, known as engineered cement composite (EEC), could be developed with the use of high volume (volume fraction of 2%) PVA fibres. EEC found to have very high ductility, low permeability and self-healing capability of cracks [9, 10]. The PVA fibres used was coated with a proprietary hydrophobic oiling agent 1.2% by weight to tailor the interfacial properties between fibre and cement matrix for strain-hardening performance. Recent research at the University of Technology Sydney showed that uncoated PVA fibres (volume fractions of 0.25% and 0.50%) had improved the mechanical properties if fibre-reinforced concrete with no significant effect on damping characteristics [11] of concrete. In this investigation, the performance of ferrocement slabs, having the dimensions of 100mm by 500mm by 19mm, was studied under increasing flexural loading until failure. All the test slabs had four layers of welded-wire steel mesh reinforcement, with or without uncoated PVA fibres. The 28-day compressive strength of mortar component in the ferrocement was about 80MPa and the PVA fibre length was either 4mm or 8mm with a fixed fibre volume fraction of 0.25%. EXPERIMENTAL INVESTIGATION Materials, mix proportion and mixing of mortar The high-strength mortar mix (80 MPa) was produced by mixing a fixed proportions of general purpose cement, river sand, silica fume, water and superplasticiser. The mix proportion was chosen from the mixes used by Toutanji and El-Korrchi [12] in their study on mortar. The mix proportion was 1 : 1.40 : 0.16 : 0.32 : 0.02 (cement : river sand : silica fume : water : superplasticiser). Fine river sand, passing 2.36mm sieve and condensed silica fume (silica content over 92%) were used. Modified polycarboxilic ether based superplasticiser used was complying with EN 934 : Part 2, to achieve workable mortar mix. Monofilament uncoated PVA fibres, either 6mm or 12mm in length, was used in two fibre mortar mixes, having the volume fraction of the fibre of 0.25%. The specific gravity and diameter of the PVA fibres were 1.29 and 0.014mm, respectively. The tensile strength of the fibre was 1500MPa with the elongation of 7%. The modulus of elasticity of the PVA fibre was 41.7GPa which is close to that for the mortar. Both the control and fibre reinforced mortar mixes were produced in a power driven Hobart 7-litre mixer. The required quantity of superplasticiser for the mortar mixes were premixed with water before adding to the dry mix. The mixing of mortar was continued until a uniform mixing was achieved. Reinforcement Welded steel wire mesh used in the preparation of ferrocement specimens had 12mm spacing and the diameter of wire was 1mm. Young s modulus of elasticity of steel was 200GPa. Making and curing of test specimens For each mortar mix, three 100mm diameter by 200mm high cylinders and one 150mm diameter by 300mm high cylinders were cast in steel moulds. A vibrating table was employed to achieve full compaction in cast test specimens. In addition, three ferrocement slab specimens, having the dimensions of 100mm by 500mm by 19mm, were cast in 112

3 wooden moulds. The ferrocement slabs had four layers of welded steel wire mesh which was cut to the required size, before placing within the mould. The bundled wire mesh reinforcement was placed on skeletal steel rods measuring 5mm in diameter. The moulded specimens were demoulded after 24 hours of casting and stored in water until testing at the age of 28 days. Testing of mortar and cast specimens Freshly mixed plain and fibre reinforced mortar mixes were tested for their flow in accordance with ASTM C1437 Mortar Flow test for their workability. At the age of 28 days, the small cylinders were tested in direct compression and the large cylinder was used to determine the static modulus of elasticity. The relevant testing procedures were in accordance with the procedures described in AS1012: Testing of concrete. Ferrocement specimens were tested in flexure over a span of 450mm in a four-point test under controlled rate of loading until failure. The mid-span deflection was continuously monitored using an LVDT. Table 1: Properties of mortar mixes Mortar Flow (%) Compressive Strength (MPa) Modulus (GPa) Plain PVA fibres (6mm) PAV fibres (12mm) RESULTS AND DISCUSSION Properties of PVA fibre mortar Table 1 shows the workability (flow), compressive strength and modulus of elasticity of the mortar mixes, with and without PVA fibres. The flow of the mortar mix was marginally increased by the addition of 6mm and 12mm PVA fibre with the volume fraction of 0.25%. The results showed that no noticeable improvement in the workability with the addition of PVA fibres was observed. While mixing some difficulty was experienced with fibre mortar mixes, due to the water absorption of hydrophilic uncoated PVA fibre. Considering the 28-day compressive strength results, it can be said that the PVA fibres had reduced the mortar strength by 8.6% and 7.5% with the addition of 6mm and 12mm PVA fibres, respectively. However, the reduction for the modulus of elasticity was less than 3% even though the modulus of PVA fibre is comparable with that of mortar. The decreases for compressive strength and modulus of elasticity could be due to increased internal porosity of the mortar, as the consequence of high water absorption of the uncoated PVA fibres. 113

4 Figure 1: Load-deflection of PVA (6mm) fibre mortar in flexure Flexural behaviour of PVA fibre reinforced mortar Figure 1 shows the mid-span deflection for the fibre reinforced high-strength mortar with 6mm PVA fibres. The maximum failure flexural load was 0.625kN. The mid-span deflection was increased with the increase in the load. The non-linear behaviour of loaddeflection curve was due the formation of multiple fine cracks. The deflection at the failure load was about 0.4mm. When the failure load was reached, the load suddenly dropped due to the formation of multiple large cracks. The mid-span deflection was increased even though the load had dropped significantly. Figure 2: Load-deflection of ferrocement with plain mortar in flexure 114

5 Figure 3: Load-deflection of ferrocement with fibre mortar (6 mm PVA fibres) Flexural behaviour of ferrocement Figure 2 shows the flexural behaviour of ferrocement with plain mortar. The maximum failure load was 0.75kN and the mid-span deflection at this load was 4mm. The mid-span deflection was increased with the increase in the applied load and ductile behaviour was clearly observed, due the formation of multiple cracks. The load-carrying capacity of ferrocement was significantly reduced after reaching the failure load as seen from the descending portion of the load-deflection curve. The mid-span deflection at 50% of the failure load was about 10mm. These results clearly showed a highly ductile behaviour of ferrocement with high-strength mortar. Figure 4: Load-deflection of ferrocement with fibre mortar (12 mm PVA fibres) 115

6 Flexural behaviour of ferrocement with PVA fibre-reinforced mortar Figures 3 and 4 show the flexural behaviour for the ferrocement with fibre- reinforced high strength mortar with 6mm and 12mm length PVA fibres, respectively. The failure load for ferrocement with 6mm long PVA fibre mortar was 0.8kN while that for with 12mm long PVA fibres was 1.15kN. The improvement of 44% for the failure load is probably due to the increased effectiveness of PVA fibre with increased aspect ratio. The results also showed that that load-carrying capacity of ferrocement with PVA fibre mortar had fluctuated noticeably before the failure load was reached. This could be due to the formation of multiple cracking of fibre mortar which had substantially reduced the stiffness of the ferrocement composite. However, the strong bond strength between the steel wire-mesh reinforcement and high-strength mortar had contributed to maintain the load-carrying capacity in the cracked ferrocement slabs. The deflection at the failure load was about 7mm and 8mm with 6mm and 12mm long PVA fibres, respectively. Figure 5: Cracking of ferrocement with PVA (12mm long) fibre mortar The results also showed that once the major cracks were developed at the failure load, the load-carrying capacity dropped suddenly similar to a brittle material. The formation of wide crack in the ferrocement with 12mm long PVA fibre reinforced mortar is shown in Figure 5. Therefore, it can be said that the ferrocement with PVA fibre reinforced highstrength mortar showed improved performance up to the failure load and reduced postfailure load ductility behaviour compared to ferrocement with plain mortar (Figure 2). 116

7 CONCLUSIONS Based on the experimental investigation on the flexural behaviour of ferrocement with high strength mortar with and without uncoated PVA fibres, the following conclusions could be made: 1. Ferrocement with high-strength mortar is a ductile material and brittleness of high strength mortar does not influence the ductility of ferrocement. 2. Addition of PVA fibres to the high strength mortar had significantly modified the flexural behaviour of ferrocement up to the failure load by increasing the ductility with the loss in the stiffness. 3. PVA fibres in high strength mortar caused the ferrocement to reduce its ductility beyond the failure load. 4. Increasing the PVA length from 6mm to 12mm had marginally increased the failure load of ferrocement but reduced the post-failure load ductility more significantly. REFERENCE [1] Sri Ravindrarajah, R. and Tam, C. T., Dimensional stability of ferrocement, J. of Ferrocement 13 (1) (1983) [2] Sri Ravindrarajah, R. and Tam, C. T., Watertightness in ferrocement, J. of Ferrocement 14 (1) (1984) [3] Paramasivam, P. and Sri Ravindrarajah, R., Effect of arrangement of reinforcement on ferrocement properties ACI Structural J. 85 (1988) [4] Lee, S. L., Tam, C. T., Paramasivam P., Das Gupta, N. C., Sri Ravindrarajah, R. and Mansur, M. A., Ideas tested on Ferrocement applications at the University of Singapore, Concrete International: Design & Construction 5 (11) (1983) [5] Sri Ravindrarajah, R., Behaviour of fibre-reinforced thin slabs under short and long-term loading, Proceedings of an International Symposium and Workshop on Ferrocement and Thin Reinforced Cement Composite, (FERRO-8), Bangkok, Thailand, February, 2006 [6] Sri Ravindrarajah, R., Fibre-reinforced and Ferrocement Car-park Pavers, Proceedings of the X International Symposium on Ferrocement and Thin Reinforced Cement Composites (FERRO 10), Havana, Cuba, October, [7] Akers, S. A. S., Studinka, J. B., Meier, P., Dobb, M. G., Johnston, D. J. and Hikasa, J., Long term durability of PVA reinforcing fibres in a cement matrix, The International J. of Cement Composites and Lightweight Concrete 11 (2) (1989) [8] Kim, D. J., Naaman, A. E. and El-Tawil, S., Comparative flexural behaviour of four fibre reinforced cementitious composites Cement & Concrete Composites 30 (2008) [9] Li V.C. On engineered cementitious composites (ECC) - a review of the material and its applications. J. Advanced Concrete Technology 1(3) (2013) [10] Lepech M. D. and Li, V. C., Water permeability of engineered cementitious composites Cement & Concrete Composites 31 (2009) [11] Noushini, A., Samali, B., and Vessalas, K., Effect of polyvinyl alcohol (PVA) fibre on dynamic and materials properties of fibre reinforced concrete Construction and Building Materials 49 (2013) [12] Toutanji. H. A. and El-Korchi. T., The influence of silica fume on the compressive strength of cement paste and mortar, Cement and Concrete Research, 25(7) (1995)

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